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This paper studies a full-duplex (FD) amplify-and-forward relaying, where a fixed-wing unmanned aerial vehicle (UAV) is dispatched as a mobile relay to serve a source-destination communication pair so as to satisfy their service requirement. Here, the UAV relay flies in a circular path and hence it must continuously change its heading of flight which leads to a bank angle of the aircraft. In order to ensure flight safety, therefore, a bank angle limit is imposed on the UAV relay. By considering a practical FD relaying, namely, there exists both residual loop interference (RLI) and processing time-delay at the relay side, novel signal to interference plus noise ratio (SINR) received at the destination is analyzed, giving a closed-form expression in addition to a concise lower-bound. Armed with the SINR analysis, the size of data received by the destination during a period of flight time of the UAV is calculated, leading to a closed-form lower-bound of the size of data. Taking bank angle limit into consideration and using the calculated lower-bound of the size of data, an optimization problem with the purpose of energy conservation is solved, leading to a novel method for joint adjustment of the UAV's flight parameters. Computer simulation experiments are conducted, and the results demonstrated that the proposed optimization method performs better in terms of energy conservation compared to the benchmark techniques regardless of the value of RLI and/or the service requirement of the source-destination communication pair.

This paper investigates an in-band full-duplex (IBFD) relaying, where a fixed-wing unmanned aerial vehicle (UAV) is dispatched as an amplify-and-forward relay to forward data from a source to a destination. By considering ideal and practical IBFD cases, new received signal-to-noise ratio (SNR) expressions at the destination are derived. Armed with the newly derived SNR expressions and the existing one in literature, three different lower-bounds of the size of data received by the destination are calculated. Using the three lower-bounds of the size of data, an optimization problem corresponding to energy minimization of the IBFD relaying system subject to a total data-bit constraint in addition to a mission completion time (MCT) constraint is solved, leading to three sets of solutions. Computer simulation experiments are conducted and the accuracy of the derived SNR expressions and the three sets of solutions of the optimization problem are compared. The results demonstrated that the newly derived SNR expressions are more accurate than the existing one in literature and are more suitable to be utilized for energy minimization, and the proposed optimization method can complete the relaying task with the minimum energy and MCT. When the residual loop-interference channel power is equal to -100dB, the scheme of ideal IBFD only consumes up to 2.5% of the energy of the compared schemes regardless of the value of the total data-bit constraint.