Single-Input Single-Output Digital Predistortion of Multi-user RF Beamforming Arrays

We investigate the application of single-input single-output (SISO) digital predistortion (DPD) to mitigate the nonlinearity in millimeter-wave multi-user RF beamforming arrays. In principle, SISO DPD may not be sufficient to linearize such arrays as a multiple-input one may be required due to coupling between the sub-array antennas. Nevertheless, in practice, the coupling between antennas of different sub-arrays may be much smaller than the coupling between antennas of the same sub-array. It is first shown mathematically that if the coupling between sub-arrays is small, then SISO DPD is sufficient to linearize such arrays. This is then confirmed in practice by linearizing two co-located 64-element sub-arrays driven by 800 MHz modulated signals at 28 GHz. Using four sets of SISO DPD coefficients, the EVM and ACPR were improved from as much as 8% and 30 dBc to better than 2% and 40 dBc, respectively, across all steering angle combinations of the two sub-arrays.