On the Effectiveness of Near-Field Feedback for Digital Pre-Distortion of Millimeter-Wave RF Beamforming Arrays

This paper investigates theoretically and experimentally the effectiveness of linearizing millimeter-wave analog beamforming arrays using a single digital pre-distortion (DPD) function trained utilizing a near-field feedback signal. First, it is shown that if the DPD is trained to minimize the error between the signal acquired in the near-field and the input to the DPD, the far-field received signal exhibits significant residual nonlinearity effects. It is then shown that a DPD function trained to minimize the difference between a properly synthesised near-field target signal and the acquired near-field signal, results in the cancellation of nonlinearity effects in the far-field.