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International Conference on Communications and Networking in China (ChinaCom)

Improved decoding of LDPC codes by variable-to-check residual belief propagation

Lingyan Song, Shujuan Hou

Published Aug 1, 2015• 1 citation
Publisher

Abstract

This paper proposes an improved variable-to-check residual belief propagation algorithm, called Lazy-RBP, in which a new informed dynamic schedule (IDS) combined with the lazy schedule (LS) is adopted. Firstly, Lazy-RBP exploits the probabilistic girth-based approach, one of the LS strategies, to decide the set of updated variable nodes in each iteration. Then among this set, a new IDS scheme is introduced, where both the unreliable variable nodes and the residual of the variable-to-check messages are utilized to locate the message to be propagated first. Simulation results show that the Lazy-RBP algorithm can not only improve the error performance but accelerate the convergence compared with the VC-RBP algorithm.

DOI
10.1109/chinacom.2015.7497930

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This paper proposes an improved variable-to-check residual belief propagation algorithm, called Lazy-RBP, in which a new informed dynamic schedule (IDS) combined with the lazy schedule (LS) is adopted. Firstly, Lazy-RBP exploits the probabilistic girth-based approach, one of the LS strategies, to decide the set of updated variable nodes in each iteration. Then among this set, a new IDS scheme is introduced, where both the unreliable variable nodes and the residual of the variable-to-check messages are utilized to locate the message to be propagated first. Simulation results show that the Lazy-RBP algorithm can not only improve the error performance but accelerate the convergence compared with the VC-RBP algorithm.
Belief-propagation (BP) algorithm and its variants are well-established methods for iterative decoding of LDPC codes. Among them, residual belief-propagation (RBP), which is the most primitive and representative informed dynamic scheduling (IDS) strategy, can significantly accelerate the convergence speed. However, RBP decoding suffers from a poor convergence error-rate performance due to its greedy property, which is one of the challenging issues in the design of IDS strategies. To tackle this problem, a novel IDS scheme, namely residual-decaying-based residual belief-propagation (RD-RBP) algorithm, is presented in this paper. In RD-RBP, a decaying mechanism is introduced to manipulate the residuals of those check-to-variable messages, preventing the decoding resources from being unreasonably occupied by a small group of edges in the Tanner graph. The greediness is therefore alleviated and better performance of convergence error-rate is achieved. Besides, a two-stage scheduling scheme combining prior-art variable-node and variable-to-check-edge RBP (V-VCRBP) with RD-RBP, named V-VC-RD-RBP, is proposed for achieving both fast convergence speed and a low convergence error-rate. The simulation results validate the advantages of the proposed schemes.
The informed dynamic scheduling (IDS) strategies for decoding of low-density parity-check codes obtained superior performance in error correction performance and convergence speed. However, there are still two problems existing in the current IDS algorithms. The first is that the current IDS algorithms only preferentially update the selected unreliable messages, but they do not guarantee the updating is performed with reliable information. In this paper, a two-step message selecting strategy is introduced. On the basis of the two reliability metrics and two types of variable node residuals, the residual belief propagation (BP) decoding algorithm, short for TRM-TVRBP, is proposed. With the algorithm, the reliability of the updating-messages can be improved. The second is the greediness problem, prevalently existed in the IDS-like algorithms. The problem arises mainly from the fact that the major computing resources are allocated to or concentrated on some nodes and edges. To overcome the problem, the reliability metric-based RBP algorithm (RM-RBP) is proposed, which can force every variable node to contribute its intrinsic information to the iterative decoding. At the same time, the algorithm can force the related variable nodes to be updated, and make every edge have an equal opportunity of being updated. The simulation results show that both the TRM-TVRBP and the RM-RBP have appealing convergence rate and error-correcting performance compared with the previous IDS decoders over the white Gaussian noise (AWGN) channel.
When residual belief-propagation (RBP), which is a kind of informed dynamic scheduling (IDS), is applied to low-density parity-check (LDPC) codes, the convergence speed in error-rate performance can be significantly improved. However, the RBP decoders presented in previous literature suffer from poor convergence error-rate performance due to the two phenomena explored in this paper. The first is the greedy-group phenomenon, which results in a small part of the decoding graph occupying most of the decoding resources. By limiting the number of updates for each edge message in the decoding graph, the proposed Quota-based RBP (Q-RBP) schedule can reduce the probability of greedy groups forming. The other phenomenon is the silent-variable-nodes issue, which is a condition where some variable nodes have no chance of contributing their intrinsic messages to the decoding process. As a result, we propose the Silent-Variable-Node-Free RBP (SVNF-RBP) schedule, which can force all variable nodes to contribute their intrinsic messages to the decoding process equally. Both the Q-RBP and the SVNF-RBP provide appealing convergence speed and convergence error-rate performance compared to previous IDS decoders for both dedicated and punctured LDPC codes.
We present an enhanced informed dynamic scheduling (IDS) strategy to avoid propagations of unreliable messages in the residual belief-propagation (RBP) algorithm for decoding low-density parity-check (LDPC) codes. To reduce the resource needed for updating the degree-one variable nodes (VNs) in an RBP-based algorithm for decoding 5G New Radio (NR) LDPC codes, we further introduce a process consisting of several simple procedures for managing the resource consumption. Simulation results demonstrate that our RBP algorithm based on the proposed IDS strategy outperforms existing RBP-based algorithms at the early decoding iterations. By using our degree-one VN resource managing approach, the performance of both the proposed and the existing decoders is significantly improved in decoding the 5G NR codes.
We researched BP decoding algorithm based on variable-to-check information residual for LDPC code (VC-RBP) in this paper. It is a dynamic scheduling belief propagation using residuals, and has some advantages, such as fast decoding, good performance, and low complexity. It is similar to residual belief propagation (RBP),but has some difference in computing the residual message. Simulation shows that it outperforms with only a maximum of ten iterations by about 0.28 dB compared with RBP at an BER of 10-4.
We propose Variable-to-Check Residual Belief Propagation (VCRBP) for LDPC decoding for faster convergence, better error correcting performance and lower complexity. It is similar to Residual Belief Propagation (RBP) that was recently applied to LDPC decoding by Vila Casado et al. because it is also a dynamic scheduling belief propagation using residuals, but it is different because its residuals are computed from variable-to-check message. Simulation shows that it out-performs with only maximum 8 iterations by about 0.3 dB compared with RBP at an FER of 10-4. Also, we propose node-wise VCRBP (N-VCRBP) for reducing complexity of VCRBP. N-VCRBP also exhibits the performance very close to VCRBP but with significantly lower decoding complexity.