Table Of ContentSource: 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
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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-
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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-
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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
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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-
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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
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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
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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.
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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
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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
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