UnderWater Acoustic Sensor Networks (UW-ASNs) are experiencing a rapid growth, due to their high relevance to commercial and military applications such as oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, and tactical surveillance. However, the design of efficient communication protocols for underwater sensor networks is still an open research problem due to the unique characteristics of the underwater acoustic communication channel such as limited bandwidth, high and variable propagation delays, and significant multipath and scattering. MIMO systems are able to exploit rich scattering and multipath fading to provide higher spectral efficiencies without increasing power and bandwidth. The objective of this research is to explore the capabilities of underwater MIMO links, and to leverage these from the perspective of higher layer protocols with a cross-layer design approach.;First, a new medium access control protocol named UMIMO-MAC is proposed. UMIMO-MAC is designed to i) adaptively leverage the tradeoff between multiplexing and diversity gain according to channel conditions and application requirements, ii) select suitable transmit power to reduce energy consumption, and iii) efficiently exploit the UW channel, minimizing the impact of the long propagation delay on the channel utilization efficiency. To achieve the objectives above, UMIMO-MAC is based on a two-way handshake protocol. Multiple access by simultaneous and co-located transmissions is achieved by using different pseudo-orthogonal spreading codes. An algorithm is proposed that, in a cross-layer fashion, jointly selects optimal transmit power and transmission mode through the cooperation of transmitter and receiver to achieve the desired level of reliability and data rate according to application needs and channel condition.;Second, distributed routing algorithms are introduced for delay-insensitive and delay-sensitive applications, with the objective of reducing the energy consumption by i) leveraging the tradeoff between multiplexing and diversity gain that characterizes MIMO links, and ii) allocating transmit power on suitable subcarriers according to channel conditions and application requirements. To achieve the objective above, each node jointly i) selects its next hop, ii) chooses a suitable transmission mode, and iii) assigns optimal transmit power on different subcarriers to achieve a target level of Quality of Service (QoS) in a cross-layer fashion. Extensive simulation results demonstrate that our proposed protocol is adaptive to the unique characteristics of the underwater acoustic communication channel, and achieves excellent performance through local cooperations between transmitter and receiver.;Finally, we present our ongoing work on developing a reconfigurable underwater networking testbed based on the Teledyne Benthos Telesonar SM-75 modem. The testbed is designed with the objective of allowing researchers and developers to advance research activities in the field of underwater networking and communications through a flexible testbed platform. This platform allows playing, processing, and recording custom-defined acoustic waveforms to support reconfigurable physical layer experimentation with arbitrary transmission schemes.
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