Increase in Capacity of an IM/DD OFDM-PON Using Super-Nyquist Image-Induced Aliasing and Simplified Nonlinear Equalization

For long reach passive optical networks, double-sideband intensity modulation and direct detection can offer the advantages of low cost and low complexity. In this paper, we investigate a joint chromatic dispersion (CD) and nonlinearity compensation technique using super-Nyquist image-induced aliasing and a Volterra series-based simplified nonlinear equalizer (SNE) in a decision feedback manner. The SNE employed in our experiments offers signal-to-noise ratio (SNR) performance comparable to the conventional nonlinear equalizers (NE), but with much fewer coefficients. The distribution of the nonlinear coefficients and the memory length of the SNE/NE are also investigated. Experimental results show that with the proposed aliasing-based CD compensation technique combined with SNE used in each down-sampled signal before per-subcarrier maximum ratio combining, the SNR can be significantly improved. The data rate of the 10-GHz optical OFDM signals is increased by 45%, 52%, and 51% after 48.8, 79.2, and 99.6 km of standard single mode fiber transmission, respectively, compared with those without CD/nonlinearity compensation.

I. Introduction

Long reach passive optical networks (LR-PONs) have gained considerable research interest as they would combine the current separate metro and access networks into a single, integrated network by extending the coverage span of the access networks from the traditional 20-km range to up to 100-km [1], which can significantly reduce the cost and simplify the architecture [2]. In LR-PONs, the optical intensity modulation and direct detection (IM/DD) scheme can offer lower cost and low complexity compared with coherent schemes. Optical orthogonal frequency-division multiplexing (O-OFDM) combined with quadrature amplitude modulation (QAM) is also a promising modulation scheme due to its high spectral efficiency, its tolerance to inter-symbol interference (ISI), and its modulation flexibility [3] –[5]. For the IM/DD OFDM system, chromatic dispersion (CD) induced power fading is one of the main bottlenecks that limit its feasibility for economical long reach operation [6]–[8]. This power fading effect may induce a frequency dependent power dip that cannot be recovered using linear equalization methods [9] or a long cyclic prefix. Single side-band (SSB) and vestigial-sideband (VSB) schemes and electronic dispersion precompensation have been employed in IM/DD systems to mitigate the power fading, which may also result in additional complexity [10], [11]. In our previous work, the CD induced power fading in optical OFDM systems was effectively compensated using super-Nyquist image induced aliasing [12] , [13].