Publication: Analysis and Design of Cell-Free Massive MIMO Systems under Spatially Correlated Fading Channels
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2021-07
Defense date
2021-09-23
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Abstract
Wireless communications have become a key pillar in our modern society. It can be hard to
think of a service that somehow does not rely on them. Particularly, mobile networks are one of
the most necessary technologies in our daily life. This produces that the demand for data rates
is by no means stopping from increasing. The cellular architecture is facing a crucial challenge
under limited performance by interference and spectrum saturation. This involves cell-edge
users experiencing poor performance due to the close vicinity of base stations (BSs) using
the same carrier frequency. Based on a combination of the coordinated multi-point (CoMP)
technique and traditional massive multiple-input multiple-output (MIMO) systems, cell-free
(CF) massive MIMO networks have irrupted as a solution for avoiding inter-cell interference
issues and for providing uniform service in large coverage areas. This thesis focuses on the
analysis and design of CF massive MIMO networks assuming a spatially correlated fading
model. A general-purpose channel model is provided and the whole network functioning is
given in detail.
Despite the many characteristics a CF massive MIMO system shares with conventional colocated
massive MIMO its distributed nature brings along new issues that need to be carefully
accounted for. In particular, the so-called channel hardening effect that postulates that the variance
of the compound wireless channel experienced by a given user from a large number of
transmit antennas tends to vanish, effectively making the channel deterministic. This critical
assumption, which permeates most theoretical results of massive MIMO, has been well investigated
and validated in centralized architectures, however, it has received little attention in the
context of CF massive MIMO networks. Hardening in CF architectures is potentially compromised
by the different large-scale gains each access point (AP) impinges on the transmitted
signal to each user, a condition that is further stressed when not all APs transmit to all users as
proposed in the user-centric (UC) variations of CF massive MIMO. In this document, the presence
of channel hardening in this new architecture scheme is addressed using distributed and
cooperative precoders and combiners and different power control strategies. It is shown that
the line-of-sight (LOS) component, spatially correlated antennas, and clustering schemes have
an impact on how the channel hardens. In addition, we examine the existent gap between the
estimated achievable rate and the true network performance when channel hardening is compromised. Exact closed-form expressions for both a hardening metric and achievable downlink
(DL) and uplink (UL) rates are given as well.
We also look into the pilot contamination problem in the UL and DL with different degrees
of cooperation between the APs. The optimum minimum mean-squared error (MMSE) processing
can take advantage of large-scale fading coefficients for canceling the interference of
pilot-sharing users and thus achieves asymptotically unbounded capacity. However, it is computationally
demanding and can only be implemented in a fully centralized network. Here,
sub-optimal schemes are derived that provide unbounded capacity with much lower complexity
and using only local channel estimates but global channel statistics. This makes them suited for
both centralized and distributed networks. In this latter case, the best performance is achieved
with a generalized maximum ratio combiner that maximizes a capacity bound based on channel
statistics only.
Description
Mención Internacional en el título de doctor
Keywords
Cell-free, Massive MIMO, Wireless communications, Rayleigh fading, Ricean fading, Channel estimation, Uplink combining, Downlink precoding, Power control, Channel hardening, Spectral efficiency, Pilot contamination