Digital predistortion of RF PAs for MIMO transmitters based on the equivalent load

Abstract

Modern transmitter architectures rely on MIMO techniques, using several RF PAs to excite an antenna array. The array elements are often coupled, creating apparent variable loads at the output of each PA. In this system, the behavior of each PA is dependent on the coupled signal from the other array elements. This is felt in terms of device output power and efficiency variation and also as an overall change of the gain characteristic. When linearization of the RF PAs feeding the antennas is performed without taking into account this coupling, some degradation of the system performance in terms of linearity is expectable. In this paper, we analyze the PA operation on a dual antenna array in terms of distortion and propose a DPD structure with adaptive coefficients based on the equivalent load.