  The low-pass characteristics of front-end elements including light-emitting
diodes (LEDs) and photodiodes (PDs) limit the transmission data rate of visible
light communication (VLC) and Light Fidelity (LiFi) systems. Using multiplexing
transmission techniques, such as spatial multiplexing (SMX) and wavelength
division multiplexing (WDM), is a solution to overcome bandwidth limitation.
However, spatial correlation in optical wireless channels and optical filter
bandpass shifts typically limit the achievable multiplexing gain in SMX and WDM
systems, respectively. In this paper, we consider a multiple-input multiple
output (MIMO) joint multiplexing VLC system that exploits available
degrees-offreedom (DoFs) across space, wavelength and frequency dimensions
simultaneously. Instead of providing a new precoder/post-detector design, we
investigate the considered joint multiplexing system from a system
configuration perspective by tuning system parameters in both spatial and
wavelength domains, such as LED positions and optical filter passband. We
propose a novel spatial clustering with wavelength division (SCWD) strategy
which enhances the MIMO channel condition. We propose to use a state-of-the-art
black-box optimization tool: Bayesian adaptive direct search (BADS) to
determine the desired system parameters, which can significantly improve the
achievable rate. The extensive numerical results demonstrate the superiority of
the proposed method over conventional SMX and WDM VLC systems.
