2018 International Symposium on Antennas and Propagation ... · This study proposed a downsized...
Transcript of 2018 International Symposium on Antennas and Propagation ... · This study proposed a downsized...
Compact Massive MIMO Antenna Using Cubic
Arrangement Suitable for Indoor Base Station
Kosei Oikawa 1, Kazunori Yuri 1, Naoki Honma 1, Kentaro Nishimori 2 1Graduate School of Engineering, Iwate University, Iwate, Japan
2Department of Information Engineering, Niigata University, Niigata, Japan
Abstract - This paper proposes a compact massive multiple-
input and multiple-output (MIMO) antenna for an indoor base
station, which uses orthogonally polarized patch antennas at five faces of a cube. A block diagonal (BD) method was applied to the MIMO channel calculated by ray-tracing simulation. The
result shows that the proposed antenna yields a higher multi-user capacity than planar array antennas and linear array antennas with the same number of elements.
Index Terms — Massive MIMO, ray-tracing, Block Diagonalization method.
1. Introduction
In recent years, with the spread of smartphones and tablets,
the amount of data traffic continues to expand with the
increase of the rich contents, e.g. high definition video.
Moreover, the various devices are connected by wireless
communication, and the problems of frequency resource
exhaustion due to an increase in connected devices are
expected. Therefore, it is necessary to improve the frequency
utilization efficiency by multiple-input, multiple-output
(MIMO) technology.
The massive MIMO base station uses a huge number of
antenna elements for the base station, enabling improvement
in channel capacity and simultaneous connection to many
terminals [1]. The massive MIMO system will be considered
for use in fifth generation mobile communications (5G). The
large shopping malls, stations, meeting places etc. are
supposed to be considered as the places, where many users
communicate simultaneously. Particularly, indoor massive
MIMO base stations are desired to be small in size. However,
the massive MIMO has the problem that the size of the base
station antenna will increases with the number of elements.
In this paper, we propose a compact massive MIMO
antenna for indoor base station, which uses orthogonally
polarized patch antennas at five faces of a cube. By using
dual polarization patch antennas, the number of elements can
be reduced to half. Although the antenna is densely arranged
in a small space, the channel capacity is improved by
utilizing the multipaths due to the reflection by the walls in
the indoor environment.
2. Simulation model
The channel capacity is calculated by using the MIMO
channel simulated by ray-tracing method, which is one of
radio wave propagation analysis method [2]. It can simulate
the radio wave propagation from the base station to the
terminal by considering reflection, transmission, refraction,
diffraction and attenuation. And we also use block
diagonalization (BD) method for multiuser channel capacity
calculation since a space division multiple access (SDMA) is
used so that all terminals share the same frequency at the
same time [3].
Figure 1 shows the ray-tracing analysis model and the
antenna configuration used for the base station. The
proposed cubic antenna for base station consists of five
subarray antennas at five faces, each of which has 2×2 dual
polarized patch antennas with a 0.5λ inter-element spacing (λ
is a wavelength in vacuum). The operation frequency of all
antenna elements is 5.0 GHz. The total number of the
antenna elements and ports of the cube are 20 and 40,
respectively. For comparison, the array antennas with a
planar 4×5 and 1×20 elements using the dual polarization
patch antennas are used. The size of the planar 4×5 and 1×20
arrays are about 120×150 mm2 and 30×600 mm2 ,
respectively, whereas the width of the proposed antenna is
about 60 mm, which is 1/2 or less of the planar arrays.
As for the ray-tracing simulation, the room size is
20×30×4 m3 and the material of the enclosing, i.e. walls,
ceiling and floor, is concrete. The base station antenna was
placed downward on the ceiling in the center of the room, the
transmission power of the base station was 1 W, the noise
power was 10−9 W, and the number of reflections was 5
times. As a terminal station antenna, a 4 element dipole array
antenna with the element spacing of 0.5λ was used. The
number of the base station’s antenna ports is 𝑁t and the
20 m30 m
4 m0.5 m1.5 m2 m
Tx
(10, 15, 3.8) m
Rx existence area
Rx
4 dipoles
xyz
Planar antenna
or
Cubic antenna
orLinear antenna
Fig. 1. Simulation model
2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea
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number of the terminal station’s elements is 𝑁r . The 𝑥𝑦
coordinates of the terminal station are uniform and random
in the room, and the 𝑧 coordinate is uniform and random in
the range between 0.5 and 2 m. The orientation of the
terminal is also randomly determined, i.e. both the elevation
angle and the azimuth angle of the terminal station array
antenna are random. The ray-tracing simulation with 200
trials is performed, i.e. 200 channels are calculated. The
Monte Carlo simulation is performed, where 300
combinations are selected from the 200 channels and
multiuser channel capacity was calculated. The channel
capacity of the multiuser MIMO is calculated by using the
BD method, where the channels for the multiple terminal
stations are randomly selected and combined. The number of
the channels is 𝑁u, where the number of the transmitters is
greater than the total number of the receivers, i.e. 𝑁t ≥ 𝑁r ×𝑁u .
3. Simulation result
Figure 2 shows an example of the visualized paths
simulated by the ray-tracing analysis. The position of the
terminal station at this time is (0.2, 23.4, 1.7) m. The figure
shows the many paths distribute on the horizontal planes that
reflect on the side walls. This means the horizontally directed
antennas are expected to transmit well to the receivers.
Figure 3 shows a color map of single user channel capacity
using the proposed antenna. The values of 20 bits/s/Hz or
more are distributed throughout the room, and besides, the
channel capacity in the vicinity of the base station shows a
high value.
Figure 4 shows the median channel capacity of multi-user
MIMO versus the number of users. The channel capacity of
the proposed antenna is always higher than that of the planar
antenna and the linear antenna. Focusing on 7-user case, the
capacity of the proposed antenna is about 22 and 8 bits/s/Hz
higher than that of the planar and linear antennas,
respectively. This effect is mainly due to the higher
directivity to the horizontal direction of the proposed antenna
since most of the reflected paths distribute on the horizontal
directions. Also, this capacity enhancement can be attained
all over the room, and this clarifies the proposed antenna
well exploits the multipath-rich environment.
4. Conclusion
This study proposed a downsized massive MIMO antenna
suitable for indoor environment, and its configuration consist
of the five array antennas in a cubic shape. Comparing to the
planar and linear antennas with the same number of elements,
the antenna width was miniaturized to 1/2 and 1/10 or less,
respectively. Also, the ray-tracing simulation reveals the
multiuser channel capacity can be improved by 22 and 8
bits/s/Hz compared to that of the planar and linear arrays
when the number of users is 7.
Acknowledgment
This work is partially supported by JSPS KAKENHI
(JP17H03262).
References
[1] E. G. Larsson, O. Edfors, F. Tufvesson, T. L. Marzetta, “Massive MIMO for next generation wireless systems,” IEEE Commun.
Magazine, vol. 52, no. 2, pp. 186-195, Feb. 2014. [2] J. W. McKown, R. L. Hamilton, “Ray tracing as a design tool for
radio networks,” IEEE Network, vol. 5, no. 6, pp. 27 -30, Nov. 1991.
[3] K. Wong, R. D. Murch, K. B. Letaief, “A joint-channel diagonalization for multiuser MIMO antenna systems,” IEEE Trans.
Wireless. CommunLett., vol. 51, pp. 190-194, 2009.
Tx
(10, 15, 3.8) m
Rx
(0.2, 23.4, 1.7) m
z [m
]
Fig. 2. Ray-tracing analysis example
2002
30
4
20 10
10
00
5
10
15
20
25
30
z [m
]
Chan
nel cap
acity[b
its/s/Hz]
Fig. 3. Color map of single user channel capacity
(Cubic antenna)
30
40
50
60
70
80
90
2 3 4 5 6 7 8 9 10
Sum
cap
acit
y [
bit
s/s/
Hz]
Number of users
Planar
LinearCubic
Fig. 4. Multiuser channel capacity versus the
number of users
2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea
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