Time reversal and the adjoint imaging method with an application in underwater communication

A direct link is established between the time‐reversal technique and the so‐called adjoint method for imaging. The latter one has been used successfully in medical imaging applications, for example for the early detection of breast cancer in ultrasound tomography. In this application, ultrasound waves are sent through the unknown medium of investigation, and the differences between detected and predicted measurements are computationally back‐propagated into the estimated medium in order to determine the correct acoustical parameters. In the application of underwater acoustic communication, the goal is to optimize a given signal, emitted by a base station, such that it focuses on the target user in the desired fashion without interfering with other users who might share the same environment. It turns out that efficient optimization schemes can be derived for achieving this goal, which are very similar to those applied in the above‐mentioned medical imaging application, but which can be realized physically in the true (unknown) ocean by iterated physical time‐reversal experiments. The close relationship, as well as some differences, between these two applications will be highlighted in this talk. [Financial support for this work from the U.S. Office of Naval Research under Grant N00014‐02‐1‐0090 is gratefully acknowledged.]