Access to spectrum is also critical for future swarm of UAVs, 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.”
The unprecedented consumer demand for wireless mobility and data consumption has resulted in reduction and fragmentation of spectrum of defence. Only 1.4 percent of the RF spectrum from 0 to 300 GHz is available exclusively to the U.S. government. This has resulted in complex Defense, spectrum management within and between the armed services, and any errors in the spectrum management plan may result in the denial of critical strategic and tactical links. The second is relatively easy for adversaries to target such a small part of the RF spectrum allocated exclusively to the government through jamming or electronic attack.
Modern networks and platforms rely on access to the radio frequency (RF) spectrum for communications, radar sensing, command and control, time transfer, and geo-location. Electromagnetic interference, due to congestion in the spectrum or malicious jamming, can have catastrophic effects. Countering such interference is particularly important for unmanned platforms.
To address this challenge, DARPA rolled it out in the summer of 2014, the Hyper-wideband Enabled RF Messaging (HERMES) program seeks to provide an assured link for essential communications by developing a jammer-countering capability that is orders of magnitude beyond the state-of-the-art. Assured access to the RF portion of the electromagnetic spectrum is critical to communications, radar sensing, command and control, time transfer, and geo-location.
To achieve this dramatic leap in jam-resistance, the HERMES program focused on systems that work with extremely wide RF bandwidths. Techniques such as direct sequence spread spectrum DARPA can exploit extremely wideband RF links, to provide a significant significant coding gain, to mitigate the effects of RF jamming and interference that threaten to disrupt important military operations while also providing operationally useful data-rates.
The broad spectral spreading of the signal assure that the transmit power in any spectral bin will be insufficient to cause unintentional interference. If successful, the HERMES program could provide unprecedentedly reliable channels of access to the RF spectrum without pre-planned spectral allocation, allowing for a new model of communications.
UCSD Researchers have developed an innovative “optical comb” receiver that retrieves sub-noise “spread-spectrum” signals has been evolving from a rough tabletop phase, to a streamlined desk-top version, and is on its way to a chip-scale finale that could became the basis of new assured channels of communication for unmanned aerial vehicles and other platforms and devices that require wireless connectivity.
DARPA’s proposal was in line with DOD’s Electromagnetic Spectrum Strategy 2013, which called for ensuring the access to the congested and contested electromagnetic environment of the future, by adopting new agile and opportunistic spectrum operations , and through systems which are more efficient, flexible and adaptable and adopting new technologies capable of more efficient use of the spectrum and reduced risk of interference.
Hyper-wideband Enabled RF Messaging (HERMES)
In recognition of these severe and mounting challenges, DARPA is soliciting innovative research proposals to explore the feasibility of a wideband spread-spectrum RF communications system with greater than 10 GHz of instantaneous bandwidth under its program, Hyper-wideband Enabled RF Messaging (HERMES). By exploring extremely wideband RF links in the HERMES, the lost bandwidth can be reclaimed, and the ultra-low power spectral densities of spread spectrum signals will avoid unintentional jamming of other users.
The goal of the program is to implement a robust wideband (>10 GHz instantaneous bandwidth) RF link residing below 20 GHz. The system will operate below 20 GHz to mitigate atmospheric absorption and employ coding gain and spectral filtering for resistance to jamming. In addition, The fundamental physics of high-power amplifiers tends to stifle attempts at delivering high power jamming signals with large fractional bandwidth.
The link will operate at a 100 kb/s rate while deploying several jammer rejection techniques—involving processing gain, integrated filters, and active cancellation—that combine to thwart jamming signals that are even 10 million times stronger than the communications signal.
The studies will focus on two Technical Areas of interest, with the overall goal of achieving more than 70 decibels of jammer suppression
The first Technical Area will define the system architecture, system design with state-of-the-art RF and integrated circuit components, with the size, weight and power (SWaP) envelope of a handheld radio. The unique channel effects of wide bandwidth like frequency-dependent effects of distortion, fading, absorption, dispersion and Doppler shifts will also be studied. The operations through representative electromagnetic background cases for benign, urban and hostile environments and their Mitigation methods and channel coding should be proposed and simulated.
This second Technical area is primarily concerned with photonic-enabled receiver technologies, The building blocks shall be chip-scale photonic devices for compact and efficient processing at the RF receiver. The enabling technologies are low-loss integrated waveguides, ultra-stable lasers, low-noise photo detectors, Integrated, tunable photonic filters, Photonic RF mixers and electronic-photonic device integration.
UCSD Researchers develop optical combs
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.”
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