IEEE Wireless Communications - April 2017 - page 108

IEEE Wireless Communications • April 2017
106
1536-1284/17/$25.00 © 2017 IEEE
Jing Zhang, Xiaohu Ge, Qiang
Li, and Yanxia Zhang are with
Huazhong University of Sci-
ence and Technology.
Mohsen Guizani is with the
University of Idaho.
The corresponding authors
are Qiang Li and Mohsen
Guizani.
A
bstract
As there has been an explosive increase in
wireless data traffic, mmw communication has
become one of the most attractive techniques
in the 5G mobile communications systems.
Although mmw communication systems have
been successfully applied to indoor scenarios,
various external factors in an outdoor environ-
ment limit the applications of mobile communi-
cation systems working at the mmw bands. In
this article, we discuss the issues involved in the
design of antenna array architecture for future
5G mmw systems, in which the antenna ele-
ments can be deployed in the shapes of a cross,
circle, or hexagon, in addition to the conven-
tional rectangle. The simulation results indicate
that while there always exists a non-trivial gain
fluctuation in other regular antenna arrays, the
circular antenna array has a flat gain in the main
lobe of the radiation pattern with varying angles.
This makes the circular antenna array more
robust to angle variations that frequently occur
due to antenna vibration in an outdoor environ-
ment. In addition, in order to guarantee effec-
tive coverage of mmw communication systems,
possible solutions such as distributed antenna
systems and cooperative multi-hop relaying are
discussed, together with the design of mmw
antenna arrays. Furthermore, other challenges
for the implementation of mmw cellular net-
works, for example, blockage, communication
security, hardware development, and so on, are
discussed, as are potential solutions.
I
ntroduction
With the rapid development of wireless transmis-
sion and mobile networking techniques, various
wireless services have emerged and smart devic-
es become more popular, which has led to an
explosive increase in the data traffic of wireless
networks [1]. In order to support the tremen-
dous demands on data traffic, the millimeter-wave
(mmw) communication technique, which can use
enormous unlicensed bandwidth beyond the tra-
ditional licensed wireless microwave bands, has
drawn great attention in the research community
and in industry, and is recognized as a key tech-
nology in fifth generation (5G) mobile communi-
cations systems [2].
In recent years, a significant amount of
research effort has been dedicated to the appli-
cations of mmw communication in outdoor
scenarios, which is mainly motivated by the fol-
lowing two reasons [3, 4]. First, the small cells
in future heterogeneous networks are expect-
ed to provide various services at rates up to
Gb/s, such as, uncompressed video, which is
almost an “impossible mission” in the current
microwave bands. This motivates research in the
mmw bands, where numerous spectrum resourc-
es are available to support extremely high data
rate transmissions. Although the effective com-
munication distance of mmw systems is limited
by a more severe signal attenuation compared
to microwave signals, it is sufficient to cover
small cells that span up to hundreds of meters.
Meanwhile, line-of-sight (LOS) propagations can
effectively mitigate the co-channel interference
to/from other systems, due to the employment
of highly directional antennas. Second, with
the fast development of hardware techniques,
for example, high gain element antennas, high
density antenna arrays, power amplifiers with
large dynamic ranges, and CMOS technology
with good reliability, the implementation of out-
door mmw communication systems is a possi-
bility in the near future. For example, based on
the recent real-world measurement results at 28
GHz and 73 GHz in New York City, the detailed
spatial statistical models of wireless channels
were derived that provide a realistic assessment
of mmw small cell networks in a dense urban
deployment [5]. Substantial coverage of up to
1.7 km was reported in [6] for mmw cellular net-
works by both indoor and outdoor mmw proto-
type tests, which asserts the feasibility of mmw
bands for cellular usage.
Although mmw technology is widely accepted
as a promising candidate for 5G systems, there
exists a gap between the existing mmw prototypes
and the expected commercial mmw cellular net-
works. Some fundamental changes are required in
the deployment of mmw cellular networks. First,
for the same communication distance, the free-
space path loss is increased by 3 dB with twice the
transmission frequency. As a result, sophisticated
antenna designs are required for mmw communi-
cation systems to overcome the severe attenuation
and path loss between the transmitters and receiv-
ers. In addition, the extra attenuation experienced
by mmw communications should also be taken
into account. For instance, under conditions of
heavy rainfall where the diameter of the raindrops
J
ing
Z
hang
, X
iaohu
G
e
, Q
iang
L
i
, M
ohsen
G
uizani
,
and
Y
anxia
Z
hang
5G M
illimeter
-W
ave
A
ntenna
A
rray
:
D
esign
and
C
hallenges
A
ccepted
from
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pen
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all
Digital Object Identifier:
10.1109/MWC.2016.1400374RP
1...,98,99,100,101,102,103,104,105,106,107 109,110,111,112,113,114,115,116,117,118,...132
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