## 8 Energy Optimization Techniques in UAV-Based Cellular Networks
### Conventional Approaches
Analytical models were developed to determine the optimal UAV trajectory and ground terminal's transmit power, resulting in the optimal energy consumption trade-off between them. The authorsin [206] studied the problem of EE and security in a cooperative multi-UAV system. A joint optimization of the transmit power, user schedule, and UAV trajectory was proposed to maximize the secure EE of the network. A three-layer heuristic algorithm was proposed based on BCD and the Dinkelbach method to solve the joint optimization problem. A UAV-BS path planning framework was proposed in [129] in order to maximize the EE of the UAV-BS. In the proposed framework, the path planning problem was first modeled using the traveling salesman problem, after which a GA was developed to determine the optimal path with minimal turns that would result in minimum energy consumption in the UAV-BS. The problem of energy-efficient UAV-based communication in the presence of several jammers was considered in [115]. To solve this problem, the UAV trajectory was optimized while considering its mobility constraints. In this regard, a heuristic algorithm based on SCA and Dinkelbach's model was proposed to determine the optimal trajectory. A flight planning mechanism for solving point of interest visiting problems involving data collection, edge computing, and surveillance operations was proposed [207] to minimize the flight energy consumption of the UAV. The flight turning and switching cost were modeled as a graph and then transferred to a travelling salesman problem after which a heuristic algorithm was proposed to determine the optimal flight path. The authors of [208] investigated the rate fairness and EE of a UAV based relay system. Two optimization objectives were considered: the first is the rate maximization while maintaining a specific power budget, and the second is optimizing the total power consumption while maintaining minimum rate requirements. Two heuristic algorithms were proposed to solve the optimization problem. In [209], a joint trajectory and transmit power optimization scheme for UAV-aided communications was investigated to maximize user average throughput. A heuristic algorithm was proposed to solve the joint optimization problem while ensuring that users' throughput is maximized. Similarly, the work in [210] considered a joint trajectory and transmit power optimization in UAV-based secure communications to maximize the throughput of the system. An iterative algorithm-based BCD, S-procedure, and SCA were proposed to solve the optimization problem. The work in [211] investigated the problem of trajectory optimization to minimize the power consumption of the UAV while ensuring that user service demands are met. The optimization problem was solved using an iterative algorithm that is based on SCA. The work in [212] investigated flight radius adjustment and routing in UAV swarm networks to minimize the total power consumption of multiple UAV systems. Two flight radius manipulation approaches were first developed to maximize the propulsion power consumption and battery lifetime of the UAVs. Then, a temporal path algorithm was proposed to minimize the power consumption of any pair of UAVs. In addition, an iterative algorithm was also developed to further adjust the flight radius to enhance the EE of the system. The authors in [213] studied the dynamic cooperation of UAVs in a multi-UAV system to enable power efficient communications. To achieve energy-efficient communications, the UAV trajectory and the cooperative beamforming were jointly optimized and then a low-complexity heuristic algorithm that is based on difference of convex (DC) was proposed to solve the optimization problem. Regarding the issue of energy consumption fairness in multi-UAV networks, trajectory optimization was considered in [214]. The problem of designing trajectories with fair energy consumption allocation was modeled using mixed-integer linear programming.