  The recent development of the massive multiple-input multiple-output (MIMO)
paradigm, has been extensively based on the pursuit of favorable propagation:
in the asymptotic limit, the channel vectors become nearly orthogonal and
inter-user interference tends to zero [1]. In this context, previous studies
have considered fixed inter-antenna distance, which implies an increasing array
aperture as the number of elements increases. Here, we focus on a practical,
space-constrained topology, where an increase in the number of antenna elements
in a fixed total space imposes an inversely proportional decrease in the
inter-antenna distance. Our analysis shows that, contrary to existing studies,
inter-user interference does not vanish in the massive MIMO regime, thereby
creating a saturation effect on the achievable rate.
