SWIPT using Hybrid ARQ over Time Varying Channels

TL;DR

This paper investigates a wireless energy harvesting system using SWIPT and HARQ over time-varying channels, proposing a Markovian framework to optimize energy and information transfer policies for reliable communication.

Contribution

It introduces a novel Markovian approach to optimize SWIPT with HARQ over time-varying channels, simplifying the problem to discrete states and actions.

Findings

Optimal policies depend on channel state and correlation.

Harvest-first-store-later policy is optimal for i.i.d. channels.

Channel correlation knowledge improves performance.

Abstract

We consider a class of wireless powered devices employing Hybrid Automatic Repeat reQuest (HARQ) to ensure reliable end-to-end communications over a two-state time-varying channel. A receiver, with no power source, relies on the energy transferred by a Simultaneous Wireless Information and Power Transfer (SWIPT) enabled transmitter to \emph{receive} and \emph{decode} information. Under the two-state channel model, information is received at two different rates while it is only possible to harvest energy in one of the states. The receiver aims to decode its messages with minimum expected number of re-transmissions. Dynamic and continuous nature of the problem motivated us to use a novel Markovian framework to bypass the complexities plaguing the conventional approaches such as MDP. Using the theory of absorbing Markov chains, we show that there exists an optimal policy utilizing the incoming RF signal solely to harvest energy or to accumulate mutual information. Hence, we convert the original problem with continuous action and state space into an equivalent one with discrete state and action space. For independent and identically distributed channels, we prove the optimality of a simple-to-implement {\em harvest-first-store-later} type policy. However, for time-correlated channels, we demonstrate that statistical knowledge of the channel may significantly improve the performance over such policies.