Enabling Near-Field Wideband Beam Focusing in ELAA Systems With Uniform Planar Arrays

Abstract

Extremely large antenna array (ELAA) system has been emerging as one of the most promising techniques in future 6G communications. With extremely massive antennas and wide bandwidth, wideband ELAA systems suffer from severe spatial wideband effect (SWE), resulting in a focus blur of the transmitted beam, especially when it comes to the near-field uniform planar array. Specifically, unlike the beam squint effect in traditional far-field communication systems which features the transmitted beam squint from the expected direction, we observe the beam defocus effect in near-field UPA systems where the SWE manifests a loss of beam focal point in the entire 3-D space that further deteriorates the beam gain. To mitigate such effect in the near-field UPA setting, in this paper, we propose two true-time-delay (TTD)-aided beamforming architectures, taking TTD hardware constraints into consideration. The first one is a piecewise TTD (PTTD) architecture, for which the TTD configuration is carefully designed by exploiting the decoupled beam defocus effect on the two dimensions, and the second one is a piecewise delay-phase-precoding (PDPP) architecture, where the configuration of phase shifters (PSs) and TTDs is formulated as a joint optimization problem subject to TTD constraints. Simulation results demonstrate the effectiveness of our proposed TTD-aided beamforming schemes in mitigating the beam defocus effect with near-optimal beam gain obtained over all subcarrier frequencies.