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Internet of Things (IoT) is an emerging network that aims to connect massive devices for exchanging information with each other. There have been various IoT access techniques, such as RFID, Blue Tooth, WiFi, LoRa and cellular IoT, in which cellular IoT has advantages in wide coverage and the economical reuse of exsiting cellular infrastructure. However, cellular base station (BS) consumes most of the energy in wireless communication networks. Therefore, improving the energy efficiency (EE) of the BS is of great importance to make cellular IoT green. In this paper, we propose a novel orbital angular momentum (OAM)-based concentric spatial division multiplexing (CSDM) downlink transmission scheme for cellular IoT terminals. Specifically, the hybrid radio frequency (RF) and baseband CSDM scheme exploits analog OAM beamforming to generate concentric annular beams for covering cell-center, cell-mediate and cell-edge groups of users, and digital space-frequency vector perturbation (SFVP) precoding and orthogonal frequency division multiple access (OFDMA) to mitigate inter- and intra-group interferences. Mathematical analysis and numerical simulations validate that the IoT BS with the proposed CSDM-SFVP scheme has better bit error rate (BER) and EE performance than traditional massive multiple-input multiple-output (MIMO) BS with full-baseband zero-forcing (FBZF) precoding in a IoT cell.

OAM-Based Concentric Spatial Division Multiplexing for Cellular IoT Terminals