**6. Conclusion**

This chapter proposes deployment and trajectory designs of UAVs for efficient resource allocation to achieve reliable wireless communication. The main features of this structure are three folded. In the first part, we optimize UAVs altitude to minimize outage probability and symbol error rate, considering pointing errors, atmospheric turbulence, and scintillation parameters where a hybrid RF-FSO channel governs the transmission environment. The second part finds the optimal deployed locations of UAVs to maximize the total instantaneous transmission rate of the devices in USSD2D network under SNR constraint. Finally, the last feature focuses on energyefficient data collection where devices' total energy consumption is minimized by jointly optimizing their association with the nearest BS or UAV, their transmitting power, and UAV trajectory while satisfying a given data rate requirements. Numerical results validate the analysis and provide insights on the optimal UAV control design for various key system parameters. Our proposed methodology significantly improves system performance compared with the benchmark techniques. This work would be extended toward a multi UAVs-assisted energy-efficient data collection system considering the age of information aspect where the users follow a certain mobility model. *Optimal Unmanned Aerial Vehicle Control and Designs for Load Balancing in Intelligent… DOI: http://dx.doi.org/10.5772/intechopen.110312*
