# 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
In [48], cellular respiration has been applied to traffic transmission between devices in cellular networks and access point. In [49], a distributed algorithm based on pricing has been used, which regards the congestion degree of the base station (BS) and the transmission environment of mobile terminals as parameters of load balancing in heterogeneous networks. Figure 11: (**a**) The comparison of non-orthogonal multiple access (NOMA) with OMA in terms of BER with QPSK; (**b**) the comparison of NOMA with OMA in terms of bit error ratio (BER) with 16 PSK [47]. On the bases of line voltage, the lines in the transmission network can be divided into high voltage, medium voltage, and low voltage in [50; 51; 52; 53]. In general, the power system includes generation, transmission, distribution, and consumption [51]. HV generates the power and transmits it. Then MV and LV regions distribute the power, and finally LV completes the consumption [51]. Typically, the range of high, medium, and low pressures varies depending on the region, industry, and so on. Generally speaking, the standard voltage classes for HV are between 110 kV and 380 kV, such as 132 kV and 220 kV, the standard voltage classes for MV are from 10 kV to 33 kV, like 13.2 kV and 33 kV, which are mentioned in [54], and the LV classes are below 1 kV, generally 0.24/0.4 kV [52; 53; 54]. Considering the rapid development of power grid in large and super-large cities in recent years, the HV is going to 500 kV. Distinct from the low-voltage power line studies above, researchers from Penn State University discussed the transmission characteristics of medium and low piezoelectric transmission media and IWOC system [50]. According to [50], both medium-voltage and low-voltage power networks can provide very high transmission capacity that cannot be achieved by any other cable network except optical fiber, and the reflection caused by the impedance mismatch in the above networks will worsen the performance of the system. At the same time, the research results show that the PLC subsystem has the possibility to realize 1 Gbps transmission rate through the reasonable network design for instance impedance matching. In the IWOC subsystem, the different influences of multipath propagation on high-speed data transmission are considered. LEDs are commonly used in indoor settings as transmitters of OWC. Therefore, in [50], the authors consider the IWOC model, and conclude that distortion are related to room size and system configuration, and the above distortion can be minimized through judicious design. This work theoretically preliminarily proves the technical potential of medium and low voltage PLC and IWOC integrated systems to provide high-speed transmission. In order to verify the actual performance of smart power OWC, research teams at domestic and abroad actively carry out the construction and test of power wireless optical system [55; 56; 57]. Furthermore, to better meet the need of hot scenes such as hospitals, shopping malls, and gymasiums, researchers from Tsinghua University are committed to applying integrated PLC and IWOC technologies to indoor broadband data broadcasting. The proposed system significantly reduces the complexity of IWOC network protocol and the degree of infrastructure transformation. Specifically, the researchers gave an experimental demonstration of LED two-light network and evaluated the performance of the proposed scheme [15], as shown in Figure 12. The high-definition television (HDTV) program is directed to the hybrid system through the time domain OFDM technology. The measurement shows that the bandwidth of the system is 8 MHz, specifically distributed between 2 MHz and 10 MHz. Figure 12: Demonstration setup of the PLC and IWOC broadcasting system in the laboratorial environment [15]. Under the condition that the visible light path length is extended to 8 m and the indoor interference lighting source is normally turned on, and the proposed hybrid system can provide a transmission performance of about 48 Mbps.