IEEE Communications Magazine - June 2017 - page 156

IEEE Communications Magazine • June 2017
ultiple-input multiple-output (MIMO) antenna tech-
nology is becoming a de facto element in all mod-
ern wireless communications networks, and has been
implemented into wireless systems such as LTE and Wi-Fi. The
theory behind MIMO is that if transmitters and receivers are
equipped with more antennas, they can take advantage of more
than one signal propagation path, which leads to better per-
formance in terms of data rate and link reliability. As always,
though, these benefits have costs associated with them; in par-
ticular, the intensive signal processing required at both ends of
the link lead to increased hardware complexity and high energy
Traditional MIMO systems may have two, four, or even eight
antennas. Massive MIMO systems, however, employ hundreds
to thousands of antennas in transmitters and receivers. The pres-
ence of a large number of antennas in massive MIMO terminals
creates many more degrees of freedom in the spatial domain.
Among other benefits, this offers increased data rates and signal-
to-noise ratios, as well as link robustness in the face of channel
degradation. A further advantage of the large number of anten-
nas is the significant amount of spatial beamforming possible.
Focusing the transmission and reception of signal energy into a
much smaller region of space brings substantial improvements
in system throughput and overall energy efficiency, particularly
when many user terminals can be coordinated and scheduled
together. Massive MIMO was originally envisioned for appli-
cation to time-division duplex (TDD) operation of cellular sys-
tems, but also has the potential to be used in frequency-division
duplex (FDD) mode.
While massive MIMO offers many advantages, it faces a
number of challenges requiring further research. To name a
few, there are: combining a huge number of low-cost, low-pre-
cision RF components that can still work together effectively; the
need for efficient acquisition of channel state information; rapid
resource allocation for newly joined terminals; the exploitation of
the many extra degrees of freedom provided by a large number
of antennas; and constraining the energy consumption of hun-
dreds of RF chains to fit the limited energy budgets of mobile
In this context, our first article, “An Overview of the Massive
MIMO Technology Component in METIS,” is written by authors
associated with the European 5G project Mobile and Wireless
Communication Enablers for the 2020 Information Society
(METIS), which has identified massive MIMO to be an enabler
for 5G cellular system deployments. The article discusses solu-
tions to two major challenges: first, channel state information
(CSI) acquisition, and second, transceiver design complexities
that arise when a very large number of channels are supported.
With regard to CSI acquisition, the authors discuss two solutions:
a low-rate multi-cell coordination for pilot contamination mitiga-
tion in TDD systems, and a random pilot access mechanism for
crowd scenarios. To address the transceiver design challenge,
they again present two separate techniques:
1. Joint user clustering and multi-user beamforming
2. Decentralized coordinated transceiver design
The article presents simulation results for all these cases.
Our next article, “Resource and Mobility Management in
the Network Layer of 5G Cellular Ultra-Dense Network,” is
also related to contributions to the METIS project. In this arti-
cle, the authors analyze and present solutions to the network
layer challenges arising from cell densification, interference, and
mobility management. This discussion is particularly germane
to next-generation (5G) cellular systems, which are expected
to gain significant system capacity improvements without con-
comitant increase in spectrum allocation. This is only possible
if ultra-dense network topologies are used in combination with
advanced radios (e.g., the massive MIMO systems described
The final article in this issue is “Linearity Challenges of LTE
Advanced Mobile Transmitters: Requirements and Potential
Solutions.” It discusses the technical challenges arising from the
severe linearity requirements for LTE–Advanced mobile termi-
nals. These linearity requirements are driven by the carrier aggre-
gation techniques used in both TDD and FDD operations. The
article surveys the requirements and the implementation prob-
lems they produce, and then present some possible solutions.
We appreciate the contributions of the authors of the arti-
cles in this issue, and would also like to take this opportunity
to express our gratitude to our many reviewers for helping us
select and improve these articles. The support and encourage-
ment of the Editor-in-Chief and the publication staff are much
appreciated as well. And, as usual, we encourage our readership
to submit articles discussing emerging trends in wireless commu-
: C
, S
Amitabh Mishra
Tom Alexander
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