Featured Research

from universities, journals, and other organizations

Making frequency-hopping radios practical

Date:
May 15, 2013
Source:
Massachusetts Institute of Technology
Summary:
New hardware could lead to wireless devices that identify and exploit unused transmission frequencies, using radio spectrum much more efficiently.

Laura Popa (left) and Dana Weinstein test their experimental chip using a cryogenic vacuum radio-frequency probe station.
Credit: M. Scott Brauer

New hardware could lead to wireless devices that identify and exploit unused transmission frequencies, using radio spectrum much more efficiently.

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. In work to be presented in June at the International Conference on Solid-State Sensors, Actuators and Microsystems, Dana Weinstein, the Steve and Renee Finn Career Development Assistant Professor of Electrical Engineering and Computer Science, and Laura Popa, a graduate student in physics, adopted the second approach.

Resonant ideas

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."

Acoustic resonators' other advantage is that, in principle, they can be packed more densely than electrical-filtration circuits. "Acoustic wavelengths are much smaller than electromagnetic wavelengths," Weinstein says. "So for a given frequency, my mechanical resonator is going to be much smaller."

But in practice, the number of acoustic resonators in a filtration bank has been limited. The heart of any device that converts electrical signals to mechanical vibrations, or vice versa, is a capacitor, which can be thought of as two parallel metal plates separated by a small distance.

"The capacitors change the impedance" -- a measure of the ease with which a wave propagates -- "that the antenna sees, so you may have unwanted reflections back into the antenna," Weinstein says. "Each capacitor from each filter is going to affect the antenna, and that's no good. 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 that turning them on or off -- a necessary step in the isolation of a particular transmission frequency -- requires giving each resonator its own electrical switch. Traditionally, an incoming radio-frequency signal has had to pass through that switch before reaching the resonator, suffering some loss of quality in the process.

Switching channels

Weinstein and Popa solve both these problems at a stroke. Moreover, they do it by adapting a technology already common in wireless devices: a gallium nitride transistor.

Almost all commercial transistors use semiconductors: materials, like gallium nitride, that can be switched between a conductive and a nonconductive state by the application of a voltage. In Weinstein and Popa's new resonator, the lower "plate" of the capacitor is in fact a gallium nitride channel in its conductive state.

Switching that channel to its nonconductive state is like removing the lower plate of the capacitor, which drastically reduces the capacitors' effect on the quality of the radio signal. In experiments, the MTL researchers found that their resonators had only one-fourteenth the "capacitive load" of conventional resonators. "The radio can now afford to have 14 times as many filters attached to the antenna," Weinstein says, "so we can span more frequencies."

Switching the channel to its nonconductive state also turns the resonator off, so the researchers' new design requires no additional switch in the path of the incoming signal, improving signal quality.

Finally, the new resonator uses only materials already found in the gallium arsenide transistors common in wireless devices, so mass-producing it should require no major modifications of existing manufacturing processes.

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."


Story Source:

The above story is based on materials provided by Massachusetts Institute of Technology. The original article was written by Larry Hardesty. Note: Materials may be edited for content and length.


Cite This Page:

Massachusetts Institute of Technology. "Making frequency-hopping radios practical." ScienceDaily. ScienceDaily, 15 May 2013. <www.sciencedaily.com/releases/2013/05/130515113914.htm>.
Massachusetts Institute of Technology. (2013, May 15). Making frequency-hopping radios practical. ScienceDaily. Retrieved October 23, 2014 from www.sciencedaily.com/releases/2013/05/130515113914.htm
Massachusetts Institute of Technology. "Making frequency-hopping radios practical." ScienceDaily. www.sciencedaily.com/releases/2013/05/130515113914.htm (accessed October 23, 2014).

Share This



More Matter & Energy News

Thursday, October 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Chameleon Camouflage to Give Tanks Cloaking Capabilities

Chameleon Camouflage to Give Tanks Cloaking Capabilities

Reuters - Innovations Video Online (Oct. 22, 2014) — Inspired by the way a chameleon changes its colour to disguise itself; scientists in Poland want to replace traditional camouflage paint with thousands of electrochromic plates that will continuously change colour to blend with its surroundings. The first PL-01 concept tank prototype will be tested within a few years, with scientists predicting that a similar technology could even be woven into the fabric of a soldiers' clothing making them virtually invisible to the naked eye. Matthew Stock reports. Video provided by Reuters
Powered by NewsLook.com
Jet Sales Lift Boeing Profit 18 Pct.

Jet Sales Lift Boeing Profit 18 Pct.

Reuters - Business Video Online (Oct. 22, 2014) — Strong jet demand has pushed Boeing to raise its profit forecast for the third time, but analysts were disappointed by its small cash flow. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
Internet of Things Aims to Smarten Your Life

Internet of Things Aims to Smarten Your Life

AP (Oct. 22, 2014) — As more and more Bluetooth-enabled devices are reaching consumers, developers are busy connecting them together as part of the Internet of Things. (Oct. 22) Video provided by AP
Powered by NewsLook.com
What Is Magic Leap, And Why Is It Worth $500M?

What Is Magic Leap, And Why Is It Worth $500M?

Newsy (Oct. 22, 2014) — Magic Leap isn't publicizing much more than a description of its product, but it’s been enough for Google and others to invest more than $500M. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:

Strange & Offbeat Stories

 

Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins