Long-reach SSB-OFDM-PON Employing Fractional Sampling and Super-Nyquist Image Induced Aliasing

We investigate frequency diversity introduced by deliberate aliasing of in-band signals using super-Nyquist images in an optical single-sideband (SSB) modulated and directly detected orthogonal frequency-division multiplexing passive optical network (OFDM-PON) system. Fractional sampling and a low-complexity per-subcarrier maximum-ratio-combining (MRC) algorithm are proposed to combine the diversity. We show that the computation complexity of the MRC can be well controlled by adding a 2× resampling after digital filtering without compromising performance. We experimentally demonstrate that for the downlink transmission of an 8 GHz, 16-QAM mapped, and SSB modulated OFDM-PON system, the residual super-Nyquist images can be used for boosting the signal-to-noise ratio of the higher frequency subcarriers at the receiver and that the receiver sensitivity can be significantly improved after 24.4 km and 80 km transmission. We also demonstrate a significant increase in system capacity for an adaptively modulated SSB OFDM-PON when using the proposed scheme without any modification in the transmitter.