Unmanned Aerial Vehicle Relay System: Performance Evaluation and 3D Location Optimization

The deployment of unmanned aerial vehicle (UAV) in wireless communication as a flying base station (BS) or relay is expected to be dominant in the following years to enhance wireless network performance in terms of coverage and capacity owing to their ability to change altitude, easy 3D movement, low cost, and easy deployment in wireless networks. In this paper, we study the performance of a wireless network in which a UAV is employed as a decode-and-forward (DF) relay linking a ground base station (BS) with multiple users in area where direct terrestrial path between the ground BS and the users is assumed to be blocked. The channel between the BS and the UAV is assumed to follow Rician channel model, while the links between the UAV and the end users are assumed to follow Rayleigh fading model with opportunistic scheduling scheme for user selection. Closed-form expressions for the outage probability and average symbol error rate (ASER) are derived. Due to complexity of the derived closed-form expressions and in order to get more insights at the system behavior in terms of system coding gain and diversity order, an asymptotic expression is derived for the outage probability at high signal-to-noise ratio (SNR) values. Furthermore, an optimization of the UAV 3D location that minimizes the asymptotic outage probability is achieved. Our numerical results show that increasing the UAV transmit power much more than the ground BS transmit power does not improve the system performance. The increase in the UAV transmit power is positive only if the ground BS transmit power is higher than the UAV transmit power. We also show that the increase of the Rician $K$-factor leads to improving the system performance when the ground BS transmit power is less than or close to the UAV transmit power. Furthermore, the proposed optimization scheme shows superior performance gain in minimizing the outage probability compared to the conventional scenarios, where the UAV is located at a fixed altitude over the center of the coverage region.