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// Demonstrates least mean-squares (LMS) equalizer (EQ) on a QPSK signal
#include <stdio.h>
#include <stdlib.h>
#include <complex.h>
#include <math.h>
#include "liquid.h"
#define OUTPUT_FILENAME "eqlms_cccf_example.m"
int main() {
unsigned int i, h_len=5, w_len=11, num_symbols=400;
float mu = 0.7f;
// modem
modemcf mod = modemcf_create(LIQUID_MODEM_QPSK);
// create channel filter (random coefficients)
float complex h[h_len];
h[0] = 1.0f;
for (i=1; i<h_len; i++)
h[i] = (randnf() + randnf()*_Complex_I) * 0.1f;
firfilt_cccf fchannel = firfilt_cccf_create(h,h_len);
// create equalizer (default initial coefficients)
float complex w[w_len];
for (i=0; i<w_len; i++) w[i] = i==0 ? 1 : 0;
eqlms_cccf eq = eqlms_cccf_create(w,w_len);
eqlms_cccf_set_bw(eq, mu);
// run equalization
float complex sym_in, sym_channel, sym_out; // modulated/recovered symbols
float rmse = 0.0f;
for (i=0; i<2*num_symbols; i++) {
// generate modulated input symbol
unsigned int sym = modemcf_gen_rand_sym(mod);
modemcf_modulate(mod, sym, &sym_in);
// apply channel filter (in place)
firfilt_cccf_push(fchannel, sym_in);
firfilt_cccf_execute(fchannel, &sym_channel);
// skip first w_len symbols
if (i < w_len) continue;
// update equalizer weights
eqlms_cccf_push (eq, sym_channel); // input symbol
eqlms_cccf_execute (eq, &sym_out); // dot product
//eqlms_cccf_step(eq, sym_in, sym_out); // ideal update
eqlms_cccf_step_blind(eq, sym_out); // blind update
// accumulate RMS error
if (i < num_symbols) continue;
float e = cabsf(sym_out - sym_in);
rmse += e*e;
}
eqlms_cccf_print(eq);
rmse = 10*log10f( rmse / (float)num_symbols);
printf("RMSE: %.3f dB\n", rmse);
eqlms_cccf_copy_coefficients(eq,w);
// clean up objects
firfilt_cccf_destroy(fchannel);
eqlms_cccf_destroy(eq);
modemcf_destroy(mod);
// export data to file
FILE * fid = fopen(OUTPUT_FILENAME,"w");
fprintf(fid,"%% %s: auto-generated file\n\n", OUTPUT_FILENAME);
fprintf(fid,"clear all; close all;\n");
fprintf(fid,"h_len=%u; w_len=%u;\n", h_len, w_len);
// save channel coefficients
for (i=0; i<h_len; i++)
fprintf(fid,"h(%3u) = %12.4e + j*%12.4e;\n", i+1, crealf(h[i]), cimagf(h[i]));
// save equalizer coefficients
for (i=0; i<w_len; i++)
fprintf(fid,"w(%3u) = %12.4e + j*%12.4e;\n", i+1, crealf(w[i]), cimagf(w[i]));
// plot results
fprintf(fid,"\n\n");
fprintf(fid,"nfft=1200;\n");
fprintf(fid,"f=[0:(nfft-1)]/nfft - 0.5;\n");
fprintf(fid,"H=20*log10(abs(fftshift(fft(h,nfft))));\n");
fprintf(fid,"W=20*log10(abs(fftshift(fft(w,nfft))));\n");
fprintf(fid,"figure;\n");
fprintf(fid,"plot(f,H,'-r',f,W,'-b', f,H+W,'-k','LineWidth',2);\n");
fprintf(fid,"xlabel('Normalied Frequency');\n");
fprintf(fid,"ylabel('Power Spectral Density [dB]');\n");
fprintf(fid,"axis([-0.5 0.5 -10 10]);\n");
fprintf(fid,"legend('channel','equalizer','composite');\n");
fclose(fid);
printf("results written to %s.\n",OUTPUT_FILENAME);
return 0;
}
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