In this paper, we consider an unmanned aerial vehicle (UAV) aided mobile relaying system under a buffer constraint. We propose a new relaying protocol employing mixed free-space optical/radio frequency (FSO/RF) communication, i.e., the source-relay and relay-destination links utilize FSO and RF links, respectively, under the buffer constraint at the UAV relay node. Taking the conditions of an imbalance in transmission rate between RF and FSO links into consideration, we study the trajectory optimization problem of buffer-constrained UAV relay node in order to maximize the end-to-end data throughput. Especially, we classify two relaying transmission schemes according to the delay requirements, i.e., i) delay-limited transmission and ii) delay-tolerant transmission. We solve the locally optimal trajectory problem of the UAV to maximize the throughput of ground user terminal. As a result, we propose an iterative algorithm that efficiently finds a local optimum solution for the throughput maximization problems. Through this algorithm, we present the resulting trajectories over the the atmospheric condition, the buffer size, and the delay requirement. Also, we show the optimum buffer size and the throughput-delay tradeoff for a given system. Our numerical results validate that the proposed buffer-aided mobile relaying scheme achieves 65.55% throughput gains compared to conventional static relaying scheme.
Index TermsThis work was presented in part at ). 2 Mixed FSO/RF communication, UAV-aided mobile relaying, throughput maximization, buffer constraint, delay-considered design. I. INTRODUCTION As it becomes easier to implement unmanned aerial vehicles (UAVs), UAV-aided mobile relaying systems have attracted great research interest [1]. Motivated by the mounting interest in the unmanned flying platforms, a UAV-enabled mobile relaying framework have been proposed to transport the backhaul/fronthaul traffic between the access and core networks [2]-[4]. Compared to a conventional static relaying system, the mobile relaying has several key advantages. Above all, cost-effectiveness and easy deployment make the mobile relaying systems especially suitable for unexpected or temporary events, such as emergency response, disaster recovery, military operation, etc. In addition, its high mobility offers new opportunities for delay-tolerant applications (e.g., periodic sensing, large data uploading/downloading) and performance improvements through the dynamic relay relocations to achieve the better communication environment. In particular, the authors in [5] identified that the mobile relaying offers a new degree of freedom for performance enhancement compared with conventional static relaying, via careful relay trajectory design. Based on the result of [5], a few works including [6]-[8] have focused on utilizing mobile relaying system in various applications. Especially in fifth-generation (5G) and future wireless networks, wireless backhaul/fronthaul networks are required to meet ultra high rate requirements, particularly in the p...