Massive MIMO for Maximal Spectral Efficiency: How Many Users and Pilots Should Be Allocated?

TLDR

Massive MIMO boosts spectral efficiency by deploying hundreds or thousands of antennas at base stations, with the common rule that the number of antennas should be roughly ten times the number of scheduled users to achieve near‑orthogonal channels. This study investigates whether that rule truly maximizes spectral efficiency and determines how the optimal number of scheduled users depends on the antenna count and other system parameters. The authors derive new spectral‑efficiency expressions that incorporate power control, arbitrary pilot reuse, and random user locations, and obtain a closed‑form optimal user count in the large‑M limit while using simulations to explore finite‑M scenarios, interference, pilot reuse, and processing schemes. They find that up to half the coherence block should be devoted to pilots, the optimal antenna‑to‑user ratio is often below ten, and the optimal user count varies strongly with the processing scheme, making it inappropriate to compare schemes using the same number of users.

Abstract

Massive MIMO is a promising technique for increasing the spectral efficiency (SE) of cellular networks, by deploying antenna arrays with hundreds or thousands of active elements at the base stations and performing coherent transceiver processing. A common rule-of-thumb is that these systems should have an order of magnitude more antennas M than scheduled users K because the users' channels are likely to be near-orthogonal when M/K 10. However, it has not been proved that this rule-of-thumb actually maximizes the SE. In this paper, we analyze how the optimal number of scheduled users K* depends on M and other system parameters. To this end, new SE expressions are derived to enable efficient system-level analysis with power control, arbitrary pilot reuse, and random user locations. The value of K* in the large-M regime is derived in closed form, while simulations are used to show what happens at finite M, in different interference scenarios, with different pilot reuse factors, and for different processing schemes. Up to half the coherence block should be dedicated to pilots and the optimal M/K is less than 10 in many cases of practical relevance. Interestingly, K* depends strongly on the processing scheme and hence it is unfair to compare different schemes using the same K.