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//
// ofdmframesync_example.c
//
// Example demonstrating the base OFDM frame synchronizer with different
// parameters and options.
//
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <getopt.h>
#include <time.h>
#include "liquid.h"
#define OUTPUT_FILENAME "ofdmframesync_example.m"
void usage()
{
printf("Usage: ofdmframesync_example [OPTION]\n");
printf(" h : print help\n");
printf(" M : number of subcarriers (must be even), default: 1200\n");
printf(" C : cyclic prefix length, default: 60\n");
printf(" T : taper length, default: 50\n");
printf(" s : signal-to-noise ratio [dB], default: 30\n");
}
// forward declaration of callback function; this will be invoked for every
// OFDM symbol received by the parent ofdmframesync object. The object will
// reset when something other than a zero is returned.
// _X : array of received subcarrier samples [size: _M x 1]
// _p : subcarrier allocation array [size: _M x 1]
// _M : number of subcarriers
// _userdata : user-defined data pointer
static int callback(float complex * _X,
unsigned char * _p,
unsigned int _M,
void * _userdata);
// custom data type to pass to callback function
struct rx_symbols {
float complex syms_bpsk[2000]; // even subcarrier symbols
float complex syms_qpsk[2000]; // odd subcarrier symbols
unsigned int num_bpsk; // counter
unsigned int num_qpsk; // counter
};
// main function
int main(int argc, char*argv[])
{
// set the random seed differently for each run
srand(time(NULL));
// options
unsigned int M = 1200; // number of subcarriers
unsigned int cp_len = 60; // cyclic prefix length
unsigned int taper_len = 50; // taper length
unsigned int num_symbols = 20; // number of data symbols
float noise_floor = -120.0f; // noise floor [dB]
float SNRdB = 30.0f; // signal-to-noise ratio [dB]
// get options
int dopt;
while((dopt = getopt(argc,argv,"hdM:C:T:s:")) != EOF){
switch (dopt) {
case 'h': usage(); return 0;
case 'M': M = atoi(optarg); break;
case 'C': cp_len = atoi(optarg); break;
case 'T': taper_len = atoi(optarg); break;
case 's': SNRdB = atof(optarg); break;
default:
exit(1);
}
}
unsigned int i;
// derived values
unsigned int frame_len = M + cp_len;
unsigned int num_samples = (3+num_symbols)*frame_len;
float nstd = powf(10.0f, noise_floor/20.0f);
float gamma = powf(10.0f, (SNRdB + noise_floor)/20.0f);
unsigned char p[M];
float complex X[M]; // channelized symbols
float complex y[num_samples]; // output time series
// initialize subcarrier allocation
ofdmframe_init_default_sctype(M, p);
// create subcarrier notch in upper half of band
unsigned int n0 = (unsigned int) (0.13 * M); // lower edge of notch
unsigned int n1 = (unsigned int) (0.21 * M); // upper edge of notch
for (i=n0; i<n1; i++)
p[i] = OFDMFRAME_SCTYPE_NULL;
// create struct for holding data symbols
struct rx_symbols data;
data.num_bpsk = 0;
data.num_qpsk = 0;
// create frame generator
ofdmframegen fg = ofdmframegen_create(M, cp_len, taper_len, p);
ofdmframegen_print(fg);
// create frame synchronizer
ofdmframesync fs = ofdmframesync_create(M, cp_len, taper_len, p, callback, (void*)&data);
ofdmframesync_print(fs);
unsigned int n=0;
// write first S0 symbol
ofdmframegen_write_S0a(fg, &y[n]);
n += frame_len;
// write second S0 symbol
ofdmframegen_write_S0b(fg, &y[n]);
n += frame_len;
// write S1 symbol
ofdmframegen_write_S1( fg, &y[n]);
n += frame_len;
// modulate data subcarriers
for (i=0; i<num_symbols; i++) {
// load different subcarriers with different data
unsigned int j;
for (j=0; j<M; j++) {
// ignore 'null' and 'pilot' subcarriers
if (p[j] != OFDMFRAME_SCTYPE_DATA)
continue;
// use BPSK for even frequencies, QPSK for odd
if ( (j % 2) == 0 ) {
// BPSK
X[j] = rand() % 2 ? -1.0f : 1.0f;
} else {
// QPSK
X[j] = (rand() % 2 ? -0.707f : 0.707f) +
(rand() % 2 ? -0.707f : 0.707f) * _Complex_I;
}
}
// generate OFDM symbol in the time domain
ofdmframegen_writesymbol(fg, X, &y[n]);
n += frame_len;
}
// add channel effects
for (i=0; i<num_samples; i++) {
// channel gain
y[i] *= gamma;
// add noise
y[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
}
// execute synchronizer on entire frame
ofdmframesync_execute(fs,y,num_samples);
// destroy objects
ofdmframegen_destroy(fg);
ofdmframesync_destroy(fs);
// estimate power spectral density of received signal
unsigned int nfft = 1024; // FFT size
float psd[nfft]; // PSD estimate output array
spgramcf_estimate_psd(nfft, y, num_samples, psd);
//
// export output file
//
FILE * fid = fopen(OUTPUT_FILENAME,"w");
fprintf(fid,"%% %s: auto-generated file\n\n", OUTPUT_FILENAME);
fprintf(fid,"clear all;\n");
fprintf(fid,"close all;\n");
fprintf(fid,"M = %u;\n", M);
fprintf(fid,"noise_floor = %f;\n", noise_floor);
fprintf(fid,"SNRdB = %f;\n", SNRdB);
fprintf(fid,"num_samples = %u;\n", num_samples);
fprintf(fid,"nfft = %u;\n", nfft);
// save received time-domain signal
fprintf(fid,"y = zeros(1,num_samples);\n");
for (i=0; i<num_samples; i++)
fprintf(fid,"y(%3u) = %12.4e + j*%12.4e;\n", i+1, crealf(y[i]), cimagf(y[i]));
// save power spectral density estimate
fprintf(fid,"psd = zeros(1,nfft);\n");
for (i=0; i<nfft; i++)
fprintf(fid,"psd(%3u) = %12.4e;\n", i+1, psd[i]);
fprintf(fid,"psd = 10*log10( fftshift(psd) );\n");
// save received symbols
fprintf(fid,"num_bpsk = %u;\n", data.num_bpsk);
fprintf(fid,"num_qpsk = %u;\n", data.num_qpsk);
fprintf(fid,"syms_bpsk = zeros(1,num_bpsk);\n");
fprintf(fid,"syms_qpsk = zeros(1,num_qpsk);\n");
for (i=0; i<data.num_bpsk; i++)
fprintf(fid,"syms_bpsk(%3u) = %12.4e + 1i*%12.4e;\n", i+1, crealf(data.syms_bpsk[i]), cimagf(data.syms_bpsk[i]));
for (i=0; i<data.num_qpsk; i++)
fprintf(fid,"syms_qpsk(%3u) = %12.4e + 1i*%12.4e;\n", i+1, crealf(data.syms_qpsk[i]), cimagf(data.syms_qpsk[i]));
// plot power spectral density
fprintf(fid,"\n\n");
fprintf(fid,"f=[0:(nfft-1)]/nfft - 0.5;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"plot(f,psd,'LineWidth',1.5,'Color',[0 0.3 0.5]);\n");
fprintf(fid,"psd_min = noise_floor - 10;\n");
fprintf(fid,"psd_max = noise_floor + 10 + max(SNRdB, 0);\n");
fprintf(fid,"axis([-0.5 0.5 psd_min psd_max]);\n");
fprintf(fid,"grid on;\n");
fprintf(fid,"xlabel('Normalized Frequency');\n");
fprintf(fid,"ylabel('Power Spectral Density [dB]');\n");
// plot received constellation
fprintf(fid,"\n\n");
fprintf(fid,"figure;\n");
fprintf(fid,"hold on;\n");
fprintf(fid,"plot(real(syms_bpsk),imag(syms_bpsk),'x','MarkerSize',4,'Color',[0 0.3 0.5]);\n");
fprintf(fid,"plot(real(syms_qpsk),imag(syms_qpsk),'x','MarkerSize',4,'Color',[0 0.5 0.3]);\n");
fprintf(fid,"hold off;\n");
fprintf(fid,"axis([-1 1 -1 1]*1.5);\n");
fprintf(fid,"axis square\n");
fprintf(fid,"grid on;\n");
fprintf(fid,"xlabel('I');\n");
fprintf(fid,"ylabel('Q');\n");
fprintf(fid,"legend('even subcarriers (BPSK)','odd subcarriers (QPSK)','location','northeast');\n");
fclose(fid);
printf("results written to %s\n", OUTPUT_FILENAME);
printf("done.\n");
return 0;
}
// callback function
// _X : array of received subcarrier samples [size: _M x 1]
// _p : subcarrier allocation array [size: _M x 1]
// _M : number of subcarriers
// _userdata : user-defined data pointer
static int callback(float complex * _X,
unsigned char * _p,
unsigned int _M,
void * _userdata)
{
// print status to the screen
printf("**** callback invoked\n");
// save received data subcarriers
struct rx_symbols * data = (struct rx_symbols*) _userdata;
unsigned int i;
for (i=0; i<_M; i++) {
// ignore 'null' and 'pilot' subcarriers
if (_p[i] != OFDMFRAME_SCTYPE_DATA)
continue;
// extract BPSK (even frequencies) and QPSK (odd frequencies) symbols
if ( (i % 2) == 0 && data->num_bpsk < 2000) {
// save at most 2000 BPSK symbols
data->syms_bpsk[data->num_bpsk] = _X[i];
data->num_bpsk++;
} else if ( (i % 2) == 1 && data->num_qpsk < 2000) {
// save at most 2000 QPSK symbols
data->syms_qpsk[data->num_qpsk] = _X[i];
data->num_qpsk++;
}
}
// return
return 0;
}
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