# Advanced Progress of Optical Wireless Technologies for Power Industry: An Overview
## 2 Classification and Progress of Power Industry Optical Wireless Communications
### Modulation Methods of Hybrid Optical Wireless Communications
#### 2.2.2 Hybrid Optical Wireless System in NOMA Mode
Since the typical ceiling height is 3 m, the receiver consisting of an avalanche diode is placed in an overlapped area covering both lights at a distance of 3 m. To evaluate the system performance under different display configurations, the researchers kept the transmitting signal power of left LED constant, and evaluated the system performance changes under different multipath propagation conditions by adjusting the power line length connected to right LED and the transmitting power level of right LED, as shown in Figure 12[15]. The result shows that the modulation error ratio (MER) is proportional to the power line length and inversely to the power of right LED. The above phenomenon can be explained as: when the secondary path increases or the power level decreases, the interference level will be correspondingly reduced. Therefore, attention must be paid to the profound influence of inter modulation interference on the performance of the receiving end. To further illustrate the various modules in Figure 12, [PERSON] et al. from Tsinghua University illustrated the different hardware devices of PLC and IWOC broadcasting system in the laboratorial environment in Figure 13, including LED temp, PLC to IWOC module, PLC modulator, and demodulator in receiver [15]. In addition, they proposed extending the above system to underground tunnel sites in the next year, as shown in Figure 14. They implemented the IWOC-PLC integrated duplex communications network, and equipment mainly consisting of the LED fixture with DF module and IWOC-PLC coupler,  
Figure 13: The hardware devices of PLC and IWOC broadcasting system in the laboratorial environment: (a) LED temp [15]; (b) PLC to IWOC module [15]; (c) PLC modulator [15]; (d) Demodulator [15]. overview of IWOC-PLC access network [17], duplex IWOC handheld terminal, and R\({}_{\rm X}\) & T\({}_{\rm X}\) modules in handheld terminals [17]. The display system is based on decoding and forwarding technology to realize duplex transmission, so as to build a symmetric duplex broadband access network to support diversified duplex high-speed communication services. In the underground tunnel scene, a plurality of light emitting diodes are used to realize the lighting function, and they are connected by power lines. In the experimental prototype, the length of the power line exceeds 100 m.  
In DF module, PLC and IWOC coupler, uplink photodetector is encapsulated into LED device. In addition, the handheld terminal is embedded in the IWOC receiving and sending modules. Experimental test results show that the distance between LED light source and receiving terminal is 3 m under the condition, the above system can support voice, video, and duplex broadband communication up to 5 Mbps. As long as 3 days of field test, BER performance can be maintained at 10\({}^{-5}\). The data rate of the system can be further extended to 30 Mbps. In the multi-user scenario, efficient and cost-effective non-interference communications are realized between different users. For the application of power tunnel, the research team of state grid also proposed the design of IWOC and PLC protocol for power tunnel inspection and the patrol scheme of LiDAR-based navigation UAV, providing new technical options for the enhancement of power wireless optical patrol technology [55; 56]. In addition, the research team from the Institute of Semiconductors of the Chinese Academy of Sciences focused on smart home scenarios and carried out home network design and a prototype  
Figure 14: The IWOC-PLC integrated duplex communications network: (**a**) The LED fixture with DF module and IWOC-PLC coupler [17]; (**b**) overview of IWOC-PLC access network [17]; (**c**) duplex IWOC handheld terminal [17]; (**d**) R\({}_{\rm X}\) & T\({}_{\rm X}\) modules in handheld terminals [17]. implementation are based on IWOC and PLC. The proposed core equipment of the home network system mainly includes home gateway, white LED driver and network appliance.