## 3 Secure Rate Maximization Scheme
### K-Means Cluster Pair Algorithm
Figure 4 shows the relationship between the secure sum rate and the number of iterations. It is observed that our proposed algorithm has faster convergence evolution and achieves better system secure rate improvement. Figure 5 shows the comparison of simulation time with different methods. Although our algorithm takes a long time to implement, it can significantly improve the system performance. Under this condition, the consumption of time cost is reasonable. Figure 4: Convergence of different algorithms. Figure 3: Optimal 3D-locations of UAVs. ### Discussion

Through our above efforts, suboptimal solutions for channel allocation, power optimization, and height optimization are obtained. Our initial optimization problem is an NP-hard problem with a difficult analytical solution to find. Although our algorithm is not optimal, by comparing it with other schemes, our algorithm has the largest sum secure rate, which is effective and guides the way for our future work. ## 5 Conclusions

In this paper, we proposed an HAP and UAVs collaboration framework in NOMA-enabled IoT networks, based on which the total secure rate maximization problem was investigated. Specifically, the BCD method was used to decouple the joint optimization problem into channel allocation, power optimization, and height optimization subproblems, where the K-means algorithm, SCA and ACO methods were applied to approach the optimal solution. Finally, the demonstration of simulation results indicated the proposed framework can achieve a higher secure rate compared with other existing schemes. []

D.W.: conceptualization, methodology, software, writing--original draft preparation, and visualization. M.W. : conceptualization, resources, writing--review and editing, supervision, project administration, and funding acquisition. Y.H. : conceptualization and writing--review and editing. L.P.: software and formal analysis. Q.X. : software, and formal analysis. R.Z. : conceptualization, resources, supervision, and funding acquisition. All authors have read and agreed to the published version of the manuscript. []

This work was supported in part by the National Natural Science Foundation of China under Grant 62271399 and Grant 61901379, in part by the Key Research and Development Program of Shaanxi Province under Grant 2022 KW-07, in part by the National Key Research and Development Program of China under Grant 2020 YFB1807003, and in part by the Foundation of the Science, Technology, and Innovation Commission of Shenzhen Municipality under Grant JCYJ20190806160218174. []

**Data Availability Statement:** Not applicable. Figure 5: Simulation duration under different algorithms. ## References

* (1) [PERSON]; [PERSON]; [PERSON]; [PERSON] Managing Time-Sensitive IoT Applications via Dynamic Application Task Distribution and Adaptation. _Remote Sens._**2021**, _13_, 4148. [CrossRef]
* (2) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Energy- and Cost-Efficient Physical Layer Security in the Era of IoT: The Role of Interference. _IEEE Commun. Mag._**2020**, _58_, 81-87. [CrossRef]
* (3) [PERSON]; [PERSON]; [PERSON] Blockchain-envisioned UAV communication using 6G networks: Open issues, use cases, and future directions. _IEEE Internet Things J._**2021**, \(8\), 5416-5441. [CrossRef]
* (4) [PERSON]; [PERSON]; [PERSON] Joint trajectory computation design for multi-UAV enabled wireless networks. _IEEE Trans. Wirel. Commun._**2018**, _17_, 2109-2121. [CrossRef]
* (5) [PERSON]; [PERSON]; [PERSON] Throughput maximization for UAV-enabled mobile relaying systems. _IEEE Trans. Commun._**2016**, _64_, 4983-4996. [CrossRef]
* (6) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Delay-Sensitive Secure NOMA Transmission for Hierarchical HAP-LAP Medical-Care IoT Networks. _IEEE Trans. Ind. Inf._**2022**, _18_, 5561-5572. [CrossRef]
* (7) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] An experimental study of uplink co-channel spectrum-sharing system between HAPS and terrestrial mobile communication networks.