Optimal Rotated QPSK Constellation for a Semi-Orthogonal Multiple Access Visible Light Communication System

Abstract

A rotated quadrature phase-shift keying (QPSK) based semi-orthogonal multiple access (SOMA) data transmission is considered for a visible light communication (VLC) system with two users. The rotation of the QPSK constellation at the transmitter followed by a data pre-processing technique at the receiver of each user ensures the elimination of the successive interference cancellation unit. An optimal maximum likelihood receiver is proposed for the system under consideration using which, the closed-form expressions for the symbol error probability (SEP) for both the users in the VLC system are derived. The optimization problem to obtain the optimal angle of rotation of the QPSK constellation which minimizes the SEP of the users is formulated and solved. The dependency of the optimality of the QPSK rotation angle on various VLC system parameters is studied via numerical results which lead to the observation of a value of the signal-to-noise ratio of the system around 8.8 dB about which the dependency of the optimal rotation angle on the other VLC system parameters interchange, thus providing design aspects for a SOMA-VLC system.