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//
// firfilt_cccf_example.c
//
// This example demonstrates the finite impulse response (FIR) filter
// with complex coefficients as a cross-correlator between transmitted
// and received sequences.
//
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>
#include <math.h>
#include "liquid.h"
#define OUTPUT_FILENAME "firfilt_cccf_example.m"
// print usage/help message
void usage()
{
printf("firfilt_cccf_example:\n");
printf(" h : print usage/help\n");
printf(" n : sequence length, default: 32\n");
printf(" s : SNR, signal-to-noise ratio [dB], default: 20\n");
}
int main(int argc, char*argv[]) {
// options
unsigned int sequence_len = 256; // sequence length
float SNRdB = 10.0f; // signal-to-noise ratio (dB)
int dopt;
while ((dopt = getopt(argc,argv,"hn:s:")) != EOF) {
switch (dopt) {
case 'h': usage(); return 0;
case 'n': sequence_len = atof(optarg); break;
case 's': SNRdB = atoi(optarg); break;
default:
usage();
exit(-1);
}
}
unsigned int i;
// derived values
unsigned int num_samples = 3*sequence_len;
// data arrays
float complex sequence[sequence_len]; // sequence
float complex x[num_samples]; // input sequence
float complex rxy[num_samples]; // correlator output
// generate random sequence
for (i=0; i<sequence_len; i++) {
sequence[i] = (rand() % 2 ? M_SQRT1_2 : -M_SQRT1_2) +
(rand() % 2 ? M_SQRT1_2 : -M_SQRT1_2) * _Complex_I;
}
// generate correlator object
firfilt_cccf q = firfilt_cccf_create(sequence, sequence_len);
// normalize by number of points
firfilt_cccf_set_scale(q, 1.0f / (float)sequence_len);
// generate the input: fill buffer with zeros then insert
// the sequence in the middle flipped and conjugated
for (i=0; i<num_samples; i++)
x[i] = 0.0f;
for (i=0; i<sequence_len; i++)
x[sequence_len + (sequence_len-i-1)] = conjf(sequence[i]);
// add noise
float nstd = powf(10.0f, -SNRdB/20.0f);
for (i=0; i<num_samples; i++)
cawgn(&x[i],nstd);
// compute cross-correlation
for (i=0; i<num_samples; i++) {
firfilt_cccf_push(q,x[i]);
firfilt_cccf_execute(q,&rxy[i]);
}
// find peak
float complex rxy_peak = 0;
for (i=0; i<num_samples; i++) {
if (i==0 || cabsf(rxy[i]) > cabsf(rxy_peak))
rxy_peak = rxy[i];
}
printf("peak cross-correlation : %12.8f, angle %12.8f\n", cabsf(rxy_peak),
cargf(rxy_peak));
// destroy allocated objects
firfilt_cccf_destroy(q);
//
// write results to 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\n");
fprintf(fid,"num_samples = %u;\n", num_samples);
// write signal to output file
for (i=0; i<num_samples; i++) {
fprintf(fid,"x(%4u) = %12.4e + j*%12.4e;\n",i+1,crealf(x[i]),cimagf(x[i]));
fprintf(fid,"rxy(%4u) = %12.4e + j*%12.4e;\n", i+1, crealf(rxy[i]), cimagf(rxy[i]));
}
fprintf(fid,"\n\n");
fprintf(fid,"t=0:(num_samples-1);\n");
fprintf(fid,"figure;\n");
fprintf(fid,"subplot(2,1,1),\n");
fprintf(fid," hold on;\n");
fprintf(fid," plot(t,real(x),'Color',[0 0.2 0.5]);\n");
fprintf(fid," plot(t,imag(x),'Color',[0 0.5 0.2]);\n");
fprintf(fid," hold off;\n");
fprintf(fid," xlabel('sample index');\n");
fprintf(fid," ylabel('received signal');\n");
fprintf(fid," legend('real','imag','location','northeast');\n");
fprintf(fid," axis([0 num_samples -1.5 1.5]);\n");
fprintf(fid," grid on;\n");
fprintf(fid,"subplot(2,1,2),\n");
fprintf(fid," plot(t,abs(rxy),'Color',[0.5 0 0],'LineWidth',2);\n");
fprintf(fid," xlabel('sample index');\n");
fprintf(fid," ylabel('cross-correlation magnitude');\n");
fprintf(fid," axis([0 num_samples -0.2 1.2]);\n");
fprintf(fid," grid on;\n");
fprintf(fid,"\n\n");
fclose(fid);
printf("data written to %s\n", OUTPUT_FILENAME);
printf("done.\n");
return 0;
}
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