  This paper investigates a wireless-powered communication network (WPCN) setup
with one multi-antenna access point (AP) and one single-antenna source. It is
assumed that the AP is connected to an external power supply, while the source
does not have an embedded energy supply. But the source could harvest energy
from radio frequency (RF) signals sent by the AP and store it for future
information transmission. We develop a discrete time-switching (DTS) protocol
for the considered WPCN. In the proposed protocol, either energy harvesting
(EH) or information transmission (IT) operation is performed during each
transmission block. Specifically, based on the channel state information (CSI)
between source and AP, the source can determine the minimum energy required for
an outage-free IT operation. If the residual energy of the source is
sufficient, the source will start the IT phase. Otherwise, EH phase is invoked
and the source accumulates the harvested energy. To characterize the
performance of the proposed protocol, we adopt a discrete Markov chain (MC) to
model the energy accumulation process at the source battery. A closed-form
expression for the average throughput of the DTS protocol is derived. Numerical
results validate our theoretical analysis and show that the proposed DTS
protocol considerably outperforms the existing harvest-then-transmit protocol
when the battery capacity at the source is large.
