Guest Editorial

Abstract

In recent years, there has been increasing interest in underwater wireless optical communication systems (UWOC) for both research and commercial use. Underwater communications has been a great challenge and one of the least developed when compared to land based systems. There is a vision to work on – that of underwater cities, where sensors and submersible vehicles will require wireless communications in that difficult underwater environment. It is desirable to employ technologies allowing long distance as well as shorter distance links. Currently, longer distance links of a few km, use acoustic technologies, but offer low data rates and there is significant delay due to the nature of acoustics. UWOC is beginning to make waves mainly for short range links. The evolution of current acoustic systems may well lead to hybrid acoustic/optical systems in the near term. Optical underwater communications, in contrast, are capable of providing high bandwidth, low delay communication within relatively short distances along with low operational cost. UWOC systems can exploit the blue/green transparency window of the seawater region – where absorption is minimum compared to other wavelengths – making these systems attractive candidates for providing real-time video communications, remote sensing and navigation, imaging and high throughput sensor networking. Although it is very difficult at this stage to operate long distance optical wireless links, very high data rate transmission seems possible at short distances. The introduction of this technology is very valuable for submarine communications. There is strong interest in increasing both the communication distance underwater and the poor data rate of existing systems. Advances in laser and LED technology, coupled with advanced system design techniques, have dramatically improved the efficiency and reliability of free space optical communication. Such technologies and designs have been catalytic in adapting them to the harsh underwater medium. Current research on UWOC systems focuses on improving knowledge of the optical properties of the underwater channel, as well as on the design of suitable components and systems tailored to high data rate underwater optical communications. In the future, we are confident we will see research growth in this area and we expect to see tangible successes in solving the significant issues of underwater optical wireless communications. In view of the above, the goal of this Special Section is to present recent results in UWOC that capture the research trends in the field. The three papers included highlight this niche but important area of application. The paper by Vali et al. considers the option of increasing the divergence of the transmitted Gaussian beam in order to reduce link's sensitivity to misalignment. It has been shown that there exists an optimum divergence angle for the maximum acceptable lateral offset with respect to the receiver sensitivity in clear waters, although this is not an efficient method in harbour waters. It has also been demonstrated that there is a design trade-off between acceptable lateral offset, power loss and channel bandwidth. Furthermore, it has been shown how the proposed scheme of beam divergence affects the maximum allowed link span as well as the channel bandwidth for a given distance. The paper by Guerra et al. analyses the effect of absorbing and non-diffracting particles in UWOC links. To this end, a geometrical model has been proposed to provide an estimation tool for shallow-water scenarios. In such links, the emitter must be analysed as an extended source, since the effect of light-blocking particles cannot be correctly modelled using the traditional point-source approximation. Various numerical results have shown that the effect of these kinds of particles is large enough to necessitate taking them into account when modelling UWOC links. Finally, it has been shown that the effect of these particles on the quality of the received optical signal depends on their size, concentration and other parameters such as the link's range and the endpoints’ surfaces. Finally, the paper by Peppas et al. provides a thorough performance analysis of MPPM receivers with spatial diversity. To this end, several setups with different water types, a number of receiving apertures and data rate requirements have been considered. The average bit error rate of thermal-noise limited systems has been investigated for UWOC systems operating over ISI conditions and log-normal oceanic turbulence. The mathematical analysis is corroborated by extensive numerical and computer simulation results. We would like to thank the authors of all submitted papers for considering our Special Section for disseminating their work. We extend our gratitude to the reviewers for their time and effort in making this Special Section a success. Last but not least, we would like to thank the devoted IET Editorial Office for their high level of professionalism, and Professor Richard Penty, the Editor-in-Chief of IET Optoelectronics, for trusting us with this important assignment and helping us to fulfill it successfully. Anthony C. Boucouvalas received the B.Sc. degree in Electrical and Electronic Engineering from Newcastle upon Tyne University, U.K., in 1978. He received his MSc and D.I.C. degrees in Communications Engineering from Imperial College, University of London, U.K., in 1979, and the PhD degree in fibre optics from Imperial College, in 1982. Subsequently, he joined GEC Hirst Research Centre, and became Group Leader and Divisional Chief Scientist working on fibre optic components, measurements and sensors, until 1987, when he joined Hewlett Packard Laboratories as Project Manager. At HP he worked in the areas of optical communication systems, optical networks, and instrumentation, until 1994, when he joined Bournemouth University. In 1996 he became a Professor in Multimedia Communications, and in 1999 became Director of the Microelectronics and Multimedia research Centre. In 2006 he joined University of Peloponnese, where he served as head of the Telecommunication Sciences and Technology Department for six years, and where he is now a Professor. His research interests lie in optical wireless communications, optical fibre communications, network protocol performance, HCI communications and interfaces. He has published over 350 papers. Prof. Boucouvalas is a Fellow of the Institution of Electrical. and Electronic Eng. (FIEEE), a Fellow of the Institute of Engineering and Technology, (FIET), a Fellow of the Royal Society for the encouragement of Arts, Manufacturers and Commerce, (FRSA). Kostas P. Peppas was born in Athens, Greece in 1975. He obtained his diploma in Electrical and Computer Engineering from the National Technical University of Athens in 1997 and the Ph.D. degree in Wireless Communications from the same department in 2004. From 2004 to 2007 he was with the University of Peloponnese, Department of Computer Science, Tripolis, Greece and from 2008 to 2014 with the National Centre for Scientific Research–’Demokritos,’ Institute of Informatics and Telecommunications as a Researcher. In 2014, he joined the Department of Telecommunication Science and Technology, University of Peloponnese, Tripoli, where he is currently a Lecturer. His current research interests include digital communications over fading channels, MIMO systems, optical wireless communication systems, wireless and personal communication networks, system level analysis and design and applied mathematics. Dr. K. P. Peppas has authored more than 80 journal and conference papers.