Space-Constrained Arrays for Massive MIMO

We analyse the behavior of a massive multi-user MIMO (MU-MIMO) system comprising a base station (BS) equipped with one of five antenna topologies whose spatial apertures are either unconstrained, or space-constrained. We derive the normalized mean interference (NMI) with a ray-based channel model, as a metric for topology comparison. Using an existing derivation for a horizontal uniform rectangular array (HURA), we provide closed-form NMI equations for a uniform linear array (ULA) and uniform circular array (UCirA). We then derive the same for a vertical URA (VURA) and uniform cylindrical array (UCylA). Results for the often-considered unconstrained case confirm the prior understanding that topologies with wider azimuth footprints aid performance. However, in the constrained case, performance is dictated by the angular resolution of the topology, particularly in elevation. We confirm the behavioral patterns predicted by the NMI by observing the same patterns in the signal-to-interference-and-noise-ratio (SINR) with minimum mean-squared error (MMSE) processing.