Low-density parity check (LDPC) codec. More...

#include <itpp/comm/ldpc.h>

Inheritance diagram for itpp::LDPC_Code:

Public Member Functions
	LDPC_Code ()
	Default constructor.

	LDPC_Code (const LDPC_Parity const H, LDPC_Generator const G=0, bool perform_integrity_check=true)
	Constructor, from a parity check matrix and optionally a generator.

	LDPC_Code (const std::string &filename, LDPC_Generator *const G=0)
	Constructor, from a saved file.

virtual	~LDPC_Code ()
	Destructor.

void	set_code (const LDPC_Parity const H, LDPC_Generator const G=0, bool perform_integrity_check=true)
	Set the codec, from a parity check matrix and optionally a generator.

void	load_code (const std::string &filename, LDPC_Generator *const G=0)
	Set the codec, by reading from a saved file.

void	save_code (const std::string &filename) const
	Save the codec to a file.

void	set_decoding_method (const std::string &method)
	Set the decoding method.

void	set_exit_conditions (int max_iters, bool syndr_check_each_iter=true, bool syndr_check_at_start=false)
	Set the decoding loop exit conditions.

void	set_llrcalc (const LLR_calc_unit &llrcalc)
	Set LLR calculation unit.

virtual void	encode (const bvec &input, bvec &output)
	Encode codeword.

virtual bvec	encode (const bvec &input)
	Encode codeword.

virtual void	decode (const bvec &, bvec &)
	Inherited from the base class - not implemented here.

virtual bvec	decode (const bvec &)
	Inherited from the base class - not implemented here.

virtual void	decode (const vec &llr_in, bvec &syst_bits)
	This function outputs systematic bits of the decoded codeword.

virtual bvec	decode (const vec &llr_in)
	This function outputs systematic bits of the decoded codeword.

void	decode_soft_out (const vec &llr_in, vec &llr_out)
	This function is a wrapper for `bp_decode()`

vec	decode_soft_out (const vec &llr_in)
	This function is a wrapper for `bp_decode()`

int	bp_decode (const QLLRvec &LLRin, QLLRvec &LLRout)
	Belief propagation decoding.

bool	syndrome_check (const QLLRvec &LLR) const
	Syndrome check, on QLLR vector.

bool	syndrome_check (const bvec &b) const
	Syndrome check, on bit vector.

QLLRvec	soft_syndrome_check (const QLLRvec &LLR) const
	Soft syndrome check.

double	get_rate () const
	Get the coderate.

int	get_nvar () const
	Get the number of variable nodes.

int	get_ncheck () const
	Get the number of check nodes.

int	get_ninfo () const
	Get the number of information bits per codeword.

std::string	get_decoding_method () const
	Return the decoding method.

int	get_nrof_iterations () const
	Get the maximum number of iterations of the decoder.

LLR_calc_unit	get_llrcalc () const
	Get LLR calculation unit used in decoder.

Protected Member Functions
void	decoder_parameterization (const LDPC_Parity *const H)
	Function to compute decoder parameterization.

void	integrity_check ()
	Function to check the integrity of the parity check matrix and generator.

void	setup_decoder ()
	Initialize decoder.

Friends
ITPP_EXPORT std::ostream &	operator<< (std::ostream &os, const LDPC_Code &C)
	Print some properties of the codec in plain text.

Related Functions
(Note that these are not member functions.)
ITPP_EXPORT std::ostream &	operator<< (std::ostream &os, const LDPC_Code &C)
	Print some properties of the LDPC codec in plain text.

Detailed Description

Low-density parity check (LDPC) codec.

This class provides the functionality for encoding and decoding of LDPC codes defined via LDPC_Parity and LDPC_Generator classes.

LDPC codecs are constructed from parity check and generator matrices. Since the procedure of constructing the codec can be time-consuming (for example, due to optimization of the parity matrix and computation of the generator matrix), codecs can be saved to a file for later use. This class provides functionality and a special file format (the file format is designed to optimize the operation of the decoder) to do this. Some examples of load and save operations follow:

Saving a codec without generator matrix:

// assume the parity matrix is already defined and stored in H
LDPC_Code C(&H);
C.save_code("filename.it");

Saving a codec with generator matrix (for the example of systematic generator):

// assume the parity matrix is already defined and stored in H
LDPC_Generator_Systematic G(&H); // create generator
LDPC_Code C(&H, &G);
C.save_code("filename.it");

Loading a codec without a generator:

LDPC_Code("filename.it");

Loading a codec with a generator (systematic in this example):

LDPC_Generator_Systematic G; // the generator object must be created first

LDPC_Code("filename.it", &G);

Note: Please refer to the tutorials Generation of LDPC codes and Simulation of LDPC codes the AWGN channel for extensive examples of how to use LDPC codes.; For issues relating to the accuracy of LLR computations, please see the documentation of LLR_calc_unit

Author: Erik G. Larsson, Adam Piatyszek and Gorka Prieto (decoder improvements)

Definition at line 731 of file ldpc.h.

Constructor & Destructor Documentation

itpp::LDPC_Code::LDPC_Code	(	const LDPC_Parity *const	H,
		LDPC_Generator *const	G = `0`,
		bool	perform_integrity_check = `true`
	)

Constructor, from a parity check matrix and optionally a generator.

This constructor simply calls set_code().

Parameters

H	The parity check matrix
G	A pointer to the optional generator object
perform_integrity_check	if true, then check that the parity and generator matrices are consistent

Definition at line 1210 of file ldpc.cpp.

References set_code(), and set_decoding_method().

itpp::LDPC_Code::LDPC_Code	(	const std::string &	filename,
		LDPC_Generator *const	G = `0`
	)

Constructor, from a saved file.

This constructor simply calls load_code().

Definition at line 1220 of file ldpc.cpp.

References load_code(), and set_decoding_method().

Member Function Documentation

void itpp::LDPC_Code::set_code	(	const LDPC_Parity *const	H,
		LDPC_Generator *const	G = `0`,
		bool	perform_integrity_check = `true`
	)

Set the codec, from a parity check matrix and optionally a generator.

Parameters

H	The parity check matrix
G	A pointer to the optional generator object
perform_integrity_check	if true, then check that the parity and generator matrices are consistent

Definition at line 1230 of file ldpc.cpp.

References decoder_parameterization(), integrity_check(), it_info_debug, and setup_decoder().

Referenced by LDPC_Code().

void itpp::LDPC_Code::load_code	(	const std::string &	filename,
		LDPC_Generator *const	G = `0`
	)

Set the codec, by reading from a saved file.

The file format is defined in the source code. LDPC codecs can be saved with the function save_code().

Parameters

filename	Name of the file where the codec is stored
G	A pointer to the optional generator object

Note: If G points at 0 (default), the generator data is not read from the saved file. This means that the encoding can not be performed.

Definition at line 1247 of file ldpc.cpp.

References itpp::it_ifile::close(), it_assert, it_info_debug, itpp::LDPC_binary_file_version, itpp::LDPC_Generator::load(), and setup_decoder().

Referenced by LDPC_Code().

void itpp::LDPC_Code::save_code ( const std::string & filename ) const

Save the codec to a file.

Parameters

filename Name of the file where to store the codec

Note: The decoder parameters (max_iters, syndr_check_each_iter, syndr_check_at_start and llrcalc) are not saved to a file.

Definition at line 1289 of file ldpc.cpp.

References itpp::it_file::close(), it_assert, it_info_debug, itpp::LDPC_binary_file_version, itpp::it_file::open(), and itpp::LDPC_Generator::save().

void itpp::LDPC_Code::set_decoding_method ( const std::string & method )

Set the decoding method.

Currently only a belief propagation method ("BP" or "bp") is supported.

Note: The default method set in the class constructors is "BP".

Definition at line 1323 of file ldpc.cpp.

References it_assert.

Referenced by LDPC_Code().

void itpp::LDPC_Code::set_exit_conditions	(	int	max_iters,
		bool	syndr_check_each_iter = `true`,
		bool	syndr_check_at_start = `false`
	)

Set the decoding loop exit conditions.

Parameters

max_iters	Maximum number of the decoding iterations
syndr_check_each_iter	If true, break the decoding loop as soon as valid codeword is found. Recommended value: `true`.
syndr_check_at_start	If true, perform a syndrome check before entering the decoding loop. If LLRin corresponds to a valid codeword, set LLRout = LLRin. Recommended value: `false`.

Note: The default values set in the class constructor are: "50", "true" and "false", respectively.

Definition at line 1330 of file ldpc.cpp.

References it_assert.

void itpp::LDPC_Code::encode	(	const bvec &	input,
		bvec &	output
	)

virtual

Encode codeword.

This is a wrapper functions which calls a proper implementation from the LDPC_Generator object.

Parameters

input	Vector of `ncheck` input bits
output	Vector of `nvar` output bits

Implements itpp::Channel_Code.

Definition at line 1347 of file ldpc.cpp.

References itpp::LDPC_Generator::encode(), it_assert, it_assert_debug, and syndrome_check().

Referenced by encode().

int itpp::LDPC_Code::bp_decode	(	const QLLRvec &	LLRin,
		QLLRvec &	LLRout
	)

Belief propagation decoding.

This function implements the sum-product message passing decoder (Pearl's belief propagation) using LLR values as messages. A fast update mechanism is used for nodes with large degrees.

Parameters

LLRin	vector of `nvar` input LLR values
LLRout	vector of `nvar` output LLR values

If the decoder converged to a valid codeword, the function returns the number of iterations performed. Otherwise the function returns the number of iterations performed but with negative sign.

One can use set_exit_conditions() method to change the number of decoding iterations and related parameters parameters. The decoding function uses LLR_calc_unit to implement table-lookup for the Boxplus operator. By setting the parameters of the LLR_calc_unit provided in set_llrcalc(), one can change the resolution, for example to use a logmax approximation. See the documentation of LLR_calc_unit for details on how to do this.

Definition at line 1393 of file ldpc.cpp.

References itpp::LLR_calc_unit::Boxplus(), it_assert, it_error, it_info_debug, it_info_no_endl_debug, and syndrome_check().

Referenced by decode(), and decode_soft_out().

bool itpp::LDPC_Code::syndrome_check ( const QLLRvec & LLR ) const

Syndrome check, on QLLR vector.

This function performs a syndrome check on a softbit (LLR vector). The function returns true for a valid codeword, false else.

Parameters

LLR	LLR-vector to check

Definition at line 1642 of file ldpc.cpp.

Referenced by bp_decode(), encode(), integrity_check(), and syndrome_check().

QLLRvec itpp::LDPC_Code::soft_syndrome_check ( const QLLRvec & LLR ) const

Soft syndrome check.

This function checks all parity constraints and computes for each one the posterior probability that it is satisfied. The result is a vector, whose i:th element is given by

$\mbox{Boxplus}_j LLR_{p_{ij}}$

where

$p_{ij}$

is the index of the j:th nonzero element of the i:th row of the code's parity check matrix.

Definition at line 1665 of file ldpc.cpp.

References itpp::LLR_calc_unit::Boxplus().

The documentation for this class was generated from the following files:

itpp/comm/ldpc.h
itpp/comm/ldpc.cpp

Public Member Functions

Protected Member Functions

Friends

Related Functions

Detailed Description

Constructor & Destructor Documentation

Member Function Documentation