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Visible light communication (VLC) has been emerged as a complementary promising technology to its radiofrequency (RF) counterpart due to its considerable merits, such as safety to human beings, wide spectrum without license and non-interference to RF communication. In VLC, white LEDs, consisting of red-green-blue (RGB) and phosphor-converted LEDs, are usually utilized to facilitate both data transmission and illumination, where RGB LEDs are preferred to support higher data rate communications. In RGB LEDs, the mixing ratio of the red, green and blue lights is flexible in the chromaticity diagram. As long as the alternation of mixing ratio lies in a MacAdam ellipse or the speed of the alternation is fast enough, the variations of emitted white light cannot be perceived by human beings. Therefore, additional information can be carried by different mixing ratio and index modulation technique is introduced firstly in RGB-LED-based VLC wireless systems in order to enhance either spectral or energy efficiency. Different from the state-of-the-art methods, the index modulation is performed in the chromaticity domain rather than the traditional frequency, spatial and time domains, which provides a new perspective of index modulation. More specifically, in the proposed chromaticity-domain index modulation (CD-IM) with white RGB LEDs, the input bits are divided into subblocks, including index bits and data bits. The index bits are used to select different mixing ratio, which are corresponding to different direct current (DC) bias of RGB channels. The data bits are further split into three parts for the pulse amplitude modulation (PAM) on each color channel independently. The generated individual alternating current (AC) is then added to its corresponding DC current of each color channel. Since DC and AC currents could convey the information concurrently, the spectral efficiency could be improved considerably. At the receiver, a maximum likelihood (ML) detector is employed to detect the index bits as well as the data bits on the RGB channels simultaneously. Simulation results illustrate that our proposed CD-IM achieves better BER performance in comparison with its traditional color shift keying (CSK) and PAM counterparts under the same spectral efficiency.
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