Wideband Beamforming in Near-Field Uniform Planar Array Systems Under TTD Constraints

In this paper wideband beamforming in near-field uniform planar array (UPA) systems are studied. Growing communication frequencies in the next-generation communication systems require frequency-dependent phase shifts for wideband beamforming, which can not be provided by commonly used phase-shifter (PS)-based beamforming architecture and will result in beam squint effect in far-field systems. However, due to the additional distance resolution brought about by the spherical wavefront, the further beam defocus effect is manifested in near-field wideband systems. Such beam defocus effect imposes challenges on beamforming, especially in UPA systems, as it may result in a loss of beam focal point in the entire 3-D domain. In this paper, we first analyze the 3-D beam defocus effect in near-field UPA systems and propose a piecewise true-time-delay (TTD) devices aided beamforming architecture to mitigate it. Considering the hardware constraints of practical TTD devices, a piecewise delay-phase-precoding under TTD constraints is proposed. Simulation results demonstrate the effectiveness of our proposed beamforming architectures.