  Efficient mitigation of power amplifier (PA) nonlinear distortion in
multi-user hybrid precoding based broadband mmWave systems is an open research
problem. In this article, we first carry out detailed signal and distortion
modeling in broadband multi-user hybrid MIMO systems, with a bank of nonlinear
PAs in each subarray, while also take the inevitable crosstalk between the
antenna/PA branches into account. Building on the derived models, we then
propose an efficient closed-loop (CL) digital predistortion (DPD) solution that
requires only a single-input DPD unit per transmit chain or subarray, despite
crosstalk, providing substantial complexity-benefit compared to the
state-of-the art multi-dimensional DPD solutions. We show that under spatially
correlated multipath propagation, each DPD unit can provide linearization
towards every intended user, or more generally, towards all spatial directions
where coherent propagation is taking place, and that the proposed CL DPD system
is robust against crosstalk}. Extensive numerical results building on practical
measurement-based mmWave PA models are provided, demonstrating and verifying
the excellent linearization performance of the proposed DPD system in different
evaluation scenarios.
