Communications Receivers: DSP, Software Radios, and Design PDF
Preview Communications Receivers: DSP, Software Radios, and Design
Source: Communications Receivers: DSP, Software Radios, and Design Chapter 1 Basic Radio Considerations 1.1 Radio Communications Systems The capability of radio waves to provide almost instantaneous distant communications withoutinterconnectingwireswasamajorfactorintheexplosivegrowthofcommunica- tionsduringthe20thcentury.Withthedawnofthe21stcentury,thefutureforcommuni- cationssystemsseemslimitless.Theinventionofthevacuumtubemaderadioapractical andaffordablecommunicationsmedium.Thereplacementofvacuumtubesbytransistors andintegratedcircuitsallowedthedevelopmentofawealthofcomplexcommunications systems, which have become an integral part of our society. The development of digital signalprocessing(DSP)hasaddedanewdimensiontocommunications,enablingsophis- ticated, secure radio systems at affordable prices. Inthisbook,wereviewtheprinciplesanddesignofmodernsingle-channelradioreceiv- ersforfrequenciesbelowapproximately3GHz.Whileitispossibletodesignareceiverto meetspecifiedrequirementswithoutknowingthesysteminwhichitistobeused,suchig- norancecanprovetime-consumingandcostlywhentheinevitableneedfordesigncompro- misesarises.Westronglyurgethatthereceiverdesignertakethetimetounderstandthor- oughlythesystemandtheoperationalenvironmentinwhichthereceiveristobeused.Here wecanoutlineonlyafewofthewidevarietyofsystemsandenvironmentsinwhichradiore- ceivers may be used. Figure 1.1 is a simplified block diagram of a communications system that allows the transferofinformationbetweenasourcewhereinformationisgeneratedandadestination thatrequiresit.Inthesystemswithwhichweareconcerned,thetransmissionmediumisra- dio,whichisusedwhenalternativemedia,suchaslightorelectricalcable,arenottechni- callyfeasibleorareuneconomical.Figure1.1representsthesimplestkindofcommunica- tionssystem,whereasinglesourcetransmitstoasingledestination.Suchasystemisoften referredtoasasimplexsystem.Whentwosuchlinksareused,thesecondsendinginforma- tionfromthedestinationlocationtothesourcelocation,thesystemisreferredtoasduplex. Suchasystemmaybeusedfortwo-waycommunicationor,insomecases,simplytoprovide informationonthequalityofreceivedinformationtothesource.Ifonlyonetransmittermay transmit at a time, the system is said to behalf-duplex. Figure1.2isadiagramrepresentingthesimplexandduplexcircuits,whereasingleblock TrepresentsalloftheinformationfunctionsatthesourceendofthelinkandasingleblockR representsthoseatthedestinationendofthelink.Inthissimplediagram,weencounterone oftheproblemswhichariseincommunicationssystems—adefinitionoftheboundariesbe- tweenpartsofthesystem.TheblocksTandR,whichmightbethoughtofastransmitterand receiver, incorporate several functions that were portrayed separately in Figure 1.1. 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 2Communications Receivers Figure 1.1Simplified block diagram of a communications link. Figure1.2Simplifiedportrayalofcommuni- cationslinks:(a)simplexlink,(b)duplexlink. Manyradiocommunicationssystemsaremuchmorecomplexthanthesimplexanddu- plexlinksshowninFigures1.1and1.2.Forexample,abroadcastsystemhasastarconfigu- rationinwhichonetransmittersendstomanyreceivers.Adata-collectionnetworkmaybe organizedintoastarwherethereareonereceiverandmanytransmitters.Theseconfigura- tionsareindicatedinFigure1.3.Aconsequenceofastarsystemisthattheperipheralele- ments,insofarastechnicallyfeasible,aremadeassimpleaspossible,andanynecessary complexity is concentrated in the central element. Examplesofthetransmitter-centeredstararethefamiliaramplitude-modulated(AM), frequency-modulated(FM),andtelevisionbroadcastsystems.Inthesesystems,high-power transmitters with large antenna configurations are employed at the transmitter, whereas mostreceiversusesimpleantennasandarethemselvesrelativelysimple.Anexampleofthe receiver-centeredstarisaweather-data-collectionnetwork,withmanyunattendedmeasur- ingstationsthatsenddataatregularintervalstoacentralreceivingsite.Starnetworkscanbe configuredusingduplexratherthansimplexlinks,ifthisprovesdesirable.Mobileradionet- workshavebeenconfiguredlargelyinthismanner,withtheshorter-rangemobilesetstrans- mittingtoacentralradiorelaylocatedforwidecoverage.Cellularradiosystemsincorporate anumberoflow-powerrelaystationsthatprovidecontiguouscoverageoveralargearea, communicating with low-power mobile units. The relays are interconnected by various meanstoacentralswitch.Thissystemusesfarlessspectrumthanconventionalmobilesys- tems because of the capability for reuse of frequencies in noncontiguous cells. Packetradiotransmissionisanotherexampleofaduplexstarnetwork.Stationstransmit atrandomtimestoacentralcomputerterminalandreceiveresponsessentfromthecom- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations Basic Radio Considerations3 Figure1.3Star-typecommunicationsnetworks:(a)broadcastsystem,(b)data-collection network. puter.Thecommunicationsconsistofbriefburstsofdata,sentasynchronouslyandcontain- ingthenecessaryaddressinformationtobeproperlydirected.Thetermpacketnetworkis appliedtothisschemeandrelatedschemesusingsimilarprotocols.Apacketsystemtypi- callyincorporatesmanyradios,whichcanserveeitherasterminalsorasrelays,andusesa flooding-typetransmission scheme. Themostcomplexsystemconfigurationoccurswhentherearemanystations,eachhav- ingbothatransmitterandreceiver,andwhereanystationcantransmittooneormoreother stationssimultaneously.Insomenetworks,onlyonestationtransmitsatatime.Onemaybe designatedasanetworkcontrollertomaintainacallingdiscipline.Inothercases,itisneces- sarytodesignasystemwheremorethanonestationcantransmitsimultaneouslytooneor more other stations. Inmanyradiocommunicationssystems,therangeoftransmissions,becauseofterrainor technologyrestrictions,isnotadequatetobridgethegapbetweenpotentialstations.Insuch acase,radiorepeaterscanbeusedtoextendtherange.Therepeatercomprisesareceiving systemconnectedtoatransmittingsystem,sothataseriesofradiolinksmaybeestablished toachievetherequiredrange.Primeexamplesarethemultichannelradiorelaysystemused bylong-distancetelephonecompaniesandthesatellitemultichannelrelaysystemsthatare usedextensivelytodistributevoice,video,anddatasignalsoverawidegeographicarea. Satelliterelaysystemsareessentialwherephysicalfeaturesoftheearth(oceans,highmoun- tains, and other physical restrictions) preclude direct surface relay. Onalink-for-linkbasis,radiorelaysystemstendtorequireamuchhigherinvestment thandirect(wired)links,dependingontheterrainbeingcoveredandthedistancesinvolved. Tomakethemeconomicallysound,itiscommonpracticeinthetelecommunicationsindus- trytomultiplexmanysinglecommunicationsontooneradiorelaylink.Typically,hundreds ofchannelsaresentoveronelink.Theradiolinksconnectbetweencentralofficesinlarge populationcentersandgatherthevarioususerstogetherthroughswitchingsystems.The hundredsoftrunksdestinedforaparticularremotecentralofficearemultiplexedtogether into one wider-bandwidth channel and provided as input to the radio transmitter. At the othercentraloffice,thewide-bandchannelisdemultiplexedintotheindividualchannels anddistributedappropriatelybytheswitchingsystem.Telephoneanddatacommoncarriers areprobablythelargestusersofsuchduplexradiotransmission.TheblockdiagramofFig- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 4Communications Receivers Figure1.4Blockdiagramofsimplifiedradiorelayfunctions:(a)terminaltransmitter,(b)re- peater (without drop or insert capabilities), (c) terminal receiver. ure1.4showsthefunctionsthatmustbeperformedinaradiorelaysystem.Atthereceiving terminal,theradiosignalisinterceptedbyanantenna,amplifiedandchangedinfrequency, demodulated, and demultiplexed so that it can be distributed to the individual users. Inadditiontothesimplecommunicationsuseofradioreceiversoutlinedhere,thereare manyspecial-purposesystemsthatalsorequireradioreceivers.Whiletheprinciplesofde- signareessentiallythesame,suchreceivershavepeculiaritiesthathaveledtotheirownde- signspecialties.Forexample,inreceiversusedfordirectionfinding,theantennasystems havespecifieddirectionalpatterns.Thereceiversmustacceptoneormoreinputsandpro- cessthemsothattheoutputsignalcanindicatethedirectionfromwhichthesignalarrived. Older techniques include the use of loop antennas, crossed loops, Adcock antennas, and otherspecializeddesigns,anddeterminethedirectionfromapatternnull.Moremodern Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations Basic Radio Considerations5 systemsusecomplexantennas,suchastheWullenweber.Othersdeterminebothdirection andrangefromthedelaydifferencesfoundbycross-correlatingsignalsfromdifferentan- tenna structures or elements. Radio ranging can be accomplished using radio receivers with either cooperative or noncooperativetargets.Cooperativetargetsusearadiorelaywithknowndelaytoreturna signaltothetransmittinglocation,whichisalsousedforthereceiver.Measurementofthe round-trip delay (less the calibrated internal system delays) permits the range to be esti- matedveryclosely.Noncooperativerangingreceiversarefoundinradarapplications.In this case, reflections from high-power transmissions are used to determine delays. The strength of the return signal depends on a number of factors, including the transmission wavelength,targetsize,andtargetreflectivity.Byusingnarrowbeamantennasandscanning theazimuthandelevationangles,radarsystemsarealsocapableofdeterminingtargetdirec- tion.Radarreceivershavethesamebasicprinciplesascommunicationsreceivers,butthey also have special requirements, depending upon the particular radar design. Anotherareaofspecializedapplicationisthatoftelemetryandcontrolsystems.Exam- plesofsuchsystemsarefoundinalmostallspacevehicles.Thetelemetrychannelsreturnto earthdataontemperatures,equipmentconditions,fuelstatus,andotherimportantparame- ters,whilethecontrolchannelsallowremoteoperationofequipmentmodesandvehicleat- titude,andthefiringofrocketengines.Theprincipaldifferencebetweenthesesystemsand conventional communications systems lies in the multiplexing and demultiplexing of a largenumberofanaloganddigitaldatasignalsfortransmissionoverasingleradiochannel. Electroniccountermeasure(ECM)systems,usedprimarilyformilitarypurposes,give risetospecialreceiverdesigns,bothinthesystemsthemselvesandintheirtargetcommuni- cationssystems.Theobjectivesofcountermeasurereceiversaretodetectactivityofthetar- gettransmitters,toidentifythemfromtheirelectromagneticsignatures,tolocatetheirposi- tions, and in some cases to demodulate their signals. Such receivers must have high detectionalsensitivityandtheabilitytodemodulateawidevarietyofsignaltypes.More- over,spectrumanalysiscapabilityandotheranalysistechniquesarerequiredforsignature determination.Eitherthesamereceiversorseparatereceiverscanbeusedfortheradio-loca- tion function. To counter such actions, the communications circuit may use minimum power,directitspowertowarditsreceiverinasnarrowabeamaspossible,andspreadits spectruminamannersuchthattheinterceptreceivercannotdespreadit,thusdecreasingthe signal-to-noiseratio(SNR,alsoreferredtoasS/N)torenderdetectionmoredifficult.This technique is referred to aslow probability of intercept(LPI). SomeECMsystemsaredesignedprimarilyforinterceptionandanalysis.Inothercases, however,thepurposeistojamselectedcommunicationsreceiverssoastodisruptcommuni- cations.Tothisend,oncethetransmissionofatargetsystemhasbeendetected,theECM systemtransmitsastrongsignalonthesamefrequency,witharandomlycontrolledmodula- tionthatproducesaspectrumsimilartothecommunicationssequence.Anotheralternative istotransmita“spoofing”signalthatissimilartothecommunicationssignalbutcontains falseorout-of-dateinformation.Theelectroniccountercountermeasure(ECCM)against spoofing is good cryptographic security. The countermeasures against jamming are high-powered, narrow-beam, or adaptive-nulling receiver antenna systems, and a spread-spectrumsystemwithsecurecontrolsothatthejammingtransmittercannotemulate it.Inthiscase,thecommunicationsreceivermustbedesignedtocorrelatethereceivedsig- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 6Communications Receivers nalusingthesecurespread-spectrumcontrol.Thus,thejammerpowerisspreadoverthe transmissionbandwidth,whilethecommunicationpowerisrestoredtotheoriginalsignal bandwidth before spreading. This provides an improvement in signal-to-jamming ratio equal to the spreading multiple, which is referred to as theprocessing gain. Specialreceiversarealsodesignedfortestingradiocommunicationssystems.Ingeneral, theyfollowthedesignprinciplesofthecommunicationsreceivers,buttheirdesignmustbe of even higher quality and accuracy because their purpose is to measure various perfor- manceaspectsofthesystemundertest.Atestreceiverincludesabuilt-inself-calibration feature.Thetestreceivertypicallyhasa0.1dBfieldstrengthmeteraccuracy.Inadditionto normalaudiodetectioncapabilities,ithaspeak,average,andspecialweightingfiltersthat areusedforspecificmeasurements.Carefullycontrolledbandwidthsareprovidedtocon- formwithstandardizedmeasurementprocedures.Thetestreceiveralsomaybedesignedfor usewithspecialantennasformeasuringtheelectromagneticfieldstrengthfromthesystem undertestataparticularlocation,andincludeorprovidesignalsforusebyanattachedspec- trumanalyzer.Whiletestreceiversarenottreatedseparatelyinthisbook,manyofourde- sign examples are taken from test receiver design. Fromthisbriefdiscussionofcommunicationssystems,wehopethatthereaderwillgain someinsightintothescopeofreceiverdesign,andthedifficultyofisolatingthetreatmentof thereceiverdesignfromthesystem.Therearealsodifficultiesinsettinghardboundariesto thereceiverwithinagivencommunicationssystem.Forthepurposesofourbook,wehave decidedtotreatasthereceiverthatportionofthesystemthatacceptsinputfromtheantenna andproducesademodulatedoutputforfurtherprocessingatthedestinationorpossiblybya demultiplexer.Weconsidermodulationanddemodulationtobeapartofthereceiver,butwe recognizethatfordatasystemsespeciallythereisanever-increasingvolumeofmodems (modulator-demodulators)thataredesignedandpackagedseparatelyfromthereceiver.For convenience,Figure1.5showsablockdiagramofthereceiveraswehavechosentotreatitin thisbook.Itshouldbenotedthatsignalprocessingmaybeaccomplishedbothbeforeandaf- ter modulation. 1.1.1 Radio Transmission and Noise LightandXrays,likeradiowaves,areelectromagneticwavesthatmaybeattenuated,re- flected,refracted,scattered,anddiffractedbythechangesinthemediathroughwhichthey propagate.Infreespace,thewaveshaveelectricandmagneticfieldcomponentsthatare mutually perpendicular and lie in a plane transverse to the direction of propagation. In commonwithotherelectromagneticwaves,theytravelwithavelocitycof299,793km/s, avaluethatisconvenientlyroundedto300,000km/sformostcalculations.Inrationalized meter,kilogram,andsecond(MKS)units,thepowerflowacrossasurfaceisexpressedin watts per square meter and is the product of the electric-field (volts per meter) and the magnetic-field (amperes per meter) strengths at the point over the surface of measure- ment. Aradiowavepropagatessphericallyfromitssource,sothatthetotalradiatedpoweris distributedoverthesurfaceofaspherewithradiusR(meters)equaltothedistancebetween thetransmitterandthepointofmeasurement.ThepowerdensityS(wattspersquaremeter) at the point for a transmitted powerP (watts) is t Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations Basic Radio Considerations7 Figure1.5Blockdiagramofacommunicationsreceiver.(RF=radiofrequency,IF=interme- diate frequency, and BB= baseband.) G ×P S = t t (1.1) 4π×R2 whereG isthetransmittingantennagaininthedirectionofthemeasurementoverauni- t formdistributionofpowerovertheentiresphericalsurface.Thus,thegainofahypotheti- calisotropic antenna is unity. Thepowerinterceptedbythereceiverantennaisequaltothepowerdensitymultipliedby theeffectiveareaoftheantenna.Antennatheoryshowsthatthisareaisrelatedtotheantenna gain in the direction of the received signal by the expression G λ2 Ae = r (1.2) r 4π When Equations (1.1) and (1.2) are multiplied to obtain the received power, the result is P G G λ2 r = r t (1.3) P 16π2 R2 t ThisisusuallygivenasalossL(indecibels),andthewavelengthλisgenerallyreplacedby velocitydividedbyfrequency.Whenthefrequencyismeasuredinmegahertz,therangein kilometers, and the gains in decibels, the loss becomes L=[32.4+20logR+20log F]–G –G ≡A –G –G (1.4) t r fs t R Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 8Communications Receivers A isreferredtoasthelossinfreespacebetweenisotropicantennas.Sometimesthelossis fs givenbetweenhalf-wavedipoleantennas.Thegainofsuchadipoleis2.15dBaboveiso- tropic, so the constant in Equation (1.4) must be increased to 36.7 to obtain the loss be- tween dipoles. Becauseoftheearthanditsatmosphere,mostterrestrialcommunicationslinkscannotbe considered free-space links. Additional losses occur in transmission. Moreover, the re- ceivedsignalfieldisaccompaniedbyaninevitablenoisefieldgeneratedintheatmosphere orspace,orbymachinery.Inaddition,thereceiveritselfisasourceofnoise.Electricalnoise limits the performance of radio communications by requiring a signal field sufficiently great to overcome its effects. Whilethecharacteristicsoftransmissionandnoiseareofgeneralinterestinreceiverde- sign,itisfarmoreimportanttoconsiderhowthesecharacteristicsaffectthedesign.Thefol- lowingsectionssummarizethenatureofnoiseandtransmissioneffectsinfrequencybands through SHF (30 GHz). ELF and VLF (up to 30 kHz) Transmissionintheextremely-lowfrequency(ELF)andvery-lowfrequency(VLF)range isprimarilyviasurfacewavewithsomeofthehigher-orderwaveguidemodesintroduced by the ionosphere appearing at the shorter ranges. Because transmission in these fre- quencybandsisintendedforlongdistances,thehigher-ordermodesarenormallyunim- portant.Thesefrequenciesalsoprovidetheonlyradiocommunicationsthatcanpenetrate theoceanssubstantially.Becausethetransmissioninsaltwaterhasanattenuationthatin- creases rapidly with increasing frequency, it may be necessary to design depth-sensitive equalizersforreceiversintendedforthisservice.Atlongranges,thefieldstrengthofthe signalsisverystable,varyingonlyafewdecibelsdiurnallyandseasonally,andbeingmin- imallyaffectedbychangesinsolaractivity.Thereismorevariationatshorterranges.Vari- ation of the phase of the signal can be substantial during diurnal changes and especially during solar flares and magnetic storms. For most communications designs, these phase changesareoflittleimportance.Thenoiseattheselowfrequenciesisveryhighandhighly impulsive.Thissituationhasgivenrisetothedesignofmanynoise-limitingornoise-can- celingschemes,whichfindparticularuseinthesereceivers.Transmittingantennasmust beverylargetoproduceonlymoderateefficiency;however,thenoiselimitationspermit theuseofrelativelyshortreceivingantennasbecausereceivernoiseisnegligibleincom- parisonwithatmosphericnoiseattheearth’ssurface.Inthecaseofsubmarinereception, thehighattenuationofthesurfacefields,bothsignalandnoise,requiresthatmoreatten- tion be given to receiving antenna efficiency and receiver sensitivity. LF (30 to 300 kHz) and MF (300 kHz to 3 MHz) Atthelowerendofthelow-frequency(LF)region,transmissioncharacteristicsresemble VLF.Asthefrequencyrises,thesurfacewaveattenuationincreases,andeventhoughthe noisedecreases,theusefulrangeofthesurfacewaveisreduced.Duringthedaytime,iono- sphericmodesareattenuatedintheDlayeroftheionosphere.Thewaveguidemoderepre- sentationofthewavescanbereplacedbyareflectionrepresentation.Asthemedium-fre- quency(MF)regionisapproached,thedaytimeskywavereflectionsaretooweaktouse. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations Basic Radio Considerations9 Thesurfacewaveattenuationlimitsthedaytimerangetoafewhundredkilometersatthe lowendoftheMFbandtoabout100kmatthehighend.Throughoutthisregion,therange is limited by atmospheric noise. As the frequency increases, the noise decreases and is minimumduringdaylighthours.Thereceivernoisefigure(NF)makeslittlecontribution to overall noise unless the antenna and antenna coupling system are very inefficient. At night,theattenuationoftheskywavedecreases,andreceptioncanbeachieveduptothou- sandsofkilometers.Forrangesofonehundredtoseveralhundredkilometers,wherethe single-hopskywavehascomparablestrengthtothesurfacewave,fadingoccurs.Thisphe- nomenoncanbecomequitedeepduringthoseperiodswhenthetwowavesarenearlyequal in strength. AtMF,theskywavefadesasaresultofFaradayrotationandthelinearpolarizationofan- tennas.Atsomeranges,additionalfadingoccursbecauseofinterferencebetweenthesur- facewaveandskywaveorbetweenskywaveswithdifferentnumbersofreflections.When fadingiscausedbytwo(ormore)wavesthatinterfereasaresultofhavingtraveledover pathsofdifferentlengths,variousfrequencieswithinthetransmittedspectrumofasignal canbeattenuateddifferently.Thisphenomenonisknownasselectivefadingandresultsin severedistortionofthesignal.BecausemuchoftheMFbandisusedforAMbroadcast, therehasnotbeenmuchconcernaboutreceiverdesignsthatwilloffsettheeffectsofselec- tivefading.However,asthefrequencynearsthehigh-frequency(HF)band,theapplications becomeprimarilylong-distancecommunications,andthisreceiverdesignrequirementis encountered.SomebroadcastingoccursintheLFband,andintheLFandlowerMFbands medium-rangenarrow-bandcommunicationsandradionavigationapplicationsarepreva- lent. HF (3 to 30 MHz) Untiltheadventofsatellite-borneradiorelays,theHFbandprovidedtheonlyradiosig- nalscapableofcarryingvoicebandorwidersignalsoververylongranges(upto10,000 km). VLF transmissions, because of their low frequencies, have been confined to nar- row-banddatatransmission.Thehighattenuationofthesurfacewave,thedistortionfrom sky-wave-reflectednear-verticalincidence(NVI),andtheprevalenceoflong-rangeinter- fering signals make HF transmissions generally unsuitable for short-range communica- tions.Fromthe1930sintotheearly1970s,HFradiowasamajormediumforlong-range voice,data,andphotocommunications,aswellasforoverseasbroadcastservices,aero- nautical,maritimeandsomegroundmobilecommunications,andradionavigation.Even today, the band remains active, and long-distance interference is one of the major prob- lems.Becauseofthedependenceonskywaves,HFsignalsaresubjecttobothbroad-band andselectivefading.Thefrequenciescapableofcarryingthedesiredtransmissionaresub- jecttoallofthediurnal,seasonal,andsunspotcycles,andtherandomvariationsofioniza- tion in the upper ionosphere. Sunspot cycles change every 11 years, and so propagation tendstochangeaswell.Significantdifferencesaretypicallyexperiencedbetweendayand nightcoveragepatterns,andbetweensummertowintercoverage.Outtoabout4000km, E-layer transmission is not unusual, but most of the very long transmission—and some downtoafewthousandkilometers—iscarriedbyF-layerreflections.Itisnotuncommon toreceiveseveralsignalsofcomparablestrengthcarriedoverdifferentpaths.Thus,fading Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Basic Radio Considerations 10Communications Receivers istherule,andselectivefadingiscommon.Atmosphericnoiseisstillhighattimesatthe low end of the band, although it becomes negligible above about 20 MHz. Receiversmustbedesignedforhighsensitivity,andimpulsenoisereducingtechniques mustoftenbeincluded.Becausetheoperatingfrequencymustbechangedonaregularbasis toobtainevenmoderatetransmissionavailability,mostHFreceiversrequirecoverageofthe entirebandandusuallyoftheupperpartoftheMFband.Formanyapplications,designs mustbemadetocombatfading.Thesimplestoftheseisautomaticgaincontrol(AGC), which also is generally used in lower-frequency designs. Diversity reception is often re- quired,wheresignalsarereceivedoverseveralroutesthatfadeindependently—usingsepa- ratedantennas,frequencies,andtimes,orantennaswithdifferentpolarizations—andmust becombinedtoprovidethebestcompositeoutput.Ifdatatransmissionsareseparatedinto manyparallellow-ratechannels,fadingoftheindividualnarrow-bandchannelsisessen- tiallyflat,andgoodreliabilitycanbeachievedbyusingdiversitytechniques.Mostofthe data sent over HF use such multitone signals. Inmodernreceiverdesigns,adaptiveequalizertechniquesareusedtocombatmultipath thatcausesselectivefadingonbroadbandtransmissions.ThebandwidthavailableonHF makespossibletheuseofspread-spectrumtechniquesintendedtocombatinterferenceand, especially, jamming. This is primarily a military requirement. VHF (30 to 300 MHz) Mostvery-highfrequency(VHF)transmissionsareintendedtoberelativelyshort-range, usingline-of-sightpathswithelevatedantennas,atleastatoneendofthepath.Inaddition toFMandtelevisionbroadcastservices,thisbandhandlesmuchofthelandmobileand somefixedservices,andsomeaeronauticalandaeronavigationservices.Solongasagood clearlineofsightwithadequateground(andotherobstruction)clearanceexistsbetween theantennas,thesignalwilltendtobestrongandsteady.Thewavelengthis,however,be- coming sufficiently small at these frequencies so that reflection is possible from ground features, buildings, and some vehicles. Usually reflection losses result in transmission oversuchpathsthatismuchweakerthantransmissionoverline-of-sightpaths.Inlandmo- bileservice,oneorbothoftheterminalsmayberelativelylow,sothattheearth’scurvature orrollinghillsandgulliescaninterferewithaline-of-sightpath.Whiletherangecanbe extended slightly by diffraction, in many cases the signal reaches the mobile station via multipathreflectionsthatareofcomparablestrengthorstrongerthanthedirectpath.The resultinginterferencepatternscausethesignalstrengthtovaryfromplacetoplaceinarel- atively random matter. Therehavebeenanumberofexperimentaldeterminationsofthevariability,andmodels havebeenproposedthatattempttopredictit.Mostofthesemodelsapplyalsointheul- tra-highfrequency(UHF)region.Forclearline-of-sightpaths,orthosewithafewwell-de- finedinterveningterrainfeatures,accuratemethodsexistforpredictingfieldstrength.In thisband,noiseisoftensimplythermal,althoughman-madenoisecanproduceimpulsive interference.Forvehicularmobileuse,thevehicleitselfisapotentialsourceofnoise.Inthe U.S.,mobilecommunicationshaveusedFM,originallyofawiderbandthannecessaryfor theinformation,soastoreduceimpulsivenoiseeffects.However,recenttrendshavere- ducedthebandwidthofcommercialradiosofthistypesothatthisadvantagehasessentially disappeared.TheotheradvantageofFMisthathardlimitingcanbeusedinthereceiverto Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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