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0e15a4cd5c
More aggressive parameters are selected with higher values of "Aggressive decoding level" on the "Advanced" tab. With settings greater than 0, be sure to set Ftol=1000 if you want to decode over the whole passband. git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6330 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
236 lines
6.5 KiB
C
236 lines
6.5 KiB
C
/*
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ftrsd2.c
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A soft-decision decoder for the JT65 (63,12) Reed-Solomon code.
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This decoding scheme is built around Phil Karn's Berlekamp-Massey
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errors and erasures decoder. The approach is inspired by a number of
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publications, including the stochastic Chase decoder described
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in "Stochastic Chase Decoding of Reed-Solomon Codes", by Leroux et al.,
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IEEE Communications Letters, Vol. 14, No. 9, September 2010 and
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"Soft-Decision Decoding of Reed-Solomon Codes Using Successive Error-
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and-Erasure Decoding," by Soo-Woong Lee and B. V. K. Vijaya Kumar.
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Steve Franke K9AN and Joe Taylor K1JT
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <time.h>
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#include <string.h>
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#include "rs2.h"
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static void *rs;
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void getpp_(int workdat[], float *pp);
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void ftrsd2_(int mrsym[], int mrprob[], int mr2sym[], int mr2prob[],
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int* ntrials0, int correct[], int param[], int ntry[])
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{
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int rxdat[63], rxprob[63], rxdat2[63], rxprob2[63];
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int workdat[63];
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int indexes[63];
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int era_pos[51];
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int i, j, numera, nerr, nn=63;
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FILE *logfile = NULL;
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int ntrials = *ntrials0;
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int verbose = 0;
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int nhard=0,nhard_min=32768,nsoft=0,nsoft_min=32768;
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int nsofter=0,nsofter_min=32768,ntotal=0,ntotal_min=32768,ncandidates;
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int nera_best=0;
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float pp,pp1,pp2;
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static unsigned int nseed;
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// Power-percentage symbol metrics - composite gnnf/hf
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int perr[8][8] = {
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{ 4, 9, 11, 13, 14, 14, 15, 15},
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{ 2, 20, 20, 30, 40, 50, 50, 50},
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{ 7, 24, 27, 40, 50, 50, 50, 50},
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{13, 25, 35, 46, 52, 70, 50, 50},
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{17, 30, 42, 54, 55, 64, 71, 70},
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{25, 39, 48, 57, 64, 66, 77, 77},
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{32, 45, 54, 63, 66, 75, 78, 83},
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{51, 58, 57, 66, 72, 77, 82, 86}};
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if(verbose) {
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logfile=fopen("/tmp/ftrsd.log","a");
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if( !logfile ) {
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printf("Unable to open ftrsd.log\n");
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exit(1);
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}
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}
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// Initialize the KA9Q Reed-Solomon encoder/decoder
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unsigned int symsize=6, gfpoly=0x43, fcr=3, prim=1, nroots=51;
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rs=init_rs_int(symsize, gfpoly, fcr, prim, nroots, 0);
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// Reverse the received symbol vectors for BM decoder
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for (i=0; i<63; i++) {
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rxdat[i]=mrsym[62-i];
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rxprob[i]=mrprob[62-i];
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rxdat2[i]=mr2sym[62-i];
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rxprob2[i]=mr2prob[62-i];
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}
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// Sort rxprob to find indexes of the least reliable symbols
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int k, pass, tmp, nsym=63;
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int probs[63];
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for (i=0; i<63; i++) {
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indexes[i]=i;
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probs[i]=rxprob[i];
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}
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for (pass = 1; pass <= nsym-1; pass++) {
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for (k = 0; k < nsym - pass; k++) {
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if( probs[k] < probs[k+1] ) {
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tmp = probs[k];
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probs[k] = probs[k+1];
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probs[k+1] = tmp;
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tmp = indexes[k];
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indexes[k] = indexes[k+1];
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indexes[k+1] = tmp;
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}
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}
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}
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// See if we can decode using BM HDD, and calculate the syndrome vector.
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memset(era_pos,0,51*sizeof(int));
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numera=0;
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memcpy(workdat,rxdat,sizeof(rxdat));
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nerr=decode_rs_int(rs,workdat,era_pos,numera,1);
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if( nerr >= 0 ) {
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// Hard-decision decoding succeeded. Save codeword and some parameters.
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nhard=0;
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for (i=0; i<63; i++) {
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if( workdat[i] != rxdat[i] ) nhard=nhard+1;
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}
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memcpy(correct,workdat,63*sizeof(int));
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param[0]=0;
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param[1]=nhard;
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param[2]=0;
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param[3]=0;
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param[4]=0;
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param[5]=0;
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param[7]=1000*1000;
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ntry[0]=0;
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return;
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}
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/*
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Hard-decision decoding failed. Try the FT soft-decision method.
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Generate random erasure-locator vectors and see if any of them
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decode. This will generate a list of "candidate" codewords. The
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soft distance between each candidate codeword and the received
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word is estimated by finding the largest (pp1) and second-largest
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(pp2) outputs from a synchronized filter-bank operating on the
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symbol spectra, and using these to decide which candidate
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codeword is "best".
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*/
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nseed=1; //Seed for random numbers
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float ratio;
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int thresh, nsum;
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int thresh0[63];
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ncandidates=0;
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nsum=0;
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int ii,jj;
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for (i=0; i<nn; i++) {
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nsum=nsum+rxprob[i];
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j = indexes[62-i];
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ratio = (float)rxprob2[j]/((float)rxprob[j]+0.01);
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ii = 7.999*ratio;
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jj = (62-i)/8;
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thresh0[i] = 1.3*perr[ii][jj];
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}
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if(nsum<=0) return;
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pp1=0.0;
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pp2=0.0;
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for (k=1; k<=ntrials; k++) {
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memset(era_pos,0,51*sizeof(int));
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memcpy(workdat,rxdat,sizeof(rxdat));
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/*
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Mark a subset of the symbols as erasures.
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Run through the ranked symbols, starting with the worst, i=0.
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NB: j is the symbol-vector index of the symbol with rank i.
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*/
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numera=0;
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for (i=0; i<nn; i++) {
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j = indexes[62-i];
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thresh=thresh0[i];
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long int ir;
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// Generate a random number ir, 0 <= ir < 100 (see POSIX.1-2001 example).
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nseed = nseed * 1103515245 + 12345;
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ir = (unsigned)(nseed/65536) % 32768;
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ir = (100*ir)/32768;
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if((ir < thresh ) && numera < 51) {
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era_pos[numera]=j;
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numera=numera+1;
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}
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}
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nerr=decode_rs_int(rs,workdat,era_pos,numera,0);
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if( nerr >= 0 ) {
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// We have a candidate coderowd. Find its hard and soft distance from
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// the received word. Also find pp1 and pp2 from the full array
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// s3(64,63) of synchronized symbol spectra.
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ncandidates=ncandidates+1;
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nhard=0;
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nsoft=0;
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nsofter=0;
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for (i=0; i<63; i++) {
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if(workdat[i] != rxdat[i]) {
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nhard=nhard+1;
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nsofter=nsofter+rxprob[i];
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if(workdat[i] != rxdat2[i]) {
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nsoft=nsoft+rxprob[i];
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}
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} else {
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nsofter=nsofter-rxprob[i];
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}
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}
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nsoft=63*nsoft/nsum;
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nsofter=63*nsofter/nsum;
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ntotal=nsoft+nhard;
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getpp_(workdat,&pp);
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if(pp>pp1) {
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pp2=pp1;
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pp1=pp;
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nsoft_min=nsoft;
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nhard_min=nhard;
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nsofter_min=nsofter;
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ntotal_min=ntotal;
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memcpy(correct,workdat,63*sizeof(int));
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nera_best=numera;
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ntry[0]=k;
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} else {
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if(pp>pp2 && pp!=pp1) pp2=pp;
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}
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// if(ntotal_min <= 81 && pp2/pp1 <= 0.87) break;
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if(nhard_min <= 41 && ntotal_min <= 71) break;
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}
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if(k == ntrials) ntry[0]=k;
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}
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if(logfile) {
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fprintf(logfile,"ncand %4d nhard %4d nsoft %4d nhard+nsoft %4d nsum %8d\n",
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ncandidates,nhard_min,nsoft_min,ntotal_min,nsum);
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fclose(logfile);
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}
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param[0]=ncandidates;
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param[1]=nhard_min;
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param[2]=nsoft_min;
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param[3]=nera_best;
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param[4]=1000.0*pp2/pp1;
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param[5]=ntotal_min;
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param[6]=ntry[0];
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param[7]=1000.0*pp2;
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param[8]=1000.0*pp1;
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if(param[0]==0) param[2]=-1;
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return;
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}
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