LED Communication Using a High Speed Digital Camera

An optical communication system using a light-emitting diode (LED) as a transmitter and a high speed digital camera as a receiver is considered. The camera collects data at a rate of 1,200 frames per second (FPS). The pixel values are then processed using equal gain combining (EGC), best pixel selection (BPS), and maximal ratio combining (MRC) techniques. The bit error rate (BER) performance of these techniques is analyzed, and the effect of using different numbers of pixels for receiver processing is considered. The effect of the camera’s pixel grid being at an angle with respect to the LED’s direct path is experimentally explored. INTRODUCTION Optical communication has become a promising technique for providing high speed data rates while maintaining a very secure link [1]. A typical optical communication system consists of optical transmitters, such as laser diodes or LEDs, as well as receivers such as photodiodes or digital cameras. Using a digital camera as a receiver has some advantages over a photodiode, as it allows the transmitter to be moving and provides a much larger field of view for receiving signals. As camera speeds continue to improve, optical communication’s potential for high speed data rates will rapidly improve in the future. LEDs operating in the visible light spectral range can serve the dual purpose of lighting a room as well as providing a high speed and secure communication link [2]. One of the challenges facing optical communication is the need to dramatically increase the data rates. One method of achieving this is to use an array of LEDs as a transmitter forming a multiple input multiple output (MIMO) system. The concept of a MIMO optical communication system using LEDs has been demonstrated in [1]. In [3], the BER performance of different diversity combining techniques is compared showing promising results for MRC. An improved receiver processing technique allows lowering of the received optical energy to achieve the same BER performance. As a result, the time duration of the transmitted pulses can be made shorter, which can allow higher data rates. At the same time, LEDs with high data rates must also meet certain room lighting standards. It is shown in [2] that LEDs in a room can maintain International Organization for Standardization (ISO) standards for lighting while also providing a data transmission link, validating the concept of a dual role room light source and communication system. In this paper, we consider a communication system with an LED as a transmitter and a digital camera as a receiver. We model the received LED image by using the Gaussian mixture model (GMM) proposed in [3]. While this paper closely follows [3], one of the main contributions of this paper is the results using a high speed complementary metal-oxide semiconductor (CMOS) Promon 501 digital camera from AOS Technologies. Note that the results in [3] were obtained using a Sony charge-coupled device (CCD) digital camera. Accordingly, our results in this paper use 1,200 FPS in contrast to 120 FPS used in [3]. We study three different receiver processing techniques: EGC, BPS, and MRC. EGC’s BER performance depends on the number of pixels used. Conditions on how many pixels to use when the receiver uses EGC are given in [3], and the conditions are verified with both simulation and experimental results. As expected, MRC outperforms the other receiver processing techniques in terms of BER performance. We also explore the effect of the LED’s angle of incidence on BER performance. SYSTEM MODEL AND LED/CAMERA PARAMETERS A single LED is used as a transmitter and a digital camera as a receiver. The camera is positioned so that the tip of the camera lens is at a distance d from the tip of the LED. The camera has a focal length f . In our model, the camera is allowed to rotate by an angle θ relative to being pointed directly at the LED as shown in Fig. 1. In [3], a GMM combined with a reference image pixel