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