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// gmsk_tracking_example.c : demonstrate symbol tracking of GMSK signal
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <math.h>
#include "liquid.h"
#define OUTPUT_FILENAME "gmsk_tracking_example.m"
int main(int argc, char*argv[]) {
// options
unsigned int k = 4; // filter samples/symbol
float beta = 0.3f; // bandwidth-time product
// create resampler object to facilitate sample rate offset
unsigned int P = 203; // output rate (interpolation factor)
unsigned int Q = 200; // input rate (decimation factor)
rresamp_crcf resamp = rresamp_crcf_create_kaiser(k*P,k*Q,12,0.5,60.0f);
// create modulator
gmskmod mod = gmskmod_create(k, 3, beta);
gmskmod_print(mod);
// frequency demodulator
freqdem dem = freqdem_create(0.01*k);
// create matched filter
firfilt_rrrf mf = firfilt_rrrf_create_rnyquist(LIQUID_FIRFILT_GMSKRX, k, 8, beta, 0);
firfilt_rrrf_set_scale(mf,1./(float)k); // compensate for 'k' samples/symbol
// create symbol synchronizer to operate after matched filter
// NOTE: can alternatively skip the matched filter and apply with the symsync object
symsync_rrrf sync = symsync_rrrf_create_kaiser(k, 4, 0/*ignored*/, 64);
symsync_rrrf_set_lf_bw(sync,0.02); // set loop filter bandwidth
symsync_rrrf_set_output_rate(sync,2); // set output rate as 2 samples/symbol
// write results to output file
FILE * fid = fopen(OUTPUT_FILENAME,"w");
fprintf(fid,"%% %s : auto-generated file\n", OUTPUT_FILENAME);
fprintf(fid,"clear all; close all;\n");
fprintf(fid,"k = %u; v = [];\n", k);
// run in blocks
float complex buf_0[k*Q]; // original GMSK input
float complex buf_1[k*P]; // resample output at a rate P/Q
float buf_2[k*P]; // freqdem output & matched filter output
float buf_3[k*P]; // symbol timing recovery output
unsigned int i, j;
for (i=0; i<25; i++)
{
// generate input GMSK signal
for (j=0; j<Q; j++)
gmskmod_modulate(mod, rand() & 1, &buf_0[k*j]);
// resample k*Q samples to k*P samples
rresamp_crcf_execute(resamp, buf_0, buf_1);
// run through frequency demod
freqdem_demodulate_block(dem, buf_1, k*P, buf_2);
// run through matched filter (in place)
firfilt_rrrf_execute_block(mf, buf_2, k*P, buf_2);
// run result through symbol timing recovery
unsigned int num_symbols_sync;
symsync_rrrf_execute(sync, buf_2, k*P, buf_3, &num_symbols_sync);
// write results to file
for (j=0; j<num_symbols_sync; j++)
fprintf(fid,"v(end+1) = %12.4e;\n", buf_3[j]);
}
fprintf(fid,"n = length(v); t = [0:(n-1)]/2; idx = 1:2:n;\n");
fprintf(fid,"figure('color','white','position',[100 100 1200 400]);\n");
fprintf(fid,"plot(t,v,'-','Color',[1 1 1]*0.6,...\n");
fprintf(fid," t(idx),v(idx),'o','Color',[0 0.2 0.4]);\n");
fprintf(fid,"axis([0 t(end) -2 2]); grid on;\n");
fprintf(fid,"xlabel('Time [symbol index]'); ylabel('symsync output');\n");
fclose(fid);
printf("results written to '%s'\n", OUTPUT_FILENAME);
// destroy objects
gmskmod_destroy(mod);
freqdem_destroy(dem);
firfilt_rrrf_destroy(mf);
symsync_rrrf_destroy(sync);
return 0;
}
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