  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.
