# Interference Avoidance through Periodic UAV Scheduling in RIS-Aided UAV Cluster Communications
## 6 Conclusions  
In this work, we studied the UAV scheduling problem within the context of an RIS-aided multi-cluster UAV system. In this system, the BS establishes periodic communication with the UAVs through the support of RIS arrays carried by an RIS-UAV. We develop a UAV interference matrix to effectively capture and describe the interference between UAV clusters. Taking into account the constraints imposed by periodic transmission and the limited number of RIS arrays, we devise UAV scheduling time slots that aim to mitigate inter-cluster interference. To solve the scheduling problem, we reformulate it as a multi-stage path-finding problem and introduce the LIM algorithm. This algorithm effectively minimizes the local interference at each stage. Furthermore, we propose the RIM algorithm, a rollout-based UAV scheduling algorithm that builds upon the LIM algorithm. The simulation results highlight the effectiveness of the RIM scheme in significantly reducing inter-cluster interference when compared to both the baseline approach and the LIM algorithm.  
Conceptualization, E.Z.; data curation, W.X.; formal analysis, W.X. and W.Y.; funding acquisition, P.L.; investigation, W.Y.; methodology, E.Z.; project administration, X.N.; resources, W.Y.; supervision, P.L. and X.N.; validation, Z.L.; writing--original draft, E.Z.; writing--review and editing, E.Z. and Z.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Science and Technology Major Project of the Tibetan Autonomous Region of China: XZ20201 ZD0006G.  
Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Notations and Abbreviations  
The following Notations and Abbreviations are used in this manuscript: Notations \(\mathbf{C_{i,i^{\prime}}}\) The UAV interference matrix between UAV cluster \(i\) and UAV cluster \(i^{\prime}\)\(\mathbf{h}_{BR}\) The wireless channel between the BS and the RIS \(\mathbf{h}_{RI}\) The wireless channel between the RIS array and UAVs \(\mathbf{d}(\theta)\) The steering vector with directional angle \(\theta\)\(\eta(i,j,i^{\prime},i^{\prime})\) The received SIR of UAV \((i,j)\) interfered by beam associated with UAV\((i^{\prime},i^{\prime})\)\(T_{i}^{i}\) The cycle to schedule UAV \(j\) in UAV cluster \(i\)\(L\) The number of RIS arrays on the RIS-UAV \(M\) The number of UAVs in a UAV cluster \(\gamma\) The system interference level \(s_{i}^{j}\) The initial time slot of UAV \(j\) in cluster \(i\)\(\mathbf{1}_{N}(\mathbf{x})\) A vector of length \(N\) that elements specified by indices \(\mathbf{x}\) equal 1 \(\mathbf{0}_{N}\) All zeros vector of length \(N\)\(\otimes\) The Kronecker product \(\mathrm{vec}\{\}\) The vectorization operation \(mod()\) The modulo operation \([\ ]\) The ceiling function \(\epsilon(a,b)\) The greatest common divisor of integer numbers \(a\) and \(b\)\(\Phi\) The empty set Abbreviations \(\mathbf{\mathrm{U}}\) UAV Unmanned aerial vehicle RIS Reconfigurable intelligent surface BS Base station UIM UAV interference matrix SIR Signal-to-noise ratio LIM Local interference minimization RIM Rollout interference minimization