New Digital Predistortion Training Method with Cross-Polarization Channel De-Embedding for Linearizing Dual-Polarized Arrays Using Far-Field Observation Receiver
This paper proposes a far-field (FF) -based digital predistortion (DPD) training method for linearizing dual-polarized (dual-pol) beamforming arrays in the presence of cross-polarization channel (XPC) interference experienced in the DPD FF-based observation receiver (OR). The cross-polarization interference (XPI) in the XPC can be attributed to the transmitter’s (TX’s) antennas, the probe used in the DPD FF OR, the OR’s over-the-air channel, as well as the mechanical misalignment between the TX antenna and the FF-based OR probe. Specifically, an XPC estimation and de-embedding technique using interleaved multi-tone test signals is proposed. Experiments conducted using a 4×4 dual-pol RF beamforming array operated at 38 GHz and excited by a 5G NR 200 MHz 256-QAM orthogonal frequency division multiplexing test signal are presented. The measurement revealed the capacity of the proposed technique to correct for the nonidealities in the XPC where the XPI was reduced from -10 dB to -40 dB. Furthermore, using the proposed DPD training method, the adjacent channel power ratio (ACPR) and error vector magnitude (EVM) improved from 26.7 dB and 10.96% to 36.38 dB and 3.3%, respectively, when a single-input-single-output DPD function was used. The ACPR and EVM were further improved by 3 dB and 1.1% when a dual-input-single-output DPD function was used.