  Characterizing the fundamental tradeoffs for maximizing energy efficiency
(EE) versus spectrum efficiency (SE) is a key problem in wireless
communication. In this paper, we address this problem for a point-to-point
additive white Gaussian noise (AWGN) channel with the transmitter powered
solely via energy harvesting from the environment. In addition, we assume a
practical on-off transmitter model with non-ideal circuit power, i.e., when the
transmitter is on, its consumed power is the sum of the transmit power and a
constant circuit power. Under this setup, we study the optimal transmit power
allocation to maximize the average throughput over a finite horizon, subject to
the time-varying energy constraint and the non-ideal circuit power consumption.
First, we consider the off-line optimization under the assumption that the
energy arrival time and amount are a priori known at the transmitter. Although
this problem is non-convex due to the non-ideal circuit power, we show an
efficient optimal solution that in general corresponds to a two-phase
transmission: the first phase with an EE-maximizing on-off power allocation,
and the second phase with a SE-maximizing power allocation that is
non-decreasing over time, thus revealing an interesting result that both the EE
and SE optimizations are unified in an energy harvesting communication system.
We then extend the optimal off-line algorithm to the case with multiple
parallel AWGN channels, based on the principle of nested optimization. Finally,
inspired by the off-line optimal solution, we propose a new online algorithm
under the practical setup with only the past and present energy state
information (ESI) known at the transmitter.
