# High-Speed Visible Light Communications: Enabling Technologies and State of the Art
## 3 Enabling Technologies for a High-Speed VLC System
### Advanced Modulation  
In VLC, the current flow through the LEDs is controlled by a driver circuit so that the brightness of the radiated light is modulated. By applying advanced high efficient modulation, the limitations of the modulation bandwidth and the transmission data rate are promising to be overcome. Common single carrier modulations include On-Off Keying (OOK), Pulse Position Modulation (PPM), Pulse Width Modulation (PWM), Pulse Amplitude Modulation (PAM) and so on, which have been widely used in IR communications. To realize high data rates, various advanced modulations, such as Orthogonal Frequency Division Multiplexing (OFDM), Carrier-Less Amplitude and Phase Modulation (CAP), Color Shift Keying (CSK), Discrete Multi-Tone (DMT), and Nyquist Single Carrier (N-SC) were proposed.  
(1) OFDM  
OFDM is a kind of digital multi-carrier modulation based on a frequency-division multiplexing (FDM) scheme. In OFDM, multiple closely spaced orthogonal subcarriers transmit data on several parallel data streams or channels simultaneously [17]. Each sub-carrier is modulated with a conventional modulation scheme like quadrature amplitude modulation (QAM) or phase shift keying (PSK), followed by a serial to parallel (S/P) converter. Then, they are converts from the frequency domain input signals into time domain output signals by an Inverse Fast Fourier Transform (IFFT). And a cyclic prefix (CP) is added to further limit the effects of inter-channel interference (ICI) and inter symbol interference (ISI). As the VLC signals are unipolar and real, the conventional OFDM needs some modifications. The Hermitian symmetry to generate real-time signals for implementation can be applied at the cost of doubling the required bandwidth. Multiple kinds of modified OFDM schemes were proposed and used in VLC, such as Asymmetrically Clipped Optical OFDM (ACO-OFDM) [33], DC-biased (Direct Current) Optical OFDM (DCO-OFDM) [34], Asymmetrically clipped DC-biased Optical OFDM (ADO-OFDM) [35], and adaptive bit and power loading OFDM [36]. Figure 6 shows the schematic diagram of DCO-OFDM modulation. In ACO-OFDM, an additional DC-bias is not required as the signal is clipped at the zero level [34]. This is because QAM symbols are only assigned to odd subcarriers, and therefore only the even subcarriers suffer from the noise generated by the asymmetrical clipping that occurs below the zero level, while the odd subcarriers are not [34]. Thus, ACO-OFDM reduces the need for a DC-bias and improves the optical power efficiency, but at the cost of giving up half of the spectrum.  
Figure 6: Schematic diagram of DCO-OFDM modulation.  
By combining with higher-order modulation, OFDM bandwidth is highly efficient and can overcome the multipath effects and ISI. However, OFDM also has some shortcomings that cannot be ignored. The non-linearity of LED needs to be considered as it introduces amplitude and phase distortion to significantly affect the performance of the VLC system. Such an impact is more severe in the high order sub-carrier modulation due to a high peak-to-average-power-ratio (PAPR), such as OFDM, and causes the increase of BER and ICI [8]. Such an impact of non-linearity can be minimized by biasing the LEDs in the optimum operation point, limiting the peak-to-peak input signals, and employing power reduction methods, such as signal clipping [37]. And the nonlinearity also can be compensated by applying pre-distortion [38], post-distortion [39], or nonlinear equalization [40].