Reliability-aware subcarrier mapping strategy of QC-LDPC encoded symbols in bandwidth-limited IM/DD OFDM systems.

We propose a simple yet effective strategy to map forward error correction (FEC) encoded symbols to subcarriers in bandwidth-limited intensity-modulation and direct-detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) systems. The design exploits the reliability diversity of OFDM subcarriers in bandwidth-limited systems and allocates FEC encoded systematic symbols and parity-check symbols to high-reliability and low-reliability subcarriers, respectively. In contrast to adaptively-loaded OFDM or systems using multiple sets of FECs with different code rates for different subcarriers, the proposed design does not induce additional complexity and avoids the round-trip delay between transceivers. We investigate the performance of the proposed design in quasi-cyclic low-density parity-check (QC-LDPC) coded OFDM system under different decoding iterations. Experimental results of 120-Gb/s signals over 2 km and 100-Gb/s signals over 5 km show that the OFDM system using the proposed mapping strategy is superior to both OFDM and discrete-Fourier-transform spread (DFT-S) OFDM using conventional mappings without considering the subcarrier diversity or using pre-equalization, regardless of the received optical power, the FEC code rate, the length of the cyclic prefix, the transmission distance, the number of decoding iterations, and the degree distribution of the QC-LDPC optimized under either an infinite or a limited number of iterations. The proposed mapping is optimal when the systematic symbols and parity-check symbols are exactly loaded to high-reliability and low-reliability subcarriers respectively. The studies enable the proposed strategy promising for bandwidth-limited IM/DD OFDM systems such as low-latency data center interconnects.