Pagina 139 - TELE-satellite-1207

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139 —
06-07-08/2012 —
TELE-satellite International —
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
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.
Figure 11.
Figure 12.