Superposed 32QAM Constellation Design for 2 x 2 Spatial Multiplexing MIMO VLC Systems

Spatial multiplexing multiple-input multiple-output (MIMO) visible light communication (VLC) has become a potential candidate for next-generation wireless communication networks. In this paper, a novel superposed odd-order 32QAM constellation scheme is proposed in 2 x 2 MIMO VLC systems. Two independent signals generated from two light emitting diode (LED)-based transmitters with a four-quadrature amplitude modulation (4QAM) and geometrically shaped square 8QAM, respectively, are transmitted over free space and spatially multiplexed in the receiver to obtain a distinctly square-shaped 32QAM signal. The high channel correlation causes failure of MIMO detection in traditional spatial multiplexing (SMP) systems owing to the rank defect of the channel matrix. However, MIMO detection can always be realized by a simple constellation de-mapping based on the concept of superposed constellations, regardless of the extent of channel correlation. Considering the non-linearity of an LED, the square-shaped 8QAM constellation is introduced to reduce the peak to average power ratio (PAPR) at the transmitter, which also benefits from its maximum minimum Euclidean distance (MED) at the receiver compared to those of traditional 8QAM constellations. Moreover, the equal-gain combining (EGC) algorithm is employed to gain the receive diversity. The proposed system is experimentally studied in detail, where the system performance is evaluated under the conditions of different driving peak-to-peak voltages (Vpps) and channel gains. The experimental demonstration also shows that the dynamic range of the driving Vpp can be improved from 0.3 V to 0.6 V when compared with that of the traditional non-geometrically-shaped 8QAM, clearly validating the superiority of the proposed system.