itpp::LDPC_Code Class Reference
[Forward Error Correcting Codes]

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

#include <ldpc.h>

List of all members.

Public Member Functions

LDPC_Code ()

Default constructor.

LDPC_Code (const LDPC_Parity *const H, LDPC_Generator *const G=0)

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)

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.

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.

Protected Attributes

bool H_defined

true if parity check matrix is defined

bool G_defined

true if generator is defined

int nvar

Number of variable nodes.

int ncheck

Number of check nodes.

LDPC_Generator * G

Generator object pointer.

std::string dec_method

Decoding method.

int max_iters

Maximum number of iterations.

bool psc

check syndrom after each iteration

bool pisc

check syndrom before first iteration

LLR_calc_unit llrcalc

LLR calculation unit.

Friends

std::ostream & operator<< (std::ostream &os, const LDPC_Code &C)

Print some properties of the 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 ldpc_gen_codes and ldpc_bersim_awgn for extensive examples of how to use LDPC codes.

Author:: Erik G. Larsson and Adam Piatyszek

Constructor & Destructor Documentation

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

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

This constructor simply calls set_code().

References set_code().

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().

References load_code().

Member Function Documentation

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.

References itpp::LLR_calc_unit::Boxplus(), H_defined, it_assert, it_error, it_info_debug, it_info_no_endl_debug, llrcalc, max_iters, ncheck, nvar, pisc, psc, and syndrome_check().

Referenced by decode(), and decode_soft_out().

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.

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

Referenced by encode().

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.

References itpp::it_ifile::close(), G, G_defined, H_defined, it_assert, it_info_debug, itpp::LDPC_Generator::load(), ncheck, nvar, 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.

References itpp::it_file::close(), G, G_defined, H_defined, it_assert, it_info_debug, ncheck, nvar, itpp::it_file::open(), and itpp::LDPC_Generator::save().

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

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

References decoder_parameterization(), G, G_defined, integrity_check(), and setup_decoder().

Referenced by LDPC_Code().

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".

References dec_method, and it_assert.

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.

References it_assert, max_iters, pisc, and psc.

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

References ncheck.

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

Friends And Related Function Documentation

std::ostream & operator<<	(	std::ostream &	os,
		const LDPC_Code &	C
	)			`[friend]`

Print some properties of the codec in plain text.

Print some properties of the LDPC codec in plain text.

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


Public Member Functions
	LDPC_Code ()
	Default constructor.
	LDPC_Code (const LDPC_Parity const H, LDPC_Generator const G=0)
	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)
	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.
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.
Protected Attributes
bool	H_defined
	true if parity check matrix is defined
bool	G_defined
	true if generator is defined
int	nvar
	Number of variable nodes.
int	ncheck
	Number of check nodes.
LDPC_Generator *	G
	Generator object pointer.
std::string	dec_method
	Decoding method.
int	max_iters
	Maximum number of iterations.
bool	psc
	check syndrom after each iteration
bool	pisc
	check syndrom before first iteration
LLR_calc_unit	llrcalc
	LLR calculation unit.
Friends
std::ostream &	operator<< (std::ostream &os, const LDPC_Code &C)
	Print some properties of the codec in plain text.

itpp::LDPC_Code Class Reference [Forward Error Correcting Codes]

Public Member Functions

Protected Member Functions

Protected Attributes

Friends

Detailed Description

Constructor & Destructor Documentation

Member Function Documentation

Friends And Related Function Documentation

itpp::LDPC_Code Class Reference
[Forward Error Correcting Codes]