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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