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// Tests simple frequency modulation/demodulation
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <getopt.h>
#include "liquid.h"
#define OUTPUT_FILENAME "freqmodem_example.m"
// print usage/help message
void usage()
{
printf("freqmodem_example [options]\n");
printf(" h : print usage\n");
printf(" n : number of samples, default: 1024\n");
printf(" S : SNR [dB], default: 30\n");
printf(" k : FM modulation factor, default: 0.1\n");
}
int main(int argc, char*argv[])
{
// options
float kf = 0.1f; // modulation factor
unsigned int num_samples = 1024; // number of samples
float SNRdB = 30.0f; // signal-to-noise ratio [dB]
int dopt;
while ((dopt = getopt(argc,argv,"hn:S:k:")) != EOF) {
switch (dopt) {
case 'h': usage(); return 0;
case 'n': num_samples = atoi(optarg); break;
case 'S': SNRdB = atof(optarg); break;
case 'k': kf = atof(optarg); break;
default:
exit(1);
}
}
// create mod/demod objects
freqmod mod = freqmod_create(kf); // modulator
freqdem dem = freqdem_create(kf); // demodulator
freqmod_print(mod);
unsigned int i;
float m[num_samples]; // message signal
float complex r[num_samples]; // received signal (complex baseband)
float y[num_samples]; // demodulator output
// generate message signal (sum of sines)
for (i=0; i<num_samples; i++) {
m[i] = 0.3f*cosf(2*M_PI*0.013f*i + 0.0f) +
0.2f*cosf(2*M_PI*0.021f*i + 0.4f) +
0.4f*cosf(2*M_PI*0.037f*i + 1.7f);
}
// modulate signal
freqmod_modulate_block(mod, m, num_samples, r);
// add channel impairments
float nstd = powf(10.0f,-SNRdB/20.0f);
for (i=0; i<num_samples; i++)
r[i] += nstd*( randnf() + _Complex_I*randnf() ) * M_SQRT1_2;
// demodulate signal
freqdem_demodulate_block(dem, r, num_samples, y);
// destroy modem objects
freqmod_destroy(mod);
freqdem_destroy(dem);
// compute RMS error (ignore first sample)
float rmse = 0.0;
for (i=1; i<num_samples; i++) {
float err = y[i] - m[i];
rmse += err*err;
}
rmse = sqrtf( rmse / (float)(num_samples-1) );
printf("rmse = %12.4e\n", rmse);
//
// 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,"n=%u;\n",num_samples);
for (i=0; i<num_samples; i++) {
fprintf(fid,"m(%3u) = %12.4e;\n", i+1, m[i]);
fprintf(fid,"r(%3u) = %12.4e + j*%12.4e;\n", i+1, crealf(r[i]), cimagf(r[i]));
fprintf(fid,"y(%3u) = %12.4e;\n", i+1, y[i]);
}
// plot time-domain result
fprintf(fid,"t=0:(n-1);\n");
fprintf(fid,"figure;\n");
fprintf(fid,"subplot(3,1,1);\n");
fprintf(fid," plot(t,m,'LineWidth',1,'Color',[0 0.2 0.5],...);\n");
fprintf(fid," t,y,'LineWidth',1,'Color',[0 0.5 0.2]);\n");
fprintf(fid," axis([0 n -1.2 1.2]);\n");
fprintf(fid," xlabel('Normalized Time [t/T_s]');\n");
fprintf(fid," ylabel('m(t), y(t)');\n");
fprintf(fid," grid on;\n");
// compute spectral responses
fprintf(fid,"nfft=2^(1+nextpow2(n));\n");
fprintf(fid,"f=[0:(nfft-1)]/nfft - 0.5;\n");
fprintf(fid,"w = hamming(n)';\n");
fprintf(fid,"g = 1 / (mean(w) * n);\n");
fprintf(fid,"M = 20*log10(abs(fftshift(fft(m.*w*g,nfft))));\n");
fprintf(fid,"R = 20*log10(abs(fftshift(fft(r.*w*g,nfft))));\n");
fprintf(fid,"Y = 20*log10(abs(fftshift(fft(y.*w*g,nfft))));\n");
// plot spectral response (audio)
fprintf(fid,"subplot(3,1,2);\n");
fprintf(fid," plot(f,M,'LineWidth',1.2,'Color',[0 0.2 0.5],...\n");
fprintf(fid," f,Y,'LineWidth',1.2,'Color',[0 0.5 0.2]);\n");
fprintf(fid," axis([-0.5 0.5 -80 20]);\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('Normalized Frequency [f/F_s]');\n");
fprintf(fid," ylabel('Audio PSD [dB]');\n");
// plot spectral response (RF)
fprintf(fid,"subplot(3,1,3);\n");
fprintf(fid," plot(f,R,'LineWidth',1.2,'Color',[0.5 0 0]);\n");
fprintf(fid," axis([-0.5 0.5 -80 20]);\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('Normalized Frequency [f/F_s]');\n");
fprintf(fid," ylabel('RF PSD [dB]');\n");
fclose(fid);
printf("results written to %s\n", OUTPUT_FILENAME);
return 0;
}
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