Johannes Stezenbach wrote:
The following applies to general DVB-S2, not just to BC mode:
Well, EN 302 307 does not explain how it is possible for
the receiver to decode the PLHEADER without knowing the
modulation first. However, TR 102 376 annex B.1 does:
"Independent from the modulation scheme of the LDPC coded block that
follows, these fields are modulated by pi/2-BPSK modulation...".
So if the frame sync and PL configuration (MODCODE etc.) is
transmitted with a different, but fixed modulation scheme,
it is clear that the receiver can indeed figure it out
by itself, and if modcod changes on a by-frame basis the receiver
can still track these changes and receive all frames.
------------------------------------------
Back to BC mode:
EN 302 307 annex F show the PLHEADER is inserted before
passing the DVB-S2 signal to the hierarchical mapper.
TR 102 376 section 4.4.1 basically says the same.
So, it seems that for hierarchical BC mode PLHEADER does
not use pi/2-BPSK. Now, I don't know if and how the
receiver could figure out that hierarchical modulation
is used, however, once it knows that, it can get the
other transmission parameters from the PLHEADER.
Well, in BC-BS mode we have the S2 satellite delivery system descriptor
I will try to explain what i understood.
According to the patent
United States Patent Application: 20050089068
Kind Code: A1
Sun, Feng-Wen ; et al, April 28, 2005
Method and apparatus for providing signal acquisition and frame
synchronization in a hierarchical modulation scheme
An approach is provided for supporting signal acquisition and frame
synchronization in a digital broadcast system utilizing Low Density
Parity Check (LDPC) codes. Hierarchical modulation is utilized to
provide backward compatibility, whereby the lower layer signal is
encoded using LDPC coding. A signal is received, whereby the signal is
modulated according to the hierarchical modulation scheme including an
upper layer and a lower layer. The signal includes a data pattern and a
coded frame. The dependency of the received signal on the upper layer
modulation is removed. The modulation removed signal is correlated with
multiple predetermined data patterns to determine the data pattern of
the signal. The code rate of the coded frame is derived based on the
determined data pattern. The above arrangement is particularly suited to
a digital satellite broadcast system.
The LP DVB-S2-compliant signal is BCH and LDPC encoded, with LDPC code
rates 1/4, 1/3, 1/2 or 3/5. Then the hierarchical mapper generates the
non-uniform 8PSK constellation:
the two HP DVB-S bits (4 symbols) define a QPSK constellation point,
while the single bit (2 symbols) from the DVB-S2 LDPC encoder sets an
additional rotation ±? before transmission.
From the patent,
#22. A method for generating a signal in a communication system
supporting a hierarchical modulation scheme, the method comprising the
steps of: generating an upper layer data stream having a first framing
structure generating a lower layer data stream having a second framing
structure, wherein the first framing structure is not a multiple of the
second framing structure, a starting point of the lower layer data
stream corresponding only to a subset of upper layer positions and
outputting the signal, wherein the signal represents a constellation
point in the hierarchical modulation scheme associated with a
combination of the data streams.
#28. A method according to claim 22, further comprising the steps of:
modulating the upper layer data stream according to a first modulation
scheme and modulating the lower layer data stream according to a second
modulation, wherein the first modulation scheme is of a higher order
than the second modulation scheme.
#29. A method according to claim 28, wherein the first modulation scheme
is Quadrature Phase Shift Keying (QPSK) and the second modulation scheme
is Binary Phase Shift Keying (BPSK).
So according to Appendix F the hierarchial modulator does ?/2 BPSK
I have tried to explain through diagrams. The diagrams
in pdf format: http://www.thadathil.net/dvb/hmod/illustration.pdf
in compressed SVG format:
http://www.thadathil.net/dvb/hmod/illustration.svgz
If someone has issues with mime types, the directory view
http://www.thadathil.net/dvb/hmod/
To enable the maximum information throughput for DVB-S2 users, yet most
importantly while maintaining backwards compatible for existing DVB-S users.
Hierarchial Modulation provides a means of increasing the data
throughput of a modulation scheme.
High and Low priority Streams
The data streams of hierarchial modulation vary in susceptibility to
noise. The better protected data stream is the DVB-S service is the High
Priority stream. DVB-S2 is the Low priority stream.
In order to limit the signal degradation (degradation in linearity) upon
the High Priority Stream, it is indeed necessary to keep the Hierarchial
modulation phase angle "?" and hence the shift in the constellation
point as small as possible. This in turn limits the performance of the
Low priority stream.
This is the reason why broadcasters prefer not to go for Hierarchial
modulation.
While STB0899 does not seem to have a register for
enabling hieararchical BC mode, it still has a (read-only)
"low priority stream detection" bit in register TSULSTKM.)
Maybe this is a hint that it can detect hieararchical
BC mode by itself.
Well for descrambling if it can do auto "n", probably it might be able
to do this some way.
The shift is relative. Somewhere i read some demodulators might be able
train themselves. Well, could be that.
For the STB0899 the information might not be needed, but could be needed
by another demod some time later.
I think for the time being we can go with the minimalist approach as it
is in multiproto7
Manu
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