Frequency-Diverse Additive Processing for Active Incoherent Millimeter-Wave Imaging

We present an approach for improving spatial frequency sampling in active incoherent millimeter-wave (AIM) imaging systems using frequency diversity. AIM imaging relies on active transmission of spatiotemporally incoherent signals to illuminate a scene, from which interferometric Fourier domain imaging can be implemented using a sparse receiving antenna array. One of the benefits of Fourier domain imaging is the sparsity of the receiving array, which can form images with equivalent resolution to traditional filled beamsteering arrays, but with a small fraction of the elements. Image reconstruction quality in Fourier domain imaging is dependent on the number of spatial frequency samples obtained by the array, which is determined by the array design, and without a sufficient number of samples the image reconstruction may be degraded when viewing complex objects. Increasing the number of samples is often accomplished with wideband hardware, but this is often expensive and requires fast baseband digitization. To address this challenge without requiring additional receiver channels, we explore the use of frequency diversity in the illuminating and receiving systems. Fourier domain spatial frequency samples are determined by the electrical spacing and rotation of the receiving elements, thus by changing the frequency the sampled spatial frequencies also change. We implement an additive technique where the spatial frequency samples are summed prior to Fourier transform image formation. Importantly, because the system is active, a consistent signal-to-noise ratio is maintained across all frequencies, which may not be possible in traditional passive Fourier-domain imagers. We implement the approach in a 24-element receiver array oriented in a circular pattern combined with a four-element noise transmitter. Image improvement is evaluated through simulation on various test scenes utilizing a structural similarity index measure (SSIM) to determine the quality of image reconstruction. Experiments were conducted in 1 GHz frequency steps ranging from 37– 40 GHz on scenes consisting of reflecting spheres and cylinders, demonstrating a reduction of spurious signals in the resultant additive image.