529
1 INTRODUCTION
Fixedandmobilesatellitecommunicationsystemsare
usingfiveprincipalformsofMAtechniques:
1 Frequency Division MultipleAccess (FDMA) is a
schemewhereeachconcernedEarthStation,such
asCoastEarthStation(CES)orShipEarthStation
(SES),isassigneditsowndifferentworkingcarrier
Radio Frequency
(RF) inside the spacecraft
transponderbandwidth.
2 Time Division Multiple Access (TDMA) is a
schemewhereallconcernedEarthstationsusethe
samecarrierRFandbandwidthwithtimesharing
andnon‐overlappingintervals.
3 Code Division Multiple Access (CDMA) is a
scheme where all concerned Earth stations
simultaneously share
the same bandwidth and
recognizethesignalsbyvariousprocesses,suchas
code identification. Actually, they share the
resourcesofbothfrequencyandtimeusingasetof
mutually orthogonal codes, such as a
PseudorandomNoise(PN)sequence.
4 Space Division Multiple Access (SDMA) is a
schemewhereallconcernedEarth
stationscanuse
the same RF at the same time within a separate
spaceavailableforeachlink.
5 Random (Packet) Division Multiple Access
(RDMA) is a scheme where a large number of
satellite users share asynchronously the same
transponderbyrandomlytransmittingshortburst
orpacketdivisions.
Currently, these
methods of multiple access are
widely in use with many advantages and
Implementation of Multiple Access Techniques
Applicable for Maritime Satellite Communications
S.D.Ilcev
DurbanUniversityofTechnology(DUT),SouthAfrica
ABSTRACT: In this paper are introduced fundamentals, characteristics, advantages and disadvantages of
MultipleAccess(MA)employedastransmissiontechniquesintheMaritimeMobileSatelliteCommunications
(MMSC) between ships and Coast Earth Station (CES) via Geostationary Earth Orbit (GEO) or Not‐GEO
satelliteconstellations.In
fixedsatellitecommunication,asarule,especiallyinMMSCmanyusersareactiveat
the same time. The problem of simultaneous communications between many single or multipoint mobile
satellite users can be solvedby using MA technique, suchas Frequency Division Multiple Access (FDMA),
Time Division Multiple Access (TDMA), Code
Division Multiple Access (CDMA), Space Division Multiple
Access(SDMA) andRandom(Packet)DivisionMultipleAccess(RDMA). Sincethe resources ofthe systems
suchasthetransmittingpowerandthebandwidtharelimited,itisadvisabletousethechannelswithcomplete
chargeandtocreateadifferentMAtothechannel.
Thisgeneratesaproblemofsummationandseparationof
signalsinthetransmissionandreceptionparts,respectively.Decidingthisproblemconsistsinthedevelopment
oforthogonal channels of transmissionin order to divide signalsfrom various users unambiguously on the
receptionpart.
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
530
disadvantages, together with their combination of
hybrid schemes or with other types of
modulations. Hence, multiple access technique
assignment strategy can be classified into three
methods as follows: (1) Preassignment or fixed
assignment; (2) Demand Assignment (DA) and (3)
RandomAccess(RA);thebitsthatmakeupthecode
words
in some predeterminedfashion, such that the
effectofanerrorburstisminimized.
In the preassignment method channel plans are
previously determined for chairing the system
resources, regardless of traffic fluctuations. This
scheme is suitable for communication links with a
large amount of traffic between receivers (Rx and
transmitters
(Tx).SincemostSESusers inMMSCdo
not communicate continuously, the preassignment
method is wasteful of the satellite resources. In
Demand Assignment Multiple Access (DAMA)
satellitechannelsaredynamicallyassignedtomobile
users according to the traffic requirements. Due to
highefficiencyandsystemflexibility,DAMAschemes
aresuitedto
MSCsystems.
Figure1.MultipleAccessTechniques
Courtesy of Book: “Global Mobile Satellite
Communications”byIlcev[01]
In RA a large number of mobile users use the
satellite resources in bursts, with long inactive
intervals. So, to increase the system throughout,
several mobile Aloha methods have been proposed.
Therefore,theMA techniquespermitmore thantwo
Earth stations to use the same satellite network for
interchanging information. In
such a way, several
transponders in the satellite payload share the RF
bands in use and each transponder will act
independently of the others to filter out its own
allocated RF and further process that signal for
transmission.Thus, thisfeatureallows anymaritime
CES located in the corresponding coverage
area to
receivecarriersoriginatingfromseveralSESandvice
versa and carriers transmitted by one SES can be
received by any CES. This enables a transmitting
Earth station to group several signals into a single,
multi‐destination carrier. Access to a transponder
maybelimitedtosinglecarrierormany
carriersmay
existsimultaneously.Thebasebandinformationtobe
transmittedisimpressedon thecarrier bythe single
processofmulti‐channelmodulation
[01,02,03,04].
2 FREQUENCYDIVISIONMULTIPLEACCESS
(FDMA)
ThemostcommonandfirstemployedMAschemefor
satellite communication systems is FDMA concept
shown in Figure 1. (FDMA), where transmitting
signalsoccupynon‐overlappingRFbandswithguard
bands between signals to avoid interchannel
interference.Thebandwidth of
a repeaterchannelis
thereforedividedintomanysub‐bandseachassigned
tothecarriertransmittedbyanSEScontinuously.In
such a way, the channel transmits several carriers
simultaneously at a series of different RF bands.
Becauseofinterchannelinterference,itisnecessaryto
provideguardintervalsbetweeneachband
occupied
by a carrier to allow for the imperfections of
oscillators and filters. The downlink Rx selects the
required carrier in accordance with the appropriate
RF. When the satellite Tx is operating close to its
saturation, nonlinear amplification produces
intermodulation (IM) products, which may cause
interferenceinthesignalsof
otherusers. Inorder to
reduceIM,itisnecessarytooperatethetransponder
byreducingthetotalinputpoweraccordingtoinput
backoffandthattheIFamplifierprovides adequate
filtering.
Therefore, FDMA allocates a single satellite
channel to one mobile user at once. In fact, if the
transmission
pathdeteriorates,thecontrollerswitches
the system to another channel. Although technically
simpletoimplement,FDMAiswastefulofbandwidth
because the voice channel is assigned to a single
conversation, whether or not somebody is speaking.
Moreover, it cannot handle alternate forms of data,
only voice transmissions. This system’s advantages
are that it is simple technique using equipment
provenoverdecadestobereliableanditwillremain
very commonly in use because of its simplicity and
flexibility.
Itdoeshavesomedisadvantageshowever:
1 An FDMA method is the relatively inflexible
system and if there are changes in the required
capacity,thentheRFplanhastochangeandthus,
involvemanyCES.
2 Multiple carriers cause IM in both the SES HPA
and in the transponder HPA. Reducing IM
requiresbackoffoftheHPApower,soitcannotbe
exploitedatfullcapacity.
3 As the number of
carriers increase, the IM
productsbetweencarriers alsoincreaseandmore
HPA back off is needed to optimize the system.
The throughput decreases relatively rapidly with
thenumber of transmissioncarriers, thereforefor
25carriersitisabout40%lessthanwith1carrier.
4 TheFMsystemcansufferfrom
whatisknownasa
capture effect, where if two received signals are
very close in RF but of different strengths, the
strongeronetendstosuppresstheweakerone.For
thisreasonthecarrierpowerhastobecontrolled
carefully.
Thus,withtheFDMAtechnique,thesignalsfrom
the
various users are amplified by the satellite
transponder in a given allocated bandwidth at the
sametimebutatdifferentfrequencies.Dependingon
the multiplexing and modulation techniques
employed, several transmission hybrid schemes can
beconsideredandingeneralmaybedividedintotwo
categories, based on the traffic demands of
Earth
stationsonMCPCandSCPC.
1 MultipleChannelsPerCarrier(MCPC)– Itsmain
elements are multiplexer, modulator and
transmitter using a satellite uplink, when CES
multiplexes baseband data is received from a
terrestrial network and destined for various SES
terminals. Then the multiplexed data are
modulated and transmitted
to the allocated RF
531
segment,whenthebandwidthofthetransponder
is shared among several SES units, each with
different traffic requirements. The transponder
bandwidthisdividedintoseveral fixedsegments,
withseveraltimefrequencydivisionsallocatedto
theseSESunitsandbetweeneachbandsegmentis
a guard band, which reduces the bandwidth
utilizationefficiencyandthelossisdirectlyrelated
tothenumberofaccessingSESinthenetwork,see
Figure 1 (FDMA). Depending on the number of
receivingSES units,atotalnumberofcarrierswill
pass through the satellite transponder.The
signals received from different SES units extract
thecarrier
containingtrafficaddressedto CESby
using an appropriate RF filter, demodulator,
baseband filter and demultiplexer. The output of
thedemodulatorconsistsinmultiplexedtelephone
channels,abasebandfilterisusedtofilteroutthe
desired baseband frequency and a demultiplexer
retrievesindividualtelephonechannels andfeeds
them into the terrestrial
network for onward
transmission. Each baseband filter of LES receive
stations in this scheme corresponds to a specific
oneintheLEStransmittingstation.
2 SingleChannel PerCarrier(SCPC) –For certain
applications, such as the provision of MMSC
service to remote areas or individual SES, traffic
requirements are
low. In reality, assigning
multiple channels to each SES is wasteful of
bandwidth because most channels remain
unutilizedforasignificantpartoftheday.Forthis
type of application the SCPC type of FDMA is
used. In the SCPC system each carrier is
modulatedbyonlyonevoiceorby
lowtomedium
bitratedatachannel.Someoldanalogsystemsuse
CompandedFMbutmostnewsystemsaredigital
PSKmodulated.IntheSCPCscheme,eachcarrier
transmits a single carrier. The assignment of
transponder channels to each SES may be fixed
Pre‐AssignedMultipleAccess(PAMA),about5
to
10 channels, or variable Demand‐Assigned
Multiple Access (DAMA), when a pool of
frequencyissharedbymanySESterminals.When
necessary, each SES requests a channel from RF
management of the Network Control Station
(NCS), which may always attempt to choose the
best availablechannel or a lower quality
channel
untilanunoccupiedchannelhasbeenfound.The
SCPS solution requires an Automatic Frequency
Control (AFC) pilot to maintain the spectrum
centering on a channel‐by‐channel basis. This is
usuallyachievedbytransmittingapilottoneinthe
centre of the transponder bandwidth. It is
transmitted by designated
reference CES and all
the SES units use this reference to correct their
transmission frequency. A receiving station uses
the pilot tone to produce a local AFC system,
which is able to control the frequency of the
individualcarriersbycontrollingthefrequencyof
the Local Oscillator (LO). This scheme is
cost‐
effective for networks consisting in a significant
number of Earth stations, each needing to be
equippedwithasmallnumberofchannels.Using
thisscheme,InmarsatsystemofA, B,C,M, Fleet
33/55/77andFleetBroadbandstandardscansimply
provideahigherusageofchannelsandcanutilize
demand‐
assignmentequipment.
There are few hybrids of multiplexed FDMA
combined with SCPS, PSK, TDM and TDMA
techniques:
1 SCPC/FM/FDMA–Thebasebandsignalsfromthe
networkoruserseachmodulateacarrierdirectly,
in either analog or digital form according to the
nature of SCPC signal in question. Each carrier
accesses
thesatelliteonitsparticularfrequencyat
the same time as other carriers on the different
frequencies from the same or other station
terminals. Informationrouting is thus,performed
according to the principle of one carrier per link
utilizing analog transmission with FM for SES
telephone channels. For calculation of
channel
capacity of this scheme it is necessary to ensure
that the noise level does not exceed specified
definedvalues.
2 SCPC/PSK/FDMA–Eachvoiceordatachannelis
modulated onto its own RF carrier using this
scheme. The only multiplexing occurs in the
transponderbandwidth,wherefrequencydivision
produces individual
channels within the
bandwidth.Varioustypesofthismultiplexscheme
are used in channels of the Inmarsat standard‐B
SES. In this case, the satellite transponder carrier
frequenciesmaybePAMAorDAMA.ForPAMA
carriers the RF is assigned to a channel unit and
the PSK modem requires a
fixed‐frequency LO
input.ForDAMA,thechannelsmaybeconnected
according to the availability of particular carrier
frequencieswithinthetransponderRFbandwidth.
Forthisarrangement,theSCPCchannelfrequency
requirement is produced by a frequency
synthesizer.TheforwardlinkassignedbyTDMin
shore‐to‐ship direction uses the
SCPC/DA/FDMA
solution for Inmarsat standard‐B voice/data
transmission.Thisstandardinthe return link for
channel request employs Aloha O‐QPSK and for
low speed data/telex uses the TDMA scheme in
ship‐to‐shore direction. The Inmarsat‐Aero in
forward ground‐to‐aircraft direction uses packet
mode TDM for network
broadcasting, signaling
anddataandthecircuitmodeofSCPS/DA/FDMA
withdistributionchannelmanagementforservice
communication links. Thus, the request for
channel assignment, signaling and data in the
return aircraft‐to‐ground direction the Slotted
AlohaBPSK(1/2–FES)of600b/sisemployedand
consequently, the TDMA scheme
is reserved for
datamessages.
3 TDM/FDMA–Thisarrangementallowstheuseof
TDM groups to be assembled at the satellite in
FDMA, while the PSK is used as a modulation
process atthe Earth station. Systems such as this
arecompatiblewith FDM/FDMA carriers sharing
the same transponders
and the terminal
requirements are simple and easily incorporated.
The Inmarsat standard‐B system for telex low
speed data uses this scheme in the shore‐to‐ship
direction only and in the ship‐to‐shore direction
uses TDMA/FDMA. The CES TDM and SES
TDMA carrier frequencies are pre‐allocated by
Inmarsat.
Each CES is allocated at least one
forwardCESTDMcarrierfrequencyandareturn
SES TDMA frequency. So, additional allocations
can be made depending on the traffic
requirements.Thechannelunitassociatedwiththe
CES TDM channel for transmission consists in a
532
multiplexer,differentencoder,frametransmission
synchronizer and modulator. So at the SES, the
receivepathofthechannelhasthecorresponding
functions to the transmitted end. The CES TDM
channelsuseBPSKwithdifferentialcoding,which
is used for phase ambiguity resolution at the
receiveend.
4 TDMA/FDMA–As
isknown,theTDMAsignals
could occupy the complete transponder
bandwidth. In fact, a better variation of this is
wheretheTDMAsignalsaretransmittedasasub‐
bandoftransponderbandwidth,theremainderof
which being available for example for
SCPC/FDMA signals. Thus, the use of a
narrowbandTDMA
arrangementiswellsuitedfor
asystemrequiringonlyafewchannelsandhasthe
alladvantagesofsatellitedigital transmissionbut
cansufferfromintermodulationwiththeadjacent
FDMA satellite channels. Accordingly, the
practical example of this multiple schemes is the
Tlx (Telex) service of the Inmarsat Standard‐B
system
in ship‐to‐shore direction, which,
depending on the transmission traffic, offers a
flexible allocation of capacity for satellite
communicationandsignalingslots[01,03,05,06].
3 TIMEDIVISIONMULTIPLEACCESS(TDMA)
TheTDMAapplicationisadigitalMAtechniquethat
permits individual Earth station transmissions to be
received
bythesatelliteinseparate,non‐overlapping
time slots, called bursts, which contain buffered
information. The satellite receives these bursts
sequentially,withoutoverlappinginterferenceandis
then able to retransmit them to the SES terminal.
Synchronizationisnecessaryandisachievedusinga
reference station from which burst position and
timinginformationcanbe usedasareference by all
otherstations. EachSES mustdetermine thesatellite
systemtimeandrangesothatthetransmittedsignal
bursts,typicallyQPSKmodulated,aretimedtoarrive
at the satellite in the proper time slots. The offset
QPSKmodulationisusedbyInmarsat
‐BSES.Soasto
ensure the timing of the bursts from multiple SES,
TDMAsystemsuseaframestructurearrangementto
supportTlxintheship‐to‐shoredirection.Therefore,
a reference burst is transmitted periodically by a
referencestationtoindicatethestartofeachframeto
control
the transmission timingof all data bursts. A
second reference burst may also follow the first in
order to provide a means of redundancy. In the
proper manner, to improve the imperfect timing of
TDMA bursts, several synchronization methods of
randomaccess,open‐loopandclosed‐loophavebeen
proposed.
In
Figure 1 (TDMA) a concept of TDMA is
illustrated,whereeachSESterminaltransmitsa data
burstwithaguardtimetoavoidoverlaps.Sinceonly
one TDMA burst occupies the full RF bandwidth of
the satellite transponder at a time, input back off,
whichisneededtoreduceIM
interferenceinFDMA,
isnotnecessaryinTDMA.Atanyinstantintime,the
transponder receives and amplifies only a single
carrier.Thus,there canbenoIM,whichpermitsthe
satellite amplifier to be operated in full HPA
saturationandthetransmittercarrierpowerneednot
be controlled. Because all
SES units transmit and
receive at the same frequency, tuning is simplified.
This results in a significant increase in channel
capacity. Another advantage over FDMA is its
flexibility and time‐slot assignments are easier to
adjustthanRFchannelassignments.Thetransmission
rate of TDMA bursts is about 4,800 b/s,
while the
frame length is about 1.74 seconds and the optimal
guardtimeisapproximately40msec,usingtheopen‐
loopburstsynchronizationmethod.
There are some disadvantages because TDMA is
morecomplexthanFDMA:
1 Two reference stations are needed and complex
computer procedures, for automated
synchronizationsbetweenSESterminals.
2 Peak power and bandwidth of individual SES
terminals need to be larger than with FDMA,
owingtohighburstbitrate.
Accordingly, in the TDMA scheme, the
transmission signals from various mobile users are
amplifiedatdifferenttimesbutatthesamenominal
frequency,beingspreadbythemodulation
inagiven
bandwidth. Depending on the multiplexing
techniques employed, two transmission hybrid
schemescanbeintroducedforuseinMMSCsystems.
1 TDM/TDMA – The Inmarsat analog standard‐A
uses the TDM/TDMA arrangement for telex
transmission. Each SES has at least one TDM
carrier and each of the carriers has
20 telex
channels of 50 bauds and a signaling channel.
Moreover, there is also a common TDM carrier
continuously transmitted on the selected idle
listening frequency by the NCS for out‐of‐band
signaling.TheSES remainstuned tothe common
TDM carrier to receive signaling messages when
the ship
is idle or engaged in a telephone call.
WhenanSESisinvolvedinatelexforwardcallit
is tuned to the TDM/TDMA frequency pair
associated with the corresponding CES to send
messages in shore‐to‐ship direction. Telex
transmissionsinthereturnship‐to‐shoredirection
form a TDMA
assembly at the satellite
transponder.EachframeofthereturnTDMAtelex
carrierhas22timeslots,whileeachoftheseslotsis
paired with a slot on the TDM carrier. The
allocation of a pair of time slots to complete the
linkisreceivedbytheSESonreceipt
ofarequest
foratelexcall.Otherwise,theInmarsat‐Ausesfor
forward signaling a telex mode, while all other
MSSInmarsatstandardsforforwardsignalingand
assignmentchannelsusetheTDMBPSKscheme.
The new generation Inmarsat digital standard‐B
(inheritor of standard‐A) uses the same
modulationTDM/TDMA
techniquebutinsteadof
AlohaBPSK(BCH)atadatarateof4800b/sforthe
return request channel used by Inmarsat‐A, new
standard‐BisusingAlohaO‐QPSK(1/2–FEC)ata
data rate of 24 Kb/s. This MA technique is also
useful for the Inmarsat
standard‐C terminal for
maritime, land and aeronautical applications. In
this case, the forward signaling and sending of
messages in ground‐to‐mobile direction use a
fixed assigned TDM carrier. The return signaling
channeluseshybrid,slottedAlohaBPSK(1/2FEC)
with a provision for receiving some capacity and
the return
message channels in the mobile‐to‐
533
ground direction are modulated by the TDMA
systematadatarateof600b/s.
2 FDMA/TDMA – The Iridium mobile system
employs a hybrid FDMA/TDMA access scheme,
which is achieved by dividing the available 10.5
MHzbandwidthinto150channelsintroducedinto
the FDMA components. Each channel
accommodates a
TDMA frame comprising eight
time slots, four for transmission and four for
reception.Eachslotlastsabout11.25msec,during
whichtimedataaretransmittedina50Kb/sburst.
Eachframelasts90msecandasatelliteisableto
support840channels.Therefore,auserisallocated
a
channel occupied for a short period of time,
duringwhichtransmissionsoccur[01,03,07,08].
4 CODEDIVISIONMULTIPLEACCESS(CDMA)
ThemodernCDMAsolutionisbasedonthe usethe
modulation technique also known as Spread
SpectrumMultipleAccess(SSMA),whichmeansthat
it spreads the information contained
in a particula r
signalofinterestoveramuchgreaterbandwidththan
theoriginalsignal.InthisMAschemetheresourcesof
bothfrequencybandwidthandtimearesharedbyall
usersemployingorthogonalcodes,showninFigure1
(CDMA).TheCDMAisachievedbyaPN(Pseudo‐
Noise) sequence generated
by irreducible
polynomials, which is the most popular CDMA
method.Inthisway,aSSMAmethodusinglow‐rate
error correcting codes, including orthogonal codes
withHadamardorwaveformtransformationhasalso
beenproposed.
Concerning the specific encoding process, each
user is actually assigned a signature sequence, with
itsown
characteristiccode,chosenfromasetofcodes
assigned individually to the various users of the
system. This code is mixed, as a supplementary
modulation, with the useful information signal. On
receptionside,fromallthesignalsthatarereceived,a
givenmobileuser isableto selectand recognize,by
itsowncode,thesignal,whichisintendedforit,and
thentoextractusefulinformation.Theotherreceived
signal can be intended for other users but they can
also originatefrom unwanted emission, which gives
CDMA a certain anti‐jamming capability. For this
operation, where it is necessary to
identify one
CDMA transmission signal among several others
sharing the same band at the same time, correlation
techniques are generally employed. From a
commercial and military perspective this MA is still
newandhassignificantadvantages.Interferencefrom
adjacentsatellitesystemsincludingjammersisbetter
solvedthanwithothersystems.This
schemeissimple
tooperateasitrequiresnosynchronizationoftheTx
andismoresuitedforamilitarySES.Smallantennas
canbeveryusefulinthese applications,without the
interferencecausedbywideantennabandwidths.
Using multibeam satellites, frequency reuse with
CDMAisveryeffectiveandallows
goodflexibilityin
the management of traffic and the orbit/spectrum
resources. The Power Flux Density (PFD) of the
CDMA signal received in the service area is
automatically limited, with no need for any other
dispersalprocesses.Italsoprovides alowprobability
of intercept of the users and some kind of privacy,
due to individual characteristic codes. The main
disadvantage of CDMA by satellite is that the
bandwidth required for the space segment of the
spread carrier is very large, compared to that of a
single unspread carrier, so the throughput is
somewhatlowerthanwithothersystems.Usingthis
scheme,
the signals from various users operate
simultaneously, at the same nominal RF, but are
spreadinthegivenallocatedbandwidthbyaspecial
encoding process. Depending on the multiplexing
techniques employed the bandwidth may extend to
the entire capacity of the transponder but is often
restrictedto itsownpart, so
CDMA can possiblybe
combined in the hybrid scheme with FDMA and/or
TDMA.
The SSMA technique can be classified into two
methods: Direct Sequence (DS) and Frequency
Hopping(FH).AcombinedsystemofDCandFHis
called a hybrid CDMA system and the processing
gaincanbeimprovedwithout
increasesofchiprate.
ThehybridsystemhasbeenusedinthemilitaryJoint
TacticalInformationDistributionSystem(JTIDS)and
OmniTRACS, which is Ku‐band mobile satellite
system,developedbytheQualcommCompany.Ina
more precise sense, the CDMA technique was
developedbyexpertsoftheQualcommCompanyin
1987.
At present, the CDMA system advantages are
practicallyeffective innew satellitesystems, such as
Globalstar, also developed by Qualcomm, which is
devoted to mobile satellite handheld terminals and
Skybridge, involved in fixed satellite systems. This
type of MA is therefore attractive for handheld and
portable satellite equipment with a
wide antenna
pattern. Antennas with large beam widths can
otherwise create or be subject to interference with
adjacentsatellites.In any case, this MA technique is
very attractive for commercial, military and even
TT&C communications because some Russian
satellites use CDMA for command and telemetry
purposes.
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 – This DS‐CDMA
technique is also called Pseudo‐Noise (PN)
modulation,wherethemodulatedsignalismultiplied
by a PN code generator, which generates a pseudo‐
randombinary sequenceof length(N)atachip rate
(R
c), much larger than information bit rate (Rb). The
chip rate sequence is introduced by the following
relation:
R
c=N∙Rb (1)
This sequence is combined with the information
signalcutintosmallchiprates(R
c),thus,speedingthe
combinedsignalinamuchlargerbandwidth(W~R
c).
TheresultingsignalhaswiderRFbandwidththanthe
original modulated signal. In such a way, the
534
transmittingsignalcanbeexpressedinthefollowing
way:
s(t)=m(t)p(t)cos(2πf
ct)=m(t)p(t)cosωct (2)
where m(t) = binary message to be transmitted and
p(t) = spreading NP binary sequence. Consequently,
at the receiver the signal is coherently demodulated
bymultiplyingthereceivedsignalbyareplicaofthe
carrier.Neglectingthermalnoise,thereceivingsignal
attheinputofthedetectorofLow‐PassFilter(LPF) is
givenbythefollowingequation:
r(t)=m(t)p(t)cosω
ct(2cosωct)=m(t)p(t)+m(t)p(t)
cos2ω
ct (3)
The detector LLF eliminates the HF components
and retains only the LW components, such as u(t) =
m(t) p(t). This component is then multiplied by the
local code [p(t)] in phase with the received code,
where the product p(t)
2
= 1. At the output of the
multiplierthisgives:
x(t)=m(t)p(t)p(t)=m(t)p(t)
2
=m(t)[V] (4)
Thesignalisthenintegratedoveronebitperiodto
filterthenoise.Thetransmittedmessageisrecovered
attheintegratoroutput,soinfact,onlythesamePN
code can achieve the despreading of the received
signalbandwidth.Inthisprocess,theinterferenceor
jammingspectrum isspreadbythePNcodes, while
otheruser’ssignals,sparedbydifferentPNcodes,are
notdespread.Interferenceorjammingpowerdensity
in the bandwidth of the received signal decreases
fromtheiroriginalpower.Otherwise,themostwidely
accepted measure of interference rejection is the
processinggain(G
p),whichisgivenbytheratioRc/Rb
and value of Gp = 20 – 60 dB. The input and output
signal‐to‐noiseratiosarerelatedasfollows:
(S/N)
Output=Gp(S/N)Input (5)
Intheforwardlink,theCEStransmitsthespread
spectrum signals spread with synchronized PN
sequencetodifferent MMSC users. Since orthogonal
codes can be used, the mutual interference in the
networkisnegligibleandthechannelcapacityisclose
to that of FDMA. In the return link, the signals
transmitted
from different SES users are not
synchronized and they are not orthogonal. The first
caseisreferredtoassynchronousandthesecondcase
asasynchronousSSMA.Thenonorthogonalitycauses
interference due to the transmission of other SES in
the satellite network and as the number of
simultaneously accessing users
increases, the
communication quality gradually degrades in a
processcalledGracefulDegradation.
2.FrequencyHopping(FH)CDMA–TheFH‐CDMA
system works similarly to the DS system, since a
correlationprocessofde‐hoppingisalsoperformedat
the receiver. The difference is that here the pseudo‐
random sequence is
used to control a frequency
synthesizer,whichresultsinthetransmissionofeach
informationbitrateintheformof(N)multiplepulses
at different frequencies in an extended bandwidth.
The transmitted and received signals have the
followingforms:
s(t)=m(t)cosω
c(t)tandr(t)=m(t)cosωc(t)t∙2
cosω
c(t)t=m(t)+m(t)cos2ωc(t)t (6)
AtRxthecarrierismultipliedbyanunmodulated
carriergeneratedunderthesameconditionsasatTx.
ThesecondterminRxiseliminatedbytheLPFofthe
demodulator.The relationof processinggain forFH
is:
G
p=W/∆f (7)
whereW=frequencybandwidthand∆f=bandwidth
of the original modulated signal. At this point,
coherent demodulation is difficult to implement in
FH receivers beca use it is a problem to maintain
phase relation between the frequency steps. Due to
the relatively slow operation of the frequency
synthesizer, DS schemes permit higher code rates
thanFHradiosystems[01,03,06,11].
5 SPACEDIVISIONMULTIPLEACCESS(SDMA)
ThesignificantfactorintheperformanceofMAina
satellite communications system is interference
caused by different factors and other users. In the
otherwords,themost
usualtypesofinterferenceare
co‐channelandadjacentchannelinterference.Theco‐
channel interference can be caused by transmissions
fromnon‐adjacentcellsorspotbeamsusingthesame
set of frequencies, where there is minimal physical
separation from neighboring cells using the same
frequencies,whiletheadjacentchannel
interferenceis
caused by RF leakage on the subscriber’s channel
fromaneighboringcellusinganadjacentfrequency.
Thiscanoccurwhentheuser’ssignalismuchweaker
than that of the adjacent channel user.Signal‐to‐
Interference Ratio (SIR) is an important indicator of
call quality; it is a measure
of the ratio between the
mobile phone signal (the carrier signal) and an
interfering signal. A higher SIR ratio means
increasingoverallsystemcapacity.
Figure2.PhasedArrayAntennaandBaseStationforSDMA
Technique
CourtesyofManual:“GlobalMobileCNS”byIlcev[3]
Taking into account that within the systems of
satellitecommunications,everyuserhasownunique
spatial position, this fact may be used for the
separationofchannelsinspaceandasaconsequence,
toincreasethe
SIRratiobyusingSDMA.Ineffect,
this method is physically making the separation of
paths available for each satellite link. Terrestrial
telecommunicationnetworkscan use separate cables
535
or radio links, but on a single satellite independent
transmission paths are required. Thus, this MA
control radiates energy into space and transmission
can be on the same frequency: such as TDMA or
CDMAandondifferentfrequencies,suchasFDMA.
5.1 SpecialEffectsofSDMAinWirelessSystems
The
new technologies are recently implemented for
the Third Generation Wireless (3G) with Future
Enhancements. Some of the key enhancements to
wireless technology include SDMA, the introduction
of 3G wireless technologies and integration with
personal and mobile applications including
aeronautical. The SDMA method is a special system
access technology that allows
a single transmitter
locationtoprovidemultiplecommunicationchannels
by dividing the radio coverage into focused radio
beams that reuse the same frequency. To allow
multipleaccesses,eachmobileradioisassigned toa
focused radio beam. These radio beams may
dynamically change with the location of the mobile
radio.Analogously,
inreception,themobileantenna
picksupsignalscomingfromalldirections,including
noise and interference. These considerations have
lead to the development of the SDMA technique,
which is based on deriving and exploiting
information on the spatial position of mobile
terminals.Theuseofanadaptiveantennaarrayat
the
base station thus allows to introduce the SDMA
technique,whosemainadvantageisthecapabilityto
increase system capacity, i.e. the number of users it
can handle. This increase can be obtained in two
different ways, and therefore the following
applicationsarepossible:
1 Reduction in Co‐channel Interference –
The
reduction in the level of co‐channel interference
betweenthedifferentcellsusingthesamegroupof
radio channels is obtained, as above seen, by
minimizingthegaininthedirectionofinterfering
mobile units. This technique, indicated with the
acronym Spatial Filtering for Interference
Reduction (SFIR) allows to
reduce frequency re‐
usedistanceandclustersize.Inthisway,eachcell
can be assigned a higher number of channels by
phasearrayantenna,aspresentedinFigure2(A).
2 Spatial Orthogonality – In conventional access
techniques, orthogonality between signals
associated with different users is obtained by
transmittingthem
indifferentfrequencybandsof
FDMA, in different time slots of TDMA orusing
different code sequences of CDMA. Using an
antennaarray,itispossibletocreateanadditional
degree of orthogonality between signals
transmitted to and from different directions. It is
thuspossibletoassignthesamephysical
channel
to several mobile units, as depicted in Figure 22
(B),whentheanglesatwhichtheyareseenbythe
basestationaresufficientlyseparated.Theresultis
an increase in the number of available channels,
since the same physical channel, for example the
samecarrierinaFDMAor
thesametimeslotina
TDMA system, can be subdivided into multiple
spatial channels, each of which is assigned to a
differentuser. So,themultipleusersbelongingto
thesamecellusethesamechannel.
5.2 SpecialEffectsofSDMAinMobileRadioSystems
InaFDMAmobileradiosystem,howevercapacityis
limited by two different factors. On one hand, a
limited number of radio channels, carriers and time
slots, are available, and they must be subdivided
among beams making up a cluster. On the other
hand, co‐channel interference limits channel re‐use.
The SDMA technique allows
to expand both these
limitsandtoenhancesystemcapacity.
Figure3.HypotheticalScenarioofSDMAforMobileRadio
Applications
CourtesyofManual:“GlobalMobileCNS”byIlcev[3]
As already described, this can occur in two
different ways: with the SFIR technique and spatial
orthogonality. In such a way, interference level is
reduced and channel re‐use distance is decreased,
whereastheactualSDMAtechniqueassignsthesame
channel to multiple, spatially separated users. In
Figure 3 is illustrated
a SDMA system, which
diagram shows a single tower that is serving many
differentusersfromthesameradiotoweronthesame
frequencyusingindependentbeamsofradioenergy.
Inpracticethisisnotpossibletoachieve,becauseeach
type ofmobilesystem has different frequency band,
but they
can be seen in separate way. On the other
hand, the SDMA technique requires an array
composedofmoreantennas thantheSFIRtechnique
does. In fact, spatial orthogonality is exploited by
eliminating,throughtheuseofspatialfiltering,intra‐
cellco‐channelinterference,whichis
Differentlytosay,intraditional
radiosystemsthe
basestation,havingnoinformationonthepositionof
mobile units, is forced to radiate the signal in all
direction,inordertocovertheentireareaofthecell.
This entails both a waste of power and the
transmission, in the directions where there are no
mobile terminals to reach, of a signal which will be
seenasinterferingforco‐channelcells,i.e.thosecells
usingthesamegroupofradiochannels.
It must be noted that the term SDMA refers,
strictly speaking, only to the latter application, in
whichSDMAisactuallyaccomplished.Inspite
ofthis
fact,theSFIRtechniqueisalsoconsideredwithinthe
SDMA technique, since it is based on the same
principles.Inadditionto theopportunityto increase
systemcapacity,theSDMAtechniquehasadditional
characteristics making its introduction in a mobile
radio system advantageous. In particular, it is
possible
toexploitthehigherreceivegainofferedby
an antenna array with respect to an omnidirectional
case, to allow mobile units to transmit at reduced
power, and therefore lower consumption. At equal
power, gain can be exploited to extend beam size.
This is useful when it is necessary to cover vast
surfaceareas,typicallyruralareas,characterizedbya
536
low mobile radio traffic density, with a limited
numberofbasestations[01,03,06,12].
Figure4.SDMAforMobileSatelliteApplications
CourtesyofManual:“GlobalMobileCNS”byIlcev[3]
Figure 5. The Beam Patterns and Adaptive Antenna
ApplicationsforSDMA
CourtesyofPaper:“SmartAntennaApplicationforSatellite
CommunicationswithSDMA”byZaharov[12]
5.3 SpecialEffectsofSDMAinMobileSatelliteSystems
TheSDMAtechnologyhasbeensuccessfullyusedin
satellitecommunicationsforseveralyears.Asstated,
the SDMA technique can also be integrated with all
the different MA techniques in use, such as FDMA,
TDMA and CDMA, and therefore can be applied to
any mobile communication system. However, we
shallseethatthewaysinwhichtheSDMAtechnique
can be introduced and the advantages it provides
differdependingonthesystemunderconsideration.
As aforesaid described, modifications required to
realizetheSDMAtechniquearelimitedtothesatellite
array, and thus do
not involve mobile units.
However, this allows to introduce this technique in
existing mobile satellite systems, with no need to
modifytheircharacteristics.
Theabilitytorejectjammerofadaptivearrayscan
be ensured for performing SDMA mode where
several mobiles are allowed to share the same
classicalaccessina
cell,leadingtoacapacityincrease.
InFigure4isillustratedthissharingwithpossibility
through the use of adaptive beam forming and
interference rejection on the satellite uplink and
downlink communication for mobiles, which are
locatedatdifferentangularsectors.
In using SDMA, either FDMA or TDMA are
needed
to allow LES to roam in the same satellite
beamorfor polarizationto enter the repeater. Thus,
the frequency reuse technique of same frequency is
effectively a form of SDMA scheme, which depends
upon achieving adequate beam‐to‐beam and
polarizationisolation. Using this system reverse line
means that interference
may be a problem and the
capacityofthebatteryislimited.
Ontheotherhand, asingle satellite may achieve
spatialseparationbyusingbeamswithhorizontaland
vertical polarization or left‐hand and right‐hand
circularpolarization. This couldallowtwo beams to
coverthesameEarthsurface
area,beingseparatedby
the polarization. Thus, the satellite could also have
multiple beams using separate antennas or using a
single antenna with multiple feeds. For multiple
satellites, spatial separation can be achieved with
orbital longitude or latitude and for intersatellite
links,byusingdifferentplanes.Exceptforfrequency
reuse, this
system provides on‐board switching
techniques,which,inturn,enhancechannelcapacity.
Additionally, the use of narrow beams from the
satellite allows the Earth station to operate with
smaller antennas and so produce a higher power
density per unit area for a given transmitter power.
Therefore, through the careful use
of polarization,
beams (SDMA) or orthogonal (CDMA), the same
spectrum may be reused several times, with limited
interferenceamongusers.
The more detailed benefits of an SDMA system
includethefollowing:
1 Thenumberofcellsrequiredtocoveragivenarea
canbesubstantiallyreduced.
2 Interferencefromothersystems
andfromusersin
othercellsissignificantlyreduced.
3 Thedestructiveeffectsofmultipathsignals,copies
of the desired signal that have arrived at the
antenna after bouncing from objects between the
signal source and the antenna can often be
mitigated.
4 Channel reuse patterns of the systems can
be
significantly tighter because the average
interference resulting from co‐channel signals in
othercellsismarkedlyreduced.
5 Separate spatial channels can be created in each
cell on the same conventional channel. In other
words,intra‐cellreuseofconventionalchannelsis
possible.
6 TheSDMAstationradiatesmuchless
totalpower
than a conventional station. One result is a
reductionin network‐wide RF pollution.Another
isareductioninpoweramplifiersize.
7 Thedirectionofeachspatialchannelisknownand
canbeusedtoaccuratelyestablishthepositionof
thesignalsource.
8 The SDMA technique
is compatible with almost
anymodulationmethod,bandwidth,orfrequency
bandincludingGSM,PHP,DECT,IS‐54,IS‐95and
other formats. The SDMA solution can be
implemented with a broad range of array
geometryandantennatypes.
Another perspective of the realization of SDMA
systems is the application of smart
antenna arrays
with different levels of intelligence consisting in the
antenna array and digital processor. Since the
frequency of transmission for satellite
communicationsishighenough(mostly6or14GHz),
that the dimensions of an array placed inorbit is
commensurablewiththedimensionsoftheparabolic
antenna,is
anecessaryconditiontoputsuchsystems
intoorbit.
537
Thus, the SDMA scheme mostly responds to the
demands of LEO and MEO constellations, when the
signals of users achieve the satellite antenna under
differentangles(±22ofortheMEO).Inthisinstance,
groundlevelmaybesplitintothenumberofzonesof
service coverage determined by switched multiple
beampatternlobesindifferentsatellitedetections,or
byadaptiveantennaseparations,illustratedinFigure
5 (A). Thus, there are two different beam‐forming
approaches in SDMA satellite communications:
(1)The multiple spot beam antennas are the
fundamental way of applying SDMA in large fixed
and mobile satellite systems and
(2) Adaptive array
antennasdynamicallyadapttothenumberofusers.
5.4 SwitchedSpotBeamAntenna
SwitchedMulti‐BeamAntennasaredesignedtotrack
each subscriber of a given cell with an individual
beam pattern as the target subscriber moves within
the cell (spot). Therefore, it is possible to use array
antennasandtocreateagroupofoverlappingbeams
thattogetherresultinomnidirectionalcoverage.This
is the simplest technique comprising only a basic
switching function between separate directive
antennasorpredefinedbeamsofanarray.
Beam‐switching algorithms and RF signal‐
processing software are incorporated into smart
antenna designs.
For each call, software algorithms
determinethebeamsthatmaintainthehighestquality
signal and the system continuously updates beam
selection,ensuringthatcustomersgetoptimalquality
for the duration of their call.One might design
overlapping beam patterns pointing in slightly
different directions, similar to the ones shown in
Figure
5(A).
Every so often, the system scans the outputs of
each beam and selects the beam with the largest
output power. The black cells reuse the frequencies
currentlyassignedtothemobileterminals,sotheyare
potentialsourcesofinterference.Infact,theuseofa
narrow beam reduces
the number of interfering
sources seen at the base station. Namely, as the
mobilemoves,thesmartantennasystemcontinuously
monitors the signal quality to determine when a
particularbeamshouldbeselected.
Switched‐beam antennas are normally used only
for the reception of signals, since there can be
ambiguityin
thesystem’sperceptionofthelocationof
the received signal. In fact, these antennas give the
bestperformance,usuallyintermsofreceivedpower
but they also suppress interference arriving from
directions away from the active antenna beam’s
centre,becauseofthehigherdirectivity,comparedto
a conventional antenna, some
gain is achieved. In
high‐interference areas, switched‐beam antennas are
further limited since their pattern is fixedand they
lacktheabilitytoadaptivelyrejectinterference.Such
anantennawillbeeasiertoimplementinexistingcell
structures than the more sophisticated adaptive
arraysbutitgivesonlylimitedimprovement.
5.5 AdaptiveArrayAntennaSystems
Adaptive Array Antenna Systems select one beam
patternforeachuseroutofanumberofpresetfixed
beam patterns, depending on the location of the
subscribers. At all events, these systems continually
monitortheir coverage areas,attempting toadaptto
their changing radio environment,
which consists in
(often mobile) users and interferers. Thus, in the
simplest scenario, that of a single user and no
interferers,thesystemadaptstotheuser’smotionby
providing an effective antenna system pattern that
followsthemobileuser,alwaysprovidingmaximum
gain in the user’s direction. The principle of
SDMA
with adaptive antenna system application is quite
different from the beam‐forming approaches
describedinFigure5(B).
The events processed in SDMA adaptive array
antennasystemsareasfollows:
1 A “Snapshot”, or sample, is taken of the
transmission signals coming from all of the
antennaelements,converted
intodigitalformand
storedinmemory.
2 The SDMAdigital processor analyzes the sample
to estimate the radio environment at this point,
identifying users and interferers and their
locations.
3 The processor calculates the combining strategy
fortheantennasignalsthatoptimallyrecoversthe
user’ssignals.Withthisstrategy,each
user’ssignal
isreceivedwithasmuchgainaspossibleandwith
theotherusers/interfererssignalsrejectedasmuch
aspossible.
4 Ananalogouscalculationisdonetoallowspatially
selectivetransmissionfrom thearray. Eachuser’s
signal is now effectively delivered through a
separatespatialchannel.
5 The system
now has the ability to both transmit
and receive information on each of the spatial
channels,makingthemtwo‐waychannels[01,03,
08,09,12].
Figure6. Block Diagram of SDMA/FDMA and
SDMA/SS/FDMA‐SDMA/SS/TDMA
CourtesyofManual:“GlobalMobileCNS”byIlcev[3]
As a result, the SDMA adaptive array antenna
system can create a number of two‐way spatial
channels on a single conventional channel, be it
frequency, time, or code. Of course, each of these
spatialchannelsenjoysthefullgainandinterference
rejectioncapabilitiesof theantennaarray. In theory,
an
antenna array with (n) elements can support (n)
spatialchannelsperconventionalchannel.Inpractice,
the number is somewhat less because the received
multipath signals, which can be combined to direct
received signals, takes place. In addition, by using
specialalgorithmsandspacediversitytechniques,the
radiationpatterncanbe
adaptedtoreceivemultipath
signals, which can be combined. Hence, these
538
techniques will maximize the SIR or Signal‐to‐
InterferenceandNoiseRatio(SINR).
5.6 SDMA/FDMA
This modulation arrangement uses filters and fixed
links within the satellite transceiver to route an
incoming uplink frequency to a particular downlink
transmissionantenna,showninFigure6 (A).Abasic
configuration of fixed links
may be set up using a
switch that is selected only occasionally. Thus, an
alternative solution allows the filter to be switched
using a switch matrix, which is controlled by a
command link. Because of the term SS (Switching
Satellite) this scheme would be classified as
SDMA/SS/FDMA, which block diagram
is shown in
Figure6 (B). Thesatellite switchesare changed only
rarely, only when it is desired to reconfigure the
satellite, to take account of possible traffic changes.
Themaindisadvantageofthissolutionistheneedfor
filters,whichincreasethemassofthepayload.
5.7 SDMA/TDMA
This solution
is similar to the previously explained
SDMA/SS/FDMA in that a switch system allows a
TDMA receiver to be connected to a single beam.
Switching again is only carried out when it is
required to reconfigure the satellite. Under normal
conditions,alink between beampairsis maintained
andoperatedunder
TDMAconditions.
Theutilizationoftimeslotsmaybearrangedonan
organized or contention basis and switching is
achieved by using the RF signal. Thus, on board
processingislikelytobeusedinthefuture,allowing
switchingtotakeplacebytheutilizationofbaseband
signals. The signal could
be restored in quality and
evenstoredtoallowtransmissioninanewtimeslot
intheoutgoingTDMAframe.
This scheme is providing up and downlinks for
the later Intelsat VI spacecraft, known as
SDMA/SS/TDMA, which block diagram is shown in
Figure6(B).ThisMAisusingto
allowTDMAtraffic
from the uplink beams to be switched to downlink
beams during the course of TDMA frame. At this
point,theconnection existsata specifictimefor the
burstdurationwithintheframetimebeforethenext
connectionismade,andsoon.
5.8 SDMA/CDMA
This arrangement allows
access to a common
frequencybandandmaybeusedtoprovidetheMA
to the satellite,wheneachstreamis decoded on the
satelliteinorder to obtainthe destinationaddresses.
Thus, on‐board circuitry must be capable of
determining different destination addresses, which
may arrive simultaneously, while also
denying
invalid users access to the downlink. However, on‐
board processors allow the CDMA bit stream to be
retimed, regenerated and stored on the satellite.
Because of this possibility the downlink CDMA
configurationsneednotbethesameasforuplinkand
theEarthlinkmaythus,beoptimized[01,03,
08,10].
6 RANDOMDIVISIONMULTIPLEACCESS
(RDMA)
For data transmission, a bit stream may be sent
continuouslyoveranestablishedchannelwithoutthe
need to provide addresses or unique words if the
channel is not charred. In fact, where charring is
implemented, data are sent in bursts, which thus,
requires unique words or synchronization signals to
enabletime‐sharingwithotherusers,tobeaffectedin
the division of channels. Each burst may consist in
one or more packets comprising data from one or
more sources that have been assembled over time,
processed and made ready for transmission.
However,
thistypeofmultiplexschemeisalsoknown
as Packet MA. Packet access can be used in special
RDMA solutions, such as Aloha, where
retransmissionofblockedpacketsmayberequired.
Randomaccesscanbeachievedtothesatellitelink
by contention and for that reason is called a
contentionaccess
scheme.Thistypeofaccessiswell‐
suitedtosatellitenetworkscontainingalargenumber
ofstations,suchasSES,whereeachstationisrequired
totransmitshortrandomly‐generatedmessageswith
long deadtimesbetween messages. The principle of
RDMA is to permit the transmission of messages
almost without
restriction, in the form of limited
duration bursts, which occupy all the bandwidth of
the transmission channel. Therefore, in other words,
this is MA with time division and random
transmission and an attribute for the synonym
RandomDivisionMultipleAccessisquiteassessable.
Ausertransmitsamessageirrespectiveofthe
fact
that theremay be other users equally in connection.
The probability of collisions between bursts at the
satellite is accepted, causing the data to be blocked
fromreceiptbytheEarthstation.Incaseofcollision,
the destination Earth station receiver will be
confronted with interference noise, which can
compromise message identification and
retransmission after a random delay period. The
retransmissionscanoccurasmanytimesasprobably
are carried out, using random time delays. Such a
scheme implies that the transmitter vies for satellite
resources on a per‐demand basis and no other
transmitterisattemptingtoaccessthe
sameresources
duringthetransmissionburstperiod,whenanerror‐
free transmission can occur. The types of random
protocolsaredistinguishedbythemeansprovidedto
overcome this disadvantage, which performance is
measured in terms of the throughput and the mean
transmission delay. Throughput is the ratio of the
volume
of traffic delivered at the destination to the
maximumcapacityofthetransmissionchannels.The
transmissiontime,i.e.,delayisarandomvariable.Its
mean value indicates the mean time between the
generationofamessage and itscorrectreception by
thedestinationstation.
6.1 Aloha
The most widely used contention
access scheme is
Aloha and its associated derivatives. This solution
wasdevelopedinthelate1960sbytheUniversityof
Hawaii and allows usage of small and inexpensive
Earthstations(includingSES)tocommunicatewitha
539
minimum of protocols and no network supervision.
This is the simplest mode of operation, which time‐
sharesasingleRF,dividedamongmultipleusersand
consists in stations randomly accessing a particular
resource that is used to transmit packets. When an
Aloha station has something to transmit, it
immediately sends
a burst of data pulses and can
detect whether its transmission has been correctly
received at the satellite by either monitoring the
retransmission from the satellite or by receiving an
acknowledgementmessagefromthereceivingparty.
Should a collision with another transmitting station
occur,resultingintheincorrectreceptionof
apacket
at the satellite, the transmitting station waits for a
random period of time, prior to retransmitting the
packet.Otherwise,aremotestation(SES)usesAloha
togetahubstation(CES)terminal’sattention.
Namely, the SES terminal sends a brief burst
requestinga frequencyortimeslotassignmentfor
the
maintransmission.Thus,oncetheassignmentforSES
is made, there is no further need for the Aloha
channel,whichbecomesavailableforotherstationsto
use.Afterthat,themaintransmissionsarethenmade
on the assigned channels. At the end, the Aloha
channel might be used again
to drop the main
channel assignments after the transmission is
completed. The advantages of Aloha are the lack of
any centralized control, giving simple, low‐cost
stations and the ability to transmit at any time,
withouthavingtoconsiderotherusers.
In the case where the user population is
homogenous,so
thatthepacketdurationandmessage
generationrateareconstant,itcanbeshownthatthe
traffic carried S (packet correctly interpreted by the
receiver),asafunctionoftotaltrafficG(originaland
retransmittedmessage)isgivenbytherelation:
S=Gexp(–2G)[packet/timeslot] (8)
where (S = transmission throughput) and (G) are
expressedasanumberofpacketspertimeslotequal
to the common packet duration. The Aloha protocol
cannot exceed a throughput of 18% and the mean
transmissiontimeincreasesveryrapidlyasthetraffic
increases due to an increasing number of
collisions
and packet retransmissions. The Aloha mode is
relativelyinefficient withamaximum throughput of
only 18.4% (1/2). However, this has to be counter‐
weight against the gains in simple network
complexity,sinceno‐coordination orcomplex timing
propertiesarerequiredatthetransmittingSES.
6.2 SlottedAloha
This form of
Aloha or S‐Aloha, where the time
domain is divided into slots equivalent to a single
packetbursttime;therewillbenooverlap, asisthe
case with ordinary Aloha. The transmissions from
differentstationsarenowsynchronizedinsuchaway
that packets are located at the satellite
in time slots
defined by the network clocks and equal to the
common packet duration. Hence, there cannot be
partialcollisions;everycollisionarisesfromcomplete
superposition of packets. In effect, the timescale of
collisionisthus,reducedtothedurationofapacket,
whereas with the Aloha protocol, this timescale
is
equal to the duration of both packets. At this point,
this situation divides the probability of collision by
twoandthethroughputbecomes:
S=Gexp(–G)[packet/timeslot] (9)
This protocol enables collisions between new
messages and retransmission to be avoided and
increases the throughput of
S‐Aloha in the order of
50–60% by introducing a frame structure, which
permits the numbering of time slots. Each packet
incorporates additional information indicating the
slot number reserved for retransmission in case of
collision. For the same value of utilization as basic
Aloha, the time delay and probability
of packet loss
are both improved. The major disadvantages of S‐
AlohaarethatmorecomplexequipmentintheEarth
station is necessary, because of the timing
requirement and because there are fixed time slots,
customerswithasmalltransmissionrequirementare
wastingcapacitybynotusingthetimeslotto
itsfull
availability.
6.3 SlotReservationAloha
This solution of an extension for the slotted‐Aloha
scheme allows time slots to be reserved for
transmissionbyanEarthstation.Ingeneraltermsthis
mode of operation is termed a Packed Reserved
Multiple Access (PRMA). Slot reservation basically
takestwoforms:
1 Implicit – When a station acquires a slot and
successfullytransmits,theslotisreservedforthat
station for as long as it takes the station to
completeits transmission.The network controller
then informs all stations on the network that the
slotisavailableforcontentiononcemore.There
is
onlytheproblemthatastationwithmuchdatato
transmitcouldblockthesystemtootherusers.
2 Explicit – Every user station may send a request
for the reservation of a time slot prior to
transmission of data. A record of all time slot
occupation and reservation
requests is kept.
Actually, a free time slot could be allocated on a
priority basis. Some kind of control for the
reservationofslotsisnecessaryandthiscouldbe
accomplished by a single or all stations being
informed of slot occupancy and reservation
requests[01,03,06,08,09,11].
7 CONCLUSION
The performances and capacities of MMSC for
CDMA, FDMA and TDMA/FDMA have been
analyzedmanyyearsagoforanL/C‐bandRFnetwork
with global coverage. For the particular MMSC
systems under discussion and for the particular
antennaconfigurations,bothCDMAandFDMAoffer
similar performance, FDMA yielding
slightly higher
channel capacities at the design point and CDMA
being slightly better at higher EIRP levels. As the
MMSC system grows and the antenna beam size
decreases,CDMAappearstobe a very efficientMA
system,becauseitisnotlimitedbyL‐bandbandwidth
540
constraints. However, CDMA is wasteful in
feederlink bandwidth, and the choice of a multiple
access system must take all parameters into
consideration,suchasoscillatorstability,interference
rejection, system complexity etc. as well as system
cost before deciding on a particular multiple access
system.
The communication satellites for MMSC provide
multiple‐beamantennasandemployfrequencyreuse
of the allocated L‐band frequency spectrum. It
appears that despite the fact that FDMA and
FDMA/TDMA are orthogonal systems, they
nevertheless suffer from bandwidth limitations and
sensitivity to interbeam interference in L‐band. The
CDMA scheme is better at absorbing Doppler and
multipath effects, and it permits higher rate coding,
butitsuffersfromself‐jammingandfrombandwidth
constraints in the feederlink. In general, all three
multipleaccesssystemsshowsimilarperformance.
However,atthechosendesignpointforaggregate
EIRP, number of beams, and allocated bandwidth,
FDMA provides still the highest
system channel
capacity. Recently is developed SDMA as an
advancedsolutionwhereallconcernedSESterminals
cansharethesamefrequencyatthesametimewithin
aseparatespaceavailableforeachlink.Ontheother
hand,theRDMAschemeissuitableforlargenumber
of users in MMSC, where
all SES terminals share
asynchronously the same transponder by randomly
transmitting short burst or packet divisions. In
additionisdevelopedseveralmobileAlohamethods,
whichsuccessfullyincreasethesystemthroughout.
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