Inter-Beam Interference Cancellation and Physical Layer Security Constraints by 3D Polarized Beamforming in Power Domain NOMA Systems

The application of BF techniques in power domain non-orthogonal multiple access (NOMA) schemes allows users to share the same single BF vector for operational reliability. The occurrence of inter-beam interference (IBI) is highly probable in a congested cell (i.e., a cell with high user density and active users). IBI cancellation by using 3D polarized BF in order to enhance the practicability of NOMA systems is the focus of this paper. An IBI cancellation scheme is proposed and evaluations of the spectrum efficiency according to the scenario congestion, as well as of the interference reduction by narrowing the generated beam to a desired beam-width, are presented. The security in the physical (PHY) layer in order to achieve confidential and authentic communication is also an important consideration. The proposed scheme checks the PHY layer security constraints on the number of users served per beam. In simulations, the robustness of the proposed scheme allows the average half power beam-width (HPBW) to be brought to about 20° for different steps in HPBW and for different user densities. Furthermore, depending on the user density, spectrum efficiency gains of approximately 4 bits/s/Hz and 9 bits/s/Hz are achieved by the described IBI cancellation scheme.