ipl-logo

2023 pp

.pdf
School
HKU**We aren't endorsed by this school
Course
ELEC 3249
Subject
Electrical Engineering
Date
Dec 17, 2024
Pages
13
Uploaded by BarristerStar15868
香港大學THEUNIVERSITYOFHONGKONGBachelorofEngineeringDepartmentofElectrical&ElectronicEngineeringELEC3243:FundamentalsofNext-GenerationCommunicationsFinalExaminationDate:18December2023UID:Time:2:30pm-5:30pmSeatnumber:SectionI/28eAnswerALLquestions.SectionIT|Ql/23ePutyouranswersontheanswerbook.Q2/23Q3/26Total=/100UseofElectronicCalculators:OnlyapprovedcalculatorsasannouncedbytheExaminationsSecretarycanbeusedinthisexamination.Itiscandidates’responsibilitytoensurethattheircalculatoroperatessatisfactorily,andcandidatesmustrecordthenameandtypeofthecalculatorusedonthefrontpageoftheexaminationscript.page1of13
Background image
ELEC3243,Dec.18,2023SectionI.Multiple-choicequestions(28marks)Foreachquestion,choosethebestoptionamongtheavailablechoices.Onemarkisgiventoacorrectanswer.Zeromarkisforanemptyorincorrectanswer.1.Fadingofthereceivedradiosignalsinamobilecommunicationenvironmentoccursbecauseof...(a)Directpropagation(b)Multipathpropagation(c)Bi-pathpropapation(d)Noneoftheabove2.Inwhichofthefollowingisfrequencyspectrumdividedintosmallerspectraandallocatedtoeachuser?(a)TDMA(b)CDMA(c)FDMA(d)FGMA3.Anantennawhichattemptstodirectallitsenergyinaparticulardirectioniscalledasa/an(a)Directionalantenna(b)One-to-oneantenna(c)Propagationantenna(d)Single-directionantenna4.Whatisthedimensionofobjectascomparedtowavelengthofpropagatingwavewhenreflec-tionoccurs?(a)Small(b)Samepage2of13
Background image
ELEC3243,Dec.18,2023(c)Verysmall(d)Large5.Whichofthefollowingisnotachannelparameter?(a)Coherencetime(b)RMSdelayspread(c)Dopplerspread(d)Bandwidth6.Whichofthefollowingisanimportantfactorindeterminingspectralefficiencyofthesystem?(a)Vocoder(b)Modulationtechnique(c)Cellsize(d)Multipleaccesstechnique7.Fadingiscausedbyinterference.(a)True(b)False(c)Itdependsonthesituations.8.Whichofthefollowingisnotaneffectcausedbymultipathinradiochannel?(a)Rapidchangesinsignalstrength(b)Randomfrequencymodulation(c)Powerofbasestation(d)Timedispersion9.TDMAallocatesasingletimeperframetodifferentusers.(a)True(b)Falsepage3of13
Background image
ELEC3243,Dec.18,2023(c)Itdependsonthesituations.10.Whatisthetimedurationofabitifdataistransmittedat270.833kbpsinthechannel?(a)270.833s(b)3ps(c)3.692pus(d)3.692s11.Networklayeratsourceisresponsibleforcreatingapacketfromdatacomingfromanother(a)Station(b)Link(c)Node(d)Protocol12.TransmissionControlProtocol/InternetProtocol(TCP/IP)have...(a)Fourlayers(b)Fivelayers(c)Sixlayers(d)Sevenlayers13.InMIMO,whichfactorhasthegreatestinfluenceondatarates?(a)Thesizeofantenna(b)Theheightoftheantenna(c)Thenumberoftransmitantennas(d)Theareaofreceiveantennas14.Thetechniquethatmakespossiblethetaskoflisteningandtalkingincommunicationsystemiscalled...page4of13
Background image
ELEC3243,Dec.18,2023(a)Simplexing(b)Duplexing(c)Modulating(d)Multipleaccesstechnique15.Whyneighbouringstationsareassigneddifferentgroupofchannelsincellularsystem?(a)Tominimizeinterference(b)Tominimizearea(c)Tomaximizethroughput(d)Tomaximizecapacityofeachcell16.Whichofthefollowingisauniversallyadoptedshapeofcell?(a)Square(b)Circle(c)Triangle(d)Hexagon17.Capacityofacellularsystemisdirectlyproportionalto...(a)Numberofcells(b)Numberoftimesaclusterisreplicated(c)NumberofBasestations(d)Numberofusers18.Adjacentchannelinterferencecanbeminimizedthrough...(a)Changingfrequencyofbasestations(b)Carefulfilteringandchannelassignments(c)Increasingnumberofbasestations(d)Increasingnumberofcontrolchannelspage5of13
Background image
ELEC3243,Dec.18,202319.WhichlayerintheInternetstackhastheroleofdeliveringpacketsfromsourcenodetointermediatenodesandeventuallytodestinationnodewhentherearemultiplelinksbetweensourceanddestination?(a)Physical(b)Transport(c)Application(d)Hardware(e)Datalink(f)Network20.Guardbandis(a)Thebandwidthallottedtothesignal(b)Thechannelspectrum(c)Thesmallunusedbandwidthbetweenthefrequencychannelstoavoidinterference(d)Thespectrumacquiredbythenoisebetweenthesignal21.Thechannelcapacityis(a)Theamplitudeofthemodulatedsignal(b)Informationcontainedinasignal(c)Themaximuminformationtransmittedbyonesymboloverthechannel(d)Alloftheabove22.GiventhatH(P)isthebinaryentropyfunction,thecapacityofabinarysymmetricchannel18(a)H(P)*-1(b)H(P)-1(c)1-H(P?(d)1—H(P)page6of13
Background image
ELEC3243,Dec.18,202323.Coherencebandwidthis(a)Channelthatpassesallspectralcomponentswithequalgain(b)Thebandwidthofmodulatedsignal(c)Channelthatpassesallspectralcomponentswithlinearphase(d)Both(a)and(c)(e)Both(a)and(b)24.Whenoneofthepaths(typicallyaline-of-sightsignalorsomestrongreflectionsignals)ismuchstrongerthantheothers,whichfadingmodelismoreappropriate?(a)Riceanfading(b)Rayleighfading(c)Both(a)and(b)(d)Noneoftheabove25.QuadraturePSKusesthephases:45°,135°,225°,315°.Ifthedurationofeachsignalelement(i.e.signalatonephase)is200ns,thenthedatarateis:(a)1Mb/s(b)2.5Mb/s(c)5Mb/s(d)10Mb/s(e)25Mb/s(f)50Mb/s(g)100Mb/s26.OFDMisatechniqueof(i)Encodingdigitaldata(ii)Multiplecarrierfrequenciespage7of13
Background image
ELEC3243,Dec.18,2023(iii)Widebanddigitalcommunication(iv)4GmobilecommunicationWhichofthefollowingstatementiscorrect?(a)(ii)and(iii)arecorrect(b)(i),(ii),and(iii)arecorrect(c)(i),(ii),and(iv)arecorrect(d)Allfourarecorrect27.Themutualinformationis(a)Symmetric(b)Alwaysnon-negative(c)Both(a)and(b)arecorrect(d)Noneoftheabove28.AsshowninFig.1,Xia,YoshiandZoyaaremovingatspeedofvx,vyandvz,respectively.YoshibroadcastssignalstoXiaandZoya.TheemittedfrequencyatYoshiisfo.ForbothXiaandZoya,theobservedfrequencyishigherthanfp.Whichstatementistrue?Figure1:Doppler’seffect.(a)Ux<Uy<UZ(b)ux>vy>uz(c)ux<vy,vg<vy(d)Noneoftheabovepage8of13
Background image
ELEC3243, Dec.18,2023SectionII.Longquestions(72marks)Question1.(23marks)(a)(10marks)LetXandYrepresentrandomvariableswithassociatedprobabilitydistributionsp(x)andp(y),respectively.Theyarenotindependent.Theirconditionalprobabilitydistri-butionsarep(z|y)andp(y|z),andtheirjointprobabilitydistributionisp(x,y).Unlessstatedotherwise,expressyouranswersforthefollowingintermsofp(x),p(y),p(zly),p(y|z),p(x,y).(i)(2marks)WhatisthemarginalentropyH(X)ofvariableX,andwhatisthemutualinformationofXwithitself,i.e.,[(X;X)?(ii)(2marks)Intermsoftheprobabilitydistributions,whataretheconditionalentropiesA(X|Y)andA(Y|X)?(iii)(2marks)WhatisthejointentropyH(X,Y),andwhatwoulditbe iftherandomvariablesXandYwereindependent?(iv)(2marks)GiveanalternativeexpressionforH(Y)—H(Y|X)intermsofthejointentropyandbothmarginalentropies.(v)(2marks)WhatisthemutualinformationI(X;Y)?(b)(13marks)TheinputsourcetoanoisycommunciationchannelisarandomvariableXoverthefoursymbolsa,b,c,d.TheoutputfromthischannelisarandomvariableYoverthesesamefoursymbols.Thejointdistributionofthesetworandomvariablesisasfollows:R=x=bxX=¢X=dy=a}1/81/161/161/4y=b|1/161/81/16Oy=c|1/321/321/160=d|1/321/321/160(i)(2marks)WritedownthemarginaldistributionforXandcomputethemarginalentropyA(X)inbits.(ii)(2marks)WritedownthemarginaldistributionforYandcomputethemarginalentropyH(Y)inbits.page9of13
Background image
ELEC3243,Dec.18,2023(iii)(2marks)WhatisthejointentropyH(X,Y)ofthetworandomvariablesinbits?(iv)(2marks)WhatistheconditionalentropyH(Y|X)inbits?(v)(2marks)WhatisthemutualinformationI(X;Y)betweenthetworandomvariablesinbits?(vi)(3marks)ProvidealowerboundestimateofthechannelcapacityCforthischannelinbits.Explainyouranswers.Question2.(23marks)(a)(8marks)NyquistBitRatev.s.ShannonCapacity.(i)(4marks)Thebitrateofasystemincreaseswithanincreaseinthenumberofsignallevelsweusetodenoteasymbol.Asymbolcanconsistofnbits.Thenumberofsignallevelsis2n.Asthenumberoflevelsgoesup,thespacingbetweenleveldecreases,leadingtoincreasingtheprobabilityofanerroroccurringinthepresenceoftransmissionimpairments.Nyquistgivestheupperboundforthebitrateofatransmissionsystembycalculatingthebitratedirectlyfromthenumberofbitsinasymbol(orsignallevels)andthebandwidthofthesystem(assuming2symbols/percycleandfirstharmonic).Nyquisttheoremstatesthatforanoiselesschannel:(NyquistBitRate)C=2Blog,(2n)whereCisthecapacityinbpsandBisthechannelbandwidthinHz.Supposewewanttotransmit500kbpsoveranoiselesschannelwitha30KHzbandwidth.Howmanysignallevelsdowerequire?(4marks)Wecanneverhavealosslesschannel.Shannonproposedthatthemaximumbitrate(i.e.,channelcapacity)dependsonthesignal-to-noiseratio:(Shannonchannelcapacity)C'=Blogo(1+S/N).Shannon’slawdoesnotmentionsignallevels.Thismeansthatnomatterwhatsignallevelsyouhave,youcannotachieveadataratehigherthanthechannelcapacity.Whatpage10of13
Background image
ELEC3243,Dec.18,2023(b)isthechannelcapacityofafaintsignalthatiscompletelyswampedbynoise?[Hint:YoushouldapplyShannon’sequationtoanswerthequestion.|(15marks)Supposewehaveawirelessnetworkwithnhops,asshowninFig.2.For1=0,1,...,n—1,node72transmitscodedmessagesatrateR;tonodei+1overanadditivewhiteGaussiannoise(AWGN)channelwithnoisevarianceN;.Assumeeachnodetransmitsinanorthogonalchannel.Nodezdecodesmessagesfromthetransmissionofnodei—1and forwardstonodei+1.Notethatthenodesmayormaynotusedifferentcodingstrategies.Node2transmitsatpowerP;andthemultihopsystemissubjecttotheconstraint—1NodeA.Y,|Nodex,Y,NodAyeoJiNodeXe:了Node012n-fnFigure2:n-hopwirelessnetwork.(i)(4marks)ExplainthecharacteristicsofanAWGNchannel.(ii)(2marks)IntermsofP;andN;, whatisthecapacityC;ofthechannelifromnodeitonodei+1?(iii)(4marks)ForagivensetofpowersPo,...,P,,whatisthecapacityofC'ofthemultihopcommunicationsystemfromnode0ton?ExpressyouranswerintermsofC;.Explainyouranswerintermsoftheend-to-enddatarateR.(iv)(5marks)WhatistheoptimalpowerallocationPo,...,Pp—1?WhatisthecorrespondingchannelcapacityC”?page11 of13
Background image
ELEC32438,Dec.18,2023Question3.(26marks)(a)(14marks)Consideratime-invariantMIMOchannelwith2transmit,2receiveantennasandchannelmatrixHisgivenby82H=25AsshowninFig.3,theMIMOchannelcanbeconvertedintoaparallelchannelthroughproperpre-processingandpost-processing.1:x一|P上一pl|olAxNyQ;Pre-processing}Channel:Post-processingFigure3:MIMOchannel.(i)(4marks)WhatarethevaluesofA;andAg?(i.e.,thesingularvaluesofchannelmatrix)ii)(6marks)FindthematricesPandQ.(iii)(4marks)SupposethetransmissionpowerforthetwoparallelchannelsarePy=2WattandP,=3Watt.ThepowerofadditivewhiteGaussiannoiseisNgo=1Watt.WhatistheMIMOchannelcapacity[inbits/s/Hz]?(b)(12marks)ConsiderthedownlinktransmissionfromtheMBStoZoyaasshowninFig.4.Assumethewirelessbackhaulhasahuge(infinite)capacity.Inthefirststep,theMBSbroadcastsitssignalm,andthesignalsreceivedbytheSBSandZoyaare,respectively:Sri =hu,smi+ng,Sri=hu,zm4+7z1.Assume|hy,s|>>|hu,z|.Afterthefirststep,SBShasperfectlydecodedthesymbolmj.Inthesecondstep,SBSsendsm,toZoyaandshereceives:z,2=hgzmi+nze2.ThetransmissionpowersoftheMBSandSBSarePyandPg,respectively,andthenoisepoweratZoya’sreceiverisNz.Assume:lhuz|?zialhs.z?=1,Py=15Nz,Ps=6Nz.page12of13
Background image
ELEC3243,Dec.18,2023(i)(2marks)Whatisbackhaul?(ii)(5marks)Findthehighestachievablerate[perchanneluse]betweentheMBSandZoya.(iii)(5marks)IfweremovetheSBS,whatisthehighestachievablerate[perchanneluse|betweentheMBSandZoya?MBSFigure 4:CooperativedownlinktransmissiontowardsZoyawithwirelessbackhaulbetweentheMBSandSBS.page13of13*ENDOFPAPER***
Background image