## 6 VLC-IoT: Existing and Proposed Approaches
### Modulation Techniques in VLC-IoT
#### 6.2.2 Proposed Approaches  
The proposed modulation schemes implemented for VLC-IoT such as DMT, and CAP which offer much higher throughputs are discussed in this section.  
1. Color shifting key (CSK)  
The CSK modulation was proposed in the latest IEEE 802.15.7 standard where transmitted bits are encoded to specific colors in the color space chromaticity diagram (the CIE 1931 coordinates). However, the major limitation of CSK is that it cannot be implemented in VLC systems where the light source is phosphorus-based LEDs [47]. In addition, the application of CSK involves complex circuit structures. Nonetheless, the IEEE 802.15.7 standard PHY III specifies the use of CSK for systems equipped with multiple sources of light and color-filtered photodetectors. Additionally, the combination of CSK modulation with OCDMA has been proposed in VLC-IoT systems to enable massive connectivity by allowing the simultaneous transmission to multiple users [34].  
2. Discrete multitone (DMT)  
The nonlinear characteristics of LEDs can impact severely on the performance of VLC systems. To mitigate this drawback, techniques such as DMT which defy non-linearity have been proposed [48; 49]. In DMT modulation, a single-carrier signal is divided into multiple independently modulated flat subcarriers that are orthogonal to their corresponding time-domain waveforms [50]. Thus, signal transmission is optimized over the subcarriers due to the flat channel response within the subcarriers by means of adaptive bit-loading. Again, in DMT, the signal power distribution over the subcarriers can be adjusted to have similar BER performance for enhanced and efficient use of available bandwidth. To further improve BER performance, a post-distortion non-linear elimination algorithm can be applied together with DMT in VLC systems [49]. Presently, a 1 Gb/s transmission rate has been demonstrated in VLC using white LED and DMT signal [50]. In addition, a 1.5 Gb/s transmission has been achieved over 10 m LED-based SI-POF link using a multilevel CAP modulation [51].  
3. Carrier-less amplitude-phase (CAP)  
CAP is a high spectral efficiency modulation scheme that is similar to OFDM and is a variant of quadruple amplitude modulation (QAM) [52; 53] in which two pulse amplitude modulated signals are filtered using pairs of orthogonal pulse shaping filters forming a Hilbert pair [53]. The deployment of two orthogonal baseband signals in CAP eliminates the need for discrete Fourier transform and inverse discrete Fourier transform [52]. Another unique feature of CAP is that it can be implemented as a single or multiband scheme. CAP has a low peak-to-average power ratio (PAPR) in comparison to DMT when deployed as a single carrier modulation [54]. For this reason, CAP has been proposed in VLC-IoT scenarios where power constraints are imposed by the transmitter front-end and design considerations. A comprehensive article on the implementation of CAP in VLC is presented in [54]. Other advantages of CAP include less complexity and easy implementation [55] as well as lower power consumption and lower cost [52].