Cell Association in Energy-Constrained Unmanned Aerial Vehicle Communications Under Altitude Consideration

In this paper, the total transmit power of Unmanned Aerial Vehicles (UAVs) which are deployed as aerial base stations is minimized by adjusting the altitudes of different UAVs. We assume that the UAVs adopt the frequency division multiple access (FDMA) technique to serve ground users that are distributed in a certain geographical area. For the given two-dimensional locations of UAVs and the distribution of ground users, we find the optimal altitude of each UAV so that it covers all the ground users nearby and provides services with the minimum transmit power. Our formulated problem is separated into two sub-problems: (1) cell association and (2) transmit power minimization. We find the optimal altitude of each UAV in the first sub-problem by using K Means clustering algorithm. In the second sub-problem, we minimize the transmit power of UAV by using the solution of the previous sub-problem. Exploiting convex optimization, we jointly find the optimal altitude and the optimal transmit power of each UAV under its QoS constraints. In essence, the resulting optimal altitudes can lead to lower transmit power of the UAVs in the network compared to the cases when UAVs are deployed at a fixed altitude.