This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES 1 Experiments of Time-Reversed Pulse Waves for Wireless Power Transmission in an Indoor Environment Rony Ibrahim, Damien Voyer, Arnaud Bréard, Julien Huillery, Christian Vollaire, Member, IEEE, Bruno Allard, Senior Member, IEEE, and Youssef Zaatar Abstract—A time reversal (TR) method is investigated for wireless power transmission in an indoor environment. Experiments performed with nanosecond pulses modulated at the frequency of 2.45 GHz reveal that the temporal and spa- tial focusing makes this technique valuable for applications of wireless power transmission. It is shown that the TR scheme avoids the fading phenomena that usually appear in an indoor environment when the power transmission is realized with a continuous wave: the voltage gain (respectively, the energy gain) can reach 30 dB (respectively, 20 dB) for the proposed scenarios. Moreover, it is theoretically proved that the TR technique is the optimal solution for an energy transmission, whatever the density of the multipath environment. In addition, simulations show that the voltage gain (respectively, the energy gain) of the TR technique is 3 dB (respectively, 9 dB) compared with the inverse filtering technique for a representative scenario. Other potential benefits of the method are discussed, notably concerning the power management of rectennas. Index Terms— Focusing gain, indoor environment, inverse filtering, rectenna, time reversal (TR), wireless power transmission. I. I NTRODUCTION I N AN indoor environment, the wireless powering of electronic devices can be achieved by exploiting the radiation of electromagnetic waves in different ways. In recent years, many systems have been designed to harvest the elec- tromagnetic energy that is disseminated by common wireless systems, such as Wi-Fi networks [1]. However, the intermittent and unpredictable nature of these ambient sources makes Manuscript received January 13, 2016; revised April 14, 2016; accepted May 7, 2016. R. Ibrahim is with the Centre National de la Recherche Scientifique, École Centrale de Lyon, Institut National des Sciences Appliquées de Lyon, Université de Lyon, Université Claude Bernard Lyon 1, Écully 69134, France, and also with the Applied Physics Laboratory, Faculty of Sciences 2, Platform for Research in NanoSciences and NanoTechnology–Ecole Doctorale des Sciences et de Technologie, Lebanese University, Beirut 6573/14, Lebanon (e-mail: ronyib@hotmail.com). D. Voyer, A. Bréard, J. Huillery, C. Vollaire, and B. Allard are with the Centre National de la Recherche Scientifique, École Centrale de Lyon, Institut National des Sciences Appliquées de Lyon, Université de Lyon, Université Claude Bernard Lyon 1, Écully 69134, France (e-mail: damien.voyer@ec-lyon.fr; arnaud.breard@ec-lyon.fr; julien.huillery@ ec-lyon.fr; christian.vollaire@ec-lyon.fr; bruno.allard@insa-lyon.fr). Y. Zaatar is with the Applied Physics Laboratory, Faculty of Sciences 2, Platform for Research in NanoSciences and NanoTechnology–Ecole Doctorale des Sciences et de Technologie, Lebanese University, Beirut 6573/14, Lebanon (e-mail: yzaatar@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2016.2572679 Fig. 1. Global system for wireless power transmission. energy harvesting critical to some applications [2]. The other possibility consists in intentionally radiating electromagnetic waves. This approach, generally referred to as the wireless power transmission, is investigated since the 1950s particularly for long-range applications [3]. Up to now, much attention has been paid to the design of the receiving part, namely, the rectenna [4], [5], while the emitting source is usually assumed to be radiating continuous waves (CWs) (see Fig. 1). However, this kind of waveform is not necessarily the most suitable one for all the applications of wireless power transmission. In [6], it is shown that the use of a chaotic waveform instead of a CW can improve the efficiency of the rectifier circuit. A judicious choice of the waveform can also lead to improvements concerning the power transmission between the emitter and the receiver. This issue is addressed in this paper. More precisely, we propose a method based on time rever- sal (TR) [7] that appears to be promising for indoor applica- tions. TR using electromagnetic waves was first demonstrated a decade ago [8], [9]. The implementation of this technique needs two stages. In the first stage called the learning stage, a short pulse is transmitted through an antenna positioned at one side of the room. At another side of the room, the receiving antenna records a signal constituted by the succession of many delayed pulses that are more or less attenuated due to the reflections in the medium. In the second stage called the TR stage, a signal built from the TR of the signal recorded during the learning stage is transmitted through one of the antennas. As a result, the time-reversed waves focus spatially and temporally on the receiving antenna. These properties have proved to be interesting for wireless communications [10]. Because of the temporal focusing, the received power is concentrated within a few taps and the problem of inter- symbol interference can be significantly reduced; in addition, 0018-9480 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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