Detection and equalization of set-partitioned offset-QAM OFDM in IMDD systems

We design the detection algorithm for orthogonal frequency division multiplexing (OFDM) based on set-partitioned offset quadrature amplitude modulation (SP-offset-QAM) in adaptively-loaded intensity modulation and direct detection (IMDD) systems. The algorithm mitigates signal-signal beating interference (SSBI), improves the equalization capability of conventional one-tap equalizers, and reduces the complexity and the required length of training sequence in the decoding of multi-dimensional SP-QAM signals. We experimentally demonstrate the adaptively-loaded SP-offset-QAM OFDM over 50-km single-mode fiber to verify the proposed algorithm. It is shown that the modified equalization significantly improves the performance while the SSBI mitigation brings benefits after transmission. The proposed decoding scheme reduces the complexity comparable to that in conventional QAM. It is also shown that the SP-offset-QAM OFDM outperforms the SP-based conventional QAM and conventional offset-QAM OFDM both at back-to-back and after 50-km transmission, when the algorithm is applied to all schemes.