Mobile CommunicationsWireless Transmission Frequencies Signals Antenna Signal propagation

Multiplexing Spread spectrum Modulation Cellular systems*

Frequencies for communicationVLF = Very Low FrequencyUHF = Ultra High FrequencyLF = Low Frequency SHF = Super High FrequencyMF = Medium Frequency EHF = Extra High FrequencyHF = High Frequency UV = Ultraviolet LightVHF = Very High Frequency

Frequency and wave length: = c/f wave length , speed of light c 3x108m/s, frequency f1 Mm300 Hz10 km30 kHz100 m3 MHz1 m300 MHz10 mm30 GHz100 m3 THz1 m300 THzvisible lightVLFLFMFHFVHFUHFSHFEHFinfraredUVoptical transmissioncoax cabletwisted pair*

Frequencies for mobile communicationVHF-/UHF-ranges for mobile radiosimple, small antenna for carsdeterministic propagation characteristics, reliable connectionsSHF and higher for directed radio links, satellite communicationsmall antenna, beam forminglarge bandwidth availableWireless LANs use frequencies in UHF to SHF rangesome 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. *

Frequencies and regulationsITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)*

Europe

USA

Japan

Cellular

Phones

GSM 450-457, 479-486/460-467,489-496, 890-915/935-960,

1710-1785/1805-1880

UMTS (FDD) 1920-1980, 2110-2190

UMTS (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

Cordless

Phones

CT1+ 885-887, 930-932

CT2

864-868

DECT

1880-1900

PACS 1850-1910, 1930-1990

PACS-UB 1910-1930

PHS 1895-1918

JCT

254-380

Wireless LANs

IEEE 802.11

2400-2483

HIPERLAN 2

5150-5350, 5470-5725

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

Signals physical representation of datafunction of time and locationsignal parameters: parameters representing the value of data classificationcontinuous time/discrete timecontinuous values/discrete valuesanalog signal = continuous time and continuous valuesdigital signal = discrete time and discrete valuessignal 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)*

Signal propagation rangesTransmission rangecommunication possiblelow error rateDetection rangedetection of the signal possibleno communication possibleInterference rangesignal may not be detected signal adds to the background noise

distancesendertransmissiondetectioninterference*

Signal propagationPropagation in free space always like light (straight line)Receiving power proportional to 1/d in vacuum much more in real environments (d = distance between sender and receiver)Receiving power additionally influenced byfading (frequency dependent)shadowingreflection at large obstaclesrefraction depending on the density of a mediumscattering at small obstaclesdiffraction at edgesreflectionscatteringdiffractionshadowingrefraction*

Real world example*

Multipath propagationSignal can take many different paths between sender and receiver due to reflection, scattering, diffractionTime 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

signal at sendersignal at receiverLOS pulsesmultipathpulses*

Effects of mobilityChannel characteristics change over time and location signal paths changedifferent delay variations of different signal partsdifferent phases of signal partsquick changes in the power received (short term fading)Additional changes indistance to senderobstacles further away slow changes in the average power received (long term fading)short term fadinglong termfadingtpower*

MultiplexingMultiplexing in 4 dimensionsspace (si)time (t)frequency (f)code (c)

Goal: multiple use of a shared medium

Important: guard spaces needed!s2s3s1ftck2k3k4k5k6k1ftcftcchannels ki*

Frequency multiplexSeparation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signals

Disadvantages:waste of bandwidth if the traffic is distributed unevenly inflexibleguard spaces

k2k3k4k5k6k1ftc*

Time multiplexA channel gets the whole spectrum for a certain amount of time

Advantages:only one carrier in the medium at any timethroughput high even for many users

Disadvantages:Precise synchronization necessaryftck2k3k4k5k6k1*

Time and frequency multiplexCombination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSM Advantages:better protection against tappingprotection against frequency selective interferencehigher data rates compared to code multiplex

but: precise coordination requiredftck2k3k4k5k6k1*

Code multiplexEach channel has a unique codeAll channels use the same spectrum at the same timeAdvantages:bandwidth efficientno coordination and synchronization necessarygood protection against interference and tappingDisadvantages:lower user data ratesmore complex signal regenerationImplemented using spread spectrum technologyk2k3k4k5k6k1ftc*

ModulationDigital modulationdigital data is translated into an analog signal (baseband)ASK, FSK, PSK - main focus in this chapterdifferences in spectral efficiency, power efficiency, robustnessAnalog modulationshifts center frequency of baseband signal up to the radio carrierMotivationsmaller antennas (e.g., /4)Frequency Division Multiplexingmedium characteristicsBasic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)*

Modulation and demodulationsynchronizationdecisiondigitaldataanalogdemodulationradiocarrieranalogbasebandsignal101101001radio receiverdigitalmodulationdigitaldataanalogmodulationradiocarrieranalogbasebandsignal101101001radio transmitter*

Digital modulationModulation 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 interference101t101t101t*

Spread spectrum technologyProblem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special codeSide effects:coexistence of several signals without dynamic coordinationtap-proofAlternatives: Direct Sequence, Frequency Hopping

detection atreceiverinterferencespread signalsignalspreadinterferenceffpowerpower*

Effects of spreading and interferencedP/dffi)dP/dffii)senderdP/dffiii)dP/dffiv)receiverfv)user signalbroadband interferencenarrowband interferencedP/df*

Spreading and frequency selective fadingfrequencychannel quality123456narrow band signalguard spacenarrowband channelsspread spectrum channels*

DSSS (Direct Sequence Spread Spectrum) IXOR of the signal with pseudo-random number (chipping sequence)many chips per bit (e.g., 128) result in higher bandwidth of the signalAdvantagesreduces frequency selective fadingin cellular networks base stations can use the same frequency rangeseveral base stations can detect and recover the signalsoft handoverDisadvantagesprecise power control necessaryuser datachipping sequenceresultingsignal0101101010100111XOR01100101101001=tbtctb: bit periodtc: chip period*

DSSS (Direct Sequence Spread Spectrum) IIXuser datachippingsequencemodulatorradiocarrierspreadspectrumsignaltransmitsignaltransmitterdemodulatorreceivedsignalradiocarrierXchippingsequencelowpassfilteredsignalreceiverintegratorproductsdecisiondatasampledsumscorrelator*

FHSS (Frequency Hopping Spread Spectrum) Discrete changes of carrier frequencysequence of frequency changes determined via pseudo random number sequenceTwo versionsFast Hopping: several frequencies per user bitSlow Hopping: several user bits per frequencyAdvantagesfrequency selective fading and interference limited to short periodsimple implementationuses only small portion of spectrum at any timeDisadvantagesnot as robust as DSSSsimpler to detect

*

FHSS (Frequency Hopping Spread Spectrum) IIuser dataslowhopping(3 bits/hop)fasthopping(3 hops/bit)01tb011tff1f2f3ttdff1f2f3ttdtb: bit periodtd: dwell time*

FHSS (Frequency Hopping Spread Spectrum) IIImodulatoruser datahoppingsequencemodulatornarrowbandsignalspreadtransmitsignaltransmitterreceivedsignalreceiverdemodulatordatafrequencysynthesizerhoppingsequencedemodulatorfrequencysynthesizernarrowbandsignal*

Cell structureImplements space division multiplex: base station covers a certain transmission area (cell)Mobile stations communicate only via the base stationAdvantages of cell structures:higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc. locallyProblems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cellsCell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies*

Frequency planning IFrequency reuse only with a certain distance between the base stationsStandard model using 7 frequencies:

Fixed frequency assignment:certain frequencies are assigned to a certain cellproblem: different traffic load in different cellsDynamic frequency assignment:base station chooses frequencies depending on the frequencies already used in neighbor cellsmore capacity in cells with more trafficassignment can also be based on interference measurements*

Frequency planning IIf4f5f1f3f2f6f7f3f2f4f5f1f3f5f6f7f2f23 cell cluster7 cell cluster3 cell clusterwith 3 sector antennas*

Cell breathingCDM 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

*

Universitt KarlsruheInstitut fr TelematikUniversitt KarlsruheInstitut fr TelematikMobilkommunikationSS 1998Prof. Dr. Dr. h.c. G. KrgerE. Dorner / Dr. J. Schiller*Prof. Dr. Dr. h.c. G. KrgerE. Dorner / Dr. J. Schiller