Non-linear equalisation techniques for high-speed step-index plastic optical fibre communication

Abstract

Step-index plastic optical fibres (SI-POF) have become a promising candidate as the media for short-range in-home and automotive networks due to their low cost and their ease of installation. However, they have the smallest bandwidth compared to the other optical fibres. Therefore, high-speed communication over SI-POF results in inter-symbol interference (ISI) that linearly distorts the signal. Moreover, there are non-linearities from the optical front-end that further degrade the SI-POF performance. A straightforward solution is to use non-linear equalisers (NLE) with the SI-POF system as they compensate for the non-linear distortions while mitigating the channel ISI. Three NLEs – transversal decision feedback equaliser (DFE), Volterra equaliser/DFE, and the multi-layer perceptron-based equaliser/DFE (MLPDFE) – have been introduced in the literature. High-order modulation formats – like pulse amplitude modulation (PAM), carrier-less amplitude and phase modulation (CAP), and discrete multi-tone (DMT) – can be used in combination with the NLE to overcome the bandwidth limitation further. Thus, the thesis deals with the performance of these NLEs for PAM, CAP, and DMT transmission in order to achieve high data rates (from several hundreds of megabits-per-second (Mbps) to gigabits-per-second (Gbps)) in SI-POF. The contributions of this research work are in threefold: firstly, a simulation model is used to evaluate and compare the performance of the NLEs for PAM and CAP schemes. The study shows that for a highly non-linear SI-POF with higher PAM (or CAP) modulation order, the MLPDFE offers higher data rates than the Volterra DFE followed by the transversal DFE. This simulation study is further verified with various experiments. For instance, the MLPDFE offers an error-free bit rate of about 6.2 Gbps over a 30 m SI-POF while the transversal DFE offers about 5 Gbps at similar SI-POF length. A computational complexity comparison of each NLE shows that the transversal DFE requires the least computing requirement, and the VOLT2DFE has higher computational order than the MLPDFE. Secondly, the work investigates a recently introduced frequency domain NLE (FD-NLE) for DMT transmission over SI-POF. It explores the performance of the FD-NLE for DMT with clipping distortion in a highly non-linear SI-POF system. The FD-NLE is shown in this case as the better choice than the conventional frequency domain equaliser. With insight from the FD-NLE for DMT, both Volterra and the MLP equalisers are translated to the frequency domain for PAM and CAP transmission over SI-POF. A computational complexity analysis shows that implementing the NLEs (with PAM and CAP) in the frequency domain reduces their complexity by at least 60% if there are more than 16 feedforward taps for the equaliser. Finally, extensive experiments are carried out to evaluate and compare the bit error rate (BER) performances and the computational complexity of the modulation schemes with their respective NLEs. The comparisons show that for a short-length SI-POF of up to 30 m, representing benign channel conditions, bit-loaded DMT with FD-NLE offers the best performance requiring the least complexity and the least transmitted electrical power. However, at longer lengths, PAM with MLPDFE gives the best performance. CAP with the MLPDFE demands the highest computational complexity and the transmitted electrical power