Experimental investigation of time -reversal techniques using electromagnetic waves

Time-reversal is a novel method to utilize the multipath components in a cluttered environment for super-resolution focusing. The work studied in this thesis is based on experimental investigation of time-reversal methods using electromagnetic waves. The ultimate aim is to demonstrate by experiments the gains achieved by electromagnetic time-reversal techniques over conventional radar methods to focus radar beams, to null the clutter environment and finally to detect targets in highly scattering environments. To that end, the main principles of time-reversal systems have been studied and the clutter channel has been analyzed to assess the feasibility of time-reversal methods in a laboratory environment. We have demonstrated physical time-reversal focusing in the frequency domain as well as in the time-domain. Time-domain experiments have been conducted in a cavity channel where we can show focusing and nulling by using a relatively small bandwidth compared to free space. In a complex lab environment, we have demonstrated computational time-reversal focusing and nulling using 6 antennas and a two dimensional grid that has 100 points on it. The results have shown that the time-reversal system performance depends on three parameters. These are bandwidth, multipath components in the medium, and the number of antennas on the time-reversal array. We have characterized a scattering environment where we have dielectric rods and copper pipes as scattering objects. The experiments have been conducted starting with a simple scenario and extended to increasingly complex propagation environments, with a progressively larger number of scatterers placed in the channel. The important parameters have been extracted and using simulations we have extended the results to larger scattering environments than permitted in the laboratory. We have worked on the detection performance of a time-reversal system and compared it with conventional detection methods. The matched filter deteriorates as we increase the complexity of the medium. On the contrary, time-reversal has better performance as the scattering environment gets more complicated. We have described experimental results using a multiple antenna detection scheme that is based on clutter nulling. The experimental results show that using time-reversal techniques, we can improve the signal-to-noise ratio of the return-echo due to the target compared to conventional change-detection radar.