The turbo equalizer uses a SISO NSC module and a SISO equalizer module. Optionally a precoder can be used at the channel input (by default the precoder is enabled).
Reference: R. Koetter, A. C. Singer, and M. Tuchler, ''Turbo equalization: an iterative equalization and decoding technique for coded data transmision,`` IEEE Signal Processing Magazine, pp. 67-80, Jan. 2004
#define USE_PRECODER
using std::cout;
using std::endl;
using std::string;
int main(void)
{
double threshold_value = 50;
string map_metric = "maxlogMAP";
ivec gen = "07 05";
int constraint_length = 3;
int ch_nb_taps = 4;
int nb_errors_lim = 3000;
int nb_bits_lim = int(1e6);
int perm_len =
pow2i(14);
int nb_iter = 10;
vec EbN0_dB = "0:0.5:10";
double R = 1.0 / 2.0;
double Ec = 1.0;
#ifdef USE_PRECODER
ivec prec_gen = "03 02";
int prec_gen_length = 2;
#endif
int nb_bits_tail = perm_len / gen.length();
int nb_bits = nb_bits_tail - (constraint_length - 1);
vec sigma2 = (0.5 * Ec / R) *
pow(
inv_dB(EbN0_dB), -1.0);
int nb_blocks;
int nb_errors;
bvec bits(nb_bits);
bvec nsc_coded_bits(perm_len);
bvec em_bits(perm_len);
ivec perm(perm_len);
ivec inv_perm(perm_len);
vec rec(perm_len);
vec eq_apriori_data(perm_len);
vec eq_extrinsic_data;
vec nsc_intrinsic_coded(perm_len);
vec nsc_apriori_data(nb_bits_tail);
nsc_apriori_data.zeros();
vec nsc_extrinsic_coded;
vec nsc_extrinsic_data;
bvec rec_bits(nb_bits_tail);
int snr_len = EbN0_dB.length();
mat ber(nb_iter, snr_len);
ber.zeros();
register int en, n;
Convolutional_Code nsc;
nsc.set_generator_polynomials(gen, constraint_length);
#ifdef USE_PRECODER
Rec_Syst_Conv_Code prec;
prec.set_generator_polynomials(prec_gen, prec_gen_length);
#endif
BPSK bpsk;
AWGN_Channel awgn;
vec ch_imp_response(ch_nb_taps);
vec ini_state =
ones(ch_nb_taps);
MA_Filter<double, double, double> multipath_channel;
SISO siso;
siso.set_generators(gen, constraint_length);
siso.set_map_metric(map_metric);
#ifdef USE_PRECODER
siso.set_precoder_generator(prec_gen(0), prec_gen_length);
#endif
BERC berc;
for (en = 0;en < snr_len;en++) {
cout << "EbN0_dB = " << EbN0_dB(en) << endl;
awgn.set_noise(sigma2(en));
siso.set_noise(sigma2(en));
nb_errors = 0;
nb_blocks = 0;
while ((nb_errors < nb_errors_lim) && (nb_blocks*nb_bits < nb_bits_lim))
{
perm = sort_index(
randu(perm_len));
inv_perm = sort_index(perm);
nsc.encode_tail(bits, nsc_coded_bits);
em_bits = nsc_coded_bits(perm);
#ifdef USE_PRECODER
prec.encode(em_bits, parity_bits);
em_bits = parity_bits.get_col(0);
#endif
ch_imp_response =
randray(ch_nb_taps);
multipath_channel.set_coeffs(ch_imp_response);
multipath_channel.set_state(ini_state);
rec = awgn(multipath_channel(bpsk.modulate_bits(em_bits)));
eq_apriori_data.zeros();
siso.set_impulse_response(ch_imp_response);
for (n = 0;n < nb_iter;n++)
{
siso.equalizer(eq_extrinsic_data, rec, eq_apriori_data, false);
nsc_intrinsic_coded =
SISO::threshold(eq_extrinsic_data(inv_perm), threshold_value);
siso.nsc(nsc_extrinsic_coded, nsc_extrinsic_data, nsc_intrinsic_coded, nsc_apriori_data, true);
rec_bits = bpsk.demodulate_bits(-nsc_extrinsic_data);
berc.clear();
berc.count(bits, rec_bits.left(nb_bits));
ber(n, en) += berc.get_errorrate();
eq_apriori_data = nsc_extrinsic_coded(perm);
}
nb_errors += int(berc.get_errors());
nb_blocks++;
}
ber.set_col(en, ber.get_col(en) / nb_blocks);
}
it_file ff("turbo_equalizer_bersim_multipath.it");
ff << Name("BER") << ber;
ff << Name("EbN0_dB") << EbN0_dB;
ff.close();
return 0;
}
static double threshold(const double &x, const double &value)
Functions used to limit values at a given +- threshold.
vec pow(const double x, const vec &y)
Calculates x to the power of y (x^y)
int pow2i(int x)
Calculate two to the power of x (2^x); x is integer.
double inv_dB(double x)
Inverse of decibel of x.
T sum_sqr(const Vec< T > &v)
Sum of square of the elements in a vector.
vec sqrt(const vec &x)
Square root of the elements.
void RNG_randomize()
Set a random seed for all Random Number Generators in the current thread.
double randu(void)
Generates a random uniform (0,1) number.
vec randray(int size, double sigma=1.0)
Generates a random Rayleigh vector.
bin randb(void)
Generates a random bit (equally likely 0s and 1s)
ITPP_EXPORT vec ones(int size)
A float vector of ones.
Include file for the IT++ communications module.
Mat< bin > bmat
bin matrix
When you run this program, the results (BER and EbN0_dB) are saved into turbo_equalizer_bersim_multipath.it file. Using the following MATLAB script
the results can be displayed.