A Critical Look at the Principles of Electromagnetic Time Reversal and its Consequences

A plethora of papers can be found in the scientific literature talking about time reversal. The term time reversal exists in the publications in a variety of different fields, such as ultrasonics, acoustics, wireless communications, electromagnetics, antennas and propagation, optics, physics, and philosophy. The term time reversal by itself sounds very interesting, and can provide many interpretations that are fascinating. However, due to a loose use of this term, it may in many cases lead to fallacious interpretations and conclusions if it is not interpreted in strict scientific terms. The main goal of this paper is to explain the appropriate definition of the term time reversal in electrical engineering in general, and in electromagnetics for wireless communications, to be specific. However, the motivation of the work presented in this paper is not to raise a controversy about time reversal and its related research. Rather, the aim of this work is to highlight the basic electrical engineering fundamentals that are necessary to study time reversal and to correctly define it, and to then accordingly interpret the results. Hence, the true and fallacious benefits of electromagnetic time reversal will be exposed. To serve this purpose, the paper is divided into two parts. First, a detailed literature review is presented. The true history of the term time reversal in electrical engineering is traced. In this part, most of the reported definitions of time reversal and its claimed capabilities are summarized. Some problems with the application of time reversal in electromagnetics are discussed, both theoretically and practically. All of the time-reversal papers talk about reciprocal networks, so the fact of the non-reciprocity of a single-antenna system in the time domain (the way we generally interpret it) is presented, to prove theoretically that there is a problem with how we interpret time reversal in electromagnetics. Finally, practical examples are shown, where time reversal is used in a wireless system and in an acoustic de-reverberation problem. The exact capabilities of time reversal in electrical engineering are exposed in those experiments.