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@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},
}