# High-Speed Visible Light Communications: Enabling Technologies and State of the Art
## 3 Enabling Technologies for a High-Speed VLC System
### Equalization  
Based on the characteristics of the VLC channel, the uneven channel will bring great obstacles to the high-speed transmission of the data. Although OFDM modulation technology can improve the system performance to a certain extent, equalization technology is necessary to further enhance the system capacity. And it can be divided into two kinds: software equalization and hardware equalization. The former mainly involves digital signal processing, while the latter one mainly relates to the circuit design. Equalization applied at the transmitter (pre-equalization) is an efficient way to increase bandwidth as well as enhance the data rate.  
(1) Hardware equalization  
According to the previous reports, hardware pre-equalization technology is proved to be an effective way to improve the bandwidth of the VLC system. In 2013, [PERSON] et al. [46] introduced pattern logic (CML) circuits and a simple resistors and capacitors (RC) pre-equalization circuit as the driver circuit of a commercial RGB LED (data transmission using red LED). A data rate of 477 Mb/s was achieved using a low-cost PIN photodiode and OOK-NRZ modulation scheme, and the bit error rate is less than \(10^{-9}\). In 2015, [PERSON] et al. [47] proposed a bridge-T amplitude equalizer, which has excellent linearity and impedance matching performance. A data rate of 2.0 Gb/s over 1.5 m free space was achieved using a commercial phosphorescent white LED with an adaptive bit and a power loading OFDM modulation scheme.  
Figure 8: Schematic diagram of the CSK signal generation.  
Figure 9: Color mapping constellations for x, y and 4-CSK in CIE 1931 (Color Space proposed by the International Illumination Organization in 1931).  
Although hardware equalization can increase the bandwidth of the system to a certain extent, it still suffers from some limitations. But as most of the hardware equalization technologies are based on an analog circuit: (i) analog circuit is susceptible to interference, and its bandwidth is limited, therefore it does not fit for high-speed signal transmission; (ii) The channel of VLC is greatly influenced by the ambient noise, which has higher requirements for the flexibility of the equalizer. However, the analog circuit cannot be debugged and adjusted according to the actual system requirements. Thus, the software equalization providing more accurate and higher flexibility is more attractive [48]. [PERSON]'s group introduced a weighted pre-equalization technique for multi-band CAP signals, and combining different post-equalization schemes, they have experimentally demonstrated a 1.35 Gb/s [49], a 4.5 Gb/s [47], and an 8 Gb/s [41] high-speed indoor VLC transmission system, respectively. In 2016, [PERSON] et al. applied power exponential software pre-equalization in VLC based on a commercial phosphorescent white LED for high speed transmission. A 2.08 Gb/s VLC transmission over a 1 m free-space distance is implemented through experimental demonstration, which is higher than the data rate record created by applying hardware pre-equalization in a phosphorescent white LED based VLC system [48].