An Modified Cole's Importance Sampling Method For Low Error Floor QC-LDPC Codes Construction
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We modified Cole's Importance Sampling (IS) method for enumerating of Trapping Sets (TS, asymmetric subgraphs) causing an error under message-passing decoder. Proposed Cole's IS modifications based on combination of several ideas: parallel TS impulse tree decomposition using unwrapping of message passing iterations, according short cycles dense and straightforward idea of Tanner Graph/ Forney's Normal Graph symmetry - Graph Authomorphism. Its allowed superior Velasquez-Subramani and Karimi-Banihashemi TS enumerating methods. Particularly proposed method under PEG (1008, 504) Mackay code for single thread implementation 5027-times (71463 times, multi-treads) faster compare to Velasquez-Subramani LP method and 43-times faster compare to original Cole's method. For TS enumerating problem under (2640, 1320) Margulis code compare to Velasquez-Subramani LP method proposed method for single thread implementation 37958 times faster, 82-times faster than Karimi-Banihashemi and 134-times faster than Cole's original method. NVIDIA Titan RTX GPU implementation of proposed method gives a further 2-30 times acceleration. FPGA device providing further acceleration from 1.25 to 44 times. We show on example of QC-LDPC codes construction how improvement of EMD spectrum, increase hamming(code) distance effect on TS spectrum and BER/FER error-floor level.
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