244 lines
6.7 KiB
C
244 lines
6.7 KiB
C
/*
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sfrsd2.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 sfrsd2_(int mrsym[], int mrprob[], int mr2sym[], int mr2prob[],
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int* ntrials0, int* verbose0, int correct[], int param[],
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int indexes[], double tt[], 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],workdat2[63];
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int era_pos[51];
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int c, i, j, numera, nmr2, nerr, nn=63, kk=12;
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FILE *datfile, *logfile;
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int ntrials = *ntrials0;
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int verbose = *verbose0;
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int nhard=0,nhard_min=32768,nsoft=0,nsoft_min=32768, ncandidates;
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int ngmd,nera_best;
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clock_t t0=0,t1=0;
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int perr[8][8] = {
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12, 31, 44, 52, 60, 57, 50, 50,
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28, 38, 49, 58, 65, 69, 64, 80,
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40, 41, 53, 62, 66, 73, 76, 81,
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50, 53, 53, 64, 70, 76, 77, 81,
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50, 50, 52, 60, 71, 72, 77, 84,
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50, 50, 56, 62, 67, 73, 81, 85,
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50, 50, 71, 62, 70, 77, 80, 85,
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50, 50, 62, 64, 71, 75, 82, 87};
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int pmr2[8][8] = {
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4, 8, 9, 7, 6, 0, 0, 0,
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13, 18, 15, 11, 9, 7, 5, 0,
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0, 23, 21, 15, 12, 10, 7, 4,
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0, 34, 28, 20, 16, 14, 11, 7,
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0, 20, 26, 25, 19, 14, 12, 9,
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0, 0, 28, 27, 22, 19, 14, 11,
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0, 0, 40, 29, 29, 23, 18, 12,
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0, 0, 40, 35, 31, 21, 20, 13};
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if(verbose) {
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logfile=fopen("sfrsd.log","a");
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if( !logfile ) {
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printf("Unable to open sfrsd.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 vector 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 the mrsym probabilities to find 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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if(verbose) fprintf(logfile," BM decode nerrors= %3d : ",nerr);
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memcpy(correct,workdat,63*sizeof(int));
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ngmd=-1;
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param[0]=0;
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param[1]=0;
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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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return;
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}
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/*
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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 potential codewords. The
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"soft" distance between each codeword and the received word is
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used to decide which codeword is "best".
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*/
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#ifdef WIN32
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srand(0xdeadbeef);
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#else
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srandom(0xdeadbeef);
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#endif
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float ratio, ratio0[63];
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int threshe, thresh2, nsum;
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int thresh0[63],thresh1[63], mr2flag;
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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];
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ratio0[i]=ratio;
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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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thresh1[i] = 0.4*pmr2[ii][jj];
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}
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if(nsum==0) return;
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for( k=0; 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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nmr2=0;
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for (i=0; i<nn; i++) {
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j = indexes[62-i];
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threshe=thresh0[i];
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thresh2=thresh1[i];
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long int ir, ir2;
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#ifdef WIN32
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ir=rand();
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ir2=rand();
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#else
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ir=random();
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ir2=random();
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#endif
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if( ((ir % 100) < threshe ) && (numera+2*nmr2) < 51 ) {
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era_pos[numera]=j;
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numera=numera+1;
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}
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if( ((ir2 % 100) < thresh2) && (numera+2*nmr2)<51) {
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workdat[j]=rxdat2[j];
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nmr2=nmr2+1;
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}
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}
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t0=clock();
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// rs=init_rs_int(symsize, gfpoly, fcr, prim, nroots, 1);
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nerr=decode_rs_int(rs,workdat,era_pos,numera,1);
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t1=clock();
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tt[0]+=(double)(t1-t0)/CLOCKS_PER_SEC;
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if( nerr >= 0 ) {
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ncandidates=ncandidates+1;
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nhard=0;
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nsoft=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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if(workdat[i] != rxdat2[i]) {
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nsoft=nsoft+rxprob[i];
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}
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}
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}
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nsoft=63*nsoft/nsum;
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if((nsoft < 33) && (nhard < 43) && (nhard+nsoft) < 74) { //???
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if( (nsoft < nsoft_min) ) {
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nsoft_min=nsoft;
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nhard_min=nhard;
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memcpy(correct,workdat,63*sizeof(int));
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ngmd=0;
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nera_best=numera;
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ntry[0]=k;
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}
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}
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if(nsoft_min < 27) break;
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if((nsoft_min < 32) && (nhard_min < 43) &&
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(nhard_min+nsoft_min) < 74) break;
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}
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if(k == ntrials-1) ntry[0]=k+1;
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}
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if(verbose) fprintf(logfile,
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"%d trials and %d candidates after stochastic loop\n",k,ncandidates);
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if( (ncandidates >= 0) && (nsoft_min < 36) && (nhard_min < 44) ) {
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if(verbose) {
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for (i=0; i<63; i++) {
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fprintf(logfile,"%3d %3d %3d %3d %3d %3d\n",i,correct[i],
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rxdat[i],rxprob[i],rxdat2[i],rxprob2[i]);
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}
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fprintf(logfile,"**** ncandidates %d nhard %d nsoft %d nsum %d\n",
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ncandidates,nhard_min,nsoft_min,nsum);
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}
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} else {
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nhard_min=-1;
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}
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if(verbose) {
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fprintf(logfile,"exiting sfrsd\n");
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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]=ngmd;
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if(param[0]==0) param[2]=-1;
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return;
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}
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