Article
A Method of Implementing a 4 × 4 Correlation Matrix for
Evaluating the Uplink Channel Properties of MIMO
Over-the-Air Apparatus
Kazuhiro Honda
Citation: Honda, K. A Method of
Implementing a 4 × 4 Correlation
Matrix for Evaluating the Uplink
Channel Properties of MIMO
Over-the-Air Apparatus. Sensors 2021,
21, 6184. https://doi.org/10.3390/s
21186184
Academic Editor: Ángela María
Coves Soler
Received: 6 August 2021
Accepted: 13 September 2021
Published: 15 September 2021
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Graduate School of Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan;
hondak@eng.u-toyama.ac.jp; Tel.: +81-76-445-6759
Abstract:
This paper presents a method of implementing a 4
×
4 correlation matrix for evaluating
the uplink channel properties of multiple-input multiple-output (MIMO) antennas using an over-
the-air measurement system. First, the implementation model used to determine the correlation
coefficients between the signals received at the base station (BS) antennas via the uplink channel
is described. Then, a methodology is introduced to achieve a 4
×
4 correlation matrix for a BS
MIMO antenna based on Jakes’ model by setting the initial phases of the secondary wave sources
in the two-dimensional channel model. The performance of the uplink channel for a four-element
MIMO terminal array antenna is evaluated using a two-dimensional bidirectional fading emulator.
The results show that the measured correlation coefficients between the signals received via the
uplink channel at the BS antennas using the proposed method are in good agreement with the
BS correlation characteristics calculated using Monte Carlo simulation and the theoretical formula,
thereby confirming the effectiveness of the proposed method.
Keywords:
uplink channel; 4
×
4 correlation matrix; initial phase; Jakes’ model; multiple-input
multiple-output (MIMO); over-the-air (OTA); bidirectional fading emulator; channel capacity
1. Introduction
Fifth-generation (5G) mobile communication systems, which will enable high speed,
low latency, and large capacity, are becoming commercially available globally. Thus far,
several new services that exploit these features, such as sports viewing [
1
] and autonomous
driving [
2
], have been considered. To transmit high-capacity data, such as video data from
a mobile terminal to a base station (BS), ultra-high-speed communication is required for the
uplink channel. In the 3rd Generation Partnership Project (3GPP), 5G supports downlink
and uplink peak rates of 20 and 10 Gbps, respectively [
3
]. Multiple-input multiple-output
(MIMO) systems are essential for achieving ultra-high-speed communication [
4
,
5
]. Hence,
evaluating the performance of MIMO terminals is necessary, not only for the downlink
channels, but also for the uplink channels.
A straightforward method for evaluating a MIMO terminal is field testing in an
actual scenario [
6
]. However, with field testing the repeatability and controllability of the
measured data cannot be observed, and, moreover, is a very time-consuming and labor-
intensive process. Hence, over-the-air (OTA) testing, which evaluates the performance of
MIMO mobile terminals by creating a realistic propagation environment in the laboratory,
is very important.
OTA measurement methods have been standardized by 3GPP and the Cellular Telecom-
munication and Internet Association (CTIA) [
7
,
8
]. In 3GPP, OTA test methodologies are
classified into three categories:
(1)
Reverberation chamber based methods,
(2)
Two-stage methods,
(3)
Multiple probe antenna based methods.
Sensors 2021, 21, 6184. https://doi.org/10.3390/s21186184 https://www.mdpi.com/journal/sensors
