Research Activities My primary research interests are in the theory of digital communications, with focus on system design and performance analysis in time-varying environments. Such environments are found in many wireless communication systems, including mobile terrestrial systems and satellite systems. Specific topics in this area include fading and multipath propagation, modulation/detection, synchronization and equalization, array processing, and multiple-access communications. Theoretical contributions that I made to this field are the analytical results for system performance on rapidly fading multipath channels. A specific area of applied research that is of great interest to me is that of underwater wireless (acoustic) communications. The major problem encountered on underwater acoustic channels is that the system bandwidth is limited, while time-varying multipath propagation causes extreme signal distortion. Consequently, the data rate at which one can transmit through these channels is severely limited. In addition, acoustic signals propagate through the water at a very low speed (1500 m/s), and suffer from attenuation that depends not only on the distance, but also on the signal frequency. Together, these facts result in a communication medium that combines the worst of radio worlds: poor link quality of a terrestrial system, and high latency of a satellite system. As a result, acoustic communication system design poses many challenging problems. My work in this area provided the first feasibility proof of bandwidth-efficient acoustic communications, and created a strong impact on the current research in the field. At present, I work on several problems in this area, ranging from the fundamental capacity analyses of future underwater networks, to specific signal processing methods and network protocols. Examples of my current research projects include adaptive signal processing for code-division multiple access (CDMA) systems, multicarrier modulation based on orthogonal frequency-division multiplexing (OFDM), the use of multiple transmit and receive elements, and efficient medium access control (MAC) and network protocols for sharing the broadcast acoustic channel. I also participate in the development of the WHOI micro-modem, the MIT software-defined acoustic modem, and an underwater acoustic network simulator.