Since its early commercial deployment in the late 1980s, optical fiber has evolved to become the predominant carrier of the globe’s communications. Yet, after accommodating the world’s exponentially growing appetite for transmitted data for more than three decades, its ability to continue doing so is being challenged by fundamental factors. In this article, we review these factors and examine their consequences in terms of information capacity. In particular, we review the difficulties that are imposed by the nonlinear nature of fiber-optic transmission on the assessment of the capacity and on the definition of fundamental concepts, such as bandwidth and spectral efficiency. We discuss relevant approximations and regimes of operation in which bounds for the capacity can be effectively assessed while covering a broad range of applications ranging from interdatacenter communications to links spanning transoceanic distances. We relate to a broad variety of transmission schemes and discuss the potential benefits of spatial multiplexing with multimode and multicore fibers. State-of-the-art transmission experiments are also reviewed and compared with theoretical capacity bounds.

Fiber's nonlinearity fundamentally bounds the achievable information rates in fiber-optic communication systems. In a wavelength-division multiplexed system it induces a nonlinear interference between adjacent channels, an interference that was recently shown to have a strong dependance on the input distribution. In this work we show that a ball shaped input constellation may significantly reduce the nonlinear effects. We study the shaping gains in the fiber-optic channel and show that in certain scenarios the maximum gains may be higher than the 1.53dB ultimate shaping gain in linear additive white Gaussian noise channels. Furthermore, the maximum gain is achieved with a finite-dimensional ball shaping region.

This paper presents a new fading model for multi-input multi-output channels: the Jacobi fading model. It asserts thatH, the transfer matrix which couples themtinputs intomroutputs, is a submatrix of anm×mrandom (Haar-distributed) unitary matrix. The (squared) singular values ofHfollow the law of the classical Jacobi ensemble of random matrices, hence the name of the channel. One motivation to define such a channel comes from multimode/multicore optical fiber communication. It turns out that this model can be qualitatively different from the Rayleigh model, leading to interesting practical and theoretical results. This paper first evaluates the ergodic capacity of the channel. Then, it considers the nonergodic case, where it analyzes the outage probability and the diversity-multiplexing tradeoff. In the case wherek=mt+mr-m> 0, it is shown that at leastkdegrees of freedom are guaranteed not to fade for any channel realization, enabling a zero-outage probability or infinite diversity order at the corresponding rates. A simple scheme utilizing (a possibly outdated) channel state feedback is provided, attaining the no-outage guarantee. Finally, noting that asmincreases, the Jacobi model approaches the Rayleigh model, the paper discusses the applicability of the model in other communication scenarios.

In the Jacobi MIMO channel the transfer matrix H which couples the mtinputs into mroutputs is a sub-matrix of an m×m random (Haar-distributed) unitary matrix. The (squared) singular values of H follow the law of the classical Jacobi ensemble of random matrices; hence the name of the channel. A motivation to define such a channel comes from multimode/multicore optical fiber communication. It turns out that this model is qualitatively different than the Rayleigh model, leading to interesting practical and theoretical results. This work first evaluates the ergodic capacity of the channel. In the non-ergodic case, it analyzes the outage probability and the diversity-multiplexing tradeoff. In the case where k = mt+mr-m >; 0 at least k degrees of freedom are guaranteed not to fade for any channel realization enabling a zero outage probability or infinite diversity order at the corresponding rates. Finally, we note that the Jacobi channel may provide a new fading model to other applications.