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// Demonstrate using qpilotsync for carrier recovery.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <time.h>
#include <getopt.h>
#include <assert.h>
#include "liquid.h"
void usage()
{
printf("%s [options]\n", __FILE__);
printf(" h : print usage\n");
printf(" n : payload length [symbols], default: 400\n");
printf(" p : pilot spacing [symbols], default: 20\n");
printf(" s : SNR [dB], default: 20\n");
printf(" m : modulation scheme, default: qam16\n");
liquid_print_modulation_schemes();
}
int main(int argc, char *argv[])
{
//srand( time(NULL) );
// options
int ms = LIQUID_MODEM_QAM16; // mod. scheme
unsigned int payload_len = 400; // payload length
unsigned int pilot_spacing = 20; // pilot spacing
float SNRdB = 20.0f; // signal-to-noise ratio [dB]
const char filename[] = "qpilotsync_example.m"; // output filename
float dphi = -0.0075f; // carrier frequency offset
float phi = 2.1800f; // carrier phase offset
float gain = 0.5f; // channel gain
// get options
int dopt;
while((dopt = getopt(argc,argv,"hn:p:s:m:")) != EOF){
switch (dopt) {
case 'h': usage(); return 0;
case 'n': payload_len = atoi(optarg); break;
case 'p': pilot_spacing = atoi(optarg); break;
case 's': SNRdB = atof(optarg); break;
case 'm': ms = liquid_getopt_str2mod(optarg); break;
default:
exit(-1);
}
}
unsigned int i;
// derived values
float nstd = powf(10.0f, -SNRdB/20.0f);
// create pilot generator and synchronizer objects
qpilotgen pg = qpilotgen_create( payload_len, pilot_spacing);
qpilotsync ps = qpilotsync_create(payload_len, pilot_spacing);
qpilotgen_print(pg);
// get frame length
unsigned int frame_len = qpilotgen_get_frame_len(pg);
// allocate arrays
unsigned char payload_sym_tx[payload_len]; // transmitted payload symbols
float complex payload_tx [payload_len]; // transmitted payload samples
float complex frame_tx [frame_len]; // transmitted frame samples
float complex frame_rx [frame_len]; // received frame samples
float complex payload_rx [payload_len]; // received payload samples
unsigned char payload_sym_rx[payload_len]; // received payload symbols
// create modem objects for payload
modemcf mod = modemcf_create(ms);
modemcf dem = modemcf_create(ms);
// assemble payload symbols
for (i=0; i<payload_len; i++) {
// generate random symbol
payload_sym_tx[i] = modemcf_gen_rand_sym(mod);
// modulate
modemcf_modulate(mod, payload_sym_tx[i], &payload_tx[i]);
}
// assemble frame
qpilotgen_execute(pg, payload_tx, frame_tx);
// add channel impairments
for (i=0; i<frame_len; i++) {
frame_rx[i] = frame_tx[i] * cexpf(_Complex_I*dphi*i + _Complex_I*phi);
frame_rx[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
frame_rx[i] *= gain;
}
// receive frame
qpilotsync_execute(ps, frame_rx, payload_rx);
// demodulate
for (i=0; i<payload_len; i++) {
unsigned int sym_demod;
modemcf_demodulate(dem, payload_rx[i], &sym_demod);
payload_sym_rx[i] = (unsigned char)sym_demod;
}
// count errors
unsigned int bit_errors = 0;
for (i=0; i<payload_len; i++)
bit_errors += count_bit_errors(payload_sym_rx[i], payload_sym_tx[i]);
printf("received bit errors : %u / %u\n", bit_errors, payload_len * modemcf_get_bps(mod));
// destroy allocated objects
qpilotgen_destroy(pg);
qpilotsync_destroy(ps);
modemcf_destroy(mod);
modemcf_destroy(dem);
// write symbols to output file for plotting
FILE * fid = fopen(filename,"w");
if (!fid) {
fprintf(stderr,"error: could not open '%s' for writing\n", filename);
return -1;
}
fprintf(fid,"%% %s : auto-generated file\n", filename);
fprintf(fid,"clear all;\n");
fprintf(fid,"close all;\n");
fprintf(fid,"payload_len = %u;\n", payload_len);
fprintf(fid,"frame_len = %u;\n", frame_len);
fprintf(fid,"frame_tx = zeros(1,frame_len);\n");
fprintf(fid,"payload_rx = zeros(1,payload_len);\n");
for (i=0; i<frame_len; i++)
fprintf(fid,"frame_rx(%6u) = %12.4e + 1i*%12.4e;\n", i+1, crealf(frame_rx[i]), cimagf(frame_rx[i]));
for (i=0; i<payload_len; i++)
fprintf(fid,"payload_rx(%6u) = %12.4e + 1i*%12.4e;\n", i+1, crealf(payload_rx[i]), cimagf(payload_rx[i]));
fprintf(fid,"figure('Color','white','position',[100 100 950 400]);\n");
fprintf(fid,"subplot(1,2,1);\n");
fprintf(fid," plot(real(frame_rx), imag(frame_rx), 'x','MarkerSize',3);\n");
fprintf(fid," axis([-1 1 -1 1]*1.5);\n");
fprintf(fid," axis square;\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('real');\n");
fprintf(fid," ylabel('imag');\n");
fprintf(fid," title('received');\n");
fprintf(fid,"subplot(1,2,2);\n");
fprintf(fid," plot(real(payload_rx),imag(payload_rx),'x','MarkerSize',3);\n");
fprintf(fid," axis([-1 1 -1 1]*1.5);\n");
fprintf(fid," axis square;\n");
fprintf(fid," grid on;\n");
fprintf(fid," xlabel('real');\n");
fprintf(fid," ylabel('imag');\n");
fprintf(fid," title('recovered');\n");
fclose(fid);
printf("results written to '%s'\n", filename);
printf("done.\n");
return 0;
}
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