Quad-LED Complex Modulation (QCM) for Visible Light Wireless Communication

In this paper, we propose a simple and novel complex modulation scheme for multiple-LED wireless communication, termed as {\em quad-LED complex modulation (QCM)}. The proposed QCM scheme uses four LEDs (hence the name `quad-LED'), one LED each to map positive real, negative real, positive imaginary, and negative imaginary parts of complex modulation symbols like QAM/PSK symbols. The QCM scheme does not need Hermitian symmetry operation to generate LED compatible positive real transmit signals. Instead it exploits spatial indexing of LEDs to convey sign information. The proposed QCM module can serve as a basic building block to bring in the benefits of complex modulation to VLC. For example, QCM with phase rotation (QAM-PR) where the complex modulation symbols are rotated in phase before mapping the signals to the LEDs achieves improved bit error performance. We also find that the proposed QCM when used along with OFDM, termed as QCM-OFDM, achieves very good performance.