Coherent RF Arrays
Jeffery Nanzer | nanzer@msu.edu | www.egr.msu.edu/emrg/
The advancements in the area of information networking in the past decades have driven society to new technological and economic heights. Distributed systems now share information at levels difficult to conceive of even a few years ago. Where this enormous expansion in networking has not yet reached is at the level of the physical aperture. 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.
Our group focuses on 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. 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, our group develops technologies with significant potential for improvements in a broad range of wireless applications.
A primary emphasis of our current work is on creating technologies for 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. Using spectrally sparse waveforms, we 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.
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.
