Signal-Dependent Noise for B-Modulation NFT-Based Transmission

Abstract

The nonlinear Fourier transform (NFT) is a potentially promising way to mitigate nonlinear signal distortions in fiber-optic communication systems [1–4]. Recently, the new approach named the b-modulation was introduced [1] that allows to control the temporal duration of NFT-generated signals. In the traditional NFT-based communication, the nonlinear spectrum, r (ξ) is used to modulate data. Then the signal duration depends on the signal energy and is not predetermined and controlled. The utilisation of the NFT scattering coefficient b (ξ) gives a possibility to control the pulse duration in time domain under condition that the Fourier spectrum of b (ξ) is localized. In particular, the OFDM format satisfies that condition. It was already shown that the properties of the effective noise in the NFT domain differ from a circular Gaussian noise. More specifically, such a noise is signal-dependent and non-circular [2, 3]. It was shown in [4] that the covariance B1 and pseudo-covariance B2 characteristics related to b-coefficient of NFT spectrum depend on the propagation distance. Here, we extend our previous work and examine noise characteristics for a novel b-modulation transmission introduced in [1]. We demonstrate that the variances behaviour is generic and do not depend on a specifics of NFT modulation. The presence of amplifier spontaneous emission in the optical link translates into the effective noise in the nonlinear spectral domain as an effective additive term N (ξ, b (ξ, 0)): $e^{-4 i \\xi^{2L}b}(\\xi, L) = b(\\xi, 0) + N(\\xi, b(\\xi, 0), L)$, where $\\xi$ is the “nonlinear frequency” (NFT spectral parameter): $\\langle N (\\xi)N^*(\\xi )\\rangle = 2DL\\pi \\delta (\\xi -\\xi )B_{1}(\\xi, L), \\langle N (\\xi)N(\\xi )\\rangle = 2DL\\pi \\delta (\\xi - \\xi )B_{2}(\\xi, L) $