This Tiny Device Could Free Up Crowded Cell Networks

The hotly contested bandwidth available to cellular phones and other pocket-sized radio wave transmitters could double, with no service provider wars required.

Using a radio wave device developed by researchers at the University of Texas, it's possible to enable true two-way signal transmission over the same frequency, toppling a long-standing limiting factor in wireless communication. The current version of the device is a few centimeters in size, but the UT physicists and engineers expect it to eventually shrink down to mere microns.

The two-way challenge is rooted in the concept of wave reciprocity. If we have a transmitter A sending a message to transmitter B, the effect is for both A and B to become, in a sense, indistinguishable. That is, A is sending the message and so B is sending the same message right back: if you can hear, then you can also be heard. A sends a message and B wants to send it right back, hogging a bunch of bandwidth.

This can be defeated by using a device known as a circulator. Imagine a ring featuring three "ports" at equal intervals, like a really half-assed sprinkler. A message goes into one port and is "circulated" around the ring to the next port down, where it exits and is received by B. The circulator allows signals to move from port to port in one direction, but not back. The effect is of isolating a transmitter's inputs from its outputs (the circulator becomes an isolator).

This is the UT team's basic design, a three-port ring. Last January, they successfully demonstrated a version using sound waves rather than radio waves, landing the project on the cover of Science.

The notion of a small-scale radio wave circulator has remained off-the-table for so long because the phenomenon is rather difficult to achieve, requiring hefty magnets and magnetic materials. The historical design for a radio wave circulator involves the usage of a powerful magnetic field to break the wave reciprocity or symmetry between transmitter A and transmitter B. This is what you'd find within the radar arrays of ships, aircraft, and satellites.

The UT physicists, led by electrical engineering professor Andrea Alu, devised an alternate way of breaking this symmetry. Their system uses a series of identical electromagnetic resonators, each one oscillating with the same amplitude, but out of phase with each other by 120 degrees (three resonators equalling the 360 degrees of the ring). This phase shift provides the ring with an electronic angular momentum, and so instead of a magnetic field breaking the A to B signal's symmetry, it's this electromagnetic wave.

It helps to imagine it in terms of sound. From a summary of the UT team's earlier experiment:

When the researchers turned the fans on and delivered a moderate airflow into the ring, with specific velocity tailored to the ring design, transmission symmetry was broken and the signal from Port 1 would flow entirely into Port 2, leaving Port 3 completely isolated […] Conversely, when a signal was sent from Port 2, it would flow into Port 3, leaving Port 1 isolated. Acoustic signals then flow from Port 1 to Port 2, from Port 2 to Port 3 and from Port 3 to Port 1, but not in the opposite directions.

There is some signal leakage back toward transmitter A, but, as the current study notes, it's some six orders of magnitude less than the original message. The result then is effective one-way transmission, leaving a whole lot of new bandwidth free to do other stuff.

"We envision micron-sized circulators embedded in cellphone technology," Nicolas Estep, a UT doctoral student, noted in a statement. "When you consider cellphone traffic during high demand events such as a football game or a concert, there are enormous implications opened by our technology, including fewer dropped calls and clearer communications."