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//
// ampmodem_test.c
//
// Tests simple modulation/demodulation of the ampmodem (analog
// amplitude modulator/demodulator) with noise, carrier phase,
// and carrier frequency offsets.
//
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <getopt.h>
#include <complex.h>
#include "liquid.h"
#define OUTPUT_FILENAME "ampmodem_example.m"
// print usage/help message
void usage()
{
printf("ampmodem_example [options]\n");
printf(" -h : print usage\n");
printf(" -m <index> : modulation index, default: 0.8\n");
printf(" -f <freq> : freq. offset [rad/sample], default: 0.05\n");
printf(" -p <phase> : phase offset, default: 2.8\n");
printf(" -n <num> : number of samples, default: 2400\n");
printf(" -S <snr> : SNR [dB], default: 30\n");
printf(" -t <type> : AM type (dsb/usb/lsb), default: usb\n");
printf(" -s : suppress the carrier, default: off (carrier enabled)\n");
}
int main(int argc, char*argv[])
{
// options
float mod_index = 0.8f; // modulation index (bandwidth)
float dphi = 0.05f; // carrier frequency offset [radians/sample]
float phi = 2.8f; // carrier phase offset [radians]
float SNRdB = 30.0f; // signal-to-noise ratio (set very high for testing)
unsigned int num_samples = 2400; // number of samples
liquid_ampmodem_type type = LIQUID_AMPMODEM_USB;
int suppressed_carrier = 0;
int dopt;
while ((dopt = getopt(argc,argv,"hm:f:p:n:S:t:s")) != EOF) {
switch (dopt) {
case 'h': usage(); return 0;
case 'm': mod_index = atof(optarg); break;
case 'f': dphi = atof(optarg); break;
case 'p': phi = atof(optarg); break;
case 'n': num_samples = atoi(optarg); break;
case 'S': SNRdB = atof(optarg); break;
case 't':
if (strcmp(optarg,"dsb")==0) {
type = LIQUID_AMPMODEM_DSB;
} else if (strcmp(optarg,"usb")==0) {
type = LIQUID_AMPMODEM_USB;
} else if (strcmp(optarg,"lsb")==0) {
type = LIQUID_AMPMODEM_LSB;
} else {
fprintf(stderr,"error: %s, invalid AM type: %s\n", argv[0], optarg);
return 1;
}
break;
case 's': suppressed_carrier = 1; break;
default: return 1;
}
}
// create mod/demod objects
ampmodem mod = ampmodem_create(mod_index, type, suppressed_carrier);
ampmodem demod = ampmodem_create(mod_index, type, suppressed_carrier);
unsigned int delay = ampmodem_get_delay_mod(mod) + ampmodem_get_delay_demod(demod);
ampmodem_print(mod);
unsigned int i;
float x[num_samples];
float complex y[num_samples];
float z[num_samples];
// generate 'audio' signal (simple windowed sum of tones)
unsigned int nw = (unsigned int)(0.90*num_samples); // window length
unsigned int nt = (unsigned int)(0.05*num_samples); // taper length
for (i=0; i<num_samples; i++) {
x[i] = 0.6f*cos(2*M_PI*0.0202*i);
x[i] += 0.4f*cos(2*M_PI*0.0271*i);
x[i] *= i < nw ? liquid_rcostaper_window(i,nw,nt) : 0;
}
// modulate signal
for (i=0; i<num_samples; i++)
ampmodem_modulate(mod, x[i], &y[i]);
// add channel impairments
float nstd = powf(10.0f,-SNRdB/20.0f);
for (i=0; i<num_samples; i++) {
y[i] *= cexpf(_Complex_I*phi);
y[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
// update phase
phi += dphi;
while (phi > M_PI) phi -= 2*M_PI;
while (phi < -M_PI) phi += 2*M_PI;
}
// demodulate signal
for (i=0; i<num_samples; i++)
ampmodem_demodulate(demod, y[i], &z[i]);
// destroy objects
ampmodem_destroy(mod);
ampmodem_destroy(demod);
// compute demodulation error
float rmse = 0.0f;
for (i=delay; i<num_samples; i++)
rmse += (x[i-delay] - z[i]) * (x[i-delay] - z[i]);
rmse = 10*log10( rmse / (float)(num_samples-delay) );
printf("rms error : %.3f dB\n", rmse);
// export results
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);
fprintf(fid,"delay=%u;\n", delay);
for (i=0; i<num_samples; i++) {
fprintf(fid,"x(%3u) = %12.4e;\n", i+1, x[i]);
fprintf(fid,"y(%3u) = %12.4e + j*%12.4e;\n", i+1, crealf(y[i]), cimagf(y[i]));
fprintf(fid,"z(%3u) = %12.4e;\n", i+1, z[i]);
}
// plot results
fprintf(fid,"t=0:(n-1);\n");
fprintf(fid,"figure('position',[100 100 800 600]);\n");
// message signals
fprintf(fid,"subplot(3,1,1);\n");
fprintf(fid," plot(t,x,t-delay,z);\n");
fprintf(fid," axis([-delay n -1.2 1.2]);\n");
fprintf(fid," xlabel('Time [sample index]');\n");
fprintf(fid," ylabel('Message Signal');\n");
fprintf(fid," legend('original','demodulated');\n");
fprintf(fid," grid on;\n");
// rf signal
fprintf(fid,"subplot(3,1,2);\n");
fprintf(fid," plot(t,real(y),t,imag(y));\n");
fprintf(fid," axis([-delay n -1.8 1.8]);\n");
fprintf(fid," xlabel('Time [sample index]');\n");
fprintf(fid," ylabel('RF Signal');\n");
fprintf(fid," legend('real','imag');\n");
fprintf(fid," grid on;\n");
// spectrum
fprintf(fid,"subplot(3,1,3);\n");
fprintf(fid," nfft=2^nextpow2(n);\n");
fprintf(fid," f=[0:(nfft-1)]/nfft - 0.5;\n");
fprintf(fid," Y = 20*log10(abs(fftshift(fft(y,nfft))));\n");
fprintf(fid," Y = Y - max(Y);\n");
fprintf(fid," plot(f,Y);\n");
fprintf(fid," axis([-0.1 0.1 -60 10]);\n");
fprintf(fid," xlabel('Normalized Frequency [f/F_s]');\n");
fprintf(fid," ylabel('Received PSD [dB]');\n");
fprintf(fid," grid on;\n");
fclose(fid);
printf("results written to %s\n", OUTPUT_FILENAME);
return 0;
}
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