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Ahmet G. Cepni. Experimental Investigation of Time-Reversal Techniques Using Electromagnetic Waves. Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, PA, USA, 2006.
Time-reversal is a novel method to utilize the multipath components in a cluttered environment for super-resolution focusing. The conventional thought about the adverse effects of multipaths on communication systems has been changing based on recent findings showing how to use mul- tipath components to create independent paths between transmitter and receiver. Time-reversal schemes are one of these recent techniques which can convert the traditionally hostile multipaths into performance-boosting elements in a two-way communication system as well as target detec- tion, and localization systems in a cluttered channel. Time-reversal techniques utilize the scattering of waves in the medium to improve the resolution of focusing in multipath rich channels. The successful demonstrations of time-reversal experiments using low frequency waveforms in acoustics and ultrasonics have generated interest in time-reversal methods using radio-frequency electro- magnetic waves. The wide-range of possible applications have ignited extensive research on this area. The work studied in this thesis is based on experimental investigation of time-reversal meth- ods 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 en- vironments. 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. Frequency domain instruments allowed us to do wideband (>1 GHz) time-reversal experiments while in the time-domain the available bandwidth was limited to 40 MHz. In the frequency domain, with the help of wideband phase shifters and network analyzer, physical time-reversal focusing and nulling experiments have been done in a laboratory environ- ment. The time-domain experiments have been conducted at 2.45 GHz in a cylindrical cavity envi- ronment. The degree of focusing and nulling depends on the multipath components in the channel as well as the bandwidth of the signal. The cavity provides a multipath-rich environment where we can show focusing and nulling by using a relatively small bandwidth compared to free space. By using 36 MHz of bandwidth, we have demonstrated single antenna time-reversal focusing and nulling. The wireless channel is reciprocal. This allows us to also do back-propagation using a computer instead of physically re-emitting the waves from the antennas. In a complex lab environ- ment, 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 (e.g. one single rod in the range of the antenna array) and extended to increasingly complex propagation environments, with a pro- gressively 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 us- ing a multiple antenna detection scheme that is based on clutter nulling. By using time-reversal, the response from the cluttered medium is first nulled. When the target enters into the medium, the electromagnetic energy focuses around the target so that a stronger echo is obtained. The ex- perimental 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.
@PHDTHESIS{cepni_thesis_2005, author = {Ahmet G. Cepni}, title = {Experimental Investigation of Time-Reversal Techniques Using Electromagnetic Waves}, school = {Carnegie Mellon University}, year = {2006}, address = {Pittsburgh, PA, USA}, month = {Dec}, abstract = {Time-reversal is a novel method to utilize the multipath components in a cluttered environment for super-resolution focusing. The conventional thought about the adverse effects of multipaths on communication systems has been changing based on recent findings showing how to use mul- tipath components to create independent paths between transmitter and receiver. Time-reversal schemes are one of these recent techniques which can convert the traditionally hostile multipaths into performance-boosting elements in a two-way communication system as well as target detec- tion, and localization systems in a cluttered channel. Time-reversal techniques utilize the scattering of waves in the medium to improve the resolution of focusing in multipath rich channels. The successful demonstrations of time-reversal experiments using low frequency waveforms in acoustics and ultrasonics have generated interest in time-reversal methods using radio-frequency electro- magnetic waves. The wide-range of possible applications have ignited extensive research on this area. The work studied in this thesis is based on experimental investigation of time-reversal meth- ods 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 en- vironments. 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. Frequency domain instruments allowed us to do wideband (>1 GHz) time-reversal experiments while in the time-domain the available bandwidth was limited to 40 MHz. In the frequency domain, with the help of wideband phase shifters and network analyzer, physical time-reversal focusing and nulling experiments have been done in a laboratory environ- ment. The time-domain experiments have been conducted at 2.45 GHz in a cylindrical cavity envi- ronment. The degree of focusing and nulling depends on the multipath components in the channel as well as the bandwidth of the signal. The cavity provides a multipath-rich environment where we can show focusing and nulling by using a relatively small bandwidth compared to free space. By using 36 MHz of bandwidth, we have demonstrated single antenna time-reversal focusing and nulling. The wireless channel is reciprocal. This allows us to also do back-propagation using a computer instead of physically re-emitting the waves from the antennas. In a complex lab environ- ment, 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 (e.g. one single rod in the range of the antenna array) and extended to increasingly complex propagation environments, with a pro- gressively 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 us- ing a multiple antenna detection scheme that is based on clutter nulling. By using time-reversal, the response from the cluttered medium is first nulled. When the target enters into the medium, the electromagnetic energy focuses around the target so that a stronger echo is obtained. The ex- perimental 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.}, owner = {henty}, pdf = {Cepni_PhD_Thesis_Dissertation_Dec2005.pdf}, timestamp = {2006.04.24}, }
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