Space-Constrained Massive MIMO: Hitting the Wall of Favorable Propagation

TL;DR

This paper investigates the limitations of massive MIMO systems when antenna spacing is constrained within a fixed space, revealing that interference does not diminish as system size grows, unlike in idealized models.

Contribution

The study introduces a realistic space-constrained model for massive MIMO, demonstrating that favorable propagation conditions are limited and interference saturation occurs in practical deployments.

Findings

Inter-user interference does not vanish in space-constrained massive MIMO.

Achievable rate saturates due to interference limitations.

Contrasts with previous models assuming fixed inter-antenna distance.

Abstract

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.