  Deploying antenna arrays with an asymptotically large aperture will be
central to achieving the theoretical gains of massive MIMO in beyond-5G
systems. Such extra-large MIMO (XL-MIMO) systems experience propagation
conditions which are not typically observed in conventional massive MIMO
systems, such as spatial non-stationarities and near-field propagation.
Moreover, standard precoding schemes, such as zero-forcing (ZF), may not apply
to XL-MIMO transmissions due to the prohibitive complexity associated with such
a large-scale scenario. We propose two novel precoding schemes that aim at
reducing the complexity without losing much performance. The proposed schemes
leverage a plane-wave approximation and user grouping to obtain a
low-complexity approximation of the ZF precoder. Our simulation results show
that the proposed schemes offer a possibility for a performance and complexity
trade-off compared to the benchmark schemes.
