使用滾動式快門相機之非同步可見光通訊系統: 實作與評估

Abstract

本論文實作利用單顆發光二極體為傳送端，滾動式快門相機為接收端之可見光通訊系統。可見光通訊是一種新型的資料傳輸技術，透過控制發光二極體的發光強度在空氣中傳遞數位訊息。 相較於無線視頻技術，可見光通訊包含了高指向性、高保密性以及高頻寬等特性。由於發光二極體具低功耗，高效率，壽命長之優點，近年來已逐漸取代傳統日光燈照明。此外對輸入訊號的快速反應及成本低，也使其極具通訊之潛在價值。 我們利用滾動式快門相機作為接收端。在現今的行動裝置中，至少都有一個內建滾動式快門相機，因此不需要額外花費或修改即能接收發光二極體訊號。我們的發想是，利用相機影像來接收發光二極體所傳遞的訊息，此技術可應用於行車安全輔助系統、室內定位，還可利用燈源在相機影像中的位置，來做與視覺結合的應用。 由於相機的每秒顯示幀數不盡相同，常會發生多收或少收到訊號的狀況；傳送端與接收端的相位差也會使得同一個幀內出現混合訊號。所以在系統設計上，我們特別解決傳送端與接收端間不同步所產生的問題。我們利用軟體無線電來實作傳送端訊號，實際測試於現有的智慧型手機相機與工業相機上，並在智慧型手機上開發一個即時解碼應用程式，以驗證使用滾動式快門相機之可見光通訊的可行性，期待未來可以廣泛運用於生活中。

The thesis presents a visible light communication (VLC) system that utilizes a single light-emitting diode (LED) light source as the transmitter and a CMOS rolling shutter camera as the receiver. VLC is a new data transmission technology. Unlike traditional RF wireless communication, VLC uses the optical signal to carry digital information by controlling the LED's light intensity in free space. VLC has the properties of high directivity, high security and high bandwidth. In recent years, with the development of the LED technology, due to its advantages of low power consumption, high efficiency, and long life, LED has gradually replaced the traditional fluorescent lighting. The quick response time and the low cost also make LED a very attractive transmitter solution for communications. In our proposed system, we use CMOS rolling shutter camera as the receiver. Today, almost every modern mobile device is equipped with at least one built-in CMOS camera, and CMOS camera can receive signals from the LEDs without any modification, and thus, any additional cost. Our idea is to use the acquired images to extract the message transmitted from the LED. This technology can be used in many applications, such as advanced driver assistance system (ADAS), indoor positioning, and visual-associated application. Due to the wide range of frames rates of the cameras, it is common to receive redundant symbols or have symbol loss; Frames with more than one data symbols, i.e., mixed frames, may also happen because of the phase offset between the transmitter and the receiver. Both of the issues are due to the unsynchronized nature of our proposed system. In this thesis, we propose a number of schemes to address the issues caused by unsynchronized transmitter and receiver. To evaluate the feasibility of these schemes, we use software-defined radio (SDR) to implement the transmitter and use the built-in camera of several recent smartphones and an industrial camera as the receiver. We also develop a real-time decoding application on the smartphone. Our result shows that we can improve the packet reception rate (PRR) from 0.6 to 0.99 when the transmitting and receiving frame rates are close. We also can improve the PRR from 0.0 to 0.8~1.0 even when the transmitting and receiving frame rates have a large difference. The overall throughput can reach 18 bytes per second. We expect this technology can be widely used for a wide range of applications in the future.