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// This example runs a bit error rate simulation for a specified modulation
// scheme and saves the resulting data points to a file for plotting.
#include <stdio.h>
#include <stdlib.h>
#include <complex.h>
#include <getopt.h>
#include <math.h>
#include "liquid.h"
// print usage/help message
void usage()
{
printf("modem_example [options]\n");
printf(" -h : print help\n");
printf(" -m <scheme> : modulation scheme\n");
liquid_print_modulation_schemes();
}
int main(int argc, char*argv[])
{
// simulation parameters
modulation_scheme ms = LIQUID_MODEM_QPSK; // modulation scheme
float SNRdB_min = -5.0f; // starting SNR value
float SNRdB_max = 40.0f; // maximum SNR value
float SNRdB_step = 1.0f; // step size
unsigned int num_trials = 1e6; // number of symbols
char filename[] = "modem_ber_example.m";
int dopt;
while ((dopt = getopt(argc,argv,"hm:")) != 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 modem objects
modem mod = modem_create(ms); // modulator
modem dem = modem_create(ms); // demodulator
// compute derived values and initialize counters
unsigned int m = modem_get_bps(mod); // modulation bits/symbol
unsigned int M = 1 << m; // constellation size: 2^m
unsigned int i, sym_in, sym_out;
float complex sample;
// iterate through SNR values
printf("# modulation scheme : %s\n", modulation_types[ms].name);
printf("# %8s %8s %8s %12s\n", "SNR [dB]", "errors", "trials", "BER");
float SNRdB = SNRdB_min; // set SNR value
FILE * fid = fopen(filename,"w");
fprintf(fid,"%% %s : auto-generated file\n", filename);
fprintf(fid,"clear all; close all; SNRdB=[]; BER=[]; ms='%s';\n", modulation_types[ms].name);
while (1) {
// compute noise standard deviation
float nstd = powf(10.0f, -SNRdB/20.0f);
// reset modem objects (only necessary for differential schemes)
modem_reset(mod);
modem_reset(dem);
// run trials
unsigned int num_bit_errors = 0;
for (i=0; i<num_trials; i++) {
// generate random input symbol and modulate
sym_in = rand() % M;
modem_modulate(mod, sym_in, &sample);
// add complex noise and demodulate
sample += nstd*(randnf() + _Complex_I*randnf())/sqrtf(2.0f);
modem_demodulate(dem, sample, &sym_out);
// count errors
num_bit_errors += count_bit_errors(sym_in, sym_out);
}
// compute results and print formatted results to screen
unsigned int num_bit_trials = m * num_trials;
float ber = (float)num_bit_errors / (float)(num_bit_trials);
printf(" %8.2f %8u %8u %12.4e\n", SNRdB, num_bit_errors, num_bit_trials, ber);
if (num_bit_errors > 0)
fprintf(fid,"SNRdB(end+1)=%12.3f; BER(end+1)=%12.4e;\n", SNRdB, ber);
// stop iterating if SNR exceed maximum or no errors were detected
SNRdB += SNRdB_step;
if (SNRdB > SNRdB_max || num_bit_errors == 0)
break;
}
fprintf(fid,"figure; semilogy(SNRdB,BER,'-x'); grid on; axis([SNRdB(1) SNRdB(end) 1e-6 1]);\n");
fprintf(fid,"xlabel('SNR [dB]'); ylabel('BER'); title(ms)\n");
fclose(fid);
printf("results written to %s\n", filename);
// destroy modem objects and return
modem_destroy(mod);
modem_destroy(dem);
return 0;
}
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