Spectrum-Stretching Tunable Antennas to the Rescue
Smart receivers seek out quiet spots in the noisy, overcrowded airwaves
New types of antennas that are smart enough to dodge interference and stretch the usable radio spectrum are slipping into our cellphones—and just in time. Mobile customers’ demand for e-mail, streaming video, and apps is pushing telecom’s existing bandwidth capacity to the brink. The big U.S. wireless companies are falling over each other trying to buy up more radio spectrum, but they also need to use the spectrum already at their disposal more efficiently.
Under today’s rules, chunks of useful spectrum are off-limits for telecommunications. A big problem is the guard band, a silent area that separates adjacent slots of usable spectrum. The U.S. Federal Communications Commission (FCC) worries that without guard bands, telecom transmissions will start jostling on the airwaves, annoying cellphone users and others with noisy interference. Or worse, as more services crowd into the finite spectrum, the price of cellphone and data service could spike. The solution could be new kinds of antennas that are tunable and smart enough to shift among many bands of the radio spectrum in search of a quieter frequency.
It used to be that transmitters took the blame for any noise that interfered with adjacent services, but a growing industry consensus now holds that radio receivers have a responsibility to protect themselves from interference.
The “transmitter is always at fault” policy was at play in a recent high-profile case involving LightSquared. That company was squeezed out of its legitimately owned spectrum in part because the transmissions from its proposed terrestrial cellular network interfered with GPS receivers in an adjacent band. The case shows the difficulties with the old approach of merely spacing transmitters widely on different frequencies. The GPS receivers causing so much trouble for LightSquared lack filters good enough to mute unwanted radio signals. “When people build receivers, they may not pay enough attention to the radio frequency environment in which they will be operating,” says Brian Markwalter, senior vice president of research and standards at the Consumer Electronics Association. Markwalter says that this should change.
Equipment makers are already working on improving receivers. In the meantime, telecom companies and the FCC are planning to double the number of spectrum bands available for cellphones by 2016. Because the equipment has to evolve to keep up, designers are focusing on improving the antenna, or rather, the antennas, because smartphones have as many as five: one for GPS, one for Wi-Fi, one for near-field communications (to make it possible, for example, to pay for items by swiping your phone over a pay point), and two for 4G LTE’s data-rate-boosting multiple-input multiple-output (MIMO) system. It’s those last two that engineers are currently tinkering with. They’re making the antennas tunable so that they can pick up a greater number of radio frequency bands more precisely.
To tune an antenna, you can change either its “electrical length”—the number of wavelengths of a given signal that can propagate along it—or its impedance. Designers include switches that lengthen the antenna by connecting an extra piece of metal, or capacitors and inductors as needed, or they integrate a tunable capacitor into the antenna’s circuitry. The extra parts can increase the price of the antenna, but that isn’t a big factor in the total cost of the device.
A tunable antenna is currently included in the Samsung Galaxy S II smartphone, on the market in Japan on the NTT Docomo network. Docomo isn’t telling exactly which antenna it’s using. But the company did say that the antenna makes the phone compatible with Japan's mobile system for terrestrial digital television.
Models for the U.S. market don’t use the technology yet, but it seems likely they will in the near future, says Markwalter, because phones there are required to handle a greater number of distinct radio-frequency bands every year. The current generation of smartphones can receive and transmit in the standard four bands of quad-band GSM, plus in four additional bands of the spectrum. According to the road map put together by the FCC’s Technological Advisory Council, phones built in 2013 should be able to send and receive in quad-band GSM, plus in six to nine additional bands. And by 2016, new phones will be responsible for at least 20 different bands, and must be able to receive and transmit through 2000 megahertz of noncontiguous spectrum. Handling that by shoehorning ever more discrete antennas and filters into handsets to deal with the new bands is too kludgy a solution.
“There’s a feeling that in the long run, handsets will become entirely tunable. In effect, you could build a ‘universal device,’ but we aren’t there yet,” Markwalter says.
Besides expanding the number of usable radio bands, tunable antennas have other benefits. “If we go from passive to active, we’ll have a more efficient antenna,” says Jeff Shamblin, chief technology officer at Ethertronics, a company that designs antennas and other parts of radio receivers used in cellphones. A tunable, or active, antenna with improved efficiency needs less transmission power from the phone’s amplifier and saves battery life, Shamblin says. Because it is more efficient, it gets better service and drops fewer calls. Improving efficiency with active antennas should also lead to higher data rates, meaning faster downloads.
What’s more, a tunable antenna can be smaller than its passive equivalent, making room for a bigger battery or display, or more components, without increasing the handset’s dimensions.
The technology is ready, but the FCC, despite its road map of ever-increasing radio bands, has no desire to mandate receiver standards. When these tunable antennas will be adopted for phones in the United States depends entirely on the cellular phone providers. The technology buyers at the big cellular companies did not respond to IEEE Spectrum’s inquiries. The real driver for adoption will likely be consumer demand for devices that perform better, Markwalter says. We keep thinking of new things we want to do with our phones. And a tunable antenna that delivers faster data rates while sucking less energy, taking up less space, and making better use of the spectrum seems like a technology whose moment is now.
A correction to this article was made on 08 March 2013.
About the Author
Kim Krieger is a freelance science writer in Norwalk, Conn.