## 1 Introduction  
UV light in the 200-280 nm wavelength range of sunlight is absorbed by ozone in the atmosphere, resulting in extremely low intensity reaching the Earth's surface. This band of UV light is utilized for communication with minimal background noise [1, 2]. Due to the scattering effect of particles in the atmosphere on UV light, UV signals can bypass obstacles, enabling UV wireless communication systems to achieve non-line-of-sight (NLOS) communication. Compared to other wireless optical communications, the transmitter and receiver of a UV wireless optical communication system can communicate without alignment [3]. Therefore, UV communication is an ideal choice in scenarios with strong solar background noise or obstacle-obstructed environments [4, 5]. Studies on UV light scattering communication systems can be divided into theoretical modeling and experimental aspects. In terms of theoretical analysis, [PERSON]. [6, 7, 8] investigated the single scattering model of UV photons. Ref. [9] analyzed the relationship among BER, transmission rate, and communication distance of a communication system under an atmospheric scattering transmission channel. Ref. [10] proposed an empirical model for channel attenuation. Ref. [11] developed Monte Carlo-based scattering channel models to study the impact of obstacles on UV scattering communication. Ref. [12] presented a theoretical framework for analyzing the performance of NLOS UV communication systems under multi-user interference (MUI), and Ref. [13] introduced a sample-based non-Monte Carlo method for UV channel analysis. Refs. [4; 6] explored photon-counting reception techniques. Ref. [14] proposed a decode-and-forward relaying-based multi-hop UV communication system to mitigate turbulence effects in short-range scenarios, and Ref. [15] analyzed the performance of cooperative relays for multiple NLOS UV links in turbulent channels. In experimental studies, Ref. [4] successfully demonstrated real-time voice transmission at 2.4 kbps, and Refs. [16; 17] carried out theoretical analysis and experimental measurements of long-distance UV light scattering communication channels. Receiving diversity techniques were investigated in Refs. [18; 19]. Ref. [20] achieved a 75 Mbps NLOS communication link using on-off keying (OOK) modulation, and Ref. [21] extended the NLOS UV communication range to 500 m using a 200 mW 266 nm solid-state laser. Ref. [22] designed a \(2\times 1\) MISO optical Pulse Position Modulation (PPM) system with a photon-counting receiver and discussed the problem of incomplete synchronization in the optical PPM MISO system equipped with a photon-counting detector. Ref. [23] studied the adaptive modulation coding and retransmission of Low-Density Parity-Check (LDPC) codes in ultraviolet communication. With the wide application of UV wireless optical communication in deep space communication and other high-demand fields, the stability and reliability of communication systems have become critical design challenges. Particularly in complex channel environments, communication signals are susceptible to noise and interference, resulting in high BER and communication interruptions. Most of the existing studies on ultraviolet scattering communication adopt OOK modulation and PPM. OOK modulation technology is relatively mature, and there is limited room for performance improvement. The PPM algorithm is rather difficult, which will increase the complexity and cost of system development. Frequency shift keying (FSK) modulation controls the transmitter to output signals of different frequencies by using different symbols, and its implementation is relatively simple. FSK is sensitive to the frequency changes in signals and has good resistance to amplitude fading and noise. As a frequency modulation method, 4 FSK, while having the above advantages, can also achieve a relatively high communication rate. Regarding the 4 FSK demodulation schemes in ultraviolet optical communication, current research is rather limited.