# Modeling and Spatial Diversity-Based Receiving Improvement of In-Flight UAV FSO Communication Links
## 1 Introduction  
Free-space optical (FSO) communication technology has been widely used in commercial and military fields due to its advantages such as large transmission capacity, rich spectrum resource, good confidentiality, and good directivity [1; 2; 3]. Moreover, with the use of unmanned aerial vehicles (UAVs) gradually shifting from the military field to the commercial and civil fields, high-speed data transmission is often needed in some special working environments, such as terrain survey and disaster detection. For the above reasons, FSO communication technology is naturally introduced into the field of UAV, so as to establish high-speed optical communication networks in ground-to-ground and air-to-air scenarios.  
The FSO communication link between UAVs is fundamental to improve the coverage range and networking flexibility of UAVs. In recent years, UAVs and high-altitude platforms are often used as relays to support wireless long-range links between two terrestrial stations [4; 5], and the channel modeling of UAV FSO communication has also been fully discussed [6; 7; 8]. In consideration of turbulence intensity, field angle, spatial position, positioning, and transmitting power, a closed-form statistical channel model, which is a mathematical statistical channel model using a finite number of standard operations of the UAV communication has been proposed, and the above parameters have been optimized to improve the communication quality [9; 10]. At the same time, a removable lens array has been used to change the beam divergence mechanism of the transmitter to enhance the stability of the link [7].  
However, previous studies were only limited to the communication between hovering UAVs without considering the influence of flying and wind resistance. In hovering states, the UAV FSO communication will only be affected by the jitter and angle deviation [9; 10; 11].  
Taking the star formation UAV group as instance, the central UAV is in the UAV hovering state, and the rest of the UAVs fly on the same level against the central UAV [12]. During the flight mission, the central UAV sends information to the other UAVs through FSO communications while the tilt of UAV bodies will cause the light beam to be non-orthogonal to the lens at the receiving end. The wind resistance is also one of the main causes of UAV's jitters. Therefore, an FSO communication link model between in-flight UAVs that includes all the jitter, deviation, receiving error, and wind resistance effects is essential to be established.  
The spatial diversity technology can greatly increase the redundancy of the system by receiving the incoherent signals at different locations and improve the stability of the link. A receiver array with multiple isometric receivers, which combines multiple groups of mutually independent fading signals at the receiving end is utilized in this technology, and a strong anti-interference effect on the beam offset and light intensity fluctuation exists [13]. Equal gain combining (EGC) is a common linear combination algorithm of combining schemes of diversity technology, which can set the weight coefficient of the signals received in each road as 1, and merge them into one road signal to achieve signal gain [13; 14; 15]. In this way, the sensitivity of the FSO link can be effectively improved.  
In this paper, a new FSO communication channel model for the in-flight receiving UAV is established. In consideration of the atmospheric turbulence and the tilt and jitter caused by the UAV flight, the bit error rate (BER) and the outage probability are simulated versus the signal-to-noise ratio (SNR), and the effect of spatial diversity technology on communication quality improvement is studied. Moreover, the improvement of communication quality by spatial diversity technology is analyzed.