期末通信原理考题A题解.docx
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期末通信原理考题A题解.docx
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期末通信原理考题A题解
学院_______________________系别____________班次_____________学号__________姓名________________
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电子科技大学2005至2006学年第1学期
<<通信原理>>课程考试题(120分钟)考试日期2006年1月10日
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评卷教师
1.Ananalogsignalwiththespectrum
istobetransmittedoveraPCMsystem.Assumethat
andthatthereisnoISI.Thepeaksignal-to-noiseratioatthereceiveroutputneedstobeatleast50dB.
(a)findthemaximumallowabletimeintervalbetweensamplevalues
.
(b)Whatwillbethenumberofquantizingstepsneeded?
(c)DeterminetheminimumbitraterequiredinthePCMsignal.
(d)WhatisthefirstnullbandwidthofthePCMsignalforthepolarNRZsignalingcase?
Solution
a.fs=2B=2*2000=4000Hz,Ts=1/fs=1/4000=0.25ms
b.50=6.02n+4.77,n=7.5,n=8,M=2n=256
c.R=nfs=8*4000=32000bit/s=32Kbit/s
d.Bnull=R=32KHz
2.AbinarylinecodethathasarectangularpulseshapeissentoverachannelwithoutISI.Assumetheimpulseresponseoftheoverallsystemis
where
.
(a)FindthezeropointsinthesystemimpulseresponseandtheminimumsymbolperiodTswithoutintroducingISI.
(b)Findthemaximumbitratethatcanbesupportedbythissystem.
(c)FindtheabsolutebandwidthBforthistransmissionsystem.
Solution:
(a)
(b)
(c)
3.AssumethatanAMtransmitterismodulatedwithavideotestingsignalgivenby
wheref1=4MHz.LetAc=100.
(a)SketchtheAMwaveform.
(b)Whatarethepercentagesofpositiveandnegativemodulation?
(c)Evaluatethetotalaveragepower.
(d)Evaluatethepeakenvelopepower.
Solution:
(a)m(t)=0.3+0.5sin1tfm=f1=4MHz
(b)
(c)
(d)
4.Considerarandomdatapatternconsistingofbinary1’sand0’s,wheretheprobabilityofobtainingeitherabinary1or0is1/2andthedurationofonesymbolis
.CalculatethePSDforpolarRZsignalingassumingpeakvaluesof
.
Solution:
5.Assumethat
isaknownsignal,where
.Forthecaseofwhitenoise,
(a)Findtheimpulseresponseofthematchedfilter.
(b)Findtheoptimalsamplingtimet0.
(c)Findtheoutputofthematchedfilterwhens(t)issent.
(d)Plottheoutputofthematchedfilter.
Solution:
(a)Theimpulseresponseofthematchedfilteris
b.Theminimumtimethatwehavetowaitbeforethemaximumsignalleveloccursatthefilteroutputis
Andt0istheoptimalsamplingtime.
c.Theoutputofthematchedfilteris
e.Theoutputofthematchedfilterisshownasinthefollowingfigure.
6.Forabinarybasebandsystem,thereceivedsignalis
AssumethechannelnoiseisWhiteGaussianNoisewiththePSDof
.Theblockdiagramofreceiverisasfollow
LetLPFhasanequivalentbandwidthofB=4Randaunitygain.
(a)EvaluatetheoptimumthresholdVT.
(b)Evaluatetheerrorbitprobabilityforthedataatthereceiveroutput.(PecanberepresentedbyQ[.])
Solution:
7.AssumethattheinputsignalpowerofLSSBreceiveris2W,asshowninthefollowingFig.,andcarrierfrequencyis100MHz.Ifthebandwidthofmodulatingsignalm(t)is4MHz,andthePSDofnoiseforchannelis
andcoherentdemodulationisused,pleasedetermine:
(a)thetransferfunctionoftheidealIFfilter;
(b)theinputratioofsignalpowerandnoisepower
ofthedetector;
(c)theouputratioofsignalpowerandnoisepower
ofthedetector.
Solution:
(a)inordertoassurethesignaltopasstheIFfilter,thebandwidthofIFfilterisequaltothatofthemodulatedsignal,Thatistosay,BIF=Bm=4MHz.thus
(b)thepowerofinputsignalbeing
andthepowerofinputnoisebeing
thustheinputratioofsignalpowerandnoisepower
(c)becausetheG=1forSSB,so
8.Abinarybasebandsignalispassedthroughamultilevelmodulatedbandpasssystem.Thewaveshapeofthebinarybasebandsignalisrectangular,andthedatarateis50Mb/s.Evaluate
(a)AssumethataQPSKsignalisusedtosendthedata.WhatistheabsolutebandwidthoftheQPSKsignal?
WhatisthenulltonullbandwidthoftheQPSKsignal?
IsthereAMcomponentontheQPSKsignal?
(b)Assumethebasebandsignalispassthrougharaisedcosine-rollofffilterwitha50%rollofffactorfirst,andthenbetransmittedusingQPSKsignal.WhatistheabsolutebandwidthoftheQPSKsignal?
WhatisthetransmissionbandwidthoftheQPSKsignal?
IsthereAMcomponentontheQPSKsignal?
(c)Assumetheavailablechannelbandwidthis20MHz,andtherollofffactoris50%.Whatistheminimummultilevelforthemultilevelmodulatedsignal?
Solution
1)TheabsolutebandwidthoftheQPSKsignalis∞.
ForQPSKsignal,
ThenulltonullbandwidthoftheQPSKsignalis
Thereisn’tAMcomponentontheQPSKsignal.
2)
TheabsolutebandwidthandthetransmissionbandwidthoftheQPSKsignalaresame,37.5MHz.
ThereisAMcomponentontheQPSKsignal.
3)
TheminimummultilevelisL=24=16
9.Assumethattheinputdatastreamis{+1,-1,+1,+1,-1,-1,+1,-1,+1,+1,-1,+1,-1,+1,+1},Ac=1.
(a)PlottheMSKtypeIImodulationwaveformsx(t)andy(t).
(b)LettheperiodofcarrierTc=Tb/2,plottheBPSKsignals(t).
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