###### Abstract
Although each sensor type has its own specific advantages, CMOS sensors are the most prevalent due to their cost-effectiveness and versatility across a wide range of applications. OCC finds its importance in the myriad of applications it offers. The spatial separation of light sources thanks to the use of image-forming optics, combined with its low cost and ease of installation, makes it an optimal choice for applications with a high density of transmitters, such as Wireless Sensor Networks (WSNs) [4], the Internet of Things (IoT) [5], Smart and Cognitive Cities [6], and Industry 4.0 [7]. Furthermore, OCC excels in applications that require high precision, such as Visible Light Positioning (VLP), with achievable accuracy on the order of centimeters [8]. It makes it also fit well with object localization tasks [9]. As a result, it also plays a vital role in Intelligent Transportation Systems (ITSs) as part of Smart Cities, where it enhances communication between autonomous vehicles and infrastructure elements such as traffic lights, street lights, and other vehicles, ultimately improving the overall system performance [10]. Figure 1 depicts a conceptual illustration on the use of OCC in the aforementioned scenario. However, in the various application domains where OCC may be a suitable option, the transmitting elements may not always be situated at such a close distance to the camera or have such large dimensions so as to occupy a significant proportion of the image pixels. This situation is intriguing for several reasons: using small transmitters can lead to energy consumption reduction, placing transmitters at distant locations can greatly expand coverage areas, and having a small projection of the transmitter on the image sensor allows increasing the number of transmitters detectable by a single camera. Consequently, it is highly likely that their proportion will be less than a few pixels, making the specific study of this scenario of great interest. The sub-pixel scenario is defined as one in which the projection of the transmitter onto the image sensor is less than one pixel [4]. It is important to consider that even if the pixel's projection on the image sensor is less than one pixel, the light projection onto it may occupy a larger number of sensor cells. This depends on various factors such as the average brightness of the scene, the brightness and directivity of the source, or distortions introduced by the channel. A higher average scene brightness leads to less light from the source being captured as it is overshadowed by ambient light. A brighter source increases the likelihood of its emission affecting multiple pixels due to channel-induced dispersion. Imperfections in the optical system are also responsible for a source occupying more than one pixel on the image sensor even if its projection is less than one pixel. This occurs because when light from a point source passes through a lens or optical system, it undergoes modifications and dispersion due to aberrations in the lens and characteristics of the optical system. This results in an extension or spreading of the original point in the projected image. To describe how an optical system or lens defocuses a point of light in an image, the Point Spread Function (PSF), also known as the Potential Spread Function, is generally used [4]. Typically, it is represented as a two-dimensional function, where each point in the output image is associated with a value indicating the light intensity at an input point and the volume of the illuminated region. In the domain of OCC, transmitter discovery tasks have received limited attention in the broader scope of OCC research. Specifically, the exploration of transmitter discovery under sub-pixel conditions has been virtually non-existent. This dearth of research not only hinders the broader application of OCC but also restricts its utility when dealing with distant or low-light transmitters.