António Tavares

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.

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.

Angle diversity and rate-adaptive transmission for indoor wireless optical communications

The main degrading factor in indoor wireless optical communication systems for bit rates up to several megabits per second is the shot noise induced by ambient light (sunlight and artificial light produced by incandescent and fluorescent lamps). Due to the directional nature of both signal and ambient light noise, the spatial distribution of the signal-to-noise ratio in indoor environments can show large variations. This article compares techniques that are able to mitigate the effect of such SNR variations: rate-adaptive transmission and angle diversity. In the first technique, the effective data rate is adjusted to the local SNR conditions by introducing different levels of redundancy. The second technique explicitly explores the directionality of the SNR by combining signals collected from different observation angles. We address the performance of rate-adaptive transmission and angle diversity techniques, and compare them based on experimental results obtained in a typical indoor environment.

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.

Performance of wireless infrared transmission systems considering both ambient light interference and intersymbol interference due to multipath dispersion

The performance of optical wireless transmission systems is mainly impaired by the shot noise induced by ambient light, interference produced by artificial light sources, transmitted optical power limitations due to high path losses and channel bandwidth limitations due to inter-symbol interference produced by the multipath dispersion of the optical signal. The contribution of these factors to the performance evaluation of infrared links have only been addressed independently and the combined effect of these channel impairments was not presented yet. The work presented in this paper extends the previous analysis by taking into account the combined effects of both optical noise (shot noise and interference) and channel impulse response. A simulation package was used to determine the indoor optical channel impulse response due to the propagation losses and multipath dispersion under various room geometries and emitter/receiver parameters. The contribution of the interference produced by incandescent and fluorescent lamps was done through the utilization of analytical models. The penalty introduced by these channel characteristics was quantified considering the modulation schemes usually considered for optical wireless communication systems: 2-, 4- and 16-PPM (pulse position modulation) at bit rates from 1 to 10 Mbps.

Experimental results of a pulse position modulation infrared transceiver

Infrared technology is a suitable alternative for the support of indoor wireless local area networks (WLANs). When compared to radio technologies, infrared offers the potential for lower cost, higher security and better resilience to interference provoked by users from adjacent cells. This paper presents experimental results of an infrared transceiver for diffuse systems based on pulse position modulation. The implementation followed the upcoming IEEE 802.11 specification. This transceiver was developed within the ESPRIT.6892 POWER (Portable Workstation for Education in Europe) project.

Design and implementation of a maximal-ratio angle-diversity receiver for optical wireless communication systems

A maximal-ratio angle-diversity receiver is composed of multiple sectors with relatively small field-of-views. Each sector estimates the signal-to-noise ratio (SNR) of the collected signal and its gain made proportional to the relation i/(sigma) 2, where i and (sigma) represent the average signal and the shot noise root mean square (rms) values, respectively. The output signals of all sectors are then combined through an adder circuit. This paper presents the design and implementation of a maximal-ratio receiver using discrete components. A major challenge is the design of the variable gain amplifier (VGA) which requires a large dynamic range because of the large fluctuations of both signal and noise in a typical office room environment. This problem was overcome through the utilization of a cascade of two VGAs where the assignment of gains to each VGA minimizes dynamic range requirements through an innovative topology. The first one provides a gain inversely proportional to the rms shot noise and the second one a gain proportional to the SNR referred to the input of the front-end. Measurements on an implemented prototype show results close to the ideal gain of a maximal-ratio receiver making the proposed techniques suitable for maximal-ratio angle-diversity receivers.

Experimental characterization of rate-adaptive transmission and angle diversity reception techniques

Ambient light is the main impairment in indoor wireless optical communication systems for data rates up to several megabits per second. Its wide dynamic range, associated with the strong directivity of wireless optical signals, produce large variations on the received signal-to-noise ratio. This article discusses experimental results obtained from a testbed developed to compare different techniques for SNR improvement. The two techniques analyzed are rate-adaptive transmission, which introduces adaptive levels of redundancy in the transmitted signal to improve connectivity, and angle diversity reception, which exploits the inherent directionality of both signal and noise to improve the SNR at the receiver. Furthermore, systems employing both techniques simultaneously were also considered. The testbed replicated a typical indoor environment with both natural and artificial light, containing incandescent and fluorescent light sources. Both the SNR and the associated coverage areas were determined for all considered techniques. Our results show that the combined use of angle diversity based on maximal ratio combining and rate adaptation through the use of repetition coding achieves very good performance with only moderate complexity, allowing connectivity at all locations with data rates close to the maximum possible. In particular, with incandescent illumination and without angle diversity, the data rate had to be decreased down to 2 and 1 Mb/s in 25.9 and 7.7 percent of the room, area, respectively, whereas with maximal ratio combining a decrease to 2 Mb/s was only needed in 0.7 percent of the room area.

A test bed for wireless optical LANs

This paper presents a test bed for wireless optical LANs. This test bed is flexible, supporting multiple implementation choices. The test bed currently covers all physical layer issues, from FEC coders and LED drivers to front-ends, clock recovery circuits, Viterbi decoders, and sectored receivers. These are implemented with multiple technologies, with DSPs and FPGAs for digital functions, and ASICs and discrete electronics for the analogue electronics. Furthermore, the test bed provides a semi-controlled real life test environment, allowing for close comparison between theoretical and practical results.