Beamforming Using Uniform Spherical Arrays: Array Construction, Beam Characteristics, and Multi-Rank Transmission

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.