# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 1 Introduction to Unguided Optical Communication (UOC)  
### Revolution of UOC  
This review paper covers the evaluation of today's free-space optical (FSO) communication, also known as unguided optical communication (UOC), and their techniques/systems, right from prehistoric fire-based signal transmission. The technology developments in thefield of UOC have significantly evolved over the past several decades, which has led, in depth, to FSO communications, underwater optical wireless communications (UOWC), and optical wireless communications (OWC). Though, primarily, these transmission schemes have a common denominator in their transceiver chains, they require different kinds of installation mechanisms, beam processing, optoelectronic and electro-optic components, signal processing, and performance indicators, due to differences in data link range, transmission medium, and applications. Due to its dual nature, light does not need any medium to travel. However, due to channel effects, in FSO systems, the link range is long, typically in the order of several kilometers, while in UOWC and OWC systems, the link span is relatively short, typically in the order of a few meters. Optical wireless technologies catering for low to high data rates are being used in a range of applications for last-meter/last-mile access networks, terrestrial long-range connectivity, ground to air/space links, air-to-water/underwater networks, and fifth-generation (5G)/emerging 6G wireless networks, etc. [1,2]. This review paper covers a broad spectrum of developments, breakthroughs, and achievements in the field of UOC techniques, starting from their origin through to today's successful artificial intelligence (AI)-supported UOC systems. The developments in optoelectronics components and optical/electrical signal processing platforms/algorithms, which are advancing the UOC techniques aiming to resolve near-future data traffic, are presented [2]. UOC systems were initially designed for only point-to-point simplex data communication and later duplex data communications, and then for full-duplex transmission [3]. With advances made in making optical sources and detectors, optoelectronic devices, wavelength selection devices, and modulation/coding techniques, UOC systems operating under weather conditions have remarkably improved [4,5], thus making them attractive for a variety of purposes. These include indoor, in-flight, underwater, and vehicular communication, as well as communications from the surface to unmanned aerial vehicles (UAVs), inter-satellite communications, etc. [6, 7, 8]. Considering the growing number of applications of these systems, it is important to address the challenges in UOC to ensure their full utilization. These challenges include the estimation/prediction of channel effects, accurate measurement of the received optical beam's temporal/spatial disturbances, development of optimum mitigation schemes, reliability analysis/prediction of system parameters, efficient transmission of radio over FSO links, design of hybrid FSO-radio frequency (RF) antennas, finding tunable modulation techniques, framing implementations and installations strategies, incorporating AI techniques, efficient UOC channel energy harvesting, designing the next generation of UOC network architecture, using UOC in the Internet of Things (IoT) as a part of 5G/6G wireless networks, etc. [5, 6, 7, 8, 9]. Note that, at the research level, only a few of the above challenges have been addressed. To address these challenges, one of the best options would be the integration of several allied research fields such as the design of high phase stable optical sources, high-data-rate modulation/demodulation optoelectronics devices, high-speed signal/data processing, AI-based optimization, design of mid-infrared/quantum-cascaded lasers, design of long-wave infrared optical sources, quantum encoding/decoding schemes, intelligent energy harvesting techniques, high-data rate optical detectors, etc. [8, 9, 10, 11].