This article presents an overview of optical wireless (OW) communication systems that operate both in the short- (personal and indoor systems) and the long-range (outdoor and hybrid) regimes. Each of these areas is discussed in terms of (a) key requirements, (b) their application framework, (c) major impairments and applicable mitigation techniques, and (d) current and/or future trends. Personal communication systems are discussed within the context of point-to-point ultra-high speed data transfer. The most relevant application framework and related standards are presented, including the next generation Giga-IR standard that extends personal communication speeds to over 1 Gb/s. As far as indoor systems are concerned, emphasis is given on modeling the dispersive nature of indoor OW channels, on the limitations that dispersion imposes on user mobility and dispersion mitigation techniques. Visible light communication systems, which provide both illumination and communication over visible or hybrid visible/infrared LEDs, are presented as the most important representative of future indoor OW systems. The discussion on outdoor systems focuses on the impact of atmospheric effects on the optical channel and associated mitigation techniques that extend the realizable link lengths and transfer rates. Currently, outdoor OW is commercially available at 10 Gb/s Ethernet speeds for Metro networks and Local-Area-Network interconnections and speeds are expected to increase as faster and more reliable optical components become available. This article concludes with hybrid optical wireless/radio-frequency (OW/RF) systems that employ an additional RF link to improve the overall system reliability. Emphasis is given on cooperation techniques between the reliable RF subsystem and the broadband OW system.

We present analytical results on the average probability of error (PER) performance of an optically pre-amplified pulse-position modulation (PPM) receiver under Malaga- $\mathcal{M}$ fading. The results are in the form of a finite sum whose number of terms depends on the PPM modulation order and the noise modes of the amplifier, enabling the efficient calculation of the average PER. In addition, we utilized the presented analysis to evaluate the performance of a equal-gain-combining (EGC) diversity receiver that operates in conjunction with optical amplification and PPM. The results show that the utilization of diverse and relatively low PPM orders achieves a drastic reduction in the average PER.

Traditionally, traffic from the access nodes is aggregated in metro-aggregation hubs over rings or horseshoes in a fixed configuration using multiple optical point-to-point (P2P) transceivers. However, this static setup limits dynamic scalability and often leads to inefficient resource usage and high costs, especially under increasing and varying traffic conditions. Coherent optical point-to-multipoint (P2MP) transceivers offer a promising solution for aggregation at this network level. These transceivers allow a single (aggregation) node to communicate with multiple (access) nodes simultaneously via digital subcarrier multiplexing (DSCM) technology. Additionally, their long-distance transmission capabilities enable the placement of the P2MP transceiver root deeper in the hierarchy. In this paper, we propose an extended access aggregation architecture that incorporates modified reconfigurable optical add–drop multiplexers (ROADMs) to support both traditional P2P and P2MP connections. This architecture allows the creation of light-trees to enable P2MP communication and hence to boost transmission flexibility and multiplexing gains. We propose a mixed-integer linear programming (MILP) model to determine the optimal placement of P2MP transceivers and the establishment of light-trees, considering physical layer impairments (PLIs) and the required quality of transmission (QoT) for the connections. To tackle the high algorithm complexity, we also introduce a best-fit decreasing heuristic to efficiently exploit the trade-off between execution time and performance. Our simulation experiments using real network topologies showcase that our proposed architecture can greatly enhance multiplexing gains while considering the operational costs tied to network expansions and upgrades.

We present novel analytical results on the optimal combiner (OC) for pre-amplified optical wireless receivers under chi-squared noise. The results show that the OC architecture that minimizes the average bit-error rate (BER) is determined by the level of the signal-spontaneous beating noise. Moreover, the OC provides increased gain to the diversity branches with the higher energies, up to the point where the beating noise becomes detrimental to the BER. The performance of the OC is simulated for practical receiver arrangements and it is shown that it performs better than the maximal-ratio and equal-gain combiners. The performance improvement amounts to an energy gain of less than 1 dB for implementations that utilize narrow optical filters and a modest number of diversity branches.

We present exact and approximate results on the average probability of error (PER) for pulse position modulation (PPM) in pre-amplified optical wireless communication systems with diversity. The approximate results are obtained by combining a new mathematical formula that we derive for binary PPM and an existing formula that associates higher-order PPM PERs with their binary PPM counterpart. The approximate results are compared with the exact in weak, moderate, and strong turbulence, and it is demonstrated that they are in good agreement under all fading conditions. Moreover, the accuracy of the approximation improves with the optical signal-to-noise ratio and the number of diversity branches that are used, which correspond to implementation scenarios that are typically anticipated in practice.

Novel results on the link layer protocol performance of future infrared personal communication links at a line rate of 100 Gbit/s are presented. The discussion aims to demonstrate the shortcomings of the currently standardised half‐duplex mode of communications in the physical layer and details its negative impact on the link layer efficiency. Results of a non‐standardised full‐duplex mode of operation that is however suitable for application in future high capacity infrared links are also presented. The results predict that the migration to this full‐duplex mode can prove advantageous for the link layer performance, as it not only yields a higher efficiency, but also requires significantly smaller frame and window sizes.