This paper presents a millimeter-wave code-modulated interferometric imaging system, which is a lens-less approach to realizing imagers using repurposed phased arrays. To use a phased array as an interferometer, incoming signals are code modulated using phase shifters, multiplexed using a power combiner, and processed through a shared receiver chain. An interference pattern is then obtained by a squaring operation, from which complex visibilities can be demodulated. Here, a four-element 60-GHz phased array chip is packaged with slot antennas, and a single 60-GHz output is measured using a power detector. This scalar measurement is then demodulated to obtain the interferometric visibilities. The four-element phased array is thinned to obtain a 13-pixel image and the system is demonstrated through a point-source detected at different locations.
Millimeter-wave (mm-wave) imaging provides compelling capabilities for security screening, navigation, and bio- medical applications. Traditional scanned or focal-plane mm-wave imagers are bulky and costly. In contrast, phased-array hardware developed for mass-market wireless communications and automotive radar promise to be extremely low cost. In this work, we present techniques which can allow low-cost phased-array receivers to be reconfigured or re-purposed as interferometric imagers, removing the need for custom hardware and thereby reducing cost. Since traditional phased arrays power combine incoming signals prior to digitization, orthogonal code-modulation is applied to each incoming signal using phase shifters within each front-end and two-bit codes. These code-modulated signals can then be combined and processed coherently through a shared hardware path. Once digitized, visibility functions can be recovered through squaring and code-demultiplexing operations. Pro- vided that codes are selected such that the product of two orthogonal codes is a third unique and orthogonal code, it is possible to demultiplex complex visibility functions directly. As such, the proposed system modulates incoming signals but demodulates desired correlations. In this work, we present the operation of the system, a validation of its operation using behavioral models of a traditional phased array, and a benchmarking of the code-modulated interferometer against traditional interferometer and focal-plane arrays.
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