Re: [OctDev] Bchenco - Communication Toolbox

[Marco M. <de...@gm...>]

Il giorno Thu, 11 Sep 2008 09:24:01 +0200
David Bateman <ad...@gm...> ha scritto:

> Marco Maso wrote:
> > Hi everybody, my name is Marco and this is my first message to the
> > ML. I've decided to subscribe because i have a little problem usign
> > the bchenco function provided with the communications toolbox of
> > octave-forge (versio 1.0.6). I'm currently using Octave 3.0.2. As i
> > can read on the Function Reference page this function:
> > 
> > "Encodes the message msg using a [n,k] BCH coding. The variable msg
> > is a binary array with k columns and an arbitrary number of rows.
> > Each row of msg represents a single symbol to be coded by the BCH
> > coder. The coded message is returned in the binary array code
> > containing n columns and the same number of rows as msg."
> > 
> > And after:
> > 
> > "By default the generator polynomial used in the BCH coding is
> > based on the properties of the Galois Field GF(2^m). This default
> > generator polynomial can be overridden by a polynomial in g.
> > Suitable generator polynomials can be constructed with bchpoly."
> > 
> > I'm developing a software that needs a BCH encoder with the
> > following parameters (among the others...):
> > 
> > - BCH uncoded word length = Kbch = 32208
> > - BCH coded word length = Nbch = 32400
> > 
> > - BCH uncoded word length = Kbch = 7302
> > - BCH coded word length = Nbch = 7200
> > 
> > In the first case i use a g polynomial obtained by multiplying 12
> > polynomials of grade 16. This polynomial generator works correctly
> > with the other 5 couples (n,k) i need to test on my simulator.
> > 
> > In the second case In the first case i use a g polynomial obtained
> > by multiplying 12 polynomials of grade 14. This polynomial
> > generator works correctly with the other 6 couples (n,k) i need to
> > test on my simulator.
> > 
> > Obviously thes are not numbers invented by myself but i'm using a
> > manual as a reference so i'm sure they are correct.
> > 
> > 
> > In both cases i obtain this error:
> > 
> > error: bchenco: can not find valid generator polynomial for
> > parameters error: evaluating assignment expression near line 140,
> > column 3
> > 
> > I can't properly understand where the problem can be even because i
> > can't find the source code of this function...
> > 
> > Can anybody explain me why i receive such an error?
> > 
> > Should i post the coefficients of my g polynomial?
> > 
> > Last question (stupid) : when working with octave a polynomial
> > insertion has always to be done from LSB to MSB am i right?
> > 
> > Thanks a lot
> > 
> > Marco  
> > 
> > 
> 
> Could you supply a short example demonstrating this issue?
> 
> D.


Ok! So i give you some consistent data:

I use this polynomial generator ( given from LSB to MSB, you don't
have answered to that question so i believe this is correct):

 Columns 1 through 20:

   1   1   1   0   0   1   1   1   1   0   1   0   1   0   1   0   0
1   0   0

 Columns 21 through 40:

   0   0   0   0   0   1   1   0   0   1   1   0   1   1   1   0   1
1   1   1

 Columns 41 through 60:

   1   0   1   0   0   0   0   1   1   1   1   0   0   0   1   0   1
1   0   0

 Columns 61 through 80:

   0   0   0   0   1   0   0   1   0   0   0   1   0   0   0   1   0
0   0   0

 Columns 81 through 100:

   1   0   1   0   1   1   0   0   0   0   1   1   1   0   1   1   0
0   0   1

 Columns 101 through 120:

   1   0   1   1   0   0   1   1   0   1   0   0   1   1   1   1   0
0   1   1

 Columns 121 through 140:

   0   0   0   0   1   0   1   0   0   0   1   1   1   0   0   0   1
0   0   0

 Columns 141 through 160:

   1   0   1   0   0   0   1   1   1   0   1   0   0   0   1   0   0
0   0   1

 Columns 161 through 180:

   1   1   0   0   0   0   0   1   0   1   1   1   0   0   0   0   0
1   1   0

 Columns 181 through 193:

   0   1   0   0   0   1   1   1   0   0   1   0   1


As you can see this polynomial has grade 192.


I call this polynomial g, with this i try to generate the BCH code with
this command:

code = bchenco(ones(1, 32208), 32400, 32208, g);

this gives:

octave:140> e = bchenco(ones(1,32208),32400,32208,g);
error: bchenco: can not find valid generator polynomial for parameters
error: evaluating assignment expression near line 140, column 3

Obviously the part "line 140" si referred to the octave prompt and
change everytime i call this function

I'm codifying a ones word but it's not important, i'm just testing.

Another valid couple (n, k) i'm trying is (48600, 48408) and so:

octave:145> code = bchenco(ones(1,48408),48600,48408,fliplr(g));
octave:146> length(code)
ans =  48600

And this is the result with other 4 known couples, no problems at all.

With another polynomial generator, let's call it g_1, i have other 7
known couples to test, and i have a problem with only one couple her
too:

g_1 = 
 
 Columns 1 through 20:

   1   0   1   0   0   1   0   1   1   0   1   0   0   0   0   0   1
0   0   1

 Columns 21 through 40:

   1   0   0   0   1   0   0   0   1   0   1   1   1   1   1   0   1
0   1   1

 Columns 41 through 60:

   1   1   1   0   0   1   1   1   1   1   1   1   0   0   0   1   0
1   0   0

 Columns 61 through 80:

   1   0   1   0   1   0   0   1   0   1   1   0   0   0   0   0   1
0   0   1

 Columns 81 through 100:

   1   1   0   0   0   1   0   1   1   1   0   0   0   1   0   0   1
0   1   1

 Columns 101 through 120:

   0   0   1   1   0   1   0   0   0   1   1   0   0   1   0   0   1
1   0   1

 Columns 121 through 140:

   1   0   0   1   0   1   1   0   0   0   0   1   1   0   0   1   0
1   0   1

 Columns 141 through 160:

   0   1   1   1   1   1   0   1   1   0   1   1   0   1   0   0   0
1   1   0

 Columns 161 through 169:

   0   0   0   0   0   0   1   0   1



This polynomial has grade 168. Again:

octave:148> code = bchenco(ones(1, 7032), 7200, 7032, g_1);
error: bchenco: can not find valid generator polynomial for parameters
error: evaluating assignment expression near line 148, column 3


Another valid couple (n, k) i'm trying is (3072, 3240) and so:

octave:148> code = bchenco(ones(1, 3072), 3240, 3072, g_1);
octave:149> length(code)
ans =  3240


And this is the result with other 5 known couples, no problems at all.


I hope this is what you asked for.

Thanks

M.

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