Daniel Stancil chose a scientific journal to report on a bit of electronic marksmanship last month, but it could just as easily have been inserted into an old Looney Tunes cartoon.

It was sort of like Yosemite Sam walking into an Old West saloon and, spying his nemesis Bugs Bunny, firing off a series of wild shots from his six-shooters. The bullets ricochet around the room until simultaneously converging -- and in true cartoon form, screeching to a halt mid-air -- around Bugs' head.

	Graphic: A closer look at time reversal focusing To view this .pdf format file, you must have Adobe Acrobat Reader, available as a free download from Adobe

Funny stuff. Stancil, on the other hand, is an electrical engineer and did his shooting with radio waves, not cartoon bullets. The results weren't nearly so hilarious but he contends they could lead to improvements in cell phone and wireless laptop communications, as well as in radar.

Using antennas instead of six-guns and his Carnegie Mellon University laboratory instead of a saloon, Stancil showed that he could bounce radio waves off of cabinets, desks and doorknobs and bring them to focus on an antenna across the room.

In fact, he and graduate student Benjamin Henty found that the more clutter in the room, the better able they were to focus their signals on a target antenna. Placing their transmitting antennas behind a metal screen to block direct transmission to the receiving antennas, they sent two different signals over the same frequency and focused each signal on a different antenna without interference between the two.

With a greater flair for the dramatic, they might have thought to don blindfolds first.

Take away the clutter, however, and their aim wasn't nearly so true. In a barren room, they could transmit only one set of signals to one antenna over the single radio frequency.

The advantages of clutter

This use of everyday clutter to focus electromagnetic waves, reported last month in the journal Physical Review Letters, is based on a principle called time-reversal focusing.

The idea behind time reversal is that electromagnetic signals often don't follow a straight path, or even a single path, when transmitted from one point to another.

In the real world, what begins as a uniform wave radiating from an antenna runs into a variety of obstacles that block, reflect or scatter the waves. By the time the signal reaches a distant antenna, it may have split into multiple paths, with the signals along each path arriving at slightly different times.

All of this results in distortion of the original signal, but, as Stancil and Henty have shown, it also presents an opportunity. By time-reversing the signals -- sending the signals out in reverse order from how they were received -- those signals will retrace their paths back to the original antenna.

That means the signals can be transmitted at lower power and over longer distances, yet result in strong reception with little interference because they are focused on a particular user. In the case of a wireless laptop user, that means greater bandwidth -- more information at higher rates.

Also, because the same transmitting antenna can focus different signals on different antennas simultaneously, the number of users on each channel can increase.

Scientists in other fields already have used time reversal to improve underwater acoustic communication devices and design telescopes that adjust for the optical distortions caused by the atmosphere.

French researchers last spring reported that they had achieved time reversal of electromagnetic waves in a reflective box, but Stancil and Henty are the first to demonstrate time-reversed electromagnetic waves in a real-world environment.

A boost for wireless

From an electronics standpoint, adding time-reversal focusing to wireless devices shouldn't be extraordinarily difficult, said Stancil, director of the Center for Wireless and Broadband Networking. It is compatible with the "smart antennas" being developed for the next generation of wireless connections.

Smart antennas allow radio waves to be pointed in one direction or another, which reduces interference in areas with multiple wireless access points.

"This is very similar in spirit to those technologies," Stancil said of time-reversal focusing, and might allow more laptops to use the same wireless access points.

In one system envisioned by Stancil, a wireless device might transmit a steady tone or pilot signal, which a wireless access point would subsequently analyze and use to time-reverse the signals sent back to the device.

As part of a project funded by the Defense Advanced Research Projects Agency, Stancil also is applying time-reversal focusing to radar. Radar detects objects when radio waves reflect off of them; in a time-reversal system, the returning radio pulses would enable the radar to not only detect the object, but to more tightly focus on the object to obtain more detailed data.

"We also may be able to enhance the initial detection," he added.