# High-Speed Visible Light Communications: Enabling Technologies and State of the Art
## 4 State of Art  
In 1999, [PERSON] et al. from Hong Kong University proposed the concept of using fast switched LEDs for communication for the first time [55]. And LED-based VLC began to take shape in the early 2000s in Japan, pioneered by [PERSON] et al. [5] at Keio University. Enthusiasm for this research topic has soon spread worldwide and attracted the attention from various institutions and universities. Active research groups include Keio University, Nagoya University, Nakagawa Laboratory, Germany Heinrich Hertz Institute, Siemens Labs, Oxford University, University of Edinburgh, University of Strathclyde, University of Sunnybird, Boston University, University of California, China Taiwan Jiaotong University, Industrial Technology Research Institute, Far East University of Science and Technology, Fudan University, Beijing University of Posts and Telecommunications, Southeast University, Institute of Semiconductors, Chinese Academy of Sciences, Huazhong University of Science and Technology, Jinan  
Figure 11: Measured BERs of downlink with no filter, common blue filter and filter versus (**a**) data data and (**b**) distance. Reproduced with permission from [2], IEEE, 2015. (Reprinted from IEEE Wireless Communications, 22, [PERSON], [PERSON] etc., A high-performance blue filter for a white-led-based visible light communication system. 61–67, Copyright (2015), with permission from IEEE.) Figure 10: Calculated and measured transmission spectra of the blue filter, as well as the normalized emission spectrum of a phosphor-based white LED and the typical spectral response of a Si-based receiver for reference [2]. Reproduced with permission from [2], IEEE, 2015. (Reprinted from IEEE Wireless Communications, 22, [PERSON], [PERSON] etc., A high-performance blue filter for a white-led-based visible light communication system. 61–67, Copyright (2015), with permission from IEEE. )University, and so on. In November 2003, the Visible Light Communication Consortium (VLCC) was established in Japan to explore different applications of VLC. After Japan, many other large research projects have also been launched, including the Omega Project (2008, European Commission), Smart Lighting Engineering Research Center (2008, USA), Center for Ubiquitous Communication by Light (UC-Light, 2010, USA), Li-Fi Consortium (2011), Center on Optical Wireless Applications (2011, USA) and Canadian Research in VLC (2012). In the recent decade, rapid developments were got, and an IEEE standard (IEEE 802.15.7) was finalized in 2011 [56], enhancing the prospects for commercializing VLC. Table 1 details the recent efforts by various groups in attaining higher speeds using VLC. Type of transmitters, receivers, modulation, and equalization used in each case are also stated for ease of comparison. As shown in Table 1, several demonstrations have shown that it is possible to construct Gb/s class VLC links. And the type of transmitter is an important factor that affects the data rate. While blue filters, modulation, and equalization technologies can improve the system performance to a certain extent. In 2016, using commercial phosphorescent white LEDs and low-cost PIN photodiodes (PDs), a high-speed VLC transmission at >1 Gb/s over a free-space link of 0.6 m in a 2 \(\times\) 2 multiple-input multiple-output (MIMO) visible light communication (VLC) system [57] was achieved. And a 2.08 Gb/s transmission data rate over 1 m free-space transmission distance is achieved by further applying adaptive bit and power loading OFDM [48]. Multi-chips can greatly improve the bandwidth of LED. Ref [58] presented the proof-of-concept of the RGB 2 \(\times\) 2 MIMO VLC system with a data rate of 6.36 Gb/s over a free-space link of 1 m in a RGB LED-based 2 \(\times\) 2 polarization-multiplexing MIMO VLC system. And an aggregate data rate of 8 Gb/s is successfully achieved over 1 m indoor free space transmission with RYGB LEDs by the hybrid equalizer [41]. A novel violet micro-LED array with two sets of inner and outer pixels is reported in Ref [28].