Broadcasting with an Energy Harvesting Rechargeable Transmitter

TL;DR

This paper studies how to minimize total transmission time in an energy-harvesting broadcast channel by optimizing power and rate allocation, revealing structural properties and proposing an optimal scheduling algorithm.

Contribution

It introduces a novel structural analysis of the optimal policy and develops a globally optimal algorithm for energy-harvesting broadcast channels.

Findings

Optimal total transmit power has a specific structure similar to single-user cases.

Existence of a cut-off power level for the stronger user.

Proposed algorithm achieves the global optimum for off-line scheduling.

Abstract

In this paper, we investigate the transmission completion time minimization problem in a two-user additive white Gaussian noise (AWGN) broadcast channel, where the transmitter is able to harvest energy from the nature, using a rechargeable battery. The harvested energy is modeled to arrive at the transmitter randomly during the course of transmissions. The transmitter has a fixed number of packets to be delivered to each receiver. Our goal is to minimize the time by which all of the packets for both users are delivered to their respective destinations. To this end, we optimize the transmit powers and transmission rates intended for both users. We first analyze the structural properties of the optimal transmission policy. We prove that the optimal total transmit power has the same structure as the optimal single-user transmit power. We also prove that there exists a cut-off power level for the stronger user. If the optimal total transmit power is lower than this cut-off level, all transmit power is allocated to the stronger user, and when the optimal total transmit power is larger than this cut-off level, all transmit power above this level is allocated to the weaker user. Based on these structural properties of the optimal policy, we propose an algorithm that yields the globally optimal off-line scheduling policy. Our algorithm is based on the idea of reducing the two-user broadcast channel problem into a single-user problem as much as possible.