# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 1 Introduction  
A Smart City is usually defined as an urban area that employs devices (general-purpose sensors) to collect data and manage assets, resources, and services by applying data analytic tools in several applications such as governance, environmental impact, security, or emergency management. These different sensors gathering information can be considered Internet of Things (IoT) elements that generally connect and communicate with other devices over short, medium, and long distances using wired or wireless networks. Due to the number of sensor devices that a Smart City would require for making accurate decisions, traditional wireless communication links, based on Radio Frequency (RF), may be insufficient. This type of communication presents issues related to the current spectrum saturation and the inter-systems interference based on the extensive use of RF links, so other complementary wireless communication techniques are needed. On account of the advantages of the Light Emitting Diodes (LED), such as energy efficiency, extended life span, cost-efficiency, and high available bandwidth, there is a trend in the use of LED-based devices for illumination, advertising, and decoration in both indoor and outdoor environments. This massive utilization of LED-based devices impulses the development and implementation of Visible Light Communications (VLC) systems [1],an optical wireless communication technology that works with the visible light portion of the electromagnetic spectrum (430 THz to 790 THz). However, photodiode-based VLC system implementations imply using potentially expensive elements and the necessity of equipment modifications. In addition, many day-to-day devices such as smartphones, wearables, and tablets, have embedded cameras presenting large resolutions and high frame acquisition speeds. In the same way, camera-based security and monitoring systems have been implemented extensively. This context derived into the development of Optical Camera Communications (OCC) [2], which is a specific VLC-based technique that uses image sensors for receiving optical signals. The use of pre-existing camera-based infrastructures such as traffic-camera networks or security systems decreases OCC deployment costs. In addition, spatially-separated sources allow the design of Multuple Input Multiple Output (MIMO) systems [3; 4], increasing OCC systems' data rate. In this way, OCC technology is suitable for deploying IoT applications and services. The use of VLC for Smart City applications has been previously proposed. For instance, a Smart City's public lighting system for ubiquitous communication ensuring broad coverage was presented in [5]. Nonetheless, the work left aside attractive possible emitters, such as traffic lights or advertising screens, and used modified devices as receivers. In [6], traffic lights and car lights were studied as potential transmitters for MIMO inter-vehicular communication solutions within a short distance range. An OCC system for rolling shutter cameras was proposed in [7], achieving a 2 m link. In [8], a long-distance OCC system, above 300 m, for slow data rate applications in Smart Cities was introduced, using pre-existing infrastructure over heterogeneous networks. However, this solution was affected by the flickering problem. Nevertheless, the use of OCC for implementations within urban environments requires an extended analysis. The distance between emitters and receivers may vary ranging from some centimeters to several tens of meters, affecting the transmitter's projection on the image sensor, and eventually the communications performance. Moreover, the majority of IoT devices have memory and processing limitations that should be taken into account. In summary, these deployments require a low-complexity communication algorithm capable of correctly functioning for a wide range of distances while maintaining human-friendly illumination. In this work, a novel flickering-free distance-independent modulation scheme is proposed.