Optimal 16-Ary APSK Encoded Coherent Optical OFDM for Long-Haul Transmission

In this letter, an optimal 4+12 amplitude and phase shift keying (APSK) modulated coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) system is proposed (the numbers 4 and 12 represent the constellation points located in the inner ring and the outer ring, respectively). Although the 4+12APSK modulation format has smaller average Euclidean distance than 16 quadrature amplitude modulation (QAM), the 4+12APSK modulated CO-OFDM system has the best transmission performance, with increased tolerance towards both amplified spontaneous emission noise and fiber nonlinearities compared with 16QAM and 8+8APSK modulated OFDM systems. The maximum reach at Q factor=8.2 dB (when bit-error rate (BER) equals 5 × 10^-3) is evaluated to assess the system performance based on a single-channel and five-channel wavelength division multiplexing (WDM) CO-OFDM system, and illustrates that the transmission performance of 4+12APSK modulated OFDM outperforms 8+8APSK modulated OFDM and 16QAM modulated OFDM by approximately 10.5%, and in the WDM system, it surpasses 8+8APSK modulated OFDM by approximately 5.6% and 16QAM modulated OFDM by 18.75%.