James D. Krieger

Dense Delta-Sigma Phased Arrays

The high cost of high-resolution phase shifters required to maintain precise control over the array beam pattern in traditional phased arrays preclude their use in a variety of emerging millimeter-wave applications. We develop a phased array architecture that obviates the need for such precise phase shifters, based on the use of sub-half-wavelength array element spacing and novel spatial domain delta-sigma processing. We characterize the performance of this architecture in terms of the array signal-to-quantization-noise ratio (SQNR) and the array power transfer efficiency, and demonstrate a tradeoff between these two metrics. As an illustrative design, we show that when constrained to two-bit phase shifters, a four-fold increase in the array density can provide a roughly 6 dB improvement in SQNR over standard design techniques, with an average efficiency loss of less than 1.5 dB with respect to a perfectly tuned ideal array. In our analysis, we account for the effects of mutual coupling, and describe a simple, practical impedance matching network for this architecture. The resulting framework allows a system designer with a given set of circuit, device, and antenna fabrication and integration technologies to choose from a spectrum of tradeoffs between array density and RF component complexity.

Design and analysis of multi-coset arrays

An efficient sparse antenna array architecture is developed for coherent imaging of sparse but otherwise unknown scenes. In this architecture, the array elements form a periodic nonuniform pattern. Using analysis that explicitly takes into account the presence of noise, we develop an efficient pattern design procedure based on co-arrays, describe an efficient scene support recovery algorithm as part of image reconstruction in the form of a modification to the MUSIC algorithm, and discuss a failure detection technique based on evaluating “back-projection” error. Since our development exploits a close connection to multi-coset sampling of bandlimited waveforms, our results may in turn may also be useful in the design of those systems.

Switched antenna array tile for real-time microwave imaging aperture

A switched array tile which is part of a large aperture for near-field microwave imaging is presented. The tile is based on the Boundary Array (BA), a sparse array topology for hardware efficient realization of imaging apertures. The larger array formed with the tile samples a scene with no redundancy, and is compatible with fast imaging techniques. Details on the design and realization of the tile are presented, as well as experimental images formed with a tile prototype.

Ultrawideband VHF/UHF dipole array antenna

A linearly-polarized ultrawideband dipole array antenna has been developed for coverage in the VHF/UHF frequency range for communications or radar applications. The antenna design utilizes a horizontally polarized array of thick tubular dipole elements above a ground plane. Numerical electromagnetic simulations were used to analyze and optimize the antenna parameters prior to fabrication. Measurements of a 24-element ultrawideband dipole array prototype in an anechoic chamber demonstrate the antennas return loss and gain pattern performance over a wide bandwidth.

Development of a high-throughput microwave imaging system for concealed weapons detection

A video-rate microwave imaging aperture for concealed threat detection can serve as a useful tool in securing crowded, high foot traffic environments. Realization of such a system presents two major technical challenges: 1) implementation of an electrically large antenna array for capture of a moving subject, and 2) fast image reconstruction on cost-effective computing hardware. This paper presents a hardware-efficient multistatic array design to address the former challenge, and a compatible fast imaging technique to address the latter. Prototype hardware which forms a partition of an imaging aperture is discussed. Using this hardware, it is shown that the proposed array design can be used to form high-fidelity 3D images, and that the presented image reconstruction technique can form an image of a human-sized domain in ≤0.1s with low cost computing hardware.

Multi-Coset Sparse Imaging Arrays

We develop an efficient structured sparse antenna array architecture for coherent imaging of sparse but otherwise unknown scenes. In this architecture, the array elements are configured in a periodic nonuniform pattern that can be viewed as the superposition of multiple sparse uniform subarrays. For such structure, we develop an efficient pattern design procedure using co-array analysis, and we describe robust and efficient algorithms implementing the required associated array processing, which comprise scene support recovery, followed by image reconstruction. In addition, we develop a practical method for detecting reconstruction failures when the scene density exceeds the level for which the array was designed, so that false images are not produced. As a demonstration of its viability, the architecture is used to reconstruct a simulated natural scene.

Low-profile dual-polarized UHF array antenna

A low-profile dual-polarized UHF array antenna has been developed for wide field-of-view dual sector coverage in the 250 to 450 MHz frequency range for communications or radar applications. The antenna utilizes a pair of parasitically-tuned dipole arrays for horizontal polarization and a pair of parasitically-tuned monopole arrays for vertical polarization, and both arrays are mounted on a common ground plane. The thickness of the antenna is 18.2 cm. Numerical electromagnetic simulations were used to analyze and optimize the antenna parameters prior to fabrication. Measurements of the dual-polarized prototype in an anechoic chamber demonstrate the antennas return loss and dual-polarized radiation gain pattern performance.

Scalable prototyping testbed for MMW imager system

A prototyping testbed for an experimental millimeter-wave multiple-input multiple-output (MIMO) radar system for security applications in high foot-traffic areas will be presented. The system is designed for flexible operation at a 10 Hz video rate, enabled by high-speed electronic scanning and real-time signal processing. Overall imaging system costs are reduced by the use of an innovative ultra-sparse multistatic radar solution and a 3-D near-field beamforming image construction technique targeted for low-cost high-throughput GPU processors. The testbed is architected with FPGAs, GPUs, CPU, storage, and networking, capable of supporting future growth in capabilities, such as interference suppression & advanced signal processing algorithms, auxiliary sensing modalities, near-sensor analytics, and integration into a system-of-systems security architecture.

Dense transmit and receive antenna arrays

Recent progress in millimeter-wave and microwave integrated circuit (IC) manufacturing technology has created new opportunities for the development of novel, low-cost antenna array architectures. In such systems, the associated specifications on RF components such as phase-shifters, oscillators, and amplifiers, can be challenging to meet in a cost-effective manner. In this paper, we develop a dense antenna array to ease such RF circuit requirements without sacrificing performance, and demonstrate its effectiveness for transmit and receive beamforming. We also investigate the effects of mutual coupling and impedance matching on the system performance.