Irregular LDPC code generator class. More...

#include <itpp/comm/ldpc.h>

Inheritance diagram for itpp::LDPC_Parity_Irregular:

Public Member Functions
	LDPC_Parity_Irregular ()
	Default constructor.

	LDPC_Parity_Irregular (int Nvar, const vec &var_deg, const vec &chk_deg, const std::string &method="rand", const ivec &options="200 6")
	Constructor that invokes `generate()` method.

void	generate (int Nvar, const vec &var_deg, const vec &chk_deg, const std::string &method="rand", const ivec &options="200 6")
	Generate an irregular LDPC code.

void	display_stats () const
	Display some information about the matrix.

int	cycle_removal_MGW (int L)
	Remove cycles (loops) from unstructured parity check matrix.

void	initialize (int ncheck, int nvar)
	Initialize an empty matrix of size ncheck x nvar.

GF2mat_sparse	get_H (bool transpose=false) const
	Get the parity check matrix, optionally its transposed form.

Sparse_Vec< bin >	get_col (int c) const
	Get a specific column from the matrix.

Sparse_Vec< bin >	get_row (int r) const
	Get a specific row from the matrix.

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

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

void	set (int i, int j, bin value)
	Set element (i,j) of the parity check matrix to value.

bin	get (int i, int j) const
	Get element (i,j) of the parity check matrix.

bin	operator() (int i, int j) const
	Get element (i,j) of the parity check matrix.

double	get_rate () const
	Get the code rate.

void	import_alist (const GF2mat_sparse_alist &H_alist)
	Import matrix from `GF2mat_sparse_alist` format.

GF2mat_sparse_alist	export_alist () const
	Export matrix to `GF2mat_sparse_alist` format.

void	load_alist (const std::string &alist_file)
	Load matrix from `alist_file` text file in alist format.

void	save_alist (const std::string &alist_file) const
	Save matrix to `alist_file` text file in alist format.

Protected Member Functions
void	generate_random_H (const ivec &C, const ivec &R, const ivec &cycopt)
	Generate a random parity check matrix.

void	compute_CR (const vec &var_deg, const vec &chk_deg, const int Nvar, ivec &C, ivec &R)
	Compute target number of columns (C) and rows (R) with a specific number of ones.

int	check_for_cycles (int L) const
	Check for cycles of length L.

int	check_connectivity (int from_m, int from_n, int to_m, int to_n, int g, int L) const
	Check for connectivity between nodes.

Protected Attributes
bool	init_flag
	Flag that indicates proper initialization.

GF2mat_sparse	H
	The parity check matrix.

GF2mat_sparse	Ht
	The transposed parity check matrix.

int	nvar
	Number of variable nodes.

int	ncheck
	Number of check nodes.

ivec	sumX1
	Actual number of ones in each column.

ivec	sumX2
	Actual number of ones in each row.

Static Protected Attributes
static const int	Nmax = 200
	Maximum node degree class can handle.

Detailed Description

Irregular LDPC code generator class.

Author: Erik G. Larsson, Mattias Andersson and Adam Piatyszek

Definition at line 332 of file ldpc.h.

Member Function Documentation

void itpp::LDPC_Parity_Irregular::generate	(	int	Nvar,
		const vec &	var_deg,
		const vec &	chk_deg,
		const std::string &	method = `"rand"`,
		const ivec &	options = `"200 6"`
	)

Generate an irregular LDPC code.

Parameters

Nvar	number of variable nodes
var_deg	vector of variable degree distributions, from an edge perspective
chk_deg	vector of check degree distributions, from an edge perspective
method	currently the only provided method is "rand" (see below)
options	Determines the level of matrix optimization.

The "rand" method generates a fully unstructured random matrix. Some stochastic optimization is performed to avoid cycles. This optimization is controlled via the parameter options. In particular, the girth for the variable-node-degree-2 part of the graph can be controlled via the parameter options(0). The girth for the rest of the nodes can be controlled via the parameter options(1). The recommended value for options is "200 6" for graphs of small size, and "100 4" for large graphs. The value "0 0" means no optimization. Double edges are always avoided.

Note: The "rand" method for graph construction provided in this function is not intended to provide the best possible error performance, but it is a simple, basic, fast tool that can easily be used to build relatively good irregular graphs. Better results in terms of performance and error-floor can be achieved by using other performance measures for the graph than cycle length (for irregular codes, the so-called ACE measure for example). Additionally, the "rand" method builds the graph edge by edge, with no possibility of removing an edge once it has been placed. Better results may be achieved by building the graphs by placing pairs or n-tuples of edges at time, for example, column by column.; Alternative (user-defined) methods for code generation can be implemented by inheriting LDPC_Parity_Irregular.

Definition at line 703 of file ldpc.cpp.

References itpp::LDPC_Parity_Unstructured::compute_CR(), itpp::LDPC_Parity_Unstructured::generate_random_H(), and it_error.

Referenced by LDPC_Parity_Irregular().

int itpp::LDPC_Parity_Unstructured::cycle_removal_MGW ( int L )

inherited

Remove cycles (loops) from unstructured parity check matrix.

This function implements the cycle removal algorithm presented by McGowan and Williamson at the IT workshop 2003. The maximum girth of the graph that will be attempted is L. The algorithm is bound to remove all loops of length L, insofar this is possible. I.e., it does not terminate until it is impossible to remove more cycles by swapping two edges.

Parameters

L	Target girth. For example, L=6 attempts to removes all 4-cycles.

Returns: The girth of the graph, i.e. the length of the shortest cycle. For example, a return value of 6 means that there are no 4-cycles.

Note: This algorithm can take a long time to run for large L or large graphs.

Definition at line 292 of file ldpc.cpp.

References itpp::elem_mult(), itpp::floor_i(), itpp::LDPC_Parity::get_col(), itpp::Sparse_Vec< T >::get_nz_index(), itpp::LDPC_Parity::init_flag, it_assert, it_assert_debug, it_info_debug, itpp::length(), itpp::LDPC_Parity::ncheck, itpp::Sparse_Vec< T >::nnz(), itpp::LDPC_Parity::nvar, itpp::randi(), itpp::randu(), and itpp::Array< T >::set_size().

void itpp::LDPC_Parity_Unstructured::compute_CR	(	const vec &	var_deg,
		const vec &	chk_deg,
		const int	Nvar,
		ivec &	C,
		ivec &	R
	)

protectedinherited

Compute target number of columns (C) and rows (R) with a specific number of ones.

Parameters

var_deg	vector of variable degree distributions, from an edge perspective
chk_deg	vector of check degree distributions, from an edge perspective
Nvar	number of variable nodes
C	number of columns with a specific number of ones
R	number of rows with a specific number of ones

The result is passed by reference and saved in C and R.

Definition at line 598 of file ldpc.cpp.

References itpp::concat(), itpp::elem_div(), itpp::elem_mult(), itpp::find(), it_info_debug, itpp::length(), itpp::linspace(), itpp::max(), itpp::LDPC_Parity::Nmax, itpp::round(), itpp::sum(), itpp::to_ivec(), itpp::to_vec(), and itpp::zeros_i().

Referenced by generate(), and itpp::LDPC_Parity_Regular::generate().

int itpp::LDPC_Parity::check_for_cycles ( int L ) const

protectedinherited

Check for cycles of length L.

This function implements a recursive routine to find loops. The function is mainly a tool for testing and debugging more sophisticated functions for graph manipulation.

Parameters

L	length of cycles to look for

Returns: The function returns the number of cycles found of length L or shorter. Cycles may be counted multiple times.

Note: This function can be very slow for large matrices. It is mainly intended as a debugging aid.

Definition at line 249 of file ldpc.cpp.

References itpp::LDPC_Parity::check_connectivity(), itpp::LDPC_Parity::get_col(), itpp::Sparse_Vec< T >::get_nz_indices(), itpp::LDPC_Parity::init_flag, it_assert, itpp::length(), and itpp::LDPC_Parity::nvar.

int itpp::LDPC_Parity::check_connectivity	(	int	from_m,
		int	from_n,
		int	to_m,
		int	to_n,
		int	g,
		int	L
	)		const

protectedinherited

Check for connectivity between nodes.

This function examines whether the point (to_m, to_n) in the matrix can be reached from the point (from_m, from_n) using at most L steps. A recursive search is used.

The function can be used to search for cycles in the matrix. To search for a cycle of length L, set from_m=to_m and from_n=to_n, and godir=0.

Parameters

from_m	starting coordinate, row number
to_m	goal coordinate, row number
from_n	starting coordinate, column number
to_n	goal coordinate, row number
g	direction: 1=start going vertically, 2=start going horizontally
L	number of permitted steps

Returns

-1 or -3 : destination unreachable
-2 : meaningless search (started in a "0" point),
-4 : meaningless search
>=0 : destination reached with certain number of steps left

Note: This function can be very slow depending on the nature of the matrix.

Note that smaller cycles may appear as longer cycles when using this method. More specifically, suppose the method is run with a given L and there are cycles in the neighborhood of (from_m,from_n) of length L-2 or less, but which do not contain (from_m,from_n). These shorter cycles may then also be reported as a cycle of length L. For example, if one of the immediate neighbors of (from_m,from_n) is part of a cycle of length 4 this method will report that (from_m,from_n) is part of a cycle of length 6, if run with L=6. However, if it is known that there are no cycles of length L-2 or smaller, and check_connectivity(from_m,from_n,from_m,from_n,0,L) returns a non-negative value, then one will know with certainty that the point (from_m,from_n) is part of a cycle of length L. (This behavior is inherent to the simple recursive search used.)

Definition at line 192 of file ldpc.cpp.

References itpp::LDPC_Parity::get_col(), itpp::Sparse_Vec< T >::get_nz_indices(), itpp::LDPC_Parity::get_row(), itpp::LDPC_Parity::init_flag, it_assert, and itpp::length().

Referenced by itpp::LDPC_Parity::check_for_cycles(), and itpp::LDPC_Parity_Unstructured::generate_random_H().

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

Protected Attributes

Static Protected Attributes

Detailed Description

Member Function Documentation