May 15, 2013
Improving Wireless Devices By Utilizing Spectrum Efficiently
Lee Rannals for redOrbit.com — Your Universe Onlinebandwidth on a single chip.
The radio spectrum is divided up into certain areas in order to allow industries like broadcast-television, radio and cellphone providers to use specific radio waves so they don't intersect. However, the current way of doing things can be inefficient, particularly because frequencies used by cellphones can become very congested while broadcast-television spectrum holds plenty of room.
In the past, researchers have proposed the idea of "cognitive radio" as a solution, which is when wireless devices scan their environments for vacant frequencies and use these for transmissions. MIT researchers said they have taken this idea a step farther using techniques already common in the production of signal-processing chips.
Their approach is to convert the radio signal to an acoustic signal and then convert it back to an electrical signal.
“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 explained in a statement. “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.”
Acoustic resonators can be packed more densely than electrical-filtration circuits, enabling the mechanical resonator to be much smaller. However, the number of acoustic resonators in a filtration bank has been limited in practice.
The team said the heart of any device that converts electrical signals to mechanical vibrations is the capacitor, which changes the impedance that the antenna sees.
“Each capacitor from each filter is going to affect the antenna, and that´s no good," Weinstein said. "It means I can only have so many filters, and therefore so many frequencies that I can separate my signal into.”
Another problem with acoustic resonators is turning them on or off, which requires giving each resonator its own electrical switch. The team solved both these problems by adapting a technology already common in wireless devices known as a gallium nitride transistor. This technology can be switched between a conductive and nonconductive state, which enabled the team to remove the lower plate of the capacity, drastically reducing the capacitors' effect on the quality of the radio signal.
“The radio can now afford to have 14 times as many filters attached to the antenna,” Weinstein said, “so we can span more frequencies.”
Thomas Kazior, a principal engineering fellow at Raytheon, said that the team's work could help with commercial adoption of cognitive radio.
“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 said. “These are all on a tiny scale."
He said this work can help with the cost problem because these resonator-type structures "almost come free."
“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," Kazior added.
The Federal Communications Commission constantly has to move around spectrum to adapt to growing technologies. In December the FCC announced it would begin sharing some of the US Government's wireless spectrum with private sector in order to free up airspace for other wireless carriers. The agency split off spectrum into 3 tiers, including a top tier for federal and military use, a second tier for hospitals and other government facilities, and a third tier to be used by the general public.