## 1 Introduction
### Previous Work  
FSO relaying transmission has been extensively investigated by many researchers in the past, e.g., [6; 7; 8; 9; 10; 11]. Refs. [6; 7; 8; 9; 10; 11] mainly focused on mixed FSO/RF transmission. More specifically, Ref. [6] considered a dual-hop multi-user relay system with mixed FSO/RF links, where closed-form expressions for the outage probability and ergodic capacity were derived accordingly. Ref. [7] considered a FSO backhaul network, where concise mathematical expressions for different performance metrics were derived accordingly, e.g., outage probability, average bit error rate (BER), and so on. Ref. [8] considered a mixed FSO/RF spectrum sharing network, where the transmit power and jamming power of a second user (SU) were jointly optimized to achieve the best secrecy performance under both a RF-dominant case and FSO-dominant case. Ref. [9] considered a mixed RF/FSO dual-hop transmission system, where closed-form expressions for outage probability and bit error probability were derived. Ref. [10] analyzed the secrecy outage probability for a dual-hop relaying transmission system involving a hybrid MIMO RF/FSO link, where four transmit antenna selection (TAS) schemes were proposed to enhance the secrecy performance under imperfect CSI. Ref. [11] investigated the throughput maximization issue in a parallel hybrid RF/FSO channel, where the optimal relay selection and time allocation policies were obtained a for buffer-aided relay and non-buffer-aided relay. However, since RF transmission has broadcasting characteristics, all sensors and eavesdroppers can still receive RF signals from an UAV, even with security authentication, and a more complicated technique is needed to ensure the privacy of information transmissions. Combining the above illustrations of the FSO technique and [12], the security performance of mixed FSO/RF transmission is usually not as good as FSO transmission alone. Refs. [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23] mainly considered a single FSO relay transmission with high security performance. Ref. [13] considered a three-node cooperative FSO transmission network with an energy harvesting (EH) decode-and-forward (DF) relay, where a novel harvest-store-use (HSU)-based relaying transmission strategy was proposed to improve the overall performance. Ref. [14] considered a single FSO-based distinct eavesdropper (ED) located near the relay, wiretapping the FSO link, where closed-form expressions for the secrecy outage probability (SOP) were derived for three different application scenarios. Ref. [15] investigated secure communications with the aid of hybrid FSO/RF links in a two-phase uplink transmission, where the trajectory and power allocation of the UAV were jointly optimized to maximize the average secrecy rate of the network. Ref. [16] investigated the physical layer security (PLS) performance of a decode and forward (DF) dual-hop mixed FSO/RF communication network in the presence of multiple eavesdroppers, where the lower bound for the secrecy outage probability (SOP) and effective secrecy throughput (EST) were derived. Ref. [17] considered a FSO communication network with a single relay, where a distance-dependent cooperative scheme was proposed to achieve a higher diversity order. Ref. [18] considered a relay-based FSO communication network, where a novel technique was further proposed to regenerate the optical signal. Ref. [19] considered a FSO communication system and proposed a novel protocol for enhancing the cooperative diversity, where a closed-form asymptotic expression for outage probability was derived. Ref. [20] considered a dynamic cooperative FSO transmission network, where two resource-allocation schemes were proposed to jointly optimize the performance. Ref. [21] proposed a novel relay selection strategy to improve the quality of transmission in a multi-relay assisted FSO transmission network, where the closed-form outage probability was derived accordingly. Ref.