## 1 Introduction  
In recent years, free-space optical (FSO) communication systems have combined the advantages of microwave communication and optical fiber communication. Theyhave the characteristics of large transmission capacity, fast network construction, being license-free, good security, and so on. They are usually used as the \"last-kilometer\" of telecommunications or LAN links between buildings [1]. However, the performance of terrestrial FSO communication systems is not only affected by atmospheric channels, such as the weather (cloud, fog, rain, snow, haze, etc.) and optical turbulence, but also affected by the pointing errors [2; 3]. In order to mitigate the impact of the atmospheric environment on the performance of terrestrial FSO communication systems and improve the reliability and availability of terrestrial FSO communication systems, a hybrid FSO/radio frequency (RF) transmission system combining an FSO link and a millimeter-wave RF link was proposed [4]. [PERSON] et al. [5] proposed a hybrid FSO/RF transmission scheme based on hard switching (only one link transmits information at the same time) and derived the expressions of the outage probability, average bit error rate (ABER), and ergodic channel capacity (ECC) of the system. [PERSON] et al. [6] proposed a hybrid FSO/RF system with a switching scheme according to the channel state information (CSI), and deduced the closed-form expressions of the ABER and outage probability under a strong turbulent environment. [PERSON] [7] derived new closed-form expressions of the BER and outage probability of a hybrid FSO/RF parallel communication system based on selective combination technology. The results showed that the performance of the hybrid FSO/RF parallel system was better than that of only the FSO system or only the RF system under different atmospheric environments. In order to improve the transmission distance and coverage of terrestrial FSO communication systems, a multi-hop network based on FSO communication was proposed [8]. [PERSON] et al. [9] analyzed the performance of the multi-hop FSO system based on amplify-and-forward (AF) transmission and the channel state information (CSI) assisted or fixed gain relay, and derived new closed-form expressions for the end-to-end ABER and ECC of the system under different binary modulation schemes. [PERSON] et al. [10] proposed a multi-hop FSO communication system based on decode-and-forward (DF) transmission and orthogonal frequency division multiplexing (OFDM), and derived closed expressions of the outage probability and ABER of the system. In order to further improve the transmission performance of multi-hop networks, a parallel multi-hop cooperative communication network was proposed, that is, a relay-aided system [11]. [PERSON] et al. [12] deduced the mathematical expressions of the ABER and outage probability of the relay-aided multi-hop network based on binary phase shift keying (BPSK) modulation under Gamma-Gamma turbulence with pointing errors. [PERSON] et al. [13] deduced the end-to-end ABER and outage probability of the multi-hop parallel FSO system under the Malaga turbulent channel with pointing errors and a DF relay scheme, and analyzed the performance of the system under different turbulent intensity, weather conditions, pointing errors, and relay-aided structures. [PERSON] et al. [14] minimized the mathematical complexity in analyzing outage and error performance by emulating a multi-hop terrestrial FSO communication system with various modulation formats as a single dual-hop scheme. [PERSON] et al. [15; 16] proposed a multi-user multi-hop hybrid FSO/RF communication system, and deduced and analyzed the mathematical expressions of the system performance. Thus, motivated by the above-mentioned facts and considering its military and disaster recovery applications, we first proposed a new communication scheme combining the hybrid FSO/RF parallel transmission and the multi-hop parallel relay-aided communication in Ref.