In order to increase the system capacity of the 5G mobile communication system, multiple-input multiple-output (MIMO) transmission techniques using a large-scale array over the millimeter-wave band have attracted a great amount of attention. To cope with various types of receivers expected in 5G communications such as user equipment (UE) in small cells, indoor Internet-of-Things (IoT) devices at diverse locations, and drones performing aerial navigation, newer types of antenna arrays require all-directional transmission capability. Existing antenna structures with typical panel arrays, however, have restrictions on their transmission angles in both horizontal and vertical directions. In this paper, we propose to employ three-dimensional (3D) array structures composed of multiple triangular panels for efficient massive MIMO transmission of the next-generation wireless systems. We analyze beamforming characteristics of a uniform triangular array (UTA) suitable for such 3D array configurations and present a basic codebook applicable to UTAs. Using antenna structures with multiple UTA panels, multiuser transmission performance is evaluated to demonstrate the effectiveness of the proposal.

5G new radio (NR) provides features such as enhanced mobile broadband (eMBB) services and massive machine-type communications (mMTC). Multiple-input multiple-output (MIMO) transmission is an essential technique to enable these features, using antenna arrays with a sufficient number of elements to generate accurate and high-gain beams to desired user locations. In newer services, various forms of mobile devices are expected to appear in diverse locations, and beamforming need to accommodate such directional diversification. However, utilization of existing array structures has a limitation in transmitting signals to arbitrary receiver positions in the three-dimensional space. Therefore, a new type of array capable of forming accurate beams in isotropic fashion is desired, and the corresponding transmission strategies need to be identified. In this paper, the construction and utilization of the three-dimensional uniform spherical array (USA) are investigated. Beamforming vectors and the basis codebook for such arrays are defined, and characteristics of the generated beams are analyzed via comparison to the beams produced by conventional arrays. Array parameters to minimize the inter-beam interference are determined, and multiuser transmission performance is evaluated using the proper configuration of the array. We also apply the radiation pattern to the antenna elements to present the sum-rate performance in practical settings. INDEX TERMS 5G NR, massive MIMO, beamforming, codebook, millimeter-wave, spherical array.

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