Beam-Space Intermodulation Suppression for LoS MIMO System with Nonlinear Power Amplifiers

In this paper, we focus on the nonlinear distortion in a massive MIMO system with nonlinear power amplifiers. It is known that the distortion generated by nonlinear power amplifiers is beamformed in the line-of-sight (LoS) channel. In particular, spurious emissions are caused by beamformed intermodulation (IM) distortion and form as IM beams. We propose a precoder for IM beam suppression in the MIMO system, which aims to suppress the intermodulation beams while maintaining beamforming for the intended receivers. First, we derive the direction of the IM beams under the free-space LoS channel condition. Then, the intermodulation suppression (IMS) precoder is proposed by maximizing the signal-to-leakage ratio. Moreover, we propose a generalized IMS precoder to achieve the trade-off between the intended signal coherent combination and IM beam suppression. Adjusting the weights of the precoder can suppress the IM beams while ensuring a coherent combination of signals at intended receivers. Simulation results show that the proposed IMS precoder can suppress the IM beams and reduce the power leakage in spurious directions. The power gain obtained by the proposed IMS precoder can approach to the maximum ratio transmission while achieving substantial suppression gain for the IM beams.