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1 INTRODUCTION

FixedandmobilesatellitecommunicationsystemsareusingfiveprincipalformsofMAtechniques:1 FrequencyDivisionMultipleAccess (FDMA) is a

schemewhereeachconcernedEarthStation,suchasCoastEarthStation(CES)orShipEarthStation(SES),isassigneditsowndifferentworkingcarrierRadio Frequency (RF) inside the spacecrafttransponderbandwidth.

2 Time Division Multiple Access (TDMA) is aschemewhereallconcernedEarthstationsusethesamecarrierRFandbandwidthwithtimesharingandnonoverlappingintervals.

3 Code Division Multiple Access (CDMA) is ascheme where all concerned Earth stationssimultaneously share the same bandwidth and

recognizethesignalsbyvariousprocesses,suchascode identification. Actually, they share theresourcesofbothfrequencyandtimeusingasetofmutually orthogonal codes, such as aPseudorandomNoise(PN)sequence.

4 Space Division Multiple Access (SDMA) is aschemewhereallconcernedEarthstationscanusethe same RF at the same timewithin a separatespaceavailableforeachlink.

5 Random (Packet) Division Multiple Access(RDMA) is a scheme where a large number ofsatellite users share asynchronously the sametransponderbyrandomlytransmittingshortburstorpacketdivisions.

Currently, these methods of multiple access arewidely in use with many advantages and

Implementation of Multiple Access Techniques Applicable for Maritime Satellite Communications

S.D.IlcevDurbanUniversityofTechnology(DUT),SouthAfrica

ABSTRACT: In this paper are introduced fundamentals, characteristics, advantages and disadvantages ofMultipleAccess(MA)employedastransmissiontechniquesintheMaritimeMobileSatelliteCommunications(MMSC) between ships and Coast Earth Station (CES) via Geostationary Earth Orbit (GEO) or NotGEOsatelliteconstellations.Infixedsatellitecommunication,asarule,especiallyinMMSCmanyusersareactiveatthe same time. The problem of simultaneous communications betweenmany single ormultipointmobilesatelliteusers canbe solvedbyusingMA technique, such asFrequencyDivisionMultipleAccess (FDMA),TimeDivisionMultipleAccess (TDMA),CodeDivisionMultipleAccess (CDMA), SpaceDivisionMultipleAccess (SDMA)andRandom (Packet)DivisionMultipleAccess (RDMA).Since the resourcesof the systemssuchasthetransmittingpowerandthebandwidtharelimited,itisadvisabletousethechannelswithcompletechargeandtocreateadifferentMAtothechannel.Thisgeneratesaproblemofsummationandseparationofsignalsinthetransmissionandreceptionparts,respectively.Decidingthisproblemconsistsinthedevelopmentoforthogonal channelsof transmission inorder todivide signals fromvarioususersunambiguouslyon thereceptionpart.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 7

Number 4

December 2013

DOI:10.12716/1001.07.04.08

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disadvantages, together with their combination ofhybrid schemes or with other types ofmodulations. Hence, multiple access techniqueassignment strategy can be classified into threemethods as follows: (1) Preassignment or fixedassignment; (2) Demand Assignment (DA) and (3)RandomAccess(RA);thebitsthatmakeupthecodewords in somepredetermined fashion, such that theeffectofanerrorburstisminimized.

In the preassignment method channel plans arepreviously determined for chairing the systemresources, regardless of traffic fluctuations. Thisscheme is suitable for communication links with alarge amount of traffic between receivers (Rx andtransmitters (Tx).SincemostSESusers inMMSCdonot communicate continuously, the preassignmentmethod is wasteful of the satellite resources. InDemand Assignment Multiple Access (DAMA)satellitechannelsaredynamicallyassignedtomobileusers according to the traffic requirements. Due tohighefficiencyandsystemflexibility,DAMAschemesaresuitedtoMSCsystems.

Figure1.MultipleAccessTechniquesCourtesy of Book: Global Mobile SatelliteCommunicationsbyIlcev[01]

In RA a large number of mobile users use thesatellite resources in bursts, with long inactiveintervals. So, to increase the system throughout,severalmobileAlohamethods have been proposed.Therefore, theMA techniquespermitmore than twoEarth stations to use the same satellite network forinterchanging information. In such a way, severaltransponders in the satellite payload share the RFbands in use and each transponder will actindependently of the others to filter out its ownallocated RF and further process that signal fortransmission.Thus, this featureallowsanymaritimeCES located in the corresponding coverage area toreceivecarriersoriginatingfromseveralSESandviceversa and carriers transmitted by one SES can bereceived by any CES. This enables a transmittingEarth station to group several signals into a single,multidestination carrier. Access to a transpondermaybelimitedtosinglecarrierormanycarriersmayexistsimultaneously.Thebasebandinformationtobetransmitted is impressedon thecarrierby the singleprocessofmultichannelmodulation[01,02,03,04].

2 FREQUENCYDIVISIONMULTIPLEACCESS(FDMA)

ThemostcommonandfirstemployedMAschemeforsatellite communication systems is FDMA conceptshown in Figure 1. (FDMA), where transmittingsignalsoccupynonoverlappingRFbandswithguardbands between signals to avoid interchannelinterference.Thebandwidthofa repeater channel isthereforedividedintomanysubbandseachassigned

to thecarrier transmittedbyanSEScontinuously. Insuch a way, the channel transmits several carrierssimultaneously at a series of different RF bands.Becauseofinterchannelinterference,itisnecessarytoprovideguardintervalsbetweeneachbandoccupiedby a carrier to allow for the imperfections ofoscillators and filters. The downlink Rx selects therequired carrier in accordancewith the appropriateRF. When the satellite Tx is operating close to itssaturation, nonlinear amplification producesintermodulation (IM) products, which may causeinterference in the signalsofotherusers. Inorder toreduce IM, it isnecessary tooperate the transponderbyreducingthetotalinputpoweraccordingto inputbackoffand that the IFamplifierprovidesadequatefiltering.

Therefore, FDMA allocates a single satellitechannel to one mobile user at once. In fact, if thetransmissionpathdeteriorates,thecontrollerswitchesthe system to another channel.Although technicallysimpletoimplement,FDMAiswastefulofbandwidthbecause the voice channel is assigned to a singleconversation,whetherornot somebody is speaking.Moreover, it cannot handle alternate forms of data,only voice transmissions. This systems advantagesare that it is simple technique using equipmentprovenoverdecadestobereliableanditwillremainvery commonly in use because of its simplicity andflexibility.

Itdoeshavesomedisadvantageshowever:1 An FDMA method is the relatively inflexible

system and if there are changes in the requiredcapacity,thentheRFplanhastochangeandthus,involvemanyCES.

2 Multiple carriers cause IM in both the SESHPAand in the transponder HPA. Reducing IMrequiresbackoffoftheHPApower,soitcannotbeexploitedatfullcapacity.

3 As the number of carriers increase, the IMproductsbetweencarriersalso increaseandmoreHPA back off is needed to optimize the system.The throughputdecreases relatively rapidlywiththenumberof transmission carriers, therefore for25carriersitisabout40%lessthanwith1carrier.

4 TheFMsystemcansufferfromwhatisknownasacapture effect,where if two received signals arevery close in RF but of different strengths, thestrongeronetendstosuppresstheweakerone.Forthis reason thecarrierpowerhas tobecontrolledcarefully.

Thus,with theFDMA technique, thesignals fromthe various users are amplified by the satellitetransponder in a given allocated bandwidth at thesametimebutatdifferentfrequencies.Dependingonthe multiplexing and modulation techniquesemployed, several transmission hybrid schemes canbeconsideredandingeneralmaybedividedintotwocategories, based on the traffic demands of EarthstationsonMCPCandSCPC.

1 MultipleChannelsPerCarrier(MCPC)Itsmainelements are multiplexer, modulator andtransmitter using a satellite uplink, when CESmultiplexes baseband data is received from aterrestrial network and destined for various SESterminals. Then the multiplexed data aremodulated and transmitted to the allocated RF

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segment,when thebandwidthof the transponderis shared among several SES units, each withdifferent traffic requirements. The transponderbandwidthisdivided intoseveralfixedsegments,withseveral time frequencydivisionsallocated totheseSESunitsandbetweeneachbandsegmentisa guard band, which reduces the bandwidthutilizationefficiencyandthelossisdirectlyrelatedtothenumberofaccessingSESinthenetwork,seeFigure 1 (FDMA).Depending on the number ofreceivingSESunits,atotalnumberofcarrierswillpass through the satellite transponder. Thesignals received from different SES units extractthecarriercontaining trafficaddressed toCESbyusing an appropriate RF filter, demodulator,baseband filter anddemultiplexer.The output ofthedemodulatorconsistsinmultiplexedtelephonechannels,abasebandfilterisusedtofilteroutthedesired baseband frequency and a demultiplexerretrieves individual telephonechannelsand feedsthem into the terrestrial network for onwardtransmission.Eachbaseband filterofLES receivestations in this scheme corresponds to a specificoneintheLEStransmittingstation.

2 SingleChannelPerCarrier (SCPC)Forcertainapplications, such as the provision of MMSCservice to remote areas or individual SES, trafficrequirements are low. In reality, assigningmultiple channels to each SES is wasteful ofbandwidth because most channels remainunutilizedforasignificantpartoftheday.Forthistype of application the SCPC type of FDMA isused. In the SCPC system each carrier ismodulatedbyonlyonevoiceorbylowtomediumbitratedatachannel.SomeoldanalogsystemsuseCompandedFMbutmostnewsystemsaredigitalPSKmodulated.IntheSCPCscheme,eachcarriertransmits a single carrier. The assignment oftransponder channels to each SESmay be fixedPreAssignedMultipleAccess(PAMA),about5to10 channels, or variable DemandAssignedMultiple Access (DAMA), when a pool offrequencyissharedbymanySESterminals.Whennecessary, each SES requests a channel from RFmanagement of the Network Control Station(NCS),whichmay always attempt to choose thebest available channelor a lowerquality channeluntilanunoccupiedchannelhasbeen found.TheSCPS solution requires an Automatic FrequencyControl (AFC) pilot to maintain the spectrumcentering on a channelbychannel basis. This isusuallyachievedbytransmittingapilottoneinthecentre of the transponder bandwidth. It istransmitted by designated reference CES and allthe SES units use this reference to correct theirtransmission frequency. A receiving station usesthe pilot tone to produce a local AFC system,which is able to control the frequency of theindividualcarriersbycontrollingthefrequencyofthe Local Oscillator (LO). This scheme is costeffective for networks consisting in a significantnumber of Earth stations, each needing to beequippedwithasmallnumberofchannels.Usingthisscheme, InmarsatsystemofA,B,C,M,Fleet33/55/77andFleetBroadbandstandardscansimplyprovideahigherusageofchannelsandcanutilizedemandassignmentequipment.

There are few hybrids of multiplexed FDMAcombined with SCPS, PSK, TDM and TDMAtechniques:

1 SCPC/FM/FDMAThebasebandsignalsfromthenetworkoruserseachmodulateacarrierdirectly,in either analog or digital form according to thenature of SCPC signal in question. Each carrieraccessesthesatelliteon itsparticularfrequencyatthe same time as other carriers on the differentfrequencies from the same or other stationterminals. Information routing is thus,performedaccording to the principle of one carrier per linkutilizing analog transmission with FM for SEStelephone channels. For calculation of channelcapacity of this scheme it is necessary to ensurethat the noise level does not exceed specifieddefinedvalues.

2 SCPC/PSK/FDMAEachvoiceordatachannelismodulated onto its own RF carrier using thisscheme. The only multiplexing occurs in thetransponderbandwidth,wherefrequencydivisionproduces individual channels within thebandwidth.Varioustypesofthismultiplexschemeare used in channels of the Inmarsat standardBSES. In this case, the satellite transponder carrierfrequenciesmaybePAMAorDAMA.ForPAMAcarriers theRF is assigned to a channelunit andthe PSK modem requires a fixedfrequency LOinput.ForDAMA,thechannelsmaybeconnectedaccording to the availability of particular carrierfrequencieswithinthetransponderRFbandwidth.Forthisarrangement,theSCPCchannelfrequencyrequirement is produced by a frequencysynthesizer.TheforwardlinkassignedbyTDMinshoretoshipdirectionuses the SCPC/DA/FDMAsolution for Inmarsat standardB voice/datatransmission.This standard in the return link forchannel request employsAlohaOQPSK and forlow speed data/telex uses the TDMA scheme inshiptoshore direction. The InmarsatAero inforward groundtoaircraft direction uses packetmode TDM for network broadcasting, signalinganddataandthecircuitmodeofSCPS/DA/FDMAwithdistributionchannelmanagement forservicecommunication links. Thus, the request forchannel assignment, signaling and data in thereturn aircrafttoground direction the SlottedAlohaBPSK(1/2FES)of600b/sisemployedandconsequently, the TDMA scheme is reserved fordatamessages.

3 TDM/FDMAThisarrangementallowstheuseofTDM groups to be assembled at the satellite inFDMA, while the PSK is used as a modulationprocess at theEarth station.Systems such as thisare compatiblewithFDM/FDMA carriers sharingthe same transponders and the terminalrequirements are simple and easily incorporated.The Inmarsat standardB system for telex lowspeed data uses this scheme in the shoretoshipdirection only and in the shiptoshore directionuses TDMA/FDMA. The CES TDM and SESTDMA carrier frequencies are preallocated byInmarsat. Each CES is allocated at least oneforwardCESTDMcarrier frequencyandareturnSES TDMA frequency. So, additional allocationscan be made depending on the trafficrequirements.ThechannelunitassociatedwiththeCES TDM channel for transmission consists in a

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multiplexer,differentencoder,frametransmissionsynchronizer and modulator. So at the SES, thereceivepathofthechannelhasthecorrespondingfunctions to the transmitted end. The CES TDMchannelsuseBPSKwithdifferentialcoding,whichis used for phase ambiguity resolution at thereceiveend.

4 TDMA/FDMAAs isknown, theTDMAsignalscould occupy the complete transponderbandwidth. In fact, a better variation of this iswheretheTDMAsignalsaretransmittedasasubbandoftransponderbandwidth,theremainderofwhich being available for example forSCPC/FDMA signals. Thus, the use of anarrowbandTDMAarrangementiswellsuitedforasystemrequiringonlyafewchannelsandhasthealladvantagesofsatellitedigital transmissionbutcansufferfromintermodulationwiththeadjacentFDMA satellite channels. Accordingly, thepractical example of thismultiple schemes is theTlx (Telex) service of the Inmarsat StandardBsystem in shiptoshore direction, which,depending on the transmission traffic, offers aflexible allocation of capacity for satellitecommunicationandsignalingslots[01,03,05,06].

3 TIMEDIVISIONMULTIPLEACCESS(TDMA)

TheTDMAapplicationisadigitalMAtechniquethatpermits individualEarth station transmissions to bereceivedby thesatellite inseparate,nonoverlappingtime slots, called bursts, which contain bufferedinformation. The satellite receives these burstssequentially,withoutoverlapping interferenceand isthen able to retransmit them to the SES terminal.Synchronization isnecessaryand isachievedusingareference station from which burst position andtiming informationcanbeusedasa referencebyallother stations.EachSESmustdetermine the satellitesystem timeandrangeso that the transmittedsignalbursts,typicallyQPSKmodulated,aretimedtoarriveat the satellite in the proper time slots. The offsetQPSKmodulationisusedbyInmarsatBSES.Soastoensure the timing of the bursts frommultiple SES,TDMAsystemsuseaframestructurearrangementtosupportTlx in theshiptoshoredirection.Therefore,a reference burst is transmitted periodically by areferencestationtoindicatethestartofeachframetocontrol the transmission timingof alldatabursts.Asecond reference burstmay also follow the first inorder to provide a means of redundancy. In thepropermanner, to improve the imperfect timing ofTDMA bursts, several synchronization methods ofrandomaccess,openloopandclosedloophavebeenproposed.

In Figure 1 (TDMA) a concept of TDMA isillustrated,whereeachSES terminal transmitsadataburstwithaguardtimetoavoidoverlaps.SinceonlyoneTDMAburstoccupies the fullRFbandwidthofthe satellite transponder at a time, input back off,which isneededtoreduceIM interference inFDMA,isnotnecessaryinTDMA.Atanyinstantintime,thetransponder receives and amplifies only a singlecarrier.Thus, therecanbeno IM,whichpermits thesatellite amplifier to be operated in full HPAsaturationandthetransmittercarrierpowerneednot

be controlled. Because all SES units transmit andreceive at the same frequency, tuning is simplified.This results in a significant increase in channelcapacity. Another advantage over FDMA is itsflexibility and timeslot assignments are easier toadjustthanRFchannelassignments.Thetransmissionrate of TDMA bursts is about 4,800 b/s, while theframe length is about 1.74 seconds and the optimalguardtimeisapproximately40msec,usingtheopenloopburstsynchronizationmethod.

There are some disadvantages because TDMA ismorecomplexthanFDMA:1 Two reference stations are needed and complex

computer procedures, for automatedsynchronizationsbetweenSESterminals.

2 Peak power and bandwidth of individual SESterminals need to be larger than with FDMA,owingtohighburstbitrate.

Accordingly, in the TDMA scheme, thetransmission signals from variousmobile users areamplifiedatdifferent timesbutat the samenominalfrequency,beingspreadbythemodulationinagivenbandwidth. Depending on the multiplexingtechniques employed, two transmission hybridschemescanbeintroducedforuseinMMSCsystems.

1 TDM/TDMA The Inmarsat analog standardAuses the TDM/TDMA arrangement for telextransmission. Each SES has at least one TDMcarrier and each of the carriers has 20 telexchannels of 50 bauds and a signaling channel.Moreover, there is also a common TDM carriercontinuously transmitted on the selected idlelistening frequency by the NCS for outofbandsignaling.TheSES remains tuned to thecommonTDM carrier to receive signalingmessageswhenthe ship is idle or engaged in a telephone call.WhenanSESisinvolvedinatelexforwardcallitis tuned to the TDM/TDMA frequency pairassociated with the corresponding CES to sendmessages in shoretoship direction. Telextransmissionsinthereturnshiptoshoredirectionform a TDMA assembly at the satellitetransponder.EachframeofthereturnTDMAtelexcarrierhas22timeslots,whileeachoftheseslotsispaired with a slot on the TDM carrier. Theallocation of a pair of time slots to complete thelink isreceivedbytheSESonreceiptofarequestforatelexcall.Otherwise,theInmarsatAusesforforward signaling a telex mode, while all otherMSSInmarsatstandardsforforwardsignalingandassignmentchannelsusetheTDMBPSKscheme. The new generation Inmarsat digital standardB(inheritor of standardA) uses the samemodulationTDM/TDMA techniquebut insteadofAlohaBPSK(BCH)atadatarateof4800b/sforthereturn request channel used by InmarsatA, newstandardBisusingAlohaOQPSK(1/2FEC)atadata rate of 24 Kb/s. ThisMA technique is alsouseful for the Inmarsat standardC terminal formaritime, land and aeronautical applications. Inthis case, the forward signaling and sending ofmessages in groundtomobile direction use afixed assignedTDM carrier.The return signalingchanneluseshybrid,slottedAlohaBPSK(1/2FEC)withaprovision for receiving some capacity andthe return message channels in the mobileto

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ground direction are modulated by the TDMAsystematadatarateof600b/s.

2 FDMA/TDMA The Iridium mobile systememploys a hybrid FDMA/TDMA access scheme,which is achieved by dividing the available 10.5MHzbandwidthinto150channelsintroducedintothe FDMA components. Each channelaccommodates a TDMA frame comprising eighttime slots, four for transmission and four forreception.Eachslotlastsabout11.25msec,duringwhichtimedataaretransmittedina50Kb/sburst.Each frame lasts90msecandasatellite isable tosupport840channels.Therefore,auserisallocateda channel occupied for a short period of time,duringwhichtransmissionsoccur[01,03,07,08].

4 CODEDIVISIONMULTIPLEACCESS(CDMA)

ThemodernCDMAsolution isbasedon theuse themodulation technique also known as SpreadSpectrumMultipleAccess(SSMA),whichmeansthatit spreads the information contained in a particularsignalofinterestoveramuchgreaterbandwidththantheoriginalsignal.InthisMAschemetheresourcesofbothfrequencybandwidthandtimearesharedbyallusersemployingorthogonalcodes,showninFigure1(CDMA). TheCDMAisachievedbyaPN(PseudoNoise) sequence generated by irreduciblepolynomials, which is the most popular CDMAmethod.In thisway,aSSMAmethodusing lowrateerror correcting codes, including orthogonal codeswithHadamardorwaveformtransformationhasalsobeenproposed.

Concerning the specific encoding process, eachuser is actually assigned a signature sequence,withitsowncharacteristiccode,chosenfromasetofcodesassigned individually to the various users of thesystem. This code is mixed, as a supplementarymodulation,with the useful information signal.Onreceptionside,fromallthesignalsthatarereceived,agivenmobileuser isable to selectand recognize,byitsowncode,thesignal,whichisintendedforit,andthentoextractusefulinformation.Theotherreceivedsignal can be intended for other users but they canalsooriginate fromunwanted emission,whichgivesCDMA a certain antijamming capability. For thisoperation, where it is necessary to identify oneCDMA transmission signal among several otherssharing the sameband at the same time, correlationtechniques are generally employed. From acommercial andmilitaryperspective thisMA is stillnewandhassignificantadvantages.Interferencefromadjacentsatellitesystems including jammers isbettersolvedthanwithothersystems.ThisschemeissimpletooperateasitrequiresnosynchronizationoftheTxandismoresuitedforamilitarySES.Smallantennascanbeveryuseful in theseapplications,without theinterferencecausedbywideantennabandwidths.

Usingmultibeam satellites, frequency reusewithCDMAisveryeffectiveandallowsgoodflexibilityinthe management of traffic and the orbit/spectrumresources. The Power Flux Density (PFD) of theCDMA signal received in the service area isautomatically limited, with no need for any otherdispersalprocesses.Italsoprovidesalowprobability

of intercept of the users and some kind of privacy,due to individual characteristic codes. The maindisadvantage of CDMA by satellite is that thebandwidth required for the space segment of thespread carrier is very large, compared to that of asingle unspread carrier, so the throughput issomewhat lower thanwithothersystems.Using thisscheme, the signals from various users operatesimultaneously, at the same nominal RF, but arespread inthegivenallocatedbandwidthbyaspecialencoding process. Depending on the multiplexingtechniques employed the bandwidthmay extend tothe entire capacity of the transponder but is oftenrestricted to itsownpart, soCDMA canpossiblybecombined in the hybrid schemewith FDMA and/orTDMA.

The SSMA technique can be classified into twomethods: Direct Sequence (DS) and FrequencyHopping (FH).AcombinedsystemofDCandFH iscalled a hybrid CDMA system and the processinggaincanbe improvedwithout increasesofchiprate.ThehybridsystemhasbeenusedinthemilitaryJointTactical InformationDistributionSystem (JTIDS)andOmniTRACS, which is Kuband mobile satellitesystem,developedby theQualcommCompany. Inamore precise sense, the CDMA technique wasdevelopedbyexpertsof theQualcommCompany in1987.At present, theCDMA system advantages arepracticallyeffective innew satellite systems, suchasGlobalstar, also developed by Qualcomm, which isdevoted tomobile satellite handheld terminals andSkybridge, involved in fixed satellite systems. Thistype ofMA is therefore attractive for handheld andportable satellite equipment with a wide antennapattern. Antennas with large beam widths canotherwise create or be subject to interference withadjacent satellites. In any case, thisMA technique isvery attractive for commercial, military and evenTT&C communications because some Russiansatellites use CDMA for command and telemetrypurposes.

The Synchronous-CDMA (S-CDMA) scheme proves efficiently to eliminate interference arising from other users sharing the same carrier and the same spot beam. Interference from other spot beams that overlap the coverage of the intended spot is still considerable. This process to ensure orthogonality between all links requires signaling to adjust transmission in time and frequency domains for every user independently.

1.Direct Sequence (DS) CDMA ThisDSCDMAtechnique is also called PseudoNoise (PN)modulation,wherethemodulatedsignalismultipliedby aPN code generator,which generates apseudorandombinary sequenceof length (N)atachip rate(Rc),much larger than information bit rate (Rb).Thechip rate sequence is introduced by the followingrelation:

Rc=NRb (1)

This sequence is combinedwith the informationsignalcutintosmallchiprates(Rc),thus,speedingthecombinedsignalinamuchlargerbandwidth(W~Rc).TheresultingsignalhaswiderRFbandwidththantheoriginal modulated signal. In such a way, the

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transmittingsignalcanbeexpressed inthefollowingway:

s(t)=m(t)p(t)cos(2fct)=m(t)p(t)cosct (2)

wherem(t) = binarymessage to be transmitted andp(t) = spreadingNPbinary sequence.Consequently,at the receiver the signal is coherentlydemodulatedbymultiplyingthereceivedsignalbyareplicaofthecarrier.Neglectingthermalnoise,thereceivingsignalattheinputofthedetectorofLowPassFilter(LPF)isgivenbythefollowingequation:

r(t)=m(t)p(t)cosct(2cosct)=m(t)p(t)+m(t)p(t)cos2ct (3)

The detector LLF eliminates theHF componentsand retainsonly theLW components, such asu(t) =m(t) p(t). This component is thenmultiplied by thelocal code [p(t)] in phase with the received code,where the product p(t)2 = 1. At the output of themultiplierthisgives:

x(t)=m(t)p(t)p(t)=m(t)p(t)2=m(t) [V] (4)

Thesignalisthenintegratedoveronebitperiodtofilterthenoise.Thetransmittedmessageisrecoveredattheintegratoroutput,so infact,onlythesamePNcode can achieve the despreading of the receivedsignalbandwidth. Inthisprocess,theinterferenceorjamming spectrum is spreadby thePNcodes,whileotheruserssignals,sparedbydifferentPNcodes,arenotdespread.Interferenceor jammingpowerdensityin the bandwidth of the received signal decreasesfromtheiroriginalpower.Otherwise,themostwidelyaccepted measure of interference rejection is theprocessinggain(Gp),whichisgivenbytheratioRc/Rbandvalue ofGp = 20 60 dB. The input and outputsignaltonoiseratiosarerelatedasfollows:

(S/N)Output=Gp(S/N)Input (5)

In the forward link, theCES transmits thespreadspectrum signals spread with synchronized PNsequence todifferentMMSCusers.Sinceorthogonalcodes can be used, the mutual interference in thenetworkisnegligibleandthechannelcapacityiscloseto that of FDMA. In the return link, the signalstransmitted from different SES users are notsynchronized and they are not orthogonal.The firstcaseisreferredtoassynchronousandthesecondcaseasasynchronousSSMA.Thenonorthogonalitycausesinterferencedue to the transmission of other SES inthe satellite network and as the number ofsimultaneously accessing users increases, thecommunication quality gradually degrades in aprocesscalledGracefulDegradation.

2.FrequencyHopping(FH)CDMATheFHCDMAsystem works similarly to the DS system, since acorrelationprocessofdehoppingisalsoperformedatthe receiver.Thedifference is that here the pseudorandom sequence is used to control a frequencysynthesizer,whichresultsinthetransmissionofeachinformationbitrateintheformof(N)multiplepulsesat different frequencies in an extended bandwidth.

The transmitted and received signals have thefollowingforms:

s(t)=m(t)cosc(t)t and r(t)=m(t)cosc(t)t2cosc(t)t=m(t)+m(t)cos2c(t)t (6)

AtRxthecarrierismultipliedbyanunmodulatedcarriergeneratedunderthesameconditionsasatTx.ThesecondterminRxiseliminatedbytheLPFofthedemodulator.The relationofprocessinggain forFHis:

Gp=W/f (7)

whereW=frequencybandwidthandf=bandwidthof the original modulated signal. At this point,coherent demodulation is difficult to implement inFH receivers because it is a problem to maintainphase relation between the frequency steps.Due tothe relatively slow operation of the frequencysynthesizer, DS schemes permit higher code ratesthanFHradiosystems[01,03,06,11].

5 SPACEDIVISIONMULTIPLEACCESS(SDMA)

Thesignificant factor in theperformanceofMA inasatellite communications system is interferencecaused by different factors and other users. In theotherwords,themostusualtypesof interferencearecochannelandadjacentchannelinterference.Thecochannel interference canbe caused by transmissionsfromnonadjacentcellsorspotbeamsusingthesameset of frequencies,where there isminimal physicalseparation from neighboring cells using the samefrequencies,whiletheadjacentchannelinterferenceiscaused by RF leakage on the subscribers channelfromaneighboringcellusinganadjacent frequency.Thiscanoccurwhentheuserssignalismuchweakerthan that of the adjacent channel user.SignaltoInterferenceRatio (SIR) is an important indicator ofcallquality; it is ameasureof the ratiobetween themobile phone signal (the carrier signal) and aninterfering signal. A higher SIR ratio meansincreasingoverallsystemcapacity.

Figure2.PhasedArrayAntennaandBaseStationforSDMATechniqueCourtesyofManual:GlobalMobileCNSbyIlcev[3]

Taking into account that within the systems ofsatellitecommunications,everyuserhasownuniquespatial position, this fact may be used for theseparationofchannelsinspaceandasaconsequence,toincreasethe SIRratiobyusingSDMA.Ineffect,thismethod is physicallymaking the separation ofpaths available for each satellite link. Terrestrialtelecommunicationnetworks canuse separate cables

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or radio links, but on a single satellite independenttransmission paths are required. Thus, this MAcontrol radiates energy into space and transmissioncan be on the same frequency: such as TDMA orCDMAandondifferentfrequencies,suchasFDMA.

5.1 SpecialEffectsofSDMAinWirelessSystems The new technologies are recently implemented forthe Third Generation Wireless (3G) with FutureEnhancements. Some of the key enhancements towireless technology includeSDMA, the introductionof 3G wireless technologies and integration withpersonal and mobile applications includingaeronautical.TheSDMAmethod is a special systemaccess technology that allows a single transmitterlocationtoprovidemultiplecommunicationchannelsby dividing the radio coverage into focused radiobeams that reuse the same frequency. To allowmultipleaccesses,eachmobile radio isassigned toafocused radio beam. These radio beams maydynamically changewith the location of themobileradio. Analogously,inreception,themobileantennapicksupsignalscomingfromalldirections,includingnoise and interference. These considerations havelead to the development of the SDMA technique,which is based on deriving and exploitinginformation on the spatial position of mobileterminals.Theuseofanadaptiveantennaarrayatthebase station thus allows to introduce the SDMAtechnique,whosemainadvantage isthecapabilitytoincrease system capacity, i.e. the number of users itcan handle. This increase can be obtained in twodifferent ways, and therefore the followingapplicationsarepossible:1 Reduction in Cochannel Interference The

reduction in the level of cochannel interferencebetweenthedifferentcellsusingthesamegroupofradio channels is obtained, as above seen, byminimizingthegaininthedirectionofinterferingmobile units. This technique, indicated with theacronym Spatial Filtering for InterferenceReduction (SFIR) allows to reduce frequency reusedistanceandclustersize.Inthisway,eachcellcan be assigned a higher number of channels byphasearrayantenna,aspresentedinFigure2(A).

2 Spatial Orthogonality In conventional accesstechniques, orthogonality between signalsassociated with different users is obtained bytransmitting them indifferent frequencybandsofFDMA, indifferent time slotsofTDMAorusingdifferent code sequences of CDMA. Using anantennaarray,itispossibletocreateanadditionaldegree of orthogonality between signalstransmitted to and fromdifferentdirections. It isthuspossible toassign thesamephysicalchannelto severalmobile units, as depicted in Figure 22(B),whentheanglesatwhichtheyareseenbythebasestationaresufficientlyseparated.Theresultisan increase in the number of available channels,since the samephysical channel, for example thesamecarrierinaFDMAorthesametimeslotinaTDMA system, can be subdivided intomultiplespatial channels, each ofwhich is assigned to adifferentuser.So, themultipleusersbelonging tothesamecellusethesamechannel.

5.2 SpecialEffectsofSDMAinMobileRadioSystemsInaFDMAmobileradiosystem,howevercapacityislimited by two different factors. On one hand, alimited number of radio channels, carriers and timeslots, are available, and they must be subdividedamong beams making up a cluster. On the otherhand, cochannel interference limits channel reuse.The SDMA technique allows to expand both theselimitsandtoenhancesystemcapacity.

Figure3.HypotheticalScenarioofSDMAforMobileRadioApplicationsCourtesyofManual:GlobalMobileCNSbyIlcev[3]

As already described, this can occur in twodifferentways:with the SFIR technique and spatialorthogonality. In such a way, interference level isreduced and channel reuse distance is decreased,whereastheactualSDMAtechniqueassignsthesamechannel to multiple, spatially separated users. InFigure 3 is illustrated a SDMA system, whichdiagram shows a single tower that is servingmanydifferentusersfromthesameradiotoweronthesamefrequencyusing independentbeamsofradioenergy.Inpracticethisisnotpossibletoachieve,becauseeachtypeofmobile systemhasdifferent frequencyband,but they can be seen in separateway.On the otherhand, the SDMA technique requires an arraycomposedofmoreantennas than theSFIR techniquedoes. In fact, spatial orthogonality is exploited byeliminating,throughtheuseofspatialfiltering,intracellcochannelinterference,whichis

Differentlytosay, intraditionalradiosystemsthebasestation,havingnoinformationonthepositionofmobile units, is forced to radiate the signal in alldirection,inordertocovertheentireareaofthecell.This entails both a waste of power and thetransmission, in the directions where there are nomobile terminals to reach, of a signalwhichwill beseenasinterferingforcochannelcells,i.e.thosecellsusingthesamegroupofradiochannels.

It must be noted that the term SDMA refers,strictly speaking, only to the latter application, inwhichSDMAisactuallyaccomplished.Inspiteofthisfact,theSFIRtechnique isalsoconsideredwithintheSDMA technique, since it is based on the sameprinciples. Inaddition to theopportunity to increasesystemcapacity, theSDMA techniquehasadditionalcharacteristics making its introduction in a mobileradio system advantageous. In particular, it ispossible toexploit thehigherreceivegainofferedbyan antenna arraywith respect to anomnidirectionalcase, to allow mobile units to transmit at reducedpower, and therefore lower consumption. At equalpower, gain can be exploited to extend beam size.This is useful when it is necessary to cover vastsurfaceareas,typicallyruralareas,characterizedbya

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low mobile radio traffic density, with a limitednumberofbasestations[01,03,06,12].

Figure4.SDMAforMobileSatelliteApplicationsCourtesyofManual:GlobalMobileCNSbyIlcev[3]

Figure 5. The Beam Patterns and Adaptive AntennaApplicationsforSDMACourtesyofPaper:SmartAntennaApplicationforSatelliteCommunicationswithSDMAbyZaharov[12]

5.3 SpecialEffectsofSDMAinMobileSatelliteSystemsTheSDMA technologyhasbeensuccessfullyused insatellitecommunications forseveralyears.Asstated,theSDMA technique can alsobe integratedwith allthedifferentMA techniques inuse, such as FDMA,TDMA andCDMA, and therefore canbe applied toany mobile communication system. However, weshallseethatthewaysinwhichtheSDMAtechniquecan be introduced and the advantages it providesdifferdependingon thesystemunderconsideration.As aforesaid described, modifications required torealizetheSDMAtechniquearelimitedtothesatellitearray, and thus do not involve mobile units.However, this allows to introduce this technique inexisting mobile satellite systems, with no need tomodifytheircharacteristics.

Theabilitytorejectjammerofadaptivearrayscanbe ensured for performing SDMA mode whereseveral mobiles are allowed to share the sameclassicalaccessinacell,leadingtoacapacityincrease.InFigure4 is illustrated thissharingwithpossibilitythrough the use of adaptive beam forming andinterference rejection on the satellite uplink anddownlink communication for mobiles, which arelocatedatdifferentangularsectors.

In using SDMA, either FDMA or TDMA areneeded to allow LES to roam in the same satellitebeamor forpolarization to enter the repeater.Thus,the frequency reuse technique of same frequency iseffectively a formofSDMA scheme,whichdependsupon achieving adequate beamtobeam and

polarization isolation.Using this system reverse linemeans that interferencemay be a problem and thecapacityofthebatteryislimited.

On theotherhand, a single satellitemay achievespatialseparationbyusingbeamswithhorizontalandvertical polarization or lefthand and righthandcircularpolarization.This couldallow twobeams tocoverthesameEarthsurfacearea,beingseparatedbythe polarization. Thus, the satellite could also havemultiple beams using separate antennas or using asingle antenna with multiple feeds. For multiplesatellites, spatial separation can be achieved withorbital longitude or latitude and for intersatellitelinks,byusingdifferentplanes.Exceptforfrequencyreuse, this system provides onboard switchingtechniques,which,inturn,enhancechannelcapacity.Additionally, the use of narrow beams from thesatellite allows the Earth station to operate withsmaller antennas and so produce a higher powerdensityperunit area for a given transmitterpower.Therefore, through the careful use of polarization,beams (SDMA) or orthogonal (CDMA), the samespectrummay be reused several times,with limitedinterferenceamongusers.

Themore detailed benefits of an SDMA systemincludethefollowing:1 Thenumberofcellsrequiredtocoveragivenarea

canbesubstantiallyreduced. 2 Interferencefromothersystemsandfromusersin

othercellsissignificantlyreduced. 3 Thedestructiveeffectsofmultipathsignals,copies

of the desired signal that have arrived at theantenna after bouncing from objects between thesignal source and the antenna can often bemitigated.

4 Channel reuse patterns of the systems can besignificantly tighter because the averageinterference resulting from cochannel signals inothercellsismarkedlyreduced.

5 Separate spatial channels can be created in eachcell on the same conventional channel. In otherwords,intracellreuseofconventionalchannels ispossible.

6 TheSDMAstationradiatesmuch lesstotalpowerthan a conventional station. One result is areduction innetworkwideRFpollution.Anotherisareductioninpoweramplifiersize.

7 Thedirectionofeachspatialchannelisknownandcanbeusedtoaccuratelyestablishthepositionofthesignalsource.

8 The SDMA technique is compatiblewith almostanymodulationmethod,bandwidth,orfrequencybandincludingGSM,PHP,DECT,IS54,IS95andother formats. The SDMA solution can beimplemented with a broad range of arraygeometryandantennatypes.

Another perspective of the realization of SDMAsystems is the application of smart antenna arrayswithdifferent levels of intelligence consisting in theantenna array and digital processor. Since thefrequency of transmission for satellitecommunicationsishighenough(mostly6or14GHz),that the dimensions of an array placed inorbit iscommensurablewiththedimensionsoftheparabolicantenna,isanecessaryconditiontoputsuchsystemsintoorbit.

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Thus, the SDMA schememostly responds to thedemandsofLEO andMEO constellations,when thesignals of users achieve the satellite antenna underdifferentangles (22o for theMEO). In this instance,groundlevelmaybesplitintothenumberofzonesofservice coverage determined by switched multiplebeampatternlobesindifferentsatellitedetections,orbyadaptiveantennaseparations,illustratedinFigure5 (A). Thus, there are two different beamformingapproaches in SDMA satellite communications:(1)The multiple spot beam antennas are thefundamentalway of applying SDMA in large fixedandmobile satellite systems and (2)Adaptive arrayantennasdynamicallyadapttothenumberofusers.

5.4 SwitchedSpotBeamAntennaSwitchedMultiBeamAntennasaredesignedtotrackeach subscriber of a given cell with an individualbeam pattern as the target subscribermoveswithinthe cell (spot). Therefore, it is possible to use arrayantennasandtocreateagroupofoverlappingbeamsthattogetherresult inomnidirectionalcoverage.Thisis the simplest technique comprising only a basicswitching function between separate directiveantennasorpredefinedbeamsofanarray.

Beamswitching algorithms and RF signalprocessing software are incorporated into smartantenna designs. For each call, software algorithmsdeterminethebeamsthatmaintainthehighestqualitysignal and the system continuously updates beamselection,ensuringthatcustomersgetoptimalqualityfor the duration of their call.One might designoverlapping beam patterns pointing in slightlydifferent directions, similar to the ones shown inFigure5(A).

Every so often, the system scans the outputs ofeach beam and selects the beam with the largestoutput power. The black cells reuse the frequenciescurrentlyassignedtothemobileterminals,sotheyarepotentialsourcesof interference. In fact, theuseofanarrow beam reduces the number of interferingsources seen at the base station. Namely, as themobilemoves,thesmartantennasystemcontinuouslymonitors the signal quality to determine when aparticularbeamshouldbeselected.

Switchedbeam antennas are normally used onlyfor the reception of signals, since there can beambiguityinthesystemsperceptionofthelocationofthe received signal. In fact, these antennas give thebestperformance,usuallyintermsofreceivedpowerbut they also suppress interference arriving fromdirections away from the active antenna beamscentre,becauseofthehigherdirectivity,comparedtoa conventional antenna, some gain is achieved. Inhighinterference areas, switchedbeam antennas arefurther limited since their pattern is fixedand theylacktheabilitytoadaptivelyreject interference.Suchanantennawillbeeasiertoimplementinexistingcellstructures than the more sophisticated adaptivearraysbutitgivesonlylimitedimprovement.

5.5 AdaptiveArrayAntennaSystemsAdaptive Array Antenna Systems select one beampatternforeachuseroutofanumberofpreset fixedbeam patterns, depending on the location of thesubscribers.At all events, these systems continuallymonitor their coverageareas,attempting toadapt totheir changing radio environment,which consists in(often mobile) users and interferers. Thus, in thesimplest scenario, that of a single user and nointerferers,thesystemadaptstotheusersmotionbyproviding an effective antenna system pattern thatfollowsthemobileuser,alwaysprovidingmaximumgain in the users direction. The principle of SDMAwith adaptive antenna system application is quitedifferent from the beamforming approachesdescribedinFigure5(B).

The events processed in SDMA adaptive arrayantennasystemsareasfollows:1 A Snapshot, or sample, is taken of the

transmission signals coming from all of theantennaelements,convertedintodigitalformandstoredinmemory.

2 TheSDMAdigitalprocessor analyzes the sampleto estimate the radio environment at this point,identifying users and interferers and theirlocations.

3 The processor calculates the combining strategyfortheantennasignalsthatoptimallyrecoverstheuserssignals.Withthisstrategy,eachuserssignalisreceivedwithasmuchgainaspossibleandwiththeotherusers/interfererssignalsrejectedasmuchaspossible.

4 Ananalogouscalculationisdonetoallowspatiallyselective transmission from thearray.Eachuserssignal is now effectively delivered through aseparatespatialchannel.

5 The system now has the ability to both transmitand receive information on each of the spatialchannels,making them twowaychannels [01,03,08,09,12].

Figure6. Block Diagram of SDMA/FDMA andSDMA/SS/FDMASDMA/SS/TDMACourtesyofManual:GlobalMobileCNSbyIlcev[3]

As a result, the SDMA adaptive array antennasystem can create a number of twoway spatialchannels on a single conventional channel, be itfrequency, time, or code. Of course, each of thesespatialchannelsenjoys the fullgainand interferencerejection capabilitiesof the antenna array. In theory,an antenna arraywith (n) elements can support (n)spatialchannelsperconventionalchannel.Inpractice,the number is somewhat less because the receivedmultipath signals,which can be combined to directreceived signals, takes place. In addition, by usingspecialalgorithmsandspacediversitytechniques,theradiationpatterncanbeadaptedtoreceivemultipathsignals, which can be combined. Hence, these

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techniques will maximize the SIR or SignaltoInterferenceandNoiseRatio(SINR).

5.6 SDMA/FDMAThismodulation arrangement uses filters and fixedlinks within the satellite transceiver to route anincominguplink frequency to aparticulardownlinktransmissionantenna,showninFigure6(A).Abasicconfiguration of fixed linksmay be set up using aswitch that is selected only occasionally. Thus, analternative solution allows the filter to be switchedusing a switch matrix, which is controlled by acommand link. Because of the term SS (SwitchingSatellite) this scheme would be classified asSDMA/SS/FDMA,which blockdiagram is shown inFigure6 (B).The satellite switchesare changedonlyrarely, only when it is desired to reconfigure thesatellite, to take account of possible traffic changes.Themaindisadvantageofthissolutionistheneedforfilters,whichincreasethemassofthepayload.

5.7 SDMA/TDMAThis solution is similar to the previously explainedSDMA/SS/FDMA in that a switch system allows aTDMA receiver to be connected to a single beam.Switching again is only carried out when it isrequired to reconfigure the satellite. Under normalconditions,a linkbetweenbeampairs ismaintainedandoperatedunderTDMAconditions.

Theutilizationoftimeslotsmaybearrangedonanorganized or contention basis and switching isachieved by using the RF signal. Thus, on boardprocessingislikelytobeusedinthefuture,allowingswitchingtotakeplacebytheutilizationofbasebandsignals. The signal could be restored in quality andevenstored toallow transmission inanew timeslotintheoutgoingTDMAframe.

This scheme is providing up and downlinks forthe later Intelsat VI spacecraft, known asSDMA/SS/TDMA,which blockdiagram is shown inFigure6(B).ThisMAisusingtoallowTDMAtrafficfrom the uplink beams to be switched to downlinkbeams during the course of TDMA frame. At thispoint, the connectionexistsata specific time for theburstdurationwithin the frame timebefore thenextconnectionismade,andsoon.

5.8 SDMA/CDMAThis arrangement allows access to a commonfrequencybandandmaybeusedtoprovidetheMAto the satellite,when each stream isdecodedon thesatellite inorder toobtain thedestinationaddresses.Thus, onboard circuitry must be capable ofdetermining different destination addresses, whichmay arrive simultaneously, while also denyinginvalid users access to the downlink.However, onboard processors allow theCDMA bit stream to beretimed, regenerated and stored on the satellite.Because of this possibility the downlink CDMAconfigurationsneednotbethesameasforuplinkandtheEarthlinkmaythus,beoptimized[01,03,08,10].

6 RANDOMDIVISIONMULTIPLEACCESS(RDMA)

For data transmission, a bit stream may be sentcontinuouslyoveranestablishedchannelwithouttheneed to provide addresses or unique words if thechannel is not charred. In fact, where charring isimplemented, data are sent in bursts, which thus,requiresuniquewordsor synchronization signals toenabletimesharingwithotherusers,tobeaffectedinthe division of channels. Each burstmay consist inone or more packets comprising data from one ormore sources that have been assembled over time,processed and made ready for transmission.However,thistypeofmultiplexschemeisalsoknownas PacketMA. Packet access can be used in specialRDMA solutions, such as Aloha, whereretransmissionofblockedpacketsmayberequired.

Randomaccesscanbeachievedtothesatellitelinkby contention and for that reason is called acontentionaccessscheme.Thistypeofaccessiswellsuitedtosatellitenetworkscontainingalargenumberofstations,suchasSES,whereeachstationisrequiredto transmitshort randomlygeneratedmessageswithlongdead timesbetweenmessages.TheprincipleofRDMA is to permit the transmission of messagesalmost without restriction, in the form of limitedduration bursts,which occupy all the bandwidth ofthe transmission channel.Therefore, inotherwords,this is MA with time division and randomtransmission and an attribute for the synonymRandomDivisionMultipleAccessisquiteassessable.

Ausertransmitsamessageirrespectiveofthefactthat theremaybeotherusers equally in connection.The probability of collisions between bursts at thesatellite is accepted, causing the data to be blockedfromreceiptbytheEarthstation.Incaseofcollision,the destination Earth station receiver will beconfronted with interference noise, which cancompromise message identification andretransmission after a random delay period. Theretransmissionscanoccurasmanytimesasprobablyare carried out, using random time delays. Such ascheme implies that the transmitter vies for satelliteresources on a perdemand basis and no othertransmitterisattemptingtoaccessthesameresourcesduringthetransmissionburstperiod,whenanerrorfree transmission can occur. The types of randomprotocolsaredistinguishedbythemeansprovidedtoovercome this disadvantage, which performance ismeasured in terms of the throughput and themeantransmission delay. Throughput is the ratio of thevolume of traffic delivered at the destination to themaximumcapacityofthetransmissionchannels.Thetransmissiontime,i.e.,delayisarandomvariable.Itsmean value indicates the mean time between thegenerationofamessageand its correct receptionbythedestinationstation.

6.1 AlohaThe most widely used contention access scheme isAloha and its associated derivatives. This solutionwasdeveloped in the late1960sby theUniversityofHawaii and allows usage of small and inexpensiveEarthstations(includingSES)tocommunicatewitha

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minimum of protocols and no network supervision.This is the simplestmode of operation,which timesharesasingleRF,dividedamongmultipleusersandconsists in stations randomly accessing a particularresource that is used to transmit packets.When anAloha station has something to transmit, itimmediately sends a burst of data pulses and candetect whether its transmission has been correctlyreceived at the satellite by either monitoring theretransmission from the satellite or by receiving anacknowledgementmessage from the receivingparty.Should a collisionwith another transmitting stationoccur,resulting in the incorrectreceptionofapacketat the satellite, the transmitting station waits for arandom period of time, prior to retransmitting thepacket.Otherwise,aremotestation (SES)usesAlohatogetahubstation(CES)terminalsattention.

Namely, the SES terminal sends a brief burstrequestingafrequencyortimeslotassignmentforthemaintransmission.Thus,oncetheassignmentforSESis made, there is no further need for the Alohachannel,whichbecomesavailableforotherstationstouse.Afterthat,themaintransmissionsarethenmadeon the assigned channels. At the end, the Alohachannel might be used again to drop the mainchannel assignments after the transmission iscompleted.The advantages ofAloha are the lack ofany centralized control, giving simple, lowcoststations and the ability to transmit at any time,withouthavingtoconsiderotherusers.

In the case where the user population ishomogenous,sothatthepacketdurationandmessagegenerationrateareconstant,itcanbeshownthatthetraffic carried S (packet correctly interpreted by thereceiver),asafunctionoftotaltrafficG(originalandretransmittedmessage)isgivenbytherelation:

S=Gexp(2G) [packet/timeslot] (8)

where (S = transmission throughput) and (G) areexpressedasanumberofpacketspertimeslotequalto the commonpacketduration.TheAlohaprotocolcannot exceed a throughput of 18% and the meantransmissiontimeincreasesveryrapidlyasthetrafficincreases due to an increasing number of collisionsand packet retransmissions. The Aloha mode isrelatively inefficientwithamaximum throughputofonly 18.4% (1/2). However, this has to be counterweight against the gains in simple networkcomplexity,sincenocoordinationorcomplex timingpropertiesarerequiredatthetransmittingSES.

6.2 SlottedAlohaThis form of Aloha or SAloha, where the timedomain is divided into slots equivalent to a singlepacketburst time; therewillbenooverlap,as is thecase with ordinary Aloha. The transmissions fromdifferentstationsarenowsynchronizedinsuchawaythat packets are located at the satellite in time slotsdefined by the network clocks and equal to thecommon packet duration. Hence, there cannot bepartialcollisions;everycollisionarisesfromcompletesuperposition of packets. In effect, the timescale ofcollision isthus,reducedto thedurationofapacket,whereas with the Aloha protocol, this timescale is

equal to thedurationofbothpackets.At thispoint,this situation divides the probability of collision bytwoandthethroughputbecomes:

S=Gexp(G) [packet/timeslot] (9)

This protocol enables collisions between newmessages and retransmission to be avoided andincreases the throughput of SAloha in the order of5060% by introducing a frame structure, whichpermits the numbering of time slots. Each packetincorporates additional information indicating theslot number reserved for retransmission in case ofcollision. For the same value of utilization as basicAloha, the timedelay andprobabilityofpacket lossare both improved. The major disadvantages of SAlohaarethatmorecomplexequipment intheEarthstation is necessary, because of the timingrequirement and because there are fixed time slots,customerswithasmalltransmissionrequirementarewastingcapacitybynotusingthetimeslotto itsfullavailability.

6.3 SlotReservationAlohaThis solution of an extension for the slottedAlohascheme allows time slots to be reserved fortransmissionbyanEarthstation.Ingeneraltermsthismode of operation is termed a Packed ReservedMultiple Access (PRMA). Slot reservation basicallytakestwoforms:

1 Implicit When a station acquires a slot andsuccessfullytransmits,theslot isreservedforthatstation for as long as it takes the station tocomplete its transmission.Thenetwork controllerthen informs all stations on thenetwork that theslotisavailableforcontentiononcemore.Thereisonlytheproblemthatastationwithmuchdatatotransmitcouldblockthesystemtootherusers.

2 Explicit Everyuser stationmay send a requestfor the reservation of a time slot prior totransmission of data. A record of all time slotoccupation and reservation requests is kept.Actually, a free time slot couldbe allocatedon apriority basis. Some kind of control for thereservationofslots isnecessaryand thiscouldbeaccomplished by a single or all stations beinginformed of slot occupancy and reservationrequests[01,03,06,08,09,11].

7 CONCLUSION

The performances and capacities of MMSC forCDMA, FDMA and TDMA/FDMA have beenanalyzedmanyyearsagoforanL/CbandRFnetworkwith global coverage. For the particular MMSCsystems under discussion and for the particularantennaconfigurations,bothCDMAandFDMAoffersimilarperformance, FDMA yielding slightly higherchannel capacities at the design point and CDMAbeing slightly better at higher EIRP levels. As theMMSC system grows and the antenna beam sizedecreases,CDMAappears tobeavery efficientMAsystem,becauseitisnotlimitedbyLbandbandwidth

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constraints. However, CDMA is wasteful infeederlink bandwidth, and the choice of amultipleaccess system must take all parameters intoconsideration,suchasoscillatorstability,interferencerejection, system complexity etc. as well as systemcost before deciding on a particularmultiple accesssystem.

The communication satellites forMMSC providemultiplebeamantennasandemployfrequencyreuseof the allocated Lband frequency spectrum. Itappears that despite the fact that FDMA andFDMA/TDMA are orthogonal systems, theynevertheless suffer from bandwidth limitations andsensitivity to interbeam interference in Lband. TheCDMA scheme is better at absorbing Doppler andmultipath effects, and itpermits higher rate coding,butitsuffersfromselfjammingandfrombandwidthconstraints in the feederlink. In general, all threemultipleaccesssystemsshowsimilarperformance.

However,atthechosendesignpointforaggregateEIRP, number of beams, and allocated bandwidth,FDMA provides still the highest system channelcapacity. Recently is developed SDMA as anadvancedsolutionwhereallconcernedSESterminalscansharethesamefrequencyatthesametimewithinaseparatespaceavailableforeach link.Ontheotherhand,theRDMAschemeissuitableforlargenumberof users in MMSC, where all SES terminals shareasynchronously the same transponder by randomlytransmitting short burst or packet divisions. InadditionisdevelopedseveralmobileAlohamethods,whichsuccessfullyincreasethesystemthroughout.

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