This thesis studies the performance of wireless in-house transmission systems using infrared optical signals. These studies focus in the physical layer of the transmission systems.

The basic principles involved in the transmission of unguided optical signals are described, as well as the major limitations of this technology. The propagation of optical signals inside a room may use one of three basic propagation modes: diffuse, quasi-diffuse and line-of-sight. Each of these propagation modes has its advantages and limitations that determine the optical power requirements and channel bandwidth, among other aspects. However, most of the technology limitations are common to all the propagation modes.

All three propagation modes suffer from the presence of ambient light. The optical spectrum of ambient light extends to the optical band used by wireless infrared transmission systems, impairing the optical signal transmission by producing shot noise and interference. In this thesis, the ambient light, both natural and artificial, is characterised through extensive measurements and models are developed for the shot noise and interference. The models and the experimental results are then used to evaluate the performance of wireless optical transmission systems operating in channels with ambient light. In this study, several modulation methods are considered. The results show that artificial light induces high power penalties and, therefore, has to be considered in the performance evaluation of optical wireless transmission systems. The models for the ambient light are also used to evaluate optical and electrical filtering techniques used to reduce the effects of the ambient light. An interference cancellation technique is proposed to reduce de effects of the interference produced by artificial light.

Many applications and communication systems have been proposed that use wireless transmission of infrared signals. One of these applications is for wireless local area networks. Since 1990, the IEEE 802.11 working group is developing a standard for wireless local area networks. This upcoming standard specifies three alternative physical layers, two of them using radio signal in the 2.4 GHz band and one of them using infrared signals. The medium access control layer is common to all the three physical layers. In this thesis this standard is described with particular emphasis on the infrared physical layer. Some of the specifications of this layer include contributions from the work described in this thesis, proposed during the participation in the activities of the IEEE 802.11 working group. This thesis describes some of these contributions, namely the proposals for a frame format and for the optical receiver sensitivity.

The theoretical studies about the performance evaluation of infrared transmission systems were complemented with the implementation of a wireless infrared transmission system closely following the IEEE 802.11 specification. This system implements the physical layer of a network interface for an optical wireless local area network. This thesis describes the implementation of this system, as well as the experimental results obtained through measurements in real environments.