Impact of 4D channel distribution on the achievable rates in coherent optical communication experiments

We experimentally investigate mutual information and generalized mutual information for coherent optical transmission systems. The impact of the assumed channel distribution on the achievable rate is investigated for distributions in up to four dimensions. Single channel and wavelength division multiplexing (WDM) transmission over transmission links with and without inline dispersion compensation are studied. We show that for conventional WDM systems without inline dispersion compensation, a circularly symmetric complex Gaussian distribution is a good approximation of the channel. For other channels, such as with inline dispersion compensation, this is no longer true and gains in the achievable information rate are obtained by considering more sophisticated four-dimensional (4D) distributions. We also show that for nonlinear channels, gains in the achievable information rate can also be achieved by estimating the mean values of the received constellation in four dimensions. The highest gain for such channels is seen for a 4D correlated Gaussian distribution

The Dispersion of Nearest-Neighbor Decoding for Additive Non-Gaussian Channels

We study the second-order asymptotics of information transmission using random Gaussian codebooks and nearest neighbor (NN) decoding over a power-limited stationary memoryless additive non-Gaussian noise channel. We show that the dispersion term depends on the non-Gaussian noise only through its second and fourth moments, thus complementing the capacity result (Lapidoth, 1996), which depends only on the second moment. Furthermore, we characterize the second-order asymptotics of point-to-point codes over $K$-sender interference networks with non-Gaussian additive noise. Specifically, we assume that each user's codebook is Gaussian and that NN decoding is employed, i.e., that interference from the $K-1$ unintended users (Gaussian interfering signals) is treated as noise at each decoder. We show that while the first-order term in the asymptotic expansion of the maximum number of messages depends on the power of the interferring codewords only through their sum, this does not hold for the second-order term.Comment: 12 pages, 3 figures, IEEE Transactions on Information Theor

A Beta-Beta Achievability Bound with Applications

A channel coding achievability bound expressed in terms of the ratio between two Neyman-Pearson $\beta$ functions is proposed. This bound is the dual of a converse bound established earlier by Polyanskiy and Verd\'{u} (2014). The new bound turns out to simplify considerably the analysis in situations where the channel output distribution is not a product distribution, for example due to a cost constraint or a structural constraint (such as orthogonality or constant composition) on the channel inputs. Connections to existing bounds in the literature are discussed. The bound is then used to derive 1) an achievability bound on the channel dispersion of additive non-Gaussian noise channels with random Gaussian codebooks, 2) the channel dispersion of the exponential-noise channel, 3) a second-order expansion for the minimum energy per bit of an AWGN channel, and 4) a lower bound on the maximum coding rate of a multiple-input multiple-output Rayleigh-fading channel with perfect channel state information at the receiver, which is the tightest known achievability result.Comment: extended version of a paper submitted to ISIT 201

Chromatic dispersion monitoring for high-speed WDM systems using two-photon absorption in a semiconductor microcavity

This paper presents a theoretical and experimental investigation into the use of a two-photon absorption (TPA) photodetector for use in chromatic dispersion (CD) monitoring in high-speed, WDM network. In order to overcome the inefficiency associated with the nonlinear optical-to-electrical TPA process, a microcavity structure is employed. An interesting feature of such a solution is the fact that the microcavity enhances only a narrow wavelength range determined by device design and angle at which the signal enters the device. Thus, a single device can be used to monitor a number of different wavelength channels without the need for additional external filters. When using a nonlinear photodetector, the photocurrent generated for Gaussian pulses is inversely related to the pulsewidth. However, when using a microcavity structure, the cavity bandwidth also needs to be considered, as does the shape of the optical pulses incident on the device. Simulation results are presented for a variety of cavity bandwidths, pulse shapes and durations, and spacing between adjacent wavelength channels. These results are verified experimental using a microcavity with a bandwidth of 260 GHz (2.1 nm) at normal incident angle, with the incident signal comprising of two wavelength channels separated by 1.25 THz (10 nm), each operating at an aggregate data rate of 160 Gb/s. The results demonstrate the applicability of the presented technique to monitor accumulated dispersion fluctuations in a range of 3 ps/nm for 160 Gb/s RZ data channel

Downlink Space–Time Spreading Using Interference Rejection Codes

In this paper, the authors will investigate the performance of a loosely synchronized (LS) code-based space–time spreading (STS) scheme in comparison to that of classic Walsh code and pseudonoise code-based STS when communicating over dispersive Nakagami-m multipath channels. Closed-form formulas are derived for characterizing the bit-error-rate performance as a function of the number of resolvable paths L and the number of users K. Our numerical results suggest that the employment of LS code-based STS scheme is beneficial in a low-user-load and low-dispersion channel scenario, where a near-single-user performance can be achieved without a multiuser detector. Index Terms—Code-division multiple access (CDMA), Gaussian approximation, interference-free window (IFW), large area synchronized (LAS) codes, loosely synchronized (LS) codes, Nakagami-m fading

Non-parametric Estimation of Mutual Information with Application to Nonlinear Optical Fibers

This paper compares and evaluates a set of non-parametric mutual information estimators with the goal of providing a novel toolset to progress in the analysis of the capacity of the nonlinear optical channel, which is currently an open problem. In the first part of the paper, the methods of the study are presented. The second part details their application to several optically-related channels to highlight their features.Comment: This work has been submited to IEEE International Symposium on Information Theor

Information capacity of optical fiber channels with zero average dispersion

We study the statistics of optical data transmission in a noisy nonlinear fiber channel with a weak dispersion management and zero average dispersion. Applying path integral methods we have found exactly the probability density functions of channel output both for a non-linear noisy channel and for a linear channel with additive and multiplicative noise. We have obtained analytically a lower bound estimate for the Shannon capacity of considered nonlinear fiber channel.Comment: 4 pages, subbmited to Phys. Rev. Let