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PostedbyleopedriniThursday,February24,20119:42:00AMCategories:Course

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We can define Frequency Hopping as a communication schemebetween a transmitter and a receiver. There are several conceptsinvolved,suchasspreadspectrummodulationandswitchingfrequencyaccordingtoaknownstandard.

FrequencyHopping(FH)iswidelyusedforexampleinGSMNetworks,solet'sunderstandalittlemoreaboutittoday.

First, a Brief History

ButbeforewetalkaboutFH:doeswhoinventedit?

Severalpeoplecompetesforthistitle,astheGermanJohannesZenneckin1908throughhiscompany:Telefunken.HesharesthetitlewithaPolishinventor,whoalsoexposedtheidea.

Butparticularly, I like theexoticversion,whereabeautifulactress (yes)HedyLamar,alongwithherneighborGeorgeAntheilwerealsoresponsible,intheSecondWorldWartime.

Hedywasmarried to aGerman armsmanufacturer. Issues like security, especially how to send textmessagesusingasignalthatcouldnotbeinterferedwithbytheenemynaturallycametomind.

Ifyousendatorpedocontrolledbyacontinuoussignal,thesignalcanbeidentifiedbytheenemy,whichinturncaninsertahighnoiseandshootitdown.

OnceatthepianothecomposerGeorgeplayedanote,andHedyrepeatedonanotherscale.Itwasthenthatsherealizeditwaspossibletoestablishacommunicationbychangingthecommunicationchannel,justforthis,theyshouldmakethechangeatthesametime,ie,followingapatternknowntoboth.

Bringingthisideatothetorpedo,itsufficedforthetransmitterinthevesselandreceiverinthetorpedoaltered (or jumped) from one frequency to another in a synchronizedmanner. That is, just that thereceiveratthetorpedoknowwhatarethepositionswherethetransmitterfrequencywilljump!

And if any of these frequencies are suffering interference?Well, we still have other channels in thesequenceofjumps,fromwhereinformationcanberetrieved!

Likeanygreatinvention,notethattheideaissimple.

Definition

Well,afterabriefhistory,IhopeyouhaveunderstoodtheideabehindFH.

After the initial contributionof inventors, the ideawasperfected,and isnowused invarioussystemssuchasGSM,asalreadymentioned.

What is Frequency Hopping FHSS?

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FHhasmainlythepurposeofavoidinginterference,andwe'llseehowhegetsit.

InFH,theinformationisspreadoverabandwidthmuchlargerthanisrequiredforitstransmission.Forthis,itisdividedintoseveralchannelsoflowerbandwidth.

Knowingthesequenceofjumpsthatmustbefollowed,thereceiverandtransmitterjumpthroughthesechannels.

Thisisapseudorandomsequence,andthat'swhatmakestheFHalsosecure,sinceunwantedreceiverscannotinterceptthesignalbecausetheydonotknowthesequence.Theonlythingtheyseearenoisesofshortduration.

For each application with its full range of frequency, it is defined as bandwidth, hop number, andmaximumaveragetimethateachfrequencymustbebusy.

We should also stress that in FH do not need continuous bands. In scenarios where the availablebandwidth is limited and not contiguous, the spectrum can be better used (Actually, this is more afeaturecomparisonbetweennarrowbandxbroadbandsystems).

FH in GSM

SpeakingspecificallyabouttheGSMnow,we'llfinallyunderstandhowtheinterferenceisavoid.

Todoso,asalways,letustakeanexampleofanetworkwith10MHzbandwidth.AsthechannelofGSMis200kHz,wehave50channelsavailable.RememberthateachGSMchannelhas8timeslots,andconsideringFullRatewehave8users.

Itmayseemtomanychannels,butbelieveme,amajorplanningchallengeistospreadthesechannelsinaGSMnetworkavoidinginterferenceproblems.Toillustrate,supposeanetworkwith100sectorseachBTS3:wehave300sectors,andonly50frequencies.Naturally,thechannelsmustbereused,whichinevitablyresultinthesamechannelsusedindifferentsectors.

Andtherewehavethecochannelinterference,amajorproblemtobesolved,especiallyindenseGSMnetworks.

Notonlytheproblemofcochannels,wealsohavetheproblemofmultipath,compoundedbythefactthatGSMbandisnarrow.Asignalcanleavethetransmitter,andduetoobstacles,bereflectedinawaythatwilleventuallyinterferewiththeoriginalsignalthatarrivesatthereceiver,sincethissignalisoutofphase,becauseithadto'travel'more.

AnditisespeciallyinthesecasesthatFHhelpsus.

Forclarity,consideransectorwithchannelsAandB.Hardlyallslotsofallchannelsareinuseallthetime.EvenifaparticularslotofchannelAisalsoinuseinanothersectorcochannelinterference,chancesarethatanotherslotofchannelBarenot!That'swhatFHdoes:changesthefrequenciesandslotsofthecall!

Thus,eachuserrunsamuchlowerriskofsufferingcochannelinterference.

Inotherwords,achannelcanbesufferinginterference,butwehaveotherchannelsinthesequenceofjumps thatmaybeno interference!When thenetworkuses FH, andmovesour call slot to slot, andfrequencytofrequency,theinterferenceisturnsintoarandomeffect.

Westill,aswespeak,theproblemsofmultipath.Andtheideaisbasicallythesame.Byjumpingfromonefrequencytoanother,theusersufferstheeffectsofmultipathproblembyaverysmallperiodsoftime.(Rememberweusenarrowbands!)

In both cases, whether cochannel interference or multipath fading, there are the error correctionalgorithms,whichachievethemostefficientwaytocleanandrecovertheoriginalsignal.

FH Basic Algorithm

Finally,weseeasimplifieddiagramshowingthesteps involvedinestablishingacommunicationusingFH.

First,thetransmittersendsarequest(1)tostarttheFHthroughthecontrolchannel.Thereceiver,afterreceivingthisrequestsendsabasenumber(2)back.Thetransmitterthenusesthatnumber,calculatesandsendstheseriesoffrequencies(3)tobeused.Withthislistoffrequencies,thereceiverreturnsasynchronization signal (4) in the first frequency of the list. Thus, communication between the two isestablished(5).

Disadvantages

AndwhatarethedisadvantagesofFH?

Likeanyspreadspectrumcommunication,weneedabiggerband thanwouldbenecessary if itwereusedonlyasinglefrequencytocarrythesignal.

Furthermore, whenever a communication is established, it takes a significant time, to establish syncbetweenthereceiverandtransmitter.

Anyway,theadvantagesoutweighthesepoints.

Conclusion

Wenowknowthesimpleideaoffrequencyhopping,aspreadspectrummodulationscheme,whereitispossibletoestablishacommunicationoverasingle logicalchannel,baseduponthetimingofchanges(jumps)infrequencyamongthem,followingapseudorandomsequenceknownbyboth.

Asa result,using theFHhaveasignalmore robust interference resistant,andsecure tobeverydifficulttointercept.