A major challenge to providing 100 Gb/s from an airborne asset to the ground is cloud cover. Free-space optical links won’t propagate through the cloud layer, which means RF is the only option. The system will be designed to provide all-weather capability enabling tactically relevant data throughput and link ranges through clouds, fog or rain. Technical advances in modulation of millimeter-wave frequencies open the door to achieving 100G’s goals.
“Providing fiber-optic-equivalent capacity on a radio frequency carrier will require spectrally efficient use of available RF spectrum,” said Dick Ridgway, DARPA program manager. “100G plans to demonstrate how high-order modulation and spatial multiplexing can be synergistically combined to achieve 100 Gigabits per second with the size, weight and power needed for a deployable system. We believe that to achieve the program’s goals requires the convergence of telecommunications system providers and the defense communications tech base.”
The most likely route to creating this sort of Skynet is to use the same sort of technology used to collect much of the data in the first place—synthetic aperture antenna technology. There have been a number of efforts to turn the Active Electronically Scanned Array (AESA) radars of fighter aircraft into dual-purpose systems capable of both acting as a radar and as a data link. Raytheon, L-3 Communications and other companies working on previous DARPA-funded projects have demonstrated the creation of airborne mobile ad-hoc networks by connecting a data modem to an AESA radar. This turns some of its transmission array into a multiplexed transmitter and establishing network connections of over 4.5 gigabits per second.
DARPA sees the next leap in data throughput coming from improvements in extreme high frequency (EHF) radio technology. Using wavelengths measured in millimeters, EHF frequencies—such as the 60 gigahertz frequency used at the top end of the WiGig standard—are typically only effective for communications at short range and within line of sight. But DARPA believes that by using techniques in the modulation of signals, including quadrature amplitude modulation (QAM), the millimeter wave band can be used over much greater distances, through cloud cover, and to achieve even higher throughput.
SOURCE - DARPA, Arstechnica
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