Photonic Imaging Receiver for Ultra-wideband Radio-frequency Communications and Sensing

University Of Delaware, Newark DE

Investigators

Abstract

A new concept in radio frequency (RF) receivers will be developed under this program. RF receivers are a vital component in a variety of systems that are yielding transformative benefits for society. Perhaps most prominent is wireless broadband internet connectivity. Enhancing the capacity of wireless networks will accelerate the benefits of worldwide connectivity among households, schools, hospitals, businesses, and governments. The photonic imaging receiver uses optical detection and processing of RF signals to enable unprecedented capabilities, from extremely broadband wireless communications to all-weather obscurant-penetrating imaging. Conventional RF receivers are limited by the intrinsic nonlinearity of the detection process, by which multiple signals mix and generate spurious responses. The unique capability of the imaging receiver system is the use of optics to"see" upconverted RF signals much like the eye or a camera sees visible light: sources of RF energy within its field of view produce distinct spots on an image plane or camera sensor, whereupon these spots can be detected separately, minimizing intermixing. This approach affords advantages in speed, complexity, linearity, and also adds new capabilities such as real-time direction finding, tracking, and processing of multiple signal sources. The imaging receiver system will offer increased capacity in wireless networks, while also improving performance and reducing infrastructure cost. The same enabling technology will also provide benefits in transportation and search/rescue operations, allowing navigation in obscurants like fog, smoke, and sand. Likewise, security checkpoints will be able to screen persons and vehicles passively at a distance, enhancing both security and convenience. The research effort will investigate the limits and optimize the performance potential of such imaging receiver systems. The photonic imaging receiver is a novel type of phased array receiver that uses the coherent properties of frequency upconversion in optical modulators to detect radio-frequency (RF) signals, and simple free-space optics to perform signal correlations between array elements, which are conventionally performed computationally after signals are detected and digitized. The optical upconversion process offers broad bandwidth, from UHF to millimeter-wave. Free-space optical processing enables signals coming from different sources to be spatially separated prior to detection, minimizing intermodulation and dramatically improving spur-free dynamic range (SFDR). It also provides the array gain of a phased array receiver, but with the advantage of forming all beams simultaneously. The recovery of data that has been modulated onto detected RF carriers is enabled by the use of a widely tunable optical local oscillator that is mixed with the upconverted signal on a photodiode. The foundation of the technical approach has been proven in the context of passive millimeter-wave imaging. This research program will explore the fundamental limitations of the approach in a broad and general sense, but will primarily focus on extending the utility of the approach toward ultra-wideband wireless communications, enhancing capacity by enabling spectrum reuse through spatial division multiple access. It will also investigate a novel concept in wideband systems, namely scale invariance, which seeks to replace conventional Fourier (time-frequency) domain formalism, in which the basis functions used for carrying information are continuous-wave (CW) carriers, with a new formalism based not on time-harmonic CW carriers but rather scale-invariant functions, which are naturally suited to applications with extremely broad bandwidth.