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//
// detector_example.c
//
// This example demonstrates the binary pre-demodulator synchronizer. A random
// binary sequence is generated, modulated with BPSK, and then interpolated.
// The resulting sequence is used to generate a detector object which in turn
// is used to detect a signal in the presence of carrier frequency and timing
// offsets and additive white Gauss noise.
//
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <math.h>
#include <time.h>
#include "liquid.h"
#define OUTPUT_FILENAME "detector_example.m"
// print usage/help message
void usage()
{
printf("detector_example -- test binary pre-demodulation synchronization\n");
printf("options:\n");
printf(" h : print usage/help\n");
printf(" n : number of sync samples, default: 128\n");
printf(" F : carrier frequency offset, default: 0.02\n");
printf(" T : fractional sample offset dt in [-0.5, 0.5], default: 0\n");
printf(" S : SNR [dB], default: 20\n");
printf(" t : detection threshold, default: 0.3\n");
}
int main(int argc, char*argv[])
{
//srand(time(NULL));
// options
unsigned int n = 128; // number of sync samples
float dt = 0.0f; // fractional sample timing offset
float noise_floor = -30.0f; // noise floor [dB]
float SNRdB = 20.0f; // signal-to-noise ratio [dB]
float dphi = 0.0f; // carrier frequency offset
float phi = 0.0f; // carrier phase offset
float threshold = 0.3f; // detection threshold
int dopt;
while ((dopt = getopt(argc,argv,"hn:T:F:S:t:")) != EOF) {
switch (dopt) {
case 'h': usage(); return 0;
case 'n': n = atoi(optarg); break;
case 'F': dphi = atof(optarg); break;
case 'T': dt = atof(optarg); break;
case 'S': SNRdB = atof(optarg); break;
case 't': threshold = atof(optarg); break;
default:
exit(1);
}
}
unsigned int i;
// validate input
if (dt < -0.5f || dt > 0.5f) {
fprintf(stderr,"error: %s, fractional sample offset must be in [-0.5,0.5]\n", argv[0]);
exit(1);
}
// derived values
unsigned int num_samples = 3*n;
float nstd = powf(10.0f, noise_floor/20.0f);
float gamma = powf(10.0f, (SNRdB + noise_floor)/20.0f);
// arrays
float complex s[n]; // synchronization pattern (samples)
float complex x[num_samples]; // transmitted signal
float complex y[num_samples]; // received signal
// generate synchronization pattern (OFDM symbol, slightly over-sampled)
float complex S[n];
for (i=0; i<n; i++)
S[i] = (i < 0.4*n || i > 0.6*n) ? randnf() + _Complex_I*randnf() : 0.0f;
fft_run(n, S, s, LIQUID_FFT_BACKWARD, 0);
float s2 = 0.0f;
for (i=0; i<n; i++)
s2 += crealf(s[i]*conjf(s[i]));
for (i=0; i<n; i++)
s[i] /= sqrtf(s2 / (float)n);
// generate transmitted signal: 0 0 0 ... 0 s[0] s[1] ... s[n-1] 0 0 0 ... 0
for (i=0; i<n; i++) {
x[0*n+i] = 0.0f;
x[1*n+i] = s[i];
x[2*n+i] = 0.0f;
}
// generate received signal (add channel impairments)
unsigned int d = 11; // fractional sample filter delay
firfilt_crcf finterp = firfilt_crcf_create_kaiser(2*d+1, 0.45f, 40.0f, dt);
for (i=0; i<num_samples+d; i++) {
// fractional sample timing offset
if (i < num_samples) firfilt_crcf_push(finterp, x[i]);
else firfilt_crcf_push(finterp, 0.0f);
if (i < d) firfilt_crcf_execute(finterp, &y[0]);
else firfilt_crcf_execute(finterp, &y[i-d]);
}
firfilt_crcf_destroy(finterp);
for (i=0; i<num_samples; i++) {
// channel gain
y[i] *= gamma;
// carrier offset
y[i] *= cexpf(_Complex_I*(dphi*i + phi));
// noise
y[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
}
// create cross-correlator
detector_cccf sync = detector_cccf_create(s, n, threshold, 0.03f);
detector_cccf_print(sync);
// push signal through detector
float tau_hat = 0.0f;
float dphi_hat = 0.0f;
float gamma_hat = 1.0f;
int signal_detected = 0;
unsigned int index = 0;
for (i=0; i<num_samples; i++) {
// correlate
int detected = detector_cccf_correlate(sync, y[i], &tau_hat, &dphi_hat, &gamma_hat);
if (detected) {
signal_detected = 1;
printf("****** preamble found, tau_hat=%8.6f, dphi_hat=%8.6f, gamma_hat=%8.6f\n",
tau_hat, dphi_hat, gamma_hat);
index = i;
}
}
// destroy objects
detector_cccf_destroy(sync);
// print results
printf("\n");
printf("signal detected : %s\n", signal_detected ? "yes" : "no");
float delay_est = (float) index + tau_hat;
float delay = (float)(2*n) + dt; // actual delay (samples)
printf("delay estimate : %8.3f, actual=%8.3f (error=%8.3f) sample(s)\n", delay_est, delay, delay-delay_est);
printf("dphi estimate : %8.5f, actual=%8.5f (error=%8.5f) rad/sample\n",dphi_hat, dphi, dphi-dphi_hat);
printf("gamma estimate : %8.3f, actual=%8.3f (error=%8.3f) dB\n", 20*log10f(gamma_hat), 20*log10f(gamma), 20*log10(gamma/gamma_hat));
printf("\n");
//
// 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", n);
fprintf(fid,"num_samples = %u;\n", num_samples);
fprintf(fid,"s = zeros(1,n);\n");
for (i=0; i<n; i++)
fprintf(fid,"s(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(s[i]), cimagf(s[i]));
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)];\n");
fprintf(fid,"figure;\n");
fprintf(fid,"subplot(2,1,1);\n");
fprintf(fid," plot(t,real(x), t,imag(x));\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('time');\n");
fprintf(fid," ylabel('transmitted signal');\n");
fprintf(fid,"subplot(2,1,2);\n");
fprintf(fid," plot(t,real(y), t,imag(y));\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('time');\n");
fprintf(fid," ylabel('received signal');\n");
fclose(fid);
printf("results written to '%s'\n", OUTPUT_FILENAME);
return 0;
}
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