In order to deal with the increasing number of mobile devices and with their demand for Internet services, particularly social media platforms, streaming video, and online gaming, Radio-Frequency (RF) wireless networks have been pushed to their capacity limits. In addition to this, 80% of the total data traffic is carried out by users inside buildings. Therefore, new technologies have started to be considered for indoor wireless communications. Visible Light Communications (VLC) can provide both illumination and communications, appearing as an alternative or complement to RF wireless networks. VLC offers high bandwidth and immunity to interference from electromagnetic sources. This manuscript reviews recent high-capacity VLC demonstrations. The main focus of this work is to present digital-signal-processing techniques used in VLC systems. Different modulation formats are analyzed, which can be divided into two large groups, namely single-carrier and multi-carrier modulation schemes. Finally, some recently proposed capacity-achieving strategies are presented. We discuss how to implement these techniques and how they will be useful for the continued development of VLC systems.

The range of applications enabled by the low-Earth orbit (LEO) constellations continues to increase, going from providing worldwide broadband internet access, to the transmission of enormous quantities of data gathered from space sensing and exploration. Therefore, it is imperative to find a technology for Earth-satellite and inter-satellite links that is able to provide ultra-high capacity and reliability. The alliance of coherent optical systems with free-space optics (FSO) can be considered a solution for these links, bringing the ultra-high capacity of fiber optic transmission systems to the wireless domain. However, the free-space channel brings some challenges to the telecommunication system, namely the atmospheric turbulence that can hinder data transmission, mainly when associated with typical satellite communication features such as the Doppler effect. Driven by this challenge, in this work, we propose a technique for mitigation of the impact of Doppler effect by dynamically adapting the transmitted signal symbol-rate while keeping the system bit-rate fixed through probabilistic constellation shaping (PCS). This methodology significantly increases the range of supported frequency shifts imposed by the Doppler effect in an experimental 600 Gbps transmission. Moreover, we address the reliability problem imposed by atmospheric turbulence, considering two different scenarios: i) a continuous transmission system with guaranteed reliability and ii) a bursty transmission without reliability constraints that solely aims at the maximization of the overall throughput of the system. In both scenarios, we show that the proposed methodology of adapting the system symbol-rate together with the transmitted constellation shaping can increase the supported bit-rate by more than 70 Gbps.

The use of free-space optics (FSO) for ultra-high-speed communications has been recently standing out as a promising alternative for fiber/wireless-based scenarios. However, FSO communications still suffer from variations in the received optical power due to atmospheric turbulence and weather instability, which limits its use in reliability-driven applications. To explore the full potential of FSO and maintain communications reliability, hybrid FSO and radio-frequency systems can be employed to leverage the strengths of the two technologies. In this letter, we demonstrate a 3m hybrid FSO and millimeter-wave system for 200G stable wireless transmission. Resorting to time-adaptive probabilistic constellation shaping and a cross-domain entropy loading scheme, we are able to compensate for up to 15% (30Gbps) of data-rate reduction in the FSO link. Comparing with FSO-only transmission, we demonstrate a cumulative capacity gain of over 150Gbps after 40 offline measurement iterations.

In response to the soaring demand for wireless internet due to the proliferation of mobile devices, the radio-frequency spectrum is severely congested. To address this issue, emerging optical transmission technologies, notably visible light communications (VLC), are gaining traction. VLC offers high bandwidth, immunity to electromagnetic interference, and combines lighting with wireless communication. This study delves into examining the performance of a high-capacity VLC system, using a setup that multiplexes and diffuses light from red, green, and blue (RGB) laser diodes (LDs) to generate white light. We achieved impressive bit rates of up to 38 Gbit/s over 1 meter in an unconstrained lighting environment. However, when meeting lighting requirements, the net bit rate ranged between 27-33 Gbit/s. By adjusting the bias current of the LDs, we optimized the color combination within a range of 2500-6500 K correlated color temperature, catering to various scenarios. Additionally, our experimental results demonstrated that the optimized RGB-VLC link effectively covers a wide 40 cm diameter area while sustaining multi-Gigabit data rates within a 30 $^\circ$ field-of-view.

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. With a 20 degree optical diffuser, the communication system is able to cover a wide area at a free-space distance of 0.90 m, while achieving a record aggregate bit rate of 31.2 Gbit/s for single-polarization RGB-VLC systems.

The tight channel filtering imposed by long cascades of reconfigurable optical add-drop multiplexers (ROADMs) represents, nowadays, one of the major performance-limiting aspects for optically-routed coherent optical fiber systems. This makes it vital to perform a ROADM-aware optimization of the network performance at the physical layer. In particular, the use of different modulation options is known to have a strong impact on the extent of the filtering-induced penalties. In that regard, a long debate between single- and multi-carrier modulation has been taking place during the last few years, sometimes leading to apparently contradictory results. Following the open scientific discussion on this topic, in this work, we investigate by simulation and experimentally the wavelength selective switch (WSS) filtering tolerance of single-carrier (SC) and digital subcarrier multiplexing (DSCM) signals. In order to promote a fair comparison, both modulation options are carefully designed to minimize the ROADM-filtering penalties, namely resorting to the use of entropy loading together with baud rate optimization. After some preliminary numerical assessment, a comprehensive set of experiments are carried out for the transmission of 21-WDM 95–105 Gbaud SC and DSCM signals over a 2040 km straight line of fiber with regularly spaced WSSs. In general, our results allow to conclude that the two modulation options yield similar performance if the overall baud rate is optimized for each filtering scenario, keeping the baud rate at $\pm$ 5% of the optimized value.