On Spatial Multiplexing of Strong Line-of-Sight MIMO With 3D Antenna Arrangements

In recent works, it has been shown that specific 2D antenna arrangements for multiple-input multiple-output (MIMO) systems can achieve similarly high spatial multiplexing gains under deterministic line-of-sight (LOS) conditions as non-line-of-sight channels with strong scattering considered in classical papers. However, the question whether 3D antenna arrays could provide an additional advantage was not addressed. In this work we show that the capacity of dominant LOS MIMO channels is invariant w.r.t. small offsets of the antenna elements along the transmit direction. This proves that the optimal 2D arrangements for point-to-point communication of LOS MIMO arrays are equivalent to 3D arrangements, whose projections of the antenna positions into a plane perpendicular to the transmit direction reproduce the optimal 2D arrangements. This insight also leads directly to the optimal designs for antenna arrays that communicate with each other along a transmit direction that is oblique w.r.t. the array plane(s).