mobile Wireless Transmission

8/14/2019 mobile Wireless Transmission

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Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.1

Mobile Communications

Chapter 2: Wireless TransmissionFrequenciesSignals

AntennaSignal propagation

MultiplexingSpread spectrum

ModulationCellular systems


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Frequencies for mobile communication

VHF-/UHF-ranges for mobile radio

simple, small antenna for carsdeterministic propagation characteristics, reliable connections

SHF and higher for directed radio links, satellite communicationsmall antenna, focusinglarge bandwidth available

Wireless LANs use frequencies in UHF to SHF spectrumsome systems planned up to EHFlimitations 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 bandsworldwide (WRC, World Radio Conferences)

Europe USA Japan

Cellular Phones

GSM 450-457, 479 -486/460 -467,489 -496, 890 -915/935 -960,1710 -1785/1805 -1880UMTS (FDD) 1920 -

1980, 2110 -2190UMTS (TDD) 1900 -1920, 2020 -2025

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

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

CordlessPhones

CT1+ 885 -887, 930 -932CT2864 -868DECT1880 -1900

PACS 1850 -1910, 1930 -1990PACS -UB 1910 -1930

PHS 1895 -1918JCT 254 -380

WirelessLANs

IEEE 802.112400 -2483HIPERLAN 25150 -5350, 5470 -5725

902 -928IEEE 802.112400 -24835150 -5350, 5725 -5825

IEEE 802.11 2471 -24975150 -5250

Others RF-Control27, 128, 418, 433,868

RF-Control315, 915

RF-Control 426, 868


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Signals I

physical representation of datafunction of time and locationsignal parameters: parameters representing the value of dataclassification

continuous time/discrete timecontinuous values/discrete values

analog signal = continuous time and continuous valuesdigital 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) = A t sin(2 f tt + t)


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Fourier representation of periodic signals

)2cos()2sin(2

1)(

11

nft bnft act g n

nn

n

=

=

++=

1

0

1

0

t t

ideal periodic signalreal composition(based on harmonics)


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Different representations of signalsamplitude (amplitude domain)frequency spectrum (frequency domain)

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

Composed signals transferred into frequency domain using Fourier transformationDigital signals need

infinite frequencies for perfect transmissionmodulation with a carrier frequency for transmission (analog signal!)

Signals II

f [Hz]

A [V]

I= M cos

Q = M sin

A [V]

t[s]


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Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission

Isotropic radiator: equal radiation in all directions (threedimensional) - only a theoretical reference antennaReal antennas always have directive effects (vertically and/or horizontally)Radiation pattern: measurement of radiation around an antenna

Antennas: isotropic radiator

zy

x

z

y x idealisotropicradiator


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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

Example: Radiation pattern of a simple Hertzian dipole

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

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simpledipole

/4

/2


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Antennas: directed and sectorized

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

top view, 3 sector

x

z

top view, 6 sector

x

z

Often used for microwave connections or base stations for mobile phones(e.g., radio coverage of a valley)

directedantenna

sectorizedantenna


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Antennas: diversity

Grouping of 2 or more antennas

multi-element antenna arraysAntenna diversity

switched diversity, selection diversityreceiver chooses antenna with largest output

diversity combining

combine output power to produce gaincophasing needed to avoid cancellation

+

/4 /2

/4

ground plane

/2 /

2

+

/2


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Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission rangecommunication possiblelow error rate

Detection rangedetection of the signalpossibleno communicationpossible

Interference rangesignal may not bedetectedsignal adds to thebackground noise


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Signal propagation

Propagation in free space always like light (straight line)Receiving power proportional to 1/d

(d = distance between sender and receiver)Receiving power additionally influenced by

fading (frequency dependent)shadowingreflection at large obstacles

refraction depending on the density of a mediumscattering at small obstaclesdiffraction at edges

reflection scattering diffractionshadowing refraction


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Real world example


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Signal can take many different paths between sender and receiver due toreflection, scattering, diffraction

Time dispersion: signal is dispersed over timeinterference with neighbor symbols, Inter Symbol Interference (ISI)

The signal reaches a receiver directly and phase shifteddistorted signal depending on the phases of the different parts

Multipath propagation

signal at sender signal at receiver

LOS pulsesmultipathpulses


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Effects of mobility

Channel characteristics change over time and locationsignal paths changedifferent delay variations of different signal partsdifferent phases of signal parts

quick changes in the power received (short term fading)

Additional changes indistance to sender obstacles further away

slow changes in the average power received (long term fading)

short term fading

long termfading

t

power


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Multiplexing in 4 dimensionsspace (s i)

time (t)frequency (f)code (c)

Goal: multiple useof a shared medium

Important: guard spaces needed!

s 2

s 3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels k i


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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 timeAdvantages:

no dynamic coordinationnecessaryworks also for analog signals

Disadvantages:waste of bandwidthif the traffic isdistributed unevenlyinflexibleguard spaces

k2 k3 k4 k5 k6k1

f

t

c


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f

t

c

k2 k3 k4 k5 k6k1

Time multiplex

A channel gets the whole spectrum for a certain amount of time

Advantages:only one carrier in themedium at any timethroughput high evenfor many users

Disadvantages:precisesynchronizationnecessary


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f

Time and frequency multiplex

Combination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSMAdvantages:

better protection againsttappingprotection against frequencyselective interferencehigher data rates compared tocode multiplex

but: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1


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Code multiplex

Each channel has a unique code

All channels use the same spectrumat the same time

Advantages:bandwidth efficient

no coordination and synchronizationnecessarygood protection against interference andtapping

Disadvantages:lower user data ratesmore complex signal regeneration

Implemented using spread spectrumtechnology

k2

k3

k4

k5

k6

k1

f

t

c


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Modulation

Digital modulationdigital data is translated into an analog signal (baseband)ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness

Analog modulationshifts center frequency of baseband signal up to the radio carrier

Motivationsmaller antennas (e.g., /4)Frequency Division Multiplexingmedium characteristics

Basic schemes

Amplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)


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Digital modulation

Modulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):

very simplelow bandwidth requirementsvery susceptible to interference

Frequency Shift Keying (FSK):needs larger bandwidth

Phase Shift Keying (PSK):

more complexrobust against interference

1 0 1

t

1 0 1

t

1 0 1

t


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Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance betweenthe carrier frequencies

special pre-computation avoids sudden phase shiftsMSK (Minimum Shift Keying)

bit separated into even and odd bits, the duration of each bit isdoubleddepending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosenthe frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK

even higher bandwidth efficiency using a Gaussian low-passfilter GMSK (Gaussian MSK), used in GSM


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Example of MSK

data

even bits

odd bits

1 1 1 1 000

t

lowfrequency

highfrequency

MSKsignal

bit

even 0 1 0 1odd 0 0 1 1

signal h n n hvalue - - + +

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

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 wavevery simple PSKlow spectral efficiencyrobust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying):2 bits coded as one symbolsymbol determines shift of sine waveneeds less bandwidth compared toBPSKmore complex

Often also transmission of relative, notabsolute phase shift: DQPSK -Differential QPSK (IS-136, PHS)

11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t


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Quadrature Amplitude Modulation

Quadrature Amplitude Modulation (QAM): combines amplitude andphase modulationit is possible to code n bits using one symbol2 n discrete levels, n=2 identical to QPSKbit error rate increases with n, but less errors compared tocomparable PSK schemes

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

different phase, but same amplitude.used in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

a


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Hierarchical Modulation

DVB-T modulates two separate data streams onto a single DVB-T streamHigh Priority (HP) embedded within a Low Priority (LP) streamMulti carrier system, about 2000 or 8000 carriersQPSK, 16 QAM, 64QAMExample: 64QAM

good reception: resolve the entire

64QAM constellationpoor reception, mobile reception:resolve only QPSK portion6 bit per QAM symbol, 2 mostsignificant determine QPSKHP service coded in QPSK (2 bit),LP uses remaining 4 bit

Q

I

00

10

00 0010 01 0101


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Spread spectrum technology

Problem of radio transmission: frequency dependent fading can wipe outnarrow band signals for duration of the interference

Solution: spread the narrow band signal into a broad band signal using aspecial codeprotection against narrow band interference

protection against narrowband interference

Side effects:

coexistence of several signals without dynamic coordinationtap-proof

Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread

signal

signal

spreadinterference

f f

power power


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Effects of spreading and interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiver f

v)

user signalbroadband interferencenarrowband interference

dP/df


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Spreading and frequency selective fading

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

22

22

2

frequency

channelquality

1

spreadspectrum

narrowband channels

spread spectrum channels


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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 signalAdvantages

reduces frequency selectivefadingin cellular networks

base stations can use thesame frequency rangeseveral base stations candetect and recover the signalsoft handover

Disadvantagesprecise power control necessary

user data

chippingsequence

resulting

signal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period


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FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequencysequence of frequency changes determined via pseudo random number

sequenceTwo versions

Fast Hopping:several frequencies per user bitSlow Hopping:

several user bits per frequencyAdvantages

frequency selective fading and interference limited to short periodsimple implementationuses only small portion of spectrum at any time

Disadvantagesnot as robust as DSSSsimpler to detect

( d )


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FHSS (Frequency Hopping Spread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping

(3 hops/bit)

0 1

tb

0 1 1 t

f

f 1

f 2

f 3

t

td

f

f 1

f 2

f 3

t

td

tb: bit period t d: dwell time

FHSS (F H i S d S ) III


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FHSS (Frequency Hopping Spread Spectrum) III

modulator user data

hoppingsequence

modulator

narrowbandsignal

spreadtransmit

signal

transmitter

receivedsignal

receiver

demodulator data

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

C ll


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Cell structure

Implements space division multiplex: base station covers a certaintransmission area (cell)

Mobile stations communicate only via the base station

Advantages of cell structures:higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc. locally

Problems:fixed network needed for the base stations

handover (changing from one cell to another) necessaryinterference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side(GSM) - even less for higher frequencies


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C ll b thi


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Cell breathing

CDM systems: cell size depends on current loadAdditional traffic appears as noise to other usersIf the noise level is too high users drop out of cells

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