This Thesis studies the propagation of optical signals in indoor environments, aiming the development of high speed and high performance wireless infrared communication systems. Special attention is given to aspects of modelling, simulation and optimisation of the optical channel.

An overview of wireless indoor optical communication systems is presented. The characteristics of the indoor optical channel are discussed, identifying the main issues that impair the performance of infrared communication systems. The main elements of the optical channel are modelled, namely, the source emitting pattern, the detector receiving pattern, the propagation of the optical signal in free-space and its reflection on indoor surfaces. This reflection is approximated through three models: the Lambert's model, the Phong's model and the Torrance-Sparrow's model. These models are used to approximate the measured reflection patterns of several surfaces. The results show that all the measured patterns are well approximated by the Phong's model and that the Lambert's model is not able to approximate correctly most f the measured reflection patterns. Three models for the propagation of optical signals in indoor channels are detailed: the line-of-sight model, the single reflection model and the multiple reflections model. Those models are used to approximate the signal propagation in the three most common system configurations: line-of-sight, quasi-diffuse and diffuse.

The models described are used to implement a simulation package, named SCOPE, that allows to simulate the propagation of optical signals in indoor channels. The simulation algorithm and the approaches/techniques used are detailed. The simulator allows to evaluate the main characteristics of the indoor optical channel, considering multiple reflections of the emitted signal. The SCOPE has reduced computation time, relatively to other existing simulators of the indoor optical channel. The effects of the first 5 signal reflections on the propagation characteristics of the indoor optical channel are evaluated.

The worst-case propagation losses of the indoor optical channel are, in general, high and change significantly with several factors, namely, the emitting and receiving patterns, the system configuration and the relative positioning of emitter, receiver and reflection surfaces in the communication cell. Using the simulator, the propagation losses in indoor spaces are studied for the three system configurations (line-of-sight, quasi-diffuse and diffuse). The dependence of the propagation losses with the main channel parameters is analytically represented through approximated equations. For each system configuration, the emitter radiation pattern is optimised to minimise the worst-case propagation losses, reducing also significantly the optical range of the received signal over the communication cell.

The IEEE 802.11 working group developed a specification for wireless local area networks, which includes an infrared physical layer. Some of the work presented in this Thesis has contributed for that specification. The IEEE 802.11 infrared physical layer is described and the specification of the emitter radiation pattern is detailed. The communication range of the systems conforming with the IEEE 802.11 standard is evaluated for a set of dissimilar indoor spaces. In all those spaces, the channel propagation characteristics degrade smoothly with the distance. It is shown that the specified emitter radiation pattern is in conformance with the most recent safety standards for laser radiation and it is safe for the user.

The multipath propagation results in time dispersion of the received signal, which may originate inter-symbol interference. The channel impulse responses for the three system configurations are compared. The multipath propagation depends on the system configuration, emitting and receiving patterns and on the relative positioning of emitter, receiver and reflection surfaces. In general, the resulting dispersion reduces the minimum channel bandwidth to values lower than about ten MHz. It is shown that the optimisation of the emitter radiation pattern to minimise the propagation losses results also in a reduction of the channel bandwidth.

The inter-symbol interference introduced by multipath propagation of the optical signal imposes a growing penalty for transmission rates above about ten Mbps. The main techniques used to combat the effects of multipath dispersion in communication systems are reviewed. By using the simulator, the use of angle diversity to combat the multipath dispersion in indoor infrared systems is investigated. The results show that the use of sectored receivers with multiple segments of sectors, associated with a best-sector selection, reduces the time dispersion of the received signal. The use of angular diversity in both emitter and receiver is proposed to combat effectively the multipath dispersion of the indoor optical channel.