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//
// bpresync_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 bpresync 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 "bpresync_example.m"
// print usage/help message
void usage()
{
printf("bpresync_example -- test binary pre-demodulation synchronization\n");
printf("options (default values in <>):\n");
printf(" h : print usage/help\n");
printf(" k : samples/symbol, default: 2\n");
printf(" m : filter delay [symbols], default: 5\n");
printf(" n : number of data symbols, default: 64\n");
printf(" b : bandwidth-time product beta in (0,1), default: 0.3\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");
}
int main(int argc, char*argv[])
{
srand(time(NULL));
// options
unsigned int k=2; // filter samples/symbol
unsigned int m=5; // filter delay (symbols)
float beta=0.3f; // bandwidth-time product
float dt = 0.0f; // fractional sample timing offset
unsigned int num_sync_symbols = 64; // number of synchronization symbols
float SNRdB = 20.0f; // signal-to-noise ratio [dB]
float dphi = 0.02f; // carrier frequency offset
float phi = 2*M_PI*randf(); // carrier phase offset
int dopt;
while ((dopt = getopt(argc,argv,"uhk:m:n:b:t:F:t:S:")) != EOF) {
switch (dopt) {
case 'h': usage(); return 0;
case 'k': k = atoi(optarg); break;
case 'm': m = atoi(optarg); break;
case 'n': num_sync_symbols = atoi(optarg); break;
case 'b': beta = atof(optarg); break;
case 'F': dphi = atof(optarg); break;
case 't': dt = atof(optarg); break;
case 'S': SNRdB = atof(optarg); break;
default:
exit(1);
}
}
unsigned int i;
// validate input
if (beta <= 0.0f || beta >= 1.0f) {
fprintf(stderr,"error: %s, bandwidth-time product must be in (0,1)\n", argv[0]);
exit(1);
} else 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_symbols = num_sync_symbols + 2*m + 10;
unsigned int num_samples = k*num_symbols;
float nstd = powf(10.0f, -SNRdB/20.0f);
// arrays
float complex seq[num_sync_symbols]; // synchronization pattern (symbols)
float complex s0[k*num_sync_symbols]; // synchronization pattern (samples)
float complex x[num_samples]; // transmitted signal
float complex y[num_samples]; // received signal
float complex rxy[num_samples]; // pre-demod correlation output
float dphi_hat[num_samples]; // carrier offset estimate
// create transmit/receive interpolator/decimator
firinterp_crcf interp = firinterp_crcf_create_prototype(LIQUID_FIRFILT_RRC,k,m,beta,dt);
// generate synchronization pattern (BPSK) and interpolate
for (i=0; i<num_sync_symbols + 2*m; i++) {
float complex sym = 0.0f;
if (i < num_sync_symbols) {
sym = rand() % 2 ? -1.0f : 1.0f;
seq[i] = sym;
}
if (i < 2*m) firinterp_crcf_execute(interp, sym, s0);
else firinterp_crcf_execute(interp, sym, &s0[k*(i-2*m)]);
}
// reset interpolator
firinterp_crcf_reset(interp);
// interpolate input
for (i=0; i<num_symbols; i++) {
float complex sym = i < num_sync_symbols ? seq[i] : 0.0f;
firinterp_crcf_execute(interp, sym, &x[k*i]);
}
// push through channel
for (i=0; i<num_samples; i++)
y[i] = x[i]*cexpf(_Complex_I*(dphi*i + phi)) + nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
// create cross-correlator
bpresync_cccf sync = bpresync_cccf_create(s0, k*num_sync_symbols, 0.05f, 11);
bpresync_cccf_print(sync);
// push signal through cross-correlator
float rxy_max = 0.0f; // maximum cross-correlation
float dphi_est = 0.0f; // carrier frequency offset estimate
int delay_est = 0; // delay estimate
for (i=0; i<num_samples; i++) {
// correlate
bpresync_cccf_push(sync, y[i]);
bpresync_cccf_execute(sync, &rxy[i], &dphi_hat[i]);
// detect...
if (cabsf(rxy[i]) > 0.6f) {
printf("****** preamble found, rxy = %12.8f (dphi-hat: %12.8f), i=%3u ******\n",
cabsf(rxy[i]), dphi_hat[i], i);
}
// retain maximum
if (cabsf(rxy[i]) > rxy_max) {
rxy_max = cabsf(rxy[i]);
dphi_est = dphi_hat[i];
delay_est = (int)i - (int)2*k*m + 1;
}
}
// destroy objects
firinterp_crcf_destroy(interp);
bpresync_cccf_destroy(sync);
// print results
printf("\n");
printf("rxy (max) : %12.8f\n", rxy_max);
printf("dphi est. : %12.8f ,error=%12.8f\n", dphi_est, dphi-dphi_est);
printf("delay est.: %12d ,error=%3d sample(s)\n", delay_est, k*num_sync_symbols - delay_est);
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,"num_samples = %u;\n", num_samples);
fprintf(fid,"num_symbols = %u;\n", num_symbols);
fprintf(fid,"k = %u;\n", k);
fprintf(fid,"x = zeros(1,num_samples);\n");
fprintf(fid,"y = zeros(1,num_samples);\n");
fprintf(fid,"rxy = 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,"rxy(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(rxy[i]), cimagf(rxy[i]));
}
fprintf(fid,"t=[0:(num_samples-1)]/k;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"subplot(2,1,1);\n");
fprintf(fid," plot(t,real(y), t,imag(y));\n");
fprintf(fid," axis([0 num_symbols -2 2]);\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('time');\n");
fprintf(fid," ylabel('received signal');\n");
fprintf(fid,"subplot(2,1,2);\n");
fprintf(fid," plot(t,abs(rxy));\n");
fprintf(fid," axis([0 num_symbols 0 1.5]);\n");
fprintf(fid," xlabel('time');\n");
fprintf(fid," ylabel('correlator output');\n");
fprintf(fid," grid on;\n");
fclose(fid);
printf("results written to '%s'\n", OUTPUT_FILENAME);
return 0;
}
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