The need for underwater wireless communications arises from several applications, such as the energy industry, the scientific study of the oceanic environment, the communication between vehicles, the recognition of the surrounding space and even more commercially oriented applications. Among these, the nature of the transmitted signals can be divided into control, telemetry, speech and video, in an ordering of increasing data rates. Although these applications have not changed much in recent years (besides the increase of commercial applications), their demands have grown continuously.
Since electromagnetic waves are widely used in many applications, they could be thought of as the natural choice to perform such communication. However, in the underwater channel, these suffer from scattering and also from the medium’s conductivity. The first effect creates the requirement of narrow laser beams and the second makes the propagation range acceptable only for very low frequencies. For these reasons, acoustic waves are the most appropriate form of underwater communication, even if they still suffer from a fair share of adverse effects. Naturally, researches attempted to transpose many important principals of wireless communications, mutatis mutandis, to their underwater acoustic counterparts. In fact, important analogies can be made, however, the particular nature of the channel, along with the associated undesirable properties, requires more specialized communication techniques. Consequently, throughout the years, a plethora of systems and techniques has been developed for this purpose, as well as chan nel models to support them. Nevertheless, "Just as in telemetry over electromagnetic channels, there is no single design of an acoustic telemetry system appropriate for all environments" [2]. In fact, as the variability of in loco conditions precludes an universal characterization of underwa ter acoustic channels, each approach is either focused only on a specific set of channels or is not exactly optimized for any usage scenario. Furthermore, both academic and commercial solutions are usually not designed with extensibility in mind neither do they allow for simple structural modifications. Therefore, nearly all the mentioned applications would benefit from the existence of a re-configurable and extensible platform, as it would allow for a straightforward parameter optimization and would also serve as a framework for further developments.
This dissertation consists of the development of a platform that will address the mentioned issues in the form of an reconfigurable and extensible underwater acoustic modem. The chosen modulation scheme will be incoherent FSK modulation as it combines simplicity with robustness to the adverse conditions of the underwater acoustic channel. This work will be based upon an acoustic modem already developed in another master's thesis, with the goal further developing this fundamental structure. More specifically, it is intendend to enhance its synchronization mechanism and to implement additional features for increased reliability and data rate, such as diversity, MT-FSK and frequency hopping.
The modem will be implemented on an FPGA as this platform is suited for the desired flexibility and extensibility. The control of the modem will be performed by a soft-coded PicoBlaze, as it allows for a simpler implementation and reconfiguration of this non-time-critical task. As for software tools, new designs will be tested through Matlab/Simlink prior to the FPGA
implementation, as these tools allow for a preliminary and rapid evaluation thereof. Software tools from Mentor and Xilinx will be used to simulate and implemented hardware designs, respectively. Finnaly, for a quick and qualitative analysis of the underwater acoustic signals, the Audacity software will be used.
Through a careful analysis of a previously developed system, we were able to identify the most critical aspects of its performance and use such understanding together with knowledge acquired thorough a study of the state-of-the-art in underwater acoustic communications to develop an appropriate set of solutions. More specifically, besides several minor modifications, we implemented frequency hopping, diversity usage and developed a novel algorithm timing acquisition and tracking, all of a highly reconfigurable nature. The combination of these features allowed for a two orders of magnitude reduction of the the error probability in two challenging environments and provided the system with a considerable resilience to Doppler induced by speeds quite difficult to reach when the nodes are simply drifting. Even so, as the number of experiments was somewhat low, further tests are required to reach an appropriate assessment of the benefits of each of these features, specially for the cases of diversity usage and MT transmission. Furthermore, neither of the implemented features is obviously optimized, i.e. each feature lacks some minor refinements that would only be permitted by a more the consistent set of tests. Unfortunately, the receiver implementation was not carried out, hence, it was not possible to obtain results regarding the actual hardware implementation. However, we believe that, due to the fidelity of the receiver replica developed in the MATLAB environment, the conclusions drawn in this dissertation will be largely transposed to the hardware implementation.
As for suggestions for future developments, among new ideas and more important requirements,
we can name:
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