  In this paper, we investigate and analyze energy recycling for a
reconfigurable intelligent surface (RIS)-aided wireless-powered communication
network. As opposed to the existing works where the energy harvested by
Internet of things (IoT) devices only come from the power station, IoT devices
are also allowed to recycle energy from other IoT devices. In particular, we
propose group switching- and user switching-based protocols with time-division
multiple access to evaluate the impact of energy recycling on system
performance. Two different optimization problems are respectively formulated
for maximizing the sum throughput by jointly optimizing the energy beamforming
vectors, the transmit power, the transmission time, the receive beamforming
vectors, the grouping factors, and the phase-shift matrices, where the
constraints of the minimum throughput, the harvested energy, the maximum
transmit power, the phase shift, the grouping, and the time allocation are
taken into account. In light of the intractability of the above problems, we
respectively develop two alternating optimization-based iterative algorithms by
combining the successive convex approximation method and the penalty-based
method to obtain corresponding sub-optimal solutions. Simulation results verify
that the energy recycling-based mechanism can assist in enhancing the
performance of IoT devices in terms of energy harvesting and information
transmission. Besides, we also verify that the group switching-based algorithm
can improve more sum throughput of IoT devices, and the user switching-based
algorithm can harvest more energy.
