DVB-S2 - BCH and LDPC coding fixed codeword length (modulation toolkit)

[diablo_17_bj]

Greetings. 

I'm doing project in LabVIEW on subject "DVB-S2 communication system modeling in LabVIEW". The system specification says that DVB-S2 information frame must have fixed length of 64800 bits (long frame) or 16200 bits (short frame) after BCH + LDPC channel coding. I use modulation toolkit's LDPC and BCH encoders, but i just can't get fixed block length, the block is always wider than 64800 (16200) bits for code rates different than 1/2 (LDPC and BCH coding parameters are attached.)

Does anybody know how to fix this problem? Thanks in advance.

Cheers 

[Esteban_B]

Hi Diablo_17_bj

Would you post a small version of your code where this behavior could be replicated?

Regards,

[diablo_17_bj]

I have an LabVIEW VI if that's what you mean...?

[RavensFan]

Yes.  Your VI is your code.

[diablo_17_bj]

Sure, here it is. My LabVIEW version is 2010. Thank you.

[diablo_17_bj]

Can you please help me? I don't have much time.

Thank you. 

[Esteban_B]

Hi!,

I tried to open the vi but it's asknig me for a MT BCH Decoder (Systematic).vi

[diablo_17_bj]

It is asking you for this VI because you need to have Modulation Toolkit installed, because the VI is part of it.

[lis@jiang]

Hi diablo_17_bj,

What are the n, k, t values you're looking to use? It looks like your final array size is dependent upon these options.

Thanks,
Lisa Jiang

National Instruments

Applications Engineering

[diablo_17_bj]

"k" is number of information bits on encoder input, "n" is overall number of bits on output, and "t" is number of bits that can be corrected by encoder.

The main problem is that "n" must be in form of 2^m-1, and it can't be defined as in DVB-S2 encoder parameters table. LDPC encoder's output must always give same number of bits - 16200 for short data block or 64800 for normal data block. When I'm changing code rate for LDPC encoder, LDPC output is always bigger than it should be.

I think that LDPC encoder is not working properly because it doesn't give correct output which is defined by number of rows / number of columns, and that's the code rate.