## 1 Introduction  
Compared with traditional optical fiber communication and microwave communication, FSOC has the advantages of high communication speed, strong anti-interference ability, high security and small size [1, 2]. It has many applications in the civil emergency information transmission, the information security transmission under electromagnetic interference and the space information network construction [3, 4, 5]. Related researches have been carried out in References [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. Besides, UAV has the advantage of flexible deployment and is developing rapidly in the direction of miniaturization and high speed [17, 18]. However, UAV command and control systems are facing challenges on interference made by a Wireless Fidelity (Wi-Fi) signal and its own interference made to other facilities, typically the influence in the airport. Using the FSOC system on the UAV helps enhance the communication ability and eliminate the strict condition on the electromagnetic environment, which brings huge impact to the UAV's communication mode [18].  
The FSOC system applied in UAV also faces many challenges. First, FSOC inevitably faces the problem of atmospheric turbulence, and UAV produces serious atmospheric turbulence in the flight process, which significantly damages the communication probability. Second, if the UAV loses its tracking during flight, it is difficult for normal FSOC equipment to complete the link establishing in a short time, which may cause the UAV to go out of control. Third, the moment of inertia generated by the rotation of the FSOC system during the search process could disturb the flight of UAVs.  
Many methods have been proposed to apply FSOC equipment on UAV. In Reference [17], the integrated optical module helps to support both the signal beam and the beacon beam. This scheme can reduce volume to a certain extent, but the static transmission distance is only 50 m. In Reference [19] and Reference [20], adaptive optics technology is used to compensate the atmospheric disturbance in real time. They realize the demonstration experiment of low-power and high-speed air to ground laser communication. However, the equipment is so heavy that can only be carried by P68 aircraft, which can carry 680 kg. A precious tracking system based on fast steering mirror is adopted in Reference [21]. The scheme also reduces the intensity jitter at the cost of increasing the volume and complexity of the receiver, but it cannot work in the vibration environment because of the poor anti-vibration performance of the fast steering mirror.  
In this paper, a scheme of omni-directional receiving is proposed, which can realize anti-jamming UAV command and control. It has the following advantages:  
1. Large area detectors and short focus lenses are used at the receiving end, which greatly improve the anti-turbulence ability of the system;
2. An omni-directional receiving communication mode is adopted, thus enhancing its abilities for anti-vibration and link establishing;
3. The receiver adopts the pure static receiving mode, which does not affect the momentum of the platform and help the UAV free from flying interference;
4. The receiver of the UAV can be miniaturized for it eliminates the gimbal mirror system, the beacon camera system and the QPD or the multi-level lens system.  
Because the command and control center on the ground has no need to be miniaturized, it can use the gimbal mirror system and some other large devices to transmit the command and control information directionally. Moreover, because the communication is a one-way link, no receiving device in required on the ground, as shown in Figure 1. Therefore, the design of the command and control center on the ground is not discussed in detail here.