awgn_bp.h

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/* -*- c++ -*- */
/*
 * Copyright 2015 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
/* -----------------------------------------------------------------
 *
 * This class defines functions for message passing mechanism for a
 * AWGN channel. Message passing (also known as belief propagation)
 * is used for decoding LDPC codes. Details of how LDPC codes are
 * decoded is available in the link below
 * - http://www.cs.utoronto.ca/~radford/ftp/LDPC-2012-02-11/decoding.html
 *
 * Belief propagation decoding is a suboptimal but efficient method of
 * decoding LDPC codes.
 *
 */
 
#ifndef AWGN_BP_H
#define AWGN_BP_H
 
#include "alist.h"
#include "gf2mat.h"
#include <gnuradio/fec/api.h>
#include <cmath>
#include <vector>
 
class FEC_API awgn_bp
{
public:
    //! Default constructor
    awgn_bp(){};
 
    //! A constructor for given GF2Mat and sigma
    awgn_bp(const GF2Mat X, float sgma);
 
    //! A constructor for given alist and sigma
    awgn_bp(alist _list, float sgma);
 
    //! Initializes the class using given alist and sigma
    void set_alist_sigma(alist _list, float sgma);
 
    //! Returns the variable Q
    std::vector<std::vector<double>> get_Q();
 
    //! Returns the variable R
    std::vector<std::vector<double>> get_R();
 
    //! Returns the variable H
    GF2Mat get_H();
 
    //! Calculates the likelihood ratios given an input vector
    void rx_lr_calc(std::vector<float> codeword);
 
    //! Returns the variable rx_lr
    std::vector<double> get_rx_lr();
 
    //! Returns the variable lr
    std::vector<double> get_lr();
 
    //! Initializes the sum product algorithm set-up
    void spa_initialize();
 
    //! Updates the check-nodes based on messages from variable nodes
    void update_chks();
 
    //! Updates the variable-nodes based on messages from check nodes
    void update_vars();
 
    //! Returns the current estimate
    std::vector<uint8_t> get_estimate();
 
    //! Computes initial estimate based on the vector rx_word
    void compute_init_estimate(std::vector<float> rx_word);
 
    //! Computes the estimate based on current likelihood-ratios lr
    void decision();
 
    //! Returns the syndrome for the current estimate
    std::vector<uint8_t> get_syndrome();
 
    //! Returns the syndrome for the input codeword
    std::vector<uint8_t> get_syndrome(const std::vector<uint8_t> codeword);
 
    //! Checks if the current estimate is a codeword
    bool is_codeword();
 
    //! Checks if the input is a codeword
    bool is_codeword(const std::vector<uint8_t> codeword);
 
    //! Sets the variable K
    void set_K(int k);
 
    //! Returns the variable K
    int get_K();
 
    //! Sets the variable max_iterations
    void set_max_iterations(int k);
 
    //! Returns the variable max_iterations
    int get_max_iterations();
 
    /*!
     * \brief Decodes the given vector rx_word by message passing.
     *
     * \param rx_word The received samples for decoding.
     * \param niterations The number of message passing iterations
     *        done to decode this codeword.
     */
    std::vector<uint8_t> decode(std::vector<float> rx_word, int* niterations);
 
private:
    //! The number of check nodes in the tanner-graph
    int M;
 
    //! The number of variable nodes in the tanner-graph
    int N;
 
    //! The dimension of the code used
    int K;
 
    //! The maximum number of message passing iterations allowed
    int max_iterations;
 
    //! The parity check matrix of the LDPC code
    GF2Mat H;
 
    //! The standard-deviation of the AWGN channel
    float sigma;
 
    //! Matrix holding messages from check nodes to variable nodes
    std::vector<std::vector<double>> R;
 
    //! Matrix holding messages from variable nodes to check nodes
    std::vector<std::vector<double>> Q;
 
    //! The array of likelihood computed from the channel output
    std::vector<double> rx_lr;
 
    //! The array for holding likelihoods computed on BP decoding
    std::vector<double> lr;
 
    //! List of integer coordinates along each column with non-zero entries
    std::vector<std::vector<int>> nlist;
 
    //! List of integer coordinates along each row with non-zero entries
    std::vector<std::vector<int>> mlist;
 
    //! Weight of each column n
    std::vector<int> num_nlist;
 
    //! Weight of each row m
    std::vector<int> num_mlist;
 
    //! The array for holding estimate computed on BP decoding
    std::vector<uint8_t> estimate;
};
#endif // ifndef AWGN_BP_H