Signal Processing for Optical Wireless Communications and Sensing

Introduction

Optical wireless is a subject which more recently has attracted much attention, as it is able to offer high bandwidth communications amidst a growing clamouring for space in the electromagnetic spectrum. It offers direct competition to terahertz technologies, as the devices used for optical wireless are, in most respects, very similar to those used in optical fibre communications, and so are well developed, cheap, and versatile. The topic is not without certain specific problems, in spite of its theoretical ease of use. The main issues are associated with the background illumination aspects, as solar radiation and even room lighting can be quite strong contenders for the usually weak infrared signature which comprises the signal to be detected. Associated with the ambient illumination is also noise. On the one hand, therefore, there is the availability of bandwidth which is compatible with the many Gigahertz available on fiber systems, and then the other aspect is that the shot noise associated with the front end receiver amplifier is powerfully related to bandwidth by a cube law, above the 3 dB point, if equalisation is applied. Equalisation can be used to flatten frequency response, as, unlike the situation for an optical fiber with a 9μm diameter core, an optical wireless receiver requires a large area device to collect the signal, resulting in a commensurate adverse capacitance. Here are ways around this, and work at Warwick University on optical antennas for example has found a way around this problem using an Optical Antenna (Green & Ramirez-Iniguez, 2000). Additionally, optimised use of the receiver amplifier as well can give significant improvements overall in terms of the SNR versus bandwidth trade-off. OFDM is an effective method for making the most of system bandwidth once this has been optimised, and it poses some interesting challenges in the optical wireless scenario, which can be evaluated, and, to some extent, mitigated against. Infrared sources tend to distribute their output over an area, and this inevitably results in an optical field which is anything but uniform. Fortunately, systems can be devised which use the genetic algorithm (GA) approach to regulation of the optical field, resulting in, not only equalisation of the SNR over the areas of interest, but also the available bandwidth over the same area. We shall firstly discuss OFDM for modulation of the optical carrier, and then examine the GA approach to regulate the optical field, and finally examine standard- and new optical receiver-amplifiers.