// create gmskdem object
// _k : samples/symbol
// _m : filter delay (symbols)
// _BT : excess bandwidth factor
gmskdem gmskdem_create(unsigned int _k,
unsigned int _m,
float _BT)
{
if (_k < 2) {
fprintf(stderr,"error: gmskdem_create(), samples/symbol must be at least 2\n");
exit(1);
} else if (_m < 1) {
fprintf(stderr,"error: gmskdem_create(), symbol delay must be at least 1\n");
exit(1);
} else if (_BT <= 0.0f || _BT >= 1.0f) {
fprintf(stderr,"error: gmskdem_create(), bandwidth/time product must be in (0,1)\n");
exit(1);
}
// allocate memory for main object
gmskdem q = (gmskdem)malloc(sizeof(struct gmskdem_s));
// set properties
q->k = _k;
q->m = _m;
q->BT = _BT;
// allocate memory for filter taps
q->h_len = 2*(q->k)*(q->m)+1;
q->h = (float*) malloc(q->h_len * sizeof(float));
// compute filter coefficients
liquid_firdes_gmskrx(q->k, q->m, q->BT, 0.0f, q->h);
#if GMSKDEM_USE_EQUALIZER
// receiver matched filter/equalizer
q->eq = eqlms_rrrf_create_rnyquist(LIQUID_RNYQUIST_GMSKRX,
q->k,
q->m,
q->BT,
0.0f);
eqlms_rrrf_set_bw(q->eq, 0.01f);
q->k_inv = 1.0f / (float)(q->k);
#else
// create filter object
q->filter = firfilt_rrrf_create(q->h, q->h_len);
#endif
// reset modem state
gmskdem_reset(q);
#if DEBUG_GMSKDEM
q->debug_mfout = windowf_create(DEBUG_BUFFER_LEN);
#endif
// return modem object
return q;
}
int main(int argc, char*argv[]) {
// options
unsigned int k=4; // filter samples/symbol
unsigned int m=3; // filter delay (symbols)
float BT=0.3f; // bandwidth-time product
unsigned int num_data_symbols=200; // number of data symbols
float SNRdB = 30.0f; // signal-to-noise ratio [dB]
float phi = 0.0f; // carrier phase offset
float dphi = 0.0f; // carrier frequency offset
int dopt;
while ((dopt = getopt(argc,argv,"uhk:m:n:b:s:")) != EOF) {
switch (dopt) {
case 'u':
case 'h': usage(); return 0;
case 'k': k = atoi(optarg); break;
case 'm': m = atoi(optarg); break;
case 'n': num_data_symbols = atoi(optarg); break;
case 'b': BT = atof(optarg); break;
case 's': SNRdB = atof(optarg); break;
default:
exit(1);
}
}
// validate input
if (BT <= 0.0f || BT >= 1.0f) {
fprintf(stderr,"error: %s, bandwidth-time product must be in (0,1)\n", argv[0]);
exit(1);
}
// derived values
unsigned int num_symbols = num_data_symbols + 2*m;
unsigned int num_samples = k*num_symbols;
float nstd = powf(10.0f,-SNRdB/20.0f); // noise standard deviation
// create modulator
gmskmod mod = gmskmod_create(k, m, BT);
gmskmod_print(mod);
// create demodulator
gmskdem demod = gmskdem_create(k, m, BT);
gmskdem_set_eq_bw(demod, 0.01f);
gmskdem_print(demod);
unsigned int i;
unsigned int s[num_symbols];
float complex x[num_samples];
float complex y[num_samples];
unsigned int sym_out[num_symbols];
// generate random data sequence
for (i=0; i<num_symbols; i++)
s[i] = rand() % 2;
// modulate signal
for (i=0; i<num_symbols; i++)
gmskmod_modulate(mod, s[i], &x[k*i]);
// add channel impairments
for (i=0; i<num_samples; i++) {
y[i] = x[i]*cexpf(_Complex_I*(phi + i*dphi));
y[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
}
// demodulate signal
for (i=0; i<num_symbols; i++)
gmskdem_demodulate(demod, &y[k*i], &sym_out[i]);
// destroy modem objects
gmskmod_destroy(mod);
gmskdem_destroy(demod);
// print results to screen
unsigned int delay = 2*m;
unsigned int num_errors=0;
for (i=delay; i<num_symbols; i++) {
//printf(" %4u : %2u (%2u)\n", i, s[i-delay], sym_out[i]);
num_errors += (s[i-delay] == sym_out[i]) ? 0 : 1;
}
printf("symbol errors : %4u / %4u\n", num_errors, num_data_symbols);
// write results to output file
FILE * fid = fopen(OUTPUT_FILENAME,"w");
fprintf(fid,"%% %s : auto-generated file\n", OUTPUT_FILENAME);
fprintf(fid,"clear all\n");
fprintf(fid,"close all\n");
fprintf(fid,"k = %u;\n", k);
fprintf(fid,"m = %u;\n", m);
fprintf(fid,"BT = %f;\n", BT);
fprintf(fid,"num_symbols = %u;\n", num_symbols);
fprintf(fid,"num_samples = %u;\n", num_samples);
fprintf(fid,"x = zeros(1,num_samples);\n");
fprintf(fid,"y = zeros(1,num_samples);\n");
for (i=0; i<num_samples; i++) {
fprintf(fid,"x(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(x[i]), cimagf(x[i]));
fprintf(fid,"y(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(y[i]), cimagf(y[i]));
}
fprintf(fid,"t=[0:(num_samples-1)]/k;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"plot(t,real(y),t,imag(y));\n");
// artificially demodulate (generate receive filter, etc.)
float hr[2*k*m+1];
liquid_firdes_gmskrx(k,m,BT,0,hr);
for (i=0; i<2*k*m+1; i++)
fprintf(fid,"hr(%3u) = %12.8f;\n", i+1, hr[i]);
fprintf(fid,"z = filter(hr,1,arg( ([y(2:end) 0]).*conj(y) ))/k;\n");
fprintf(fid,"figure;\n");
fprintf(fid,"plot(t,z,t(k:k:end),z(k:k:end),'or');\n");
fprintf(fid,"grid on;\n");
fclose(fid);
printf("results written to '%s'\n", OUTPUT_FILENAME);
return 0;
}
// Design (root-)Nyquist filter from prototype
// _type : filter type (e.g. LIQUID_FIRFILT_RRRC)
// _k : samples/symbol
// _m : symbol delay
// _beta : excess bandwidth factor, _beta in [0,1]
// _dt : fractional sample delay
// _h : output coefficient buffer (length: 2*k*m+1)
void liquid_firdes_prototype(liquid_firfilt_type _type,
unsigned int _k,
unsigned int _m,
float _beta,
float _dt,
float * _h)
{
// compute filter parameters
unsigned int h_len = 2*_k*_m + 1; // length
float fc = 0.5f / (float)_k; // cut-off frequency
float df = _beta / (float)_k; // transition bandwidth
float As = estimate_req_filter_As(df,h_len); // stop-band attenuation
// Parks-McClellan algorithm parameters
float bands[6] = { 0.0f, fc-0.5f*df,
fc, fc,
fc+0.5f*df, 0.5f};
float des[3] = { (float)_k, 0.5f*_k, 0.0f };
float weights[3] = {1.0f, 1.0f, 1.0f};
liquid_firdespm_wtype wtype[3] = { LIQUID_FIRDESPM_FLATWEIGHT,
LIQUID_FIRDESPM_FLATWEIGHT,
LIQUID_FIRDESPM_FLATWEIGHT};
switch (_type) {
// Nyquist filter prototypes
case LIQUID_FIRFILT_KAISER:
liquid_firdes_kaiser(h_len, fc, As, _dt, _h);
break;
case LIQUID_FIRFILT_PM:
// WARNING: input timing offset is ignored here
firdespm_run(h_len, 3, bands, des, weights, wtype, LIQUID_FIRDESPM_BANDPASS, _h);
break;
case LIQUID_FIRFILT_RCOS:
liquid_firdes_rcos(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_FEXP:
liquid_firdes_fexp(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_FSECH:
liquid_firdes_fsech(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_FARCSECH:
liquid_firdes_farcsech(_k, _m, _beta, _dt, _h);
break;
// root-Nyquist filter prototypes
case LIQUID_FIRFILT_ARKAISER:
liquid_firdes_arkaiser(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_RKAISER:
liquid_firdes_rkaiser(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_RRC:
liquid_firdes_rrcos(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_hM3:
liquid_firdes_hM3(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_GMSKTX:
liquid_firdes_gmsktx(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_GMSKRX:
liquid_firdes_gmskrx(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_RFEXP:
liquid_firdes_rfexp(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_RFSECH:
liquid_firdes_rfsech(_k, _m, _beta, _dt, _h);
break;
case LIQUID_FIRFILT_RFARCSECH:
liquid_firdes_rfarcsech(_k, _m, _beta, _dt, _h);
break;
default:
fprintf(stderr,"error: liquid_firdes_prototype(), invalid root-Nyquist filter type '%d'\n", _type);
exit(1);
}
}
