Optimized Diagonal and Pseudo-Random Phase Precoding Schemes for MIMO VLC Systems

Dual-LED complex modulation (DCM) and quad-LED complex modulation (QCM) are state-of-the-art MIMO modulation schemes which efficiently transmit complex-valued modulation symbols in visible light communication (VLC). DCM uses two LEDs to transmit complex symbols exploiting their polar form of representation. QCM uses four LEDs to transmit complex symbols through intensity modulation to convey the magnitude of real and imaginary parts of a complex symbol and spatial modulation to convey the sign information. In this paper, we introduce efficient precoding schemes for DCM and QCM schemes. The first proposed precoder is for DCM, and it is termed as optimized diagonally precoded DCM (ODP-DCM). In ODP-DCM, the magnitude and phase information are weighted such that the normalized minimum distance is maximized. The second precoder is for both DCM and QCM, and it is termed as pseudo-random phase precoded DCM/QCM (PRPP-DCM/QCM). In PRPP-DCM/QCM, pseudo-random phase matrices which do not need any channel knowledge at the transmitter for their construction are used as the precoding matrices. Numerical results show that the proposed precoding schemes achieve good performance and alleviate the effect of spatial correlation in MIMO VLC channels.