移动通信第二版答案.docx

移动通信第二版答案.docx

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移动通信第二版答案

SolutionManualforMobileCommunications2nded.

JochenH.Schiller,FreieUniversitätBerlin,Germany

schiller@computer.org,www.jochenschiller.de

1.Introduction

1.1Goodsourcesforsubscribernumbersandotherstatisticsare,e.g.,

www.3gpp.org,www.3gpp2.org,www.emc-,

www.3g.co.uk,www.regtp.de…

1.2Today’sGSMoperatorsaddthenew3GairinterfacesofUMTStotheirexisting

GSM/GPRSinfrastructurenetworks.CurrentGSM/GPRSnetworksalreadyoffer

packetandcircuitswitcheddatatransmissionfollowingtheRelease99ofUMTS.The

operatorshavetoinstallnewradioaccessnetworks,i.e.,antennas,radionetwork

controlleretc.asdescribedinchapter4.Thesituationissimilarforoperatorsusing

cdmaOne（IS-95）technology.However,theseoperatorsgoforcdma2000asthis

systemallowsthemtoreusetheiralreadyexistinginfrastructure.Thus,basedonthe

separationofthemobilephonesystemsinto（veryroughly）CDMAandGSM

operatorswillleadtotwodifferentmajor3Gsystems,cdma2000andUMTS（and

theirfuturereleases）.Rightnow,itdoesnotseemthatthereisaplaceforathird3G

system.CurrentTDMAoperatorsmightmovetoEDGEenhancedsystemsorjointhe

UMTSsystem.However,itisstillopenwhatwillhappeninChina–theChinese

systemTD-SCDMAwaspushedbythegovernment,butnetworksanddevicesare

stillmissing.Currently,themajorityofChinesesubscribersuseGSM,someoperators

offerCDMA.

2.WirelessTransmission

2.1CheckalsotheWRCsthattrytoharmonizeglobalfrequencyplans.

2.2Below2MHzradiowavesfollowtheground（groundwavepropagation）.Onefactor

forthisisdiffraction（wavesareboundtowardsobstaclesthathavesizesintheorder

ofthewavelength）,anotherfactoristhecurrentinducedintheEarth’ssurface,which

slowsthewavefrontneartheearth,causingthewavefronttotiltdownward.Several

reasonsmakelowfrequenciesunusableincomputernetworks:

•LowerfrequenciesalsomeanlowerdataratesaccordingtoNyquist/Shannonas

theavailablebandwidthisless.

•Lowerfrequenciesalsorequirelargeantennasforefficienttransmissionand

reception.Thismightworkforsubmarines,notformobilephones.

•Lowerfrequenciespenetratematerialmoreeasily.ThusSDMismoredifficult–

cellsizewouldincreasedramaticallyandfrequencyreusewouldbealmost

impossible.

2.3FrequenciesintheTHzrange,e.g.,infrared,visiblelight,areeasilyblockedby

obstaclesand,thus,donotinterferewithothertransmissions.Inthiscase,onlythe

standardsafetyregulationsapply（e.g.,laseremission）.Mostradiosystemsstaywell

beyond100GHzasitisnotthatsimpletogeneratehigherfrequencies（inthelower

THzrange）.

2.4TheclassicalEuropeanapproachwasbasedonstandardisationandregulation

beforeanyproductswereavailable.TheEUgovernmentsfoundedETSIto

harmonizeallnationalregulations.ETSIcreatedthestandards,allcountrieshadto

follow.IntheUScompaniesdevelopsystemsandtrytostandardizethemorthe

marketforcesdecideuponsuccess.TheFCC,e.g.,onlyregulatesthefairness

amongdifferentsystemsbutdoesnotstipulateacertainsystem.Theeffectsofthe

twodifferentapproachesaredifferent.Many“governmental”standardsinEurope

failedcompletely,e.g.,HIPERLAN1,somesucceededonlyinEurope,e.g.,ISDN,

andhowever,somesoonbecameaworldwidesuccessstory,e.g.,GSM.Formost

systemstheUSapproachworkedbetter,firstsomeinitialproducts,thenstandards.

OnegoodexampleisthewirelessLANfamily802.11,agoodcounterexampleisthe

mobilephonemarket:

severaldifferent,incompatiblesystemstrytosucceed,many

features,wellestablishedinEuropesincemanyyears,arenotevenknownintheUS

（freeroaming,MMS,GPRSroaming,nochargesforbeingcalledetc.）.

2.5Computers,incontrastto,e.g.,TVsets,travelaroundtheworldaslaptops,PDAsetc.

Customerswanttousethemeverywhere.Thusitisveryimportanttobeabletouse

built-inWLANadaptersaroundtheglobewithoutreconfigurationandwithout

licensing.Furthermore,itismuchcheaperforWLANmanufacturerstodesignasingle

commonsystemcomparedtomanydifferentsystemsforprobablysmallmarkets.

2.6No.Loss-freetransmissionofanaloguesignalsisnotpossible.Attenuation,

dispersionetc.willalwaysdistortthesignal.Additionally,eachdigitalsignalis

transmittedas“bundle”ofanaloguesinewaves（thinkofFourier!

）.Aperfectdigital

signalwithrectangularshaperequiresaninfinitenumberofsinewavestobe

preciselyrepresented（thedigitalsignalcanberepresentedasinfinitesumofsine

wavesaccordingtoFourier）.However,nomediumcantransmitinfinitehigh

frequencies.Thus,thedigitalsignalcanneverbetransmittedwithoutanyloss.

2.7Withoutanyadditional“intelligence”directionalantennasarenotusefulinstandard

mobilephonesasusersmaynotwanttodirectthephonetoacertainantenna.Users

move,rotate,flipthephonesetc.Phonesareinbags,pockets,…whileoperated

hands-free.Thereisnochanceofdirectedtransmission.However,new

developmentscomprisingfastsignalprocessorsandmultipleantennasmayexploit

directedcharacteristicsofantennas（beamforming）.Thereareseveralwaysof

improvingthegainofanantenna:

rightdimensioning（e.g.,halfthewavelength）,

multipleantennasplusasignalprocessorcombiningthesignals,activeandpassive

componentsattachedtotheantenna（comparewithtraditionalTVantennas,satellite

dishesetc.）.

2.8Problems:

attenuation,scattering,diffraction,reflection,refraction.Exceptfor

attenuationallothereffectscandivertthewavesfromastraightline.Onlyinvacuum

andwithoutgravitationaleffectsradiowavesfollowastraightline.Withoutreflection

radioreceptionintownswouldbealmostimpossible.Aline.-of-sightalmostnever

exists.However,reflectionisthemainreasonformultipathpropagationcausingISI.

2.9ISImitigation:

largeenoughguardspacesbetweensymbols/lowsymbolrate（usedin

OFDM:

distributethesymbolstreamonmanydifferentcarriers）,channel

estimation/calculatethenstrongestpathsandadaptthereceiveraccordingly.Using

higherfrequenciesreducestheeffectsofmultipathpropagationandthusISI（waves

moreandmorebehavelikelight）.ThehigherthesymbolratethestrongertheISI.If

sendersand/orreceiversmovefastthechancesforISIarehigherbecausethe

locationofobstacleschanges,hencethenumber,magnitude,andtimingofthe

secondarypulses–itisdifficulttofollowthesignalsandadjustthedelaysfor

recombination.ISIlowersthebandwidthofaTDMschemeastheguardspaces

requiresometime.

2.10Severalmechanismsexisttomitigatenarrowbandinterference（whichmightbe

causedbyothersenders,too）:

•DynamicFrequencySelection:

Senderscansensethemediumforinterference

andchooseafrequencyrangewithlower/nointerference.HiperLAN2and

802.11husethisscheme.Networkoperatorscanalsothisschemetodynamically

assignfrequenciestocellsinmobilephonesystems.DFShasarelativelylow

complexity.

•Frequencyhopping:

Slowfrequencyhopping（severalsymbolsperfrequency）

mayavoidfrequencieswithinterferencemostofthetimewithacertainprobability.

ThisschememaybeusedinGSM.Furthermore,wirelesssystemscanusethis

principleformultiplexingasitisdoneinBluetoothsystems（stillslowhoppingas

Bluetoothsendsmanysymbols,indeedawholepacket,onthesamefrequency）.

Fasthoppingschemestransmitasymboloverseveralfrequencies,thuscreating

aspreadspectrum.FHsystemshavemediumcomplexity.Maintopicis

synchronisationofthedevices.

•Directsequencespreadspectrum:

DataisXORedwithachippingsequence

resultinginaspreadsignal.ThisisdoneinallCDMAsystems,butalsoinWLANs

using,e.g.,Barkersequencesforspreading（e.g.,802.11b）.Thesignalisspread

overalargespectrumand,thus,narrowbandinterferenceonlydestroysasmall

fractionofthesignal.Thisschemeisverypowerful,butrequiresmorepowerful

receiverstoextracttheoriginalsignalfromthemixtureofspreadsignals.

2.11WorldwideregulationalwaysusesFDMforseparatingdifferentsystems（TV,WLAN,

radio,satellite,…）.Thus,allradiosystemsmustmodulatethedigitalsignalontoa

carrierfrequencyusinganaloguemodulation.Themostprominentsystemisthe

traditionalradio:

allmusicandvoiceusefrequenciesbetween,e.g.,10Hzand22

kHz.However,manydifferentradiostationswanttotransmitatthesametime.

Therefore,alltheoriginalsignals（whichusethesamefrequencyrange）mustbe

modulatedontodifferentcarrierfrequencies.Othermotivationsfordigitalmodulation

areantennaandmediumcharacteristics.Importantcharacteristicsfordigital

modulationarespectralefficiency,powerefficiencyandrobustness.Typicalschemes

areASK,PSK,FSK.

2.12Thereceivermay“check”thedistancebetweenthereceivedpointandthe

neighbouringpoints.Thereceiverthenchoosestheclosestneighbourandassumes

thatthesenderoriginallytransmitteddatarepresentedbythechosenpoint.Themore

pointsaPSKschemeusesthehigherarechancesthatinterference（noise）shiftsa

transmitted“point”ontoanother.Ifthegapsbetweenthepointsaretoosmall,in

particularsmallerthannoiseaddedduringtransmission,chancesareveryhighthat

thereceiverwillmapreceiveddataontothewrongpointintheconstellationdiagram

（pleasenote:

dataiscodedusingPSK,thepointsintheconstellationdiagram

representcodes,thesecodesarethentransmitted–itisjustsimplertothinkin

“points”…）.

2.13Mainbenefits:

veryrobustagainstinterference,inherentsecur