## 5 Simulation Results  
### Simulation of SUUIL Model  
The Minimum Shift Keying (MSK) modulation method is adopted in the traditional Loo model. However, there are a lot of obstacles in the communication of ultra-low altitude UAVs. The fading and shadow in the channel will cause the phase change of the signal. The traditional coherent demodulation method of MSK is not suitable for this situation. Therefore, the binary phase shift keying (BPSK) modulation method is adopted in this paper. In the simulation experiment, the influence of three physical parameters in the UAV channel is considered: the average signal-to-noise ratio per unit bit, the shadowing standard deviation, and the average Rice factor. In addition, BER is an important evaluation index in the evaluation of channel performance. Especially in the field of emergency management, urgent communication process, link interruption and data error will have a great impact on the whole system. As a result, the BER performance of the three channel models were compared under the same parameter conditions. Figure 5: The multipath effect of the signal and the combined effect of multipath and Doppler of the signal. (**a**) No Doppler frequency shift; (**b**) With Doppler frequency shift. The relationship between BER and average signal-to-noise ratio per bit is shown in Figure 6. The three curves in the figure are: the BER curve of the Loo model in MSK mode, ideal channel curve and SUUL channel curve. When the SNR is approximately 10 dB, the BER of SUUL model is \(10^{-5}\). At this point, the BER of LOO model is \(10^{-2}\). It can be seen that in the same communication environment, the bit error rate corresponding to the reliability of UAVs in emergency communications of the SUUL model is much lower than that of the LOO model. High reliability means that the communication link is more stable and less prone to packet loss and data distortion, so the BER is essential for emergency communications. The relationship between bit error rate and shadow standard deviation is shown in Figure 7. \(K_{r}\) is the Rice factor, which is defined as the ratio of the power of the main signal to the multipath component. The \(K_{r}\) is used to reflect the influence of small-scale fading. When \(K_{r}\) changes from 0 dB to 20 dB, it can be seen that the overall change trend of the channel error rate is similar. This means that the SUUL channel has good stability in different small-scale fading environments. On the other hand, the variation range of the BER curve is quite different under different SNR scenarios. When the SNR is small, the bit error rate changes relatively smoothly with the increase of the shadow standard deviation. At this time, the shadow has little effect on the UAV channel performance. When the SNR reaches a medium or high value, the change in the bit error rate is obvious, which means that the shadow has a greater impact on the performance of the communication channel. In other words, when the SNR is large, the channel performance is affected by both shadow and small-scale fading. When  
Figure 6: Curve of relationship between BER and average signal-noise ratio per bit. Figure 7: Relationship between BER and shadowing standard deviation. the SNR of the received signal is small, corresponding to long-distance communication, the influence of the shadow on the channel can be approximately ignored at this time. The relationship between the uncertainty of SUUL model and shadow standard deviation is shown in Figure 8. Uncertainty refers to the difference of BER when \(K_{r}\) is 0 dB and 20 dB under the same condition of signal-to-noise ratio. It is the influence of small-scale fading on the received signal under the same shadow fading environment and the same signal-to-noise ratio. It can be seen that the system uncertainty decreases non-linearly as the shadowing standard deviation increases. In the near field communication environment, when the signal-to-noise ratio reaches a larger value of about 18 dB to 30 dB, the system uncertainty value is very large.