On-Off Transmission Policy for Wireless Powered Communication with Energy Storage

TL;DR

This paper analyzes an energy harvesting wireless node's on-off transmission policy, considering practical limitations like unknown energy profiles and buffer imperfections, providing insights into energy distribution and communication performance.

Contribution

It introduces a Markov chain-based analysis of the energy buffer dynamics under a simple transmission policy, accounting for buffer imperfections and circuit power consumption.

Findings

Limiting distribution of energy buffer derived in closed form.

Diversity order remains unaffected by finite buffer capacity.

Optimal transmit power is below average harvested power and increases with buffer size.

Abstract

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 remains silent 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, for medium to high signal-to-noise ratio (SNRs), the optimal target transmit power of the EH node is less than the average harvested power and increases with the capacity of the energy buffer.