Examples of wireless communication systems

Examples of wireless communication systems

1. By,
Dr. D. Veera Vanitha,
Asso. Prof. /ECE,
School of Engineering,
Avinashilingam Institute for Home Science and Higher
Education for Women, Coimbatore

2. Pagingsystemsarecommunicationsystemsthatsendbriefmessages
toasubscriber
Dependingonthetypeofservice,themessagemaybeeithera
numericmessage,analphanumericmessageoravoicemessage
Pagingsystemsaretypicallyusedtonotifyasubscriberoftheneed
tocallaparticulartelephonenumberortraveltoaknownlocationto
receivefurtherinstruction
Inmodernpagingsystem,newsheadlines,stockquotationandfaxes
maybesent
Amessageissentapagingsubscriberviathepagingsystemaccess
number(usuallyatoll-freetelephonenumber)withatelephone
keypadormodem.Theissuedmessageiscalledapage

3. Thepagingsystemthentransmitsthepagethroughouttheservice
areausingbasestationwhichbroadcastthepageonaradiocarrier
Pagingsystemvarywidelyintheircomplexityandcoveragearea
Whilesimplepagingsystemsmaycoveralimitedrangeof2kmto
5km,ormayevenbeconfinedtowithinindividualbuildings,wide
areapagingsystemscanprovideworldwidecoverage
Thoughpagingreceiversaresimpleandinexpensive,the
transmissionsystemrequiredisquitesophisticated.
Wideareapagingsystemsconsistsofanetworkoftelephonelines,
manybasestationtransmittersandlargeradiotowersthat
simultaneouslybroadcastapagefromeachbasestationthisiscalled
simulcasting.

4. Simulcasttransmittersmaybe
locatedwithinthesameservice
areaorindifferentcitiesor
countries.
Pagingsystemsaredesignedto
providereliablecommunications
tosubscriberswherevertheare;
whetherinsideabuilding,driving
inahighway,orflyinginan
airplane.
Thisnecessitateslargetransmitter
power(ontheorderofkilowatts)
andlowdatarates(acoupleof
thousandbitspersecond)for
maximumcoveragefromeach
basestation.

5. Cordlesstelephonesystemsarefullduplexcommunicationsystems
thatuseradiotoconnectaportablehandsettoadedicatedbase
station,whichistheconnectedtoadedicatedtelephonelinewitha
specifictelephonenumberonthepublicswitchedtelephonenetwork
(PSTN)
Infirstgenerationcordlesstelephonesystem(manufacturedin
1980s),theportableunitcommunicatesonlytothededicatedbase
unitandonlyoverdistanceofafewtensofmeters
Earlycordlesstelephonesoperatesolelyasextensiontelephonestoa
transceiverconnectedtoasubscriberlineonthePSTNandare
primarilyforinhomeuse
Secondgenerationcordlesstelephoneshaverecentlybeenintroduced
whichallowsubscriberstousetheirhandsetsatmanyoutdoor
locationswithinurbancenters

6. Moderncordlesstelephonearesometimescombinedwithpaging
receiverssothatasubscribermayfirstbepagedandthenrespondto
thepageusingthecordlesstelephone
Cordlesstelephonesystemsprovidetheuserwithlimitedrangeand
mobility,asitisusuallynotpossibletomaintainacalliftheuser
travelsoutsidetherangeofthebasestation
Typicalsecondgenerationbasestationprovidecoveragerangesupto
afewhundredmeters

7. Acellulartelephonesystemprovidesawirelessconnectionstothe
PSTNforanyuserlocationwithintheradiorangeofthesystem
Cellularsystemsaccommodatealargenumberofusersoveralarge
geographicarea,withinalimitedfrequencyspectrum
Cellularradiosystemsprovidehighqualityservicethatisoften
comparabletothatofthelandlinetelephonesystems
Highcapacityisachievedbylimitingthecoverageofeachbase
stationtransmittertoasmallgeographicareacalledacellsothatthe
sameradiochannelsmaybereusedbyanotherbasestationlocated
somedistanceaway
Asophisticatedswitchingtechniquecalledahandoffenablesacallto
proceeduninterruptedwhentheusermovesfromonecelltoanother

8. Thediagramshowsabasiccellularsystemwhichconsistsofmobile
stations,basestationandamobileswitchingcenter(MSC)
Themobileswitchingcenterissometimescalledamobiletelephone
switchingoffice(MTSO)sinceitisresponsibleforconnectingall
mobilestothePSTNinacellularsystem

9. Eachmobilecommunicationsviaradiowithoneofthebasestation
andmaybehanded-offtoanynumberofbasestationthroughoutthe
durationofacall
Themobilestationcontainsatransceiver,anantenna,andcontrol
circuitry,andmaybemountedinavehicleorusedasaportable
hand-heldunit
Thebasestationsconsistsofseveraltransmittersandreceiverswhich
simultaneouslyhandlefullduplexcommunicationandgenerallyhave
towerswhichsupportsseveraltransmittingandreceivingantennas
Thebasestationservesasabridgebetweenallmobileusersinthe
cellandconnectsthesimultaneousmobilecallsviatelephonelinesor
microwavelinkstotheMSC
TheMSCcoordinatestheactivitiesofallofthebasestationsand
connectstheentirecellularsystemtothePSTN

10. AtypicalMSChandles100,000cellularsubscribersand5,000
simultaneousconversationsatatime,andaccommodatesallbilling
andsystemmaintenancefunctions,aswell
Inlargecities,severalMSCsareusedbyasinglecarrier
Communicationbetweenthebasestationandthemobilesisdefined
byastandardcommonairinterface(CAI)thatspecifiesfourdifferent
channels
Thechannelsusedforvoicetransmissionfromthebasestationtothe
mobilesarecalledforwardvoicechannel(FVC)andthechannels
usedforvoicetransmissionsfrommobilestothebasestationare
calledreversevoicechannel(RVC)

11. Thetwochannelsresponsibleforinitiatingmobilecallsarethe
forwardcontrolchannels(FCC)andreversecontrolchannels(RCC)
Controlchannelsareoftencalledsetupchannelsbecausetheyare
involvedinsettingupacallandmovingittoanunusedvoice
channel
Controlchannelstransmitandreceivedatamessagesthatcarrycall
initiationsandservicesrequest,andaremonitoredbymobileswhen
theydonothaveacallinprogress
Forwardcontrolchannelsalsoserveasbeaconswhichcontinually
broadcastallofthetrafficrequestsforallmobilesinthesystem
Thetowersrepresentsbasestationswhichprovideradioaccess
betweenmobileusersandthemobileswitchingcenter(MSC)
(Ref.:T.S.Rappaport,“WirelessCommunicationsPrinciplesand
Practice”,2
nd
Edition,)