Load and Power Indexed Predistortion for Improved Robustness to MIMO Channel Conditions

This letter presents an effective digital predistortion (DPD) solution to the problem of channel-dependent variations in the linearity of multiple-input-multiple-output (MIMO) transmitters. Such variations arise because: 1) conventional MIMO precoding results in large disparities in the average-power levels across chains and 2) antenna crosstalk causes significant active load modulation at the power amplifier (PA) output. Consequently, PA nonlinearity is channel dependent and cannot be compensated using static DPD. In this work, we propose a DPD solution that is a function of the estimated average reflection coefficient at the antenna as well as the PA average input power, which thus accounts for the channel conditions and antenna crosstalk. The proposed solution is validated through experiments on a MIMO transmitter prototype across varying channel conditions.