Robust Beamforming for Uplink RSMA in UAV Communication Systems With Jittering

This letter investigates a robust beamforming (BF) scheme for a rate-splitting multiple access-enabled unmanned aerial vehicle (UAV) uplink communication system under jittering effects. Considering multiple two-user pairs, we formulate a worst-case achievable sum rate maximization problem by jointly optimizing the transmit power of ground users and the receive BF of UAV. The difficulties of this problem arise from the complex trigonometric form of the angle-of-arrival (AoA) uncertainties caused by UAV jitter. To address these challenges, we first derive an approximation method based on the second-order Taylor series expansion to simplify the AoA uncertainties, and then convert the resulting random quadratic forms into deterministic forms by applying S-Procedure. Based on these, we propose a penalty function-based successive convex approximation algorithm to obtain a suboptimal solution to the original problem. Simulation results demonstrate the robustness and superiority of the proposed robust BF scheme.

I. Introduction

Due to the high maneuverability and line-of-sight (LoS) link, the unmanned aerial vehicle (UAV)-assisted uplink communication systems can efficiently collect information from ground users (GUs) [1]. With the rapid growth of GUs in 5G and beyond UAV applications, inter-user interference (IUI) management plays a crucial role in fulfilling the high data rate, ultra-low latency and massive connectivity requirements. Benefiting from dynamically managing IUI to achieve the optimal capacity region without time sharing [2], the uplink rate splitting multiple access (RSMA) has been recognized as a promising multiple access technology for UAV uplink communications [3]. The main concept of uplink RSMA is to utilize rate splitting at the transmitters and then apply successive interference cancellation (SIC) at the receivers [4]. In M-user uplink RSMA, only one user transmits its whole message without splitting and other users split their messages into two sub-messages each [5]. Since the uplink RSMA generates sub-messages, layers of SIC are required at the receiver to decode all sub-messages. As M increases, both the complexity of detection and the size of decoding order set grow significantly. To prevent overloading the UAV, by employing the orthogonal frequency resources, the multi-user uplink RSMA can be organized into several user-pairs, with each user-pair consisting of only two users [6]. However, since each orthogonal frequency resource is allocated to a specific user-pair, only two users can be served simultaneously, which limits spectral efficiency (SE) of uplink RSMA systems. To enhance SE, beamforming (BF) technologies can be adopted to enable simultaneous service for all user-pairs through effectively exploiting the spatial domain resources and advanced multi-antenna processing techniques at the receiver [7].