Despite more than 70% of our planet surface is covered by water, today the underwater world can still be considered largely unknown. Rivers, lakes, seas and oceans have always been a fundamental resource for human life development, but at the same time they have often represented natural obstacles very hard to surmount. The most impressive example is probably given by the ocean, whose vastness severely limited geographical explorations and discoveries for tens of centuries. Anyway, the growing curiosity about what happens below the water surface has gradually led man to immerse in this unknown environment, trying to overcome its inaccessibility and figure out its secrets. Underwater investigation and exploring have been increasingly supported by technology, advanced over time for different purposes (military, commercial, scientific). In this regard, providing a communication link between remote users has been recognized as one of the main issues to be addressed. The first significant solutions derived from the radio-frequency world, subject of study since the 19th century. Unfortunately both wired and wireless RF inspired signal propagation strategies were not evaluated as successful. The former ones, since considering the deployment of meters (up to kilometers) of cable in depth, were too costly and difficult, while the latter ones did not offer good performance in terms of communication range due to signal attenuation. An alternative way, examined with particular interest from the beginning of the 20th century, has been that one offered by acoustics. Actually, the study of sound and its propagation through different media has been an intriguing topic since the Old World Age, hence the attempt of messaging underwater has seemed to be a great opportunity to convey theoretical principles in a real application. In addition, not only humans but also marine animals use acoustic waves to communicate, even over several kilometers distances as demonstrated by whales. So, since already existing in nature, acoustic communications have been considered as potentially successful, furthermore representing an effective trade-off between feasibility and performance, especially if compared to the other electromagnetic signals-based methods. Communication over RF channels has been extensively investigated so as to become a mature technology. The thorough knowledge about OSI (Open Systems Interconnection) model physical layer issues has allowed the researchers attention to be drawn to the upper layers. Following this direction, the recent advances in technology in this field have been accomplished mainly due to novelties in networks managing rather than to enhancements in the signal propagation study. Moving to acoustics, unfortunately this approach results to be failing if applied in the underwater scenario, as the major challenges rise indeed from physics matters. The underwater environment is varied and variable, so understanding the mechanisms that govern the propagation of sound in water is a key element for the design of a well-performing communication system. In this sense, the physical layer has therefore regained the centrality that has been diminished in other contexts. The underwater acoustic communications can be adopted in a wide range of applications. The best-known are coastal monitoring, target detection, AUVs (Autonomous Underwater Vehicles) remote control, tsunami alarm, environmental data collection and transmission. Those ones are very specific activities, so the devices to be employed must sometimes meet very strict requirements. In this regard, the solutions commercially available provide good performance (that are paid in terms of high costs). On the other hand, the fact that hardware and software are usually copyrighted leads to have a closed system. Having reconfigurable devices is instead an opportunity to match the technology with the environment features and variations, especially in real-time applications. Recently, the need to overcome these constraints has encouraged the debate about underwater technology challenges. The work by Demirors et al.  reports an interesting discussion about the implementation of software-defined underwater acoustic networks (UWANs), highlighting how this solution can provide enhancements in terms of software portability, computational capacity, energy efficiency and real-time reconfigurability. Furthermore, the authors propose the architecture of a software-defined acoustic modem and evaluate its performance and capabilities with tank and lake experiments. Considering the comments outlined above, the following dissertation deals with the design of an acoustic communication system. The preliminary theoretical analysis regarding physical layer concerns, such as signal propagation and channel behavior, represents the starting point from which several proposals regarding the implementation of UWANs are introduced. In particular the context of Multiple-Input Multiple-Output (MIMO) communications is investigated, presenting several solutions about transmission schemes and receiver implementation. Furthermore, concerning UWANs management, some strategies for access and error control, established at the data link layer level, are detailed. It is worth highlighting that the goal of this contribution is not to present a disjointed discussion about the topics just listed. The objective is instead to propose practical solutions developed hand in hand with theory, making choices firstly by looking at what nature allows.
|Titolo:||MIMO underwater acoustic communications over time-varying channels: from theory to practice|
|Data di discussione:||22-feb-2018|
|Appartiene alla tipologia:||07a Tesi di Dottorato|