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A short article on DTH(Direct to Home) . In direct-to-home (DTH) telecast, TV channels/programmes are directly distributed via satellite to the subscribers’ homes without the intervention of a cable operator.
  ENTERTAINMENT ELECTRONICS FOR YOUAUGUST 2003   ENTERTAINMENT GP. CAPT. K.C. BHASIN I n direct-to-home (DTH) telecast, TVchannels/programmes are directly dis-tributed via satellite to the subscrib-ers’ homes without the intervention of acable operator. The signals are transmit-ted in Ku band (10.7 GHz to 18 GHz) andare received by the subscribers through asmall dish antenna (about 45cm in dia.)and a set-top box (or an integrated re-ceiver decoder). The DTH system can also providemany value-added services such as theInternet, e-mail, data casting, e-commerce,and interactive multimedia. It has the pro-vision for a subscriber management sys-tem similar to the one for conditional ac-cess system (CAS). The current means of broadcasting in India don’t provide qual-ity reception in shadow areas, particularlyin the north-eastern region. The DTH canfill this void easily. All in all, DTH offersimmense opportunities to both the broad-casters and the viewers. The detailed guidelines for starting theDTH service in India were issued by the Direct-to-home (DTH) TV is allset to revolutionise the Indianentertainment scenario. Itpromises quality TV receptioneven in shadow areas andmany value-added servicessuch as the Internet andinteractive multimedia. Findout how the DTH systemworks, followed by differentarchitectures for DTHreception DIRECT-TO-HOME TV: TRANSMISSIONANDRECEPTION government on March 15,2001, followed by guidelinesin March 2003 for uplinkingof foreign-owned news chan-nels. So far only three compa-nies have applied for startingthe DTH broadcastingservice. These are Space TV, A.S.C. Enterprises, andEssel Shyam Communications.Essel Shyam hopes to up-link around 250 channels fromits teleport facility at Noida intwo to three years.Doordarshan is also planningto launch DTH television sat-ellite with Prasar Bharati allo-cating Rs 5000 million for theproject, says Ravi ShankarPrasad, Information & Broad-casting Minister. SESAT satellite from Eutelsat connecting East and West DTH receiver from Eutelsat   ENTERTAINMENT ELECTRONICS FOR YOUAUGUST 2003 The DVB core system Generally, a digital video broadcasting(DVB) system valid for all media carries aflexible combination of MPEG-2 video, au-dio, and data using the common MPEG-2transport stream multiplex. The commonservice information system gives detailsof the programmes. Modulation and addi-tional channel coding system, if any, arechosen to meet the requirements of differ-ent transmission media. A common scram-bling system and a common conditionalaccess interface are available.DVB-S is the DVB standard for satel-lite delivery. It is an extension of MPEG-2standard with specific instructions forimplementing the satellite links. This stan-dard is widely adopted. DVB-S transmission Many of the formats and transmission as-pects of satellite DVB services arestandardised by international bodies suchas the International Organisation forStandardisation (ISO) and the International Telecommunications Union (ITU). The standard developed by the Euro-pean Telecommunication StandardOrganisation (part of ITU) applies to Ku-band satellites operating at 11/12 GHz. Itis designed to provide quasi error-free(QEF) service at bit error rates (BERs) of 10 -10  to 10 -11 . By using a fairly robusterror-prevention scheme, which can bevaried depending on the channel environ-ment, it can provide this QEF rate to chan-nels with non-corrected error rates of 10 -1 to 10 -2 . The functional block diagram of aDVB-S channel is shown in Fig. 1. The MPEG encoder unit can take inseveral compressed video channels (in-cluding the programme audio and otherdigital data). All the data is compressed toproduce a single MPEG data block of 188bytes. Latest MPEG encoders can compresstogether up to ten regular video channels. The whole DVB-S system operates in thetime-division multiplexing (TDM) mode. The input data must be in 188-byte blockswith 1-byte sync word at the beginning. Energy dispersal.  The MPEG blocksare shuffled to improve the output spec-trum. Data coding.   A Reed Solomon code(204/188) is applied to the data. This cod-ing can correct up to eight errors. In it, 16bytes of overhead are added to the 188bytes from the MPEG encoder. On the re-ceive side, the Reed Solomon decoder cantake in data coming at a BER of approxi-mately 10 -4  and convert it into a BER of 10 -10 or lower. Interleaving.  The data is then option-ally Fornay interleaved (convolutional in-terleaving with depth 12). It is delimitedby occasional sync packets. On the re-ceive side, the interleaver provides a gainof approximately 3 dB. This enhances theability to correct burst errors that havebeen missed by the inner convolutionaldecoder. Inner code.    The data is thenconvolutionally coded depending on thetransponder size and channel quality de-sired. (Increasing the code rate reducesthe redundancy from the base rate. In-creasing the code rate increases the infor-mation rate and hence the error rate butreduces Eb/N0 requirements.) The basiccode rate is ½ with K=7. But this ratecan be increased by puncturing the codeat code rates of 2/3, 3/4, 5/6, 7/8, andothers. Each code rate is tried and thenlocked using the sync data. On the re-ceive side, the convolutional decoder cantake in a service quality of 10 -2  and im-prove it to an error rate of 10 -4 . Baseband pulse shaping.   Basebandpulses are then gray-coded and root-raisedcosine filtered. The roll-off rate is 0.35. QPSK modulation.  This single carrieris now quaternary phase shift keying(QPSK) modulated. The table shown below provides datarates and transponder sizes. This is anexample, and the parameters for individualsystems may vary. The code rate is dy-namically variable. So when the link isclean, the transmitter may be transmittingat a high code rate (less overhead). But if the link deteriorates, say, due to rainfall,the transmitter switches to a higher coderrate to provide the same BER. This meansthat the system must be designed to oper-ate in the worst condition. To guarantee acertain link quality, the system must pro-vide the highest Eb/N0 listed in the table. Characteristics of DVB-Scarriers Satellite access modes.    The multiple-chan-nel per carrier (MCPC) mode of operationis used for transmission of a high-rate mul-tiplex (typically, 38Mbps) comprising fourto eight standard definition digital televi-sion programmes. Usually, such transmis-sions use a dedicated (single-carrier) tran-sponder. The single-channel per carrier (SCPC) Fig. 1: Functional block diagram of a DVB-S channel Data Rates and Transponder Sizes TransponderQPSKCodedConvolutionalReedInformationEb/N0bandwidthsymbol ratebitratecode rateSolomonbitrate(dB)(MHz)(MS/s)(Mbps)code rate(Mbps) 24-2719.5395/11=0.45188/20416.304.001/2=0.50(=0.922)18.004.003/5=0.6021.604.502/3=0.6724.404.803/4=0.7527.005.004/5=0.8028.805.605/6=0.8330.006.207/8=0.8831.407.00  ENTERTAINMENT ELECTRONICS FOR YOUAUGUST 2003 mode of operation is used for transmis-sion of a medium-rate multiplex (typically,4-8Mbps), which usually comprises asingle digital television programme. Suchtransmissions use part of the satellitetransponder’s bandwidth and power. Inother words, the transponder is accessedin the multi-carrier mode, i.e. the tran-sponder resources are shared amongst sev-eral users (uplink carriers). The modulation, coding, and multiplex-ing structures of MCPC and SCPC carriersare identical and are as specified in theDVB-S standard. However, SCPC and MCPCtransmissions differ in transmission (sym-bol) rates; radiated power levels; frequencyaccuracy, frequency stability, and phasenoise performance of reception equipment;and ‘channel hopping’ response times. These may also employ different degrees of forward error correction (FEC). Symbol rates (refer the box).    Typi-cally,   MCPC transmissions have a symbolrate of 20 to 30 symbols per second (S/s), depending upon the bandwidth avail-able for transmission. A symbol rate of 27.5 MS/s is compatible with 33MHz sat-ellite transponder bandwidth and is themost commonly used. A symbol rate of 30 MS/s is compatible with 36MHz tran-sponder bandwidth. SCPC transmissionscan have a symbol rate as low as 3 MS/s,depending upon the amount of informa-tion carried, for example, one or moredigital TV programmes. The upper limiton the symbol rate extends beyond thelower limit for MCPC, which means thatthe equipment capable of receiving bothSCPC and MCPC carriers would need tobe able to process symbol rates over thefull 3-30MS/s range. Radiated power levels.   Effective iso-tropic radiated power (EIRP) is a measureof the signal strength that a satellite trans-mits towards the earth below. The EIRP ishighest at the centre of the beam and de-creases at angles away from the bore-sight.MCPC transmissions essentially em-ploy all of the available satellite tran-sponder power with little or no transpon-der ‘back-off’. (Transponder back-off means operation of the satellite’s high-power amplifier below its maximum out-put level in order to reduce the adverseeffects of channel non-linearities on thetransmission quality.)SCPC transmissions share the satellitetransponder with other SCPC transmissionsand consequently the EIRP assigned toeach carrier is significantly less than thetransponder’s maximum EIRP capability. The back-off per carrier is at least 5 dB The Bandwidth Available and the Symbol Rate The carrier on the satellite is made up of a sequence of pulses joined together to make acontinuous signal. Each pulse is a symbol. According to the modulation method, each symbolrepresents 1, 2, 3, etc bits of transmission rate data. In phase shift keying (PSK) modulation, each pulse is a burst of carrier signal with itssinewave-zero crossing point timing adjusted forwards or backwards in time to constitute aphase shift. Phase shifts of 180° apply in BPSK and 90° in QPSK. A phase shift of 90° representsa time shift of 1/4 of a full cycle of the sine wave. The closer the spacing phase shifts, themore difficult the distinction between them at the receive end. So for each higher-order PSKscheme, a higher carrier-to-noise ratio is required. As a general rule, if you have bandwidth to spare, use a lower-order modulation or a low-rate FEC (like 1/2 or 2/3) to spread out the signal. If you have power to spare, use a higher-order modulation and/or a higher-rate FEC (like 3/4 or 7/8).Ideally, you would want to use all of the available bandwidth and power simultaneously. Ifyou use larger receive dishes, you will always be able to increase the system capacity. If you aredoing a point-to-point link, it is worth using larger dishes. If you have thousands of receivedishes, the aggregate cost of these is significant and you will want to allow smaller sizes eventhough this reduces system capacity and increases space segment costs.FEC is applied to the customer’s information data at the transmit end, so transmissiondata rate = customer information rate x 1/FEC rate FEC rate is typically 0.5 to 0.9, so the transmission data rate is always significantly higherthan the customer information rate. The symbol rate is related to other quantities as per thefollowing relationship: where SR is the symbol rate, DR is the data rate (or the customer information rate), CRv is theViterbi FEC code rate (typically, 1/2, 2/3, 3/4, 5/6, or 7/8), CRrs is the Reed Soloman FEC coderate (typically, 168/204), and ‘m’ is the modulation factor or transmission rate bits per symbol(BPSK=1, QPSK=2, 8PSK=3, etc). On a spectrum analyser, the 3dB bandwidth is approximately the same as the symbol rate.SR =DRm x CRv x CRrs  ENTERTAINMENT ELECTRONICS FOR YOUAUGUST 2003 and is frequently much higher, dependingupon the number of SCPC transmissionsthat share the transponder. Consequently,for a given satellite coverage (EIRP), alarger antenna may be required to receivean SCPC transmission thanis required to receive aMCPC transmission. It isfor this reason that MCPCaccess is preferred for DTHapplications. Forward error correc- tion.   Both MCPC and SCPCtransmissions employ thefull range of FEC ratesspecified in the DVB-Sstandard (1/2, 2/3, 3/4, 5/6, and 7/8). MCPC trans-missions frequently employan FEC rate of 3/4 but arenot constrained to do so.SCPC signals are transmit-ted with a lower power andconsequently these oftenemploy a higher FEC rate(2/3 or 1/2) in order tominimise the size of the an-tenna needed for service recep-tion. Frequency stability and phase noise considerations.  The frequency stability andphase noise performance of outdoor reception systems de-signed for FM TV services maybe adequate for reception of MCPC digital TV transmissions.However, use of a digital-readylow-noise block converter(LNB) will guarantee receptionof all MCPC transmissions.When choosing a digital-readyLNB for SCPC reception, themost important parameters to be consid-ered are the initial frequency accuracy andthe temperature stability. The deviceshould be selected for the best initial fre-quency precision and the best tempera- Fig. 2: Functional block diagram of a typical Ku-band satellite transmit terminal ture stability (<±1 to ±2 MHz over theoperating temperature range). Channel hopping considerations.  Theresponse time during channel hopping canbe acceptably short with MCPC access, solong as the service information is deliv-ered at an adequate rate. The maximumdelay is likely to occur when switchingfrom one multiplex to another multiplex,which requires retuning of the receiver tothe new carrier frequency.With SCPC access, the response timecan be as high as 5 seconds. This is partlybecause the data transmission rate is muchlower than for MCPC transmissions, lead-ing to a lower rate of service informationtransfer for the same degree of overhead(percentage of the capacity allocated tothe service information). Switching be-tween different multiplexes, and hence re-ceiver retuning, will also occur more fre-quently, as SCPC transmissions will gen-erally carry only one or very few digital TV programmes. Spectrum inversion.  TheDVB-S standard specifies theuse of QPSK for transmissionvia satellite and stipulatesthe correct way to map bitsfrom the in-phase (I) andquadrature (Q) basebandbitstreams onto the fourphase states. Whilst mosttransmissions comply withthis mapping, some trans-missions are made with theI and Q data streams inter-changed. The result is ‘spec-trum inversion’, which af-fects both broadcasters andreceiver manufacturers.Spectrum inversion can bepresent regardless of the ac- Fig. 3: Antenna for the transmit terminalPanAmSat’s Castle Rock Teleport ground facility 
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