## 5 Detection of Received Signal Data Processing and ISI/BER Analysis  
In this study, ISI simulation and BER statistical analysis of airborne laser communication links in the airborne air-to-air channel were conducted employing a 2.5 Gbps baud rate (OOK modulation). This was achieved by utilizing the established ISI analysis model and integrating the simulated channel impulse response derived from the air-to-air channel transmission simulation. Furthermore, the MATLAB simulation tool was employed to calculate and perform a comprehensive analysis of the bit error rate (BER). Taking the results of the signal time-response curve obtained from the MCRT simulation as input parameters, combined with the design and development of the communication system, the simulation results show that the average single pulse spreading statistic is \(\Delta\tau=74\) ps when the standard deviation of the fluctuation in the atmospheric refractive index is \(\sigma_{n}=1\times 10^{-8}\). The total pulse width statistic after sending 10,101 fixed code elements averages about 3000 ps and there is a certain ISI effect on the system (shown in Figure 10a). This study modeled and simulated the changes in ISI in the front-end signal of the 2.5 Gbps airborne communication system's receiving system based on OOK modulation. Figure 10: ISI and BER simulation of laser transmission over airborne air-to-air channel under 50 km visibility condition. (**a**) Standard deviation of fluctuation in atmospheric refractive index \(\sigma_{n}=1\times 10^{-8}\), threshold \(VT=0.38\); (**b**) eye diagram and BER, SNR = 0.009 dB; (**c**) standard deviation of fluctuation in atmospheric refractive index \(\sigma_{n}=1\times 10^{-7}\), threshold \(VT=0.39\); and (**d**) eye diagram and BER, SNR = 0.007 dB. The ISI waveforms of the received pulse signals, eye diagrams, and BER performance simulation results for the airborne channel with a visibility of \(V=50\) km and different standard deviations of atmospheric refractive index fluctuation (\(\sigma_{n}\)) are shown in Figure 10. (Note: the transmitting codeword element is 10,101 and the number of BER statistical samples is 10,000). The simulation results show that the average single pulse spreading statistic is \(\Delta\tau=74\) ps when the standard deviation of the fluctuation in the atmospheric refractive index is \(\sigma_{n}=1\times 10^{-8}\). The total pulse width statistic after sending 10,101 fixed code elements averages about 3000 ps and there is a certain ISI effect on the system (shown in Figure 10a). Moreover, the receiving eye diagram is slightly open, with a BER of \(9\times 10^{-4}\) (shown in Figure 10b). After the system implements equalization, anti-jamming coding, and other forms of technical processing to suppress inter-symbol interference, the quality of the communication link can reach the goal of high-performance communication. When the standard deviation of the fluctuation in the atmospheric refractive index is \(\sigma_{n}=1\times 10^{-7}\), the statistical average of a single pulse spread is \(\Delta\tau=97\)ps. The total pulse width statistics after sending 10,101 fixed code elements averages about 3000 ps and the system has a certain ISI influence (as shown in Figure 10c). Moreover, the receiving eye diagram shows a microlosed state and the bit error rate is \(1.5\times 10^{-3}\) (as shown in Figure 10d). The system cannot establish a high-performance communication link under these conditions without using technical processing means such as equalization to suppress the inter-symbol interference.