llr.h

Go to the documentation of this file.

#ifndef LLR_H
#define LLR_H

#include <limits>
#include <itpp/base/vec.h>
#include <itpp/base/mat.h>
#include <itpp/base/specmat.h>
#include <itpp/base/matfunc.h>
#include <limits>

namespace itpp
{

typedef signed int QLLR;

typedef Vec<QLLR> QLLRvec;

typedef Mat<QLLR> QLLRmat;

const QLLR QLLR_MAX = (std::numeric_limits<QLLR>::max() >> 4);
// added some margin to make sure the sum of two LLR is still permissible

class LLR_calc_unit
{
public:
  LLR_calc_unit();

  LLR_calc_unit(short int Dint1, short int Dint2, short int Dint3);

  void init_llr_tables(short int Dint1 = 12, short int Dint2 = 300,
                       short int Dint3 = 7);

  QLLR to_qllr(double l) const;

  QLLRvec to_qllr(const vec &l) const;

  QLLRmat to_qllr(const mat &l) const;

  double to_double(QLLR l) const;

  vec to_double(const QLLRvec &l) const;

  mat to_double(const QLLRmat &l) const;

  inline QLLR jaclog(QLLR a, QLLR b) const;
  // Note: a version of this function taking "double" values as input
  // is deliberately omitted, because this is rather slow.

  QLLR Boxplus(QLLR a, QLLR b) const;

  inline QLLR logexp(QLLR x) const;

  ivec get_Dint();

  friend std::ostream &operator<<(std::ostream &os, const LLR_calc_unit &l);

private:
  ivec construct_logexp_table();

  ivec logexp_table;

  short int Dint1, Dint2, Dint3;
};

std::ostream &operator<<(std::ostream &os, const LLR_calc_unit &lcu);


// ----------------------------------------------------------------------
// implementation of some inline functions
// ----------------------------------------------------------------------

inline double LLR_calc_unit::to_double(QLLR l) const
{
  return static_cast<double>(l) / (1 << Dint1);
}

inline QLLR LLR_calc_unit::to_qllr(double l) const
{
  double QLLR_MAX_double = to_double(QLLR_MAX);
  // Don't abort when overflow occurs, just saturate the QLLR
  if (l > QLLR_MAX_double) {
    it_info_debug("LLR_calc_unit::to_qllr(): LLR overflow");
    return QLLR_MAX;
  }
  if (l < -QLLR_MAX_double) {
    it_info_debug("LLR_calc_unit::to_qllr(): LLR overflow");
    return -QLLR_MAX;
  }
  return static_cast<QLLR>(std::floor(0.5 + (1 << Dint1) * l));
}


inline QLLR LLR_calc_unit::logexp(QLLR x) const
{
  it_assert_debug(x >= 0, "LLR_calc_unit::logexp(): Wrong LLR value");
  int ind = x >> Dint3;
  if (ind >= Dint2) // outside table
    return 0;

  it_assert_debug(ind >= 0, "LLR_calc_unit::logexp(): Internal error");
  it_assert_debug(ind < Dint2, "LLR_calc_unit::logexp(): internal error");

  // With interpolation
  // int delta=x-(ind<<Dint3);
  // return ((delta*logexp_table(ind+1) + ((1<<Dint3)-delta)*logexp_table(ind)) >> Dint3);

  // Without interpolation
  return logexp_table(ind);
}


inline QLLR LLR_calc_unit::jaclog(QLLR a, QLLR b) const
{
  QLLR x, maxab;

  if (a > b) {
    maxab = a;
    x = a - b;
  }
  else {
    maxab = b;
    x = b - a;
  }

  if (maxab >= QLLR_MAX)
    return QLLR_MAX;
  else
    return (maxab + logexp(x));
}

}

#endif

---