# The Optimization of UAV-Assisted Downlink Transmission Based on RSMA
## 1 Introduction  
Compared to ground-based equipment such as communication vehicles and fixed base stations, Unmanned Aerial Vehicles (UAVs) offer a superior environment for signal transmission, as they experience less interference from external factors. In complex disaster scenarios, deploying ground equipment can be challenging and it is often difficult to find suitable locations. UAVs, on the other hand, can be easily deployed and positioned according to real-time demands, providing service to a wider range of users and overcoming coverage challenges in such environments.  
One of the key challenges in UAV-assisted emergency communication is ensuring high Quality of Service (QoS). Enhancing QoS in emergency communication networks can be addressed at various levels, including network architecture [1], protocols at the network layer [2], and data link layer [3] designs. Previous studies [4] have explored routing protocols at the network layer for UAV-assisted emergency communication systems, while this paper focuses on improving QoS through a physical-layer-assisted Medium Access Control (MAC) design.  
MAC protocols can be broadly categorized into orthogonal and non-orthogonal types. Traditional orthogonal MAC protocols, such as Time Division Multiple Access (TDMA) [5], Frequency Division Multiple Access (FDMA) [6], and Code Division Multiple Access (CDMA) [7], have been widely used in earlier generations of mobile communication systems (1G to 4G). However, these protocols struggle to meet the demands of large-scale user access and ultra-low latency scenarios, making them less suitable for modern communication needs. To address these limitations, the fifth generation (5G) of mobile communication systems has adopted non-orthogonal MAC protocols, such as Rate-Splitting Multiple Access (RSMA) [8], Sparse Code Multiple Access (SCMA) [9], and Non-Orthogonal Multiple Access (NOMA) [10], to manage user access.  
This paper focuses on UAV-assisted emergency communication networks where UAVs serve as communication base stations. Specifically, we study the rate optimization problem in RSMA-based UAV communication networks. RSMA, as a non-orthogonal MAC protocol, differs from Spatial Division Multiple Access (SDMA) [11], which treats interference as noise, and from NOMA, which decodes all interference. RSMA partially decodes interference and treats part of it as noise, allowing users in the same group to share time-frequency resources. A key design aspect of non-orthogonal MAC protocols is beamforming, which, when optimized, can significantly enhance network throughput. Poor beamforming design, however, can lead to resource wastage and fail to meet rate requirements. Therefore, optimizing RSMA system parameters is crucial for achieving higher data transmission rates.  
Additionally, both emergency communication systems and regular communication systems exhibit asymmetry between uplink and downlink communications, with the QoS demands typically higher for downlink. Therefore, optimizing the downlink system in RSMA-based UAV-assisted networks is of particular importance.  
In this study, we address the rate optimization problem in the downlink of RSMA-based UAV communication systems, with the objective of maximizing transmission rates. We decompose the problem into three subproblems: UAV parameter optimization, RSMA parameter optimization, and user group parameter optimization. To solve these subproblems, we propose an optimization algorithm that combines the Augmented Lagrange Method (ALM) with the Artificial Fish Swarm Algorithm (AFSA) [12]. A ternary cross-iterative algorithm is designed to solve the original problem and achieve optimal performance in the downlink system.