  This paper proposes a hybrid dimming scheme based on joint LED selection and
precoding design (TASP-HD) for multiple-user (MU) multiple-cell (MC) visible
light communications (VLC) systems. In TASP-HD, both the LED selection and the
precoding of each cell can be dynamically adjusted to reduce the intra- and
inter-cell interferences while satisfying illumination constraints. First, a
MU-MC-VLC system model is established, and then a sum-rate maximization problem
under dimming level and illumination uniformity constraints is formulated. In
this studied problem, the indices of activated LEDs and precoding matrices are
optimized, which result in a complex non-convex mixed integer problem. To solve
this problem, the original problem is separated into two subproblems. The first
subproblem, which maximizes the sum-rate of users via optimizing the LED
selection with a given precoding matrix, is a mixed integer problem solved by
the penalty method. With the optimized LED selection matrix, the second
subproblem which focuses on the maximization of the sum-rate via optimizing the
precoding matrix is solved by the Lagrangian dual method. Finally, these two
subproblems are iteratively solved to obtain a convergent solution. Simulation
results verify that in a typical indoor scenario under a dimming level of 70%,
the mean bandwidth efficiency of TASP-HD is 4.8 bit/s/Hz and 7.13 bit/s/Hz
greater than AD and DD, respectively.
