## 2 State-of-the-Art in Light Dimming and Lights-Off Visible Light Communications
### Related Work
#### 2.2.1 Single Carrier Modulations  
The most common single-carrier modulations used in VLC are On-Off Keying (OOK), Pulse Position Modulation (PPM), and Pulse Amplitude Modulation (PAM). Some of the most important advantages of the OOK modulation are the low cost and simplicity. OOK is the simplest Amplitude Shift Keying (ASK) modulation method. Usually, it has a square signal, and the amplitude is on two levels: high power and low power. OOK can use different coding formats, such as Non-Return to Zero (NRZ), Return to Zero (RZ), or Manchester coding. The RZ code ensures the returning to a low power level even when a series of \"1\" bits are needed to be transmitted. The big advantage here is the easier recovery of the clock from the data signal, with the exception of long series of \"0\" bits, a situation where Manchester code is more suitable. In order to have dimming capability, one possible approach is Variable On-Off Keying (VOOK), through the insertion of compensation time [18]. In [25], the authors argue that in order to be perceived as \"off,\" the illumination limit of an LED device in indoor scenarios is dependent on the ambient light, which means that in broad daylight the VLC emitter can communicate with an increased level of power, without affecting the lights-off perception in the room. One drawback, however, is the fact that in vehicle scenarios, the LED emitter is in many cases directly observed, so the glowing must be much lower than in the indirect observance. PPM is an attractive modulation technique for optical communications, offering an increased efficiency in power transmission. There are different types of PPM that can support dimming control, such as VPPM, and Multiple Pulse Position Modulation (MPPM) with its variants: Overlapping Pulse Position Modulation (OPPM), Differential Amplitude Pulse Position Modulation (DAPPM), Digital Pulse-Interval Modulation (DPIM), and so on. Although is easy to implement the VPPM scheme, which is a combination of PWM brightness control with a 2-PPM modulation for communication proposed by the IEEE 802.15.7 standard group, the data rate limitation imposed by the binary implementation asks for a better method of digital dimming. One study [26] evaluated the performance of an _M-ary_ VPPM scheme with dimmable capability for VLC, with increased performance of achievable data rate being demonstrated for a large modulation order. In order to counteract the rapid brightness fluctuations, a novel VPPM scheme was proposed in [27], where the targeted dimming level is achieved based on a step-by-step change in the  
Figure 3: VLC modulation schemes. brightness of the LED while moving average correlation masks are used to keep up with the change. Controlling the number of pulses in each signal block, an MPPM communication with dimming support was proposed in [28], the authors arguing that this modulation is more efficient in terms of power and spectrum requirements than VOOK or VPPM. These results confirm the potential of this type of modulation, as seen in other studies, such as [29], where the band-utilization improved efficiency of MPPM over the PPM was demonstrated. As opposed to classical solutions, where two different modulations are used when dimming is necessary, a Variable-Rate Multi-Pulse Position Modulation (VR-MPPM) was proposed in order to jointly control the brightness control and data transmission in a VLC system based on white LEDs [30]. A novel dimming control scheme for VLC systems was proposed in [31]. Based on MPPM, the system can keep an average light intensity to attain different dimming targets. Another design was made in [32] to provide \"lights-off\" communication at 1.6-kbps data rate, over a distance of 1.3 m, using OPPM (encoding 10 times more bits per light pulse than OOK).