## 3 O-RAN with 200 m FSO Link
### Preoperation and Measurement of 200 m FSO Link Election  
To ensure the normal operation of the FSO link and its related covenant, a basic test was conducted after the FSOs were set up. Since FSO devices support 1 Gbps and 2.5 Gbps transmission rates, the received power of the FSOs is an important indicator to ensure proper operation. Figure 7a shows the throughput of the FSO device at 1 Gbps and 2.5 Gbps for different power levels, which represents the lowest power that the FSO device requires at 1 Gbps and 2.5 Gbps at about \(-25\) dBm and \(-20\) dBm. The measurement of throughput was conducted using third-party speed measurement software (Iperf3) via a PC. Iperf3 is a tool for measuring network performance over a local network. It creates a data stream between a client and a server. Users can change settings like the buffer length and window size to help monitor and improve network performance. The configurations can impact the throughput measured during testing, enabling users to simulate various network conditions and assess how these changes influence performance. Iperf3 shows information on bandwidth and other important metrics. To conduct a test, two computers are set up: one as the client and the other as the server. The client sends requests to the server, which then transmits data streams to the client through a wireless optical communication system, completing the transmission test. The signal-to-noise ratio (SNR) is the reason why the attenuation of 2.5 Gbps transmission is less acceptable than that of 1 Gbps transmission. Higher receiving power is needed for a higher SNR, assuming that the background noise remains constant. As a result, 2.5 Gbps requires a higher SNR than 1 Gbps, and it naturally requires a higher receiving power than 1 Gbps. In addition, when there is an obstacle in  
Figure 6: The actual setup of 200 m FSOs across the freeway, a scene taken from Google Earth Pro [34]. Schematic diagram of O-RAN using 200 m FSOs as backhaul and fiber optic as fronthaul. the FSO link path for a certain time to block the passage of the beam, it will also affect the normal operation of the FSOs, and Figure 7a shows the effect of different masking times (MTs) on the FSO transmission. When the MT is below 30 s, the recovery time (RT) of FSO transmission is within 10 s, while when the MT is above 40 s, the RT of FSO transmission will be greatly increased or even unrecoverable, so this operation should be avoided. Moreover, measurements were carried out on sunny, cloudy, and rainy days to ascertain the impact of weather conditions on the FSO transmission. The measurement scene, which is the same as the test scene in Figure 7a, is depicted in the upper half of Figure 6. As shown in Figure 6, the integrated technology building (ITB) is where the transmitter of FSO transmission is located, and the everlight building (EB) is where the receiver of FSO transmission is located. The data sampling was done every minute, and the results reveal that the throughput was similar and close to 2.5 Gbps for sunny, cloudy, and rainy days, as shown in Figure 7b. The FSO system exhibited considerable transmission attenuation during rainy weather conditions. Conversely, on sunny days, one would anticipate minimal transmission attenuation; however, this expected outcome was not consistently observed due to higher wind speeds recorded during such weather. While variations in weather can introduce discrepancies in the received power of FSO transmissions, it has been empirically demonstrated that these fluctuations do not significantly impact the overall throughput. This assertion is supported by the substantial levels of received power, as corroborated by the test results in Figure 7a. Hence, FSOs can function normally on sunny, cloudy, and rainy days. In hefty rain or strong winds, the transmission can be enhanced using the previously mentioned methods [26; 27; 28; 29]. To assess network performance parameters, the following experiments were undertaken, disregarding variations in weather conditions.