EE Maximization With Imperfect CSI at Transmitter in BackCom NOMA System

This paper presents a novel model within a nonorthogonal multiple access (NOMA) system comprising a multiantenna access point (AP) and multiple single-antenna backscatter devices (BDs) and user equipments (UEs), where the channels from AP to UEs and BDs to UEs are characterized by imperfections. The core objective of this study is to optimize energy efficiency (EE) while ensuring the quality of service (QoS) for both uplink and downlink users. To achieve this, the paper employs the Dinkelbach method to address the fractional programming challenge, leverages the S-procedure and Bernstein-type inequality (BTI) for managing channel uncertainties, utilizes semidefinite relaxation (SDR) to tackle the nonconvexity arising from the quadratic form of the beamforming matrix, and applies sequential rank-one constrained relaxation (SROCR) to circumvent the computational complexity introduced by Gaussian randomization. Additionally, for the specific case of resource allocation with a single BD, a closed-form expression is derived. The simulation results demonstrate that the proposed algorithm outperforms the non-robust scheme in terms of EE, significantly enhancing the system's stability and robustness.