Manu Abraham wrote:
Johannes Stezenbach wrote:
Well, since the LP stream is a DVB-S2 stream it could in theory
be either a SIS or a MIS stream (except that figure F.1
only shows a single input to the DVB-S2 outer & inner coding
block).
However, I think that MIS/stream_id filter is not used to
seperate the HP and LP stream.
Stream ID filter is described (somewhere that) IS is used for the
PLFrame to be demodulated.
The points that which i am a bit confused was this:
1) The presence of the S2 satellite delivery system descriptor is not
always, it appears in some cases and in some cases it doesn't.
2) Now, BBHEADER contains the info that is supposed to be there in the
S2 satellite delivery system descriptor
except for
(a) Backwards Compatibilty Indicator (This one is obvious since in NBC
mode, there is no Backward Compatibility)
(b) Scrambling sequence indicator
Whether it is in NBC or BC mode descrambling has to be done. For the
STB0899 it is fine , since index, n=0 is default, but in another case
the index can be possibly be changed, but in that case i don't
understand from where i can change the Scrambling Sequence Index in NBC-BS
Ok, got some more clarity
According to the patent,
" This application claims the benefit of the earlier filing date under
35 U.S.C. .sctn. 119(e) of U.S. Provisional Application Ser. No.
60/583,410 filed Jun. 28, 2004, entitled "Scrambling of Physical Layer
Header and Pilot Symbol in DVB-S2 to Reduce Co-Channel Interference,"
and U.S. Provisional Application Ser. No. 60/585,654 filed Jul. 6, 2004,
entitled "Scrambling of Physical Layer Header and Pilot Symbol in DVB-S2
to Reduce Co-Channel Interference"; the entities of which are
incorporated herein by reference."
According to one embodiment of the present invention, different
initialization seeds are supplied to a Gold sequence generator for each
of the co-channels to produce the different scrambling sequences. The
above arrangement advantageously reduces the impact of co-channel
interference, thereby enhancing receiver performance.
This truly matches to the scrambling_sequence_index in S2 satellite
delivery system descriptor.
Now, the confusion was that the S2_satellite_delivery_system_descriptor
is not always present. I really got lost as to what to do in such a
case. Initially i randomly went into a wild conclusion that it can be
possible only from the Pilot Symbols otherwise, but was not very sure
about the same.
Now, myself and Christoph Pfister argued on IRC, how this could possibly
work.
It eventually turned out to be like this .. From TR 102 376 p 71,
"The differential detection scheme described above using information
embedded in the PLSCODE is therefore preferred. In fact as long as the
difference, not necessarily a constant, is known a /priori/, receivers
can take advantage of it. For this reason we are able to further
scramble the codeword of PLSCODE to improve the auto-correlation
property. The specific sequence is as follows: 0111000 ................
..........010
The scrambling sequence is essentially just an extended m-sequence.
In Annex "C", the algorithms for rapid frame synchronization by
utilizing SOF and PLSCODE are described.
So it finally turns out that "masochistic" devices can do "auto-n"
sequences. In such a case, wondering "now" whether all DVB-S2 demods are
powerful to do this ..
Thanks to Christoph for helping to clear this confusion. I thought it
would be helpful to others as well as to how it might work, hence the post.
Manu
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