[linux-dvb] Some unscrambled data in DVBS-2

Manu Abraham abraham.manu at gmail.com
Wed Apr 4 15:28:41 CEST 2007


Richard Mayo wrote:
> Manu Abraham wrote:
>> Richard Mayo wrote:
>>   
>>> I am trying to find out about a feature of DVBS-2, the 90 symbols of
>>> un-scrambled data, Physical Layer Signalling (PLS) code, which occurs a
>>> regular intervals. Is the content constant or predictable? 
>>>     
>>
>> PLSCODE aka Pilot tones are used to identify the Physical Layer params.
>> This is part of the DVB-S2 specification at the Physical Layer, nothing
>> abnormal. Some demods allow you to extract the Header, while some do
>> not. So it depends a lot on which demodulator you are using. Some handle
>> it in firmware while some do it in the driver.
>>
>> The PLPARMS cannot be scrambled, since it contains very basic
>> information. According to the specification, the MODCOD can change at
>> any given instance of time. This information is present in the Physical
>> Layer. So therefore a lot will depend on how constant your MODCOD is "at
>> the least".
>>
>>
>> Manu
>>
>>
>>   
> Thanks Manu,
>     I'm not demodulating the signal, I just wish to identify it with a
> receiver with very poor S/N. So what information does it contain?


The PLHEADER is used for receiver synchronization and PL signalling.

According 302 307,

The PLHEADER (one slot of 90 symbols) constitutes of:

SOF (26 symbols), identifying the Start of Frame.

PLS code (64 symbol): PLS (Physical Layer Signalling) code shall be a
non-systematic binary code of length
64 and dimension 7 with minimum distance dmin = 32. It is equivalent to
the first order Reed-Muller under
permutation. It transmits 7 bits for physical layer signalling purpose.

These 7 bits consists of two fields:
MODCOD and TYPE defined as follows:

MODCOD (5 symbols), identifying the XFECFRAME modulation and FEC rate;

TYPE (2 symbols), identifying the FECFRAME length (64 800 bits or 16 200
bits) and the
presence/absence of pilots.
The PLHEADER, represented by the binary sequence (y1, y2,...y90) shall
be modulated into 90 π/2 BPSK symbols


Manu




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