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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 two common relaying protocols. Based on the optimum placement, the two relaying uses are then compared in terms of outage and bit error rate. Numerical results show that the dual-hop option is better when the source-to- destination distance is small. Also, decode-and- forward UAVs provide better performances than amplify-and-forward UAVs. The investigation has also revealed the effects of important system parameters on the optimum UAV …