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

By Emil Björnson, Erik G. Larsson, Merouane Debbah. IEEE Transactions on Wireless Communications, 2014

537 citations recorded by the source corpus.

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.

Subfields: Computer Networks and Communications, Electrical and Electronic Engineering

View Paper: https://www.semanticscholar.org/paper/60d78829142e0fdef21e40b977247bd4f9e9c17c

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Citations in the corpus

Toward Massive MIMO 2.0: Understanding Spatial Correlation, Interference Suppression, and Pilot Contamination (2019, methodology): “Fourthly, the sum capacity is often maximized when M/K is fairly small [1], [53], which implies that the traditional large-system limit where M,K → ∞ with a fixed ratio is of more practical interest [54].”
A Comprehensive Survey of Pilot Contamination in Massive MIMO—5G System (2016, background): “affected and does not give a true picture of a dynamic system as presented in [15], [115].”
5G-Advanced Toward 6G: Past, Present, and Future (2023, methodology): “for MU-MIMO precoding [22], [23] have been introduced.”
Making Cell-Free Massive MIMO Competitive With MMSE Processing and Centralized Implementation (2019, background): “The SE gain comes from that each BS has a compact array with a hundred or more antennas, which are used for digital beamforming and, particularly, to spatially multiplex many user equipments (UEs) on the same time-frequency resource [8].”
Scalable Cell-Free Massive MIMO Systems (2019, background): “8This is consistent with cellular networks, where it is a common practice to let the DL precoders be equal to the UL combiners, except for a scaling factor [50]–[52].”
Massive MIMO: ten myths and one critical question (2015, background): “…work [1] on Massive MIMO studied the asymptotic regime where the number of service antennas M → ∞ . closed-form Numerous later works, including [4]–[6], have derived closed-form achievable spectral efﬁciency expressions (unit: bit/s/Hz) which are valid for any number of antennas and terminals,…”
Massive MIMO Detection Techniques: A Survey (2019, background): “Numerous antennas create more spatial data-streams, more throughput, more multiplexing gain, and hence achieve high spectral efﬁciency [24].”
Massive MIMO is a Reality - What is Next? Five Promising Research Directions for Antenna Arrays (2019, background): “are the spectral efficiency analysis [18]–[21], system design for high energy efficiency [22]–[24], pilot contamination and decontamination [25]–[29], and power optimization [30]–[32].”