Experimental Trials of 5G Ultra High-Density Distributed Antenna Systems

Fifth-generation mobile communication systems (5G) require significantly higher system capacity than 4G in order to accommodate rapidly increasing mobile data traffic. Cell densification is considered to be one of the most effective ways to increase the system capacity. However, severe inter-cell interference degrades the system capacity due to the increase of the line-of-sight (LOS) environment between the transmission point (TP) and the user equipment (UE). We have proposed large-scale coordinated multi-user multiple-input multiple-output (LSC-MU-MIMO), which is the combination of MU-MIMO and joint transmission from all the TPs connected to a centralized baseband unit, to increase the system capacity in dense cell environments. In this paper, we introduce our recent results of indoor experimental trials with up to sixteen TPs coordination and show that the system capacity is almost proportional to the number of TPs with LSC-MU-MIMO. We also study how to locate TPs in order to exploit potential performance of LSC-MU-MIMO. Both simulation and experimental results show that the throughput is significantly improved by deploying TPs to make at least a channel of TP LOS with a high probability. Therefore TPs are preferable to be located that UEs have at least one TP with LOS condition.

I. Introduction

In fifth-generation mobile communication systems (5G), it is necessary to significantly increase system capacity defined as bps/Hz/km² compared with 4G in order to accommodate rapidly increasing mobile data traffic. The following three measures can be considered to increase system capacity: 1) Utilization of wider bandwidth; 2) Improvement of spectral efficiency; 3) Cell densification. The contribution of cell densification is most effective because the system capacity is expected to increase in proportion with the number of cells per unit area. However, when the cell density becomes much higher, severe interference from the surrounding cells slows system capacity improvement [1]. Therefore, inter-cell coordination technology, which can mitigate inter-cell interference, is important. We have been studying ultra high-density distributed antenna systems with coordinated radio resource scheduling [2]. Our final goal is the realization of a dynamic virtual cell concept that enables user equipment (UE) to communicate while always being located at the center of a virtual cell by controlling the transmission antenna weight and UE scheduling, in contrast to the conventional cell configuration with fixed cell borders. In ultra high-density distributed antenna systems, a large number of transmission points (TPs) densely set in a low-super high frequency (low-SHF) band (3–6 GHz) cover a hotspot area demanding high data rates. Each TP is connected to a centralized baseband unit (C-BBU) via optical fiber. Centralized baseband processing in a C-BBU enables more enhanced inter-TP coordination technologies than 4G (LTE-A).