signal processing techniques for software radio-chp6

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1、Chapter6AnOverviewofTransceiverSystemsThischapterplaystheroleofanintroductorychaptertotherestofthisbook.Thekeyideahereistopointouttheproblemsthatonemayencounterwhenapairoftransmitterandreceiverareimplementedandseparatedbyanon-ideal(i.e.,areal)channel.Samplesimulationprogramsth

2、athelpuswithdemonstrationoftheimpactofachannelandimplementationtolerances(e.g.,acarrieroffsetbetweentransmitterandreceiver)aredeveloped.Variousmethodsofcombatingthechanneldistortionsandimplementationtolerancesarethesubjectofthesubsequentchapters.Theprogramsdevelopedinthischapte

3、rwillbeextendedandusedextensivelyintherestofthebooktodemonstratetheperformanceofthedevelopedmethods.6.1BuildingBlocksFigure6.1presentsablockdiagramofacompletetransceiver(transmitter+chan-nel+receiver)digitalcommunicationsystem.Theinformationbits,b[n],arefirstcodedbyaddingredund

4、antbitsthatprotect(toacertainextent)theinformationagainstpossibleerrorsduetochannelnoise/distortion.Thecodedbitsarethengroupedandconvertedto(possiblycomplex)datasymbols,s[n].Next,thedatasymbolsarepulse-shaped,throughtransmitfilterpT(t),andmodulateacarriertogenerateanRF(radiofre

5、quency)signalfortransmissionthroughapassbandchan-nel.Thechannelmayintroducesomedistortiontothesignalandaddsomenoisetoit.Interferencefromotherusersmayalsoexist.Theseareusuallyoverthebandsthatarenon-overlappingwiththedesiredsignalbandandthuscouldberemovedthroughlinearfiltering.At

6、thereceiver,thesignalpickedupbytheantennafirstpassesthroughalownoiseamplifier(LNA)andamplifiedtoalevelappropriatefortherestofthecircuit.Thesignallevelatthisstageiscontrolledbyanautomaticgaincontrol(AGC)mechanism.Next,thedemodulatorshiftsthedesiredsignalto151152QuadratureAmplitude

7、ModulatedTransceiverSystemsChap.6thebaseband.Thereceive/matchedfilterpR(t)(usuallychosentobematchedtopT(t))removesanyout-of-bandsignals,includingtheinterferencefromotherusersandchannelnoise.Thematchedfilteroutputisthensampledandpassedthroughanequalizertocompensateforthedistortio

8、ncausedbythechannel.Theoutputoftheequalizerisinputtoaslicer/d