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