Optical Wireless Communications

  The optical spectrum can serve as a good spectrum resource for wide⁃band wireless communications. The advantages of optical wireless communications (OWC) mainly lie in two aspects: the potential large transmission bandwidth due to the high⁃frequency carrier, and the communication security due to no radio⁃frequency radiation. Thus OWC can be applied in the scenarios where the radio silence is required or the radio frequency radiation may cause explosions, for example in the battle field or some special areas in the storehouses.

  OWC can be performed in the infrared spectrum, the visible light spectrum, and the ultra⁃violet spectrum. Daily OWC applications typically cater for the visible light spectrum, which can be achieved using the lighting emitting diode (LED) as the transmitter and the photodiode (PD)/avalanche photodiode (APD) as the receiver. A typical application of the visible light communication (VLC) is the indoor VLC auto⁃cell network, where the LEDs are mounted on the ceilings, and the user equipments (UEs) include handheld terminals, robots, and intelligent furniture and appliances. Information and control messages are transmitted from the LEDs to the mobile UEs in an indoor VLC intelligent network. Moreover, the visible light spectrum can be adopted for equipment identification. This can be achieved via a camera. The camera emits the light to the optoelectronic tags, and then the tags can be charged and identified.

  Against those application requirements, the indoor VLC has attracted significant research interests from both academia and industrial areas. Due to the non⁃coherent characteristic of the LED⁃based VLC signals, the intensity⁃modulation direct⁃detection (IM/DD) communication signal processing has been adopted. To increase the transmission rate, the orthogonal⁃frequency division⁃multiplexing protocols have been tailored for the indoor VLC. To mitigate the interference between the adjacent LEDs, various inter⁃cell interference cancellation protocols have been adopted, such as the beamforming, fractional⁃frequency reuse, and soft⁃frequency reuse. As the LEDs are lighting equipments, the communication should also yield the light constraints, which forms the catalogue of VLC under lighting constraints.

  This special issue includes six papers. The first paper, “Subcarrier Intensity Modulated Optical Wireless Communications: A Survey from Communication Theory Perspective” by Julian Cheng et al., provides an overview on the communication theoretic aspects on the subcarrier intensity modulated optical wireless communications. The second paper, “Short⁃Range Optical Wireless Communications for Indoor and Interconnects Applications” by WANG Ke et al., introduces the short⁃range OWC for indoor and M2M communications. The third paper, “Optical Transmission Power in a Nonlinear VLC system” by QIAN Hua et al., optimizes the transmission power for the VLC system with nonlinearity. The fourth paper, “Modulation Techniques for Li⁃Fi” by Mohamed Sufyan Islim and Harald Haas, shows the modulation techniques for Li⁃Fi communications. The fifth paper, “LDPC Decoding for Signal Dependent Visible Light Communication Channels” by ZHAO Chunming et al., provides the decoding techniques for the indoor VLC with signal⁃dependent noise.