In this paper, we propose a monolithic optical front-end for soft-decision low-density parity-check (LDPC) decoders
based on 112 Gb/s polarization division multiplexed quadrature shift keying (PM-QPSK) coherent detection. The novel
front-end is a low complexity and low cost solution with a performance improvement of 6.4 dB over system without the
use of coding. The front-end also outperforms the conventional RS(255,239) based codes by 1.3 dB at the BER of 10<sup>-8</sup>.