Linearization for Hybrid Beamforming Array Utilizing Embedded Over-the-Air Diversity Feedbacks

In this article, a modified beam-oriented digital predistortion (BO-DPD) utilizing the over-the-air diversity feedbacks (OTA DFBs) is proposed for linearizing the hybrid beamforming (HBF) large-scale array. The proposed DPD can solve the hardware implementation issue of conventional DPD architecture in HBF array by estimating and linearizing the main beam signal while avoiding the complex feedback configurations. Different from the far-field OTA measurements, the OTA feedback receivers are placed symmetrically inside the transmit array according to its geometric structure. Theoretical analysis proves that it is possible to derive the main beam signal from the OTA feedback information directly by adjusting the phase shifter of each branch using a relative simple procedure which allows the predistorter to be identified based on the estimated main beam signal. Furthermore, an iterative algorithm for eliminating uncorrelated coupling interference from other subarrays in the feedback signal is proposed. Measurement results on a 1 × 4 transmitter and a 2 × 3 transmitter as well as simulations on a 4 × 64 array are presented to benchmark the proposed DPD technique against the existing techniques. Better than 15-dB adjacent channel power ratio (ACPR) improvement has been realized with the proposed DPD.