A. M. de Oliveira Duarte

Optical interference produced by artificial light

Wireless infrared transmission systems for indoor use are affected by noise and interference induced by natural and artificial ambient light. This paper presents a characterisation (through extensive measurements) of the interference produced by artificial light and proposes a simple model to describe it. These measurements show that artificial light can introduce significant in‐band components for systems operating at bit rates up to several Mbit/s. Therefore it is essential to include it as part of the optical wireless indoor channel. The measurements show that fluorescent lamps driven by solid state ballasts produce the wider band interfering signals, and are then expected to be the more important source of degradation in optical wireless systems.

The infrared physical layer of the IEEE 802.11 standard for wireless local area networks

The new IEEE 802.11 standard for wireless local area networks defines a specification for an infrared physical layer. This article gives an overview of infrared technology and describes the IEEE 802.11 specification in detail, presenting a historical perspective of its development. The infrared physical layer was designed for diffuse systems supporting two data rates (1 and 2 Mb/s) and includes provisions for a smooth migration to higher data rates. The specification is suitable for low-cost transceivers but allows interoperability with higher-performance systems. The main application envisaged for IEEE 802.11 infrared wireless local area networks is ad hoc networks.

Characterisation and modelling of artificial light interference in optical wireless communication systems

Wireless indoor infrared transmission systems are affected by noise and interference induced by natural and artificial ambient light. While the shot noise induced on the receiver photodiode by steady ambient light has been extensively described and included in system models, the interference produced by artificial light has only been mentioned as a source of degradation and quite simple descriptions have been presented. This paper presents a characterisation (through extensive measurements) of the interference produced by artificial light and proposes a simple model to describe it. These measurements show that artificial light can introduce significant in-band components for systems operating at bit rates up to several Mbit/s. Therefore it is essential to include it as part of the optical wireless indoor channel. The measurements show that fluorescent lamps driven by solid state ballasts produce the wider band interfering signals, and are then expected to be the more important source of degradation in optical wireless systems.

Sectored receivers for indoor wireless optical communication systems

This document proposes and studies a novel receiver structure for indoor optical wireless communication systems that exploits the directional nature in both signal and noise propagation through the use diversity techniques. In this document we concentrate on the ability of the proposed sectored receiver in combating the ambient noise. Significant optical power gains are demonstrated and are seen to increase with the relative weight of the directional noise within the cell, with the sharpness of the directional noise source beam width and in environments where there are noise sources positioned outside the cell. Also the SNR of a sectored receiver is seen to be much less sensitive to the position and beamwidth of the noise sources than the SNR of a non-sectored receiver allowing for more universal transceiver designs.

Sectored receivers to combat the multipath dispersion of the indoor optical channel

This paper presents the first results of a study developed to evaluate the performance of sectored receivers in combating the multipath dispersion of the indoor optical channel. We intend to: (1) evaluate the performance of sectored receivers in minimizing multipath dispersion and, (2) conclude if sectored receivers may be used successfully to reduce both ambient noise and multipath dispersion, by estimating only the ambient noise at each sector of the receiver. The results show that sectored receivers can reduce significantly the multipath dispersion, but selection of the sector with the best impulse response is required. Also, to achieve significant gains the field-of-view of the sectors have to be smaller than 50/spl deg/.

Modulation methods for wireless infrared transmission systems: performance under ambient light noise and interference

The major aspects that impair the performance of optical wireless transmission systems are the shot noise induced by the steady ambient light level, transmitted optical power limitations (high path losses), channel bandwidth limitations due to multipath dispersion and the interference produced by artificial light sources. Several modulation and encoding schemes have been proposed for this channel and their performance has been studied and presented by several authors. The work reported in this paper extends the previous analysis by taking into account the optical power penalty induced by artificial light interference. An analytical approach is used to estimate the optical power penalty induced by artificial light interference. In practical systems, the effect of the interference is usually mitigated using electrical high- pass filters. In this paper the combined effect of interference and high-pass filter is evaluated by resorting to simulation. The presented results show that interference produced by fluorescent lamps driven by electronic ballasts induce high power penalties in OOK and PPM systems, even when high-pass filtering is used. For the interference produced by incandescent lamps and fluorescent lamps driven by conventional ballasts, the power penalty induced in OOK systems can be effectively reduced using high-pass filtering, while PPM is very tolerant to that interference even without any high-pass filtering.

Performance of an optical sectored receiver for indoor wireless communication systems in presence of artificial and natural noise sources

This paper gives special attention to wireless local area networks using infrared technology mainly with respect to the reception techniques and presents the performance evaluation of optical sectored receivers for indoor wireless communication systems in the presence of artificial and natural noise sources. Performance evaluation was extended to four distinct sectored receiver configurations which result in significant gains over a non-sectored optical receiver. A characterization of the ambient light noise distribution due artificial light was performed. Also, the radiation patterns of some directional incandescent lamps were measured and modeled through a generalized Lambertian function. The feasibility of optical sectored receivers in the presence of directional light sources is demonstrated.

Efficient simulation of the impulse response of the indoor wireless optical channel

The use of wireless local area networks (WLANs) is experiencing a significant growth. Infrared technology has an extensive bandwidth of free use, and has the potential to be the support of high-baud rate WLANs. For that, it is necessary to understand the propagation characteristics of the indoor optical channel. This paper presents a computationally efficient algorithm for the simulation of indoor optical channels, considering multiple reflections of the emitted signal. The models used to approximate the emitter pattern, the propagation environment and the receiver pattern are described. Two new procedures (called time-delay agglutination and time and space indexed tables) are introduced to increase the efficiency of the simulation of the impulse response of the indoor wireless optical channel. The structure of the algorithm is presented and the approaches used are discussed. The new algorithm was implemented in a simulation package, named SCOPE, which is very efficient and allows the evaluation of the main parameters of indoor optical channels considering multiple reflections of the emitted signal. Results obtained for several channels are compared with experimental and published values, and good agreement is verified. Copyright

Angle Diversity to Combat the Ambient Noise in Indoor Optical Wireless Communication Systems

We propose and study the use of angle diversityto combat the ambient noise in indoor optical wirelesscommunication systems. Models for the radiation patternof spot lamps and for the spatial distribution of the ambient light based on an isotropic anda directional noise component are derived. Performanceevaluation of receivers using angle diversity is carriedout and significant optical power gains are demonstrated. The optical gains are seen toincrease with the relative weight of the directionalnoise within the cell, with the sharpness of thedirectional noise source beam width, and in environments where there are noise sources positionedoutside the cell. Also, the signal-to-noise ratio (SNR)of a sectored receiver is seen to be much less sensitiveto the position and beam width of the noise sources than the SNR of a nonsectored receiver,allowing for more universal transceiverdesigns.

Propagation Losses and Impulse Response of the Indoor Optical Channel: A Simulation Package

In this paper we present a simulation package developed to evaluate and optimize both the channel propagation losses and the multipath dispersion of the indoor optical channel. The simulation package is based on a model of the indoor optical channel which includes the emitter radiation pattern, the channel propagation characteristics and the receiver collecting pattern. We illustrate the use of the simulation package by means of two case studies corresponding to: i) a satellite based cell and ii) a passive reflection based cell. It is seen that optimization of the emitting pattern by tuning the number, orientation and radiation pattern of the LEDs can significantly reduce the worst-case channel losses and increase the channel bandwidth. The channel impulse response changes with the receiver position, therefore, techniques to combat the multipath dispersion have to be used for bit rates higher than a few Mbit/s.