Hello Achilleas,
I went through the document gr-trellis. In my case (BPSK) 2-aray ,1-D
modulation:[D{1},constellation{C11 , C21}]
Since the output of encoder is 2 bit packed leading to output symbol
cardinality O=4, { 0 ,1,2,3} and as per your suggestion i created
constellations as follows:
Â
 std::vector< float > constellation;
          constellation.push_back( 1);
     constellation.push_back(1);
     constellation.push_back( 1);
     constellation.push_back(-1);
     constellation.push_back( -1);
     constellation.push_back(1);
     constellation.push_back(- 1);
     constellation.push_back(-1);
  and dimensionality D = 2;
  Now the result appears to be as i expected both encoding and
decoding.
 Thank you very much
 Ram
— On Tue, 20/7/10, Achilleas A. [email protected] wrote:
From: Achilleas A. [email protected]
Subject: Re: [Discuss-gnuradio] gr-trellis : problem in using viterbi
decoder
To: “Raman O” [email protected], [email protected]
Date: Tuesday, 20 July, 2010, 1:55 AM
you will have to make the constellation a 2D array (essentially
representing the 2 BPSK symbols).
It is all explained in the examples and the documentation of gr-trellis
and in the examples…
please read it; i spend a lot of time writing it…
Achilleas
On Mon, Jul 19, 2010 at 8:19 PM, Raman O [email protected] wrote:
Hello Achilleas,
I made changes to vector source and sink, encoding works fine with
awgn1o2_128.fsm.
But the result of encoder is 2 bit packed and is not suitable for me.
Since i want to employ BPSK modulation (0-> -1 / 1 ->1), I need encoder
output 1 bit at a time.
Since awgn1o2_128.fsm dictates numInputbits: 2 and   numOutputbits: 4
, Modulator throws following error. If i am not wrong it says,
constellation length is not enough.
lt-director: gr_chunks_to_symbols_sf.cc:66: virtual int
gr_chunks_to_symbols_sf::work(int, gr_vector_const_void_star&,
gr_vector_void_star&): Assertion `((unsigned int)in[i]*d_D+d_D) <=
d_symbol_table.size()’ failed.
Aborted
I am using following Constellation:
 std::vector constellation;
  Â
      constellation.push_back(1);
         constellation.push_back(-1);
So how do i achieve BPSK modulation with encoder using generator poly
in awgn1o2_128.fsm.
Many many thanks,
Ram
— On Mon, 19/7/10, Achilleas A. [email protected] wrote:
From: Achilleas A. [email protected]
Subject: Re: [Discuss-gnuradio] gr-trellis : problem in using viterbi
decoder
To: “Raman O” [email protected], [email protected]
Date: Monday, 19 July, 2010, 9:50 PM
As I mentioned in my earlier email, you have misunderstood the
functionality of the
gr_make_vector_source_s(vec,false)
block. It takes a vector “vec” as an argument and
produces A STREAM OF SHORTS at its output!!!
It is NOT producing a stream of vectors at its output…unless you
specify this as the third parameter,
which has nothing to do with the size of the input vector “vec” you are
using…
Please remove both stream2vector and vector2stream from your code and
modify the statement
gr_vector_source_s_sptr bit_src =
gr_make_vector_source_s(in_bits,false,K );
as
gr_vector_source_s_sptr bit_src =
gr_make_vector_source_s(in_bits,false);
Please see the python program test_tcm1.py in the
gr-trellis/src/examples directory to see
how the blocks are arranged…
best,
Achilleas
On Mon, Jul 19, 2010 at 4:19 PM, Raman O [email protected] wrote:
Hello Achilleas,
 Thanks for your response.
 I have connected encoder block , it disappeared during editing mail by
mistake.
Input is in the form of an array of size K and vector2stream feeds
encoder bit a time ,so i am using following syntax :
gr_vector_source_s_sptr bit_src =
gr_make_vector_source_s(in_bits,false,K );
  Â
gr_vector_to_stream_sptr vector2stream =
gr_make_vector_to_stream(sizeof(short),K)------------------------------------------------------------------------------------------------------------
int K=100; Â
 short inputbits[100]={ …contains bits… };
std::vector in_bits;
     Â
       for (int i = 0; i <K ; i++) { // K - no
of bits
      in_bits.push_back(inputbits[i]); // input bits
       }
       std::vector constellation; // BPSK mapping
        Â
constellation.push_back(1);
         constellation.push_back(-1);
  Â
       Â
        gr_vector_source_s_sptr bit_src =
gr_make_vector_source_s(in_bits,false,K );
 Â
Â
        gr_vector_to_stream_sptr vector2stream=
gr_make_vector_to_stream(sizeof(short),K);
  Â
   // encoder  Â
          fsm f=
fsm(“…/…/…/gr-trellis/src/examples/fsm_files/awgn1o2_128.fsm”);
          trellis_encoder_ss_sptr encoder = trellis_make_encoder_ss (f,
0);
  Â
   // modulate
 gr_chunks_to_symbols_sf_sptr mod = gr_make_chunks_to_symbols_sf
(constellation, 1);
   // ----------- ideal channel -------------
  Â
   // decoder
  Â
   trellis_metric_type_t type= TRELLIS_EUCLIDEAN;
   trellis_metrics_f_sptr metrics = trellis_make_metrics_f (f.O(), 1,
constellation, type);
   trellis_viterbi_s_sptr viterbi =trellis_make_viterbi_s ( f,K,
0,-1);
 Â
   gr_stream_to_vector_sptr stream2vector=
gr_make_stream_to_vector(sizeof(short), K);
   d_dst = gr_make_vector_sink_s (K);
connect( bit_src ,  0, vector2stream,  0);
   connect( vector2stream ,  0,encoder, 0);
connect( encoder ,  0,mod, 0);
   connect(mod,  0,metrics, Â
0);
   connect(metrics,  0, viterbi,  0);
   connect( viterbi,  0,stream2vector,  0);
   connect(stream2vector,  0, d_dst, 0);
Â
— On
Mon, 19/7/10, Achilleas A. [email protected] wrote:
From: Achilleas A. [email protected]
Subject: Re: [Discuss-gnuradio] gr-trellis : problem in using viterbi
decoder
To: “gnuradio
mailing list” [email protected], [email protected]
Date: Monday, 19 July, 2010, 6:47 PM
I also realized that the way you call
gr_make_vector_source_s(in_bits,false,K );
is incorrect.
I think you have misunderstood what this block does.
You should
use:
gr_make_vector_source_s(in_bits,false,1 );
or simply
gr_make_vector_source_s(in_bits,false);
and you really don’t need the vector2stream block.
Achilleas
In your code the “encoder” block is not connected to any other block…
It should be:
info bits → encoder → modulator → channel → metrics → viterbi
Achilleas