Iterative Detection and Phase-Noise Compensation for Coded Multichannel Optical Transmission

The problem of phase-noise compensation for correlated phase noise in coded multichannel optical transmission is investigated. To that end, a simple multichannel phase-noise model is considered and the maximum a posteriori detector for this model is approximated using two frameworks, namely factor graphs (FGs) combined with the sum–product algorithm (SPA), and a variational Bayesian (VB) inference method. The resulting pilot-aided algorithms perform iterative phase-noise compensation in cooperation with a decoder, using extended Kalman smoothing to estimate the a posteriori phase-noise distribution jointly for all channels. The system model and the proposed algorithms are verified using experimental data obtained from space-division multiplexed multicore-fiber transmission. Through Monte Carlo simulations, the algorithms are further evaluated in terms of phase-noise tolerance for coded transmission. It is observed that they significantly outperform the conventional approach to phase-noise compensation in the optical literature. Moreover, the FG/SPA framework performs similarly or better than the VB framework in terms of phase-noise tolerance of the resulting algorithms, for a slightly higher computational complexity

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

Pilot Distributions for Joint-Channel Carrier-Phase Estimation in Multichannel Optical Communications

Joint-channel carrier-phase estimation can improve the performance of multichannel optical communication systems. In the case of pilot-aided estimation, the pilots are distributed over a two-dimensional channel--time symbol block that is transmitted through multiple channels. However, suboptimal pilot distributions reduce the effectiveness of the carrier-phase estimation and thus result in unnecessary pilot overhead, which reduces the overall information rate of the system. It is shown that placing pilots identically in all channels is suboptimal in general. By instead optimizing the pilot distribution, the mean squared error of the phase-noise estimates can be decreased by over 90% in some cases. Moreover, it is shown that the achievable information rate can be increased by up to 0.05, 0.16, and 0.41 bits per complex symbol for dual-polarization 20 GBd transmission of 64-ary, 256-ary, and 1024-ary quadrature amplitude modulation over 20 four-dimensional channels, respectively, assuming a total laser linewidth of 200 kHz.Comment: Submitted to Journal to Lightwave Technolog

Pilot Distributions for Joint-Channel Carrier-Phase Estimation in Multichannel Optical Communications

Joint-channel carrier-phase estimation can improve the performance of multichannel optical communication systems. In the case of pilot-aided estimation, the pilots are distributed over a two-dimensional channel-time symbol block that is transmitted through multiple channels. However, suboptimal pilot distributions reduce the effectiveness of the carrier-phase estimation and thus result in unnecessary pilot overhead, which reduces the overall information rate of the system. It is shown that placing pilots identically in all channels is suboptimal in general. By instead optimizing the pilot distribution, the mean squared error of the phase-noise estimates can be decreased by over 90% in some cases. Moreover, it is shown that the achievable information rate can be increased by up to 0.05, 0.16, and 0.41 bits per complex symbol for dual-polarization 20 GBd transmission of 64-ary, 256-ary, and 1024-ary quadrature amplitude modulation over 20 four-dimensional channels, respectively, assuming a total laser linewidth of 200 kHz

Compensation of Laser Phase Noise Using DSP in Multichannel Fiber-Optic Communications

One of the main impairments that limit the throughput of fiber-optic communication systems is laser phase noise, where the phase of the laser output drifts with time. This impairment can be highly correlated across channels that share lasers in multichannel fiber-optic systems based on, e.g., wavelength-division multiplexing using frequency combs or space-division multiplexing. In this thesis, potential improvements in the system tolerance to laser phase noise that are obtained through the use of joint-channel digital signal processing are investigated. To accomplish this, a simple multichannel phase-noise model is proposed, in which the phase noise is arbitrarily correlated across the channels. Using this model, high-performance pilot-aided phase-noise compensation and data-detection algorithms are designed for multichannel fiber-optic systems using Bayesian-inference frameworks. Through Monte Carlo simulations of coded transmission in the presence of moderate laser phase noise, it is shown that joint-channel processing can yield close to a 1 dB improvement in power efficiency. It is further shown that the algorithms are highly dependent on the positions of pilots across time and channels. Hence, the problem of identifying effective pilot distributions is studied.The proposed phase-noise model and algorithms are validated using experimental data based on uncoded space-division multiplexed transmission through a weakly-coupled, homogeneous, single-mode, 3-core fiber. It is found that the performance improvements predicted by simulations based on the model are reasonably close to the experimental results. Moreover, joint-channel processing is found to increase the maximum tolerable transmission distance by up to 10% for practical pilot rates.Various phenomena decorrelate the laser phase noise between channels in multichannel transmission, reducing the potency of schemes that exploit this correlation. One such phenomenon is intercore skew, where the spatial channels experience different propagation velocities. The effect of intercore skew on the performance of joint-core phase-noise compensation is studied. Assuming that the channels are aligned in the receiver, joint-core processing is found to be beneficial in the presence of skew if the linewidth of the local oscillator is lower than the light-source laser linewidth.In the case that the laser phase noise is completely uncorrelated across channels in multichannel transmission, it is shown that the system performance can be improved by applying transmitter-side multidimensional signal rotations. This is found by numerically optimizing rotations of four-dimensional signals that are transmitted through two channels. Structured four-dimensional rotations based on Hadamard matrices are found to be near-optimal. Moreover, in the case of high signal-to-noise ratios and high signal dimensionalities, Hadamard-based rotations are found to increase the achievable information rate by up to 0.25 bits per complex symbol for transmission of higher-order modulations