## 3 System Design with Simulation and Experimental Results
### System Simulation and Experimentation
#### 3.3.2 Experimental Test
The experimental results are presented in Figure 12.  
Figure 12 demonstrates that as the transmitter elevation angle increases from near 0\({}^{\circ}\) to 45\({}^{\circ}\), BER rises correspondingly. This phenomenon stems from two key factors: increasing path loss reduces the number of received ultraviolet photons at the receiver, and frame header synchronization becomes more challenging, thereby increasing data transmission failure rates. Additionally, the 4 FSK-LDPC system consistently outperforms the uncoded 4 FSK system in terms of BER across all tested angles. Notably, the BER growth trend of the 4 FSK-LDPC system remains more gradual compared to the uncoded counterpart as the elevation angle increases. Furthermore, both experimental and simulation results exhibit consistent trends, with simulation BER generally outperforming experimental BER. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Values** \\ \hline UV light wavelength & 254 nm \\ Center wavelength of the optical filter & 261 nm \\ Power of UV light & 3 W \\ Bit Rate & 5 kbps \\ Receiver elevation angle (NLOS) & \(\approx\)45\({}^{\circ}\) \\ Transmitter elevation angle (NLOS) & 10–45\({}^{\circ}\) \\ Transmission range (NLOS) & 60 m \\ Height of obstacle (NLOS) & 4–5 m \\ Transmission range (LOS) & 500–900 m \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of outdoor NLOS field test. Figure 11: NLOS test: (**a**) test equipment of TX; (**b**) test equipment of RX; (**c**) test scenario. In the LOS experiment, BER tests were performed at line-of-sight distances ranging from 500 to 900 m in 50 m increments, with the results presented in Figure 13. The results demonstrate that as transmission distance increases from 500 m to 900 m, the system BER exhibits a continuous increasing trend. Notably, the rate of BER growth accelerates with longer propagation ranges. At equivalent distances, the LDPC-4 FSK system achieves a BER approximately one order of magnitude lower than the uncoded 4 FSK system. The experimental BER of both systems is consistently higher than simulated values, primarily due to two factors: (1) high particle density in air causes multiple scattering effects that are not captured by the single scattering model used in simulations, with ultraviolet photons interacting with atmospheric molecules and aerosol particles leading to increased path loss; (2) wavelength mismatches between mercury lamp emissions and optical filters combined with temperature-induced efficiency degradation of the light source reduce reception efficiency, as demonstrated by photon loss at the receiver and power output decline under prolonged operation. Synthesizing the results of the NLOS and LOS experiments, we can evaluate the communication capability of the UV communication system in this paper. In NLOS communication, increasing transmitter elevation angles lead to significant attenuation of scattered energy, resulting in corresponding BER increases. In LOS communication, prolonged propagation distances exacerbate path loss, degrade receiver signal-to-noise ratio (SNR), and thereby elevate BER. The exponential decay of received optical power with distance causes the BER growth rate to accelerate progressively as the communication range increases. In communication scenarios where angular changes exist, the 4 FSK-LDPC system shows superior anti-angle variation interference performance compared to the uncoded 4 FSK system. Specifically, when the transmitter elevation angle ranges from 10\({}^{\circ}\) to 45\({}^{\circ}\) at a propagation distance of 60 m, the 4 FSK-LDPC system maintains a BER below 5.04 \(\times\) 10\({}^{-4}\). For long-distance LOS communication, LDPC coding effectively enhances the  
Figure 12: BER of different transmitting elevation angles for NLOS. Figure 13: The relationship between BER and distance. anti-interference capability and transmission reliability of the 4 FSK modulation scheme.