The future of wireless communication requires the unique capabilities of free-space optics (FSO). Therefore, it is crucial to develop methods for achieving reliable long-distance FSO communications. This study presents a field trial of a 1.8 km FSO link that can achieve 4 Tbps+ using coherent optics, wavelength division multiplexing (WDM), atmospheric turbulence mitigation, and optimized forward error correction (FEC) coding. Our study focuses on the impact of atmospheric turbulence on FSO communication and the methods used to mitigate its effects. We compensate for the turbulence-induced power fluctuations by using an optical pre-amplification technique with automatic power control (APC) to stabilize the received power. This technique reduces the perceived Rytov variance by a factor of ten, making the FSO communication more reliable and efficient. Furthermore, we explore the maximization of net bit-rate by optimising the transmitted channels' FEC overhead, considering two different architectures: individual and joint wavelength processing. The proposed optimization techniques are shown to provide significant gains both in terms of capacity and reliability, making FSO technology a more practical solution for long-range wireless communication.

The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized.

The quest for high-speed optical wireless communications is now entering a new and ambitious phase of pursuing fiber-like capacity over the air. However, legacy optical wireless technologies relying on simple intensity modulation and direct detection will not be up to the challenge of providing Terabit datarates for wireless communications. Benefitting from the latest developments on coherent optical fiber transceivers, unprecedented optical wireless capacity can now be achieved, effectively enabling the wireless delivery of Terabit-range data-rates. In this work, we exploit advanced features provided by state-of-the-art coherent transceivers, such as symbol-rate and bit-rate adaptability to enhance the tradeoff between capacity and reliability in optical wireless systems impaired by atmospheric turbulence, pointing errors and Doppler shift. Targeting the digital mitigation of these challenges, a set of different field trials is provided, demonstrating the viability of per-channel bit-rates in the range of 400 Gbps to 1 Tbps, applicable to both terrestrial and SatCom scenarios.

In this work, we experimentally exploit the use of free-space optics (FSO) transmission with multi-mode fiber (MMF) coupling towards robust optical wireless communication system, with enhanced flexibility and misalignment tolerance. Such analysis is carried out considering a 2.5 GHz cost-effective fiber-coupled multi-mode receiver together with advanced modulation techniques, such as multi-carrier modulation, probabilistic constellation shaping and entropy loading, achieving an overall bit rate of >40 Gbps with 6.7 bit/s/Hz spectral efficiency. Results show that the exploited MMF-coupling enables a 2-fold bit rate increase over a standard single-mode fiber (SSMF) for a 0.25 mrad pointing error. Finally, the interplay between FSO pointing errors and inter-modal dispersion is experimentally assessed for MMF lengths up to 500 m. Our results demonstrate that MMF can effectively be exploited in a quasi-single-mode propagation regime provided that the air-to-fiber coupling is performed with sufficient beam alignment accuracy.

We exploit the frequency diversity offered by digital subcarrier-multiplexing to overcome the coherent combining challenge associated with the use of FMF in FSO systems. Experimental validation at 200 Gbps in an atmospheric chamber reveals reliability gains of >20% compared with an equivalent single-mode coupling system.

The increasing demand for high-capacity communications has positioned free-space optics (FSO) as a future-proof technology, promising unmatched wireless data-rates. However, the widespread deployment of high-capacity FSO requires the development of ultra-precise and highly-dynamic beam alignment solutions, capable of being easily integrated in a low-power and low-footprint all-optical system. This article presents a system that exploits multi-core fiber (MCF) coupling to retrieve the position of the received beam and feed it to the pointing, acquisition and tracking (PAT) system, thereby enabling to perform optical beam alignment without the need for an auxiliary opto-electronic position sensing device. The beam alignment methodology is based on a computationally-efficient lookup table (LUT) approach, with its effectiveness analyzed as a function of the number of illuminated cores. The obtained results reveal a trade-off between the number of utilized cores and number of alignment iterations (and hence, alignment latency). Starting with a single-core transmission and 7-core reception scheme, i.e. a 1×7 single-input multiple-output (SIMO) system, nearly 100% alignment success rate has been achieved after 5 iterations. Expanding the system towards a multiple-input multiple-output (MIMO) 7×7 architecture, the system alignment can be successfully completed after only 3 iterations, supporting a total field of view (FOV) of 4.5 mrad.