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Author(s): Sheikh Muhammad Sajid | Zabih Ghassemlooy

Journal: Journal of Communications
ISSN 1796-2021

Volume: 4;
Issue: 8;
Start page: 521;
Date: 2009;
Original page

Keywords: special issue | Optical Wireless Communications

ABSTRACT
Welcome to this Special Issue on Optical Wireless Communications, in the Journal of Communications, in which we explore the latest research and development in the area. We are seeing a growing demand for ultra broadband wireless access networks coming from the end users such as hospitals, teaching environments, software houses and retailers etc. Currently there are two main technologies, radio and optical, capable of offering these services. The former offers mobility and has the advantage of being available ubiquitously, both outdoors and indoors, but suffers from bandwidth bottleneck. Whereas, the later offers deployment flexibility, inherent security as well as ultra-high bandwidth. During the last few years, we have seen a considerable interest in both outdoor and indoor optical wireless communications.The application areas of optical wireless include optical interconnects, terrestrial links, Inter-satellite links, inter-high altitude platforms (HAP), inter-UAV, HAP-GEO links and links from HAP/aircraft to ground, visible light based LANs, high definition TV entertainment systems, and high speed communication within high-speed trains and passenger aircrafts. The purpose of this special issue is to bring together the latest development in the area of optical wireless communications systems. The articles selected, except the invited paper, are the extended version of the papers presented at the “First IEEE Colloquium on Optical Wireless Communications” organized at the 6th IEEE sponsored CSNDSP 2008 conference at Graz, Austria. The special issue covers diverse topics and is intended for wider audience within the growing optical wireless research community, and is composed of 7 standard papers and one invited review paper.Free space optical (FSO) communications have the potential for creating a three dimensional global communication grid of inter-connected ground and airborne terminals. The promised enormous data rates of FSO are achievable under clear weather conditions. Atmospheric phenomenon such as fog, rain, snow, clouds and turbulence degrade the performance of the links dramatically.The invited article by Kavehard et al. discusses application of multiple-inputs-multiple-outputs (MIMO) with aperture averaging for free space optical communication links. The paper outlines MIMO transceiver design for FSO terminals and shows the bit error rate performance for both single-input-single-output and MIMO systems in the presence of atmospheric turbulence. Aperture averaging and MIMO techniques are suggested for better performance in turbulent environment. Terrestrial FSO links are most affected by the atmospheric attenuators particularly the fog. It is often not easy to obtain an analysis of the attenuation caused by the atmospheric aerosols. In the second article by Saleem et al., full analysis of experimental results for the attenuation for foggy and snowy environment carried out at the Graz University of Technology and in other European cities namely Milan (Italy) and Nice (France) are presented. The results presented provide valuable insights for improved system design of the next generation FSO systems.Random variations in the refractive index of the Earth’s atmosphere are responsible for random fluctuations in the signal carrying light beam intensity commonly termed as turbulence which remains a major challenge in establishing highly reliable optical wireless links. The paper by Popoola et al. employs a sub-carrier intensity modulation scheme as against the widely used on-off keying FSO for improved performance in atmospheric turbulence channels. By exploiting the reduced spatial coherence of the atmospheric channel during turbulence, the spatial diversity technique is adopted as way of mitigating the deleterious fading effect. Closed form expressions for the bit error rate and the outage probability performance indicators are presented for a differential phase shift keying pre-modulated sub-carrier scheme. Most optical wireless systems are based on the intensity modulation direct detection schemes (IM/DD) due to their simplicity. El Tabach et al. in their article presents a comparative study of modified coded OFDM and modified coded OFDM/OQAM schemes adapted to IM/DD over a diffuse channel based on experimental results. The COFDM/OQAM benefits from spectral shaping and mitigates the multipath induced intersymbol interference (ISI) effects. The paper utilizes a channel model based on different optical wireless propagation topologies and analysis the channel impulse response for a diffuse indoor multi-path environment.One of the prime difficulties with non-directed indoor wireless infrared systems is the ISI caused by multiple reflections of the received signal which limits the maximum achievable data rate. There are a number of techniques to combat ISI including the zero forcing equalizer and the minimum mean square equalizer. The paper by Ntogari et al. investigates an equalized optical wireless link. Different equalization techniques such as the maximum likelihood sequence estimation, the linear mean square error equalizer and the decision feedback equalizer (DFE) are considered for PPM, OOK and PAM modulation schemes. It is shown that a data rate of 100 Mbps can be achieved in a medium sized room using equalized indoor optical wireless links. Full exploitation of the bandwidth capabilities of optical wireless links requires the line of sight channel approach. To achieve a high optical gain and a wide field of view (FOV) in line of sight links a large detector area with associated capacitance is required at the receiver. Joyner et al. have proposed a CMOS based analogue front-end receiver design for fully integrated optical wireless transceivers. The trans-impedance amplifier based design utilizes capacitive feedback and is designed for integration with wide FOV receiver. The design demonstrates the key merit of capacitive feedback structure over conventional resistive feedback whereby an increase in the input capacitance from 0.5 pF to 5 pF keeps the change in the -3dB bandwidth within 5%. The trans-impedance amplifier utilized implements an automatic gain control and achieves a dynamic range from 52 dBΩ to 36 dBΩ.Deep space and inter-satellite communications is an area of growing interest for the application of FSO links due to their high data rate capability. In March 2008, a 5.5 Gbps inter-satellite link was successfully demonstrated representing a step ahead after the success of the famous SILEX experiment. Morio et al. in their article have proposed the concept of a compact free space laser communication terminal employing a vertical cavity surface emitting laser (VCSEL) array. A non-mechanical compact optical transceiver architecture utilizing an 8 x 8 VCSEL array is presented that promises robustness and a lifelong operation when deployed in the orbit. FSO links can be used to establish an optical transport network interconnecting HAPs in the stratosphere as well as HAPs and ground stations in various network topologies. The final paper by Fortuna et al. outlines a HAP-based optical transport network. Wavelength requirements are evaluated using a numerical approach for different topologies taking into account the physical constraints imposed by FSO for a proposed dense wavelength division multiplexing (DWDM) system. Using analytical formulae for the wavelength requirements, the authors determine the closest match, star and bus topologies. The performance analysis of different routing and wavelength assignment algorithms in a representative network is discussed. Comparison of different routing algorithms including the random assignment algorithm, the first fit assignment algorithm and the adaptive routing algorithm shows that adaptive routing yields smallest blocking probability with smaller number of burdened links. The simulation platform predicts that in a DWDM system with 102 wavelengths per link, there would be no blocked connection requests.We hope that this special issue would be beneficial for the optical wireless research community and would provide an interesting reading and value addition. The Guest Editors would like to thank all the authors for their contributions, the reviewers for their valuable inputs and insight and the Editor-in-chief Professor Kia Makki of the Journal of Communications. Finally, our thanks go to all those in the Publishing Department of the Journal for their co-operation and assistance in putting together this special issue. We hope that the readers would enjoy reading the contribution in this special issue and it may stimulate future research activities in optical wireless communications.
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