Research On High-Resolution Imaging And Wireless Power Transmission Based On Time Reversal

Based on the basic theory and the property of temporal and spatial focusing of time reversal(TR) technique, this dissertation investigates high-resolution imaging algorithms and wireless power transmission(WPT) techniques. In the existing imaging techniques, numerous detection elements are needed in order to obtain the clear image of targets, and noise affects the imaging results seriously. Especially, signal distortion happens in propagating in the lossy and dispersive media, so that the targets are more difficult to be located accurately. Additionally, in the hot research field-WPT, the long transmission distance can be obtained by using electromagnetic waves to transmit power wirelessly, but the power collection efficiency(PCE) is low. While, in the existing methods capable of improving PCE, most is suitable for these power transmitting arrays which consist of antennas with the same structure and regular arrangement. And the control of beam pattern can be only achieved at a single frequency by a single run of optimization.This makes calculation cost increase and limits the scope of applications. Aim at the problems above, this dissertation proposes solutions. The main innovative contributions are summarized as follows:Firstly, aim at some representative TR algorithms, classification and research are carried on, including traditional time reversal mirror, iterative time reversal mirror, decomposition of the time reversal operator and time reversal multiple signal classification, and compared with back projection which is another important imaging method in imaging field.Secondly, aim at the problems that a lot of detection elements are needed by using traditional imaging methods, which increases the complexity of systems and the amount of processing data, and noise affects the imaging results seriously, two solutions are put forward. One is TR imaging method based on signal waveform similarity, this method makes the use of three ways to construct sub-arrays of receiving and transmitting systems, the position of target can be obtained by using the waveform similarity of TR echo signals received by sub-arrays. In addition, among the signals received by three sub-arrays, noise and noise are uncorrelated, noise and useful signals are uncorrelated, while focusing signals obtained from the position of targets are correlative. Thus, noise can be suppressed effectively by using waveform similarity. The other one is from the perspective of focusing time, combine with the similarity between TR signal obtained from the position of target and detection pulse to suppress noise effectively. TR signals reflected by target and detection pulse are similar. However, TR signals reflected by off-target and detection pulse are dissimilar. Noise can be suppressed by choosing the detection pulse which is uncorrelated to noise. Furthermore, this method uses the sum of the mean difference of focusing time as imaging function to enhance imaging resolution. Both methods exhibit good performance in experiments, when only a few detection elements can be used.And then, aim at the problem that lose the position of target because of signal distortion in the lossy and dispersive media, propose imaging method based on time reversal channel compensation.The phase aberration correction at target position is automatically compensated by the TR operation via phase conjugation, which does not happen at the position of off-target.According to the similarity discussed above, weight amplitude compensation to construct new compensation function, utilize this new compensation function, the effect of compensation at the position of target is larger than that at the position of off-target. Thus, the targets can be located accurately in the lossy and dispersive media even when only a few detection elements can be used. Moreover, the signal amplitude corresponding to focal time observed at the position of target is used as imaging function which can enhance imaging resolution further.At last, aim at the low PCE in WPT, and considered the superiority of TR technique in imaging area, propose that using TR technique to enhance PCE of WPT. Choose indoor environment as the scene of this study, and utilize IEEE indoor non-line of sight model to simulate indoor environment. In 2D indoor space, employ different kinds of signal as charging request signal of powered device, analyze the PCE and power map when sending pulse directly and sending TR electromagnetic wave. It is shown that, compared with WPT directly, the average power which incidents on powered device is enhanced about one order by sending TR electromagnetic wave, and PCE is also enhanced obviously. Meanwhile, the ability of TR technique to improve the PCE is unrelated to the kind of incident pulse. In 3D indoor environment, radition pattern can be matched automatically towards the direction of powered devices by using TR technique. Thus, this method can be suitable for these power transmitting arrays which consist of omnidirectional antennas with different structure and irregular arrangement,which increases the universality of this method. And the array excitation over a wide frequency range can be obtained at once by a single run of TR and Fourier operation, compared with traditional method which performs time-consuming multi-objective optimizations of beam pattern cost-functions at each frequency, this method can reduce optimization time and calculation cost greatly. In addition, by weighting the time reversed charging request signal, the interference caused by different observation elevation angles can be suppressed effectively. Furthermore, multi-powered devices placed arbitrarily can be charged simutaneously by using the proposed method.