The Incredible Loudness of Whispering

Estimated reading time: 3 minutes

In a vision shared by innovators, entrepreneurs, and planners in both defense and civilian contexts, the skies of the future will be busy with unmanned aerial vehicles (UAVs). Unseen but central to the realization of this vision is wireless communication within and between those future fleets of UAVs that is reliable and resistant to both unintentional and ill-willed interference. “If these UAVs can’t communicate, they don’t take off or they don’t operate the way we want them to,” said Josh Conway, a program manager in DARPA’s Microsystems Technology Office. “As wireless communication becomes part and parcel of all kinds of platforms and devices in the coming years, we will need assured communications, especially for command and control, but for other things too, like data transfer.”

In IEEE's Journal of Lightwave Technology, researchers at the University of California, San Diego, report results of work conducted for DARPA’s Hyper-wideband Enabled RF Messaging (HERMES) program that could become the technological foundation for this interference-resistant communications necessity.

“This paper shows that there is a way to get there,” said Conway, who has been overseeing the HERMES program since DARPA rolled it out in the summer of 2014. The same technology could provide an exciting opportunity to make fuller use of not only unlicensed Wi-Fi bands but also huge swaths of otherwise license-restricted radio frequencies. Said Conway: “This advance in HERMES means we might have a new way to tap into all of this spectrum and in a way in which you won’t jam anyone else and they won’t be able to jam you.”

In the IEEE article, UCSD Professor Stojan Radic and four colleagues describe their use of “optical combs” residing within a single hair-thin glass fiber to perform an amount of high-speed signal processing that normally would require a power-hungry supercomputer, which is not the sort of equipment that fits well onto small UAVs.

The new receiver opens the way to a new channel of assured communication because it can retrieve direct-sequence, spread-spectrum (DSSS) signals—a category of signals modified with a coding protocol that confers several benefits, including increasing the signals’ resistance to jamming and interception—so faint they fall within the sea of always-present radio noise.

To demonstrate what has become possible, Radic and his colleagues created these radio whispers by recasting a narrowband, 20 MHz radio signal across an optical comb of hundreds of frequencies—each one carrying the same signal but within a much wider, 6 GHz spectrum—that all can simultaneously travel within a single optical fiber. Their system also features a unique optical “key” technique both on the front end (to imprint the information in the original radio signal into all of the frequencies of the spread-spectrum analog) and on the back end at the receiver (to reconvert the sub-noise, spread-spectrum signal back into the original information-bearing radio signal).

“Our system can reconstruct the signal at almost no energy expenditure,” Radic said. And because the optical key steps do not modify jamming and other RF power in the overall spread-spectrum signal, “they do not get snapped back upon receipt and they remain spread out into noise that you can filter out,” Radic added. With the addition of narrow-band filtering, sub-noise command and control signals could be received even in the presence of jamming power up to 100,000 times stronger. This is akin to extracting one faint voice from a football stadium of cheering fans. Radic and his colleagues now are working methodically to increase the spectral spreading to 10 GHz or more and to shrink the heart of the receiver technology down to a chip level, a final step toward a lightweight means of the assured communication technology that UAVs would be able to carry and power.

Because the new technology works with radio signals so weak that links can be designed without signal interference, and because the receivers could be chip-sized and power efficient, the technology could end up transforming mobile communications by opening up previously restricted frequencies and increasing the longevity of battery-run wireless links. The engineering advance points toward a new means for accessing the vast quantities of underutilized electromagnetic spectrum with higher levels of security and privacy.

“From a military perspective, we want this for assured communications as we move toward future unmanned systems,” Conway said. “From a civilian side, it also could allow you to use the spectrum more effectively and freely.”