### Related Works  
In terms of FSO communication systems based on UAV relays, current related research primarily focuses on channel modeling and system parameter optimization. A tractable performance indicator expression is crucial for an effective system performance analysis. Specifically, the outage probability (OP) of an FSO system under a log-normal channel was analyzed [10]. An integral form of the OP for the multi-hop FSO communication system was obtained [11]. In [12], the authors analyzed the throughput of a UAV-based hybrid FSO/radio frequency (RF) system with buffer constraints. In [13], the authors studied the performance of a dual-hop fixed-gain FSO relaying system with differential detection and direct detection. In [14], a theoretical expression of the OP for FSO communication systems based on a Markov chain was obtained. However, these studies [10; 11; 12; 13; 14] fail to account for the impact of the angle of arrival (AoA) fluctuations resulting from the orientation deviations of hovering UAVs. In [15], a multi-rotor UAV-based air channel was established by comprehensively considering path loss, atmospheric turbulence, pointing error, and AoA fluctuation. For FSO communication with a high-altitude UAV, the weak atmospheric turbulence, path loss, pointing deviation, and AoA fluctuation were considered while modeling the UAV-to-UAV link [16]. In [17], an FSO channel model with weak turbulence, pointing errors, and AoA fluctuation was considered. For the multi-rotor UAV relay-based air channel, the AoA fluctuation was also discussed to demonstrate the impact of the UAV jitter [18]. However, the derived system performance expressions in [15; 16; 17; 18] are cumbersome and intractable. Therefore, to obtain further insights, it is necessary to develop tractable theoretical expressions. Moreover, the majority of current research on UAV-based FSO communication has primarily focused on serial relaying. For instance, in a serial UAV-based multi-hop relaying FSO system, exact and asymptotic expressions of OP were derived [19]. The performance of a triple-hop FSO system with amplification-and-forward relaying has been analyzed [20]. In [21], a regenerate-and-forward relaying scheme was proposed for an FSO serial relay-assisted system to expand its transmit distance. Using the decode-and-forward (DF) scheme, the authors of another paper optimized key parameters to minimize the outage probability of a UAV-based FSO system [22]. The throughput of a dual-hop FSO/RF hybrid system based on serial UAV relays, was maximized in [23]. When multiple UAV relays exist in the system and cooperate and [PERSON], the use of parallel relays can further enhance the overall performance of the system. However, the performance of a parallel-UAV-relay-based FSO system has not been extensively investigated in the available literature. [PERSON] Contributions and Organization  
Motivated by the issues mentioned above, this paper focuses on the error performance analysis of an FSO communication system based on parallel UAV relays. The main focus in this paper is error probability, which is a key aspect of reliable communication in [PERSON]'s information theory. The main contributions of this paper are as follows:* We establish a system model for parallel-UAV-relay-based FSO communication systems. We consider an FSO communication scenario with a source node, \(M\) relay nodes, and a destination node. The received signal and signal-to-noise ratio (SNR) are analyzed. We establish an FSO channel model that considers atmospheric loss, atmospheric turbulence, pointing error, and AoA fluctuation. * We analyze the performance of a system for parallel-UAV-relay-based FSO communication. For the considered system, we obtain tractable closed-form expressions of the OP and average bit error rate (ABER)[PERSON] Our numerical results indicate that the simulation results are in good agreement with the theoretical results, which verifies the correctness of the expressions we derived. * We analyze the asymptotic performance of the considered system.