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IEEE International Symposium on Circuits and Systems proceedings

Modified layered message passing decoding with dynamic scheduling and early termination for QC-LDPC codes

Yeong-Luh Ueng, Yu-Lun Wang, Chi-Yu Lin, Jen-Yuan Hsu, Pangan Ting

Published May 1, 2009• 9 citations
Publisher

Abstract

The convergence speed of layered message-passing decoding (LMPD) is faster than that of standard two phase message passing (TPMP) decoding. We propose to use dynamic scheduling and early termination to reduce computational complexity for a modified LMPD algorithm which was proposed for quasi-cyclic low-density parity-check (QC-LDPC) codes. We dynamically skip or redo the decoding operations for some layers based on appropriate criteria. An early termination strategy which is efficient in hardware implementation is also proposed in this paper. The modified MPD using these two techniques simultaneously can reduce the computational complexity with similar error performance as compared to the case of not using these two techniques.

DOI
10.1109/iscas.2009.5117700

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The convergence speed of layered message-passing decoding (LMPD) is faster than that of standard two phase message passing (TPMP) decoding. We propose to use dynamic scheduling and early termination to reduce computational complexity for a modified LMPD algorithm which was proposed for quasi-cyclic low-density parity-check (QC-LDPC) codes. We dynamically skip or redo the decoding operations for some layers based on appropriate criteria. An early termination strategy which is efficient in hardware implementation is also proposed in this paper. The modified MPD using these two techniques simultaneously can reduce the computational complexity with similar error performance as compared to the case of not using these two techniques.
Low-density parity-check (LDPC) codes are one of the most popular linear block codes have ever been used in the communication system.The standard two-phase message passing decoding (TPMPD) algorithm and the layered message-passing decoding (LMPD) algorithm are widely used in LDPC decoder.However, the convergence speed of LMPD algorithm is much faster than that of TPMPD algorithm, particularly in the condition of high signal noise ratio (SNR).In this paper, we propose an early termination criterion for quasi-cyclic low-density parity-check (QC-LDPC) codes based on LMPD algorithm with fast convergence speed.The strategy is to dynamically reprocesses the uncorrected related layers while skipping the corrected ones.Simulations show that 3% reduction in terms of iteration numbers can be achieved with no performance loss compared to the traditional termination strategies, such as hard decision aided (HDA) strategy and parity check equations (PCE) strategy.Furthermore, the optimized early termination strategy achieves low complexity in hardware implementation.
Quasi-cyclic low-density parity-check codes (QC-LDPC) are selected as the channel coding scheme for the data channel of the new radio (NR) enhanced mobile broadband (eMBB) in fifth-generation (5G) mobile communication technology. Traditional layered scheduling strategies used in 5G LDPC codes improve decoding convergence speed, but fixed scheduling schemes cannot fully utilize dynamic changes in decoded messages. To address this issue, this paper proposes a layered scheduling scheme for 5G LDPC codes based on edge information. By prioritizing updates to layers with minimal changes in edge information, this method reduces decoding complexity and accelerates convergence with minimal performance loss. Simulation results demonstrate that this approach enhances decoding performance by approximately 0.05dB compared to other improved hierarchical scheduling strategies.
The implementation complexity of the decoder for low-density parity-check codes (LDPC) is dictated by memory and interconnect requirements. We propose new LDPC decoder architectures that reduce the need of message passing memory by 80% (for standard message passing)-55%(for layered decoding) and the router requirements by more than 50%. These novel architectures are based on scheduling of computation that results in "on the fly computation" of variable node and check node reliability messages. These architectures are targeted for quasi- cyclic LDPC codes such as array LDPC codes (regular QC-LDPC) and block LDPC codes (irregular QC-LDPC). FPGA and ASIC implementation results show substantial gains when compared to the existing work in the literature.
Entering the 5G era, the hardware design of low-density parity-check (LDPC) codes under the new standard has increasingly higher requirements on throughput. According to the geometric characteristics of the check matrices of the quasi-cyclicQC) LDCP codes under the CCSDS standard, this paper designs a layered dynamic normalized minimum sum algorithm(LDNMSA) under the pre-termination decoding based on de-layering scheme. The check matrix structure of this code is easy for hardware design and the storage resource consumption is low. The approximate replacement of the minimum sum algorithm(MSA) reduces the computational complexity of the check node update process. The determination of correction factors and inter-layer deletion thresholds through computer simulation improves the decoding performance and the speed of a single iteration, and adopt the layered scheduling scheme with optimized update order reduces the number of iterations required for decoding. Experimental results show that when the bit error rate(BER) is 10-5, the designed algorithm has a gain of approximately 0.5 dB compared to the MSA. The speed and calculation amounts to a single iteration are much lower than the log-likelihood-ratio(LLR) belief propagation(BP) algorithm and the performance is only less than 0.1dB.
Layered decoding of Low Density Parity Check (LDPC) codes has been shown to give faster convergence and use less storage than the conventional Two Phase Message Passing (TPMP) algorithm in decoding such codes. To achieve higher throughput, multiple rows of the parity check matrix may be decoded in parallel, as in the Parallel-Turbo Decoding Message Passing (P-TDMP) algorithm. However, contention for globally shared memory permits only a subset of rows to be decoded in parallel thus limiting overall throughput. This paper uses a near optimal scheduling algorithm, based on graph colouring, to overcome memory contention, allowing processing of multiple rows without code modification, resulting in significant throughput gains over the standard P-TDMP algorithm. For the LDPC codes studied, throughput gains compared to the P-TDMP algorithm, of between ten and fifteen are shown to be possible.
An efficient dynamic schedule for layered belief-propagation (LBP) decoding of low-density parity-check (LDPC) codes is presented, in which the check nodes connecting to a variable node with maximum relative message residual are chosen in each dynamic decoding iteration, and then LBP is performed for these check nodes. In this schedule we combined the features of lazy schedule and node-wise residual belief propagation (NWRBP) together while keeping the extra computational complexity very low. Simulation results show that the new schedule speeds up the convergence rate and greatly improves error performance at medium to high signal-to-noise ratio (SNR) when compared to the standard LBP decoding algorithm.