Multiuser MIMO-OFDM for Visible Light Communications

Visible light communication (VLC) is emerging as a promising technique to provide ubiquitous wireless connection. In this paper, a multiuser VLC system utilizing multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) is investigated. Since the distances of the multiple transmitter-receiver links are different, their temporal delays are also different, resulting in complex channel gain and phase differences when transformed to the frequency domain. For each subcarrier in OFDM, the corresponding precoding matrix is calculated in the frequency domain to eliminate multiuser interference. Phase information in the frequency domain is first considered, where complex, instead of real, channel matrices are used for precoding, which reduces the channel correlation and achieves better performance. Moreover, minimum dc bias, unified dc bias, and asymmetrically clipped optical OFDM-based schemes are proposed to generate real-valued nonnegative signals for intensity modulation, and their performances are validated via simulations with zero forcing and minimum mean-squared error (MMSE) precoding techniques.