In this paper, we consider an energy harvesting (EH) node which harvests
energy from a radio frequency (RF) signal broadcasted by an access point (AP)
in the downlink (DL). The node stores the harvested energy in an energy buffer
and uses the stored energy to transmit data to the AP in the uplink (UL). We
consider a simple transmission policy, which accounts for the fact that in
practice the EH node may not have knowledge of the EH profile nor of the UL
channel state information. In particular, in each time slot, the EH node
transmits with either a constant desired power or a lower power if not enough
energy is available in its energy buffer. For this simple policy, we use the
theory of discrete-time continuous-state Markov chains to analyze the limiting
distribution of the stored energy for finite- and infinite-size energy buffers.
Moreover, we take into account imperfections of the energy buffer and the
circuit power consumption of the EH node. For a Rayleigh fading DL channel, we
provide the limiting distribution of the energy buffer content in closed form.
In addition, we analyze the average error rate and the outage probability of a
Rayleigh faded UL channel and show that the diversity order is not affected by
the finite capacity of the energy buffer. Our results reveal that the optimal
desired transmit power by the EH node is always less than the average harvested
power and increases with the capacity of the energy buffer.