Upper bound of outage probability in unmanned aerial vehicle-assisted cellular networks over fading channels

In this paper, we consider a multi-tier cellular network in which a hovering Unmanned Aerial Vehicle (UAV) assists the network in the absence of the terrestrial Macrocell base station. The orthogonal sub channels are assumed for communication between the UAV and its attached users. The Femtocell users and Device-to-Device (D2D) pairs transmit their data to the corresponding receivers in the same sub-channels. Achieving the outage probability of the ground users, is a challenge for the operators considering the dominant small scale and large scale fading over the channels, Line-of-Sight and None-Line-of-Sight conditions together. The mentioned problem becomes worse in the presence of cross-tier interferences. We investigate the outage probability of the ground UAV users to evaluate the performance of the network. Due to intractability of the calculations to derive the exact outage probability, the closed-form expressions are derived for the upper bound of outage probability under Rayleigh and Nakagami-m fading. The effect of UAV altitude, density of D2Ds and corresponding transmission powers are discussed. The results verify the simulations and confirm that the proposed approach outperforms the existing upper bound methods.