  A continuous goal in all communication systems is to enhance the users
experience and provide them with the highest possible data rates. Recently, the
concept of cell-free massive MIMO (CF-mMIMO) systems has been considered to
enhance the performance of systems that operate merely with Radio Frequency
(RF) or visible light communication (VLC) technologies. In this paper, a hybrid
VLC/RF cell-free massive MIMO system is proposed where an RF cell-free network
and a VLC cell-free network coexist to serve users. The idea is to utilize the
benefits of each network and balance the load aiming at maximizing the system's
sum-rate. The system is evaluated using zero-forcing (ZF) precoding scheme. Two
user association algorithms are proposed to assign users to either the VLC or
the RF networks. In addition, two user-centric clustering approaches are
proposed and evaluated. Simulation results show that the proposed association
algorithms significantly outperform a random network association of users in
terms of sum-rate. Results also show great potential for the proposed system
compared to standalone cell-free networks.
