Abstarct: Abstract Recently, unmanned aerial vehicles (UAVs) have attracted lots of attention in wireless communication networks because of their high mobility and low cost and high robustness of line-of-sight (LoS) channel. In this thesis, we describe two different communication systems. The first communication system, includes a network with fixed users. In this system, investigates a communication system assisted by multiple UAV-mounted baxse stations (BSs), aiming to minimize the number of required UAVs in communicating between fixed and moving users in the network and to improve the coverage rate by optimizing the three-dimensional (3D) positions of UAVs, Clustering of users using different algorithms, and frequency band allocation. We provide a solution for deploying multiple UAVs as baxse stations to serve ground users according to the required quality of service (QoS) and the service ability of each UAV. The second communication system, includes a network with mobility users. In this system, a new method for mobility management (displacement) for connected drones in future mobile networks, including telecommunications and satellite networks, is presented. In this system, the investigated area is examined from different perspectives, including the movement behavior of users and drones, deployment in specific places, received power and interference power in users and coverage rate. In this situation, the movement behavior and movement path of users is created according to historical information or randomly, and the position of each user is calculated along with the duration of its deployment in different positions. During this research, in the first system, each of the algorithms is tested and evaluated, and the best result is clearly extracted from them. The simulation results show that after obtaining the optimal position and height of drones, the RLP algorithm can achieve superior performance compared to other algorithms in terms of minimizing the number of drones and improving the coverage of users.