A two-stage coded modulation scheme based on the 8-QAM signal for optical transmission systems

Abstract A novel two-stage coded modulation (TSCM) scheme based on the 8-QAM signal is proposed. In the proposed scheme, three bits in the same symbol are protected by different codes in order to fully utilize 8-QAM’s ring characteristic and obtain the more coding gain in optical transmission systems. The two simulation experiments with two groups of quasi-cyclic low-density parity-check (QC-LDPC) codes are performed. The simulation results show that the proposed scheme, compared with the traditional parallel independent scheme which has similar code-rate, can bring about 0.5dB and 0.1dB net coding gain (NCG) respectively at the bit error ratio (BER) of 10-6. In addition, the applied QC-LDPC codes need less times of iterations and have low error floor. The proposed scheme can implement the transmission without time delay, and the number of addition operations is also only 2/3 of the traditional scheme while calculating the log-likelihood ratio (LLR) algorithm of a symbol. Therefore, the proposed TSCM scheme combined with the QC-LDPC codes has the better comprehensive performance and can better be suitable for high-speed optical transmission systems.