134 135TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 06-07-08/2012 — www.TELE-satellite.com www.TELE-satellite.com — 06-07-08/2012 — TELE-satellite International — 全球发行量最大的数字电视杂志

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H-8PSK Hierarchical Modulation

FEATURE Satellite Transmission Technology

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136 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 06-07-08/2012 — www.TELE-satellite.com

Hide the SD inside the HDJacek Pawlowski

FEATURE

If you examine satellite charts, you will see that DVB-S2 transponders usually use 8PSK or QPSK modula-tion. However these two modulations are not the only possibilities the DVB-S2 standard specifies. Except for the four basic modulation schemes: QPSK, 8PSK, 16APSK, 32APSK, the norm gives you other interesting op-tions. In another Feature story in TELE-satellite we report on the Vari-able & Adaptive Coding and Modula-tion (VCM/ACM). In this article, we would like to discuss yet another pos-sibility. It is called H-8PSK and it is a form of a hierarchical modulation.

In the classical DVB-S, one tran-sponder is able to transmit just one transport stream. Of course, there can be a number of TV and radio pro-grams multiplexed in one transport stream. However, everything in this transport stream is transmitted with one modulation and one forward er-ror correction (FEC) setting, for ex-ample: QPSK 3/4.

H-8PSK is a method to transmit two transport streams in one trans-mission channel and thus by one transponder. These two transport streams are modulated in different ways onto the radio frequency car-rier and may have quite different FEC values. The transport streams are designated as the high priority (HP) stream and the low priority (LP) stream. H-8PSK is a rather unique concept in digital TV as it provides backward compatibility what is not so common in digital world. Namely, every DVB-S receiver should be able to receive a HP stream. Only the re-ception of the LP stream requires a H8-PSK compatible DVB-S2 receiver. How does it work?

Let’s start with a reminder of a classical QPSK modulation. Figure 1 shows an unmodulated RF carrier and in Figure 2 this carrier is QPSK modulated with all possible combina-tions of a symbol: 00, 10, 11 and 01.

Satellite Transmission Technology

A symbol in QPSK is simply 2 suc-cessive bits in a bit stream. So we have just four possibilities – as men-tioned above. For each symbol value, one specific phase shift is assigned. These are respectively: 45, 135, 225 and 315 degrees. Figure 3 shows the phase shift corresponding to the modulation shown in the example in Figure 2.

Your receiver converts the received phase shift and converts each phase shift to a pair of a bit. So the output of a QPSK demodulator supplied with

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Bit stream after QPSK demodulation

QPSK phase shift [deg]

QPSK modulated carrier

Unmodulated carrier

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138 139TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 06-07-08/2012 — www.TELE-satellite.com www.TELE-satellite.com — 06-07-08/2012 — TELE-satellite International — 全球发行量最大的数字电视杂志

stellation diagrams, it would look like Figure 11.

Now, a regular QPSK receiver should recognize the 8-PSK modula-tion as a somewhat noisy QPSK. Of course, this “noise” is caused by the deviations of θ° introduced by the LP component. Such a receiver will be able to extract only the HP bit stream.

An H-8PSK compatible receiver, af-ter decoding QPSK and learning from the information it contains that there is

in eight possible phase shifts in H-8PSK instead of four in QPSK - see Figure 7. In practice, θ should be something between 7-12°.

3 bits are assigned to every possi-ble phase shift in a H-8PSK constella-tion. Except for the 2 bits that create a 2-bit symbol like in QPSK, there is one additional bit shown in blue ink in Figure 7. The HP stream is still coded like in a QPSK modulation. For example, if we need to send a sym-bol 00 we use 45° but we additionally take into account one bit from the LP stream and depending on its value, we either increase or decrease 45° by θ°. As you can see, for every 2 bits of the HP stream, we send one bit of the LP stream.

Let’s examine an example in which our original QPSK stream is enhanced with LP stream. In our example, we increased the QPSK shifts by θ° for the first 2 symbols and decreased it by θ° for the successive 2 symbols. The result is shown in Figure 8. Fig-ure 9 and 10 show the HP and LP bit streams resulting from demodula-tion.

Should we wanted to present our H-8PSK example with a series of con-

the signal as in Figure 2 would look like the picture in Figure 4.

Almost always, the QPSK modula-tion is explained with a constellation diagram as the one shown in Figure 5.

The phase shift changes shown in Figure 3 could be represented as four vector positions presented in Figure 6.

Now H-8PSK is a modification of QPSK. Each of the phase shifts de-fined for QPSK are replaced with two

new values: greater by θ° and less by θ°. For example instead of 45°, we get 45°- θ° and 45°- θ°. This results

an LP stream available, will try to lock to the H-8PSK phase shifts instead of QPSK and it will extract both streams.

It is important to note that H-8PSK is different than the regular 8PSK. Regular 8PSK is presented in Figure 12. The phase shifts for 8PSK are different than those for H-8PSK. So a receiver compatible with a typi-cal DVB-S2 modulation: 8PSK, will not necessarily be able to decode H-8PSK. The LP stream has a twice lower bit rate than the HP stream. The ratio of useful bit rates between HP and LP streams may be not neces-sarily equal to 2:1 because they can have different FEC settings. Usually, the LP will need more error correc-tion than HP.

H-8PSK is also quite different from the VCM mentioned at the beginning of this article. In VCM, we have one bit stream transmitted with a given modulation, symbol rate and FEC for a period of time then another bit stream with (possibly) different modulation, symbol rate and FEC transmitted for another period of time, and so on. If

we have N bit streams, in the N+1 peri-od of time, the first bit stream appears again and so on. At any given moment, only one bit stream is transmitted. It is called “time multiplexing”. In H-8PSK, there are exactly two bit streams and both are transmitted simultaneously. They can differ in FEC.

There are already receivers (or tun-ers in the form of PC cards) available on the market that can demodulate H-8PSK. But what about real trans-missions? One of the satellite charts available on the Internet denoted three transponders on HOTBIRD 13° East as using H-8PSK modulation. The other charts did not confirm this information. So, we connected 3 dif-ferent signal analyzers that we had in our lab to an antenna aimed at HOT-BIRD to check how the constellation of those signals look. We have to say, that those signals were NOT H-8PSK modulated. We checked later a few more European satellites to see if anybody transmitted with H-8PSK but did not find any transponder. If you have a satellite signal analyzer capa-ble of showing signal constellation, it should be able to lock to H-8PSK as to normal QPSK with symbol rate equal to that of the HP stream. If the symbol rate is not known and your analyzer has no blind scan feature it might be time consuming for you to guess the true HP symbol rate. But once your analyzer is locked to the QPSK com-ponent of a H-8PSK signal, you should see a constellation like that in Figure 7 instead of that in Figure 5.

There is one interesting use for H-8PSK, which indeed reminds us about the VCM feature: providers could transmit a SD version of their program on the HP (high priority) stream together with a HD version on the LP (low pirority) stream. The provider would only need one single transponder for this and a regular DVB-S2 satellite receivers would get the SD version of program. Upgrad-ing to the HD version would only re-quire a H-8PSK capable receiver, all the rest (satellite transponder and satellite reception system at the re-ceiver) would remain identical as before. Program providers in North America are already conducting tests with 8-8PSK.

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Figure 7.

Figure 8.

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Figure 11.

Figure 12.

Low priority bit stream after H-8PSK demodulation

High priority bit stream after H-8PSK demodulation

H-8PSK phase shift