Channel Hardening and Favorable Propagation in Cell-Free Massive MIMO With Stochastic Geometry

By Zheng Chen, Emil Björnson. IEEE Transactions on Communications, 2017

341 citations recorded by the source corpus.

Cell-free (CF) massive multiple-input multiple-output (MIMO) is an alternative topology for future wireless networks, where a large number of single-antenna access points (APs) are distributed over the coverage area. There are no cells but all users are jointly served by the APs using network MIMO methods. Prior works have claimed that the CF massive MIMO inherits the basic properties of cellular massive MIMO, namely, channel hardening and favorable propagation. In this paper, we evaluate if one can rely on these properties when having a realistic stochastic AP deployment. Our results show that channel hardening only appears in special cases, for example, when the pathloss exponent is small. However, by using 5–10 antennas per AP, instead of one, we can substantially improve the hardening. Only spatially well-separated users will exhibit favorable propagation, but when adding more antennas and/or reducing the pathloss exponent, it becomes more likely for favorable propagation to occur. The conclusion is that we cannot rely on the channel hardening and the favorable propagation when analyzing and designing the CF massive MIMO networks, but we need to use achievable rate expressions and resource allocation schemes that work well also in the absence of these properties. Some options are reviewed in this paper.

Subfields: Computer Networks and Communications, Electrical and Electronic Engineering

View Paper: https://www.semanticscholar.org/paper/bb7a0b574780b2e8f3a41016d5d0139d79d88402

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Computer Networks and Communications

In Electrical and Electronic Engineering

Citations in the corpus

6G and Beyond: The Future of Wireless Communications Systems (2020, background): “Speciﬁcally, channels under a coordinated scheme of this kind will satisfy the conditions of favorable propagation [160].”
Prospective Multiple Antenna Technologies for Beyond 5G (2019, background): “Later works have studied more realistic setups, such as single-antenna APs with Rician fading channels [41], [42], multi-antenna APs with uncorrelated [43], [44] or correlated [45], [46] fading, multi-antenna UEs [47], [48], and hardware impairments [49], [50].”
Making Cell-Free Massive MIMO Competitive With MMSE Processing and Centralized Implementation (2019, background): “However, this may not occur when the number N of antennas at the APs is relatively small [26].”
Scalable Cell-Free Massive MIMO Systems (2019, methodology): “The SE expression in (17) holds for any receive combining vector v k , spatial correlation matrices, and selection of the DCC, and is thus a generalization of [7], [30], [47], [48].”
User-Centric Cell-Free Massive MIMO Networks: A Survey of Opportunities, Challenges and Solutions (2021, methodology): “On the other hand, some beamforming techniques like local MMSE combining [3], the weighted MMSE [20, 201], and PPZF [9] can be a better candidate because they limit the CSI sharing.”
Foundations of User-Centric Cell-Free Massive MIMO (2020, background): “Hence, even if favorable propagation effects can be observed also in cell-free networks with many distributed antennas [51], it remains important to design the signal processing schemes to actively suppress interference.”
Cell-Free Massive MIMO: A New Next-Generation Paradigm (2019, background): “Recent results in [28] reveal that the conditions of channel hardening can only be observed for very low pathloss exponents (e.”
Massive MIMO is a Reality - What is Next? Five Promising Research Directions for Antenna Arrays (2019, background): “The positive side is that the user channels become richer and more likely to be nearly orthogonal when the antennas are well separated [41], [43], [44], which is known as favorable propagation in the Massive MIMO literature.”