## 4 Numerical Results
The reason is that the barrier between the UAV and the receiver can no longer affect covert communication, which leads to a slow increase in the covert rate. Second, we can observe that the detection rate of covert communication of Willie can significantly limit the performance of covert communication. For example, the covert rate can achieve a 10% increase when the detection rate varies from 0.1 to 0.125 and from 0.125 to 0.15. On the other hand, when \(H\) is large enough beyond a threshold, the covert decreases quickly. This is due to the fact that as \(H\) is larger than the threshold, reliable covert communication between the UAV and the receiver cannot be guaranteed owing to the limitation of the transmit power. In addition, when the UAV is far away from Willie beyond a certain threshold, the detection probability decreases, leading to the constraint formulated in Equation (38) no longer playing a role. In this case, the transmit power decreases, which also decreases the covert rate. Figure 5 demonstrates the covert rate of the proposed UAV-assisted CCS with NOMA versus different finite LTMB with different noise variances and detection rates for each per channel, denoted as \(\eta/N\). From Figure 5, we first observe that as N increases, \(\eta/N\) also increases smoothly, which is caused by the optimization of finite blocks and the transmit power, as described in Section 3. When the \(\eta\) is optimal, and covertness is satisfied, the maximum value of the finite block transmission channel is \(N\). Second, when varying the value of the covertness constraint constant \(\varepsilon\), there is a significant change in \(\eta/N\) with a slight increase in \(\varepsilon\). Therefore, at the same height, the covert rate increases with \(\varepsilon\), which indicates that the maximum achievable rate is sensitive to the covertness requirement. Third, we compare the covert rate versus three different noise variances, where the noise variance is the variance value of the U2. In this simulation, the variance value at Willie is set as \(\sigma_{w}^{2}=1\). We observe that the smaller the noise variance of the U2 the larger the covert channel of each channel can be achieved. And this phenomenon increases with the increase in the \(\eta/N\) variance at different \(\sigma^{2}\). Figure 6 plots the covert rate \(\eta\) when \(\xi^{*}\geq 1-\varepsilon\) is satisfied against the decoding error probability \(\delta\). In Figure 6, we first observe that \(\eta\) reaches a maximum value when \(\delta\) increases up to a certain value, which can help us to find the optimal value of \(\delta\). After obtaining the optimal  
Figure 5: The covert rate of UAV-assisted CCS with NOMA versus detection rate for different finite block lengths, different noise variances, and per channel. value of \(\delta\), the covert decreases slowly as the detection rate of Willie keeps increasing. Second, the maximum throughput becomes larger as the finite block length increases with the same decoding error probability, which proves the result shown in Figure 5.  
In Figure 7, the performance is compared to the proposed SGPA algorithm with two benchmark schemes. One benchmark is the fixed altitude optimization scheme (FH), where the hover height is set to the lowest altitude (i.e., \(H=H_{\text{min}}\)) [36]. The second benchmark is the fixed power optimization scheme (FP), where the UAV is launched at the maximum transmit power (i.e., \(P_{u_{1}}+P_{u_{2}}=P_{\text{max}}\)) [10]. From Figure 7, we can observe that \(\eta\) increases with \(\varepsilon\) for all three schemes. However, the covert rate \(\eta\) does not increase with \(\varepsilon\) in the FP when \(\varepsilon\) is larger than 0.2, tending to be flat. The gradual increase in FP is due to the gradual relaxation of the covertness constraint. For the FP scheme, the covert tends to be a constant, since [PERSON]'s detection no longer  
Figure 6: The covert rate \(\eta\) for different decoding error probabilities \(\delta\) when \(\xi^{*}\geq 1-\varepsilon\).