# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 3 Theoretical Description of the Proposed Modulation  
As it was aforementioned, there exist several modulation schemes developed or adapted for OCC. However, there are three main constraints for these modulation techniques: data rate, limited distance range, and algorithm complexity. Furthermore, OCC implementations should avoid the flickering problem since the lights may be used also for illumination, decoration or advertising purposes without disturbing people. Flickering-free modulations with low complexity algorithms only perform adequately for medium distance ranges since the number of bits that can be allocated in a single frame decreases with distance. In the case of long-distance modulation schemes [24; 25; 26; 37; 39; 40], these techniques rely on high-complexity algorithms that require adaptive thresholding method and a calibration process for identifying the light state. Moreover, the most advanced schemes use several bits in each transmission header for assuring the calibration, decreasing the throughput [29; 30]. In general terms, the modulation methods presented in the previous section work correctly for either short or long distances but not for both cases simultaneously. In this work, a flickering-free and distance-independent modulation based on frame comparison that overcome these constraints is proposed and evaluated. This scheme can be defined as a rolling shutter effect-based modulation that requires a reduced minimum number of light bands for extracting the information. Hence, it can work for short, medium, and long distances. As a brief description, the proposed technique selects different signals as symbols for the transmitted data (in this work, 2 bits per symbol were chosen). These signals use specific switching frequencies, as shown in Figure 2, based on the system's characteristics to improve the link capabilities (flickering avoidance and operation range enhancement). Additionally, the system employs constant voltage as input signal of the emitter when it is not transmitting. In this was, the proposed modulation reduces the detection process's complexity, since the 2D standard deviation is directly used during the frame synchronization, to detect a change from constant light to switching signal. Similarly, this scheme mitigates the problems related to the system's sensitivity to background light by using other statistical function, Pearson's Correlation Coefficient (PCC), for symbol detection, instead of determining the frame illumination level which usually requires thresholding techniques and an additional calibration process. At this point, the scheme is similar to a traditional Frequency Shift Keying (FSK) modulation, but the demodulation procedure is based on two main assumptions that ease the detection procedure and reduce the needed frequency variation on the transmitter side. On the one hand, thanks to the rolling shutter scanning mode of CMOS cameras, the apparent movement of a given frequency between two consecutive frames is determined by the  
Figure 1: Analysis of Optical Camera Communication modulation schemes distributed by complexity and achieved link range.  
modulo operation with respect to the frame acquisition rate of the camera. In the same way, the likeliness of consecutive frames, obtained from the PCC value, can be used for determining the represented symbol. In the next section, all the procedures will be in-depth described and explained.