  In this paper, we investigate the spectral efficiency (SE) of massive
multiple-input multiple-output (MIMO) systems with a large number of antennas
at the base station (BS) accounting for physical space constraints. In contrast
to the vast body of related literature, which considers fixed inter-element
spacing, we elaborate on a practical topology in which an increase in the
number of antennas in a fixed total space induces an inversely proportional
decrease in the inter-antenna distance. For this scenario, we derive exact and
approximate expressions, as well as simplified upper/lower bounds, for the SE
of maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-squared
error receivers (MMSE) receivers. In particular, our analysis shows that the
MRC receiver is non-optimal for space-constrained massive MIMO topologies. On
the other hand, ZF and MMSE receivers can still deliver an increasing SE as the
number of BS antennas grows large. Numerical results corroborate our analysis
and show the effect of the number of antennas, the number of users, and the
total antenna array space on the sum SE performance.
