On the Behavior of Radiated Nonlinear Distortion With Digital Predistortion in Wideband Large Arrays With Near-Field Beamfocusing

Communication in the radiating near-field will be an essential part of future wireless systems with large-scale antenna arrays. In the near field, the channel delay differences are a function of the observation point in space. This, in conjunction with the differences in the nonlinear behavior of the parallel power amplifier (PA) branches, causes the overall radiated array nonlinear response to vary nonlinearly with respect to both observation distance and angle. This paper investigates the nonlinear behavior of the array with digital predistortion (DPD) as a function of the observation location in a 2D space, considering both distance and direction in the near-field and far-field. It is shown that the DPD performance can be optimized at a location of interest in a 2D space and provides focused linearization. The DPD performance in terms of adjacent channel power ratio (ACPR) and error vector magnitude (EVM) deteriorates in space when evaluated at a location other than its trained location. Moreover, the wideband channel effects are evaluated, and it is shown that the frequency-dependent channel adds linear and nonlinear memory to the overall array responses.