Combined Sidelobe Reduction and Omnidirectional Linearization of Phased Array by Using Tapered Power Amplifier Biasing and Digital Predistortion

Abstract

Power amplifier (PA) efficiency and linearity are among the key drivers to reduce energy consumption while enabling high data rates in the fifth-generation (5G) millimeter-wave phased array transmitters. Analog per-branch phase and amplitude control is used to steer the beam, suppress the sidelobes, and form zeros to the desired spatial directions. The amplitude control of individual PA inputs makes nonlinearity vary from antenna to antenna, which challenges the common digital predistortion (DPD) used to linearize the array. In this article, we implement an amplitude control for beamforming by tuning the PA gate bias. Varying the output powers via PA biasing makes the nonlinear characteristics observed at the individual PA outputs similar that helps the array DPD to linearize also individual PAs. The technique is validated by both simulations and measurements. As a measurement platform, we use a 28-GHz phased array transceiver equipped with 64 antenna elements and 16 radio frequency chains. The desired beam shape is synthesized by controlling the per-antenna over-the-air-power with PA gate bias. Then, the system is linearized by training DPD with a reference antenna. The DPD is demonstrated with 100-MHz-wide 5G new radio modulated waveform. The best example case showed −23.5-dB maximum sidelobe level (SLL) with 4.9% error vector magnitude and −40.8-dB total radiated adjacent channel power ratio with DPD. The proposed approach enables simultaneous reduction of beam pattern SLL, achieves good linearity in all directions, and maintains the PA efficiency.

Broadband Cross-Slotted Patch Antenna for 5G Millimeter-Wave Applications Based on Characteristic Mode Analysis

“…With the other three unit cells terminated to 50 Ω, the pattern of the array prototype was obtained by sequentially exciting each unit cell fed by coaxial cables. Thereafter, the four individual patterns were combined to obtain the array pattern radiating at boresight by summing the measured four field vectors with equal phase and amplitude weighting in MATLAB as implemented in [57].…”

Over-the-Air Phase Calibration Methods for 5G mmW Antenna Array Transceivers

“…Nevertheless, in TX mode, the amplitude of individual antenna ports can be controlled by varying the PA gate bias. However, varying the PA operation point has an impact on the phase response of the antenna path [29].…”

“…Furthermore, as the TRX path did not have path-specific amplitude control, the impact of amplitude errors was studied on the TX, only. The TX amplitude control of the antenna array was implemented by varying the bias currents of the PAs by controlling their gate voltages [29]. However, due to the fact that the PA biasing has an impact on the phase of the signal, the bias values were set prior to the phase calibration.…”

Beam-Level Frequency-Domain Digital Predistortion for OFDM Massive MIMO Transmitters

“…L INEARIZATION of active array radio transmitters (TXs) with large numbers of antennas and power amplifier (PA) units is one timely topic in digital predistortion (DPD) research, posing many new challenges compared to classical single PA linearization [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The operation principle and specific implementation of DPD solutions tailored for antenna arrays depend heavily on the considered TX architecture.…”