# 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
#### 2.2.1 Hybrid Wireless Optical System in OFDM Modulation  
In view of the fact that multiple light sources with visible light communication functions overlap the service area, [PERSON] research team from Canada proposed collaborating with and provide data for IWOC emitters through power lines. Specifically, they proposed a system architecture named hybrid IWOC-PLC, and gave the corresponding analytical framework [14; 27; 28]. In this scheme,  
Figure 7: The measured channel frequency response (CFR) of a typical integrated PLC and IWOC system [19]. researchers apply spatial optical-orthogonal frequency division multiplexing (SO-OFDM) to multiple sources, and propose an OFDM scheme to explore frequency selectivity of IWOC and PLC channels, as shown in Figure 8[14; 23]. Compared with the integration of traditional IWOC and PLC, this scheme improves the peak to average power ratio (PAPR) problem caused by the application of OFDM to LED emitter, and overcomes the interference between different light sources through the cooperation between spatial distributed light sources. In the meantime, the robustness of the system against local link occlusion is enhanced. Specifically, in the above scheme, the basic mathematical model of the proposed system is to be submitted, specifically considering static continuous noise, periodic static continuous noise, and periodic pulse noise. In the visible light link, only the contribution of line of sight (LOS) component and the influence of stray light scatter noise and receiver preamplifier noise are considered. At the same time, the effect of multipath fading on the performance of the system is ignored since the detector size is much larger than the optical wavelength. Through the quantitative analysis results, the team further confirmed that the IWOC link can provide SNR up to 22 dB to meet the functional requirements of IWOC and basic lighting. Therefore, the overall performance of the integrated system is to a great extent affected by the PLC link. Researchers also try to introduce OFDM and color shift keying (CSK) technologies into the IWOC channel, and study the behavior of these series channels under the condition of full link transmission [29]. Specifically, in the PLC link, researchers sought to apply the quadrature phase shift keying (QPSK) and OFDM modulation schemes. In IWOC link, CSK scheme is utilized to improve system performance. The researchers are determined to the mapping scheme design of OFDM modulation constellation map of PLC link to CSK constellation map in IWOC link. Facing the communications scene between outdoor buildings, researchers in South Africa proposed the integrated scheme of PLC and IWOC to conduct targeted exploration. Similarly, specific system schemes introduce phase shift keying (PSK) and CSK modulation respectively in PLC and IWOC links [30]. The mapping method between PSK and CSK modulation symbols is given and verified by simulation and experiment. At the same time, the interference effect of solar rays on the red, green, and blue symbols of CSK is analyzed and emphasized. Based on the above research work, investigators further try a different modulation scheme combines to obtain the further improvement of the system performance. Specifically, spread frequency shift keying (S-FSK) is processed in PLC link, and an on off keying (OOK) modulation is tried in IWOC. The significant advantages of the scheme depended on its low cost and low complexity. Correspondingly, the disadvantage is the case that the transmission rate is extremely limited. Through the prototype experiment based on this scheme, the researchers measured the impact of sunlight exposure on the system performance at four different points in the day, and determined that sunlight irradiance is the main source of system noise [31; 32]. Figure 8: Block diagram of the hybrid IWOC and PLC for indoor multiuser downlink [14; 23]. The various applications of hybrid PLC and IWOC technologies have been studied in [1; 33; 34; 35; 36].