Researchers are exploring the possibilities of combining the operations of distributed mobile systems with coherence at the radio frequency (RF) level, called coherent RF arrays, and the microwave and millimeter-wave technologies necessary to get there. The term “distributed” makes explicit that either the transmission or the reception function is divided among two or more separate and distinct sets of transmitter/receiver hardware.
These coherent arrays offer significant potential for improvements in a broad range of wireless applications from distributed remote sensing on cubesats for improved measurements of the Earth, to distributed UAV arrays for better soil moisture mapping in agriculture, to ad-hoc arrays of cell phones or personal radios for increased range and throughput.
For military “Distributed Coherent RF Operations.” Would enable NCW and IO. Information operations (IO) and network-centric warfare (NCW) are two of the capabilities considered critical for military. IO gives emphasis to the criticality of information in conflict: there is obvious advantage to knowing the enemy’s capabilities, disposition, and intentions while denying him any such knowledge of your own forces. IO may be either offensive or defensive in nature, and includes aspects such as electronics support (ES, intelligence gathering, and threat warning) and electronic attack (EA, jamming, spoofing, deception). NCW gives emphasis to the synergistic operation of multiple entities distributed across the battle-space. In NCW, the whole is much greater than the sum of the parts.
There are a number of military RF operations that could conceivably be executed using “Distributed Coherent RF Operations.” The military systems which would benefit from Distributed RF Operations would be radar, communications and Electronic warfare or jamming. The general rule of thumb is that the incoherent combination of data from N entities will enhance the signalto-noise ratio (SNR) by a factor of square root of N , whereas truly coherent combination of data from N entities will enhance performance by a factor of N.
Simple examples would include bi-static and multi-static radar, and radio direction-finding nets. By coherently coordinating such systems to perform operations as a single wireless system, new capabilities in remote sensing and communications can be gained, while using lower-cost individual systems. For example, N radars on UAVs operating coherently can detect targets at longer ranges due to a power gain of N3. Furthermore, the expense of designing a single high-performance sensor is much greater than that associated with building several lower-performance sensors.
The potential advantages for RF communications using “Distributed Coherent RF Operations” relate to the use of multiple transceivers to form ad-hoc beam-forming arrays. As envisioned, this behavior would be available for both transmit and receive ends of the communications link. The implementation of beam-forming provides several distinct advantages over simple isotropic operation: 1) the beam-formed link can achieve a higher SNR and, hence, higher data rate operation, 2) the higher SNR can be used to enable more robust encryption, 3) the direction of the beam-formed link can be steered so as to minimize the potential for interception, 4) the beam-forming algorithm can be used to null jamming and interference inclusive of co-channel interference. Further, coherently received data can be processed using any number of known advantageous multi-channel signal processing techniques, such as multiple user detection (MUD) and multiple signal interference cancellation (MUSIC).
The potential advantages for non-communications operations similarly relate to the use of multiple transceivers to form ad-hoc beam-forming arrays and to provide fully coherent data for multi-channel signal processing. Additional nuanced advantages may be possible with regard to advanced jamming techniques. Again, we note that the basic concepts behind “Distributed Coherent RF Operations” apply to both low-cost systems with simple antenna elements as well as to substantially more advanced radar and electronic support measures systems.
Distributed systems will also enable Distributed Electronic Warfare that will provide the following objective capabilities: wide area, real-time location determination of adversary emitters; automated recognition of threat emitter operating modes; adaptive electronic attack response to threat emitters; wide area camouflaging to deny target detection or cause misclassification of targets; wide-area deception through synchronized decoy control; denial or corruption of enemy sensing capabilities by synthetic generation of high-density clutter environments; seamless operability and graceful degradation of network- enabled functions in dense EM environments; and simplified scalability and ability to upgrade through modular and open systems architecture design
Perhaps the clearest example of a developing military application of “Distributed Coherent RF Operations” is the DARPA Wolfpack program. The stated goal of Wolfpack is to develop a “close-in distributed, autonomous, ground-based jamming system to selectively deny enemy use of the RF spectrum.” The system’s underlying concept is equivalent to “Distributed Coherent RF Operations.”
One of the critical technology for enabling coherent operations is high-precision, high-accuracy inter-node coordination between moving systems. To enable a collection of systems to operate as a coherent phased array, the relative node positions must be known accurately and clocks on each platform must be synchronized precisely, among other challenges.
At the present time, GPS is relied upon to provide ubiquitous position and timing to many DoD and civilian users. The military value of precise position and timing is reflected in the broader application GPS receivers to more and more military platforms and systems. However, these installations are not sufficient to enable “Distributed Coherent RF Operations:” 1) these installations do not provide axial orientation for most static or slow-moving ground-based systems, 2) these installations do not provide take advantage of GPS to provide a common frequency reference, and 3) these installations do not consider a common phase reference.
Michigan state University researchers using spectrally sparse waveforms, have developed a novel high-accuracy ranging method that enables microwave wireless positioning between nodes with accuracies below 1 mm using low-rate, low-cost digitizers. “ By leveraging our high-accuracy ranging method, we implemented a low-overhead distributed clock synchronization approach, where a master node simply broadcast its clock signal. Knowing the distance to the slave node with the ranging method, the propagation delay can be simply adjusted, directly enabling distributed beamforming without direct clock feedback from the slave nodes.”
“Our group demonstrated the first open-loop distributed transmitter using these methods, showing for the first time that distributed beamforming is possible on moving platforms without feedback from the receiver. Current work is focused on the development of jointly optimized coordination methods and system miniaturization for small platform integration.”
Distributed systems hold the potential for significant gains from coherent RF networking. However, many of the techniques enabling system-level benefits are applicable in single-system coherent arrays. Digital arrays, comprised of element-level digitizers, are in essence spatially-fixed coherent distributed systems. Our work on millimeter-wave photonic arrays can enhance digital array capabilities by adding significant bandwidth and frequency ubiquity to coherent RF network nodes. Utilizing the spatial diversity principles of distributed arrays, we also developed a new interferometric radar which can directly measure the angular velocity of moving objects, write Jeffery Nanzer of MSU.
Coherent RF networks will allow significant capabilities that are not possible with existing technologies and approaches. Significant spatial diversity along with fast measurement techniques will allow spatial resolution and range extent well beyond what can currently be achieved with platforms that are in motion, creating arrays of low-cost, moderate capability elements, which when combined in a coherent RF network will operate as a single, highly capable system. We anticipate a future where individually small devices for communications, remote sensing, and other wireless applications can be operated as a cooperative, distributed system, allowing significantly improved wireless capabilities at lower cost.
Jeffrey Nanzer Selected for Darpa Director’s Fellowship
Jeffrey Nanzer of Michigan State University has been awarded a director’s fellowship from the Defense Advanced Research Projects Agency, or DARPA. Nanzer is the Dennis P. Nyquist assistant professor in MSU’s Department of Electrical and Computer Engineering, or ECE.
In 2017, Nanzer won a $400,000 DARPA Young Faculty Award, or YFA, to create technologies enabling separate, small wireless systems to collaborate as a single system. Rather than redesigning new and larger systems when performance increases are needed, he said adding small and cheap nodes to the distributed system will reduce both costs and development time.
“The aim is to develop methods for precise coordination between multiple nodes and explore the new capabilities enabled by coherent distributed arrays,” Nanzer said.
MSU foundation professor and ECE chair John Papapolymerou said the award is an acknowledgment of Nanzer’s potential to expedite the capabilities of wireless and remote sensing applications.
“Jeff’s work is significantly advancing the state-of-the art in modern radar and wireless communication systems and will enable a new generation of such systems with unprecedented capabilities,” Papapolymerou said. “His work will not only benefit our armed forces but also people’s everyday lives. This is a great recognition of his high quality and groundbreaking work and a great honor for him and our department.”
Digital Radar Tech to Enable Distributed Sensing
A new digital radar architecture in development at Sandia National Laboratories is intended to shift the paradigm for military sensing. Researchers are working to replace legacy analog radars commonly used by the military with a new, digital, software-defined system called Multi-Mission Radio Frequency Architecture. The overhauled design promises U.S. warfighters unprecedented flexibility and performance during intelligence, surveillance and reconnaissance operations, even against sophisticated adversaries.
The new technology replaces analog signal processing hardware with digitally based signal processing firmware and software that can be downloaded for different missions to a small toolbox-sized piece of equipment that meshes with existing and future sensors. It takes radars “from a few exquisite sensors to a distributed [electronic warfare], distributed communications, distributed intelligence paradigm,” said Jacques Loui, technical lead for Sandia’s multi-mission radar frequency architecture.
The architecture allows a single sensor to perform multiple tasks “like a Swiss Army knife,” diminishing space, weight and power requirements, Loui said. Hypothetically, the sensors could be placed in a variety of unmanned aircraft, manned aircraft and even weapons to form a distributed sensing network of platforms that can communicate and collaborate. The digital architecture’s flexibility could allow a sensor to tackle an EW mission on one sortie, then perform intelligence, surveillance and reconnaissance on the next, he said. It has the potential to help realize the military’s concept of replacing expensive, mission-dedicated aircraft like the EA-18G Growler with large numbers of inexpensive drones and smart weapons, he said.
The rapidly upgradeable, reconfigurable architecture employs advanced electronic components developed for 5G cellphone systems. The 5G components also allow sensors to receive and transmit massive amounts of data over a much broader bandwidth and process it in real time at the sensor.
Advanced wireless technology also enables the new digital architecture to operate multiple radio-frequency channels simultaneously, either working together on a single function or working independently on several different functions. Sandia is using it to create digital processing tools that convert massive amounts of analog data to digital signals and vice versa, such as a digital version of synthetic aperture radar, a remote radio frequency imaging technology widely used for many national security missions.
“Digital, software-based radar systems do exist on small scales,” Jacques said, but his team is using advanced electronic components developed for 5G cellphone systems to reap major advantages in performance and agility over similar technologies. “Our aim is to deliver outstanding sensors to our customers in the most efficient manner possible.” “The data is processed to output as actionable products,” Loui noted. “It doesn’t require post-processing.”
“The new architecture will be harder for an adversary to jam or disrupt,” Jacques said. Someone who knows they’re being watched by a radar can deploy countermeasures that degrade the radar’s performance, Jacques explained. But Sandia’s system enables users to digitally change characteristics of their transmitted signal in real-time, making it harder to recognize. In addition, the high-performance system can be used to analyze a complex radio-frequency environment — one that has many kinds of signals, including those of an adversary. It “can generate very high-resolution imagery, but we are no longer tied to ‘chirp’ waveforms. Any adversary that sees a chirp knows they’re being imaged,” Loui said, referring to sweep signals.
“The use of commercially available electronics is driving down the cost of these sophisticated systems, providing a clear path of upgrades as the technology continues to advance,” said Steven Castillo, recently retired Sandia senior manager who worked with the project. “The new architecture also sets the stage for utilizing new, highly agile antennas of the future.”
Kurt said Sandia radars require extreme high performance. Now, technology is finally at a point where the lab can make the switch from analog to digital and preserve the extreme fidelity. The technology is currently being tested aboard a de Havilland Twin Otter aircraft, and could be fielded as early as 2025, according to the lab.
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