Light-emitting diodes (LEDs) have been deployed for various applications in our daily lives. Similarly, image sensors or cameras integrated into mobile phones have become common. Hence, visible light communication (VLC) using LED and mobile phone cameras is attractive and provides low-cost wireless communication.
In this paper, we propose and demonstrate a VLC system using color-shift-keying (CSK) modulation and code-division multiple-access (CDMA) technology simultaneously for the first time; a mobile phone camera is used as the receiver (Rx). CSK is used to enhance the VLC system capacity and to mitigate the single color light interference, whereas CDMA allows multiple users to access the network.
The system design and operation mechanism of the proposed CSK–CDMA VLC system are discussed. A proof-of-concept demonstration is performed, and error-free transmission is achieved for multiple-access users. A 3-dB transmission gain is also obtained in each user when compared with the traditional on–off keying (OOK) modulation.
SYSTEM DESIGN AND OPERATION MECHANISM
Fig. 1 shows the proposed scenario of VLC using CSK and CDMA. The CDMA Data1 combines with the CDMA Data2 to produce the CSK signal. The CSK signal is transmitted by an array of RGB-LED. User1 and user2 receive the CSK signal by mobile-phone camera1 and mobile-phone camera2 respectively. Each user then decodes the individual data by its own CDMA spreading code.
As shown on Table1, the CSK scheme used in this proposed system is consisted by 4 color magnitude subsets. Since the number of bit used for each color [red (R), green (G), and blue (B)] is 8 bits (color depth), the magnitude levels represented by of the R, G, and B color are within the range of 0-255.
In the Tx as shown in Fig.2, the magnitude of Spread-signal1 and Spread-signal2 are added to produce the total red magnitude signal. The signal level is adjusted to the level 0–255 by adding the offset + 120. That is for fitting the 8-bit data depth (0–255). Finally, the output of Tx.
Experiment of the CSK-CDMA VLC System, Results, and Discussion
Fig. 5 shows the experiment result of the CSK-CDMA VLC system and the comparison with OOK modulation. The system serves two users simultaneously in this proof-of-concept demonstration; however more users can be supported by using different orthogonal spreading-codes. As shown in the BER measurement in Fig. 5, low error transmission (BER < 10-6) for both users can be achieved. Each user has a 3-dB gain comparing with the OOK modulation. Since we use spreading-code with spreading factor = 4, a coding gain of 6-dB is excepted theoretically.
A novel CSK-CDMA VLC system was proposed and a proof-of-concept experiment using RGB-LED and mobile-phone camera was demonstrated. CSK scheme enhanced the system through-put and avoid the single color interference; while CDMA allowed multiple users to access to the network. The system design and operation mechanism of the proposed CSK-CDMA VLC system were discussed.
A proof-of-concept demonstration having two users to receive data in CSK simultaneously was preformed, and the proposed scheme can support more users simultaneously by using more orthogonal spreading-codes. BER measurement was performed, and the experiment result showed the proposed system can be operated in low error (BER < 10-6) transmission with a 3-dB transmission gain compared with the traditional OOK modulation.
Source: National Chiao Tung University
Authors: Shih-Hao Chen | Chi-Wai Chow