Energy Cooperation in Energy Harvesting Communications

TL;DR

This paper introduces energy cooperation in energy harvesting communications, allowing users to wirelessly share energy to optimize system performance, and develops algorithms to maximize throughput in multi-user networks.

Contribution

It proposes the concept of energy cooperation, formulates optimal energy management policies, and develops a two-dimensional directional water-filling algorithm for multi-user energy harvesting networks.

Findings

Energy cooperation improves system throughput.

The proposed algorithm optimally manages energy flow in time and among users.

A generalized algorithm achieves the capacity boundary of the two-way channel.

Abstract

In energy harvesting communications, users transmit messages using energy harvested from nature during the course of communication. With an optimum transmit policy, the performance of the system depends only on the energy arrival profiles. In this paper, we introduce the concept of energy cooperation, where a user wirelessly transmits a portion of its energy to another energy harvesting user. This enables shaping and optimization of the energy arrivals at the energy-receiving node, and improves the overall system performance, despite the loss incurred in energy transfer. We consider several basic multi-user network structures with energy harvesting and wireless energy transfer capabilities: relay channel, two-way channel and multiple access channel. We determine energy management policies that maximize the system throughput within a given duration using a Lagrangian formulation and the resulting KKT optimality conditions. We develop a two-dimensional directional water-filling algorithm which optimally controls the flow of harvested energy in two dimensions: in time (from past to future) and among users (from energy-transferring to energy-receiving) and show that a generalized version of this algorithm achieves the boundary of the capacity region of the two-way channel.