Reliability-based quantization for soft decoding of LDPC codes in short-reach optical links

Efficient quantizer design is vital in high-speed optical receivers. In this paper, the authors propose a three-level (1.5-bit) reliability-based quantizer for low-complexity soft decoding of low-density parity-check (LDPC) codes, and show that it outperforms a conventional quantizer employing eight levels (3 bits). Different approaches for obtaining the quantization thresholds are discussed, and it is shown that the achievable channel capacity with the proposed quantizer is within 0.4 dB of the Shannon limit of the unquantized channel. Rate-compatible LDPC codes are also designed tailored to the asymmetric optical channel terminated with the proposed reliability-based quantizer, and it is shown that the resulting quantizer-aware LDPC codes significantly outperform other existing codes of similar rates.