## 3 Design  
The receiving sub modules are composed of short focus lenses, optical filters, detectors and amplification circuits, as shown in Figure 4. The signal light transmitted in space converges to the back of the filter after passing through the short-focus lens. By doing so, it can make the normal incident light spot larger in case of burning the filter and detector. What is more, the detector can receive more oblique incident light energy. It also enables the effective reception of optical signals in the range of 0-30\({}^{\circ}\) incident angle. The signal light is collected by the detector after passing through the filter. The detector converts the signal light into current signal. Then the receiving sub module outputs the signal to the core board circuit in the form of voltage signal after two-stage amplification circuit. Since the window angle of a receiving sub module is only 30\({}^{\circ}\), multiple receiving sub modules are needed to realize field of view splicing. According to the calculation, 12 receiving sub modules can realize \(360^{\circ}\times 30^{\circ}\Big{(}\theta_{\parallel}\times\theta_{\perp} \Big{)}\) circular reception, and 52 receiving sub modules can realize real omnidirectional reception. These sub modules are fixed in different positions of the spherical support and work independently to achieve omnidirectional receiving function, as shown in Figure 5.  
Each voltage signal is input to the core board circuit after the amplifying and the converting to digital signal by Analog-to-Digital Converter (ADC). Field Programmable Gate Array (FPGA) selects the strongest input signal while monitoring multiple signals in real time, and it is the command and control signal which is needed. If the signal strength of one channel exceeds the signal being collected in the detection process, and the strongest signal is similar to the signal being collected, the current strongest signal is used to replace the previous signal to make sure that the command and control signal being collected is the strongest one from all signals. The whole system continues monitoring the signal in the whole process. Figure 4: Receiving sub module model. According to the structural design of field of view splicing, if the whole system vibrates due to external influence, the incident signal light will move from one receiving sub module to another. In this process, the light received by the former module gradually becomes weak, and the light received by the latter module gradually becomes stronger, and the two are very similar. When the output signal strength of the latter exceeds that of the former, the signal collected by the whole system will also be switched to ensure that the collected command and control signal is the strongest effective one. Therefore, the influence of external vibration on the whole system can be compensated by the design of the system. The process of switching command and control signals in the system is realized by FPGA program design. During the process of FPGA, the pure static switching can be realized without generating the moment of inertia and interference to the flight process of UAV, which ensures the flight safety of UAV. Because of the omnidirectional receiving design, the optical signal transmitted can be effectively received in any direction. Once the communication link being connected is interrupted, the receiver of omnidirectional receiving can receive the retransmitted link signal from the ground control center quickly, which can realize the fast link establishment. It will not occur when the receiver cannot receive the signal effectively for lacking of alignment at the receiver when the link is interrupted. When the parallel light passes through the short focus lens, due to the influence of the non-uniform temperature distribution in the environment, the lens will undergo thermal deformation. It will cause the wave-front distortion of the parallel light, and result in the intensity and weakness of the focused spot [25].