llr.cpp
Go to the documentation of this file.
#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;
}
}
itpp::Array
General array class.
Definition array.h:105
itpp::LLR_calc_unit
Log-likelihood algebra calculation unit.
Definition llr.h:125
itpp::LLR_calc_unit::QLLR
signed int QLLR
Definition llr.h:46
itpp::LLR_calc_unit::Boxplus
QLLR Boxplus(QLLR a, QLLR b) const
Hagenauer's "Boxplus" operator.
Definition llr.cpp:123
itpp::LLR_calc_unit::init_llr_tables
void init_llr_tables(short int Dint1=12, short int Dint2=300, short int Dint3=7)
Set the quantization and table parameters.
Definition llr.cpp:55
itpp::LLR_calc_unit::get_Dint
ivec get_Dint()
Retrieve the table resolution values.
Definition llr.cpp:46
itpp::LLR_calc_unit::to_qllr
QLLR to_qllr(double l) const
Convert a "real" LLR value to an LLR type.
Definition llr.h:245
itpp::LLR_calc_unit::logexp
QLLR logexp(QLLR x) const
Logexp operator.
Definition llr.h:261
itpp::LLR_calc_unit::to_double
double to_double(QLLR l) const
Convert an LLR type to a "real" LLR.
Definition llr.h:240
itpp::LLR_calc_unit::LLR_calc_unit
LLR_calc_unit()
Constructor, using the default table resolution.
Definition llr.cpp:36
itpp::LLR_calc_unit::QLLR_MAX
const QLLR QLLR_MAX
Definition llr.h:61
itpp::Mat
Matrix Class (Templated)
Definition mat.h:202
itpp::Mat::rows
int rows() const
The number of rows.
Definition mat.h:237
itpp::Mat::cols
int cols() const
The number of columns.
Definition mat.h:235
itpp::Vec
Vector Class (Templated)
Definition vec.h:245
it_info_debug
#define it_info_debug(s)
Print information message if NDEBUG is not defined.
Definition itassert.h:163
it_assert
#define it_assert(t, s)
Abort if t is not true.
Definition itassert.h:94
itpp::pow2
double pow2(double x)
Calculate two to the power of x (2^x)
Definition log_exp.h:55
itpp::length
int length(const Vec< T > &v)
Length of vector.
Definition matfunc.h:51
llr.h
Class for numerically efficient log-likelihood algebra.
itpp
itpp namespace
Definition itmex.h:37
itpp::operator<<
std::ostream & operator<<(std::ostream &output, const bin &inbin)
Output stream of bin.
Definition binary.cpp:36

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