Cylindrical Massive MIMO System with Low-Complexity Angle-Based User Selection for High-Altitude Platform Stations

Atsushi NAGATE

IEICE TRANSACTIONS on Communications   Vol.E105-B    No.4    pp.449-460
Publication Date: 2022/04/01
Publicized: 2021/10/15
Online ISSN: 1745-1345
DOI: 10.1587/transcom.2021WWP0003
Type of Manuscript: Special Section PAPER (Special Section on Wired-and-Wireless Network System Technologies in Beyond 5G Era)
Category: Adaptive Array Antennas/MIMO
high-altitude platform station (HAPS),  massive MIMO,  cylindrical array antenna,  user selection,  service link,  6G,  

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High-altitude platform stations (HAPSs) are recognized as a promising technology for coverage extension in the sixth generation (6G) mobile communications and beyond. The purpose of this study is to develop a HAPS system with a coverage radius of 100km and high capacity by focusing on the following two aspects: array antenna structure and user selection. HAPS systems must jointly use massive multiple-input multiple-output (mMIMO) and multiuser MIMO techniques to increase their capacity. However, the coverage achieved by a conventional planar array antenna is limited to a circular area with a radius of only tens of kilometers. A conventional semi-orthogonal user selection (SUS) scheme based on the orthogonality of channel vectors achieves high capacity, but it has high complexity. First, this paper proposes a cylindrical mMIMO system to achieve an ultra-wide coverage radius of 100km and high capacity. Second, this paper presents a novel angle-based user selection (AUS) scheme, where a user selection problem is formulated as a maximization of the minimum angular difference between users over all user groups. Finally, a low-complexity suboptimal algorithm (SA) for AUS is also proposed. Assuming an area with a 100km radius, simulation results demonstrate that the proposed cylindrical mMIMO system improves the signal-to-interference-plus-noise ratio by approx. 12dB at the boundary of the area, and it achieves approx. 1.5 times higher capacity than the conventional mMIMO which uses a planar array antenna. In addition, the results show that the proposed AUS scheme improves the lower percentiles in the system capacity distribution compared with SUS and basic random user selection. Furthermore, the computational complexity of the proposed SA is in the order of only 1/4000 that of SUS.