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