A Channel-Aware Adaptive System for Underwater Acoustic Communications

In recent years, underwater communications have seen a growing interest pushed by marine research, oceanography, marine commercial operations, offshore oil industry and defense applications. Generally, underwater communications employ audio signals which can propagate relatively far. However, acoustic underwater channels are very challenging, because of limited bandwidth, long propagation delays, extended multipath, severe attenuation, rapid time variation and large Doppler shifts. A plethora of underwater communication techniques have been developed for dealing with such a complexity, mostly tailoring specific applications scenarios which can not be considered as one-size-fits-all solutions. Indeed, the design of environment-specific solutions is especially critical for modulations with high spectral efficiency, which are very sensitive to channel characteristics. In this thesis, we design and implement a software-defined modem able to dynamically estimate the acoustic channel conditions, tune the parameters of a OFDM modulator as a function of the environment, or switch to a more robust JANUS/FSK modulator in case of harsh propagation conditions. To achieve this ambitious goal, various topics have been addressed and will be presented in this thesis. We study how to compensate for the Doppler effect in transmission employing the JANUS standard, that is a popular modulation scheme for underwater communication developed by the NATO Center for Maritime Research. Although JANUS is resistant to multipath thanks to frequency hopping, a specific correction must be done to cope with the distortions caused by Doppler effect and make it a more reliable solution for harsh environments. With the aim of developing an adaptive system, we implemented FLUMO, a flexible Software-Defined Acoustic (SDA) underwater modem. The system architecture follows two key ideas. First, the modulation/demodulation part runs fully in software and is completely decoupled from the rest of the system. The system can be used to send/receive modulated signals from different systems and, on the other side, it can be used to modulate signals that are sent from different systems. Second, we provide to the users a flexible and tunable system where the different communication parameters can be adjusted in order to reach the best performance in every scenario. In order to develop a system supporting high data rate communication, we also implemented an OFDM transceiver which use a feedback chain to follow channel variations and correct the transmission recursively. We show the results obtained using the simulated channels in Watermark and how in these channels our technique achieves good performance compared to a traditional receiver. We also show realtime test performed in indoor (aquarium) and outdoor environments (at sea). Finally, to allow our platform to decide between different solutions we address the problem of the channel characterization. Since the temporal variability of the channel behavior can be summarized in terms of maximum delay spread and Doppler spread, we present a solution for deriving these parameters and discuss the limit conditions under which the OFDM modulator can work. In such scenarios, we also calibrate the prefix length and the number of subcarriers for limiting the inter-symbol interference and signal distortions due to the Doppler effect. We validate our estimation and adaptation techniques by using both a custom-made simulator for time-varying underwater channels and the Watermark simulator, as well as real in field experiments. Our results show that, for many practical cases, a dynamic adjustment of the prefix length and number of subcarriers may enable the utilization of OFDM modulations in underwater communications, while in harsher environments JANUS can be used as a fall-back modulation.