Indoor optical wireless communication systems and networks

Welcome to this Special Issue on Indoor Optical Wireless Communications, in which we explore recent research and development in the area. There is an increasing demand for ultra broadband wireless access networks and the wireless multimedia applications and video which exploit their capability, coming from end users such as hospitals, teaching environments, and retailers. Currently there are two main technologies, radio and optical, capable of offering these services. Radio has the advantage of being available ubiquitously, both outdoors and indoors, and it also offers mobility, but will be challenged to provide the required high bandwidth. On the other hand, the optical wireless option could provide a cost effective, flexible, secure and ultra-high speed solution to the emerging challenges facing the system and service providers. The last few years have seen a considerable interest in both outdoor and indoor optical wireless communications from materials and devices through to systems and networks. It has already been used in applications from more complex wireless LANs such as those being specified by the IEEE 802.11 and ETSI (HIPERPLAN) to simple remote controllers for home appliances. The purpose of this special issue is to bring together the latest development in the area of indoor optical wireless communications systems and networks. In order to increase the speed of optical wireless systems multiple-subcarrier (MS) systems have been used. However since the average optical power is proportional to this d.c. bias, it is important to minimize the bias signal. The paper by Kitamoto and Ohtsuki discusses MS schemes and proposes new parallel combinatory multiple-subcarrier (PC-MS) system. PC-MS not only can decrease the d.c. bias but it can also offer higher information per symbol interval than the conventional MS system with N subcarriers. Indoors wireless optical channels are limited not only in transmitted optical power, but also in signalling bandwidth. This bandwidth constraint arises due to multipath dispersion in indoor settings as well as due to response time limitations of optoelectronic components. The paper by Hranilovic presents an overview of theoretical and practical issues in the design of signalling for bandwidth constrained intensity modulated direct detection optical wireless channels. A survey of modulation design is also presented together with results on the channel capacity of indoor wireless optical channels with particular emphasis placed on recently derived asymptotically exact bounds. The use of multiple emitters and receivers in wireless optical channels is also discussed with particular emphasis placed on techniques, which exploit spatial dimensions to improve spectral performance. As in RF communication system, noise and path loss are also major problems in indoor optical wireless communications. The use of large area photodetectors in conjunction with narrow bandwidth optical filters can minimize the problems of path loss and noise, but at the cost of reduced bandwidth. A paper by Ramirez-Iniguez and Green overcomes this problem by using an optical antenna (which is basically an optical concentrator (OC)) in conjunction with a small area photodetector. This improves the collection efficiency by transforming light rays