## 1 Introduction
To avoid flicker and the resulting interference with the displayed content, these DCC systems use different modulation schemes for the raw pixel intensities of images and videos. Due to the distortion from the interframe interference problem [9], the schemes for embedding data in video content are more complex with regard to the design of channel coding and demodulation, which leads to higher computational complexity and difficulty in real-time communication. In 2021, a real-time DCC system based on LED displays and smartphones with an Alternate Bit-flipping Repeat Coding (ABRC) scheme for the synchronization problem between LED displays and the smartphones' cameras, and a fast image processing algorithm to decrease the computational complexity of image processing were proposed [14]. The previous work modulates a single pixel on a 16 \(\times\) 16 point LED display, which can achieve a data transmission rate of 30 bps, and the data decoding latency on the Android smartphone for data extraction is only 11.83 ms. However, the maximum data transmission distance is only 80 cm under a BER of 0.5 and the data transmission distance is 15 cm under the 7% forward error correction (FEC) limit, limiting its practical use. The main reason for the short communication distance is that the real-time processing capability of the fast image processing algorithm cannot support the real-time information extraction of larger area pixel points, which limits the quality of the received visible light signal. Toincrease the data transmission distance, it is important to enhance the quality of the received VLC signals, i.e., either increasing the DCC light-emitting area on the transmitter side or increasing the resolution of the image sensor on the receiver side in the LED-DCC system. However, both solutions will multiply the computational latency for image processing several times and will eventually degrade the real-time data transmission performance of the LED-DCC system. In this paper, we propose a long-distance, real-time DCC system based on LED point clustering and lightweight image processing. First, we propose a LED-DCC point clustering scheme, which uses multiple LED display points to cluster together to increase the light-emitting area for sending information, as well as the data transmission distance. Then, to solve the problem of processing latency introduced by point clustering and improve the reliability of information extraction, a lightweight, back-forth, fast image processing algorithm is proposed, which can quickly realize the high-precision positioning and data extraction of the LED-DCC area using the adaptive scanning method and variable step lengths. The proposed LED-DCC system has been experimentally verified and demonstrated on a 2.2-inch 16 \(\times\) 16 point LED display with a refresh rate of 150 fps and on a commercial Android smartphone with a camera image sensor with a resolution of 3840 \(\times\) 2160. Experimental results show that the proposed LED-DCC system can reach the maximum data transmission distance of 7 m under a BER of 0.5, which is about 9 times that of the previous LED-DCC system, and can reach a data transmission distance of 175 cm under the 7% FEC limit, which is about 12 times that of the previous LED-DCC system. Additionally, the data decoding latency caused by extracting information from each video frame is only 13.26 ms, which is similar to the previous LED-DCC system [14], even though the proposed system needs to process many more pixels (about 10 times) than the previous LED-DCC system. Therefore, the proposed system has advantages not only in data transmission distance, but also in data transmission rate and system reliability. The remainder of this paper is structured as follows: Section 2 presents a detailed description of the proposed LED-DCC system, including the system architecture, the proposed LED-DCC point clustering scheme, and the proposed lightweight, back-forth, fast image processing algorithm. The experimental results and discussion are provided in Section 3.