Wireless data services and systems represent a rapidly growing and increasingly important segment of the communications industry. In the paper the authors present an overview of this field, emphasizing three major elements: (1) technologies utilized in existing and currently planned wireless data services, (2) issues related to the performance of these systems, and (3) discernible trends in the continuing development of wireless data systems. While the wireless data industry is becoming increasingly diverse and fragmented, one can identify a few mainstreams which relate directly to users requirement for data services. On one hand, there are requirements for relatively low-speed data services supporting mobile users over wide geographical areas, as provided by mobile data networks. On the other hand, there are requirements for high-speed data services in local areas, as provided by wireless LANs. The system-level issues are somewhat different for these two categories of services, and this has led to different technology choices in the two domains, which the authors discuss in the paper. >

Wireless data communications

A simple analytical model of the wireless infrared communication channel in indoor environments is presented. The infrared signal is modeled as the combination of a diffuse component and a line-of-sight (LOS) or direct component. For the diffuse component alone, the properties of the channel are found using Ulbricht's integrating sphere. When a LOS component is also present, the transfer function depends upon the Rician factor K given by the ratio of the electrical power in the LOS and diffuse signals after the detector. For small K, the transfer function shows notches down to low frequencies, but due to the nature of light never for zero frequency. We confirm that a K-factor /spl ges/13 dB is required also in infrared wireless links in order to support distortionless data transmission beyond 100 Mbit/s. Increasing the directivity at the receiver and/or at the transmitter improves the effective value of K. Here, we show that a moderate directivity will be sufficient for high-speed infrared communication in typical indoor scenarios.

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A physical model of the wireless infrared communication channel

The emergence of portable computing devices such as laptops, palmtops and personal digital assistants (PDAs) has fuelled the demand for mobile connectivity and hence, led to the development of wireless local area networks (LANs). Wireless LANs offer users increased mobility and flexibility compared with traditional wired networks, and may be classified as either infrastructure wireless or ad hoc wireless networks [1]. Untethered from conventional network connections, infrastructure wireless LANs allow users to maintain network connectivity whilst roaming anywhere within the coverage area of the network. This configuration requires the use of access points, or base stations, which are connected to the wired LAN and act as interfaces to the wireless devices. Each access point may accommodate multiple clients. Examples of practical applications for wireless infrastructure LANs which are often cited include medical professionals accessing patient records, real-time vital signs and other reference data at the patient bedside, and factory floor workers accessing part specifications and process information as and when required. In contrast, ad hoc wireless LANs are simple peer-to-peer networks in which each client only has access to the resources of the other clients on the network and not a central server. Ad hoc wireless LANs require no administration or pre-configuration, and are created on demand only for as long as the network is needed. Examples of practical applications for wireless ad hoc LANs include employees sharing information during a meeting, and colleagues electronically swapping business cards.

Indoor Optical Wireless Communication Systems - Part I: Review

This study investigates the use of error control code, discrete wavelet transform (DWT) and artificial neural network (ANN) to improve the link performance of an indoor optical wireless communication in a physical channel. The key constraints that barricade the realization of unlimited bandwidth in optical wavelengths are the eye-safety issue, the ambient light interference and the multipath induced intersymbol interference (ISI). Eye-safety limits the maximum average transmitted optical power. The rational solution is to use power efficient modulation techniques. Further reduction in transmitted power can be achieved using error control coding. A mathematical analysis of retransmission scheme is investigated for variable length modulation techniques and verified using computer simulations. Though the retransmission scheme is simple to implement, the shortfall in terms of reduced throughput will limit higher code gain. Due to practical limitation, the block code cannot be applied to the variable length modulation techniques and hence the convolutional code is the only possible option. The upper bound for slot error probability of the convolutional coded dual header pulse interval modulation (DH-PIM) and digital pulse interval modulation (DPIM) schemes are calculated and verified using simulations. The power penalty due to fluorescent light interference (FL I) is very high in indoor optical channel making the optical link practically infeasible. A denoising method based on a DWT to remove the FLI from the received signal is devised. The received signal is first decomposed into different DWT levels; the FLI is then removed from the signal before reconstructing the signal. A significant reduction in the power penalty is observed using DWT. Comparative study of DWT based denoising scheme with that of the high pass filter (HPF) show that DWT not only can match the best performance obtain using a HPF, but also offers a reduced complexity and design simplicity. The high power penalty due to multipath induced ISI makes a diffuse optical link practically infeasible at higher data rates. An ANN based linear and DF architectures are investigated to compensation the ISI. Unlike the unequalized cases, the equalized schemes don‘t show infinite power penalty and a significant performance improvement is observed for all modulation schemes. The comparative studies substantiate that ANN based equalizers match the performance of the traditional equalizers for all channel conditions with a reduced training data sequence. The study of the combined effect of the FLI and ISI shows that DWT-ANN based receiver perform equally well in the present of both interference. Adaptive decoding of error control code can offer flexibility of selecting the best possible encoder in a given environment. A suboptimal ?soft‘ sliding block convolutional decoder based on the ANN and a 1/2 rate convolutional code with a constraint length is investigated. Results show that the ANN decoder can match the performance of optimal Viterbi decoder for hard decision decoding but with slightly inferior performance compared to soft decision decoding. This provides a foundation for further investigation of the ANN decoder for convolutional code with higher constraint length values. Finally, the proposed DWT-ANN receiver is practically realized in digital signal processing (DSP) board. The output from the DSP board is compared with the computer simulations and found that the difference is marginal. However, the difference in results doesn‘t affect the overall error probability and identical error probability is obtained for DSP output and computer simulations.

Application of wavelets and artificial neural network for indoor optical wireless communication systems

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.

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

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.

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

The design and implementation of a wireless infrared Ethernet link is considered. It is shown that, using commercially available devices, it is technical and economically feasible to implement wireless infrared extensions of Ethernet networks, maintaining the degree of complexity in transceiver electronics at very low level. In particular, it is shown that Manchester coding can still be used in the infrared path without impairing system range and performance. A laboratory prototype was developed and implemented using Manchester coding, achieving a bit error rate of 10/sup -10/ over a range typical of normal office environments. The utilization of low cost lenses, both for the LED's radiation pattern correction and as optical power collecting elements, was investigated and shown to provide significant performance improvements. >

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A. M. Oliveira Duarte

Papers

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

Design and implementation issues of a wireless infrared Ethernet link