Research Activities My primary research interests are in the theory of digital communications and signal processing, with focus on system design and performance analysis in time-varying channels. Such channels are found in many wireless communication systems, such as mobile terrestrial systems and satellite systems. My research has a particular focus on underwater wireless (acoustic) communications. The major problem encountered on underwater acoustic channels is that the system bandwidth is limited, while the low speed of sound (1500 m/s in water) and time-varying multipath propagation cause extreme signal distortion. Acoustic signals also 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. At present, I work on several problems in this area, ranging from statistical channel characterization and fundamental capacity analyses, to specific signal processing methods and network protocols. Examples of my current research projects include adaptive signal processing for multi-carrier modulation, such as orthogonal frequency-division multiplexing (OFDM) over Doppler-distorted channels; the use of multiple input (transmit) and multiple output (receive) elements (MIMO), and efficient medium access control (MAC) and network protocols for sharing the broadcast acoustic channel. I also work closely with the scientists and engineers at the Woods Hole Oceanographic Institution, and participate in the development of the WHOI micro-modem.