High sensing-capacity multi-dimension-coded millimeter-wave multiple-in-multiple-out imaging system

Abstract

Millimeter-wave (mm-wave) multiple-in-multiple-out (MIMO) imaging systems have been explored to use more and more complicated radar waveforms to achieve advanced multiplexing and high-performance imaging. As the complexity of the radar waveform increases, conventional systems inevitably suffer from higher design difficulty and cost. In spite of the radar waveform design, existing mm-wave imaging systems are still suboptimal due to the fact that the sensing matrix is not tailored properly to achieve its maximum capacity, which often results in large mutual information between successive measurements, and limited imaging performance. This dissertation is to design high sensing-capacity mm-wave MIMO systems with efficient multi-dimension coding to image on-the-move objects in 4D (1-D velocity and 3-D profile). As the first effort, a 70-77 GHz frequency-modulated continuous wave (FMCW) MIMO imaging system with massive measurement channels is evaluated. To achieve a high sensing-capacity, the first compressive reflector antenna (CRA) imaging prototype is designed where the CRA is fed by a 4-by-4 time-switching multistatic array. The use of the CRA introduces the spatial coding to increase the sensing capacity and achieve a better imaging resolution. Both static and slowly on-the-move objects are imaged. As the second effort for more efficient multi-dimension coding, an 81-86 GHz software-defined mm- wave (SDMMW) multistatic imaging system is designed, which makes use of commercial cost-effective software-defined radios (SDRs) and mm-wave mixers, a stepped FMCW modulator, and a metal-printed 8-by-8 waveguide array. Due to the great baseband flexibility of the SDRs, efficient space-time coding orthogonal frequency-division multiplexing is designed as the radar waveform to achieve simultaneous MIMO operation for a high receiving signal-to-noise ratio. The experiments are performed to image an array of corner reflectors with good imaging performance. To further enhance the sensing capacity and accelerate the image formation, the 4-D imaging of on-the-move objects is developed by leveraging the CRA and the SDMMW MIMO with a fast continuous FMCW modulation of 5.02GHz/50μs. Thus, a large maximum unambiguous velocity of ±2.23m/s and a high raw image data acquisition rate of 2500 volumetric frames per second are achieved, showing the fastest raw image data acquisition rate ever reported in state-of-the-art mm-wave imaging systems. In addition to the high sensing-capacity multi-dimension-coded mm-wave imaging, the material characterization is of great importance, especially in the security screening where both the object profile and its material information are desired for detecting potential threats. Thus, as the third effort, the material characterization method based on the Geometrical Optics forward model is developed for reflectarray imaging systems, which can be generalized and applied to other imaging systems with known geometrical setups. Simulations and measurements have shown the effectiveness and efficiency of the material characterization method to retrieve the object complex relative permittivity and thickness.--Author's abstract