GPU-Based Linearization of MIMO Arrays

In this paper, we present a graphics processing unit (GPU)-based implementation for linearizing the power amplifiers (PAs) in massive multiple-input multiple-output (MIMO) arrays leading to lower error vector magnitude for the users and lower adjacent channel leakage ratio at the output of each antenna. In wireless transmitters, the nonlinearities of PAs can cause undesired spectral regrowth into the adjacent channels. For single antenna communications, this is corrected by digitally predistorting the transmit signal with the inverse nonlinearities of the power amplifier. However, in 5G and beyond, MIMO systems may have over one-hundred antennas and PAs that need to be linearized. Scaling up digital predistortion so that it can be performed on every transmit chain in large antenna arrays creates a significant computational burden for the base station. The parallel processing structure of GPUs provides a commercially available off-the-shelf solution that can be used to efficiently implement digital predistortion across many PAs in a massive MIMO basestation. Such a software-based solution is particularly attractive in virtual radio access networks or other software-defined radio scenarios. In this paper, we examine how the widely used memory polynomial scales on a GPU as the number of antennas scales up to 128, the number of memory taps scales up to four, and the polynomial degree scales up to nine. We find that a mid-range GPU can support predistortion for a sample rate of 100 MSps for up to thirty-two antennas while using a seventh-order polynomial with two memory taps.