Doppler Radar Sensing of Multiple Subjects in Single and Multiple Antenna Systems

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Abstract
Doppler radar life sensing has shown promise in medical and security applications, however the problems of motion artifacts and presence of multiple subjects limit the usefulness of this technique. By leveraging recent advances in signal processing and wireless communications technologies, the Doppler radar technique has the potential to overcome these limitations. We will explore the single and multiple antenna systems and SIMO/MIMO signal processing to isolate desired radar return signals from multiple subjects. It has been experimentally demonstrated that up to two subjects can be separated in a single antenna systems. Simulations have also shown that in case two subjects have identical cardiovascular behavior, it is possible to distinguish them using MIMO techniques. background for both of these approaches, experimental results for a single antenna system, and simulation results for the MIMO system. Single Doppler radar antenna systems have been described in (4-8). In these systems, analog signal conditioning was used to amplify baseband output, cut-off the DC component, and avoid aliasing, while digital signal processing was used to separate breathing and heart signals from a single subject, and determine heart and breathing rates. In a single antenna system described here digital signal processing is additionally used to identify spectral signatures from multiple subjects based on FFT approach. The multiple antenna approach is based on recent developments in multiple input multiple output (MIMO) signal processing techniques. Lucent Technologies has been the leader in this field with Bell-Labs LAyered Space-Time (BLAST) techniques (9). We propose to use techniques based on BLAST to separate useful scattered radio signals from multiple individuals. When more than one target is in view, multiple transmitters and receivers providing multiple signal copies could be used to distinguish between the different sources of Doppler motion, isolate the desired signal, and determine a number of targets. In this paper, we present MIMO system simulation results that indicate that it is possible to separate multiple subjects even with identical breathing and heart rates.

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