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//
// gmskmodem_example.c
//
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <math.h>
#include "liquid.h"
#define OUTPUT_FILENAME "gmskmodem_example.m"
// print usage/help message
void usage()
{
printf("gmskmodem_example -- Gaussian minimum-shift keying modem example\n");
printf("options (default values in <>):\n");
printf(" u/h : print usage/help\n");
printf(" k : samples/symbol, default: 4\n");
printf(" m : filter delay [symbols], default: 3\n");
printf(" n : number of data symbols, default: 200\n");
printf(" b : bandwidth-time product, 0 <= b <= 1, default: 0.3\n");
printf(" s : SNR [dB], default: 30\n");
}
int main(int argc, char*argv[]) {
// options
unsigned int k=4; // filter samples/symbol
unsigned int m=3; // filter delay (symbols)
float BT=0.3f; // bandwidth-time product
unsigned int num_data_symbols=200; // number of data symbols
float SNRdB = 30.0f; // signal-to-noise ratio [dB]
float phi = 0.0f; // carrier phase offset
float dphi = 0.0f; // carrier frequency offset
int dopt;
while ((dopt = getopt(argc,argv,"uhk:m:n:b:s:")) != EOF) {
switch (dopt) {
case 'u':
case 'h': usage(); return 0;
case 'k': k = atoi(optarg); break;
case 'm': m = atoi(optarg); break;
case 'n': num_data_symbols = atoi(optarg); break;
case 'b': BT = atof(optarg); break;
case 's': SNRdB = atof(optarg); break;
default:
exit(1);
}
}
// validate input
if (BT <= 0.0f || BT >= 1.0f) {
fprintf(stderr,"error: %s, bandwidth-time product must be in (0,1)\n", argv[0]);
exit(1);
}
// derived values
unsigned int num_symbols = num_data_symbols + 2*m;
unsigned int num_samples = k*num_symbols;
float nstd = powf(10.0f,-SNRdB/20.0f); // noise standard deviation
// create modulator
gmskmod mod = gmskmod_create(k, m, BT);
gmskmod_print(mod);
// create demodulator
gmskdem demod = gmskdem_create(k, m, BT);
gmskdem_set_eq_bw(demod, 0.01f);
gmskdem_print(demod);
unsigned int i;
unsigned int s[num_symbols];
float complex x[num_samples];
float complex y[num_samples];
unsigned int sym_out[num_symbols];
// generate random data sequence
for (i=0; i<num_symbols; i++)
s[i] = rand() % 2;
// modulate signal
for (i=0; i<num_symbols; i++)
gmskmod_modulate(mod, s[i], &x[k*i]);
// add channel impairments
for (i=0; i<num_samples; i++) {
y[i] = x[i]*cexpf(_Complex_I*(phi + i*dphi));
y[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
}
// demodulate signal
for (i=0; i<num_symbols; i++)
gmskdem_demodulate(demod, &y[k*i], &sym_out[i]);
// destroy modem objects
gmskmod_destroy(mod);
gmskdem_destroy(demod);
// print results to screen
unsigned int delay = 2*m;
unsigned int num_errors=0;
for (i=delay; i<num_symbols; i++) {
//printf(" %4u : %2u (%2u)\n", i, s[i-delay], sym_out[i]);
num_errors += (s[i-delay] == sym_out[i]) ? 0 : 1;
}
printf("symbol errors : %4u / %4u\n", num_errors, num_data_symbols);
// write results to output file
FILE * fid = fopen(OUTPUT_FILENAME,"w");
fprintf(fid,"%% %s : auto-generated file\n", OUTPUT_FILENAME);
fprintf(fid,"clear all\n");
fprintf(fid,"close all\n");
fprintf(fid,"k = %u;\n", k);
fprintf(fid,"m = %u;\n", m);
fprintf(fid,"BT = %f;\n", BT);
fprintf(fid,"num_symbols = %u;\n", num_symbols);
fprintf(fid,"num_samples = %u;\n", num_samples);
fprintf(fid,"x = zeros(1,num_samples);\n");
fprintf(fid,"y = zeros(1,num_samples);\n");
for (i=0; i<num_samples; i++) {
fprintf(fid,"x(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(x[i]), cimagf(x[i]));
fprintf(fid,"y(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(y[i]), cimagf(y[i]));
}
fprintf(fid,"t=[0:(num_samples-1)]/k;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"plot(t,real(y),t,imag(y));\n");
// artificially demodulate (generate receive filter, etc.)
float hr[2*k*m+1];
liquid_firdes_gmskrx(k,m,BT,0,hr);
for (i=0; i<2*k*m+1; i++)
fprintf(fid,"hr(%3u) = %12.8f;\n", i+1, hr[i]);
fprintf(fid,"z = filter(hr,1,arg( ([y(2:end) 0]).*conj(y) ))/k;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"plot(t,z,t(k:k:end),z(k:k:end),'or');\n");
fprintf(fid,"grid on;\n");
fclose(fid);
printf("results written to '%s'\n", OUTPUT_FILENAME);
return 0;
}
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