Run-time reconfigurable adaptive LDPC coding for optical channels.

In this paper, we proposed a class of large-girth QC-LDPC codes designed to maximize the girth property with code rates ranging from 0.5 to 0.8, which leads to well-structured parity-check matrix and generator matrix. Instead of implementing several FEC encoder and decoder engines in hardware, we design an efficient unified FPGA-based architecture enabling run-time reconfigurable capability. Apart from four principle LDPC codes being incorporated into a unified design, shortening is adopted to bridge the rate gap between principle codes. With our proposed unified LDPC engine, the signal-to-noise ratio (SNR) limits of -1 dB to 2.2 dB have been demonstrated at BER of 10-12 in additive white Gaussian noise (AWGN) channel by FPGA emulation. It is desirable for the application to both free-space optical (FSO) and fiber optics communications. Large code rate range is preferred to deal with various channel impairments. To further verify the proposed unified code engine for FSO applications, we tested the scheme through a spatial light modulator (SLM)-based FSO channel emulator. We showed that in medium atmospheric turbulence regime, a post-FEC BER below 10-8 can be achieved without any interleaver and adaptive optics.