## 2 State-of-the-Art in Light Dimming and Lights-Off Visible Light Communications  
### Light Dimming in IEEE 802.15.7 Standard for Optical Communications  
One of the many obstacles in the development of the Visible Light Communications technology is standardization [17; 19; 20; 21], so different attempts to standardize this technology have been made. Among the different standards that have been proposed, the IEEE 802.15.7 [16] seems to be the one that has the highest complexity and the widest acceptance. This standard has been developed with various challenges in mind, such as avoidance of flickering, dimming support, and maintaining communication when the lights are necessary to be off. As debated in [17], although the standard has drawbacks and vulnerabilities, considering the importance of standardization, the IEEE 802.15.7 standard and its revisions remain for the moment the most covering solution for VLC applications. The lights-off communication can be seen as a particular case of communication in dim conditions, but with an average brightness so low, that it cannot be perceived by the human eye, and therefore, the LED is considered to be \"off.\" The IEEE 802.15.7 standard specifies various methods for dimming support during data transmission, but also during idle periods when no data are being transmitted. Inserting compensation symbols (i.e., idle patterns and compensation time), controlling pulse width, adjusting the amplitude of the signal, and/or controlling in the out-of-band frequency (including the option of using the particular case of an un-modulated DC bias) are the major techniques proposed for dimming support [16; 17; 18]. Figure 2 illustrates the light dimming mechanisms for these techniques, which can simultaneously be used for flicker mitigation.  
Broadly speaking, there are two main techniques for dimming the lighting of an LED source: analog and digital. The analog dimming technique is based on the Continuous Current Reduction (CCR), where the luminous flux is decreased by directly diminishing the forward current through an LED. This type of dimming control can be used with various modulations, such as Pulse-Amplitude Modulation (PAM). The main issue of this technique is the chromaticity shift of the light, due to the under-voltage applied to the LED [22]. Although it has been established that the use of digital dimming is recommended to avoid this issue, a relatively recent study [23] showed that color deviation makes its presence felt even in the digital case, due to different junction temperatures at various fill factor values.  
The digital dimming techniques can be applied to many modulation methods for VLC data transmission, such as Pulse Position Modulation (PPM). The modulated VLC data signal is employed over a digital control signal used in a dimming technique, such as Pulse-Width Modulation (PWM). On these grounds, the IEEE 802.15.7 standard has introduced the Variable Pulse Position Modulation (VPPM), which combines the ability of PWM to provide simple light dimming with the data encoding mechanism of PPM. The resulting VPPM technique is able to provide high-resolution dimming and flicker-free communication while providing constant data rates.  
Figure 2: _Cont._