# NOMA-Based Rate Optimization for Multi-UAV-Assisted D2D Communication Networks
## 1 Introduction  
With the emergence of new devices and innovative applications, the global demand for mobile data is growing at an accelerated pace. As a result, the need for high-quality services has become increasingly urgent worldwide. Furthermore, the rapid growth of the Internet of Things (IoT) requires ubiquitous connectivity [1]. To satisfy users' high data throughput, mobile networks must significantly increase their capacity. However, because radio spectrum resources are scarce, there is an urgent need for new spectrum management and power optimization approaches to fulfill the rapidly expanding demand for wireless connectivity. Driven by the above challenges, Non-Orthogonal Multiple Access (NOMA) has received significant attention [2; 3] and is recognized by both industry and academia. Unlike traditional multiple-access techniques, which require independent or orthogonal band protection, NOMA allows a single resource block to be multiplexed by multiple users who have different allocated power factors through flexible user scheduling and bandwidth allocation strategies. Moreover, the use of overlay coding techniques [4] and serial interference cancellation techniques [5] not only effectively mitigates the rapidly growing capacity problem but also significantly improves network performance in terms of fairness and security. In addition, as one of the key technologies of 5G, Device-to-Device (D2D) communication refers to the direct exchange of information between devices without requiring transmission through a base station (BS) [6; 7]. In traditional cellular networks, D2D users are able to communicate by multiplexing the communication links of cellular users, which improves the spectrum utilization of the system, which in turn improves the network throughput. However, the introduction of D2D technology in cellular networks increases the inter-user cochannel interference in the network, which affects the quality of user communication; furthermore, the quality of communication is ensured by introducing NOMA technology [8], which utilizes a decoding mechanism to eliminate the inter-user interference caused by D2D communication [9; 10]. Unmanned Aerial Vehicles (UAVs) have been gradually applied to assisted communication scenarios due to their flexible maneuverability and high-altitude-coverage capability [11; 12]. A UAV can be used as an airborne base station or relay device to expand network coverage and enhance communication quality. When UAV technology is combined with D2D and NOMA technologies, wireless resource allocation can be further optimized, especially in high-density device distribution and complex channel environments, to effectively enhance system performance. Specifically, the dynamic deployment capability of UAVs can achieve accurate coverage of D2D user clusters while combining the spectrum multiplexing characteristics of NOMA to maximize spectrum efficiency. Although the combination of NOMA, D2D technology, and UAVs demonstrates significant performance improvement potential, its converged application still faces a number of challenges. First, in dynamic environments, it is a complex issue to realize UAV trajectory design to cover multiple D2D user clusters while ensuring communication stability and efficiency [13]. Second, the introduction of NOMA technology further increases the complexity of channel assignment and power control. In multi-user coexistence scenarios, how to achieve fairness among users under dynamic channel conditions and effectively suppress interference has become a core problem that needs to be solved [14]. Moreover, in UAV-assisted D2D-NOMA scenarios, there is strong coupling among trajectory design, channel assignment, and power control. Designing joint optimization algorithms to maximize system performance under limited resources while reducing the computational complexity is the focus and challenge of current research.