Joint Design of User Scheduling and Precoding for Interference Management in Cell-Free VLC Network

Visible light communication (VLC) is a promising technology for short-range communications with the benefits of wide spectrum, low power consumption, and high security. Since access points (APs) in VLC network are usually densely and randomly located, cell-free network model can be utilized, where APs cooperate to serve users without frequent handover. However, when massive APs transmitting signals simultaneously, managing the inter-user interference is critical to ensure the network performance. Precoding schemes can eliminate inter- user interference in spatial domain. Nevertheless, the number of supportable users in each time slot cannot exceed the number of APs and it can be further reduced if the VLC channel correlation is high, which occurs commonly when APs or users are closely located. Therefore, user scheduling scheme should be developed to decide the proper user group for precoding in each time slot, in order to mitigate the impact of high channel correlation and provide satisfying service for users. In this paper, an interference management (IM) algorithm for cell-free VLC network is proposed via the joint design of user scheduling and precoding. Specifically, to adapt to the various channel conditions, feasible user groups with different group sizes are considered. Given a user group, SLNR-based precoding is employed to reduce the inter-user interference and improve users' data rates. Moreover, the user scheduling scheme is proposed to optimize the time proportions for different user groups being served based on the VLC channel conditions, aiming to improve users' data rates while considering user fairness.