# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 2 Related Work  
In this section, the current state-of-the-art solutions for OCC systems are analyzed focusing on the proposed modulation techniques to verify the feasibility of employing OCC for outdoor long-distance links, including Smart City applications. Several modulation schemes for OCC have been proposed and can be mainly divided into four categories: based on OOK, polychromatic modulations, undersampling techniques, and based on space-time coding. At first, the researchers adapted OOK modulation directly from VLC, where the on-light state represented a \"1\" bit whilst the off-light state corresponded to a \"0\" bit. In general, the implementation of this method is simple. In [9] the LED's switching frequency was defined as half the camera's frame rate, and two consecutive frames were required for representing each symbol, but the implementation involved a synchronization stage. For avoiding synchronization delimiters, an asynchronous OOK was presented [10], two out of three consecutive frames that represented two bits were selected for the demodulation process based on a predefined initial sequence. Later on, the authors improved this technique [11] by using an adaptive threshold for flickering-free links within 0.2 m. In [12; 13; 14], the OOK modulation was combined with Manchester codification and the rolling shutter effect for representing several bits per frame. However, these modulations did not tolerate environmental noise or distances greater than 1 m. [PERSON] et al. [15; 16] introduced a binary Frequency Shift Keying (FSK) scheme (frequencies separated by 200 Hz) that supported several transmitters with different pair of frequencies. This technique required a synchronization delimiter with a flag and a pilot signal for each transmission decreasing the throughput. In [17], frequencies from a predefined cluster depending on the camera's characteristics were assigned for each symbol for improving the performance. Nevertheless, communication was feasible for short distances (less than 1 m) and required a fixed frequency splitter before each symbol, a parity symbol, and two delimiters. To improve link range, [PERSON] et al. [18] proposed the application of image processing in the receiver for using the illuminated area outside the light source. However, the maximum achieved distance was 0.4 m. Finally, in [7] the application of an adaptive thresholding technique ([PERSON]'s algorithm) for Non-Line-Of-Sight (NLOS) links was carried out with successful results in a 2-m link, although the packets required to be transmitted twice including a 12 bit header, ultimately decreasing the throughput. Several authors worked with polychromatic systems for taking advantage of Red-Green-Blue (RGB) cameras. At the beginning, the schemes were based on Color Shift Keying (CSK) modulation, which is primarily used for VLC. For example, in [19] a 4-CSK modulation was presented for short range links (less than 1 m), while [PERSON] et al. [20] introduced a high level constellation modulation (8-CSK, 16-CSK, and 32-CSK), that needed a synchronization delimiter and periodic calibration packets during the transmission, and complex image processing for the demodulation. In [21; 22], an asynchronous CSK scheme was presented for distances below 1 m, and an 8-CSK modulation [23] was presented along with a beacon-jointed packet reconstruction scheme for longer packets, but the system's distance range was strongly limited (less than 5 cm). In [24] the authors defined their technique as Binary CSK. The modulation scheme is based on a light polarization change that was perceived by the receiver as two different colors due to the use of dispersion films. This technique achieved 10 m range but required physical modifications on the receivers. Another polychromatic modulation is Generalized Color Modulation (GCM), which is a generalization of CSK [25; 26]. This scheme is based on creating different color constellations to achieve a specific target color working in the CIELUV space for long-range links (up to 75 m).