Variable Pulse Width Unipolar Orthogonal Frequency Division Multiplexing for Visible Light Communication Systems

Visible light communication (VLC) systems working as high-speed and short-range wireless connection methods have attracted more and more attention. This paper proposes a variable pulse width unipolar optical orthogonal frequency-division multiplexing (VPW-OFDM) scheme for indoor VLC systems. Similar to the polar-based optical OFDM, Hermitian symmetric data are not required to generate real signals. Instead, the magnitude and phase components of the complex-valued OFDM signal are transmitted successively. Unlike the polar-based optical OFDM, the proposed VPW-OFDM allows using variant pulse widths to reduce the effects of the noise added on the phase component. By adaptively optimizing the pulse widths, the proposed VPW-OFDM outperforms the polar-based OFDM and other state-of-the-art optical OFDM techniques, such as dc-biased optical OFDM, asymmetrically clipped optical OFDM, and unipolar-OFDM. For indoor VLC systems, illumination requirement is crucial and needs to be considered. In this paper, we analyze the effects of the illumination requirements on system performance. Bit error rate and transmitted bit rate are used as criteria to evaluate the communication performances for the techniques tested. In addition, the system channel with bandwidth limitation is taken into account. To enhance the data throughput, a single-tap equalizer at the receiver and the bit loading algorithm are applied. From the numerical results, the proposed VPW-OFDM outperforms DCO-, ACO-, U-, and polar-based OFDM for a wide channel bandwidth.