Optical Wireless Communications

The use of mobile devices, e.g., laptops, cell phones and PDAs, has become ubiquitous. Users expect to access very high-speed networks from anywhere and be able to communicate with other users, and share common resources such as printers, at high data rates. As a consequence, wireless local area networks (WLANs) have become the subject of intense research. WLANs are subject to more noise and interference than wired systems. Because users share the same high-speed network channel, bandwidth and data security are in great demand. From a user point of view, power consumption, size, weight and cost are major concerns. Thus, WLAN design is much more challenging than for a wired network. In general, WLANs have adopted radio wave-based technology, for example Bluetooth, IEEE802.11 and the emerging ultra wideband (UWB) systems. The demand for network access is growing rapidly, yet radio bandwidth is limited. In the future, radio frequencies may be insufficient to accommodate a large number of users or devices. As a consequence, infrared indoor wireless communications has been proposed as an alternative to provide additional capacity [1]. The most attractive feature of infrared WLANs is virtually unlimited and worldwide unregulated bandwidth because the infrared frequencies range from 300 GHz to approximately 300 THz. The optical transceiver can be made smaller and cheaper than an RF transceiver module. In addition, the optical circuitry consumes little power, therefore, battery life can be very long. A comparison of different communications standards, e.g., IEEE802.11, UWB, Bluetooth and an optical link in terms of power consumption and bit rate [2] has shown that optical systems achieve the highest bit rate and the least energy consumption normalized by the bit rate. Since optical signals cannot penetrate through walls or other opaque barriers, the security of infrared WLANs is very high and there is no interference between rooms. Consequently, cell planning is simple and easy, and the potential capacity of an optical-based network in a building is extremely high. Infrared WLANs can be used where radio signals are prohibited such as hospitals, airplanes and laboratories as there is no electromagnetic interference (EMI). An optical system can potentially operate at very high data rates (several Gbps) within approximately a 10-meter range. However, the crucial factor that hinders the data rate is multipath dispersion caused by reflections from ceilings and walls. or LD LED /2 λ