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Feature 8psk


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Feature 8psk

  1. 1. FEATURE Satellite Transmission Technology H-8PSK Hierarchical • backwards compatible to regular DVB-S2 Modulation • useful to pack SD and HD version of a tv program into one transponder • very sensitive to misalignment • may remain a niche technology134 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 06-07-08/2012 — — 06-07-08/2012 — TELE-satellite International — 全球发行量最大的数字电视杂志 135
  2. 2. FEATURE Satellite Transmission TechnologyHide the SD inside the HDJacek Pawlowski If you examine satellite charts, you ■ Figure 1.will see that DVB-S2 transponders Unmodulated carrierusually use 8PSK or QPSK modula-tion. However these two modulationsare not the only possibilities the DVB-S2 standard specifies. Except forthe four basic modulation schemes:QPSK, 8PSK, 16APSK, 32APSK, thenorm gives you other interesting op-tions. In another Feature story inTELE-satellite we report on the Vari-able & Adaptive Coding and Modula- ■ Figure 2.tion (VCM/ACM). In this article, we QPSK modulated carrierwould like to discuss yet another pos-sibility. It is called H-8PSK and it is aform of a hierarchical modulation. In the classical DVB-S, one tran-sponder is able to transmit just onetransport stream. Of course, therecan be a number of TV and radio pro-grams multiplexed in one transportstream. However, everything in this ■ Figure 3. QPSK phase shift [deg]transport stream is transmitted withone modulation and one forward er-ror correction (FEC) setting, for ex-ample: QPSK 3/4. H-8PSK is a method to transmittwo transport streams in one trans-mission channel and thus by onetransponder. These two transportstreams are modulated in differentways onto the radio frequency car- Bit stream after QPSK demodulation ■ Figure 4.rier and may have quite differentFEC values. The transport streamsare designated as the high priority(HP) stream and the low priority (LP)stream. H-8PSK is a rather uniqueconcept in digital TV as it provides A symbol in QPSK is simply 2 suc-backward compatibility what is not cessive bits in a bit stream. So we ■ Figure common in digital world. Namely, have just four possibilities – as men-every DVB-S receiver should be able tioned above. For each symbol value,to receive a HP stream. Only the re- one specific phase shift is assigned.ception of the LP stream requires a These are respectively: 45, 135, 225H8-PSK compatible DVB-S2 receiver. and 315 degrees. Figure 3 showsHow does it work? the phase shift corresponding to the modulation shown in the example in Let’s start with a reminder of a Figure 2.classical QPSK modulation. Figure1 shows an unmodulated RF carrier Your receiver converts the receivedand in Figure 2 this carrier is QPSK phase shift and converts each phasemodulated with all possible combina- shift to a pair of a bit. So the outputtions of a symbol: 00, 10, 11 and 01. of a QPSK demodulator supplied with136 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 06-07-08/2012 —
  3. 3. in eight possible phase shifts in H- we have N bit streams, in the N+1 peri- ■ Figure 6. 8PSK instead of four in QPSK - see ■ Figure 11. od of time, the first bit stream appears Figure 7. In practice, θ should be again and so on. At any given moment, something between 7-12°. only one bit stream is transmitted. It is called “time multiplexing”. In H-8PSK, 3 bits are assigned to every possi- there are exactly two bit streams and ble phase shift in a H-8PSK constella- both are transmitted simultaneously. tion. Except for the 2 bits that create They can differ in FEC. a 2-bit symbol like in QPSK, there is one additional bit shown in blue ink There are already receivers (or tun- in Figure 7. The HP stream is still ers in the form of PC cards) available coded like in a QPSK modulation. For on the market that can demodulate example, if we need to send a sym- H-8PSK. But what about real trans- bol 00 we use 45° but we additionally missions? One of the satellite charts take into account one bit from the LP available on the Internet denoted stream and depending on its value, three transponders on HOTBIRD 13° we either increase or decrease 45° East as using H-8PSK modulation. by θ°. As you can see, for every 2 The other charts did not confirm this bits of the HP stream, we send one information. So, we connected 3 dif- bit of the LP stream. ferent signal analyzers that we had in our lab to an antenna aimed at HOT- Let’s examine an example in which BIRD to check how the constellation our original QPSK stream is enhanced of those signals look. We have to say, with LP stream. In our example, we that those signals were NOT H-8PSK increased the QPSK shifts by θ° for modulated. We checked later a few the first 2 symbols and decreased it more European satellites to see if by θ° for the successive 2 symbols. anybody transmitted with H-8PSK but The result is shown in Figure 8. Fig- did not find any transponder. If you ure 9 and 10 show the HP and LP bit have a satellite signal analyzer capa- streams resulting from demodula- ble of showing signal constellation, it tion. should be able to lock to H-8PSK as tothe signal as in Figure 2 would look new values: greater by θ° and less stellation diagrams, it would look like an LP stream available, will try to lock normal QPSK with symbol rate equallike the picture in Figure 4. by θ°. For example instead of 45°, we Should we wanted to present our Figure 11. to the H-8PSK phase shifts instead of to that of the HP stream. If the symbol get 45°- θ° and 45°- θ°. This results H-8PSK example with a series of con- QPSK and it will extract both streams. rate is not known and your analyzer Almost always, the QPSK modula- Now, a regular QPSK receiver has no blind scan feature it might betion is explained with a constellation should recognize the 8-PSK modula- It is important to note that H-8PSK time consuming for you to guess the H-8PSK phase shift ■ Figure 8.diagram as the one shown in Figure 5. tion as a somewhat noisy QPSK. Of is different than the regular 8PSK. true HP symbol rate. But once your course, this “noise” is caused by the Regular 8PSK is presented in Figure analyzer is locked to the QPSK com- The phase shift changes shown in deviations of θ° introduced by the LP 12. The phase shifts for 8PSK are ponent of a H-8PSK signal, you shouldFigure 3 could be represented as four component. Such a receiver will be different than those for H-8PSK. So see a constellation like that in Figurevector positions presented in Figure 6. able to extract only the HP bit stream. a receiver compatible with a typi- 7 instead of that in Figure 5. cal DVB-S2 modulation: 8PSK, will Now H-8PSK is a modification of An H-8PSK compatible receiver, af- not necessarily be able to decode There is one interesting use forQPSK. Each of the phase shifts de- ter decoding QPSK and learning from H-8PSK. The LP stream has a twice H-8PSK, which indeed reminds usfined for QPSK are replaced with two the information it contains that there is lower bit rate than the HP stream. about the VCM feature: providers The ratio of useful bit rates between could transmit a SD version of their High priority bit stream after H-8PSK ■ Figure 9. HP and LP streams may be not neces- program on the HP (high priority) ■ Figure 7. ■ Figure 12. sarily equal to 2:1 because they can stream together with a HD version demodulation have different FEC settings. Usually, on the LP (low pirority) stream. The the LP will need more error correc- provider would only need one single tion than HP. transponder for this and a regular DVB-S2 satellite receivers would get H-8PSK is also quite different from the SD version of program. Upgrad- the VCM mentioned at the beginning ing to the HD version would only re- ■ Figure 10. of this article. In VCM, we have one quire a H-8PSK capable receiver, all Low priority bit stream after H-8PSK bit stream transmitted with a given the rest (satellite transponder and demodulation modulation, symbol rate and FEC for a satellite reception system at the re- period of time then another bit stream ceiver) would remain identical as with (possibly) different modulation, before. Program providers in North symbol rate and FEC transmitted for America are already conducting tests another period of time, and so on. If with 8-8PSK.138 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 06-07-08/2012 — — 06-07-08/2012 — TELE-satellite International — 全球发行量最大的数字电视杂志 139