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Broadcast andServices

UNIT-V

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Syllabus

Broadcast: DBS-Orbital spacing's-powerratings-re!uency and polari"ation-transponder capacity-bit rate-#$%&-orward error correction( ODU-IDU-downlin)

analysis-uplin)-satellite *obile services:VS+T-&$S

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Broadcast servicesSatellites provide broadcast transmissions

in the fullest sense of the word, becauseantenna ootprints can be *ade to coverlarge areas o te eart( Te idea o using

satellites to provide direct trans*issionsinto te o*e as been around or *anyyears, and te services provided are )nowngenerally as direct broadcast satellite (DBS)

services.Broadcast services include audio,

television, and Internet services(

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Orbital spacing

It is )nown tat te orbital spacing is ./ orte ig power satellites, so ad0acentsatellite intererence is considerednone1istent(

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Orbital spacing234It sould be noted tat altoug te DBS

services are spaced by ./, clusters ofsatellites occupy

Te no*inal orbital positions( or e1a*ple,te ollowing satellites are located at55./6 longitude: %coStar VI, launced on

7uly 58, 3999%coStar IV, launced on #ay;, 5..; %coStar II, launcedSepte*ber59, 5..

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Power Rating and Numberof Transponders

ro* Table 5(8 it will be seen tat satellites pri*arilyintended or DBS ave a iger >%I?$@ tan or te otercategories, being in te range =5 to

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US-DBS syste*s

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Frequencies andPolarizationTe re!uencies or direct broadcast satellites vary

ro* region to region trougout te world, altoug

tese are generally in te u band( Table 5(8 sowstat or ig-power satellites, te pri*ary use o

wic is or DBS, te uplin) re!uency range is

5(Ato 5(; &E", and te downlin) range is 53(3 to53( &E"(

Te *ediu* power satellites listed in Table 5(8 also

operate in te u band at 58 to 58(= &E" uplin) and

55( to 53(3 &E" downlin)(Te pri*ary use o tese satellites, owever, is or

point-to-point applications, wit an allowedadditional use in te DBS service(

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Frequencies andPolarization(2)Te available bandwidt 2uplin) and downlin)4 is seen to

be =99#E"( + total nu*ber o A3 transponder cannels,eac o bandwidt 38 #E", can be acco**odated( Tebandwidt is so*eti*es speciCed as 3 #E", but tisincludes a A-#E" guard band allowance(

Tereore, wen calculating bit-rate capacity, te 38 #E"value is used(

Te total o A3 transponders re!uires te use o botrighthand

circular polarization (RC!) and left"hand circular

polarization2FEG$4 in order to per*it re!uency reuse,and guard bands are inserted between cannels o agiven polari"ation

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Transponder capacityTe 38-#E" bandwidt o a transponder is capable o

carrying one analog television cannel( To beco**ercially viable, DBS television also )nown as direct"to"home (D#) television re$uires many more cannels,and tis re!uires a *ove ro* analog to digital television(

Digiti"ing te audio and video co*ponents o a televisionprogra* allows signal compression to be applied% &hichgreatly reduces the band&idth re!uired(

Te signal co*pression used in DBS is a igly co*ple1process, and only a brie overview will be given ere o

te process( Beore doing tis, an esti*ate o te bit rate tat can be carried

in a 38-#E" transponder will be *ade(

The symbol rate that can be transmitted in a gienbandwidth is Rsym!"if#$%

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BIT ?+T%S O? DI&IT+F T%F%VISION

Te bit rate or digital television dependsvery *uc on te picture or*at( One wayo esti*ating te unco*pressed bit rate isto *ultiply te nu*ber o pi1els in a ra*e

by te nu*ber o ra*es per second, and*ultiply tis by te nu*ber o bits used toencode eac pi1el(

Te nu*ber o bits per pi1el depends on

te color dept per pi1el, or e1a*ple 5