cellular system

A PROJECT REPORT ON

“CELLULAR SYSTEM” Head of the Department

Project In charge

Er. Vishal Awasthi Deptt. of E.C.E U.I.E.T., C.S.J.M University, Kanpur

Mr. Ajeet Srivastava Deptt. of E.C.E U.I.E.T., C.S.J.M University, Kanpur

Project Guide Er. Prashant Bisht Deptt. of E.C.E U.I.E.T., C.S.J.M University, Kanpur

Submitted By Ashita Srivastava Awakash Dixit Payal Sharma Supreet Bagga

(673019) (673021) (673039) (673053)

M.Sc. (Final Year) 2006 – 2007 Deptt. of E.C.E., U.I.E.T., Kanpur

Acknowledgment I wish to express my sincere thank, deep sense of gratitude to my chief supervisor esteemed teacher Er. Vishal Awasthi (Head of Department),

Mr.

Ajeet

srivastava

(project

Incharge)

and

Er.

Prashant Bisht (project guide) of Department of Electronics and Communication Engineering, U.I.E.T., C.S.J.M. University, Kanpur for

their

keen

Interest,

valuable

suggestions,

guidance

and

encouragement throughout the course of this study. I would also like to express my heart felt gratitude to the institute staff & my friends for their valuable support in study & for their suggestions. Last but not least I would like to vote a thank to my parents for being affectionate, caring and homely throughout my stay during the course of the study of this project. Ashita Srivastava M.Sc. (Electronics)


Mr.

Ajeet

srivastava

(project

Incharge)

and

Er.


their

keen

Interest,

valuable

suggestions,

guidance

and

encouragement throughout the course of this study. I would also like to express my heart felt gratitude to the institute staff & my friends for their valuable support in study & for their suggestions. Last but not least I would like to vote a thank to my parents for being affectionate, caring and homely throughout my stay during the course of the study of this project. Awakash Dixit M.Sc. (Electronics)


Mr.

Ajeet

srivastava

(project

Incharge)

and

Er.


their

keen

Interest,

valuable

suggestions,

guidance

and

encouragement throughout the course of this study. I would also like to express my heart felt gratitude to the institute staff & my friends for their valuable support in study & for their suggestions. Last but not least I would like to vote a thank to my parents for being affectionate, caring and homely throughout my stay during the course of the study of this project. Payal Sharma M.Sc. (Electronics)


Mr.

Ajeet

srivastava

(project

Incharge)

and

Er.


their

keen

Interest,

valuable

suggestions,

guidance

and

encouragement throughout the course of this study. I would also like to express my heart felt gratitude to the institute staff & my friends for their valuable support in study & for their suggestions. Last but not least I would like to vote a thank to my parents for being affectionate, caring and homely throughout my stay during the course of the study of this project. Supreet Bagga M.Sc. (Electronics)

U.I.E.T., C.S.J.M. UNIVERSITY, KANPUR (Electronics & Communication Engineering Department)

Certificate This is to certify that Ashita Srivastava (Roll No. 673019) the student of M.Sc. Electronics (Final Year) has completed the project on “cellular system” in the academic session of 2006-2007. This project is submitted in fulfillment of describe course of study. She has worked with proper sincerity and devotion in completing this project. We wish her all the success for her bright future.

Head of the Department

Project In charge





Certificate This is to certify that Awakash Dixit (Roll No. 673021) the student of M.Sc. Electronics (Final Year) has completed the project on “cellular system” in the academic session of 2006-2007. This project is submitted in fulfillment of describe course of study. He has worked with proper sincerity and devotion in completing this project. We wish him all the success for his bright future.


Project In charge





Certificate This is to certify that Payal Sharma (Roll No. 673039) the student of M.Sc. Electronics (Final Year) has completed the project on “cellular system” in the academic session of 2006-2007. This project is submitted in fulfillment of describe course of study. She has worked with proper sincerity and devotion in completing this project. We wish her all the success for her bright future.


Project In charge





Certificate This is to certify that Supreet Bagga (Roll No. 673053) the student of M.Sc. Electronics (Final Year) has completed the project on “cellular system” in the academic session of 2006-2007. This project is submitted in fulfillment of describe course of study. She has worked with proper sincerity and devotion in completing this project. We wish her all the success for her bright future.


Project In charge




ABSTRACT This project is based on simulation of cellular communication system using MATLAB. In a mobile communication system, the cellular telecommunication model is frequently used as the basic access scheme, because it can obtain high utilization efficiency of frequencies. In our project we used computer simulations to evaluate for total system performance, such as system capacity, and blocking probability or forced termination probability of a call. Our simulation consists of three main parts: 1. Preparation part 2. Programming part 3. The output part In preparation part, several pieces of information needed for the simulation are introduced, such as the cell layout or traffic parameters, before the main loop is started. In programming part of our simulation, each user causes several events, such as call initiation, channel searching, channel allocation, channel reallocation, and call termination based on status matrix. Finally in the output part, measured and accumulated data in the main loop are organized into output, in the form of matrix.

Table of contents

1. Introduction

1

2. The cellular concept - system Design Fundamentals

9

3. GSM

34

4. CDMA

69

5. Conclusion

94

6. Software part

96

a) Objective of cellular concept b) Objective of CDMA c) Programming part 7. References

Introduction The prosperous progress of mobile communication has built the main road of the history of wireless communication. When a mobile communication system was to be standardized, many system proposals were submitted to standardization bodies. Then, these proposals were equally evaluated by using computer simulations or developed prototypes. Thee history of mobile communication can be categorized in to three periods: 1. The pioneer era, 2. The pre cellular era. 3. The cellular era summarizes several representative events in each era.

In the pioneer era, a great deal of the fundamental research and development in the files of wireless communication took place.

Time

Significant Event

Pioneer Era 1860s

James clark Maxwell’s electromagnetic(EM) wace postulates

1880s

Proof of the existence of EM waves by heinrich Rudolf Hertz

1890s

First use wireless and first patent of wireless communication by gugliemo Marconi.

1905

first transmission of speech and music via a wireless link by Reginald fessenden

1912

Sinking of the Titanic highlights the importance of wireless communication on the seaways, in the following year’s marine radio telegraphy is established.

1

From the above fundamental research and the resultant developments in wireless telegraphy, the application of wireless telegraphy to mobile communication systems started from the 1920s, the period, called the precellular era.

Precellular Era 1921

Detroit Police Department conducts fields tests with mobile radio In the United States, four channels in the 30-40 MHz range.

1933

In the United states, rules for regular services Wireless communication is stimulated by world war II

1938

First commercial mobile telephone system operated by the Bell

1940

system and deployed in St.Louis. First commercial fully automatic mobile telephone system is

1946

deployed in Richmond. Virginia, in the United States Microwave telephone and communication links are developed

1948

Introduction of trunked radio system with automatic channel allocation allocation capabilities in the United states.

1950s

Commercial

mobile

telephone

system

operated

in

many

countries(e.g. 100 million moving vehicles on U.S. highways, “BNetz” In West germany)

1960s

1970

The cellular zone concept was developed to overcome this problem by using the propagation characteristics of radio waves. A frequency channel in one cellular zone is used in another zone. However, the distance between the cellular zones that use the same frequency channels is sufficiently long to ensure that the probability of interference is quite low. The use of the new cellular zone concept launched the third era, know as the cellular era.

2

Cellular Era 1980s

Deployment of analog cellular systems

1990s

Digital Cellular deployment and dual-mode operation of digital systems

2000s

Future

public

land

system(FPLMTSs)/international

mobile

telecommunication

mobile

telecommunications-

2000(IMT-200)/ universal telecommunication systems (UMTS) will be deployed with multimedia services

2010s

Fixed-point (FP)-based wireless broadband communication and software radio will be available over the internet. Radio over fiber (such as fiber-optic microcells) will be available.

2010s+

Classification of mobile communication systems: Analog cellular: Analog cellular radio systems include the system such as AMPS, NMT450, NMT-900, TACS, ETACS, C-450, RTMS, radiocom-2000, JTACS/NTACS, NTT etc. The highest frequency range is of JTACS/ NTACS i.e. 915-925/869-870 MHz & radio-com 2000 has the lowest frequency range i.e. 165.2-168.4/ 169.8-173 MHz. AMPS has the highest channel spacing of 30 KHz as compared to other system are deployed are Australia, china, Europe, UK, America, Japan etc.

DIGITAL CELLULAR RADIO SYSTEM: Digital cellular system includes system viz. GSM DCS-1800, 15-95, 1595, PDC. Here, PDC has the highest frequency range comprising of Tx:14,429-1,453, Rx:1,477-1,501 MHz. also, 15-95 has largest channel spacing of 1,250 MHz All the above said system have the multiple access to TDMA/FDMA. & duplex of FDD. The 3

type of modulation involved is GMSK,  /4 DQPSK, QPSK or BPSK. the regions where such system are developed are N. America, Australia, S.E Asia, etc. IMT-2000: This system includes systems viz. WCDMA, CDMA 2000. WCDMA has the frequency of 3-GHz band. Such systems have asynchronous type of synchronization between base station. Such systems have GSM N/W & ANSI-41 type exchange. DSRC SYSTEM: It has the active & backscatter system. Such types of systems have ARIB & CEN type of organization respectively. Backscatter has the 10.7 MHz RF carriers spacing & the largest band width of 10MHz for backscatter type system. The type of modulation involved is ASK type. & data coding is of Manchester code.

PHS/DECT/PCS

Digital Cellular

Analog cellular

IMT- 2002

ITS (dsrc)

High-speed wireless access system

UltraHighspeed Wireless Access system

Evaluation by computer simulation The performances of several wireless communication systems can be evaluated by computer simulation without the need to develop prototype and perform field experiments. This book focuses on the evaluation of digital wireless communication systems. The typical models of digital wireless communication systems are categorized into

three

types:

(1)

point-to-point

communication,

(2)

communication, and (3) multipoint-to-multipoint communication.

4

point-to-multipoint

Point-to-point communication In point-to-point communication, information data is first fed into a source encoder. In this encoder, the information data is digitized if it is analog data. If the volume of digitized data is large, the data is compressed by one of several encoding methods. Motion picture experts group(MPEG) and adaptive differential code modulation (ADPCM). Then the source-encoded digital data is fed into a channel encoder to reduce the occurrence of bit errors under sever radio communication channels. Next the channelencoded digital data is fed into a digital modulator and converted to a radio signal.

The process of modulation is performed is in the lower frequency band as well as the carrier radio frequency (RF) band. In some cases, we use a quite low-frequency band in which digital signal processing can be performed. Then the digital modulated signal is transmitted to the receiver through a radio channel. In the receiver the received signal is fed into the digital demodulator and down converted to base band digital data. And finally the transmitted digital data is recovered. Next the detected and phases caused by the radio channel are compensated and the ECC used in the transmitter is decoded. Finally the channel-decoded data is fed into the source decoder, and the transmitted information data is recovered.

5

Point-to-multipoint Communication In point-to-multipoint communication, an access point (AP) communication with several user terminals (UTs). First user sends their information data by using some protocols. The protocols are defined between an AP and several UTs. In some case, collisions and counting the number of collisions, we simulate the throughput of transmission data. Moreover, the average delay times to transmit information data difference between levels of received signals are large. In this case, the data that has the largest signal level is received at the AP even if collisions occur. This effect is called a capture effect.

Multipoint-to-multipoint communication In the multipoint-to-multipoint communication, several APs and UTs are installed, and call-blocking probability is evaluated as a new evaluation topic. Call blocking occurs when the cellular system shown figure considered. In this cellular system, frequencies used in a cellular zone are reused at another cellular zone. Transmission signals generated by UTs located in one cellular zone sometimes interfere with APs located in other cellular zones using large. By making a simulation model of multipoint-to-multipoint communication, we can evaluate the probability of call blocking. 6

First of all the position of UTs and APs, the number UTs, and the size of the cellular zone are determined. At the same time, the traffic model, a date generation model in each terminal is defined. Then under a programmed resource management technique such as frequencyallocation methods, a cellular zone is configured , and the ratios between the desired signal level and undesired signal levels are the measured at all APs. The ratio is calculated by considered such factors as the position of APs and UTs, the transmitted signal power, the antenna gain and pattern, and the height of antenna. By comparing the ratio and the defined threshold level, we can understand whether call blocking occurs. Finally we obtain the call-blocking probability.

7

start Preparation, status initialization

Time loop

Finishing call is terminated

Connected call is checked, if necessary, reallocation New call arrival and channel assignment

Calculation of CIR at each access points and user terminals

Calculation of CIR at each access point

Time loop

Output data

End

8

The cellular conceptsystem design fundamental Introduction The design objective of early mobile radio systems was to achieve a large coverage area by using a single, high-powered transmitter with an antenna mounted on a tall tower. While this approach achieved very good coverage, it also meant that it was impossible to reuse those same frequencies throughout the system, since any attempts to achieve frequency reuse would result in interference. Faced with the fact that government regulatory agencies could not make spectrum allocations in proportion to the increasing demand for

mobile

services,

radiotelephone

it

become

imperative

to

restructure

the

system to achieve high capacity with limited radio

spectrum while at the same time covering very large areas. The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum allocation without any major technological changes. The cellular concept is a system-level idea which calls for replacing a single, high power transmitter (large cell) with many low power transmitters (small cells), each providing coverage to only a small portion of the service area. Each base station is allocated a portion of the total number of channels available to the entire system, and nearby base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations. Neighboring base stations are assigned different groups of channels so that the interference between base stations is minimized. By systematically spacing base stations and their channel groups throughout a market, the available channels are distributed throughout the geographic region and may be reused as 9

many times as necessary so long as the interference between cochannels stations is kept below acceptable levels. As the demand for service increases (i.e., as more channels are needed within a particular market), the number of base stations may be increased (along with a corresponding decrease in transmitter power to avoid added interference), thereby providing additional radio capacity with no additional increase in radio spectrum. This fundamental principle is the foundation for all modern wireless communication systems, since it enables a fixed number of channels to serve an arbitrarily large number of subscribers by reusing the channels throughout the coverage region.

Multiple Access Techniques This scheme is used to allow many mobile users to share simultaneously a finite amount of radio spectrum. Different multiple access tech. are used in different wireless system. In wireless comm. system it is desirable to have full duplex system e.g. in conventional telephone system, it is possible to talk & listen simultaneously, this effect is known as duplexing. It may be done using frequency or time domain tech. Types of multiple access techniques 

FDMA, TDMA and CDMA are the three major access tech. used to share the available b.w. in a wireless common system.



These can be grouped as narrowband and wideband system depending upon how the available b.w. is allocated to the user.

Narrow band system In narrow band sys. and wideband system the spectrum is divided into a large no of narrowband channels. 

In narrowband sys. channels are usually operated using FDD. Then sys. is called FDMA/FD. 10

In narrowband FDMA, a user is assigned a particular channel which is not shared by other users in the vicinity. Narrowband TDMA, on the other hand, allows user to share the same radio channels but allocates a unique times slot to each user in a cyclic fashion on the channel. Wide band system In wide band sys. the transmission b.w. of a single channel is much larger than the coherence b.w. of the channel. In wideband multiple access sys., a large no. of transmitters are allowed to transmit on the same channel. TDMA and CDMA fall in this category. FDMA allocates time slots the many transmitters on the same channel and allows only one transmitter to access the channel at any instant of time where as CDMA allows all of the transmitters to access the channel at the same time. Another multiple access tech. is used called spread spectrum M.A.

11

Frequency Division Multiple Access (FDMA) assigns individual channels to the individual users. In it each user is allocated a unique frequency band or channel. These channels are assigned o demand to users who request service during the period of call, no other user can share channel. In it the users are assigned a channel as a pair of frequencies, one frequency is used for forward channel, while other is used for reverse channel. Time Division Multiple Access (TDMA) System divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive. In it each user occupies a cyclically repeating time slot, so a channel may be thought

of as a particular

time slot that reoccurs every frame. TDMA systems transmit data in a buffer and burst method, thus the transmission for any user is noncontinuous. In Code Division Multiple Access (CDMA) systems, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise code sequence that has a chip rate which is order of magnitudes greater than the data rate of the message. In CDMA system all user use the same carrier frequency and may transmit simultaneously. Each user has its own pseudorandom codeword which is approximately orthogonal to all other 12

code words. The receiver performs a time correlation operation to detect only the specific desired codeword. All other code words appear as noise due to decorrelation. CDMA uses spread spectrum technology to break up speech into small, digitized segments and encodes them to identify each call. While GSM and other TDMA based systems have become the dominant 2G wireless technologies, CDMA technology is recognized as providing clearer voice quality with less background noise, fewer dropped calls, enhanced security, greater reliability and greater network capacity.

Frequency Reuse Cellular radio systems rely on an intelligent allocation and reuse of channels throughout a coverage region. Each cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell. Base stations in adjacent cells are assigned channel groups, which

contain completely different channels

than neighboring cells. The base station antennas are designed to achieve the desired coverage within the particular cell. By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits. The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.

13

Figure (a) illustrate the concept of cellular frequency reuse, where cells labeled with the same letter use the same group of channels. The frequency reuse plan is overlaid upon a map to indicate where different frequency channels are used. The hexagonal cell shape shown in figure is conceptual and is a simplistic model of the radio coverage for each base station, but it has been universally adopted since the hexagon permits easy and manageable analysis of a cellular system. The actual radio coverage of a cell is known as the footprint and is determined from field measurements or propagation prediction models. A Cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell. For a given distance between the centre of a polygon and its farthest perimeter points, the hexagon has the largest area. When using hexagons to model coverage areas, base station transmitters are depicted as either being in the centre of the cell (center-excited cells) or on three of the six cell vertices (edge-excited cells). Normally, omni-directional antennas are used in centre-excited cells and sectored directional antennas are used in corner-excited cells. Practical considerations usually do not allow base stations to be placed exactly as they appear in the hexagonal layout. Most system designs permit a base stations to be positioned up to one-fourth the cells radius away from the ideal location. To understand the frequency reuse concept, consider a cellular system, which has a total of S duplex channels available for use. If each 14

cell is allocated a group of k channels (k < S), and if the S channels are divided among N cells into unique and disjoint channel groups which each have the same number of channels, the total number of available radio channels can be expressed as S = kN

(1)

The N cells which collectively use the complete set of available frequencies is called a cluster. If a cluster is replicated M times within the system, the total number of duplex channels, C, can be used as a measure of capacity and is given by C = MkN = MS

(2)

As seen from equation (2) the capacity of a cellular system is directly proportional to the number of times a cluster is replicated in a fixed service area. The factor N is called the cluster size and is typically equal to 4, 7 or 12. If the cluster size N is reduced while the cell size is kept constant, more clusters are required to cover a given area, and hence more capacity is achieved. A large cluster size indicates that the ratio between the cell radius and the distance between co-channel cells is small. Conversely, a small cluster size indicates that co-channel cells are located much closer together. The value of N is a function of how much interference a mobile or base station can tolerate while maintaining a sufficient quality of communications. From a design viewpoint, the smallest possible value of N is desirable in order to maximize capacity over a given coverage area. The frequency reuse factor of a cellular system is given by 1/N, since each cell within a cluster is only assigned 1/N of the total available channels in the system. Due to the fact that the hexagonal geometry of figure (a) has exactly six equidistant neighbors and that the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees, there are only certain cluster sizes and cell layouts which are possible. In order to tessellate–to connect without gaps between

15

adjacent cells–the geometry of hexagons is such that the numbers of cells per cluster, N, can only have values which satisfy equation (3) N = i2 + ij + j2

(3)

Where i and j are non-negative integers. To find the nearest co-channel neighbors of a particular cell, one must do the following: (1) move i cells along any chain of hexagons and then (2) turn 60 degrees counter clockwise and move j cells. This is illustrated in figure (b) for, i=3 and j=2 (example, N = 19).

Channel Assignment Strategies

In Figure(c) the four base stations are R0, R1, R2 and R3 and the four users are T0, T1, T2 and T3. Here, it is assumed that user Ti is connected to base stations Ri, and that all of four users are allocated the same traffic channels. On this assumption, we can derive the carrier to noise plus interference ratio (C/(N+I) Rcni for user T0 as follows: 16

Rcni 

 0

0

AP0 d 10 3

10

N   APi d 10 i 1

 i

i 10

Here,  is a path loss factor, A is a proportional coefficient, Pi is the transmitted power of user Ti, di is the distance between user Ti and base station R0. In addition, i is the distortion caused by shadowing between Ti and R0, the value of which is denoted by decibels. We can see that the longer distances of d1, d2, d3 result in a higher C/(N+I) ratio, which is equivalent to a higher quality of transmission. Therefore, to achieve an adequate C/(N+I) ratio for preliminary service quality, several channel assignment algorithms are being used in the current systems. Channel assignment strategies can be classified as either fixed or dynamic. The choice of channel assignment strategy impacts the performance of the system, particularly as to how calls are managed when a mobile user is handed off from one cell to another. An example of FCA is shown in figure (d), where four channels are allocated to each base station so that users in a cell suffer from minimum co-channel interference from other cells.

17

In the case of FCA, as shown in figure (e1), the second user in a cell faces call blocking, because the channels per base station are fixedly restricted. In a dynamic channel assignment strategy, voice channels are not allocated to different cells permanently. Instead, each time a cell request is made, the serving base station requests a channel from the MSC. The switch then allocates a channel to the requested cell following an algorithm that takes into account the likelihood of future blocking within the cell, the frequency of use of the candidate channel, the reuse distance of the channel, and other cost functions. The MSC only allocates a given frequency if that frequency is not presently in use in the cell or any other cell, which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. Dynamic channel assignment reduce the likelihood of blocking, which increases the trunking capacity of the system, since all the available channels in a market are accessible to all of the cells. Dynamic channel assignment strategies require the MSC to collect realtime data on channel occupancy, traffic distribution, and radio signal strength indications (RSSI) of all channels on a continuous basis. This increases the storage and computational load on the system but provides the advantage of increased channel utilization and decreased probability of a blocked call.

18

For performance measure we are focusing on two measures, blocking probability and forced termination probability. The blocking probability is defined as the statistical probability that a new call will fail to find suitable channels that satisfy the C/(N+I) ratio condition. Although the blocking probability is the measure pertaining to new calls, a connected call can be interrupted before it finishes due to rapid degradation of the C/(N+I) ratio condition. Thus, we define forced termination probability as the statistical probability that a connected call will be interrupted before its conclusion. If we further define callnum, blocknum, and forcenum as the number of generated calls, blocked calls, and forced terminated calls, respectively, we truly introduce the variable of the same names in the simulation as shown in following sections. Blocking probability Pbl and forced termination probability Pfo are given as follows:

Pbl  Pfo 

blocknum call num

forcenum call num  blocknum

Introducing those two performance measures enables several potential evaluations of cellular systems.

Handoff strategies When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This handoff operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station.

19

Processing handoffs is an important task in any cellular radio system. Many handoff strategies prioritize handoff requests over call initiation requests when allocating unused channels in a cell site. Handoffs must be performed successfully and as infrequently as possible. Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the base station receiver (normally taken as between –90dBm and –100dBm), a slightly stronger signal level is used as a threshold at which a handoff is made. This margin, given by  = Pr handoff – Pr minimum usable, cannot be too large or too small. If  is too large; unnecessary handoffs which burden the MSC may occur, and if  is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal condition. Figure (f1) demonstrates the case where a handoff is not made and the signal drops below the minimum acceptable level to keep the channel active. This dropped call event can happen when there is an excessive delay by the MSC in assigning a handoff or when the threshold  is set too small for the handoff time in the system. Excessive delays may occur during high traffic conditions due to computational loading at the MSC or due to the fact that no channels are available on any of the nearby base station.

20

Improper handoff Situation

Proper handoff situation

In deciding when to handoff, it is important to ensure that the drop in the measured signal level is not due to momentary fading and that the mobile is actually moving away from the serving base station. In order to ensure this, the base station monitors the signal level for a certain period of time before a handoff is initiated. This running average measurement

of

signal

strength

should

be

optimized

so

that

unnecessary handoffs are avoided. If the slope of the short-term average received signal level in a given time interval is steep, the handoff should be made quickly. Information about the vehicle speed, which can be useful in handoff decisions, can also be computed from the statistics of the received short-term fading signal at the base station. The time over which a call may be maintained within a cell, without handoff, is called the dwell time. The dwell time of a particular user is governed by a number of factors, including propagation, interference, distance between the subscriber and the base station, and 21

other time varying effects. In first generation analog cellular systems, signal strength measurements are made by the base stations and supervised by the MSC. Each base station constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect, to the base station tower. In today’s second-generation systems, handoff decisions are mobile assisted. In mobile assisted handoff (MAHO), every mobile station measures

the

received

power

from

surrounding

base

stations

and continually reports the results of these measurements to the serving base station. A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time. The MAHO method enables the call to be handed over between base station at a much faster rate than in first generation analog systems since the handoff measurements are made by each mobile, and the MSC no longer constantly monitors signal strengths. MAHO is particularly suited for microcellular environments where handoffs are more frequent.

Interference and System Capacity Interference is the major limiting factor in the performance of cellular radio systems. Sources of interference include another mobile in the same cell, a call in progress in a neighboring cell, other base stations operating in the same frequency band, or any non-cellular system, which inadvertently leaks energy into the cellular frequency band. Interference on voice channels causes cross talk, where the subscriber hears interference in the background due to an undesired transmission. interference

The are

two

major

co-channel

types

of

interference

system-generated and

adjacent

cellular channel

interference. Even though interfering signals are often generated within the cellular system, they are difficult to control in practice (due to 22

random

propagation

effects).

Even

more

difficult

to

control

is

interference due to out-of-band users, which arises without warming due to front-end overload of subscriber equipment or intermittent intermodulation products. Frequency reuse implies that in a given coverage areas there are several cells that use the same set of frequencies. These cells are called co-channel cells, and the interference between signals from these cells is called co-channel interference. Unlike thermal noise, which can be overcome by increasing the signal-to-noise ratio (SNR), co-channel interference cannot be combated by simply increasing the carrier power of a transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells. To reduce co-channels interference, co-channel cells must be physically separated by

a

minimum

distance

to

provide

sufficient

isolation

due

to

propagation. Co-channel interference ratio is independent of the transmitted power and becomes a function of the radius of the cell (R) and the distance between centers of the nearest co-channel cells (D). Thus interference is reduced from improved isolation of RF energy from the co-channel cell. The parameter Q, called the co-channel reuse ratio, is related to the cluster size. For a hexagonal geometry

Q

D  3N R

A small value of Q provides larger capacity since the cluster size N is small, whereas a large value of Q improves the transmission quality, due to a smaller level of co-channel interference. A trade-off must be made between these two objectives in actual cellular design. Co-channel Reuse Ratio for some values of N

23

Cluster size (N)

Co-channel reuse ratio (Q)

i = 1, j = 1

3

3

i = 1, j = 2

7

4.58

i = 2, j = 2

12

6

i = 1, j = 3

13

6.24

Let i0 be the number of co-channel interfering cells. Then, the signal–to-interference ratio (S/I or SIR) for a mobile receiver which monitors a forward channel can be expressed as

S  I

S i0

I i 1

i

Where S is the desired signal power from the desired base station and Ii is the interference power caused by the ith interfering co-channel cell base station. If the signal levels of co-channel cells are known, then the S/I ratio for the forward link can be found using Equation above. Average received signal strength at any point decays as a power law of the distance of separation between a transmitter and receiver. The average received power Pr at a distance d from the transmitting antenna is approximated by-

d  Pr  P0    d0 

n

d  Pr (dBm)  P0 (dBm)  10n log   d0  Where P0 is the power received at a close in reference point in the far field region of the antenna at a small distance d0 from the transmitting antenna and n is the path loss exponent. Now consider the forward link where the desired signal is the serving base station and where the interference is due to co-channel base stations. If Di is the distance of the ith interferer from the mobile, the received power at 24

a given mobile due to the ith interfering cell will be proportional to (Di)-n. the path loss exponent typically ranges between two and four in urban cellular system. When the transmit power of each base station is equal and the path loss exponent is the same throughout the coverage area, S/I for a mobile can be approximated as

S  I

R n i0

 (D ) i 1

n

i

Considering only the first layer of interfering cells, if all the interfering base stations are equidistant from the desired base station and if this distance is equal to the distance D between cell centers, then equation simplifies to

S ( D / R) n ( 3N ) n   I i0 i0 Equation relates S/I to the cluster size N, which in turn determines the overall capacity of the system. The cluster size N should be at least 6.49, assuming a path loss exponent n = 4. Thus a minimum cluster size of seven is required to meet an S/I requirement of 18 dB. For some frequency reuse plans (e.g., N=4), the closest interfering cells vary widely in their distances from the desired cell. Using

the

approximate

geometry

shown

in

figure

(g)

and

assuming n=4, the signal-to-interference ratio for the worst case can be closely approximated as

S R 4  I 2( D  R) 4  2( D  R) 4  2 D 4 Equation can be rewritten in terms of the co-channel reuse ratio Q, as

S 1  4 I 2(Q  1)  2(Q  1) 4  2Q 4 25

For N=7, the co-channel reuse ratio Q is 4.6, and the worst case S/I is approximated as 49.56 (17 dB). Hence for a seven-cell cluster, the S/I ratio is slightly less than 18 dB for the worst case. To design the cellular system for proper performance in the worst case, it would be necessary to increase N to the next largest size, which from equation (3) is found to be 12 (corresponding to i=j=2). This obviously entails a significant decrease in capacity, since 12-cell reuse offers a spectrum utilization of 1/12 within each cell, whereas seven-cell reuse offers a spectrum utilization of 1/7. In practice, a capacity reduction of 7/12 would not be tolerable to accommodate for the worst case situation which rarely occurs. From the above discussion, it is clear that cochannel interference determines link performance.

Interference

resulting

from

signals,

which

are

adjacent

in

frequency to the desired signal, is called adjacent channel interference. Adjacent channel interference results from imperfect receiver filters, which allow nearby frequencies to leak into the pass band. The problem can be particularly serious if an adjacent channel user is transmitting in very close range to a subscriber’s receiver, while the receiver 26

attempts to receive a base station on the desired channel. This is referred to as the near-far effect, where a nearby transmitter (which may or may not be of the same type as that used by the cellular system) captures the receiver of the subscriber. Adjacent channel interference can be minimized through careful filtering and channel assignments. Since each cell is given only a fraction of the available channels, a cell need not be assigned channels, which are all adjacent in frequency. By keeping the frequency separation between each channel in a given cell as large as possible, the adjacent channel interference may be reduced considerably. Thus instead of assigning channels, which form a continuous band of frequencies within a particular cell, channels are allocated such that the frequency separation between channels in a given cell is maximized. If the frequency reuse factor is large (e.g. small N), the separation between adjacent channels at the base station may not be sufficient to keep the adjacent channel interference level within tolerable limits. For example, if a close-in mobile is 20 times as close to the base station as another mobile and has energy spill out of its passband, the signal-tointerference ratio at the base station for the weak mobile (before receiver filtering) is approximately

S  (20) n I For a path loss exponent n=4, this is equal to -52dB. If the intermediate frequency (IF) filter of the base station receiver has a slope of 20dB/octave, then an adjacent channel interferer must be displaced by at least six times the passband bandwidth from the center of the receiver frequency passband to achieve 52dB attenuation. Here, a separation of approximately six channel bandwidths is required for typical filters in order to provide 0 dB SIR from a close-in adjacent channel user. This implies more than six channel separations are needed to bring the adjacent channel interference to an acceptable level. 27

Cell layout and Cell-Wrapping Technique Used We have used 19 hexagonal cells having a cell radius of 1. Such cells are determined by the position of the 19 base stations. In our simulation regulated numbers of users are scattered in each of the 19 cells from which data are taken.

In the case of a cellular system using the DCA algorithm, we should take into account not only one sample cell, but also neighboring cells, because co-channel interference from neighboring cells has a significant effect on the performance of the sample cell. For example, the 5th cell is subject to interference from the 1st, 4th, 14th, a 15th, 16th and 6th cell. Cells located farther away, such as the 2nd or 3rd ones, could interfere with the 5th cell. However, it is assumed that such interference is decreased enough by the distance that it can be ignored, and we take into account only the six immediate neighboring cells in the simulation. On the other hand, in the case of the 9th cell, which is located on the boundary of the cell layout, it has only three neighboring cells, the 10th, 2nd and 8th. Such a “boundary cell” has different performance than an “inner-located cell”, for example the 9th cell, which

28

would show better performance than the 2nd cell, because fewer cells cause interference in the 9th cell. Consequently, taking user activity in the boundary cells as well as that in an inner cell into account does not adequately evaluate DCA performance. Thus, to avoid such a problem, two solutions can be used. One is to take data only from inner cells such as the 1st, 2nd, 3rd, 4th, 5th, 6th and 7th cells in figure, and exclude boundary cells. These inner cells are all subject to interference from six neighboring cells and are expected to reveal effective performances of the DCA algorithm. However, because boundary cells do not contribute to output data, we need a larger number of cells to construct the entire cell layout to obtain wellaveraged data, making the simulation burden heavy. Figure shows a concept of cell wrapping technique, boundary cells are regarded as neighbors of the boundary cells located almost directly opposite the cell layout. In figure given below, only the 19 shaded cells are cells that really exist, and the other cells are copies of the real cells having the same number. As a result, the 9th cell suffers from interference not only from the 10th, 2nd and 8th cells, but also copies of the 13th, 17th and 14th cells in the neighboring positions. On this assumption, every cell in the cell layout can be regarded as being an “inner-located cell” having six neighbors.

29

Cell Mesh Construction In our simulation, user distribution is considered to be uniform over one cell as well as over the entire cell layout. Such a condition is realized by cells distributed into a log of small meshes. Figure shows a cell and meshes used in our simulation.

Set to “Fineness = 50” in our simulation. Here, discrete meshes are used to allocate a certain amount of traffic into the specially shaped area such as a hexagonal cell. If we introduce a larger value of “Fineness”, such a discrete mesh structure gets close to the continuous distribution model, as expected in figure. In the simulation, when a user initiates a call, we generated a random integer “Mesh” whose value uniformly fluctuates from 1 to “meshnum” and locates a user on a location of “meshposition (mesh, :)” in a cell. As a result, each user is scattered in a certain point of “meshnum” points in a cell with equal probability at its call initiation, and we can obtain uniform user distribution over a cell.

30

Holding time Every initiated call has its own call holding time, and the initiated call is terminated after such a time. The holding time of each call is subject to exponential distribution with a mean value of “ht” (second). In our simulation, we obtain the value of the holding time as the output of the function “holdtime(ht)”, which outputs a random value subject to exponential distribution with an average value “ht”.

31

Attenuation due to distance We assume the transmitted signal is subject to path loss with a decay factor of , which is provided as variable “alpha” and set “alpha = 3.5” in the simulation. Calculating this path loss is conducted using the program “dist.m”. This program provides a function “dist (a, b, alpha)”, where “a” and “b” area 1*2 matrix and respectively denote coordinates of two points. Then, value of “Dist (a,b,alpha)” reveals the path loss between these two points, a and b.

Attenuation due to Shadowing Shadowing is assumed to be subject to log-normal distribution with a standard deviation of “sigma”, corresponding to i. This “sigma” is set to “Sigma = 6.5” in the simulation. We also obtain the value of shadowing as the output of the function “shadow (sigma)”, which outputs a random value subject to log-normal distribution with its standard deviation “sigma”. The symbol “shadow(sigma)” is provided by 32

program “shadow.m”, and figure shows a pdf of the “shadow(sigma)” output, which is measured by MATLAB.

Conclusion This chapter focuses on an access scheme simulation to evaluate the total system performance considering multipoint situations. After explaining the concept of a cellular system and a channel assignment algorithm, we introduced an example of a simulation program for a cellular system having DCA, as well as several key techniques that are very useful for conducting these kinds of simulations. Moreover, we also introduced DCA with an array antenna and described the effectiveness of constructing dynamic zones.

33

GSM INTRODUCTION The existence of incompatible analog air interfaces for the cellular networks already in operation in various countries was certainly a deterrent that had to be removed if a truly economic integration were to be achieved in Europe. Full roaming in all countries was then a target to be pursued but this was impossible in light of the different standards adopted by each country. A system that would serve this purpose would also have to provide for different service plans to accommodate different needs and different policies. In the early 1980s, stimulated by the authorities, the Conference

of

European

Postal

and

Telecommunications

(CEPT)

administrations created the Group Special Mobile with the aim of developing

a

pan

–

European

standard

for

digital

communications, The project was named GSM and

cellular

the system

implementing the corresponding standard, Global System for mobile Communications, was also refereed to as GSM. Global System for Mobile (GSM) is a second-generation cellular system standard that was developed to solve the fragmentation problems of the first cellular systems. GSM was the world first cellular system to specify digital modulation and network level architectures and services, and is the worlds most popular 2G technology. GSM was first introduced into the European market in 1991. By the end of 1993, several non – European countries in South America, Asia and Australia had adopted GSM the technically equivalent offshoot, DCS 1800, which supports Personal Communication Services (PCS) in the 1.8 GHz to 2.0 GHz radio bands recently created by governments throughout the world.

34

Form

that

time,

GSM

networks,

either

in

the

original

GSM

conception or as an evolution of it, spread worldwide and are unanimously considered a very successful project. GSM system are found operating in frequency bands around 900 MHz(GSM-9000), 1.8 GHz(GSM-1800), or 1.9GHz (1900). GSM-900 is the original GSM cellular network initially conceived to serve large areas (macro cells) and to operate with high power terminals. GSM1800 and GSM-1900 incorporate the personal communication service concepts. GSM-1800 is designed to operate in Europe and GSM-1900 is designed to operate in America, and both comprise low power terminal and serve small areas (micro cells). A new revision of the GSM specification defines an E-GSM system. In E-GSM, the original GSM-900 operating band is extended and lower power terminal and smaller serving areas (micro cells) are specified. GSM-900, E-GSM, GSM-1800, and GSM-1900 comprise the GSM family and their respective operating bands are shown in table. The number of channels shown in table indicates the maximum possible number of GSM channels within the available band. In fact, because of interference problems, a guard is recommended and fewer channels are actually used.

Features and Services The GSM project embraces an ambitious set of targets: 

International roaming 35



Open architecture



High degree of flexibility



Easy installation



Interoperation with ISDN (Integrated Services Digital Networks),

CSPDN

(Circuit-Switched

Network),

PSPDN(Packed

Network),

and

PSTN

Switched

Public

Data

Public

Data

(Public-Switch

Telephone

Network) 

High-quality signal and link integrity



High-spectral efficiency



Low-cost infrastructure



Low-cost, small terminals



Security features

These objectives have been gradually achieved and broad collections of services are provided. GSM services follow ISDN guide lines and are classified as either teleservices or data services. Teleservices include standard mobileoriginated or base originated traffic. Data services include computer-tocomputer communication and packet-switched traffic. User services may be divided into three major categories: Teleservices Regular telephony, emergency calls voice messaging are within TS. Telephony, the old bidirectional speech calls, is certainly the most popular of all services. An emergency call is a feature that allows the mobile subscriber to contact a nearby emergency service, by dialing a unique number. Bearer services Data services, short massage service (SMS), cell broadcast, and local features are within BS, Rates up to 9.6 Kbit/s are supported. With a suitable data terminal or computer connected directly to the mobile apparatus, data may be sent through circuitswitched or packet-switched networks. Short massages containing as 36

many as 160 alphanumeric characters can be transmitted to or from a mobile phone. Data may be transmitted using either a transparent mode or nontransparent mode. Supplementary ISDN service Supplementary ISDN service are digital in nature and include call diversion, closed user groups, and caller identification, and are not available in analog mobile networks. Some of the SS are as follows: 

Advice of charge. This SS details the cost of a progress.



Barring of all outgoing calls. This SS blocks outgoing calls.



Barring of international calls. This SS block incoming or outgoing international calls as a whole or only those associated with a specific basic service, as desired.



Barring of roaming calls. This SS blocks all the incoming calls or only those associated with a specific service.



Call forwarding. This SS forwarding all incoming calls, or only those associated with a specific basic service, to another directory number. The forwarding may be unconditional or may be performed when the mobile subscriber is busy, when there is no reply, when the mobile subscriber is not reachable, or when there is radio congestion.



Call

hold.

This

communication

on

SS

allows

an

existing

interruption call.

reestablishment of the call is permitted.

37

of

Subsequent



Call waiting. This SS permits the notification of an incoming call when the mobile subscriber is busy.



Call transfer. This SS permits the transference of an established incoming or outgoing call to a third party.



Completion of calls to busy subscribers. This SS allows notification of when a busy called subscriber becomes free. At this time, if desired, the call is reinitiated.



Closed user group. This SS allows a group of subscribers to communicate only among themselves.



Calling

number

identification

presentation/

restriction. This SS permits the presentation or restriction. This SS permits the presentation or restricts

the

identification

presentation number

of

(or

the

calling

additional

party’s address

information). 

Connected number identification presentation. This SS indicates the phone number that has been reached.



Free phone service. This SS allocates a number to a mobile subscriber and all calls to that number are free of charge for the calling party.



Malicious call identification. This SS permits the registration of malicious, nuisance, and obscene incoming calls.



Three-party service. This SS permits SS permits the establishment of conference calls.

38

Architecture The public Land Mobile Network system (PLMN) with GSM architecture is consists of four major blocks: 1. Mobile Station Subsystem (MSS) 2. Base Station Subsystem (BSS) 3. Network and Support Subsystem (NSS) 4. Operation and Support Subsystem (OSS) The MSS consists of the apparatus supporting the interface between the user and the PLMN. The BSS provides and manages radio access between the MSS and the rest of the GSM network. The NSS is responsible for communication within the same PLMN or with other network, such as another PLNM,PSTN,ISDN,CSPDN, and PSPDN. The OSS provides the means for operation and maintenance of the GSM networks; which includes monitoring, diagnoses, and troubleshooting.

39

Mobile station subsystem The MSS is basically a human-machine interface performing function to connect the user and the PLMN. These functions include voice and data transmission, synchronization, monitoring of signal quality, equalization, and display of short messages. Location updates, and others. To carry out all its function, an MSS includes the terminal equipment (TE), the terminal adapter (TA), the mobile termination (MT), and a subscriber identity module (SIM). TE, TA, and MT compose the mobile equipment, and SIM enables the use of the mobile equipment. Mobile Equipment A TE may be a fax, a computer, or another nonspecific GSM device. An MT is the equipment that realizes the standard GSM mobile terminal function. A TA works as an interface between the TE and the MT. These devices may all be integrated into one piece or just partially

integrated.

In

addition

to

its

international

equipment

identification, the mobile equipment is identified through its class mark, which include the following information: 

Revision level. This indicates the GSM specification version implemented within the terminal



Encryption capability. This indicates the type of cryptography supported by the equipment



Frequency capability. This indicates the frequency band in which the equipment can operate.



Short message capability. This indicates whether or not the equipment is able to receive/send short messages.



RF power capability. This indicates the maximum RF power of the equipment.

40

Subscriber Identity Module A GSM MSS is not associated with the subscriber, i.e., subscriber identity and equipment identity are independent element. This is achieved through the SIM card, which is a removable card containing all subscriber specific data, such as identification numbers, contracted services, and personal features, as chosen by the subscriber. The user may never own an equipment set. It suffices to have an SIM card that can be inserted into any type of GSM MSS. A validation process is utilized with the insertion of SIM. This is accomplished when the subscriber provides the four-digit personal identification number (PIN), which should match that number stored within the SIM card. An SIM card can be found in credit card-sized or plug-in-sized format. The SIM card makes it possible for the subscriber to be charged

according

to

the

subscriber’s

identification.

Except

for

emergency calls, any other call may only be performed with the use of the SIM card. Without a SIM installed, all GSM mobiles are identical and nonoperational. It is the SIM that gives GSM subscriber units their identity. Subscribers may plug their SIM into any suitable terminal such as public phones, hotel phones, mobile phones and are then able to have all incoming GSM calls routed to that terminal and have all outgoing calls billed to their home phones, no matter where they are in the world.

Base station Subsystem The BSS response for the radio coverage of a given geographic region and for appropriate signal processing. It is function and physically separated into two component: a base transceiver station (BTS) and base station controller (BSC). A third element within the BSS

41

(not shown in is the transcoder (XCDR), also named transcoder/ rate adapter unit (TRAU) Base station Transceiver A BTS is responsible for the radio coverage it self and basically consists of the radio equipment. The BSC is responsible for function concerning network operation and signal processing. Base Station Controller A BSC control one or more (typically several) BTSs. Its functions include radio resource management, signaling transmission, power control, handover control, frequency hopping control, and others. Transcoder/Rate Adapter Unit A TRAU is a device placed between two GCM element- BTS, BSC, or mobile switching center (MSC)- and is used to conserve transmission resources. It takes four 13-kbit/s speech channels ( to be defined later), inserts overhead information to bring each channel to 16 kbit/s, and combines the four channels into one data stream of 64 kbit/s. channels with lower data rates are also raised be multiplexed to from a 2.048Mbit/s data stream. A TRAU can be physically located within a BTS, a BSC; the second option is the most common configuration.

Network and Switching Subsystem The NSS handles the switching of GSM calls between external networks and the BSCs in the radio subsystem and is also responsible for managing and providing external access to several customer database. Its functions include coordination of call setup, paging, resource allocation, location registration, encryption, interfacing with other networks, handover control, billing, synchronized, and others. The NSS consists of the MSC, the home location register (HLR), the visitor location register (VLR), the authentication center (AuC), and the equipment identity register (EIR). 42

Mobile Switching Center MSC is central unit in NSS and controls traffic among all the BSCs. The MSC performs the switching functions and coordinates the calls and routing procedures within GSM. Thus, It is responsible for traffic management and the radio coverage of a given geographic area, the MSC area. A GSM network may have one or more MSCs, depending on the traffic to be controlled. An MSC is responsible for several functions such as paging, coordination of call setup, allocation of resources, interworking with other networks, handover management, billing, encryption, echo canceling, control, synchronization with BSSs, and others. It interfaces the GSM PLMN with the external networks such as PSTN, ISDN, CSPDN, and PSPDN. Such interfacing may be carried out through a gateway MSC connected to a serving MSC. Home Location Register The

HLR

is

a

database

containing

a

list

of

those

subscribers belonging to one or more MSC areas within which they have originally been registered. An HLR, therefore, defines the subscription area. In the HLR, These subscribers are associated with information records relevant to call management. Both permanent and temporary data are held within the HLR. The Permanent data constitute data that are modified only for administrative reason and are kept for every call. The temporary data comprise data that are modified to accommodate the transient status of the subscriber’s parameters and can be changed from call. The permanent data include IMSI, MS-ISDN, and information on

roaming

restriction,

permitted

supplementary

services,

and

authentication key for security procedures. The temporary data consist of MSRN, data related to ciphering, VLR address, MSC address, and information on roaming restriction. An HLR is usually centralized, but it can also be distributed within the network, with the configuration chosen in accordance with the operator’s needs. 43

Visitor Location Register The VLR is a database containing a list of those subscribers belonging to another subscription area, but who are now in a roaming condition to this MSC area. In the VLR, these roaming subscribers are associated with information records relevant to the call management. In essence, the VLR contains the same information associated with the HLR. In fact, when a subscriber roams into another MSC area, the relevant data belonging to this subscriber stored in the HLR are transferred to the corresponding VLR. The data of the roaming subscriber, retrieved from the HLR, remain in the respective VLR as long as the subscriber is found in a roaming condition. A VLR is usually co-located with the MSC. Authentication Center The AuC is a strongly protected database which handless the authentication & encryption keys for every single subscriber in HLR & VLR. An authentication key, kept in the SIM card and in the AuC, is provided for each subscriber in the system and is never transmitted over the air. Instead, a random challenge and the response to this challenge are transmitted. The random challenge and the respective response are based on authentication keys and on some ciphering algorithms. The contents of the information exchanged in such a procedure may change for each call. This, in conjunction with the TMSI, constitutes an interesting procedure that renders GSM robust with respect to unauthorized accesses. On the other hand, vulnerability is present when the authentication key must be transmitted form an HLR to a VLR in a roaming situation. EQUIPMENT IDENTITY REGISTER The authentication center contains a register called the EIR which identifies stolen or fraudulently altered phones that transmit 44

identity data that does not match with information contained in either HLR or VLR. The EIR is a database containing the IMEIs of all subscribers. The IMEIs are grouped in three categories as follows: 1) White list, containing the IMEIs known to belong to equipment with no problems. 2) Black list, containing the IMEIs of equipment reported stolen 3) Gray list, containing the IMEIs of equipment with some problems not substantial enough to warrant barring

OPERATION AND SUPPORT SUBSYSTEM The

operation

and

support

subsystem

(OSS)

perform

the

operation and maintenance function through two entities, namely, the operation and maintenance center (OMC) and the network management center (NMC). The OSS has three main functions, which are 1) to maintain all telecommunications hardware and network operations with

a

particular

market,

2)

manage

all

charging

and

billing

procedures, and 3) manage all mobile equipment in the system. Within each GSM system,an OMC is dedicated to each of these tasks and has provisions for adjusting all base station parameters and billing procedures, as well as for providing system operator with the ability to determine the performance and integrity of each piece of subscriber equipment in the system. In general, the operation and management functions performance by the OSS include alarm handling, fault management, performance management, configuration control, traffic data acquisition, etc. In many

circumstances,

the

actions

may

be

taken

remotely

and

automatically and, upon detection of an abnormal operation, tests, diagnoses, and fault removal can be carried out to place the system back in service.

45

Operation and Maintenance Center The network resources may be activated or deactivated via the OMC functions. There may be one or several OMCs depending on the size of the network. An OMC is regional entity used for daily maintenance activities. Networks management Center The NMC serves the entire network. It performs a centralized network management and is used for long-term planning.

Open Interfaces A large number of open interfaces are specified within the GSM architecture. Open interfaces favor market competition with operators able to choose equipment from different venders. The various interfaces are identified as follows. 

A-Interfaces: The interfaces between BSC and MSC. It supports signaling and traffic (voice and data) information transmitted by means of one or more 2.048-Mbit/s-transmission systems.



Abis-Interface: The interface between BTS and BSC. It handles common control functions within a BTS. IT is physically supported by a digital link using the link access data protocol (LAPD).



B-Interface: The interface between MSC and VLR.



C-Interface: The interface between MSC and HLR.



D-Interface: The interface between HLR and VLR.



E-Interface: The interface between MSCs.



F-Interface: The interface between MSC and EIR.



G-Interface: The interface between VLRs.



H-Interface: The interface between HLR and AuC.



Um-Interface: The interface between MSS and BSS.

The great majority of the GSM interfaces make use of those wellconsolidated protocols such as SS7, X.25, and LAPD (the ISDN data 46

link layer). The control information may use protocols such as SS7, LAPD, and X.25.

Multiple Access GSM is a fully system with a multiple-access architecture based on the narrowband FDMA/TDMA/FDD technology. GSM-900 uses a total 50-MHz band, divided into two 25-MHz bands, with uplink and downlink separated by 45 MHz. with a carrier spacing of 200 kHz the maximum number of carries per direction is 25,000 » 200 = 125. EGSM adds 10MHz, and correspondingly 50 carries, to each direction of transmission; the separation between uplink and downlink (45 MHz) is maintained. GSM – 1800 uses of a total 150- MHz band, divided into two 75-MHz bands. The same 200- kHz carries spacing is used, this leading to a maximum of 75,000 » 200 = 375 carries per direction, with the separation between uplink and downlink 95 MHz. GSM–1900 makes use of a total 120-MHz bands, divided into two 30-MHz bands. The number of carriers per direction is 60,000 » 200 = 300, with a separation between uplink and downlink of 80 MHz. In all cases, each carrier is shared in time by eight accesses.

Signal Processing The signal processing operations in each direction (uplink downlink) are briefly described as follows. At the mobile station, and for the uplink direction, the voice signal is A/D converted at a sampling rate of 8 kHz with each sample using 13 bits for uniform encoding. The operation can be understood with help of given figure.

47

Multiple Access GSM is a fully digital system with a multiple access architecture based on the narrowband FDMA/TDMA/FDD technology. GSM -900 uses a total 50-MHz band, divided into two 25-MHz bands, with uplink and downlink separated by 45 MHZ. with a carrier spacing of 200 kHz the maximum number of carriers per direction is 25,000-200=E-GSM adds 10 MHz , and correspondingly 50 carriers, to each direction of transmission; the separation between uplink and downlink (45 MHz) is maintained. GSM-1800 uses of a total 150-MHz band, divided into two 75-MHz bands. The same 200-KHz carrier spacing is used, this leading to a maximum of 75, 00 – 200=375 carriers per direction, with the separation between uplink and downlink 95 MHz. GSM -1900 makes use of a total 120-MHz band, divided into two 30-MHz bands. The number of carriers per direction is 60,000-200=300, with a separation between uplink and downlink of 80 MHz. In all cases, each carrier is shared in time by eight accesses. The FDMA/ TDMA/ FDD multiple-access scheme allows the terminal to transmit and receive simultaneously. Transmission and reception make use of different carriers. GSM, on the other hand, adopts the solution of aligned frames on the downlink and uplink but with the time reference for the reverse link retarded by three time slots with respect to the time

48

reference of the forward direction. This eliminates the requirement for the terminal to transmit and receive simultaneously. GSM makes use of two types of channels: physical channel and logical channel. The physical channels constitute the physical medium through which information flows. The logical channel supports the logical functions within the network. The combination of a TS number and an ARFCN constitutes a Physical channel for both the forward and reverse link. Each physical channel in a GSM system can be mapped into different logical channels at different time. That is each specific time slot or frame may be dedicated to either handing traffic data (user data such as speech, facsimile, or tele-text data), signaling data (required by the internal workings of the GSM system), or control channel data (from the MSC, base station or mobile user). The GSM specification defines a wide variety of logical channels which can be used to link the physical layer with the data link layer of the GSM network. These logical channels efficiently transmit user data while simultaneously providing control of the network on each ARFCN.

PHYSICAL CHANNELS Although the 25MHz spectrum per direction for GSM-900 admits 125 200-KHz, carriers, in order to allow for 100 kHz of guard bands at both edges of the spectrum, only 124 carriers are effectively specified. The

absolute

radio

frequency

channel

number

(N)

and

the

corresponding centre frequency, in MHz, for the uplink and for the downlink are related to each other as f uplink = 0.2 N + 890 f downlink = 0.2 N + 935 with 1  N  124.

For GSM-1800, the corresponding formulas are f uplink = 0.2 N + 1607.8 49

f downlink = 0.2 N + 1702.8

with 512  N  885. and for GSM-1900, these are f uplink = 0.2 N + 1747.8 f downlink = 0.2 N + 1827.8 with 512  N  810

The multiple-access structure is defined in terms of slot, frame, multiframe, superframe, and hyperframe, as illustrated in figure.

One time slot contains 156.25 bits and has duration of 577  s. therefore; the bit duration is 3.69  s. eight-time slots compose one 4.651-ms frame. There are two types of multiframes: traffic multiframe, with 26 frames and a duration of 120 ms. And control multiframe, with 51 frames and a duration of 235.4 ms. In the same way, there two 50

types of superframes, both with the same 6.12s duration: traffic superframe, with 51 multiframes, and control superframes, with 26 multiframes.

Finally,

2048

superframes

compose

a

hyperframe

(encryption hyperframe) with a total duration of 3h 28m 53.76s. The exact 120 ms traffic multiframe duration conforms to two basic time intervals: that of some digital networks (125 ms for ISDN, for example), and that of the duration of each block of speech (20ms, as is the case of GSM speech coding). The GSM timing structure is very elaborate, with definitions of time intervals ranging from one quarter of a bit

(900ns)

to

3,394,560,000

bits

(3h

28m

53.76s).

The

GSM

transmission rate can be obtained by dividing the number of bits per traffic multiframe (156.25 x 8 x 26 = 32,500) by the traffic multiframe duration (120 ms, a round number), which gives 270,83333…. Kbit/s. a physical channel is then specified by a carrier and a time slot within the whole multiple access structure.

BURST FORMAT GSM specifies five different time slot format (156.25 bit), or burst formats, to comply with various functions to be performed: 1. Normal burst 2. Frequency correction 3. Synchronization burst 4. Access burst 5. Dummy burst The different logical channel according to specific task of these channels uses these bursts. The normal burst is used to carry user information or network control information. The synchronization burst is used for time synchronization of the terminal, i.e., it helps terminals synchronization their operations with the base station.

51

The frequency correction burst is used for frequency synchronization purpose of the terminal with respect to the base station, during this burst; the BTS transmits a signal whose frequency is 67.7 KHz above that of the carrier. The terminals to initiate a dialogue with the system through a signaling protocol use the access burst. The dummy burst carries no information and is transmitted by the BTS. The formats these five bursts are shown in figure.

The following is a brief description of the various fields within the bursts. 

Data: - Except for the normal burst, in which the data field may convey user information or network control information, in all the other burst formats this field conveys network control information only.



Flag: - The flag indicates the type of information that is being transmitted (user information or control network information).



Training: - The training field carries a training sequence used by the adaptive equalizer to estimate the channel.



Tail: - The tail field contains all-zero bits used to indicate the start and the end of the burst. The

52

adaptive equalizer as a start/stop bit pattern also uses them. 

Guard: - The guard field serves several purposes. It uses a ramp time for the transmitter to turn off at the end of the time slot and to turn on at the beginning of the next time slot. It is also used to avoid overlapping between adjacent time slots. In the case of the access burst, it makes it possible for mobile transmissions from all parts of the cell to arrive at the base station within the duration of the burst.



Synchronization: - The synchronization field conveys a synchronization sequence (a known bit pattern), which is used for time synchronization purposes.

LOGICAL CHANNELS The tasks performed in a GSM platform are supported by a number of functional channels, the logical channels. These logical channel

are

two

types:

traffic

channels,

which

convey

payload

information (speech, data ), and signaling channel, which carry overhead (control) information. The traffic channels carrying speech information are two types. FULL-RATE TCH The following full rate speech and data channel are supported: 

Full-rate speech Channel (TCH/FS): - The full-rate speech channel carries user speech which is digitized at a raw data rate of 13kbps. With GSM channel coding added to the digitized speech, the full rate

speech channel carries 22.8

kbps 

Full-Rate Data Channel for 9600 bps (TCH/F9.6):- The full rate traffic data channel carries raw user data which is sent

53

at 9600 bps. With additional forward error correction coding applied by the GSM standard, the 9600 bps data is sent at 22.8 kbps. 

Full-Rate Data Channel For 4800 bps (TCH/F4.8): -The Fullrate traffic data channel carries raw user data, which is sent at 4800 bps. With additional forward error correction coding applied by the GSM standard, the 4800 bps is sent at 22.8 kbs.



Full-Rate Data Channel for 2400 bps (TCH/ F2.4): - the fullrate traffic data channel carries raw user data which is sent at 2400 bps. With additional forward error correction coding applied by the GSM standard, the 2400 bps is sent at 22.8 kbps.

HALF-RATE TCH The following half-rate speech and data channels are supported: 

Half-Rate Speech Channel (TCH/HS):- The half-rate speech channel has been designed to carry digitized speech which is sampled at a rate half that of the full-rate channel. GSM anticipates the availability if speech coders which can digitizes speech at about 6.5 kbs. With GSM channel coding added to the digitized speech, the half-rate speech cannel will carry 11.4 kbps.



Half-Rate Data Channel for 4800 bps (TCH/H4.8): - The half-rate traffic data channel carries raw user data, which is

sent

at

4800

bps.

With

additional

forward

error

correction coding applied by the GSM standard, the 4800 bps data is sent at 11.4 kbps. 

Half-Rate Data Channel for 2400 bps (TCH/H2.4): - the halfrate traffic data channel carries raw user data, which is sent at 2400 bps. With additional forward error correction 54

coding applied by the GSM standard, the 2400 bps data is sent at 11.4 kbps.

The signaling channels are grouped into three categories: broadcast channels, common control channels, and dedicated control channels. The following are the broadcast channels: frequency correction channel (FCCH), synchronization channel (SCH), and broadcast control channel (BCCH). The following are the common control channels: paging channel (PCH), access grant channel (AGCH) and random-access channel (RACH). And the following are the dedicated the control channel stand –alone dedicated control channel (SDCCH), slow associated control channel (SACCH) and fast associated control channel (FACCH)

Broadcast Channels (BCHs) The broadcast channel operates on the forward link of a specific ARFCN within each cell, and transmits data only in the first time slot 55

(TS 0) of certain GSM frames. Unlike TCHs which are duplex, BCHs only use the forward link. just as the forward control channel (FCC) in AMPS is used as a beacon for all nearby mobiles to camp on to, the BCH serves as a TDMA beacon channel for any nearby mobile to identify and lock on to. The BCH provides synchronization for all mobiles with the cell and is occasionally monitored by mobiles in neighboring cells so that received power and MAHO decisions may be made by out-of-cell users. Although BCH data is transmitted in TS 0, the other seven timeslots in a GSM frame for that same ARFCN are available for TCH data, DCCH data, or are filled with dummy bursts. Furthermore, all eight timeslot on all other ARFCNs within the cell are available for TCH or DCCH data. The BCH is defined by three separate channels which are given access to TS 0 during various frames of the 51-frame sequence. Frequency Correction Channel The FCCH bears information for frequency acquisition purposes. The FCCH is a one–way channel operating in the forward direction and using the frequency correction burst format. It occupies TS 0 for the very first GSM frame (frame 0) and is repeated every ten frames within a control channel multiframe. The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station. The 142 all–zero bits in this burst causes the GMSK modulator to deliver an un-modulated carrier for the entire duration of the time slot. The modulator produces a carrier with an offset of 1625 » 24 kHz above the nominal carrier frequency. Upon detecting this sine wave, the terminal can adjust its frequency reference appropriately, and is thus able to keep track of the occurrences of the time slots.

Synchronization Channel 56

The SCH bears data information for timing synchronization and BTS identification purposes. The SCH is a one–way channel operating in the forward direction and using the synchronization burst format. SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is used to identify the serving base station while allowing each mobile to frame and is used to identify the serving base station while allowing each mobile to frame synchronize with the base station. The frame number (FN), which ranges from 0 to 2,715,647, is sent with the base station identity code (BSIC) during the SCH burst. The BSIC is uniquely assigned to each BST in a GSM system. Since a mobile may be as far 30 km away from a serving base station, it is often necessary to adjust the timing of a particular mobile user such that the received signal at the base station is synchronized with the base station clock. The BS issues coarse timing advancement commands to the mobile stations over the SCH, as well. The SCH is transmitted once every ten frames within the control channel multiframe, Broadcast Control Channel The BCCH bears data information for call setup purposes. The BCCH is a one-way channel operating in the forward direction and using the normal burst format. More specifically, it conveys the following information: 

Cell identity



Network identity



Control channel



List of channels structure



List of channel in use



Details of the access protocol The raw message containing this information uses 184 bits.

These 184 bits are protected by an error-correcting block code-a fire code- adding 40 bits to the message .A fire code is specially used to 57

detect and correct bursty errors. The resulting 224 bits together with 4 tail bits are half-rate convolutionally encoded, yielding 456 bits. Finally, these 456 bits are split into 4*114 bits and sent in four time slots.

Common Control Channel (CCCHs) On the broadcast (BCH) ARFCN, the common control channels occupy TS 0 of every GSM frame that is not otherwise used by the BCH or the idle frame. CCCH consist of three different channel: the paging channel (PCH), which is a forward link channel, the random access channel (RACH) which is a reverse link channel, and the access grant channel (AGCH), which is a forward link channel. CCCHs are the most commonly used control channels and are used to page specific subscribers, assign signaling channels to specific users, and receive mobile request for service. These channels are described below. Paging channel The PCH bears data information for paging purposes. The PCH is a one-way channel operating in the forward direction and using the normal burst format. The same coding scheme used for the BCCH is also used for the PCH. Therefore each paging message contains 4 X 114 bits and occupies four time slots. The PCH provides paging signals from the base station to all mobiles in the cell, and notifies a specific mobile of an incoming call which call which originates from the PSTN. The PCH transmits the IMSI of the target subscriber, along with a request for acknowledgment from the mobile unit on the RACH. Alternative, the PCH may be used to provide cell broadcast ASCII text messages to all subscribers, as part of the SMS feature of GSM. Access Grant Channel (AGCH) The AGCH is used by the base station to provide forward link communication to the mobile, and carries data which instructs the mobile to operate in a particular physical channel (time slot and 58

ARFCN) with a particular dedicated control channel. The AGCH is the final CCCH message sent by the base station before a subscriber is moved off the control channel. The AGCH is used by the base station to respond to a RACH sent by a mobile station in a previous CCCH frame. Random Access Grant Channel (RACH) The RACH is a reverse link channel used by a subscriber unit to acknowledge a page from the PCH, and is also used by mobiles to originate a call. The RACH uses a slotted ALOHA access scheme. All mobiles must request access or respond to a PCH alert within TS 0 of a GSM frame. At the BTS, every frame (even the idle frame) will accept RACH transmissions from mobiles during TS 0. In establishing service, the GSM base station must respond to the RACH transmission by allocating a channel and assigning a stand-alone dedicated control channel (SDCCH) for signaling during a call. This connection is confirmed by the base station over the AGCH.

Dedicated Control channels (DCCHs) There are three types of dedicated control channels in GSM, and, like traffic channels, they are bidirectional and have the same format and function on both the forward and reverse links. Like TCHs. DCCHs may exist in any time slot and on any ARFCN except TS 0 of the BCH ARFCN. The stand-alone dedicated control channels (SDCCHs) are used for providing signaling services required by the users. The Slowand Fast-Associated Control Channels (SACCHs and FACCHs) are used for supervisory data transmission between the mobile station and the base station during a call. Stand-alone Dedicated Control Channels (SDCCHs) – The SDCCH carries signaling data following the connection of the mobile with the base station, and just before a TCH assignment is issued by the base station. The SDCCH ensures that the mobile station and the base station remain connected while the base station and MSC verify the 59

subscriber unit and allocate resources for the mobile. The SDCCH can be thought of as intermediate and temporary channel which accepts a newly completed call from the BCH and holds the traffic while waiting for the base station to allocate a TCH channel. The SDCCH is used to send authentication and alert messages (but not speech) as the mobile synchronizes itself with the frame structure and waits for a TCH. SDCCHs may be assigned their own physical channel or may occupy TS 0 of the BCH if there is low demand for BCH or CCCH traffic. Slow Associated Control Channel (SACCH) – The SACCH is always associated with a traffic channel or a SDCCH and maps onto the same physical channel. Thus, each ARFCN systematically carries SACCH data for all of its current users. As in the USDC standard, the SACCH carries general information between the MS and BTS. On the forward link, the SACCH is used to send slow but regularly changing control information to the mobile, such as transmit power level instructions and specific timing advance instructions for each user on the ARFCN. The reverse SACCH carries information about the received signal strength and quality of the TCH, as well as BCH measurement results from neighboring cells. The SACCH is transmitted during the thirteenth frame (and the twenty-sixth frame when half-rate traffic is used) of every speech/dedicated control channel multiframe, and within this frame. The eight timeslots are dedicated to providing SACCH data to each of the eight full–rate (or sixteen half-rate) users on the ARFCN. Fast Associated Control Channels (FACCHs) – FACCH carries urgent messages, and contains essentially the same type of information as the SDCCH. A FACCH is assigned whenever a SDCCH has not been dedicated for a particular user and there is an urgent message (such as a handoff request). The FACCH gains access to a time slot by “stealing” frames from the traffic channel to which it is assigned. This is done by setting two special bits, called stealing bits, in a TCH forward channel

60

burst. If the stealing bits are set, the time slot is known to contain FACCH data, not a TCH, for that frame.

Call Management This section outlines some call management procedures, namely, mobile initialization, location update, authentication, ciphering, mobile station termination, mobile station origination, handover, and call clearing. Mobile Initialization There are three main goals of the mobile initialization procedure: (1) Frequency synchronization (2) Timing synchronization (3) Overhead information acquisition Frequency Synchronization- As the terminal is switched on; it scans over the available GSM RF channels and takes several readings of their RF levels to obtain an accurate estimate of the signal strengths. Starting with the channel with the highest level, the terminal searches for the frequency correction burst on the BCCH. If no frequency correction burst is detected, it then moves to the next highest level signal and repeats the process until it is successful. In this event, the terminal will then synchronize its local oscillator with the frequency reference of the base station transceiver. Timing synchronization:- After frequency synchronization has been achieved, the terminal will search for the synchronization burst for then for the timing information present on the process stating from the frequency synchronization procedure until it is successful. In this event, it moves to the BCCH to acquire overhead system information. Overhead Information acquisition:- After timing synchronization has been achieved, the will search for overhead information on the BCCH. If the BCCH information dose not includes the current BCCH number, it will restart the mobile initialization procedure. In a successful event, the terminal will have acquired, form the BCCH and through the 61

system information massage present on the BCCH, the following main information.       

Country code Network code Location area code Cell identity Adjacent cell list BCCH location Minimum received signal strength The terminal cheeks if the acquired identification codes coincide

with those in the SIM card. In a successful event, it will maintain the link and monitor the PCH. Otherwise, it will start a location update procedure. Location Update A location update procedure is carried out in one of the following events: 

The terminal is switched on and verifies that the identification codes present on the current BCCH do not coincide with those in the SIM card.



The terminal moves into a location area different form that within which it is currently registered.



There has been no activity for a pre-established amount of time. As part of the process used to speed the paging procedure, location reports are used. These location reports are periodic reports used to update the location of the terminal so that, in the event of a page, the latest reported location is used an initial guess to locate the terminal. The time span between location reports constitutes a system parameter whose value is indicated on the BCCH, varying in accordance with the network loading. The location update procedure starts with the uplink channel

request message on the RACH. The network answers with an immediate assignment message on the AGCH indicating the SDCCH 62

number to be used throughout the location update procedure. The terminal moves to this SDCCH and sends a location updating request message

with

authentication authentication

its

identification

procedure is

is

(IMSI

then

unsuccessful,

the

or,

preferably,

carried

out.

procedure

is

In

TMSI).

An

case

the

aborted.

In

a

successful event, the ciphering procedure is performed. The network then uses the location updating accept message to assign a new TMSI to the terminal. The terminal stores its TMSI and responds with a TMSI allocation complete message. The location update is concluded with a channel release message from the network to the terminal. The terminal then resumes its PCH monitoring procedure. Authentication An authentication procedure may be required at the location update

procedure

authentication

or

at

procedure

the

request

starts

with

of the

a

new

network

service. sending

The an

authentication request message to the terminal; the message conveys a 128-bit random number (RAND). The terminal uses the RAND, the secret key, Ki, stored at SIM, and the encryption algorithm, referred to as A3, to compute a 32-bit number, referred to as a signed response (SRES). Another 64-bit key, the ciphering key, Kc, is computed using another encryption algorithm, referred to as A8. The Kc parameter is later used in the ciphering procedure. After these computations, the terminal responds with an authentication response message, which contains the SRES. The network uses the same parameters and the same algorithm to compute another SRES. The terminal SRES and the network SRES are then compared with each other. If a match occurs, the network accepts the user as an authorized subscriber. Otherwise, the authentication is rejected. Ciphering Ciphering

(or

encryption)

is

usually

required

for

user

transactions over the RF link after authentication has been successful. 63

The network transmits a ciphering mode message to the terminal indicating whether or not encryption is to be applied. In case ciphering is to be performed, the secret key Kc (64 bits), which was generated previously in the authentication procedure, the frame number (22 bits), and an encryption algorithm, referred to as A5, are used to compute a 114-bit encryption mask. This mask is modulo-2 added to the 2x57=114 bits of the data fields, in the bursts. Deciphering is obtained at the base station by performing the same procedure. The terminal answers with a ciphering mode acknowledgment message. Note that the ciphering to be used is continuously changing (on a frame-by-frame basis), because it depends on the current frame number. Mobile Station Termination After the mobile initialization procedure, the terminal camps on the PCH. It eventually detects a paging request message conveying its TMSI. This impels the terminal to access the RACH to transmit a channel request message. An immediate assignment with the SDCCH number is sent by the network on the AGCH. The terminal moves to SDCCH and the following occurs. The terminal transmits a paging response message indicating the reason for the specific message (response to a paging). An authentication procedure is carried out, as already described. In a successful event, a ciphering procedure is accomplished, as already described. The base station then sends a setup message. The terminal responds with a call confirmed message followed by an alerting message to indicate that the subscriber is being alerted. At the subscriber’s call acceptance, the terminal sends a connect message and removes the alerting tone. The network responds with an assignment command message indicating the traffic channel number to be used for the conversation. The subscriber, still on the SDCCH, responds with an assignment acknowledgement message and moves to the traffic channel that has been assigned. The network confirms the acceptance of the call by the other party by means of a 64

connect acknowledgement message on the FACCH of the assigned TCH. And the conversation proceeds on the TCH. Mobile Station Origination The terminal detects a user-originated call. It then access the RACH to send a channel request message. An immediate assignment with the SDCCH number is sent by the network on the AGCH. The terminal moves to this channel and the following occurs. The terminal transmits a paging response message indicating the reason for the specific message (call setup). The base station responds with an unnumbered acknowledgement message. An authentication procedure is carried out, as already described. In a successful event, a cheering procedure is performed, as already described. The terminal then sends a setup message. The base station responds with a call confirmed message followed by an alerting message in which case the terminal applies the ring-back tone. At the called party’s call acceptance, the network sends an assignment command message informing the traffic channel number to be used for the conversation. The subscriber, still on the SDCCH, responds with an assignment acknowledgement message and moves to the traffic channel that has been assigned. The network confirms the acceptance of the call by the other party by means of a connect acknowledgement message on the FACCH of the assigned TCH. And the conversation proceeds on the TCH. Handover The handover process in a GSM network has the mobile terminal as an integral part of the procedure. The whole process is named mobile-assisted handover (MAHO). While making use of the traffic channel, the mobile monitors the signal levels of its own channel, of the other channels of the same cell, and of the channels of six surrounding cells. The measurements are then reported to the base on the SACCH. Concerning the control of the process, handovers may occur: 65

   

Within the same BTS or between BTSs controlled by the same BSC Between different BSCs controlled by the same MSC Between different BSCs controlled by the same MSCs Between different BSCs controlled by different MSCs belonging to different PLMNs In addition, there are two modes of the handovers: synchronous

or asynchronous. In the synchronous mode, the origin cell and the destination cell are synchronized. By measuring the time difference between their respective time slots, the mobile itself may compute the timing advance. This is used to adjust its transmissions on the new channel,

therefore,

speeding

up

the

handover

process.

In

the

asynchronous mode, the origin cell and the destination cell are unsynchronized. The timing advance, in this case, must be acquired by means of a procedure involving the terminal and the new BTS, as follows. The mobile terminal sends a series of access burst with a zero timing advance through several handover access messages. The BTS then computes the required timing advance using a round-trip time delay of the messages. On the average, the handover processing time in the synchronous mode (200 ms) is twice as long as that of the synchronous mode (100 ms). Next a simple asynchronous handover procedure occurring between BTSs of the same BSC is described. While in conversation on a TCH, the terminal monitors the signal levels of several channels. These measurements are reported to the base station on a periodic basis by means of the measurement report message running on the SACCH. Whenever suitable, the base sends a handover command message on the FACCH, indicating that a handover is to take place. The number of the new TCH is included within the message. The terminal then moves to this new channel and sends a series of handover access to the terminal. This is done in the physical information message transmitted to the terminal on the FACCH. The

66

timing adjustment is carried out and the terminal responds with a handover complete message. Call clearing The call clearing process may be initiated either by the network or by the mobile. In either case, the channel used for the exchange of information is the BCCH. Assuming the network initiates the clearing, the base sends a disconnect message to the terminal. The terminal responds with a release message. The base replies with a release complete message. If the terminal initiates the clearing, then the same messages flow, but in the opposite direction. Power control GSM employs power control techniques to adjust the power of both mobile stations and base stations for better performance. Power control reduces co-channel interference and increases battery life. Mobile stations can have their power adjusted in steps of 2 dB with the power levels ranging over 30 dB, i.e., 16 power levels are permitted. The time span between power adjustments is 60 ms, corresponding to 13 frames. Base stations can also control their own power, this process involving the mobile station; the mobile station monitors the signal received from the base station and the base station transmitting power can be changed to improve the signal reception at the mobile station. Spectral Efficiency GSM uses powerful interference counteraction techniques such as adaptive equalization, powerful error correcting codes, efficient modulation, speech frame extrapolation and others. These render GSM system robust and capable of operating at a low carrier-to-interference ratio, in which case, reuse factors of three or four cells per cluster, depending on the environment, are admissible. A number of spectral efficiency definitions are available. In accord with reference 12, the spectral efficiency parameter  is defined as

67

 = conversations / cell / spectrum The umber of physical channels in the 50-MHz GSM spectrum is 124 carriers x 8 channels / carrier = 992 (GSM–900). It can also be assumed that all these 992 channels can be used for conversation. Therefore, for a reuse factor of 4:



992  4.96 4  50

And for a reuse factor of 3:



992  6.61 3  50

The same calculations can be performed for the other GSM systems. The results will be very close to these.

Conclusion GSM has emerged as a digital solution to the incompatible analog air interfaces of the differing cellular networks operating in Europe. Among the set of ambitious targets to be pursued, full roaming was indeed a very important one. In addition, a large number of open interfaces have been specified within the GSM architecture. Open interfaces favor market competition with operators able to choose equipment from different vendors. GSM was the first system to stimulate the incorporation of the personal communication services philosophy into a cellular network. These and other innovative features rendered GSM network, either in the original GSM conception or as an evolution of it’s, a very successful project with worldwide acceptance. GSM systems are found operating in frequency bands around 900 MHz (GSM-900), 1.8 GHz (GSM-1800), or 1.9 GHz (GSM-1900). A new revision of the GSM specifications defines an E-power terminals and smaller serving areas.

68

CDMA INTRODUCTION In the cellular telephone industry, CDMA is primarily an airinterface or radio transmission technology and access technique that is bases on direct sequence spread spectrum techniques. Error control coding, spreading of the spectrum, soft handoffs, and strict power control pay a very important role in the design and operation of CDMAbased systems. CDMA is both an access method and an air-interface. The rest of the network and system is very similar to GSM. Radio resources management, mobility management, and security of the CDMA systems are all implemented in the same way as in GSM system. There are difference in terms of handling the power control and employing soft handoffs. With CDMA, all user data, and in most implementations the control channel and signaling information are transmitted at the same frequency at the same time. All the CDMA systems employ direct sequence spread spectrum and powerful error control codes. The primary significance of CDMA is that by employing a variety of physical layer schemes, it is possible to reuse frequencies in all cells. CDMA, as it has been implemented in the IS-95 standard, has demonstrated an increase in system capacity compared with the analog and TDMA systems. CDMA improves quality of voice by using a better voice coder, has resistance to multipath and fading, implements soft handoffs, has less power consumption (6-7 mW on average), that is, about 10 percent of analog or TDMA phones because of implementation of power control, and does not require frequency planning because all cells employ the same frequency at the same time.

69

The air-interface in CDMA systems is by far the most complex of all systems, and it is not symmetrical on the forward and reverse channels.

SSMA SSMA used signals which have a transmission b.w. that is several orders of magnitude greater than the minimum required RF b.w. It is not very b.w. efficient when used by a single user. How ever, since many users can share the same spread spectrum b.w. without interfering with one another, spread spectrum become b.w. efficient in a multiple user enjoinment. There are two types of SSMA tech : Frequency happed MA (FHMA) and direct sequence M.A. (DSMA). DSMA is also called CDMA. Spread spectrum modulation In spread spectrum modulation channel; bandwidth and transmit power are sacrificed for the sake of secure communication. The primary advantage of a spread spectrum communication system is its ability to reject interference whether it be the unintentional interference by another user simultaneously attempting to transmit through the channel, or the intentional interference by a hostile transmitter attempting to jam the transmission. The definition of spread spectrum modulation may be stated in two parts : 1.

Spread – spectrum is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it.

2.

The spectrum spreading is accomplished before transmission through the use of a code that is independent of the data source. The same code is used by receiver to dispread the received signal so that the original data sequence may be recovered. The standard modulation tech. i.e. FM, PCM satisfy the 1st part of

the definition but do not satisfy the 2nd part of the definition. 70

SS modulation was originally developed for military applications, but it is also used to provide multipath rejection in a ground based mobile radio environment./ yet another application is in multiple access common indications. Now, spreading the spectrum can be achieved in two different ways one is direct sequence and other is frequency hopping techniques. In a direct sequence spread – spectrum tech. two stage of modulation reused. First, the incoming data sequence (Narrowband) is used

to

modulate

a

wideband

code.

This

code

transforms

the

narrowband data sequence into a noise like wideband signal. The resulting signal under goes a second modulation using a phase shift keying tech. In a frequency hop spread spectrum tech, on the other hand, the spectrum of a data modulated carrier is widened by changing the carrier frequency in a pseudo – random manner. For their operation, both of these techniques depend upon pseudo-noise sequence. Pseudo-Noise Sequence It is a periodic binary sequence with a noise like wave form that is generated by feed back shift form that is generated by feed back shift register. A pseudo-noise sequence converts the a narrowband signal to a wideband noise – like signal before transmission In

a

CDMA

(Code

Division

Multiple

Access)

system

the

narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. the spreading signal is pseudo-noise code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message. All user in a CDMA system, as seen

from

figure

use

the

same

frequency

and

may

transmit

simultaneously. Each user has its own pseudorandom code word which is approximately orthicons to other codeword. The receiver performs a time correlation operation to detect only the specific desired codeword. All other codeword appears as a noise due to decor relation. For 71

detection of the message signal, the receiver needs to know the codeword used by the transmitter. Each user operates independently with no knowledge of the other users. In CDMA the power of multiple user at a receiver determines the noise floor after decor relation. If the power of each user within a cell is not controlled such that they do not appear equal at the base station receiver then the near far problem occurs. In CDMA, stranger received signal levels raise the noise floor at the base station demodulators for the weaker signals, thereby decreasing the probability that weaker signals will be received. To combat the near for problem, power control is provided by each base station in a cellular system and assure that each mobile within the base station area provides that same signal level to the base station receiver. Despite of power control within each cell, out of cell mobiles provides interference which is not under the control of receiving base station. The features of CDMA include the following.  

 





Many users of CDMA system share the same frequency. Unlike TDMA or FDMA, CDMA has soft capacity limit. Therefore there is no absolute limit on the number of users ion CDMA. Rather, the system performance gradually degrades for all users as the number of users are increased. Multipath fading may be substantially reduced because the signal is spread over a large spectrum. Channel data rotes are very high in CDMA system consequently the symbol (chip) duration is very short and usually much les than the channel delay spread. Since PN sequence has low autocorrelation multipart which is delayed by more than a chip will appear as noise. Since CDMA uses co-channel cells, it can use macroscopic spatial diversity to provide soft hand off. It is performed by MSC, which can simultaneously monitor a particular user from two or more base station. Self jamming is a problem is CDMA system. Self jamming arises from the fact that the spreading sequence of different users are 72

not exactly orthogonal, hence in the dispreading of a particular PN-code, non-zero contributions to the receiver decision statistic for a desired user arise from the transmission of other users in the system.  The near – far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user. CDMA Power Control In CDMA system, the system capacity is maximized if each mobile transmitter power level is controlled so that its signal arrives at the cell site within minimum required signal to interference ratio. It the signal power of all mobile transmitters within an area covered by a cell site are controlled then the total signal power received at the cell site from all mobile will be equal to the average received power times the number of mobile operating in the region of coverage. Frequency and channel specifications IS (Interim Standard) – 95 is specified for reverse link operation in the 824 – 849 MHz band and 869 – 894 MHz for the forward link. A forward and reverse channel pair is separated by 45 MHz for cellular band operation. The maximum user data rate is 9.6 Kb/s. Forward CDMA Channel The forward CDMA channel consists of a pilot channel, a synchronization channel up to seven paging channels, and up to 63 forward traffic channels. The pilot channel allows a mobile station to acquire timing for a forward CDMA channel. The synchronization channel broadcasts synchronization message to the mobile stations and a pirates at 1200 bps. The paging channel is used to send control information and paging message from the base station to the mobiles and operates at 9600, 4800 and 2400 bps. Reverse CDMA Channel The reverse channel modulation process shown in fig, user data on the reverse channel are grouped into 20 ms frames. The reverse 73

channel CDMA are made up to access channels (ACs) and reverse traffic channels (RTCs). Both

share

the

same

frequency

assignment,

and

each

traffic/access channel is identified by a distinct user long code. The access channel is used by the mobile to initiate communication with the base station and the respond to paging channel messages. The access channel is random access channel with each channel user uniquely identified by their long codes. The Reverse CDMA channel may contain a maximum of 32 ACs per supported paging channel. While RTC operates on a variable data rate, the AC works at a fixed data rate of 4800 bps. CDMA protocols constitute a class of protocols in which multipleaccess capability is primarily achieved by means of coding. In CDMA, each user is assigned a unique code sequence that he or she uses to encode his or her information signal. The receiver, knowing the code sequences of the user, decodes the received signal after reception and recovers the original data. Because the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information signal, the encoding process enlarges (spreads) the spectrum of the signal and is therefore also known as spread-spectrum (SS) modulation. The resulting encoded signal is also called as SS signal, and CDMA protocols are often denoted as SS multiple-access (SSMA) protocols.

Concept Behind CDMA Transmission Scheme CDMA, protocols can be classified in two different ways: by the concept behind the protocols or by the modulation method used. The first classification gives us two groups of protocols, namely, averaging systems and avoidance systems. Averaging systems reduce interference by averaging the interference over a long time interval. Avoidance systems reduce interference by avoiding it for a larger part of the time.

74

Classifying the CDMA protocols by the modulation method gives us five groups of protocols: (1) direct-sequence (DS) (or pseudo-noise), (2) frequency-hopping, (3) time-hopping, (4) chirp SS, and (5) hybrid. Here we discuss the CDMA protocols in terms of the modulation technique on which they are based.

Direct Sequence In the DS-CDMA protocols, the modulated information signal (the data signal) is directly modulate by a digital code signal. The data signal can be either an analog or a digital signal. In most cases, it is digital. In the case of a digital signal, the data modulation is often omitted, the data signal is directly multiplied by the code signal, and the resulting signal modulates the wideband carrier. It is from this direct multiplication that the DS-CDMA protocol gets its name.

Figure (a) shows a block diagram of a DS-CDMA transmitter. The binary data signal modulates an RF carrier. The modulated carrier is then modulated by the code signal. The code signal consists of a number of code bits called “Chips” that can be either +1 or -1. To obtain the desired spreading of the signal, the chip rate of the code signal must be much higher than the chip rate of the information signal. For the code modulation, various modulation techniques can be used, but forms of PSK, such as BPSK, or MSK are employed. If we omit the data modulation and use BPSK for the code modulation, we get the block diagram shown in Figure (b) 75

. The DS-SS signal resulting from this transmitter is shown in Fig. (c). in the figure, 10 code chips per information signal are transmitted (the code chip rate is 10 times the information chip rate), so the processing gain is equal to 10. In practice, the processing gain will be much larger.

After transmission of the signal, the receiver uses coherent demodulation to dispreads the SS signal by using a locally generated code sequence. To be able to perform the dispreading operation, the receiver must not only know the code sequence used to spread that signal, but also synchronize the codes of the received signal and the locally generated code. This synchronization must be accomplished at the beginning of the reception and maintained until the whole signal has been

received.

A

synchronization/

tracking

block

performs

this

operation. After dispreading, a data-modulated signal is generated and after demodulation, the original data can be recovered.

76

Here we discuss these four properties for the case of DS-CDMA. 

Multiple access: If multiple users use the same channel at the same time there will be multiple DS signals overlapping in time and frequency. At the receiver, coherent demodulation is used to remove the code modulation. This operation concentrates the power of the desired user in the information bandwidth. If the cross-correlation between the code of the desired user and the code of the interfering user is small, coherent detection will put only a small part of the power of the interfering signals in the information bandwidth.



Multipath interference: If the code sequence has an ideal autocorrelation function, then the correlation, function is zero outside the interval [-Tc, Tc] were Tc is the chip duration. This means that if the desired signal and the copied signal that is delayed for more than 2Tc, are received, coherent demodulation will treat the delayed signal as an interfering signal, putting only a small pat of the power of this signal in the information bandwidth.



Narrowband interference: The coherent detection at the receiver involves a multiplication of the received signal by a locally generated code

sequence.

However,

at

the

transmitter,

multiplying

a

narrowband signal by a wideband code sequence spreads the spectrum of the narrowband signal so that its power in the information bandwidth decreases by a factor equal to the processing gain. 

LPI: Because the DS signal uses to whole signal spectrum all the time, it will have a very low transmitted power per hertz. This makes it very difficult to detect a DS signal. 77

The DS–CDMA protocols have a number of advantageous (+) and disadvantageous (-) which are as –

Advantageous +

The generation of the coded signal is easy. It can be done by a simple multiplication.

+

Because only one carrier frequency has to be generated, the frequency synthesizer (carrier generator) is simple.

+

Coherent demodulation of the SS signal is possible.

+

No synchronization among users is necessary.

Disadvantageous -

Obtaining and maintaining the synchronization of the locally generated

codes

signal

and

received

signal

are

difficult.

Synchronization has to take place within a fraction of the chip time. -

For correct reception, the locally generated code sequence and the received code sequence must be synchronized within a fraction of the chip time. Combined with the non-availability of large contiguous frequency bands, this property practically limits the spread bandwidth to 10-20 MHz.

-

The power received from users close to the base station is much higher than that received from users further away. Because a user continuously transmits over the whole bandwidth, a user close to the base station will be constantly creating a lot of interference for users far from the base station, making their reception impossible. This near-far effect can be eliminated by using a power control algorithm so that the signals of all users can be received by the base station with the same average power. However, this control mechanism, which is called power control, is quite difficult to attain.

78

Frequency Hopping In frequency-hopping (FH) CDMA (FH-CDMA) protocols, the carrier frequency of the modulated information signal is not constant but changes periodically. During time intervals T, the carrier frequency remains the same, but after each time interval the carrier hops to another frequency. The hopping pattern is determined by the code signal. The set of available frequencies that the carrier can attain is called a hop set. The frequency occupation of an FH-SS system differs considerably from that of a DS-SS system. A DS system occupies the whole frequency band when it transmits, whereas an FH system uses only a small part of the bandwidth when it transmits, but the location of this part differs in time.

Fig. (d) Shows a block diagram of an FH-SS transmitter and an FH-SS receiver. The data signal is base band modulated on a carrier. Several modulation techniques can be used for the modulation, and it does not really matter which one is used for application of frequency hopping. Usually, FM modulation is used for analog signals, and FSK modulation is used for digital signals. Using a fast frequency synthesizer controlled by the code signal, the carrier frequency is converted up to the transmission frequency. The inverse process takes place at the receiver. Using a locality generated sequence; the received signal is converted down to the baseband-modulated carrier. The data are recovered after the (baseband) 79

demodulation. The synchronization/tracking circuit ensures that the hopping of the locally generated carrier is synchronized with the hopping pattern of the received carrier, so that the signal is despread correctly. Within the FH-CDMA protocols, a distinction is made that is based on the hopping rate of the carrier. If the number of hops is (much) greater than the data rate, one speaks of a fast FH (F-FH) CDMA protocol. In this case, the carrier frequency changes a number of times during the transmission of one bit, so that one bit is transmitted at different frequencies. If the number of hops is (much) smaller than the data rate, one speaks of slow FH (S-FH) CDMA protocols. In this case, multiple bits are transmitted at the same frequency. The occupied bandwidth on the signal on one of the hopping frequencies depends not only on the bandwidth information signal but also on the shape of the hopping signal and the hopping frequency. If the hopping frequency is much smaller than the information bandwidth is the main factor that determines the occupied bandwidth. If, however, the hopping frequency is much greater that the information bandwidth, the pulse shape of the hopping signal will determine the occupied bandwidth at one hopping frequency. If this pulse shape is very abrupt, the frequency band will be very broad, limiting the number of hop frequencies. If we make sure that the frequency changes are smooth, the frequency band at each hopping frequency will be about 1/Tb times the frequency bandwidth where Tb is equal to the hopping frequency. the frequency changes can be smooth by decreasing the transmitted power before a frequency hop and increasing it again when the hopping frequency has changed. Here we discuss the properties of FH-CDMA. 

Multiple access : In the F-FH protocol; one bit is transmitted in different frequency bands. If the desired user is the only one to transmit in most of the frequency bands, the received power of the

80

desired signal will be much higher than the interfering power, and the signal will be received correctly. In the S-FH protocol, multiple bits are transmitted at one frequency. If the probability of other users transmitting in the same frequency band is rather low, the desired user is signed correctly most of the time. For the times when interfering users transmit in the same frequency band, errorcorrecting codes are used to recover the data transmitted during that period. 

Multipath interference: In the F-FH CDMA protocol, the carrier frequency changes a number of times during the transmission of one bit. Thus, a particular signal frequency will be modulated and transmitted on a number of carrier frequencies. The multipath effect is different at the different carrier frequencies. As a result, signal frequencies that are amplified at one carrier frequency will be attenuated at another carrier frequency and vice verse. At the receiver, the responses at the different hopping frequencies are averaged, thus reducing the multipath interference. This is not as effective as the multipath interference rejection in a DS-CDMA system but it does improve the transmission.



Narrowband interference: Suppose that a narrowband signal is interfering on one of the hopping frequencies. If there are Gp hopping frequencies (where Gp is the processing gain), the desired user will (on average) use the hopping frequency where the interferer is located 1/Gp percent of the time. The interference is therefore reduced by a factor of Gp.



LPI: The difficulty in intercepting an FH signal lies not in its low transmission power. During a transmission, it uses as much power per hertz as does a continuous transmission. However, the frequency at which the signal is going to be transmitted is unknown and the duration of the transmission at a particular frequency is quite small.

81

Therefore, although the signal is more readily intercepted than a DS signal, it is still a difficult task to perform. The FH–CDMA protocols have a number of other specific properties that we can divide into advantageous (+) and disadvantageous (-) which are as –

Advantageous +

Synchronization is much easier with FH-CDMA than it is with DSCDMA. With FH-CDMA, synchronization has to be within a fraction of the hop time. Because spectral spreading is obtained not by using a very high hopping frequency but by using a large hop set, the hop time will be much longer than the chip time of a DS-CDMA system. Thus, an FH-CDMA system allows for a larger synchronization error.

+

The different frequency bands that an FH signal can occupy do not have to be continuous, because we can make the frequency synthesizer easily skip over certain parts of the spectrum. Combined with the easier synchronization, this allows for much wider SS bandwidths.

+

Because FH-CDMA is an avoidance SS system, the probability of multiple users transmitting in the same frequency band at the same time is small. A user transmitting far from the base station will be received by the base station even if users close to the base station are transmitting, because those users will probably be transmitting at other frequencies. Thus, the near-far performance of FH-CDMA is much better than that of DS.

+

Because of the larger possible bandwidth an FH system can use, it offers a greater possible reduction of narrowband interference than does a DS system.

Disadvantageous -

A highly sophisticated frequency synthesizer is necessary. 82

-

An abrupt change of the signal when changing frequency bands will lead to an increase in the frequency band occupied. The avoid this; the signal has to be turned off and on when changing frequency.

-

Coherent demodulation is difficult because of the problems in maintaining phase relationships during hopping.

Time Hopping In time-hopping (TH) CDMA (TH-CDMA) protocols, the data signal is transmitted in rapid bursts at time intervals determined by the code assigned to the user. The time axis is divided into frames, and each frame is divided into M time slots. During each frame, the user will transmit in one of the M time slots. Which of the M time slots in transmitted depends on the code signal assigned to the user. Because a user transmits all of his or her data in 1, instead of the M time slots, the frequency the user needs for the transmission increases by a factor of M. a block diagram of a THCDMA system is shown in Figure (a). Figure b shows a time-frequency plot of the TH-CDMA system. TH-CDMA protocol uses the whole wideband spectrum for short periods instead of using parts of the spectrum all of the time.

Here we discuss the properties of TH-CDMA.

83



Multiple access : The multiple access capability of TH-SS signals is acquired in the same manner as that of the FH-SS signals, namely, by making the probability of users’ transmissions in the same frequency band at the same time is small. In the case of TH, all transmissions are in the same frequency band, so the probability of more than one transmission at the same time is small. This is again achieved by assigning different codes to different users. If multiple transmissions do occur, error-correcting codes ensure that the desired signal can still be recovered. It there is synchronization among the users, and the assigned codes are such that no more than one user transmits at a particular slot, then the TH-CDMA protocol changes to a TDMA protocol where the slot in which a user transmits is not fixed but changes from frame to frame.



Multipath interference: In the TH-CDMA protocol, a signal is transmitted in reduced time. The signaling rate, therefore, increases, and the dispersion of the signal will now lead to an overlap of adjacent bits. Therefore, no advantage is to be gained with respect to multipath interference rejection.



Narrowband interference: A TH-CDMA signal is transmitted in reduced time. This reduction is equal to 1/Gp where Gp is the processing gain. At the receiver, we will only receive an interfering signal during the reception of the desired signal. Thus, we only receive the interfering signal 1/Gp percent of the time, reducing the interfering power by a factor of Gp.



LPI: With TH-CDMA, the frequency at which a user transmits is constant, but the times at which the user transmits are unknown, and the duration of the transmissions is very short. Particularly when multiple users are transmitting, it is difficult for an intercepting receiver to distinguish the beginning and end of a transmission and to determine which transmissions belong to which user. 84

The TH–CDMA protocols have a number of advantageous (+) and disadvantageous (-) which are as –

Advantageous +

Implementation of the TH-CDMA is simpler that that of FH-CDMA protocols.

+

It is a very useful method when the transmitter is an averagepower-limited but not peak-power-limited because the data are transmitted in short bursts at high power.

+

As with the FH-CDMA protocols, the near-far problem is much less of a problem because TH-CDMA is an avoidance system, which means that most of the time an terminal far from the base station can transmit alone and is not hindered by transmissions from the stations close by.

Disadvantageous -

Code synchronization takes a long time, and the time in which receiver has to perform the synchronization is short.

-

If multiple transmissions occur, al lot of data bits are lost, so a good error-correcting code and data interleaving are necessary.

Chirp Spread Spectrum Chirp SS systems have not yet been adapted as a CDMA protocol. A chirp SS system spreads the bandwidth by linear frequency modulation of

the

carrier.

This

is

shown

in

figure.

The

processing

gain,

Gp, is a product of bandwidth B, in which the frequency varies, and duration T of a given signal waveform:

Gp=BT

85

Hybrid Systems The hybrid CDMA system includes all CDMA systems that employ a combination of two or more of the above mentioned SS modulation techniques. If we limit our discussion to the DS, FH, and TH modulations, we have four possible hybrid systems: DS/FH, DS/TH, FH/TH, and DS/FH/TH. The idea behind the hybrid system is to combine the specific advantages of each of the modulation techniques. A combined DS/FH system, have the advantage of the anti-multipath property of the DS system combined with the favorable near-far operation of the FH system. Of course, the disadvantage lies in the increased complexity of the transmitter and receiver. Figure shows a block diagram of a combined DS/FH CDMA transmitter and receiver. Fig. shows a block diagram of a combined DS/FH CDMA transmitter. The data signal is first spread using a DS code signal. The SS is then modulated on a carrier whose frequency hops according to a different code sequence. A code clock ensures a fixed relationship between the two codes.

Generation of a Spreading Code To overcome the interference, severally requirements must be satisfied: 1. Each code sequence generated from a set of code-generation functions must be periodic with a constant length. 2. Each code sequence generated from a set of code-generation functions must be easy to distinguish from its time-shifted code. 3. Each code sequence generated from a set of code-generation functions must be easy to distinguish from other code sequences. 86

The first and second requirements are important with respect to the multipath propagation effects that occur in mobile outdoor and indoor radio environments. The third requirement is important with respect to the multiple access capability of communications systems. To measure the distinction level of the codes for requirements (1) and (2), and autocorrelation function and a cross-correlation function are used, respectively. The autocorrelation function is used to measure the distinction level, and it is defined as follows:

rxx(t)=

1 T

T

 X (t)X(t+ )d 0

For the MATLAB function showing the autocorrelation function, autocorr.m must be used. The arguments of this function are the name of the sequence and the number of periods of the code for which the autocorrelation function is to be obtained. In contrast, the cross-correlation function is a value of correlation between distinct code sequences X(t) and Y(t), and it is defined as follows:

1 rxx(t)= T

T

 X (t)Y(t+ )d 0

For the MATLAB function showing the cross-correlation function, cross-corr.m must be used in Program. The arguments of this function are the name of the sequence and the number of periods of the code for which the autocorrelation function is to be obtained. 87

By using the autocorrelation and cross-correlation functions, the spread code is evaluated. This section introduced three code sequences as examples of such spreading codes. These are (1) and M-sequence, (2) a Gold sequence, and (3) an orthogonal Gold sequence.

Code Generation by Liner Feedback Shift Registers There are several ways to generate code sequences. One is by the use of feedback shift registers, and this method is the one generally used in CDMA systems. A shifted register consists of a number of cells (numbered from 1 to r), and each cell is a storage unit that, under the control of a clock pulse, moves its contents from its input. In the standard configuration of a feedback shift register, the input cell m will be a function of the output of cell m-1, and the output of cell r (the last cell of the shift register) forms the desired code sequences.

The function combining the outputs of cell m-1 and cell r with the input of cell m can be either a liner or a nonlinear function. This gives us to types of feedback shift registers: liner feedback shift registers (linear FSRs) and nonlinear. Fig. shows a single linear binary shift register, which can generate a sequence from generation polynomial h(x) = x5 +x2 +1. In general, the configuration of a linear binary shift register of n sections is described by a generator polynomial, which is a binary polynomial of degree n. Number n is the number of sections of the shift register:

b(x)= bnxn+bn-1xn-1+…..b1x1+1

(bi  {0,1})

In given figure b(x)= x5+x2+1, b0=b2=1 and bn=0. By using these shift registers, most spread code sequences are generated. 88

M-Sequence M-Sequences are generated by a single LSR. In particular, a sequence with the maximum possible period, (Nc=2n-1), is generated by an n-stage binary shift register with linear feedback. To generate an Msequence, the generator polynomial must be a generation polynomial of degree n. Thus, the periodic autocorrelation function of an M-sequence is giver: by

r

xx(t)=

{  11/ N

t  0 mod N c c

otherwise

If n0 mod 4, there exist pairs of maximum-length sequences width a three-valued. Cross-correction function, where the two values are {-t(n), t(n)-2} with

t(n)=

{1  2

1  2 ( n 1) / 2 ( n 2) / 2

n : odd n : even

The function to generate M-sequence is mseq.m. The number of registers, the initial values of the registers, and the position of the feedback taps are given as arguments of mseq.m.

Suppose that the number of registers is 3, the initial values of the registers are [1, 1, 1], and the position of the feedback tap is in the first and third taps. The generation polynomial is expressed as h(x) = x3 + x + 1, and the shift register to configure an M-sequence is shown in Figure. When an M-sequence is generated by using a generation polynomial, the following command is performed. >> m1 = mseq (3, [1, 3], [1, 1, 1])

89

As a result, three-stage M-sequence [1, 1, 1, 0, 1, 0, 0] is generated as a vector. Here, mseq.m has the fourth argument, which denotes the number of outputs. If the number of outputs, N, is given, we can obtain N one-chip shifted M-sequence. For example another three stage M-sequence is generated by the following command: >> m2= mseq (3, [2, 3], [1, 1, 1],3) When this command is performed, three one-chip shifted Msequences are obtained. Ans= 1 1 1 0 0 1 0 0 1 1 1 0 0 1 1 0 1 1 1 0 0

Shift.m used in function mseq.m is one of the functions that shift the number of chips given by the users for the vector of matrix.

Gold Sequence The

M-Sequence

has

good

autocorrelation

characteristics.

However, the number of mobile communication system that uses the MSequence as a function is very small. This is because the number of Msequences that have the same code length and the same correlation characteristics is limited. When a CDMA system where many users communicate to each other is realized, we need sequences with many different codes that have the same correlation value. The Gold sequence is one of such sequences. The gold sequence was developed by gold [13]. It is generated by exclusive OR (EXOR) of two M-sequences.

The generation circuit is

shown in figure where a three stage shifter is used. The number of gold sequences generated by a generation circuit is 2n + 1[1, 11], and it is obtained by changing the initial value of register

and adding two M-

sequences when we use an n-stage shift register. In the gold sequence generated from two preferred-pair M-sequences, the cross- correlation value takes three values, namely, {-1, -t(n), t(n)-2}. 90

The generation functions of a gold sequence are performed by goldseq.m function.

To generate a Gold sequence, tow preferred-pair M-

sequences are given in goldseq.m. For example, to generate a threestage Gold sequence from two M-sequences, the following commands must be performed. >> m1=mseq (3, [1, 3], [1, 1, 1]); >> m2=mseq (3, [2, 3], [1, 1, 1]); >> g1=goldseq (m1, m2) as result, we can obtain a three-stage gold sequence of [0 0 0 0 1 1 0] with a length of 7 vector. By changing the initial data, a different gold sequence can be obtained. To calculate the cross correlation value, another gold sequence is generated. >> m3=mseq (3, [1, 3], [1, 0, 0]); >> m4=mseq (3, [2, 3], [1, 0, 1]); >> g2=goldseq (m3, m4) here, goldseq.m has the third argument, which denotes the number of outputs. If the number of outputs, N, is given as an argument, we can obtain N one-chip shifted gold sequences. For example, when the following command is performed: >> g1=goldseq (m1, m2,3) Three one chip shifted gold sequences are obtained. 91

Ans = 0 0 0 0 1 1 0 1 0 0 1 1 0 1 0 1 0 1 0 0 0 by using functions autocorr.m and crosscorr.m programmed in programs given at the back, The characteristic of an M-sequence can be evaluated. First, the autocorrelation function of three-stage Gold sequence g1(1,:) is calculated by the following command: >> g1=g1*2-1; >>autocorr (g1(1,:)) as a result, a cross-correlation value of [7, 3, -1, -1, -1, 3] is obtained.

The

autocorrelation

value

is

a

large

value

at

the

synchronization point. However, at other points, the data fluctuate. Next, the cross-correlation function between g1(1,:) and g1(2,:) is obtained by the following command: >>crosscorr (g1(1,:),g1(2,:)) as a result, a cross-correlation value of [-1, 3, -1, -5, -1, 3, -1] is obtained. This result takes value of {-1, -t(n), t(n)-2},where t(n)=3.

Orthogonal Gold Sequence The gold sequence has many different codes compared to those of M-sequence. However, there are several problems associated with the gold sequence. 

The proportion of 0 to 1 not always balanced.



The cross-correlation value of the Gold sequence is not 0 in a synchronized environment.



The code length is an odd number. As a result, a special clock is needed to generate the Gold sequence.

92

To solve this problem, one chip is added to the gold sequence to balance the proportion of 0 to 1. This sequence is called an orthogonal gold sequence. The cross-correlation value of the orthogonal gold sequence is 0 at the synchronous point. At other points, the characteristics of the sequence are similar to those of the gold sequence.

Conclusion Here

we

have

discussed

some

methods

to

evaluate

the

performance of CDMA systems. We have described basic CDMA & analyzed three well known code sequences. The BER performance of a synchronized DS-CDMA was evaluated by computer simulations with the help of this chapter anyone can design their own code sequences or new DS-CDMA systems. CDMA is one of the key systems for III & IV generation wireless communication system.

93

Conclusion

For simulation of cellular system, our first objective was to set the position of base stations. We used 19 hexagonal cells having a cell radius of 1. Such cells were determined by the position of 19 base stations. The second objective was to take into account not only one sample cell but also neighboring cells, because co-channel interference from neighboring cells has a significant effect on the performance of sample cell. For this we used the cell wrapping technique, where the boundary cells were regarded as neighbors of boundary cells located almost directly opposite the cell layout. The strength of received signals was one of most important issues in our simulation. The transmitted signal suffers from attenuation caused by factors such as distance and obstruction. Therefore, we introduced path loss & shadowing as such attenuation factors. Taking all these parameters into account, we accumulated the data & calculated the essential numerical values such as callnum, blocknum & forcenum thereby simply calculating the performance measures as blocking probability & forced termination probability. Therefore, this simulation evaluates the suitable performance of a cellular system using DCA. The availability of cost competitive dig. & radio technology and constantly increasing demand for wireless services formed the 2 major features for success of IInd generation mobile communication. The fast world wide acceptance and deployment of GSM is the solution Several industry partners believe that network inter working based on enhanced second generation cellular & cordless systems such as GSM

94

& DECT will offer a valid and economically viable platform for achieving the above stated objective. The common air interface standard, 15-95-A, prescribes, in considerable detail, the behavior of dual-mode CDMA/AMPS subscriber station, and to lesser extent, the base stations. A 15-95-A compliant subscriber station can obtain service by communicating with either an AMPS or CDMA base station. 15-95-A emphasizes subscriber station requirements because the subscriber side reflects all can process features, and because the specification is simpler in context of a single user. In contrast to very detailed subscriber station requirements, base station requirements are sketchy & incomplete.

95

SOFTWARE PART

96

Objective of cellular concept 2.

To determine the position of base station (BS).

3.

To determine relationship of base station positions and introducing cell wrapping.

4.

To distribute a cell into meshes and store the data about each mesh coordinates.

5.

To generate call holding time this is exponentially distributed.

6.

To generate attenuation due to shadowing, this has log-normal distribution.

7.

To generate a path loss due to distance.

8.

To determine the information of station.

9.

To determine characteristics of the antenna gain.

10.

To realize DCA algorithm.

97

Objective of CDMA 1.

To obtain the sequence of Auto correlation function.

2.

To obtain the sequence of cross correlation function.

3.

To generate the M-sequence by using generating function.

4.

To shift the contents of the register.

5.

To generate the gold sequence by using generating function.

6.

To obtain the data spread function.

7.

To obtain the data dispread function.

8.

To obtain sample time by using sample function.

9.

To perform the convolution between signal & filter

10.

To add white Gaussian noise by using function.

11.

To realize DS-CDMA system

98

PROGRAMMING PART

CELLULAR CONCEPT PROGRAM

[1]-A function[out]=basest() baseinfo(1,1)=0.0; baseinfo(1,2)=0.0; baseinfo(2,1)=-0.5*sqrt(3.0); baseinfo(2,2)=1.5; baseinfo(3,1)=-sqrt(3.0); baseinfo(3,2)=0.0; baseinfo(4,1)=-0.5*sqrt(3.0); baseinfo(4,2)=-1.5; baseinfo(5,1)=0.5*sqrt(3.0); baseinfo(5,2)=-1.5; baseinfo(6,1)=sqrt(3.0); baseinfo(6,2)=0.0; baseinfo(7,1)=0.5*sqrt(3.0); baseinfo(7,2)=1.5; out=baseinfo; plot(baseinfo(:,1),baseinfo(:,2),'r*')

[1]-B function[out]=basest() baseinfo(1,1)=0.0; baseinfo(1,2)=0.0; baseinfo(2,1)=-0.5*sqrt(3.0); baseinfo(2,2)=1.5; baseinfo(3,1)=-sqrt(3.0); baseinfo(3,2)=0.0; baseinfo(4,1)=-0.5*sqrt(3.0); baseinfo(4,2)=-1.5; baseinfo(5,1)=0.5*sqrt(3.0); baseinfo(5,2)=-1.5; baseinfo(6,1)=sqrt(3.0); baseinfo(6,2)=0.0; baseinfo(7,1)=0.5*sqrt(3.0); baseinfo(7,2)=1.5; out=baseinfo; plot(baseinfo(:,1),baseinfo(:,2),'r*')

[2]-A function[out]=wrap(inputmat) inputmat(1,1:7)=1:7; inputmat(2,1:7)=[2,4,6,3,1,7,5]; inputmat(3,1:7)=[3,6,5,7,4,1,2]; inputmat(4,1:7)=[4,3,7,6,2,5,1]; inputmat(5,1:7)=[5,1,4,2,7,3,6]; inputmat(6,1:7)=[6,7,1,5,3,2,4]; inputmat(7,1:7)=[7,5,2,1,6,4,3]; out=inputmat;

[2]-B function[out]=wrap(inputmat) inputmat(1,1:19) =1:19; inputmat(2,1:19) =[2,9,10,3,1,7,8,14,13,17,16,11,12,4,5,6,18,19,15]; inputmat(3,1:19) =[3,10,11,12,4,1,2,9,17,16,15,19,18,13,14,5,6,7,8]; inputmat(4,1:19) =[4,3,12,13,14,5,1,2,10,11,19,18,17,9,8,15,16,6,7]; inputmat(5,1:19) =[5,1,4,14,15,16,6,7,2,3,12,13,9,8,19,11,10,17,18]; inputmat(6,1:19) =[6,7,1,5,16,17,18,19,8,2,3,4,14,15,11,10,9,13,12]; inputmat(7,1:19) =[7,8,2,1,6,18,19,15,14,9,10,3,4,5,16,17,13,12,11]; inputmat(8,1:19) =[8,14,9,2,7,19,15,5,4,13,17,10,3,1,6,18,12,11,16]; inputmat(9,1:19) =[9,13,17,10,2,8,14,4,12,18,6,16,11,3,1,7,19,15,5]; inputmat(10,1:19)=[10,17,16,11,3,2,9,13,18,6,5,15,19,12,4,1,7,8,14]; inputmat(11,1:19)=[11,16,15,19,12,3,10,17,6,5,14,8,7,18,13,4,1,2,9]; inputmat(12,1:19)=[12,11,19,18,13,4,3,10,16,15,8,7,6,17,9,14,5,1,2]; inputmat(13,1:19)=[13,12,18,17,9,14,4,3,11,19,7,6,16,10,2,8,15,5,1]; inputmat(14,1:19)=[14,4,13,9,8,15,5,1,3,12,18,17,10,2,7,19,11,16,6]; inputmat(15,1:19)=[15,5,14,8,19,11,16,6,1,4,13,9,2,7,18,12,3,10,17]; inputmat(16,1:19)=[16,6,5,15,11,10,17,18,7,1,4,14,8,19,12,3,2,9,13]; inputmat(17,1:19)=[17,18,6,16,10,9,13,12,19,7,1,5,15,11,3,2,8,14,4]; inputmat(18,1:19)=[18,19,7,6,17,13,12,11,15,8,2,1,5,16,10,9,14,4,3]; inputmat(19,1:19)=[19,15,8,7,18,12,11,16,5,14,9,2,1,6,17,13,4,3,10]; out=inputmat;

[3] function [meshnum,meshposition]=cellmesh() fineness=50; k=1; j=0; ds=sqrt(3.0)/fineness; xmesh=-0.5*sqrt(3.0); while xmesh