Frequency-domain sonar processing in FPGAs and DSPs

Beamforming is a spatial filtering operation performed on data received by an array of sensors, such as antennas, microphones, or hydrophones. It provides a system with the ability to "listen" directionally even when the individual sensors in the array are omnidirectional. Over the past year we have been exploring the use of FPGA based custom computing machines for several sonar beamforming applications, including time domain beamforming (P. Graham nd B. Nelson, 1998), frequency domain beamforming, and matched field processing. In many ways sonar processing fits the criteria found by W. Mangione-Smith and B. Hutchings (1997) for good FPGA applications-the computations are data parallel, they require little control, the data sets are large (infinite streams), and the raw sensor data is at most 12 bits. However, they have three characteristics which make them challenging. First, they involve intensive arithmetic (multiply accumulates and trigonometric functions) on real and/or complex data. Second, they require significant memory support, far beyond that indicated in much previously published work. Third, the scale of the computation is large, requiring (possibly) hundreds of FPGAs and high bandwidth interconnections to meet real time constraints. We address the first issue.