Page 55 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 6 – Wireless communication systems in beyond 5G era

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 6

MMWAVE MASSIVE ANALOG RELAY MIMO
Kei Sakaguchi , Takumi Yoneda , Masashi Iwabuchi , Tomoki Murakami
2
1
1
2
1 Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, Japan.
2 NTT Access Network Systems Laboratories, 1-1 Hikari-no-oka, Yokosuka-shi, Kanagawa, Japan.
NOTE: Corresponding author: Kei Sakaguchi, sakaguchi@mobile.ee.titech.ac.jp

Abstract – Millimeter-Wave (mmWave) communications are a key technology to realize ultra-high data-
rate and ultra-low latency wireless communications. Compared with conventional communication systems
in the microwave band such as 4G/LTE, mmWave communications employ a higher frequency band which
allows a wider bandwidth and is suitable for large capacity communications. It is expected to be applied to
various use cases such as mmWave cellular networks and vehicular networks. However, due to the strong
diffraction loss and the path loss in the mmWave band, it is difficult or even impossible to achieve high
channel capacity for User Equipment (UE) located in Non-Line-Of-Sight (NLOS) environments. To solve the
problem, the deployment of relay nodes has been considered. In this paper, we consider the use of massive
analog Relay Stations (RSs) to relay the transmission signals. By relaying the signals by a large number of
RSs, an artificial Multiple-Input Multiple-Output (MIMO) propagation environment can be formed, which
enables mmWave MIMO communications to the NLOS environment. We describe a theoretical study of a
massive relay MIMO system and extend it to include multi-hop relays. Simulations are conducted, and the
numerical results show that the proposed system achieves high data rates even in a grid-like urban
environment.

Keywords – 5G, mmWave, multi-hop, relay, massive MIMO

1. INTRODUCTION In mmWave bands, due to the propagation loss, the
influence of reflected paths and diffracted paths is
In recent years, the enhanced Mobile Broadband small and the direct path is dominant. As a result,
(eMBB) services in the 5th-Generation mobile the number of streams tends to be small in Single-
communication system (5G) are expected to satisfy User (SU) Multiple-Input Multiple-Output (MIMO)
the increasing traffic demands [1][2]. One of their [9]. One of the most intuitive and common ways to
key enablers is to explore the wireless solve this challenge is to deploy a large number of
communication techniques in higher frequency Base Stations (BSs) as distributed antennas [10].
bands, such as the millimeter-Wave (mmWave) However, the large-scale deployments of
band, where the ultra-wide bandwidth enables distributed antennas require the high financial cost
ultra-high data-rate and ultra-low latency in both Capital Expenditure (CAPEX) and Operating
communications [3][4]. MmWave communications Expenditure (OPEX). Moreover, it also has to face
are also expected to be applied into and deeply physical limitations due to the large required
transform a variety of wireless applications such as installation spaces and power consumption.
Vehicle-to-Everything (V2X) communications and
Internet of Things (IoT) [5][6]. Instead of deploying multiple BSs, we focus on
distributed relays to solve the above-mentioned
Though mmWave has relatively high path loss, it issues in mmWave bands. There are several
can perform long-range communications under a methods for the wireless relay. The Decode-and-
condition of Line-Of-Sight (LOS) [7]. However, the Forward (DF) relay method is one of the typical
received power of signals in mmWave bands is methods [11], and has been discussed as Integrated
dramatically reduced by Non-Line-Of-Sight (NLOS) Access and Backhaul (IAB) in the standardization of
locations or a blockage effect [8]. Since the NLOS the 3rd Generation Partnership Project (3GPP)
probability increases as the communication Release 16 [12]. In the DF method, since relay nodes
distance increases, it is difficult to introduce perform the decoding and re-encoding operations,
mmWave bands for macro-cells without expanding noise can be reduced and flexible radio resource
LOS coverage. Another important issue in mmWave control including bandwidth is possible [13].
bands is highly spatially correlated channels. However, the method has to tolerate the delay due

© International Telecommunication Union, 2021 43