Capacity improvement of TWDM-PONs exploiting the 16-QAM technique for downstream side with a nonlinearity effect study for upstream DML

Currently, the main goal of developing time-wavelength- division multiplexing passive optical networks (TWDM-PONs) is to acquire capacity improvement in a scalable and cost-effective manner. In response to the challenge, this paper demonstrates a full-system TWDM-PON through physical layer simulation that provides 56 Gb/s downstream and 40 Gb/s upstream bandwidths. We propose a simple and cost-effective scheme to improve the capacity of the downstream signal by utilizing highly spectrally efficient 16 quadrature amplitude modulation (QAM) with 20 GHz radio-over-fiber signal distribution using a single sideband signal that is generated by an optical dual-arm modulator. In addition, we study the effect of nonlinearity on a directly modulated laser (DML) as the upstream laser of an optical network unit. The outcomes obtained revealed good transmission quality in both directions, where bit error ratios of 10^-6 and 10^-5 are obtained for the downstream and upstream transmissions, respectively. This leads to the ability to support up to 128 users over 40 km of single-mode fiber.