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DensE: A MATLAB toolbox for density evolution

This toolbox is a collection of MATLAB classes and routines that can be used to analyze iterative decoding schemes via density evolution.

The toolbox is currently limited to deterministic generalized product codes under iterative bounded-distance decoding. It can be used to predict the asymptotic performance of product codes, staircase codes, or other related code classes.

Getting Started & Examples

All MATLAB files can be found in the folder DensE. To install the toolbox, simply download this folder and add it to MATLAB's path variable via addpath('path_to_DensE').

In the following, several examples are provided to illustrate the basic functionality of the toolbox.

Computing Thresholds

The toolbox can be used to numerically compute decoding thresholds. An example for so-called half-product codes is given below (see examples/thresholds_hpcs.m). The threshold is given in terms of the expected number of erasures per component code c, which is related to the erasure probability as p=c/n, where n is the component code length.

max_iterations = 1000;
target_error_rate = 1e-10; % target error rate that counts as "successful decoding"
t = 4; % erasure-correcting capability of the component codes

code = DE_half_product_code(t); 
scheme = DE_scheme_detgpc(code);

% the base class combines the channel and coding scheme information
DE_obj = DE_base(DE_channel_gpc_bec, scheme, max_iterations, target_error_rate); 
thr = DE_obj.find_threshold() % thr = 6.79

Decoding thresholds of half-product codes (and product codes) correspond exactly to the existence thresholds of cores in random graphs. For example, 6.79 is the threshold for a 5-core and, in general, decoding with t-erasure-correcting component codes corresponds to the (t+1)-core. This was first observed in a 2007 paper by Justesen and Høhold.

Predicting The Waterfall Behavior in Bit Error Rate Simulation Plots

Density evolution can be used to predict the waterfall behavior of iterative coding schemes. For example, the following figure shows the performance of a product code together with the density evolution prediction (see examples/waterfall_pcs.m):

The product code threshold that can be inferred from the above figure is roughly 0.013*512=6.66. This is slightly lower than the above value of 6.79 due to the lower number of iterations.

Another example for a staircase code is shown below.

The simulations assume idealized decoding where no miscorrections occur in the component decoding.

More Advanced Topics

The toolbox can also be used to visualize and analyze decoding schedules. For example, the following animation shows the wave-like decoding behavior of a staircase code when decoded with a window schedule below the threshold (see examples/staircase_wave.m):

When decoding above the threshold, the window moves too fast in relation to the speed of the decoding wave and the decoder gets stuck eventually:

Additional Information

The toolbox is based on joint work with Henry D. Pfister, Alexandre Graell i Amat, and Fredrik Brännström. If you decide to use the toolbox for your research, please make sure to cite our paper:

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