Sampling frequency offset compensation in non-Hermitian symmetric optical OFDM

Non-Hermitian symmetric optical orthogonal frequency-division multiplexing (NHS-OFDM) can make the best use of fast Fourier transform (FFT) functions. Hence, hardware implementation of NHS-OFDM has a low complexity when maintaining the same bandwidth granularity. And the similar peak-to-average power ratio (PAPR) and bit error rate (BER) performances can be obtained in comparison with the conventional Hermitian symmetric optical orthogonal frequency-division multiplexing (HS-OFDM). However, the sampling clock frequency offset (SFO) between digital-to-analog converters (DAC) and analog-to-digital converters (ADC) always exists in asynchronous HS/NHS-OFDM systems. It will degrade BER performance seriously without compensation. In this paper, the mechanism of SFO in NHS-OFDM is analyzed and discussed. The inter-symbol differential detection (ISDD) with residual phase compensation (RPC) is applied for SFO compensation (SFOC) in the NHS-OFDM system. Moreover, enhanced residual phase estimation (ERPE) is also proposed to improve the tracking speed and accuracy of residual phase estimation. The optimal FFT size for NHS-OFDM is first identified in both additive white Gaussian noise (AWGN) channel and a 20-km standard single-mode fiber (SSMF) link. The BER performance of the ISDD with RPC/enhanced RPC (ERPC) enabled HS/NHS-OFDM is investigated by means of both numerical simulation and offline experiments. With the help of the ISDD with RPC/ERPC methods, the SFO can be compensated. Also, a slight improvement in BER performance can be achieved with the ERPC method compared to the RPC method. And the BER performance penalties can be ignored in the presence of SFO up to 200 ppm. •The ISDD-based SFO compensation is studied in optical NHS-OFDM.•An ERPE is proposed to obtain faster RPE tracking and more accurate RPE estimate.•Up to 200 ppm SFO can be compensated with a negligible BER performance penalty.