Superposed three-dimensional 64QAM constellation design for MIMO-OFDM visible light communication systems

Multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) has been widely used to improve data rate in visible-light communication (VLC) systems. However, the high correlation of channels restricts the application of MIMO. A superposed constellation, combined with MIMO, can achieve considerable diversity gain even in highly correlated MIMO channels. In this study, what we believe to be novel superposed three-dimensional 64-quadrature amplitude modulation (3D-64QAM) constellation schemes are proposed for MIMO-OFDM VLC systems. First, a superposed 3D-64QAM constellation scheme using two transmitted light emitting diodes (LEDs) is proposed, where two independent signals with 3D-4QAM and 3D-16QAM modulation formats are superposed to form a 3D-64QAM signal at the receiver. Then, for what we believe is the first time, we expand the superposed constellation solution to three-LED application scenarios, wherein the 3D-64QAM constellation is superposed by three different 3D-4QAM constellations. Both schemes benefit from a higher minimum Euclidean distance of 3D-64QAM constellation, 1.67 times that of traditional two-dimensional (2D) 64QAM constellation. Meanwhile, the equal-power superposition design of transmitted signals reduces the nonlinearity of LEDs and power competition of photodiodes. Moreover, the three-LED scheme further improves the transmitted power without increasing the risk of nonlinear distortion. To improve spectral efficiency and reduce complexity, we also propose a 3D OFDM modulation scheme. The superposed 3D-64QAM schemes are first studied through theoretical analysis and computer simulation. Then, an experimental demonstration is established to investigate the system performance comprehensively. Experimental results prove that the superposed 3D-64QAM constellation schemes achieve a superior bit error rate (BER) performance than the traditional superposed 2D-64QAM constellation scheme. Compared with two-LED scheme, the three-LED scheme not only obtains a lower BER, but also improves the dynamic range of driving peak-to-peak voltage significantly.