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.
Highlights
M ILLIMETER-WAVE transceivers (TRXs) equipped with large antenna arrays have become the mainstream of wireless research in academia and industry
This makes the common linearization of multiple transmitter (TX) branches challenging with a single digital predistortion (DPD) linearizer due to the fact that the Power amplifier (PA) have different nonlinear responses
total radiated ACPR (TRACPR) was improving 11 dB and error vector magnitude (EVM) was improved from 8.5% without DPD to 4.9% with DPD
Summary
M ILLIMETER-WAVE (mmW) transceivers (TRXs) equipped with large antenna arrays have become the mainstream of wireless research in academia and industry. DPD measurement results in the input amplitude tapering case are studied in [25] by emulating the array with a multichannel oscilloscope These papers use a traditional analog beamforming approach where the amplitude control is done prior to the biased PAs. these papers use a traditional analog beamforming approach where the amplitude control is done prior to the biased PAs This selection makes it challenging to linearize the array in terms of TRACPR and achieve good EVM simultaneously. We present an alternative approach for per-branch amplitude control combined with OTA DPD for RF beamforming array [see Fig. 1(b)]. A common DPD is applied for the array in Section V together with the OTA-measured sidelobe reduction, linearity, and efficiency analysis.
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