Basics of Wireless and Mobile Communications

Basics of Wireless and Mobile Communications Wireless Transmission  Frequencies  Signals  Antenna  Signal propagation  Multiplexing  Modulation  Spread spectrum  Cellular systems

Media Access Schemes  Motivation  SDMA, FDMA, TDMA, CDMA  Comparison Basic Functions in Mobile Systems  Location management  Handover  Roaming

References 



Jochen Schiller: Mobile Communications (German and English), 2nd edition, AddisonWesley, 2003 (most of the material covered in this chapter is based on the book) Holma, Toskala: WCDMA for UMTS. 3rd edition, Wiley, 2004

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2

Mobile Communication Systems – the Issues: What does it require?  Provide telecommunition services voice (conversation, messaging)  data (fax, SMS/MMS, internet)  video (conversation, streaming, broadcast) 

anywhere  coverage  anytime  ubiquitous connectivity, reachability  wireless  without cord/wire  mobile  in motion, on the move (terrestrial)  secure  integrity, identity, privacy, authenticity, non-repudiation (Unleugbarkeit)  reliable  guaranteed quality of service 

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Frequencies for communication (spectrum) twisted pair

GSM, DECT, UMTS, WLAN

coax cable

1 Mm 300 Hz

10 km 30 kHz

VLF

LF

100 m 3 MHz

MF

1m 300 MHz

HF

VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency

VHF

UHF

10 mm 30 GHz

SHF

EHF

optical transmission

100 m 3 THz

infrared

1 m 300 THz

visible light UV

UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light

Frequency and wave length:

 = c/f wave length , speed of light c  300 x 106 m/s, frequency f

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Electromagnetic Spectrum

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100 MHz:

UKW Radio, VHF TV

400 MHz:

UHF TV

450 MHz:

C-Netz

900 MHz:

GSM900

1800 MHz:

GSM1800

1900 MHz:

DECT

2000 MHz:

UMTS (3G)

2400 MHz:

WLAN, Bluetooth

2450 MHz:

Mikrowellenherd

3500 MHz:

WiMax

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Frequencies for mobile communication VHF-/UHF-ranges for mobile radio  simple, small antennas  good propagation characteristics (limited reflections, small path loss, penetration of walls)  typically used for radio & TV (terrestrial+satellite) broadcast, wireless telecommunication (cordless/mobile phone) SHF and higher for directed radio links, satellite communication  small antenna, strong focus  larger bandwidth available  no penetration of walls Mobile systems and wireless LANs use frequencies in UHF to SHF spectrum  some systems planned up to EHF  limitations due to absorption by water and oxygen molecules (resonance frequencies) weather dependent fading, signal loss caused by heavy rainfall etc.

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Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Examples of assigned frequency bands (in MHz): Europe

USA

Japan

GSM 450-457, 479486/460-467,489-496, 890-915/935-960, 1710-1785/1805-1880 UMTS (FDD) 19201980, 2110-2190 UMTS (TDD) 19001920, 2020-2025 CT1+ 885-887, 930932 CT2 864-868 DECT 1880-1900 IEEE 802.11 b 2400-2483 802.11a/HIPERLAN 2 5150-5350, 5470-5725

AMPS, TDMA, CDMA 824-849, 869-894 TDMA, CDMA, GSM 1850-1910, 1930-1990

PDC 810-826, 940-956, 1429-1465, 1477-1513

PACS 1850-1910, 19301990 PACS-UB 1910-1930

PHS 1895-1918 JCT 254-380

902-928 IEEE 802.11 2400-2483 5150-5350, 5725-5825

IEEE 802.11 2471-2497 5150-5250

Others

RF-Control 27, 128, 418, 433, 868

RF-Control 315, 915

RF-Control 426, 868

WiMax (IEEE 802.16, licensed)

2.3GHz, 2.5GHz and 3.5GHz

2.3GHz, 2.5GHz and 3.5GHz

2.3GHz, 2.5GHz and 3.5GHz

Cellular Phones (licensed)

Cordless Phones (unlicensed)

Wireless LANs (unlicensed)

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Abbreviations: AMPS Advanced Mobile Phone System CDMA Code Division Multiple Access CT Cordless Telephone DECT Digital Enhanced Cordless Telecommunications GSM Global System for Mobile Communications HIPERLAN High-Performance LAN IEEE Institute of Electrical and Electronics Engineers JCT Japanese Cordless Telephone NMT Nordic Mobile Telephone PACS Personal Access Communications System PACS-UB PACS- Unlicensed Band PDC Pacific Digital Cellular PHS Personal Handyphone System TDMA Time Division Multiple Access WiMAX Worldwide Interoperability for Microwave Access

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UMTS Frequency Bands (FDD mode only) Operating Band

Frequency UL Frequencies Band UE transmit (MHz)

DL Frequencies UE receive (MHz)

Typically used in region ...

I

2100

1920 - 1980

2110 - 2170

EU, Asia

II

1900

1850 - 1910

1930 - 1990

America

III

1800

1710 - 1785

1805 - 1880

EU (future use)

IV

1700

1710 - 1755

2110 - 2155

Japan

V

850

824 - 849

869 - 894

America, Australia, Brazil

VI

800

830 - 840

875 - 885

Japan

VII

2600

2500 - 2570

2620 - 2690

„Extension Band“

VIII

900

880 - 915

925 - 960

EU (future use)

IX

1800

1749.9 - 1784.9

1844.9 - 1879.9

Japan

X

1700

1710 - 1770

2110 - 2170

America/US

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UMTS Frequency Bands (FDD mode only), Germany Operator

Uplink (MHz)

Downlink (MHz)

Carriers

Auction Price

Vodafone

1920,3 – 1930,2

2110,3 – 2120,2

2x10 MHz

16,47 Mrd. DM (8,42 Mrd. €)

Currently spare

1930,2 – 1940,1

2120,2 – 2130,1

2x10 MHz

16,45 Mrd. DM Group 3G (Marke Quam)

E-Plus

1940,1 – 1950,0

2130,1 – 2140,0

2x10 MHz

16,42 Mrd. DM (8,39 Mrd. €)

Currently spare

1950,0 – 1959,9

2140,0 – 2149,9

2x10 MHz

O2

1959,9 – 1969,8

2149,9 – 2159,8

2x10 MHz

16,52 Mrd. DM (8,45 Mrd. €)

T-Mobile

1969,8 – 1979,7

2159,8 – 2169,7

2x10 MHz

16,58 Mrd. DM (8,48 Mrd. €)

(16,37 Mrd. DM Mobilcom; returned)

In 2000, the UMTS frequency bands were auctioned in Germany. 6 operators won 10 MHz each, for total 50 B€ UMTS Networks

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Basic Lower Layer Model for Wireless Transmission Transmit direction Data link layer – media access – fragmentation – frame error protection – multiplexing Physical layer – encryption – coding, forward error protection – interleaving – modulation – D/A conversion, signal generation – transmit

Receive direction – reassembly

Digital Signal Processing

– frame error detection – demultiplex – decryption – decoding, bit error correction – deinterleaving – demodulation – A/D conversion; (signal equalization) – receive

Wireless Channel (path loss)

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– IntersymbolInterference (distortion of own signal) – Intercell-Interference (multiple users) – Intracell-Interference (multiple users) –Thermal Noise Andreas Mitschele-Thiel, Jens Mückenheim

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Signals in general    



physical representation of data function of time and location signal parameters: parameters representing the value of data classification  continuous time/discrete time  continuous values/discrete values  analog signal = continuous time and continuous values  digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift   sine wave as special periodic signal for a carrier:

s(t) = At sin(2  ft t + t) amplitude

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phase shift

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Signal representations amplitude (time domain)

frequency spectrum (frequency domain)

Q = M sin 

A [V]

A [V]

phase state diagram (amplitude M and phase  in polar coordinates)

t[s]

 I= M cos 







f [Hz]

Composed signals transferred into frequency domain using Fourier transformation Digital signals need  infinite frequencies for perfect transmission  modulation with a carrier frequency for transmission (analog signal!)

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Fourier representation of periodic signals Every periodic signal g(t) can be constructed by





1 g (t )  c   an sin( 2nft )   bn cos(2nft ) 2 n 1 n 1

1

1

0

0 t

ideal periodic signal

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t

real composition (based on harmonics)

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Signal propagation Propagation in free space always like light (straight line, line of sight) Receiving power proportional to 1/d² (ideal), 1/dα (α=3...4 realistically) (d = distance between sender and receiver) Receiving power additionally influenced by  fading (frequency dependent)  shadowing  reflection at large obstacles  scattering at small obstacles  diffraction at edges

shadowing UMTS Networks

reflection

scattering

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diffraction October 2012

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Radio Propagation: Received Power due to Pathloss

1m

10m

100m

(d-2):

1

1:100

1:10000

Realistic propagation

1

1:3000 to 1:10000

Ideal line-of sight

(d-3.5…4): UMTS Networks

35-40 dB

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1:10 Mio to 1:100 Mio 15

Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

signal at sender signal at receiver

Time dispersion: signal is dispersed over time  interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted  distorted signal depending on the phases of the different parts Delayed signal rec’d via longer path Signal received by direct path UMTS Networks

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Effects of mobility – Fading Channel characteristics change over time and location  signal paths change  different delay variations of different signal parts (frequencies)  different phases of signal parts  quick changes in the power received (short-term fading or fast fading) Additional changes in  distance to sender power  obstacles further away  slow changes in the average power received (long-term fading or slow fading)

long-term fading

short-term fading

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t

17

Fast Fading 

simulation showing time and frequency dependency of Rayleigh fading

V = 110km/h 900MHz

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Signal propagation ranges 





Transmission range  communication possible  low error rate Detection range  detection of the signal possible  no communication possible Interference range  signal may not be detected  signal adds to the background noise

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sender

transmission distance detection interference

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Interference

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Carrier to Interference Ratio (CIR, C/I) (Uplink Situation) 

Ratio of Carrier-to-Interference power at the receiver

C CIR  Ij  N





The minimum required CIR depends on the system and the signal processing potential of the receiver technology

Typical in GSM: C/I=15dB (Factor 32)

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Range limited systems (lack of coverage) 







Mobile stations located far away from BS (at cell border or even beyond the coverage zone) C at the receiver is too low, because the path loss between sender and receiver is too high

C/I is too low No signal reception possible

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Interference limited systems (lack of capacity)



Mobile station is within coverage zone C is sufficient, but too much interference I at the receiver



C/I is too low





No more resources / capacity left

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Information Theory: Channel Capacity (1) 

  



Bandwidth limited Additive White Gaussian Noise (AWGN) channel Gaussian codebooks Single transmit antenna Single receive antenna (SISO) Shannon (1950): Channel Capacity >1 (high signal-to-noise ratio), approximate

Observation: Bandwidth and S/N are reciproke to each other This means:  With low bandwidth very high data rate is possible provided  S/N is high enough  Example: higher order modulation schemes  With high noise (low S/N) data communication is possible if  bandwidth is large  Example: spread spectrum  Shannon channel capacity has been seen as a “unreachable” theoretical limit, for a long time. However:  Turbo coding (1993) pushs practical systems up to 0.5 dB to Shannon channel bandwidth

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Link Capacity for Various Rate-Controlled Technologies 6 Shannon bound Shannon bound with 3dB margin (3GPP) HSDPA (3GPP2) EV-DO (IEEE) 802.16

achievable rate (bps/Hz)

5

4

3

2

1

0 -15  

-10

-5

0 5 required SNR (dB)

10

15

20

The link capacity of current systems is quickly approaching the Shannon limit (within a factor of two). Future improvements in spectral efficiency will focus on intelligent antenna techniques and/or coordination between base stations.

Link Performance of OFDM & 3G Systems are Similar and Approaching the (Physical) Shannon Bound UMTS Networks

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Antennas: isotropic radiator 







Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna

y

z

z y

ideal isotropic radiator

x

x

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Antennas: simple dipoles Real antennas are not isotropic radiators but, e.g.  dipoles with lengths /4 on car roofs or  /2 as Hertzian dipole  shape of antenna proportional to wavelength /4

/2

Example: Radiation pattern of a simple Hertzian dipole y

y

x side view (xy-plane)

z

z side view (yz-plane)

x

simple dipole

top view (xz-plane)

Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) UMTS Networks

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Antennas: directed and sectorized Often used for microwave connections (narrow directed beam) or base stations for cellular networks (sectorized cells)

y

y

z

x

z

side view (xy-plane)

x

side view (yz-plane)

top view (xz-plane)

z

z

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sectorized antenna

x

x

top view, 3 sector

directed antenna

top view, 6 sector

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Antenna 3-sectorized downtilt

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Real world propagation examples

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Antennas: diversity Grouping of 2 or more antennas  multi-element antenna arrays Antenna diversity  switched diversity, selection diversity 



receiver chooses antenna with largest output

diversity combining  

combine output power to produce gain cophasing needed to avoid cancellation /2 /4

/2

+

/4

/2

/2

+

ground plane

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Multiplexing Goal: multiple use of a shared medium Multiplexing in 4 dimensions  space (si)  time (t)  frequency (f)  code (c) Multiple use is possible, if resource (channel) is different in at least one dimension

channels ki k1

k2

k3

k4

k5

k6

c c

t

t s1

f s2

f

c t

s3

f

Important: guard spaces needed! UMTS Networks

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Frequency multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages:  

no dynamic coordination needed applicable to analog signals

k1

k2

k3

k4

k5

k6

c

Disadvantages:  waste of bandwidth if the traffic is distributed unevenly  inflexible  guard space

f

t

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Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages:  only one carrier in the medium at any time  throughput high even for many users Disadvantages:  precise synchronization needed

k1

k2

k3

k4

k5

k6

c f

t

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Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM (frequency hopping) Advantages:  some (weak) protection against tapping  protection against frequency selective interference but: precise coordination required

k1

k2

k3

k4

k5

k6

c f

t

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Code multiplex Each channel has a unique code All channels use the same spectrum at the same time

k1

k2

k3

k4

Advantages:  bandwidth efficient  no coordination and synchronization necessary  good protection against interference and tapping

k5

k6

c

f

Disadvantages:  complex receivers (signal regeneration) Implemented using spread spectrum technology

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t

October 2012

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Cellular systems: Space Division Multiplex Cell structure implements space division multiplex:  base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures:  higher capacity, higher number of users  less transmission power needed  more robust, decentralized  base station deals with interference, transmission area, etc. locally Disadvantages:  fixed network needed for the base stations  handover (changing from one cell to another) necessary  interference with other cells Cell sizes vary from 10s of meters in urban areas to many km in rural areas (e.g. maximum of 35 km radius in GSM) UMTS Networks

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Cellular systems: Frequency planning I Frequency reuse only with a certain distance between the base stations Typical (hexagon) model: f5 f4 f1

reuse-3 cluster:

f1

reuse-7 cluster:

f3

f3

f2 f1 f3 f2

f4

f6 f1

f3 f2

f7 f2

f5 f1

f6

f3

f5 f4

f7 f2

f6 f1

f3

f7 f2

Other regular pattern: reuse-19  the frequency reuse pattern determines the experienced CIR Fixed frequency assignment:  certain frequencies are assigned to a certain cell  problem: different traffic load in different cells Dynamic frequency assignment:  base station chooses frequencies depending on the frequencies already used in neighbor cells  more capacity in cells with more traffic  assignment can also be based on interference measurements UMTS Networks

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Cellular systems: frequency planning II

f3

f3 f2

f1

f3 f2

f1 f3

f2

f1 f3

f2

f2 f1

f1 f3

3 cell cluster

f3

f3 f2

f3 f5

7 cell cluster

f4

f2 f6

f1 f3

f7

f6

UMTS Networks

f5 f4 f1 f3

f2

f2 f2 f2 f1 f f1 f f1 f h h 3 3 3 h 2 h 2 g2 1 h3 g2 1 h3 g2 g1 g1 g1 g3 g3 g3

f7

f5

f2

3 cell cluster with 3 sector antennas

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Spread spectrum technology: Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code  protection against narrow band interference interference

power

power

spread signal

signal (despreaded) spread interference

detection at receiver

Side effects:  

f

f

coexistence of several signals without dynamic coordination tap-proof

Alternatives:  Direct Sequence (UMTS)  Frequency Hopping (slow FH: GSM, fast FH: Bluetooth) UMTS Networks

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Effects of spreading and interference i) narrow band signal

ii) spreaded signal (broadband signal)

dP/df

dP/df

user signal broadband interference narrowband interference f

f

sender iii) addition of interference

iv) despreaded signal

dP/df

v) application of bandpass filter dP/df

dP/df

f

f

f

receiver UMTS Networks

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42

Spreading and frequency selective fading channel quality

1

2

5

3

6

narrowband interference without spread spectrum

4 frequency narrow band signal

guard space

channel quality

1

spread spectrum

UMTS Networks

2

2

2

2

2

spread spectrum to limit narrowband interference

frequency

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DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence)  many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages  reduces frequency selective fading  in cellular networks 





base stations can use the same frequency range several base stations can detect and recover the signal soft handover

Disadvantages  precise power control needed

tb user data 0

1

XOR

tc chipping sequence 01101010110101

= resulting signal

01101011001010

tb: bit period tc: chip period

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44

DSSS (Direct Sequence Spread Spectrum) II spread spectrum signal

user data X

transmit signal modulator

chipping sequence

radio carrier transmitter

correlator lowpass filtered signal

received signal demodulator radio carrier

products

sampled sums data

X

integrator

decision

chipping sequence receiver

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Modulation “The shaping of a (baseband) signal to convey information”. Basic schemes  Amplitude Modulation (AM)  Frequency Modulation (FM)  Phase Modulation (PM) Digital modulation  digital data is translated into an analog signal (baseband)  ASK, FSK, PSK  differences in spectral efficiency, power efficiency, robustness Motivation for modulation  smaller antennas (e.g., /4)  medium characteristics  Frequency Division Multiplexing  spectrum availability UMTS Networks

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Modulation and demodulation

digital data 101101001

digital modulation

analog baseband signal

analog modulation

radio transmitter

radio carrier

analog demodulation

analog baseband signal

synchronization decision

digital data 101101001

radio receiver

radio carrier

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Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK):  very simple  low bandwidth requirements  very susceptible to interference

1

0

1

t

1

0

1

Frequency Shift Keying (FSK):  needs larger bandwidth

Phase Shift Keying (PSK):  more complex  robust against interference

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t

1

0

1

t

October 2012

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Advanced Frequency Shift Keying 

bandwidth needed for FSK depends on the distance between the carrier frequencies

Idea:  special pre-computation avoids sudden phase shifts  MSK (Minimum Shift Keying) MSK technique:  bit stream is separated into even and odd bits, the duration of each bit is doubled  depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen  the frequency of one carrier is twice the frequency of the other, eliminating abrupt phase changes 

even higher bandwidth efficiency using a Gaussian low-pass filter  GMSK (Gaussian MSK), used for GSM and DECT

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Example of MSK 1

0

1

1

0

1

0

data

Transformation scheme

even bits

bit

odd bits

low frequency

even

0101

odd

0011

signal value

h l l h - - ++

h: high frequency l: low frequency +: original signal -: inverted signal

high frequency

MSK signal

t No phase shifts!

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Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying):  bit value 0: sine wave  bit value 1: inverted sine wave  very simple PSK  low spectral efficiency  robust, used e.g. in satellite systems QPSK (Quadrature Phase Shift Keying):      

Q

1

10

0

Q

2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK 00 more complex used in UMTS and EDGE (8-PSK) often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS)

I

11

I

01

Puls filtering of baseband to avoid sudden phase shifts => reduce bandwidth of modulated signal UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

51

Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM)  combines amplitude and phase modulation  it is possible to code n bits using one symbol  2n discrete levels: e.g. 16-QAM, 64-QAM n=2: 4-QAM identical to QPSK  bit error rate increases with n, but less errors compared to comparable PSK schemes

Example: 16-QAM (1 symbol = 16 levels = 4 bits) Symbols 0011 and 0001 have the same phase, but different amplitude 0000 and 1000 have different phase, but same amplitude

also: 64-QAM (1 symbol = 64 levels = 6 bits) QAM is used in  UMTS HSDPA (16-QAM)  UMTS LTE (64-QAM)  standard 9600 bit/s modems UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

Q

0010 0011

0001 0000 I 1000

October 2012

52

Media Access Schemes 

 

 

Motivation  limits of CSMA/CD  hidden and exposed terminals  near-far problem TDD vs. FDD TDMA  Aloha, slotted Aloha  Demand Assigned Multiple Access (DAMA) CDMA theory and practice Comparison

Media Access: Motivation The problem: multiple users compete for a common, shared resource (medium) Can we apply media access methods from fixed networks? Example CSMA/CD  Carrier Sense Multiple Access with Collision Detection (IEEE 802.3)  send as soon as the medium is free (carrier sensing – CS)  listen to the medium, if a collision occurs stop transmission and jam (collision detection – CD) Problems in wireless networks  signal strength decreases (at least) proportional to the square of the distance  the sender would apply CS and CD, but the collisions happen at the receiver  it might be the case that a sender cannot “hear” the collision, i.e., CD does not work  furthermore, CS might not work if, e.g., a terminal is “hidden”

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

54

Motivation - hidden and exposed terminals Hidden terminals  A sends to B, C cannot receive A  C wants to send to B, C senses a “free” medium -> CS fails  collision at B: A cannot detect the collision -> CD fails  A is “hidden” for C

Exposed terminals   



A

B

C

B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B

A UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

B

C October 2012

55

Motivation - near and far terminals Terminals A and B send, C receives  signal strength decreases proportional to the square of the distance  the signal of terminal B therefore drowns out A’s signal  C cannot receive A

A

B

C

Severe problem for CDMA-networks – precise power control needed!

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

56

Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access)  segment space into sectors, use directed antennas  cell structure FDMA (Frequency Division Multiple Access)  assign a certain frequency to a transmission channel between a sender and a receiver  permanent (e.g., radio broadcast), slow hopping (e.g. GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access)  assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

The multiplexing schemes presented previously are now used to control medium access!

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

57

Communication link types Each terminal needs an uplink and a downlink Types of communication links: 





Simplex  unidirectional link transmission Half Duplex  Bi-directional (but not simultaneous) Duplex  simultaneous bi-directional link transmission, two types:  Frequency division duplexing (FDD)  Time division duplexing (TDD)

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

58

Duplex modes Td Tu Td

Fd

Tu

Fu Frequency Division Duplex (FDD)

Time Division Duplex (TDD)

Separate frequency bands for up- and downlink

Separation of up- and downlink traffic on time axis

+ separation of uplink and downlink interference

+ support for asymmetric traffic

- no support for asymmetric traffic

- mix of uplink and downlink interference on single band

Examples: UMTS, GSM, IS-95, AMPS

Examples: DECT, UMTS (TDD)

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

59

FDD/FDMA - general scheme, example GSM

f 960

935.2

124

200 kHz

1 20

915

890.2

124

1

t

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

60

TDD/TDMA - general scheme, example DECT

417 µs 1 2 3

11 12 1 2 3

downlink

UMTS Networks

11 12

uplink

Andreas Mitschele-Thiel, Jens Mückenheim

t

October 2012

61

Aloha/slotted aloha Mechanism  random, distributed (no central arbiter), time-multiplex  Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries collision

Aloha sender A sender B sender C

t

Slotted Aloha

collision

sender A sender B sender C t UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

62

DAMA - Demand Assigned Multiple Access Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length) Reservation can increase efficiency to 80%  a sender reserves a future time-slot  sending within this reserved time-slot is possible without collision  reservation also causes higher delays  typical scheme for satellite links  application to packet data, e.g. in GPRS and UMTS Examples for reservation algorithms:   

Explicit Reservation (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

63

Access method DAMA: Explicit Reservation Explicit Reservation (Reservation Aloha): Two modes:  ALOHA mode for reservation: competition for small reservation slots, collisions possible  reserved mode for data transmission within successful reserved slots (no collisions possible) synchronisation: it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time

collision

Aloha

UMTS Networks

reserved

Aloha

reserved

Aloha

Andreas Mitschele-Thiel, Jens Mückenheim

reserved

October 2012

Aloha

t

64

Access method CDMA CDMA (Code Division Multiple Access)  all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel  each sender has a unique random number, the sender XORs the signal with this random number  the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function Advantages:  all terminals can use the same frequency, less planning needed  huge code space (e.g. 232) compared to frequency space  interference (e.g. white noise) is not coded  forward error correction and encryption can be easily integrated Disadvantages:  higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)  all signals should have the same strength at a receiver (power control) UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

65

CDMA Principle

sender (base station)

receiver (terminal)

Code 0

Code 0 data 0

data 0 Code 1

Code 1



data 1

Transmission via air interface

data 1 Code 2

Code 2

data 2

data 2

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

66

CDMA by example data stream A & B

UMTS Networks

spreading

spreaded signal

Source 1

Code 1

Source 1 spread

Source 2

Code 2

Source 2 spread

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

67

CDMA by example Despread Source 1

Su m o f So u rce s Sp re a d

Su m o f So u rce s Sp re a d + N o ise

decoding and despreading

+

Despread Source 2

overlay of signals

UMTS Networks

transmission and distortion (noise and interference)

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

68

CDMA in theory Sender A 

sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)



sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender B 

sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)



sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space 

interference neglected (noise etc.)



As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender A 



apply key Ak bitwise (inner product) 

Ae = (-2, 0, 0, -2, +2, 0)  Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6



result greater than 0, therefore, original bit was „1“

receiving B 

UMTS Networks

Be = (-2, 0, 0, -2, +2, 0)  Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“ Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

69

CDMA on signal level I data A

1

0

Ad

1

key A key sequence A data  key

0

1

0

1

0

0

1

0

0

0

1

0

1

1

0

0

1

1

1

0

1

0

1

1

1

0

0

0

1

0

0

0

1

1

0

0

Ak

As

signal A

Real systems use much longer keys resulting in a larger distance between single code words in code space

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

70

CDMA on signal level II

As

signal A

1

data B key B key sequence B data  key

0

Bd

0

0

0

0

1 1

0

1 0

1 0

0

0

0

1 0

1

1

1

1

1

1

0 0

1

1 0

1 0

0

0

0

1 0

1

1

1

Bk

Bs

signal B

1 0 As + Bs

UMTS Networks

-1

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

71

CDMA on signal level III data A

1

0

1

Ad 1 0

As + Bs

-1 1 Ak

-1 1 0

(As + Bs) * Ak

-1

integrator output comparator output

UMTS Networks

1

0

Andreas Mitschele-Thiel, Jens Mückenheim

1

October 2012

72

CDMA on signal level IV data B

1

0

0

Bd 1

As + Bs

0 -1 1

Bk

-1 1 0

(As + Bs) * Bk

-1

integrator output comparator output

UMTS Networks

1

0

Andreas Mitschele-Thiel, Jens Mückenheim

0

October 2012

73

CDMA on signal level V 1 As + Bs

0 -1 1

wrong key K

-1 1

(As + Bs) *K

0 -1

integrator output comparator output

(0)

(0)

?

Assumptions  orthogonality of keys  neglectance of noise  no differences in signal level => precise power control UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

74

Comparison SDMA/TDMA/FDMA/CDMA Approach Idea

SDMA

TDMA

segment space into cells/sectors

Terminals

only one terminal can be active in one cell/one sector

Signal separation

cell structure, directed antennas

segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain

FDMA segment the frequency band into disjoint sub-bands

CDMA spread the spectrum using orthogonal codes

every terminal has its all terminals can be active own frequency, at the same place at the uninterrupted same moment, uninterrupted filtering in the code plus special frequency domain receivers

Advantages very simple, increases established, fully

simple, established, robust

inflexible, antennas Disadvantages typically fixed

inflexible, frequencies are a scarce resource

flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders

typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)

still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA

capacity per km²

Comment

UMTS Networks

only in combination with TDMA, FDMA or CDMA useful

digital, flexible

guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

75

Basic Functions in Mobile Systems

   

Location management Handover Roaming Authentication (see later)

Location Management The problem: locate a mobile user from the network side (mobile-terminated call) Two extreme solutions: 



Mobile registers with each visited cell (e.g. direct call to the hotel room to reach a person) – signaling traffic to register mobile when cell is changed – network has to maintain location information about each mobile + low signaling load to page mobile (i.e. in one cell only) Page mobile using a network- or worldwide broadcast message (e.g. broadcast on TV or radio to contact a person) – heavy signaling load to page the mobile (i.e. in all cells) + no signaling traffic while mobile is idle

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

77

Location Management The issue: Compromise between  minimizing the area where to search for a mobile  minimizing the number of location updates

TOTAL Signalling Cost

Solution 2: Small paging area

RA

RA

Location RA Update

RA

RA

Location RA Update

RA

RA

RA

=

Paging  Signalling Cost

+

Solution 1: Large paging area

Paging Area Update Signalling Cost

Location Update UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

Location Update Location Update

October 2012

78

Handover The problem: Change the cell while communicating

cell 2

cell 1

Link quality

Reasons for handover:  Quality of radio link deteriorates  Communication in other cell requires less radio resources  Supported radius is exceeded (e.g. Timing advance in GSM)  Overload in current cell  Maintenance

cell 1 Handover margin (avoid ping-pong effect) cell 2

Link to cell 1 UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

Link to cell 2

time 79

Roaming The problem: Use a network not subscribed to Roaming agreement needed between network operators to exchange information concerning:  Authentication  Authorisation  Accounting Examples of roaming agreements:  Use networks abroad  Use of T-Mobile network by O2 (E2) subscribers in area with no O2 coverage

UMTS Networks

Andreas Mitschele-Thiel, Jens Mückenheim

October 2012

80