# Special Issue on Light Communications: Latest Advances and Prospects
###### Abstract  
The wireless information carrier shift from the radio frequencies to optical frequencies has been studied in increasing detail since the invention of the laser in the 1960s. With the advent of optical fiber communications and their global use, we have seen a growing interest in the field of optical wireless communications (OWC) for front- and back-haul access networks and beyond as part of fifth-generation (5G) wireless networks. OWC has found niche applications in a wide range of systems, from short-range indoor to long-range outdoor and very long deep space and inter-satellite links [1]. New components, such as visible light-emitting sources, image sensors, advanced signal processing techniques, multi-input multi-output, and machine learning are pushing the OWC technology into new application areas such as vehicular communications, smart environments, medical, manufacturing, etc., as part of 5G wireless networks and the emerging 6G networks and beyond. This Special Issue was put together with the goal of bringing together the latest research and development in wireless light communications. A total of 53 submissions were received, and 24 were selected for publication in this Special Issue. The published works present novel results mainly on outdoor and indoor applications of light communications, while a few papers also address broader areas. We briefly present these works in the following sections. 202220202202202202202202202  
Viterbi-Viterbi phase offset estimation method with Kalman filter, which leads to lowering the estimation error by no less than 60%, even when the signal to noise ratio (SNR) is low. [PERSON], et al. [4] have discussed the performance of optically amplified FSO systems when an orthogonal modulation format such as PPM is utilized to partially mitigate atmospheric-induced fading. Novel analytical approximations are presented for the average bit error rate in these systems, which are shown to be in good agreement with the exact values. This theoretical work also addresses the applicability of diversity reception, identifying fading scenarios where multiple amplifiers can be used despite the associated increase in optical noise. [PERSON] and co-authors [5] have conducted experimental research on the effects of weather conditions and the chemical composition of the atmosphere on FSO communications. The authors set up their experiment in Qatar, a region with arid and harsh weather, and monitored the system performance during the four seasons. Their results indicate that the FSO link performs better in winter than in summer, mainly since higher temperatures during summer introduce increased atmospheric scintillations. The next two papers also report on the impact of atmospheric effects on FSO communications, with the addition of intermediate relays between the transmitter and the receiver. [PERSON], et al. [6] have studied the performance of three relay selection schemes (source to relay, relay to destination, and source to destination). The obtained results show that no optimum technique exists for all channel conditions, and the relay selection algorithm needs to take into account the severity of atmospheric induced fading and the pointing errors. [PERSON], et al. [7] have evaluated the performance of a relay-assisted FSO communication system with a blind detection scheme. The authors derive novel analytical expressions for the average bit error rate and demonstrate that this technique performs similar to more complex systems requiring channel state information. The optimal relay placement is also investigated using the analytical expressions. A promising future practical application of outdoor FSOs, namely, space optical communications, is the topic of the next two papers. [PERSON] and co-researchers [8] have analyzed the technique of aperture averaging with a goal to partially correct the adverse effects of atmospheric turbulence in a coherent LEO satellite-to-ground binary phase shift keying system.