Multi-Stream Spatial Digital Predistortion for Fully-Connected Hybrid Beamforming Massive MIMO Transmitters

In this paper, a novel multi-stream spatial digital predistortion (DPD) technique is proposed to model and linearize the fully-connected (FC) hybrid beamforming (HBF) transmitters. The proposed scheme solves the DPD implementation issue in the FC HBF array by estimating and linearizing the beam signals instead of the individual PAs. In FC HBF systems, significant intermodulation (IMD) between different transmit signals will be generated due to the analog beamforming and combining network upstream of the power amplifiers (PAs). The IMD beams will end up being radiated in different directions and some of them might fall in the linear beam directions. Therefore, multi-input DPD blocks using a practical multi-variable model are constructed for each RF chain to eliminate the complicated inner- and cross-channel IMDs of the beam signals. Simulations on a 4-stream 64-element FC HBF array and experimental tests on a 2-stream 4-element system are carried out to benchmark the proposed DPD technique against the conventional techniques. Better than 13 dB adjacent channel power ratio (ACPR) improvement and 12 dB normalized mean square error (NMSE) improvement have been achieved by the proposed DPD technique.