# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 3 Theoretical Description of the Proposed Modulation
### Wake up Process
All the rows are bright pixels so the 2D standard deviation of the ROI is lower than the threshold (\(\sigma_{F_{1}}<3\%\)) and the transmission is not detected on \(F_{1}\). The ROIs collection is triggered on frame \(F_{2}\)  
Figure 8: Transmission’s examples. (**a**): the transmission started previous to the emitter’s acquisition; (**b**): the emission began after the projection of the transmitter on the frame; (**c**): the first bit was transmitted during the acquisition of the emitter’s pixel rows. so the ROI extracted from \(F_{2}\), \(F_{3}\), and \(F_{4}\) should be stored for demodulation. Unlike the previous scenario, the demodulation from frames \(F_{2}\) and \(F_{3}\) is correct (\(PCC(F_{2},F_{3})=1\)), but the accuracy of the symbol extracted from frames \(F_{3}\) and \(F_{4}\) can not be assured (\(-1<PCC(F_{3},F_{4})<1\)), due to the presence of the next transmitted symbol. Therefore, when the transmission starts during the acquisition process, an additional operation is required. In order to avoid the demodulation uncertainty that a transmission starting during the acquisition process may produce, all the emissions begin with a known symbol. For this first symbol, the decodification is done independently from the possible frame's combination. If the decoded symbol from the first two ROIs, based on \(PCC(F_{1},F_{2})\), is different to the defined one, the emission started during the acquisition process, as shown in Figure 8c, generating the first scenario where the ROI has dark strips. In this case, the first ROI of each frame group is ignored during the average PCC calculation (\(\overline{PCC}=average(PCC(F_{2},F_{3}),...,PCC(F_{m-1},F_{m}))\)). However, if the decoded symbol from the last two ROIs, based on \(PCC(F_{m-1},F_{m})\), is different to the defined one, the emission started during the acquisition process without generating black bands (second scenario). In this case, the last frame's ROI of each frame group is ignored, and the data extraction is applied to the remaining frames (\(\overline{PCC}=average(PCC(F_{1},F_{2}),...,PCC(F_{m-2},F_{m-1}))\)). As it was shown above, the number of successive frames (\(m\)) used for transmitting each symbol can improve the detection results in the reception stage. By employing the average PCC from \(m\) consecutive frames, the impact of noise on the PCC calculation is mitigated. However, the use of several consecutive frames for transmitting one symbol generates throughput degradation. Therefore, three consecutive frames have been defined as a trade off between reception performance and system's throughput. Still, a higher number of frames used for the transmission of each symbol can improve the success probability. To prove this, 3 to 5 frames were used to determine the system's improvement during the experimental evaluation. After explaining the wake up procedure that determines whether the transmission started, and selecting which frames should be analyzed for data extraction, the modulation and demodulation techniques will be introduced.