Unmanned aerial vehicles (UAVs) have found many important applications in communications. They can serve as either aerial base stations or mobile relays to improve the quality of services. In this paper, we study the use of multiple UAVs in relaying. Considering two typical uses of multiple UAVs as relays that form either a single multi-hop link or multiple dual-hop links, we first optimize the placement of the UAVs by maximizing the end-to-end signal-to-noise ratio for three useful channel models and two common relaying protocols. Based on the optimum placement, the two relaying setups are then compared in terms of outage and bit error rate. Numerical results show that the dual-hop multi-link option is better than the multi-hop single link option when the air-to-ground path loss parameters depend on the UAV positions. Otherwise, the dual-hop option is only better when the source-to-destination distance is small. Also, decode-and-forward UAVs provide better performances than the amplify-and-forward UAVs. The investigation also reveals the effects of important system parameters on the optimum UAV positions and relaying performances to provide useful guidelines.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The work of N. Zhao was supported in part by the Open Research Fund of the National Mobile Communications Research Laboratory, Southeast University, under Grant 2018D03, and in part by the Fundamental Research Funds for the Central Universities under Grant DUT17JC43. The work of Z. Ding was supported in part by the U.K. EPSRC under Grant EP/P009719/1 and in part by H2020-MSCA-RISE-2015 under Grant 690750. Part of this paper was presented at the IEEE 87th Vehicular Technology Conference, Porto, Portugal, June 2018 . The associate editor coordinating the review of this paper and approving it for publication was R. Dinis.