## 3 O-RAN with 200 m FSO Link
### The 200 m FSO Link Backhaul for O-RAN with 4 m Fronthaul for Indoor Network  
After the successful establishment of FSO normal operation, the FSO link can be integrated into the O-RAN system. As depicted in Figure 6, the servo data center network is routed through a switch to the FSO, which then transmits across the physical barrier to the receiving FSO. Subsequently, the receiving FSO connects to the O-CU and O-DU sides of the RAN via a switch. The repeater building's O-CU and O-DU sides can facilitate the signal transition to the O-RU side through fiber-optic fronthaul. The O-RU can directly serve mobile phone users or be converted to Wi-Fi by CPE for cellular phones or other electronic devices. Notably, direct communication between the O-RU and cell phone users necessitates a proprietary 5G chip, whereas electronic device users via CPE do not require this proprietary chip. Incidentally, the frequency band used by O-RAN is n79 (4.8 GHz to 4.9 GHz), the bandwidth is 100 MHz, and the downlink and uplink ratio is DSUUU (2:3). Based on this setup, the maximum uplink throughput is 181 Mbps, and the maximum downlink throughput is 410 Mbps, as provided by the supplier of O-RAN equipment. Similarly, it should be noted that the experiment was conducted on a university campus, and due to network traffic limitations, the uplink and downlink throughput limit was 100 Mbps, as shown in Figure 8. Figure 9 shows the network performance measurement of the O-RU via 4 m fiber-optic fronthaul directly to cell phone users. There were four scenarios of the distance between the O-RU antenna and the mobile phone: 10 m, 20 m, 30 m, and 40 m (measured within an indoor corridor). The O-RU antenna was about 2 m above the ground, the cell phone was about 1 m above the ground, and the sampling interval was once every two minutes. From Figure 9a, it can be seen that, irrespective of the distance between the cell phone and the O-RU antenna, the downlink throughput stayed around 80 to 90 Mbps. However, the uplink throughput was smaller and more variable than the downlink throughput, which is mainly attributed to the multipath interference generated by the non-open space, the smaller packets processed by the uplink than the downlink, and the upper limit of the transmission power of the mobile device. In addition, there were also antenna directivity issues, such as a significant height difference between the antenna and  
Figure 8: Throughput and ping records before and after FSOs. the cell phone, which indirectly led to more path loss and multipath effects interference, whereas downlinks are usually designed to be resilient against interference and fading and are less susceptible to antenna directivity issues. In Figure 9b, the performance of jitter (J) and ping are shown. Ping is the average latency, jitter is a measure of the variation in the time it takes for a packet to arrive, and jitter reflects the consistency of the latency. The ping stayed around 20 to 30 ms, while the jitter stayed around 0 to 11 ms. These indicators demonstrate that the network transmission is functioning well, and the values are consistent with the tests conducted by the supplier without the FSO link. Figure 10 shows the O-RU antenna via 4 m fronthaul to the CPE, followed by the CPE feeds the network to the cell phone. The distance between the CPE and the O-RU antenna was arranged in three scenarios--0 m, 20 m, and 40 m--while the distance between the cell phone and the CPE was arranged in four scenarios--5 m, 10 m, 15 m, and 20--so there were a total of 12 measurement scenarios. Moreover, the O-RU antenna distance was about 2 m from the ground, and the CPE distance was about 0.5 m from the ground. The sampling was also done every two minutes, and the measurements were also conducted in the indoor corridor.