  In this paper, we study the time allocation optimization problem to maximize
the sum throughput in a dual-hop heterogeneous visible light communication
(VLC)/radio frequency (RF) communication system. Two scenarios are investigated
in this paper. For the first scenario, we consider an optical wireless powered
communication network (WPCN) in which all users harvest energy from the
received lightwave over downlink (DL), and then they use the harvested energy
to transmit information signals in the uplink (UL) channels based on the time
division multiple access (TDMA) scheme. The optimal time allocation in the UL
is obtained to maximize the sum throughput of all users. For the second
scenario, the time switching simultaneous lightwave information and power
transfer (TS-SLIPT) based on the dual-hop VLC/RF is assumed that the LED
transmits information and power simultaneously in the first hop DL (i.e., VLC
link). The harvested energy at the relay is used to transmit information
signals over the UL in the second hop (i.e., RF link). We propose a
multi-objective optimization problem (MOOP) to study the trade-off between UL
and DL sum-rate maximization. The non-convex MOOP framework is then transformed
into an equivalent form, which yields a set of Pareto optimal resource
allocation policies. We also illustrate the effectiveness of the proposed
approaches through numerical results.
