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/*!
* \file
* \brief Class for numerically efficient log-likelihood algebra
* \author Erik G. Larsson and Martin Senst
*
* -------------------------------------------------------------------------
*
* IT++ - C++ library of mathematical, signal processing, speech processing,
* and communications classes and functions
*
* Copyright (C) 1995-2008 (see AUTHORS file for a list of contributors)
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*
* -------------------------------------------------------------------------
*/
#include <itpp/comm/llr.h>
namespace itpp {
LLR_calc_unit::LLR_calc_unit()
{
init_llr_tables();
}
LLR_calc_unit::LLR_calc_unit(short int d1, short int d2, short int d3)
{
init_llr_tables(d1,d2,d3);
}
ivec LLR_calc_unit::get_Dint()
{
ivec r(3);
r(0) = Dint1;
r(1) = Dint2;
r(2) = Dint3;
return r;
}
void LLR_calc_unit::init_llr_tables(short int d1, short int d2, short int d3)
{
Dint1 = d1; // 1<<Dint1 determines how integral LLRs relate to real LLRs (to_double=(1<<Dint)*int_llr)
Dint2 = d2; // number of entries in table for LLR operations
Dint3 = d3; // table resolution is 2^(-(Dint1-Dint3))
logexp_table = construct_logexp_table();
}
ivec LLR_calc_unit::construct_logexp_table()
{
ivec result(Dint2);
for (int i=0; i<Dint2; i++) {
double x = pow2(static_cast<double>(Dint3 - Dint1)) * i;
result(i) = to_qllr(std::log(1 + std::exp(-x)));
}
it_assert(length(result)==Dint2,"Ldpc_codec::construct_logexp_table()");
return result;
}
QLLRvec LLR_calc_unit::to_qllr(const vec &l) const {
int n=length(l);
ivec result(n);
for (int i=0; i<n; i++) {
result.set(i,to_qllr(l(i)));
}
return result;
}
vec LLR_calc_unit::to_double(const QLLRvec &l) const {
int n=length(l);
vec result(n);
for (int i=0; i<n; i++) {
result.set(i,to_double(l(i)));
}
return result;
}
QLLRmat LLR_calc_unit::to_qllr(const mat &l) const {
int m=l.rows();
int n=l.cols();
imat result(m,n);
for (int i=0; i<m; i++) {
for (int j=0; j<n; j++) {
result.set(i,j,to_qllr(l(i,j)));
}
}
return result;
}
mat LLR_calc_unit::to_double(const QLLRmat &l) const {
int m=l.rows();
int n=l.cols();
mat result(m,n);
for (int i=0; i<m; i++) {
for (int j=0; j<n; j++) {
result.set(i,j,to_double(l(i,j)));
}
}
return result;
}
// This function used to be inline, but in my experiments,
// the non-inlined version was actually faster /Martin Senst
QLLR LLR_calc_unit::Boxplus(QLLR a, QLLR b) const
{
QLLR a_abs = (a > 0 ? a : -a);
QLLR b_abs = (b > 0 ? b : -b);
QLLR minabs = (a_abs > b_abs ? b_abs : a_abs);
QLLR term1 = (a > 0 ? (b > 0 ? minabs : -minabs)
: (b > 0 ? -minabs : minabs));
if (Dint2 == 0) { // logmax approximation - avoid looking into empty table
// Don't abort when overflowing, just saturate the QLLR
if (term1 > QLLR_MAX) {
it_info_debug("LLR_calc_unit::Boxplus(): LLR overflow");
return QLLR_MAX;
}
if (term1 < -QLLR_MAX) {
it_info_debug("LLR_calc_unit::Boxplus(): LLR overflow");
return -QLLR_MAX;
}
return term1;
}
QLLR apb = a + b;
QLLR term2 = logexp((apb > 0 ? apb : -apb));
QLLR amb = a - b;
QLLR term3 = logexp((amb > 0 ? amb : -amb));
QLLR result = term1 + term2 - term3;
// Don't abort when overflowing, just saturate the QLLR
if (result > QLLR_MAX) {
it_info_debug("LLR_calc_unit::Boxplus() LLR overflow");
return QLLR_MAX;
}
if (result < -QLLR_MAX) {
it_info_debug("LLR_calc_unit::Boxplus() LLR overflow");
return -QLLR_MAX;
}
return result;
}
std::ostream &operator<<(std::ostream &os, const LLR_calc_unit &lcu)
{
os << "---------- LLR calculation unit -----------------" << std::endl;
os << "LLR_calc_unit table properties:" << std::endl;
os << "The granularity in the LLR representation is "
<< pow2(static_cast<double>(-lcu.Dint1)) << std::endl;
os << "The LLR scale factor is " << (1 << lcu.Dint1) << std::endl;
os << "The largest LLR that can be represented is "
<< lcu.to_double(QLLR_MAX) << std::endl;
os << "The table resolution is "
<< pow2(static_cast<double>(lcu.Dint3 - lcu.Dint1)) << std::endl;
os << "The number of entries in the table is " << lcu.Dint2 << std::endl;
os << "The tables truncates at the LLR value "
<< pow2(static_cast<double>(lcu.Dint3 - lcu.Dint1)) * lcu.Dint2
<< std::endl;
os << "-------------------------------------------------" << std::endl;
return os;
}
}
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