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// Demonstate soft demodulation of linear modulation schemes.
#include <stdlib.h>
#include <stdio.h>
#include <getopt.h>
#include "liquid.h"
#define OUTPUT_FILENAME "modem_soft_example.m"
// print usage/help message
void usage()
{
printf("modem_soft_example [options]\n");
printf(" h : print help\n");
printf(" m : modulation scheme (qpsk default)\n");
liquid_print_modulation_schemes();
}
// print a string of bits to the standard output
// _x : input symbol
// _bps : bits/symbol
// _n : number of characters to print (zero-padding)
void print_bitstring(unsigned int _x,
unsigned int _bps,
unsigned int _n)
{
unsigned int i;
for (i=0; i<_bps; i++)
printf("%1u", (_x >> (_bps-i-1)) & 1);
for (i=_bps; i<_n; i++)
printf(" ");
}
int main(int argc, char*argv[])
{
// create mod/demod objects
modulation_scheme ms = LIQUID_MODEM_QPSK;
int dopt;
while ((dopt = getopt(argc,argv,"uhm:")) != EOF) {
switch (dopt) {
case 'h':
usage();
return 0;
case 'm':
ms = liquid_getopt_str2mod(optarg);
if (ms == LIQUID_MODEM_UNKNOWN) {
fprintf(stderr,"error: %s, unknown/unsupported modulation scheme '%s'\n", argv[0], optarg);
return 1;
}
break;
default:
exit(1);
}
}
// create the modem objects
modemcf mod = modemcf_create(ms);
modemcf demod = modemcf_create(ms);
// ensure bits/symbol matches modem description (only
// applicable to certain specific modems)
unsigned int bps = modemcf_get_bps(mod);
modemcf_print(mod);
unsigned int i; // modulated symbol
unsigned int s_hard; // demodulated symbol (hard)
unsigned char soft_bits[bps];
unsigned int s_soft; // demodulated symbol (soft, compacted)
unsigned int num_symbols = 1<<bps;
float complex x;
unsigned int num_sym_errors = 0;
unsigned int num_bit_errors = 0;
printf("\n");
printf(" %-11s %-11s %-11s : ", "input sym.", "hard demod", "soft demod");
for (i=0; i<bps; i++)
printf(" b[%1u]", i);
printf("\n");
for (i=0; i<num_symbols; i++) {
// modulate symbol
modemcf_modulate(mod, i, &x);
// demodulate, including soft decision
modemcf_demodulate_soft(demod, x, &s_hard, soft_bits);
// re-pack soft bits to hard decision
liquid_pack_soft_bits(soft_bits, bps, &s_soft);
// print results
printf(" ");
print_bitstring(i, bps,12);
print_bitstring(s_hard,bps,12);
print_bitstring(s_soft,bps,12);
printf(" : ");
unsigned int j;
for (j=0; j<bps; j++)
printf("%7u", soft_bits[j]);
printf("\n");
num_sym_errors += i == s_soft ? 0 : 1;
num_bit_errors += count_bit_errors(i,s_soft);
}
printf("num sym errors: %4u / %4u\n", num_sym_errors, num_symbols);
printf("num bit errors: %4u / %4u\n", num_bit_errors, num_symbols*bps);
modemcf_destroy(mod);
modemcf_destroy(demod);
return 0;
}
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