2,263 research outputs found
Optimal Save-Then-Transmit Protocol for Energy Harvesting Wireless Transmitters
In this paper, the design of a wireless communication device relying
exclusively on energy harvesting is considered. Due to the inability of
rechargeable energy sources to charge and discharge at the same time, a
constraint we term the energy half-duplex constraint, two rechargeable energy
storage devices (ESDs) are assumed so that at any given time, there is always
one ESD being recharged. The energy harvesting rate is assumed to be a random
variable that is constant over the time interval of interest. A
save-then-transmit (ST) protocol is introduced, in which a fraction of time
{\rho} (dubbed the save-ratio) is devoted exclusively to energy harvesting,
with the remaining fraction 1 - {\rho} used for data transmission. The ratio of
the energy obtainable from an ESD to the energy harvested is termed the energy
storage efficiency, {\eta}. We address the practical case of the secondary ESD
being a battery with {\eta} < 1, and the main ESD being a super-capacitor with
{\eta} = 1. The optimal save-ratio that minimizes outage probability is
derived, from which some useful design guidelines are drawn. In addition, we
compare the outage performance of random power supply to that of constant power
supply over the Rayleigh fading channel. The diversity order with random power
is shown to be the same as that of constant power, but the performance gap can
be large. Furthermore, we extend the proposed ST protocol to wireless networks
with multiple transmitters. It is shown that the system-level outage
performance is critically dependent on the relationship between the number of
transmitters and the optimal save-ratio for single-channel outage minimization.
Numerical results are provided to validate our proposed study.Comment: This is the longer version of a paper to appear in IEEE Transactions
on Wireless Communication
Throughput Maximization for Mobile Relaying Systems
This paper studies a novel mobile relaying technique, where relays of high
mobility are employed to assist the communications from source to destination.
By exploiting the predictable channel variations introduced by relay mobility,
we study the throughput maximization problem in a mobile relaying system via
dynamic rate and power allocations at the source and relay. An optimization
problem is formulated for a finite time horizon, subject to an
information-causality constraint, which results from the data buffering
employed at the relay. It is found that the optimal power allocations across
the different time slots follow a "stair-case" water filling (WF) structure,
with non-increasing and non-decreasing water levels at the source and relay,
respectively. For the special case where the relay moves unidirectionally from
source to destination, the optimal power allocations reduce to the conventional
WF with constant water levels. Numerical results show that with appropriate
trajectory design, mobile relaying is able to achieve tremendous throughput
gain over the conventional static relaying.Comment: submitted for possible conference publicatio
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