Augmenting a Smartphone Camera with a Telephoto Lens for Enhanced LED-to-Camera Communication Omer Dalgic * , Jagdeep Singh † , Tim Farnham † , Daniele Puccinelli * * University of Applied Sciences and Arts of Southern Switzerland (SUPSI); † Toshiba Europe Limited, BRIL, UK. Email: {omer.dalgic, daniele.puccinelli}@supsi.ch {jagdeep.singh, tim.farnham}@toshiba-bril.com ABSTRACT—In LED-to-Smartphone Camera communication, the camera detects the status of the light source (ON or OFF) in each frame to receive information. If the light source is modulated faster than a single frame duration, there are multiple white & dark stripes corresponding to the transmitted information bits. Further, the receiver’s region of interest in LED-to-Smartphone Camera communication is limited by the size of the light source in the frame in terms of the number of pixels. As the distance between the LED and the camera increases, the light source does not cover the whole frame, resulting in a significant amount of data loss. Therefore, LED-to-Smartphone Camera communication is limited by distance, making it difficult to transmit data from several meters away. On the other hand, there is a loss of data even at relatively short ranges due to small region of interest, especially for tiny light sources. In this work, we enhance the smartphone camera with a telephoto lens that provides optical zoom on the light source and increases the region of interest. The optical zoom from a telephoto lens helps the user focus on the light source and reduces data loss. We compare the data transfer rate with and without a telephoto lens at short distances (15-80 cm). The initial results show that a telephoto lens improves the data transfer rate at short distances and also expands the possible range for LED-to-Smartphone Camera links. Index Terms—visible light communication, CMOS sensors, optics. I. I NTRODUCTION Wireless communication highly uses the radio frequency (RF) spectrum to transfer valuable information from device to device. However, the increasing use of wireless devices is putting pressure on the limited RF spectrum [1], leading to a phenomenon known as ”spectrum crunch.” New ways to optimize spectrum allocation or use other parts of the electromagnetic spectrum, such as visible light or infrared, are needed to overcome these limitations and provide additional capacity for wireless communication. One example is Visible Light Communication (VLC) [2], which uses the wider visible light spectrum to transmit data. VLC can use various types of transmitters such as light bulbs, screens, and liquid crystal cells, and receivers such as photodiodes, cameras, and solar cells to transfer information. A key combination for IoT applications is the use of LEDs as transmitters and smartphone cameras as receivers, known as LED-to-Smartphone Camera (L2C) communication, due to the widespread use of both. According to market estimates, there are over 6.2 billion smartphone users worldwide [3] and the Fig. 1: Two consecutive frames: data loss in L2C communica- tion due to small ROI (red arrows) and inter-frame duration. compound annual growth rate (CAGR) of the global LED mar- ket size is 11.0% from 2023 to 2030 based on a recent report, thanks to its low energy consumption and long lifetime [4]. Smartphone users can interact with light sources both indoors and outdoors, enabling various applications such as human- computer interaction, sensing, and augmented reality [5]–[11]. Examples include home and city automation applications that use small LEDs in appliances, parking meters, or thermostats to communicate their status [5], [6]. In L2C communication, digital information is transmitted mainly through changes in light intensity or color. The smart- phone camera records the modulated light source information as a video, which is then separated into frames. Digital infor- mation is extracted through image processing techniques from each frame. However, the inter-frame-gap duration defined as the time gap between two consecutive frames affects the achievable data rate, because the camera does not capture information bits during this period (Figure 1). Furthermore, the appearance of the LED image on each frame is critical for decoding the transmitted information [12]. The image sensors on the camera create this appearance corresponding to the incident light. Most smartphones use Complementary Metal- Oxide-Semiconductor (CMOS) image sensors, which consist of a 2D array of pixel sensors [13]. Each pixel sensor converts the incident light into a voltage value. In CMOS sensors, each row of pixel sensors sequentially collects the photons