# Zeroes: Robust Derivative-Based Demodulation Method for Optical Camera Communication
## 1 Introduction  
The development of Light Emitting Diode (LED) technology has brought a revolution in the lighting field, with greater energy efficiency than incandescent and halogen light bulbs [1]. Moreover, it also represents a tremendous opportunity for alternative and complementary wireless communication to Radio Frequency (RF) systems, which are now subject to spectral crowding [2] due to the massive use of new technologies such as Internet of Things (IoT) or cloud computing. Visible Light Communication (VLC) [3], a subset of Optical Wireless Communication (OWC) [4], takes advantages of the fast switching state of LED to transmit data at a frequency higher than the critical flicker fusion frequency of human eye in order to provide both communication and illumination at the same time. While a VLC transmission is generally based on a LED at the emission and a photodiode (PD) at the reception, Optical Camera Communication (OCC) [5] is exclusively restricted to the use of a camera as a receiver, directly benefiting from the omnipresence of smartphones for a rapid, large-scale and low-cost deployment of OWC systems in daily life as it does not require any hardware modifications, only software implementation. Even though OCC promises great accessibility, the achievable data rate remains the major limitation of this technology, considering the low frame rate of most cameras (about 30 or 60 fps) in accordance with the Nyquist frequency. However, a clever approach was proposed to overcome this limitation by taking advantage of the architecture of Complementary Metal-Oxide Semiconductor (CMOS) cameras [6], making smartphones cheap and equipping them with ease of integration compared to Charge-Coupled Device (CCD) cameras. While the picture-taking of CCD cameras is based on the global shutter, meaning that all the pixels composing the image sensor are exposed simultaneously to the light, CMOScameras are based on the rolling shutter, where pixels are illuminated column-wise [7]. Although rolling shutter is considered as a major drawback in photography since causing the deformation of fast-moving objects due to the column shift mechanism, the latter becomes an opportunity for OCC systems to enhance the data rate, considering the column shift frequency as sampling rate instead of frame rate. In doing so, the achievable data rate increases from a few bits per second to several thousand bits per second [8], which becomes enough to perform a lot of communication applications. Indeed, by setting the transmission frequency of the light source up to twice as low the column-shift frequency, bright and dark strips will appear on the resulting image, respectively, corresponding to ON and OFF states of the LED during image capture. This sequence of bright and dark strips is extracted from the image using specifically designed Region-Of-Interest (ROI) techniques [9; 10], and the original information is thereafter recovered by demodulating the signal. However, the main difficulty of this operation comes from the blooming effect, characterized by the amplitude variation of the extracted signal due to a difference in light exposure intensity of all pixels, the most aligned to the source being the brightest. Since a demodulation based on constant-value thresholding is therefore no longer adapted, adaptive threshold methods [11] have been developed in order to correctly demodulate the extracted signals, but their performance still remains limited on large and rapid amplitude variations signals. Moreover, other approaches based on the first-order derivative of the extracted signal have been proposed in order to directly identify the state changes by determining both local maximum and minimum values, named peaks and valleys, respectively [12; 13], or even the second-order derivative [14].