In state-of-the-art fibre-optics communication systems, the forward error correction (FEC) and high-order constellation formats are employed. It is important to approach the Shannon limit by using turbo product codes (TPC) and low-density parity-check (LDPC) codes with soft-decision decoding (SDD) algorithms. In this paper, we first study the rate-adaptive irregular LDPC coding technique in wide range of code rates enabling the flexible adjustment according to channel conditions and the simulation results show an improved SNR gain of 0.15 ∼ 0.44 dB compared with regular LDPC codes under BPSK transmission. Then we propose a bit-loading rule for irregular LDPC codes combined with 16QAM and demonstrate additional 0.3 dB SNR improvements in low code rate regime, making it a viable solution for next generation high-speed optical communication systems.