# 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  
Non-orthogonal multiple access (NOMA) is deemed to be another promising technology that can significantly improve spectral efficiency and connectivity. Different from orthogonal multiple access (OMA), multiple users in NOMA could transmit on the same frequency resources at the same time by superimposing the signals of multiple users in the air. At the receiving terminal, successive interference cancellation (SIC) could be applied to sequentially decode each individual signal from the received composite signal. It has been proven that NOMA could increase spectral efficiency and promote large-scale connectivity. Meanwhile, NOMA signaling can reduce the cost of communications systems to help provide low-latency services. In [41; 42; 43], researchers analyzed the signal-to-noise (SNR) about different OFDM modulation methods of PLC and IWOC system. It is anticipated that the application of NOMA to the IWOC systems will further increase the system capacity and user connectivity [44; 45]. In [45], it was shown that the spectrum efficiency of IWOC system with NOMA function is superior to that of IWOC system with conventional orthogonal frequency division multiple access (OFDMA). In [45], the authors analyze the performance of the coverage probability and the ergodic rate. The results show that using NOMA could enhance these performance metrics compared to OMA. Though there have been some pioneering works in the NOMA-enabled IWOC systems, these studies mainly focus on low order modulation schemes. Furthermore, [PERSON] and his researchers in [46] proposed a NOMA scheme based on VLC based on optical power domain to address the deficiencies of the traditional PD-NOMA scheme. The maximum drive current of the scheme is significantly reduced, and the gain bandwidth product of the drive circuit is also obviously reduced. Experimental results show that the OPD-NOMA scheme provides better bit error rate performance. The experiments are as shown in Figures 9 and 10. Where Figure 9 shows the experimental setup of the proposed IWOC-OPD-NOMA for two users [46]. Use arbitrary waveform generator (AWG, RIGOLDG5352) to convert digital OFDM signal to an analog format. Note that the AWG is also used as a driver amplifier. The detail block diagram is shown in Figure 10. In T\({}_{\mathsf{X}}\), two 1.7 Mbaud-based band OFDM signals are mapped to QPSK and converted to an RF subcarrier whose frequency is 1.25 MHz. When the symbol rate is 1.7 m, the maximum output peak voltage (Vpp) is 20 V. For the driver amplifier, the gain-bandwidth product is a constant [46]. As the symbol rate increases, the maximum output value decreases. The LEDs in the LED array are divided into two groups, and each group is assigned to a user. The OFDM electrical signals of different Vpp use the offset tee module to perform DC level shift before the IM of the grouped LEDs. The modulation bandwidth of the LED is 2.5 MHz [46]. In addition, the bit error ratio (BER) comparison between NOMA and OMA is shown in Figure 11. As can be seen, it could be used in the OMA low SNR regime [47]. On the contrary, the simulation results of NOMA show that compared with OMA, the BER is derived under the condition of low SNR regime more accurately. Furthermore, it should be noted that NOMA has a worse BER than OMA owing to the multi-user interference in NOMA. Specifically, for the user that is decoded in NOMA first, the influence of this mutual interferences is more obvious. For the user that is decoded last in NOMA, if the SIC performance is good enough, its bit error rate performance is similar to that of OMA. In addition, for users that are decoded first in NOMA, when a higher modulation scheme is utilized, the BER gap between OMA and NOMA will  
Figure 10: Block diagram for the downlink IWOC-OPD-NOMA [46]. Figure 9: Experiment setup for IWOC-OPD-NOMA [46]. increase. This is because under the same transmission SNR, the constellation of high-order modulation schemes is more susceptible to noise [47].