# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 1 Introduction  
It is projected that the arrival of the sixth-generation (6G) communications era will result in both an unparalleled rise in the quantity of internet devices connected wirelessly and a sharp growth in mobile data traffic [1]. This not only highlights a major obstacle for future communication networks' transmission capacity, but it also shows how inadequate present communication technologies are to handle spectrum congestion [2, 3], and resolvethe problem of limited spectrum resources [4]. Addressing these challenges has become a focal point of research within the field of communications. Due to their large bandwidth and fast wireless data transmission capabilities, free-space optical (FSO) and terahertz (THz) communication have garnered a lot of attention in the face of spectrum shortage and congestion problems. These technologies provide promising solutions to alleviate the current shortage of radio frequency (RF) communication bands [5; 6]. The future generation of high-speed wireless communication systems will be supported by FSO in particular because of its many advantages, which include low implementation costs, high security, license-free spectrum use, large bandwidth, and ease of deployment [7]. Furthermore, FSO has demonstrated its distinct benefits in resolving the \"last mile\" issue with post-5G (B5G) communication systems [8]. However, in practical applications, FSO systems still face challenges such as atmospheric turbulence, mis-aimed shots, and interference from extreme weather conditions (such as rain and fog), which can lead to unreliable transmission links. Research and application of THz communication are particularly important to alleviate these problems that FSO encounters. THz communication shows great potential in high-data-rate propagation. Its frequency band is between millimeter waves and infrared light bands, which can provide high security, large directional gain, and wide operating bandwidth [3; 9; 10]. THz supports line-of-sight (LoS) and non-LoS (NLoS) propagation. Under conditions of haze and atmospheric turbulence, THz propagation performs better than FSO. However, in clear weather, the performance of THz communication may not be as good as FSO [11], mainly because of multipath fading caused by molecular interactions and energy absorption in the propagation medium [12]. To sum up, FSO and THz communications each have their own advantages and limitations. The strategy of parallel deployment of FSO and THz can not only give full play to the complementary advantages of the two technologies under different environmental conditions, but also provide an effective solution to the scarcity of spectrum resources and spectrum congestion problems faced by future wireless communication systems. Nevertheless, both THz and FSO communications encounter significant challenges when it comes to real-world implementations [13; 14], particularly with regard to severe path fading and channel degradation. To counteract the detrimental impacts of these factors, the adoption of relay technology has proven to be an efficacious method for enhancing the wireless transmission rate and broadening the scope of wireless coverage. Taking into account the presence of obstacles, the strategic placement of relays at optimal locations is critical in the system's architecture. Moreover, given the high flexibility of unmanned aerial vehicles (UAVs), utilizing them as mobile relays represents an innovative and promising approach. Relative to conventional ground-based relay systems, UAV-mounted relays offer enhanced adaptability to identify the most favorable communication conditions and rapidly establish line-of-sight connections. In [15], the authors delve into modeling and evaluating the performance of a UAV-based FSO communication system, considering atmospheric turbulence, pointing inaccuracies, and angle-of-arrival (AOA) variations.