  Multi-colour light-emitting diode (LED) based visible light communication
(VLC) benefits from wavelength diversity while providing indoor illumination.
Colour shift keying (CSK) is a well-researched IEEE standardised multi-colour
VLC modulation technique. This paper presents an investigation into the
performance of tri-chromatic LED (TLED) CSK standardised in IEEE 802.15.7 and a
quad-chromatic LED (QLED) CSK, over a range of diffuse optical wireless
channels, and proposes the use of frequency domain equalisation (FDE) at the
receiver to combat multipath dispersion. The investigation results show that
the FDE enables the higher order CSK modulation modes to achieve a bit error
rate (BER) of 10\textsuperscript{-6} for finite amounts of optical power while
operating over highly dispersive channels and hence provide data links of up to
85.33 and 256 Mbit/s through TLED and QLED CSK, respectively, for a system
bandwidth of 24 MHz. The overall optical power requirements of the CSK schemes
can be reduce by up to 12.6 dB with the use of FDE at the cost of a small
overhead due to cyclic prefix. The optical channel model used in this
investigation includes the cross-talk and insertion losses caused by the
optical properties of commercially available system front end devices.
