EVOLUTION OF CELLULAR WIRELESS NETWORKS: ROLE OF MEDIA ACCESS CONTROL PROTOCOLS Vishwas Lakkundi Institute of Radio Electronics, Brno University of Technology, 118 Purkynova, Brno CZ-61200
[email protected] Abstract: This paper presents the role of salient features of MAC protocols in realizing high data rate communications, leading the transition to next generation wireless networks. A comparative analysis among various networks from 1G through 3G, has been carried out to illustrate these features, with emphasis on access procedures & data frame formats.
1 Introduction The ever-increasing demand for new applications has lead to tremendous advances in cellular networks during the last two decades. From first generation analog systems to present day third generation systems, they have come quite a long way. Higher data rates, advanced multimedia services & ubiquitous access to services exemplify this evolution, made possible to a large extent, by enhanced air interfaces, multiple access procedures & coding techniques. The following section contains a brief overview of transition from first to third generation cellular networks. Their relevant access procedures & frame formats are explained next. Graphical illustrations highlighting these effects on the data rates are presented subsequently. Finally, conclusions have been drawn. 1.1 First Generation (1G) networks They are analog circuit-switched systems, examples of which include NMT, AMPS, & TACS mostly operating in the 800 MHz range. NMT (Nordic Mobile Telephone) was an analog system used in the Scandinavia in the 450 MHz band with a very low data rate of 1.2 kbps. AMPS (Advanced Mobile Phone System) is the original standard for analog cellular phone service primarily used in the Americas, Australia, and parts of Russia and Asia with data rates upto 10 kbps [1]. TACS (Total Access Communication System) was used in Britain in the 900 MHz band. TACS was based on the AMPS system and was later adopted in other countries, including Hong Kong and Japan providing a data rate of 8 kbps. 1.2 Second Generation (2G) networks Digital systems such as GSM, IS-54 TDMA, IS-95 CDMA, & PDC came to the fore in the early 1990s. They are best suited for voice services although simple text messages can be sent via them. GSM (Global System for Mobile communications) is a digital transmission technique widely adopted in Europe and supported in North America. It uses 900 MHz and 1800 MHz bands in Europe & 1900 MHz band in North America. IS-54 TDMA (Time Division Multiple Access) standard defines traffic on digital voice channels, using 800 MHz and 1900 MHz bands. Capacity enhancements to the tune of 6-15 times that of AMPS are possible. IS-95 CDMA (Code Division Multiple Access) standard uses spread spectrum techniques to scatter a radio signal across a wide range of frequencies. 2.5G systems, such as GPRS, are packet switched 2G networks designed for bursty data traffic. They provide a bridging path between voice-centric 2G & data-centric 3G networks. GPRS (General Packet Radio Service) is a packet-based radio technology for GSM
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networks. It enables ubiquitous wireless Internet and other high-speed data communications such as SMS, MMS, email, games, and WAP applications [2]. 1.3 Third Generation (3G) networks 3G networks are designed to deliver next-generation services with transmission rates beyond 2.5G systems that can support multimedia, data & video along with voice. UMTS, cdma2000, and EDGE support 3G services. UMTS (Universal Mobile Telecommunications System) is the European 3G wireless standard offering indoor data rates up to 2 Mbps. cdma2000 is a North American standard. It uses a multicarrier technique & different chip rates & is similar to, but incompatible with W-CDMA. EDGE (Enhanced Data rates for GSM Evolution) allows current GSM networks to offer 3G services within existing frequencies. As an evolution of GSM/GPRS, it enhances their voice & data networks to provide data rates upto 3 times that of GPRS.
2 Access Procedures and Frame Formats MAC procedures mainly deal with resource allocation among users & overall flow control, achieved through efficient access procedures & frame formats [2]. From this perspective, three networks, one each from 1G, 2G & 3G are considered & explained in the following sections. 2.1 AMPS (Advanced Mobile Phone System) 1G networks such as AMPS use FDMA (Frequency Division Multiple Access) to support multiple users. Each cell is allocated a total spectrum of 2M channels of δ Hz each. Half of the channels are used on the reverse link (mobile unit to base station): fc, fc+δ, fc+2δ,..., fc+(M-1)δ, where fc is the centre frequency of the lowest-frequency channel. The other half of the channels are used on the forward link: fc, fc+δ+∆, fc+2δ+∆,..., fc+(M-1)δ +∆, where ∆ is the spacing between forward & reverse channels. When a connection is set up, the mobile user is assigned 2 channels, f and f+∆, which is not an efficient use of the spectrum, as one or both channels are idle most of the time [1]. 48 bits precursor
240 bits word 1
word 1
bit sync 30 bits
word 1
word sync 11
240 bits word 1
word 1
word 2
word 2
word 2
240 bits word 2
word 2
. ..
word n
word n
word n
DCC 7
Fig.1 AMPS reverse control channel (RCC) frame format: mobile unit to base station 21 bits
400 bits word A
bit sync 10 bits
word B
word A
word B
word A
word B
word A
word B
word A
word B
word sync 11
Fig.2 AMPS forward control channel (FCC) frame format: base station to mobile unit
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word n
word n
The RCC frame contains 1 to 6 words of data, with 36 data bits each. 12 check bits are added to form a 48-bit word & each word is transmitted five times in the same frame to enhance reliability. Thus the data rate, taking into account the entire overhead, is only a few 100b/s. At 10 kbps signalling rate, the FCC data rate (excluding overhead) is 56 data bits × 10 4 = 1.2 kbps 463 bits / frame 2.2 GSM (Global System for Mobile communications) GSM utilizes a combination of FDMA & TDMA access mechanisms [4]. Logical channels are defined using TDMA hierarchy. In TDMA too, the capacity is allocated in reverse & forward channels. However, each physical channel is further divided into a number of logical channels. A repetitive sequence of frames, each divided into a number of time-slots is used for transmission. In GSM, 125 full-duplex channels with 200 KHz carriers are modulated at a bit rate of 270.833 kbps [1]. The normal burst slot format is shown below. 0
1 multiframe=26 frames=120 ms 2 . . .
1
0
1 frame=8 time slots=4.615 ms 2 3 4 5
1
.
25
6
7
1 timeslot=156.25 bits=0.577 ms 8.25 3t
57 data
1f 26 synch
1f
57 data
3t
guard
Fig.3 GSM frame format
TDMA frames, with 8-slots each, are organised into a 26-frame multiframe, of which every 13th frame is either used for control signalling or currently unused. That leaves 24 frames for data traffic. Hence, each traffic channel is given 1 slot per frame & 24 frames per multiframe. The resulting data rate is
114 bits / slot × 24 slots / multiframe = 22.8 kbps 120 ms / multiframe
2.3 UMTS (Universal Mobile Telecommunications System) It includes 2 standards: W-CDMA & IMT-TC, also known as TD-CDMA, which is a combination of CDMA & TDMA technologies. CDMA uses unique spreading codes to spread data before the signal is transmitted in a channel that is below noise level. The receiver then uses a correlator to de-spread the wanted signal. Each user gets the allocation of whole spectrum all of the time. Wideband-CDMA exploits Direct Spread CDMA technology to provide high data rates with efficient spectrum usage. It supports multirate by which a user has the provision of multiple data rate logical channels [3]. UMTS frame structure used in uplink & downlink directions is shown below. Control time-slot is not shown here for simplification. UMTS proposed bit rates: at least 144 kbps – rural outdoors at least 384 kbps – urban/suburban outdoors at least 2048 kbps – indoor/low range outdoor
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0
0
1 superframe=72 frames=720 ms 2 . . .
1
1
2
1 radio frame=15 time slots=10 ms 7 4 5 6 8 9 10
3
.
71
11
12
13
14
1 timeslot=666.7 µs (2560 chips) Bits per slot: 10 * 2k (k=0,1...6) Spreading factor: 256/2k
Data Ndata bits
Fig.4 UMTS frame format
3 Illustrations The figures shown below illustrate the growth of wireless networks in terms of data rates. The time durations required to download a typical 3MB MP3 song by various networks are demonstrated as an example, which testify this phenomenon. 350
Download Time in minutes (3MB MP3 Song)
2500 2048
Data Rate in kbps
2000
1500
1000
384
500 8
1.2
9.6
45
330
300
250
200
150
100 50 50
42
64
9
6
1
0.2
0
0 AMPS TACS 1G
GSM GPRS IS 95 2G
EDGE UMTS
AMPS TACS
3G
1G
GSM
GPRS
IS 95
2G
EDGE UMTS 3G
Cellular Network
Cellular Network
4 Conclusions The results reaffirm the role played by MAC features in the evolution of wireless networks, thereby fuelling new applications with higher data rates. Further improvements have already given shape to next generation networks such as B3G (Beyond 3G) & 4G. Integration across networks has a great potential in taking this process onto a faster lane.
Acknowledgements This work has been supported by the Grant Agency of Czech Republic under grants no.102/03/H109-‘Methods, structures & electronic components for wireless communications’ and no.102/04/0557-‘Development of digital wireless communication resources’.
References [1] STALLINGS, W. Wireless Communications & Networking, Pearson Education, Inc. 2002 [2] LAKKUNDI, V., KASAL, M. A novel MAC layer adaptation for General Packet Radio Service-Satellite mode (GPRS-S) data communication Proc. of 4th WSEAS International Conference on Automation & Information, Spain, Dec 2003, ISBN 960-8052-92-0 [3] 3GPP specifications at www.3gpp.org [4] www.gsmworld.com
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