Amplifier Nonlinearities in an Antenna Array During Spatially-Multiplexed Transmissions

The effects of nonlinearities in radio frequency (RF) power amplifiers (PAs) driving a N
ant
element array antenna are investigated. Initially, it is assumed that only the phase of each antenna path is adjusted and that the phase shift between neighbouring elements is equal. The N
ant
phase shifts used is referred to as the precoding vector. It is shown that when two (or more) data streams are spatially-multiplexed using different precoding vectors, the PA nonlinearities intermodulate the precoding vectors. This creates IMD beams in directions following the linear beam directions, as well as additional IMD beams whose directions are determined by the spatial separation of the linear beams. This case is generalized to allow amplitude shifts that are distributed to create Gaussian shading (tapering) within the precoding vectors. Digital predistortion is applied to reduce the IMD beams in the dominant directions.