The way in which radio spectrum is currently allocated to different wireless technologies can lead to gross inefficiencies. In some regions, for instance, the frequencies used by cellphones can be desperately congested, while large swaths of the broadcast-television spectrum stand idle.
One solution to that problem is the 15-year-old idea of “cognitive radio,” in which wireless devices would scan their environments for vacant frequencies and use these for transmissions. Different proposals for cognitive radio place different emphases on hardware and software, but the chief component of many hardware approaches is a bank of filters that can isolate any frequency in a wide band.
Researchers at MIT’s Microsystems Technology Laboratory (MTL) have developed a new method for manufacturing such filters that should improve their performance while enabling 14 times as many of them to be crammed on a single chip. That’s a vital consideration in handheld devices where space is tight. But just as important, the new method uses techniques already common in the production of signal-processing chips, so it should be easy for manufacturers to adopt. There are two main approaches to hardware-based radio-signal filtration: one is to perform the filtration electronically; the other is to convert the radio signal to an acoustic signal — a physical vibration — and then convert it back to an electrical signal.
Both types of filtration use devices called resonators, and acoustic resonators have a couple of clear advantages over electronic ones. One is that their filtration is more precise.
“If I pluck a guitar string — that’s the easiest resonator to think of — it’s going to resonate at some frequency, and it’s going to die down due to losses,” Weinstein explains. “That loss is related to, basically, energy leaked away from that resonance mode into all other frequencies. Less loss means better frequency selectivity, and mechanical acoustic resonators have less loss than electrical resonators.”
Commercial adoption of cognitive radio has been slow for a number of reasons. “Part of it is being able to get the frequency-agile components and do it in a cost-effective manner,” says Thomas Kazior, a principal engineering fellow at Raytheon. “Plus the size constraint: Filters tend to be big to begin with, and banks of tunable filters just make things even bigger.”
The MTL researchers’ work could help with both problems, Kazior says. “We’re talking about making filters that are directly integrated onto, say, a receiver chip, because the little resonator devices are literally the size of a transistor,” he says. “These are all on a tiny scale.”
“They can help with the cost problem because these resonator-type structures almost come for free,” Kazior adds. “Building them is part of the semiconductor fabrication process, using pretty much the existing fabrication steps that you’re using to build the transistor and the rest of the circuits. You just may need to add one, or two at the most, additional steps — out of 100 or more steps."