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Time Reversal Signal Processing

Time Reversal Signal Processing

Time reversal signal processing uses a mirroring technique to amplify and concentrate wave forms in specific mediums. When some forms of waves travel through some mediums, they can accumulate reflections that distort and diffuse the original signal. Think of an acoustic sonar ping under water bouncing off of every surface in a cluttered environment. The ping ends up looking like it originated from many sources and there are many return echoes. The more cluttered and confusing the environment becomes, the more difficult it becomes to establish a clear signal that can be used for communications of any type. A time reversal mirror can be created using an array of receivers and playing back the signals they absorb, but in a time reversed sequence. This allows the scattered signals to travel back along their original paths and create a stronger, more focused signal.

While most of the research is being done with acoustic waves in water, it is possible to use other waveforms in other mediums, such as electromagnetic waves in space. Combining techniques such as synthetic aperture radar with time reversal signaling seems intuitive.

‘Time reversal’ allows wireless broadband under the sea – [newscientist.com]

Time reversal exploits the way undersea acoustic signals typically arrive clouded by echoes that travel at different speeds. For example, a “ping” may arrive as three separate sounds – one that travelled directly, an echo from the surface and then an echo from the ocean floor.

If the receiver transmits the same sequence of sounds backwards, they will take the same routes back to the original source. But because the sound that took the longest to travel is sent first, the second-slowest next, and the fastest last, all three will arrive at about the same time at the original source.

Applications of Time-Reversal to Underwater Acoustics – [navy.mil]

The rapidly developing field of time-reversal acoustics is yielding results as robust and productive in large-scale ocean experiments as in table-top laboratory experiments. The operative physical principle is that acoustic waves can be turned around (time-reversed) and sent back to their source, no matter how complex the environment. Thus, unlike conventional sonar signals that disperse as they propagate away from their source, a time-reversal mirror can focus sound energy in the ocean. In practice, time-reversal mirrors are realized by constructing an array of collocated source and receiver elements. The Naval Research Laboratory has developed and deployed a source-receive array (SRA) that is being used to test time-reversal methods on problems of interest to the Navy. These experiments seek to enhance the target echo levels relative to conventional sonar by using time-reversal processing. By putting more energy on the target and keeping it away from the ocean boundaries, these techniques promise significant increases in echo-to-reverberation level.

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