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//
// fec_soft_example.c
//
// This example demonstrates the interface for forward error-correction
// (FEC) codes with soft-decision decoding. A buffer of data bytes is
// encoded before the data are corrupted with at least one error and
// noise. The decoder then attempts to recover the original data set
// from the soft input bits. The user may select the FEC scheme from
// the command-line interface.
//
// SEE ALSO: fec_example.c
// packetizer_soft_example.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>
#include "liquid.h"
// print usage/help message
void usage()
{
printf("fecsoft_example [options]\n");
printf(" u/h : print usage\n");
printf(" v/q : verbose/queit (print soft bits?)\n");
printf(" n : input data size (number of uncoded bytes)\n");
printf(" c : coding scheme, (h74 default):\n");
liquid_print_fec_schemes();
}
int main(int argc, char*argv[])
{
// options
unsigned int n = 4; // data length (bytes)
unsigned int nmax = 2048; // maximum data length
fec_scheme fs = LIQUID_FEC_HAMMING74; // error-correcting scheme
int verbose = 1; // verbose?
int dopt;
while((dopt = getopt(argc,argv,"uhvqn:c:")) != EOF){
switch (dopt) {
case 'h':
case 'u': usage(); return 0;
case 'v': verbose = 1; break;
case 'q': verbose = 0; break;
case 'n': n = atoi(optarg); break;
case 'c':
fs = liquid_getopt_str2fec(optarg);
if (fs == LIQUID_FEC_UNKNOWN) {
fprintf(stderr,"error: unknown/unsupported fec scheme \"%s\"\n\n",optarg);
exit(1);
}
break;
default:
exit(1);
}
}
// ensure proper data length
n = (n > nmax) ? nmax : n;
// create arrays
unsigned int n_enc = fec_get_enc_msg_length(fs,n);
printf("dec msg len : %u\n", n);
printf("enc msg len : %u\n", n_enc);
unsigned char data[n]; // original data message
unsigned char msg_enc[n_enc]; // encoded data message
unsigned char msg_cor_soft[8*n_enc]; // corrupted data message (soft bits)
unsigned char msg_cor_hard[n_enc]; // corrupted data message (hard bits)
unsigned char msg_dec[n]; // decoded data message
// create object
fec q = fec_create(fs,NULL);
fec_print(q);
unsigned int i;
// create message
for (i=0; i<n; i++)
data[i] = rand() & 0xff;
// encode message
fec_encode(q, n, data, msg_enc);
// convert to soft bits and add 'noise'
for (i=0; i<n_enc; i++) {
msg_cor_soft[8*i+0] = (msg_enc[i] & 0x80) ? 255 : 0;
msg_cor_soft[8*i+1] = (msg_enc[i] & 0x40) ? 255 : 0;
msg_cor_soft[8*i+2] = (msg_enc[i] & 0x20) ? 255 : 0;
msg_cor_soft[8*i+3] = (msg_enc[i] & 0x10) ? 255 : 0;
msg_cor_soft[8*i+4] = (msg_enc[i] & 0x08) ? 255 : 0;
msg_cor_soft[8*i+5] = (msg_enc[i] & 0x04) ? 255 : 0;
msg_cor_soft[8*i+6] = (msg_enc[i] & 0x02) ? 255 : 0;
msg_cor_soft[8*i+7] = (msg_enc[i] & 0x01) ? 255 : 0;
}
// flip first bit (ensure error)
msg_cor_soft[0] = 255 - msg_cor_soft[0];
// add noise (but not so much that it would cause a bit error)
for (i=0; i<8*n_enc; i++) {
int soft_bit = 0.8*msg_cor_soft[i] + (int)(20*randnf());
if (soft_bit > 255) soft_bit = 255;
if (soft_bit < 0) soft_bit = 0;
msg_cor_soft[i] = soft_bit;
}
// convert to hard bits (printing purposes)
for (i=0; i<n_enc; i++) {
msg_cor_hard[i] = 0x00;
msg_cor_hard[i] |=(msg_cor_soft[8*i+0] >> 0) & 0x80;
msg_cor_hard[i] |=(msg_cor_soft[8*i+1] >> 1) & 0x40;
msg_cor_hard[i] |=(msg_cor_soft[8*i+2] >> 2) & 0x20;
msg_cor_hard[i] |=(msg_cor_soft[8*i+3] >> 3) & 0x10;
msg_cor_hard[i] |=(msg_cor_soft[8*i+4] >> 4) & 0x08;
msg_cor_hard[i] |=(msg_cor_soft[8*i+5] >> 5) & 0x04;
msg_cor_hard[i] |=(msg_cor_soft[8*i+6] >> 6) & 0x02;
msg_cor_hard[i] |=(msg_cor_soft[8*i+7] >> 7) & 0x01;
}
// decode message
fec_decode_soft(q, n, msg_cor_soft, msg_dec);
printf("original message: [%3u] ",n);
for (i=0; i<n; i++)
printf(" %.2X", (unsigned int) (data[i]));
printf("\n");
printf("encoded message: [%3u] ",n_enc);
for (i=0; i<n_enc; i++)
printf(" %.2X", (unsigned int) (msg_enc[i]));
printf("\n");
// print compact result
printf("corrupted message: [%3u] ",n_enc);
for (i=0; i<n_enc; i++)
printf("%c%.2X", msg_cor_hard[i]==msg_enc[i] ? ' ' : '*', (unsigned int) (msg_cor_hard[i]));
printf("\n");
if (verbose) {
// print expanded result (print each soft bit value)
for (i=0; i<n_enc; i++) {
printf("%5u: ", i);
unsigned int j;
for (j=0; j<8; j++) {
unsigned int bit_enc = (msg_enc[i] >> (8-j-1)) & 0x01;
unsigned int bit_rec = (msg_cor_soft[8*i+j] > 127) ? 1 : 0;
//printf("%1u %3u (%1u) %c", bit_enc, msg_cor_soft[i], bit_rec, bit_enc != bit_rec ? '*' : ' ');
printf("%4u%c", msg_cor_soft[8*i+j], bit_enc != bit_rec ? '*' : ' ');
}
printf(" : %c%.2X\n", msg_cor_hard[i]==msg_enc[i] ? ' ' : '*', (unsigned int) (msg_cor_hard[i]));
}
} // verbose
printf("decoded message: [%3u] ",n);
for (i=0; i<n; i++)
printf("%c%.2X", msg_dec[i] == data[i] ? ' ' : '*', (unsigned int) (msg_dec[i]));
printf("\n");
printf("\n");
// count bit errors
unsigned int j, num_sym_errors=0, num_bit_errors=0;
unsigned char e;
for (i=0; i<n; i++) {
num_sym_errors += (data[i] == msg_dec[i]) ? 0 : 1;
e = data[i] ^ msg_dec[i];
for (j=0; j<8; j++) {
num_bit_errors += e & 0x01;
e >>= 1;
}
}
//printf("number of symbol errors detected: %d\n", num_errors_detected);
printf("number of symbol errors received: %3u / %3u\n", num_sym_errors, n);
printf("number of bit errors received: %3u / %3u\n", num_bit_errors, n*8);
// clean up objects
fec_destroy(q);
return 0;
}
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