Seeking Signals of Interest

By Karl Hill

It’s like noise-canceling headphones on steroids. That’s one way of thinking of the signal processing system that could emerge from an ambitious collaboration among New Mexico State University and Los Alamos National Laboratory researchers.

Their goal is to create a hybrid analogdigital microsystem that can cancel out strong but unwanted radio-frequency signals so that weaker signals of interest can be detected.

Noise-canceling headphones can cancel out the drone of an airplane’s engines so the user can enjoy soothing music, but this system of the future will have a much more complicated task. The signal to eliminate might be the chirp of a Navy vessel’s radar, for instance, and the signal of interest might be covert communications between terrorists.

“It’s the same principle but the technology has to be different,” said Christopher Brislawn, a LANL scientist and affiliated faculty member in the NMSU Department of Mathematical Sciences. “For one thing, we’re doing it at RF frequencies instead of audio frequencies – and moreover, at frequencies that are propagating at the speed of light rather than the speed of sound.”

Working with Brislawn are Charles Creusere and Jaime Ramirez-Angulo of NMSU’s Klipsch School of Electrical and Computer Engineering, Joseph Lakey of Mathematical Sciences and two graduate students – Scott Izu in mathematics and Jose Luis Ruiz Chavira in electrical and computer engineering.

The project, one of several collaborations between the university and the national lab, received initial funding from the Defense Advanced Research Projects Agency (DARPA), a Department of Defense organization that “pursues research and technology where risk and payoff are both very high and where success may provide dramatic advances for traditional military roles and missions.”

LANL provided seed money to NMSU to initiate the collaboration through the LANL-NMSU Research Memorandum of Understanding, which enabled the team to compete for the DARPA funding.

Commercially available noise-canceling headphones use a technology called active noise control. A microphone on the outside of the headphones picks up the ambient noise, which is synthesized by the system’s electronic circuitry and fed into the headphones 180 degrees out of phase to cancel itself out.

“At audio frequencies this is a pretty tried-and-true method,” Lakey said. “We’re doing this at frequencies that are much higher, and we’re trying to do this directly on the radio frequency signal.”

The researchers are working with frequencies ranging from about 500 megahertz to 2 gigahertz and higher. Cell phones, satellite broadcasts, wireless Internet connections and unlicensed devices like garage door openers, cordless telephones and baby monitors operate at frequencies within that broad spectrum.

“One of the main problems that we have is there’s so many different types of signals,” Izu said. “There’s so many different ways to communicate things on a channel.”

Most of the group’s work so far has been developing algorithms and computer simulations for synthesizing and canceling different types of signals. “I think we’ve made good progress toward demonstrating proof of concept,” Brislawn said. “Our next goal, if we get additional funding, is developing a hardware prototype.”

“This would definitely be pushing the state of the art,” Creusere said. “It would probably require designing custom analog integrated chips. Silicon’s not fast enough to handle it, so we’ll be looking at using some exotic semiconductor material.

Ultimately such a system could have commercial as well as defense applications.

“The spectrum at some frequencies is getting pretty crowded with more and more devices causing problems with interference,” Lakey said. “Probably the biggest potential application would be to allow multiple devices to operate in the same part of the spectrum – but that would be further down the road.”