Article
Investigation on Beam Alignment of a Microstrip-Line Butler
Matrix and an SIW Butler Matrix for 5G Beamforming Antennas
through RF-to-RF Wireless Sensing and 64-QAM Tests
Munsu Jeon
1,2
, Yejune Seo
1,2
, Junghyun Cho
1,2
, Changhyeong Lee
3
, Jiyeon Jang
1,2
, Yejin Lee
1,2
,
Hyung-Wook Kwon
2
and Sungtek Kahng
1,2,
*
Citation: Jeon, M.; Seo, Y.; Cho, J.;
Lee, C.; Jang, J.; Lee, Y.; Kwon, H.-W.;
Kahng, S. Investigation on Beam
Alignment of a Microstrip-Line Butler
Matrix and an SIW Butler Matrix for
5G Beamforming Antennas through
RF-to-RF Wireless Sensing and
64-QAM Tests. Sensors 2021, 21, 6830.
https://doi.org/10.3390/s21206830
Academic Editor: Ángela
María Coves Soler
Received: 2 September 2021
Accepted: 8 October 2021
Published: 14 October 2021
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1
Department of Information & Telecommunication Engineering, Incheon National University,
Incheon 22012, Korea; Jeon@inu.ac.kr (M.J.); M.June@inu.ac.kr (Y.S.); elsa@inu.ac.kr (J.C.);
fhtk02@inu.ac.kr (J.J.); yeeejjin@inu.ac.kr (Y.L.)
2
Convergence Research Center for Insect Vectors, Incheon National University, Incheon 22012, Korea;
hwkwon@inu.ac.kr
3
Center for Advanced Meta-Materials, Korea Institute of Machinery & Materials, Daejeon 34103, Korea;
antman@kimm.re.kr
* Correspondence: s-kahng@inu.ac.kr; Tel.: +82-32-835-8288
Abstract:
In this paper, an intuitive approach to assessing advantages of beamforming in 5G wireless
communication is proposed as a novel try and practical demonstration of importance of alignment
between the transmitter’s and receiver’s beams working in millimeter-wave frequency bands. Since
the diffraction loss of millimeter-wave signals matters seriously in propagation, the effects of the
misalignment and alignment between beams need to be checked for, which was conducted with
a horn antenna and the 4
×
4 Butler matrix which mimic the relationship of the base station and
handset antennas. Designing and using the microstrip-line and the substrate integrated waveguide
(SIW) Butler matrices, RF-to-RF wireless connectivity between the horn and the microstrip line
beamformer as case 1 and the horn and the SIW beamformer as case 2, concerning the changing angle
of the beam from either of the two Butler matrices, was tested, showing over 12 dB enhancement
in received power. This direct electromagnetic link test was accompanied by examining 64-QAM
constellations for beam-angle changing from
−
30
◦
to +30
◦
for the two cases, where the error vector
magnitude in the QAM-diagram becomes less than 10% by beam-alignment for the changing angle.
Keywords: millimeter-wave antenna; 5G antenna; beamforming antenna; Butler matrix; 64-QAM
1. Introduction
The 5th generation (5G) mobile communication is featured by technological fascina-
tion such as several Gbps data transfer-rate, low latency and low interference [
1
–
3
]. These
three keywords can be attained by a macroscopic measure that the system architecture of
the device is optimized; channel models are set up and monitored in real time; neighboring
heterogeneous networks are found and connected with compatibility and adaptiveness;
and a microscopic one that a wide-bandwidth of the millimeter-wave frequency is em-
ployed, and a wide-beam from the low-frequency handset is replaced by a narrow-beam
from the high-frequency smartphone. Because the wireless signal should be emanated
from the mobile device over the air and travels over the space on the net to the receiver,
electromagnetic connectivity is very crucial. In order to realize the wireless connectivity
for 5G, antennas operable in millimeter-wave frequency bands are needed. Making use
of millimeter-wave antennas, the wide-bandwidth and narrow beamwidth would be ac-
complished by designing them to be arrays whose footprint is relatively small for even the
commercial wireless phone. The beamwidth becomes narrow and pointy to have higher
directionality in the far-field pattern. This is so-called beamforming.
Sensors 2021, 21, 6830. https://doi.org/10.3390/s21206830 https://www.mdpi.com/journal/sensors
