IEEE Communications Magazine - June 2017 - page 157

IEEE Communications Magazine • June 2017
0163-6804/17/$25.00 © 2017 IEEE
As the standardization of full-dimension MIMO
systems in the Third Generation Partnership Proj-
ect progresses, the research community has start-
ed to explore the potential of very large arrays as
an enabler technology for meeting the require-
ments of fifth generation systems. Indeed, in its
final deliverable, the European 5G project METIS
identifies massive MIMO as a key 5G enabler
and proposes specific technology components
that will allow the cost-efficient deployment of
cellular systems taking advantage of hundreds of
antennas at cellular base stations. These technol-
ogy components include handling the inherent
pilot-data resource allocation trade-off in a near
optimal fashion, a novel random access scheme
supporting a large number of users, coded chan-
nel state information for sparse channels in fre-
quency-division duplexing systems, managing
user grouping and multi-user beamforming, and a
decentralized coordinated transceiver design. The
aggregate effect of these components enables
massive MIMO to contribute to the METIS objec-
tives of delivering very high data rates and manag-
ing dense populations.
Multiple-input multiple-output (MIMO) systems
involving a number of antenna elements an order
of magnitude larger than in the early releases
of wireless standards is a quickly maturing tech-
nology. Indeed, an ongoing work item of the
Third Generation Partnership Project (3GPP) for
Release 13/14 of the Long Term Evolution (LTE)
and 3GPP New Radio is identifying the technolo-
gy enablers and performance benefits of deploy-
ing up to 64 antenna ports and an even greater
number of antenna elements at wireless access
points and base stations (BSs) [1]. While this is
a significant increase of the number of antenna
ports compared to today’s typical deployments,
to fully realize the promises of scaling up MIMO
to very large (massive) arrays in practice requires
further research and system development work
[2]. Recent developments in the industrial and
academic research communities in related fields
such as 3D MIMO, hybrid beamforming (BF)
based on combining analog and digital precoding
techniques, and understanding the asymptotic
behavior of random matrices suggest that massive
MIMO can bring unprecedented gains in terms of
spectral and energy efficiency and robustness to
hardware failures and impairments. Also, as point-
ed out in [3], higher frequency bands, like millime-
ter-wave (mmWave), are appealing for large-scale
antenna systems, since the physical array size can
be greatly reduced due to the decrease in wave-
The METIS technology components (TCs)
address two essential issues in massive MIMO:
channel state information (CSI) acquisition and
transceiver structure [6]. CSI acquisition in mas-
sive MIMO is challenging because of the many
channel links that need to be estimated and the
problem of pilot contamination. Likewise, the
very large number of channel links represents
one major impediment in transceiver design as
it sharply increases the computational complex-
ity, calls for robust designs against CSI errors to
achieve the desired gains, and increases the traffic
data transport over the backhaul in coordinated
The massive MIMO TCs in METIS address two
major fifth generation (5G) goals defined in the
project: the ability to deliver very large data rates
to each user, and the ability to deliver a high qual-
ity of service to a very dense population of users.
Note that the second goal is rarely addressed,
while it is becoming more and more relevant in
view of the capability of massive MIMO to spatial-
ly multiplex a large number of users. Furthermore,
the METIS technology components target lega-
cy bands below 6 GHz. This focus is justified by
the allocation of frequency bands below 6 GHz
by the recent International Telecommunication
Union World Radio Conference WRC-15, while
for higher frequency bands no allocations for 5G
have been made so far.
While time-division duplexing (TDD) is the
widely preferred solution for massive MIMO sys-
tems, as it scales with the number of antennas
at the BS, frequency-division duplexing (FDD)
remains an attractive solution for operators.
Therefore, METIS developed TCs for both TDD
and FDD systems.
In TDD systems, one of the main impediments
specific to massive MIMO is pilot contamination
An Overview of Massive MIMO Technology
Components in METIS
Gábor Fodor, Nandana Rajatheva, Wolfgang Zirwas, Lars Thiele, Martin Kurras, Kaifeng Guo, Antti Tölli,
Jesper H. Sørensen, and Elisabeth de Carvalho
In its final deliverable,
the European 5G project
METIS identifies mas-
sive MIMO as a key 5G
enabler and proposes
specific technology com-
ponents that will allow the
cost-efficient deployment
of cellular systems taking
advantage of hundreds of
antennas at cellular base
Gábor Fodor is with Ericsson Research and also with the Royal Institute of Technology;
Nandana Rajatheva and Antti Tölli are with the University of Oulu; Wolfgang Zirwas is with Nokia Siemens Networks;
Lars Thiele and Martin Kurras are with Fraunhofer Heinrich Hertz Institute; Kaifeng Guo is with RWTH Aachen University;
Jesper H. Sørensen and Elisabeth de Carvalho are with Aalborg University.
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