CodewordLength for BCH encoder

Hi colleagues,

The bch-encoder (Communication Tool box, https://www.mathworks.com/help/comm/ref/comm.bchencoder-system-object.html) accepts two main integer parameters, CodewordLength and MessageLength which can not be arbitrary.

CodewordLength must satisfy 2^M-1 (2<M<17) and MessageLength can take on several possible values depending on error correction capability of the code.

In the same time, the DVB-S.2 demo (https://www.mathworks.com/help/comm/examples/dvb-s-2-link-including-ldpc-coding.html)

uses CodewordLength=43200 and MessageLength=43040 which are not compliant with the above mentioned requirements.

How is it possible?

Answer 1

[Chidvi Modala]

Hi,

Here are few points which may be of help to you

• According to the documentation of comm.BCHEncoder default value of M is 15. But in this case, the value of M (the degree of the primitive polynomial) is calculated using the following relation

M=ceil(log2(CodewordLength+1)).

• In addition to the above, "BCHPrimitivePoly" property of dvb object is set to a binary vector of length 17 which results in the value of M being 16.

• The documentation page for comm.BCHEncoder mentions that the codeword length must use the form (2^M-1) for full-length BCH codes. The one which we use in DVBS2 is shortened so the codeword length(43200) should be less than or equal to 65535(2^16-1)

[Igor Poloyko]

Thanks Chidvi!

I first though about shortened codes, but my mistake was conclusion that if the comm.BCHEncoder is called in demo example just with message-argument (without ShortMessageLength argument) so, no shortening occures.

Now I see that this is not true.

Thanks a lot again.