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//
// symsync_crcf_example.c
//
// This example demonstrates the basic principles of the symbol timing
// recovery family of objects, specifically symsync_crcf. A set of random
// QPSK symbols are generated and interpolated with a timing offset. The
// resulting signal is run through the symsync_crcf object which applies a
// matched filter and recovers timing producing a clean constellation.
//
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <getopt.h>
#include <time.h>
#include <assert.h>
#include "liquid.h"
#define OUTPUT_FILENAME "symsync_crcf_example.m"
// print usage/help message
void usage()
{
printf("symsync_crcf_example [options]\n");
printf(" h : print help\n");
printf(" k : filter samples/symbol, default: 2\n");
printf(" m : filter delay (symbols), default: 5\n");
printf(" b : filter excess bandwidth, default: 0.5\n");
printf(" B : filter polyphase banks, default: 32\n");
printf(" n : number of symbols, default: 400\n");
}
int main(int argc, char*argv[]) {
srand(time(NULL));
// options
unsigned int k = 2; // samples/symbol (input)
unsigned int m = 5; // filter delay (symbols)
float beta = 0.5f; // filter excess bandwidth factor
unsigned int num_filters = 32; // number of filters in the bank
unsigned int num_symbols = 400; // number of data symbols
int dopt;
while ((dopt = getopt(argc,argv,"hk:m:b:B:s:n:")) != EOF) {
switch (dopt) {
case 'h': usage(); return 0;
case 'k': k = atoi(optarg); break;
case 'm': m = atoi(optarg); break;
case 'b': beta = atof(optarg); break;
case 'B': num_filters = atoi(optarg); break;
case 'n': num_symbols = atoi(optarg); break;
default:
exit(1);
}
}
// validate input
if (k < 2) {
fprintf(stderr,"error: k (samples/symbol) must be at least 2\n");
exit(1);
} else if (m < 1) {
fprintf(stderr,"error: m (filter delay) must be greater than 0\n");
exit(1);
} else if (beta <= 0.0f || beta > 1.0f) {
fprintf(stderr,"error: beta (excess bandwidth factor) must be in (0,1]\n");
exit(1);
} else if (num_filters == 0) {
fprintf(stderr,"error: number of polyphase filters must be greater than 0\n");
exit(1);
} else if (num_symbols == 0) {
fprintf(stderr,"error: number of symbols must be greater than 0\n");
exit(1);
}
unsigned int i;
// static values
liquid_firfilt_type ftype = LIQUID_FIRFILT_ARKAISER;
float bandwidth = 0.02f; // loop filter bandwidth
float dt = -0.5f; // fractional sample offset
// derived values
unsigned int num_samples = k*num_symbols;
float complex s[num_symbols]; // data symbols
float complex x[num_samples]; // interpolated samples
float complex y[num_symbols + 64]; // synchronized symbols
// generate random QPSK symbols
for (i=0; i<num_symbols; i++) {
// random signal (QPSK)
s[i] = (rand() % 2 ? -M_SQRT1_2 : M_SQRT1_2) +
(rand() % 2 ? -M_SQRT1_2 : M_SQRT1_2) * _Complex_I;
}
// design interpolating filter with 'dt' samples of delay
firinterp_crcf interp = firinterp_crcf_create_prototype(ftype,k,m,beta,dt);
// run interpolator
firinterp_crcf_execute_block(interp, s, num_symbols, x);
// destroy interpolator
firinterp_crcf_destroy(interp);
// create symbol synchronizer
symsync_crcf sync = symsync_crcf_create_rnyquist(ftype, k, m, beta, num_filters);
// set bandwidth
symsync_crcf_set_lf_bw(sync,bandwidth);
// execute on entire block of samples
unsigned int ny=0;
symsync_crcf_execute(sync, x, num_samples, y, &ny);
// destroy synchronizer
symsync_crcf_destroy(sync);
// print last several symbols to screen
printf("output symbols:\n");
printf(" ...\n");
for (i=ny-10; i<ny; i++)
printf(" sym_out(%2u) = %8.4f + j*%8.4f;\n", i+1, crealf(y[i]), cimagf(y[i]));
//
// export output file
//
FILE* fid = fopen(OUTPUT_FILENAME,"w");
fprintf(fid,"%% %s, auto-generated file\n\n", OUTPUT_FILENAME);
fprintf(fid,"close all;\n");
fprintf(fid,"clear all;\n");
fprintf(fid,"ny=%u;\n",ny);
for (i=0; i<ny; i++)
fprintf(fid,"y(%3u) = %12.8f + j*%12.8f;\n", i+1, crealf(y[i]), cimagf(y[i]));
fprintf(fid,"i0 = 1:round(0.5*ny);\n");
fprintf(fid,"i1 = round(0.5*ny):ny;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"hold on;\n");
fprintf(fid,"plot(real(y(i0)),imag(y(i0)),'x','MarkerSize',4,'Color',[1 1 1]*0.7);\n");
fprintf(fid,"plot(real(y(i1)),imag(y(i1)),'o','MarkerSize',4,'Color',[0 0.25 0.5]);\n");
fprintf(fid,"hold off;\n");
fprintf(fid,"axis square;\n");
fprintf(fid,"grid on;\n");
fprintf(fid,"axis([-1 1 -1 1]*1.6);\n");
fprintf(fid,"xlabel('In-phase');\n");
fprintf(fid,"ylabel('Quadrature');\n");
fprintf(fid,"legend(['first 50%%'],['last 50%%'],'location','northeast');\n");
fclose(fid);
printf("results written to %s.\n", OUTPUT_FILENAME);
// clean it up
printf("done.\n");
return 0;
}
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