Block-Segmented Spatial-Spectrum Imaging for Near-Field Millimeter Waves with Microwave-Photonic Broadband Reception

Active millimeter-wave (mm-wave) imaging uses non-ionizing electromagnetic radiation to detect concealed metallic and dielectric objects and is an important technology for personnel security screening. However, practical security-screening scenarios typically involve short standoff distances and small apertures: the limited physical aperture constrains the two-dimensional imaging resolution, while the short antenna-to-target distance introduces non-negligible high-order nonlinear phase errors for scatterers far from the global coordinate origin, leading to image defocusing. This paper presents a block-segmented spatial-spectrum near-field imaging method combined with a Ka-band microwave-photonic broadband reception prototype. Microwave-photonic broadband reception provides broadband spectral resources to expand the two-dimensional spatial-frequency support, thereby improving imaging resolution under limited-aperture conditions. Meanwhile, the full imaging plane is divided into multiple subblocks, each of which establishes its own local coordinate system. Scattering points within each subblock are kept close to the reference origin to suppress high-order nonlinear phase errors, thereby enhancing the accuracy of local linear spatial-frequency mapping. Finally, global near-field imaging is achieved through sub-image stitching. Simulations with point targets and an aircraft-shaped extended target show that, compared with spatial-spectrum imaging based on a single global coordinate origin, the proposed method effectively suppresses nonlinear phase errors and enables large-area near-field imaging. A discrete-component laboratory prototype implements microwave-photonic broadband reception for a time-division 3-transmit/13-receive acquisition configuration operating from 32 to 38 GHz. The method extends the effective linear imaging span from a 0.2 m × 0.2 m locally focused region to a 1 m × 1 m imaging plane, corresponding to a fivefold increase in linear span and a 25-fold increase in area. Based on the antenna arrangement geometry and the 32–38 GHz operating frequency band, the theoretical two-dimensional imaging resolution of the prototype system is estimated to be approximately 3 cm × 4 cm. Experiments using aluminum-foil strip resolution targets demonstrate that the prototype can resolve adjacent strip scattering regions spaced approximately 3.3 cm and 4.7 cm apart in two orthogonal directions, with a corresponding measured imaging resolution of approximately 3.3 cm × 4.7 cm. The prototype is also capable of reconstructing the primary scattering structure of gun-shaped threat simulation targets. These results indicate that combining block-segmented spatial-spectrum imaging with microwave-photonic broadband reception provides a potentially feasible approach for high-resolution, wide-area millimeter-wave security-screening imaging.