Blue-Sky Research Aims at the Ultraviolet

March 1, 2020
By Robert K. Ackerman
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One of the more exotic technologies being explored for Army networking is ultraviolet communications. It can be employed for non-line-of-sight (LOS) communications or direct LOS, and it is harder for an adversary to detect, as atmospheric absorption limits its range. This also reduces an enemy’s ability to jam the signal.

Robert Drost, branch chief for the ARL’s tactical network assurance branch, explains that establishing a free-space optical communications link can be achieved using infrared or visible light—the wavelength is largely irrelevant. But the deep ultraviolet (UV) frequency—about 280 nanometers—generates more atmospheric scattering and absorption. In turn, a receiver can detect that scattered light when the UV is shined into the sky. This enables a non-LOS optical link between two points.

Drost continues that the scattering currently is relatively weak, so any communications would be short range and low rate—useful for voice about 200 meters away. A meshnet relay network could expand that range into greater use, he suggests.

With most conventional communication methods vulnerable to adversary action, UV looms as an alternative that could complement or back up traditional links. One advantage UV offers over radio frequency (RF) is that, at longer ranges, UV suffers from increased absorption. The absorption increases exponentially, so an adversary has less ability to jam or intercept a signal. This effect begins to dominate around 1 to 2 kilometers away from the transmitter, Drost adds, and an adversary 5 kilometers or more away has very little chance of jamming a signal.

A UV signal could be transmitted a couple of ways. The signal could be modulated using conventional approaches such as on/off keying or pulse position modulation, Drost allows. “You turn your [light emitting diode] LED on when you want to send a 1 bit, and you turn your LED off when you want to send a 0,” he explains. “Knowing the range you’re trying to communicate, you know how long you have to keep it on or off. You know the data rate that you can support.”

Some research questions remain regarding how to multiplex the same environment, Drost admits. In a complex environment such as a local area network with multiple UV links operating simultaneously, planners would have to consider media access and multiplexing. One option might be to spatially multiplex the sky so that links are pointed at different spots above the area. A receiver that wants to see one particular link would be able to point in a specific direction and avoid interference from other UV signals. But a single link avoids this problem and permits single on/off keying at a low data rate, he reiterates.

The low data rate does limit applications, but it can free other types of networking technologies. Activities such as media access control layer and networking can be shifted out of RF and into UV. Eventually, as with all new types of communication technologies, warfighters could find new applications that take advantage of UV networking.       

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