Joint-Polarization Phase-Noise Estimation and Symbol Detection for Optical Coherent Receivers

The problem of optimal symbol detection in the presence of laser phase noise is studied, for uncoded polarization-multiplexed fiber-optic transmission. To this end, the maximum a posteriori (MAP) symbol detector is presented. Specifically, it is emphasized that obtaining phase-noise point estimates, and treating them as the true values of the phase noise, is in general suboptimal. Furthermore, a pilot-based algorithm that approximates the MAP symbol detector is developed, using approaches adopted from the wireless literature. The algorithm performs joint-polarization phase-noise estimation and symbol detection, for arbitrary modulation formats. Through Monte Carlo simulations, the algorithm is compared to existing solutions from the optical communications literature. It is demonstrated that joint-polarization processing can significantly improve upon the single-polarization case, with respect to linewidth tolerance. Finally, it is shown that with less than 3% pilot overhead, the algorithm can be used with lasers having up to 6 times larger linewidths than the most well-performing blind algorithms can tolerate

Phase-Noise Compensation for Space-Division Multiplexed Multicore Fiber Transmission

The advancements of popular Internet-based services such as social media, virtual reality, and cloud computing constantly drive vendors and operators to increase the throughput of the Internet backbone formed by fiber-optic communication systems. Due to this, space-division multiplexing (SDM) has surfaced as an appealing technology that presents an opportunity to upscale optical networks in a cost-efficient manner. It entails the sharing of various system components, such as hardware, power, and processing resources, as well as the use of SDM fibers, e.g., multicore fibers (MCFs) or multimode fibers, which are able to carry multiple independent signals at the same wavelength in parallel.Higher-order modulation formats have also garnered attention in recent years as they allow for a higher spectral efficiency, an important parameter that relates to the throughput of communication systems. However, a drawback with increasing the order of modulation formats is the added sensitivity to phase noise, which calls for effective phase-noise compensation (PNC). This thesis studies the idea of sharing processing resources to increase the performance of PNC in SDM systems using a particular type of fiber, namely uncoupled, homogeneous, single-mode MCF.Phase noise can be highly correlated across channels in various multichannel transmission scenarios, e.g., SDM systems utilizing MCFs with all cores sharing the same light source and local oscillator, and wavelength-division multiplexed systems using frequency combs. However, the nature of the correlation in the phase noise depends on the system in question. Based on this, a phase-noise model is introduced to describe arbitrarily correlated phase noise in multichannel transmission. Using this model, two pilot-aided algorithms are developed using i) the sum–product algorithm operating in a factor graph and ii) variational Bayesian inference. The algorithms carry out joint-channel PNC and data detection for coded multichannel transmission in the presence of phase noise. Simulation results show that in the case of partially-correlated phase noise, they outperform the typical PNC approach by a wide margin. Moreover, it is shown that the placement of pilot symbols across the channels has a considerable effect on the resulting performance.Focusing on SDM transmission through an uncoupled, homogeneous, single-mode MCF with shared light source and local oscillator lasers, the performance benefits of joint-channel PNC are investigated. A significant gain in transmission reach is experimentally demonstrated, and the results are shown to agree strongly with simulations based on the introduced phase-noise model. In addition, the simulations show that dramatic improvements can be made for phase-noise limited systems in terms of power efficiency, spectral efficiency, and hardware requirements

Digital signal processing for coherent optical fibre communications

In this thesis investigations were performed into digital signal processing (DSP) algorithms for coherent optical fibre transmission systems, which provide improved performance with respect to conventional systems and algorithms. Firstly, an overview of coherent detection and coherent transmission systems is given. Experimental investigations were then performed into the performance of digital backpropagation for mitigating fibre nonlinearities in a dual-polarization quadrature phase shift keying (DP-QPSK) system over 7780 km and a dual-polarization 16- level quadrature amplitude modulation (DP-QAM16) system over 1600 km. It is noted that significant improvements in performance may be achieved for a nonlinear step-size greater than one span. An approximately exponential relationship was found between performance improvement in Q-factor and the number for required complex multipliers. DSP algorithms for polarization-switched quadrature phase shift keying (PS-QPSK) are then investigated. A novel two-part equalisation algorithm is proposed which provides singularity-free convergence and blind equalisation of PS-QPSK. This algorithm is characterised and its application to wavelength division multiplexed (WDM) transmission systems is discussed. The thesis concludes with an experimental comparison between a PS-QPSK transmission system and a conventional DP-QPSK system. For a 42.9 Gb/s WDM system, the use of PS-QPSK enabled an increase of reach of more than 30%. The resultant reach of 13,640 km was, at the time of publication, the longest transmission distance reported for 40 Gb/s transmission over an uncompensated link with standard fibre and optical amplification

Study on Advanced Modulation Formats in Coherent Optical Communication Systems

Ph.DDOCTOR OF PHILOSOPH