1. Introduction
In the recent literature on wireless communications and electromagnetics (from 2004 to the time of the writing of this paper), time reversal has been evolving as a new technology that provides solutions to wireless channel impairments in wideband wireless communications systems. There is a belief that the technology of time reversal has been successfully exploited in the ultrasonic domain since the early 1990s, and efforts are being exerted to exploit it in electromagnetics and wireless communications. To define what is meant by time reversal in this paper, and all of the considered references, we start with a simple example.