Entropy loading for capacity maximization of RGB-based visible light communications.

In this work, we propose and experimentally demonstrate an entropy loading technique based on probabilistic constellation shaping for a visible light communication (VLC) system. The aggregated achievable bit rate of a multi-carrier signal is maximized, considering a given pre-estimated signal-to-noise ratio. A study of the ideal number of subcarriers and signal bandwidth was performed using multiplexed red, green and blue lasers diodes with a bandwidth of 1 GHz.

Adaptive optical beam alignment and link protection switching for 5G-over-FSO.

Free-space optics (FSO) convey an enormous potential for ultra-high-capacity seamless fiber-wireless transmission in 5G and beyond communication systems. However, for its practical exploitation in future deployments, FSO still requires the development of very high-precision and robust optical beam alignment. In this paper, we propose two different methods to achieve tight, precise alignment between a pair of FSO transceivers, using a gimbal-based setup.

Simplified high-order Volterra series transfer function for optical transmission links.

We develop a simplified high-order multi-span Volterra series transfer function (SH-MS-VSTF), basing our derivation on the well-known third-order Volterra series transfer function (VSTF). We notice that when applying an approach based on a recursive method and considering the phased-array factor, the order of the expression for the transfer function grows as 3 raised to the number of considered spans. By imposing a frequency-flat approximation to the higher-order terms that are usually neglected in the commonly used VSTF approach, we are able to reduce the overall expression order to the typical third-order plus a complex correction factor.

Digital signal processing for fiber nonlinearities [Invited].

This paper reviews digital signal processing techniques that compensate, mitigate, and exploit fiber nonlinearities in coherent optical fiber transmission systems.

Nonlinear compensation with DBP aided by a memory polynomial.

Using digital backpropagation (DBP) based on the split step Fourier method (SSFM) aided by a memory polynomial (MP) model, we demonstrate an improved DBP approach for fiber nonlinearity compensation. The proposed technique (DBP-SSFM&MP) is numerically validated and its performance and complexity are compared against the benchmark DBP-SSFM, considering a single-channel 336 Gb/s PM-64QAM transmission system. We demonstrate that the proposed technique allows to maintain the performance achieved by DBP-SSFM, while decreasing the required number of iterations, by over 60%.

Experimental demonstration of a frequency-domain Volterra series nonlinear equalizer in polarization-multiplexed transmission.

Employing 100G polarization-multiplexed quaternary phase-shift keying (PM-QPSK) signals, we experimentally demonstrate a dual-polarization Volterra series nonlinear equalizer (VSNE) applied in frequency-domain, to mitigate intra-channel nonlinearities. The performance of the dual-polarization VSNE is assessed in both single-channel and in wavelength-division multiplexing (WDM) scenarios, providing direct comparisons with its single-polarization version and with the widely studied back-propagation split-step Fourier (SSF) approach. In single-channel transmission, the optimum power has been increased by about 1 dB, relatively to the single-polarization equalizers, and up to 3 dB over linear equalization, with a corresponding bit error rate (BER) reduction of up to 63% and 85%, respectively.

Mitigation of intra-channel nonlinearities using a frequency-domain Volterra series equalizer.

We address the issue of intra-channel nonlinear compensation using a Volterra series nonlinear equalizer based on an analytical closed-form solution for the 3rd order Volterra kernel in frequency-domain. The performance of the method is investigated through numerical simulations for a single-channel optical system using a 20 Gbaud NRZ-QPSK test signal propagated over 1600 km of both standard single-mode fiber and non-zero dispersion shifted fiber. We carry on performance and computational effort comparisons with the well-known backward propagation split-step Fourier (BP-SSF) method.