Ergodic Sum Rate Analysis of UAV-Based Relay Networks With Mixed RF-FSO Channels

Abstract

Unmanned aerial vehicle (UAV)-based communications is a promising new technology that can add a wide range of new capabilities to the current network infrastructure. Given the flexibility, costefficiency, and convenient use of UAVs, they can be deployed as temporary base stations (BSs) for ondemand situations like BS overloading or natural disasters. In this work, a UAV-based communication system with radio frequency (RF) access links to the mobile users (MUs) and a free-space optical (FSO) backhaul link to the ground station (GS) is considered. In particular, the RF and FSO channels in this network depend on the UAV's positioning and (in)stability. The relative position of the UAV with respect to the MUs impacts the likelihood of a line-of-sight (LOS) connection in the RF link and the instability of the hovering UAV affects the quality of the FSO channel. Thus, taking these effects into account, we analyze the end-to-end system performance of networks employing UAVs as buffer-aided (BA) and non-buffer-aided (non-BA) relays in terms of the ergodic sum rate. Simulation results validate the accuracy of the proposed analytical derivations and reveal the benefits of buffering for compensation of the random fluctuations caused by the UAV's instability. Our simulations also show that the ergodic sum rate of both BA and non-BA UAV-based relays can be enhanced considerably by optimizing the positioning of the UAV. We further study the impact of the MU density and the weather conditions on the end-to-end system performance.

Communication and networking technologies for UAVs: A survey

“…Effective iterative lowcomplexity algorithms have been proposed to solve the optimization problems associated with these types of relay networks [93]. Work by Ajam et al [94] established the ergodic sum rate of a UAV-based relay networks with mixed RF and free-space optical channels. Their analysis showed that these networks are able to provide high rate, which can be further enhanced by the optimal positioning of the UAV.…”

On UAV Selection and Position-Based Throughput Maximization in Multi-UAV Relaying Networks

“…This section presents a study of two UAV selection schemes (HM and BDS) that guarantee a certain quality-of-service (QoS). In extreme physical conditions and high terrain areas UAVs can be used as moving relay nodes to relay the information from one location to another location [28]- [31]. In such scenarios, the efficient UAV selection and the position optimization of UAVs are of utmost importance in order to attain a good performance for the data transmission.…”

Buffer‐aided relaying: a survey on relay selection policies

“…The advantage of using BA UAV‐based relay has been discussed in [118]. In [118], the communication system considered is a DF UAV‐relay connecting MUs via RF links to a ground station (GS) via an FSO back‐haul link. Due to the mutual orthogonality of the links, the UAV‐based relay can simultaneously transmit and receive.…”

“…The RF and FSO channels depend on the UAV's location and instability, respectively, where the relative position of the UAV with respect to the MUs affects the LOS connection in the RF link, whereas the instability of the UAV affects the FSO channel's quality. Ajam et al [118] analyse the E2E system performance of networks employing UAVs as BA and non‐BA relays, in terms of ergodic sum rate. Results show that, when the weather conditions worsen and the atmospheric loss in the FSO channel intensifies, positioning the UAV closer to the GS improves the back‐haul channel's quality.…”

“…For a higher density of MUs and larger amount of data received via the RF channel, performance can be enhanced if the UAV's position becomes closer to the GS. Results reveal that the variations of the FSO channel due to the UAV's instability can be diminished by equipping the UAV relay with a buffer, resulting in larger ergodic sum rate, compared with non‐BA relaying, at the expense of additional delay [118]. Depending on the network setup (position of relays), parallel relaying [109] is better in some cases, while serial relaying is better in others.…”