Guest editorial: optical wireless communications

the last two decades, wireless communications has gained enormous popularity, offering attractive options for many personal and organizational communication needs due to major intrinsic characteristics such as flexibility, cost effectiveness, and mobility. Two transmission techniques for wireless communications have been deployed so far; Radio Frequency (RF) and Optical Communications. While the vast majority of research efforts so far have been dedicated to RF wireless communications, there is a rapidly growing interest in Optical Wireless Communications (OWC) due to intrinsic advantages such as extremely high data rates, inherent security, and licence-free operation. However, OWC also poses several challenges that have to be overcome before widespread deployment of OWC systems is possible. As a consequence, the field of OWC promises enormous opportunities for basic and applied research and development. Thisspecial issue is an attempt to capture the current state of the art in OWC and to open up new avenues for research in this area. RESEARCH CHALLENGES IN INDOOR AND OUTDOOR OPTICAL WIRELESS SYSTEMS Optical wireless systems can be classified into long and short range systems, according to the distance between the transmitter and receiver. Long range systems are used for outdoor optical wireless, while short range systems can be applied for both outdoor and indoor environments. Indoor and outdoor propagation environments are inherently different from each other and pose unique challenges which need to be carefully taken into account for optical wireless system design. Early research in OWC has focused on design issues related to indoor infrared wireless technologies. These systems are characterized by smaller distances and are free from major outdoor environmental degradations such as fog, rain, snow, and mist. The loss in the indoor link iscaused only by the free space loss. There are two indoor optical wireless transmission techniques: Direct line of sight and diffuse. The direct line of sight configuration has superior power efficiency, low multi-path dispersion, and lower path loss and can achieve higher transmission rates. The drawback of this configuration is that it suffers from shadowing and a concern for eye safety that limits the average transmitter power, hence affecting overall power efficiency. On the other hand, the diffuse configuration does not require the alignment of transmitter and receiver and as a result is robust against shadowing and easy to use allowing more mobility. The diffuse system, however, suffers from higher path loss and requires higher transmitter power levels and larger size photo-detecting area at …