24,986 research outputs found
Spectral Efficiency and Energy Efficiency of OFDM Systems: Impact of Power Amplifiers and Countermeasures
In wireless communication systems, the nonlinear effect and inefficiency of
power amplifier (PA) have posed practical challenges for system designs to
achieve high spectral efficiency (SE) and energy efficiency (EE). In this
paper, we analyze the impact of PA on the SE-EE tradeoff of orthogonal
frequency division multiplex (OFDM) systems. An ideal PA that is always linear
and incurs no additional power consumption can be shown to yield a decreasing
convex function in the SE-EE tradeoff. In contrast, we show that a practical PA
has an SE-EE tradeoff that has a turning point and decreases sharply after its
maximum EE point. In other words, the Pareto-optimal tradeoff boundary of the
SE-EE curve is very narrow. A wide range of SE-EE tradeoff, however, is desired
for future wireless communications that have dynamic demand depending on the
traffic loads, channel conditions, and system applications, e.g.,
high-SE-with-low-EE for rate-limited systems and high-EE-with-low-SE for
energy-limited systems. For the SE-EE tradeoff improvement, we propose a PA
switching (PAS) technique. In a PAS transmitter, one or more PAs are switched
on intermittently to maximize the EE and deliver an overall required SE. As a
consequence, a high EE over a wide range SE can be achieved, which is verified
by numerical evaluations: with 15% SE reduction for low SE demand, the PAS
between a low power PA and a high power PA can improve EE by 323%, while a
single high power PA transmitter improves EE by only 68%.Comment: to be published, IEEE J. Sel. Areas Commu
Data Offloading in Load Coupled Networks: A Utility Maximization Framework
We provide a general framework for the problem of data offloading in a
heterogeneous wireless network, where some demand of cellular users is served
by a complementary network. The complementary network is either a small-cell
network that shares the same resources as the cellular network, or a WiFi
network that uses orthogonal resources. For a given demand served in a cellular
network, the load, or the level of resource usage, of each cell depends in a
non-linear manner on the load of other cells due to the mutual coupling of
interference seen by one another. With load coupling, we optimize the demand to
be served in the cellular or the complementary networks, so as to maximize a
utility function. We consider three representative utility functions that
balance, to varying degrees, the revenue from serving the users vs the user
fairness. We establish conditions for which the optimization problem has a
feasible solution and is convex, and hence tractable to numerical computations.
Finally, we propose a strategy with theoretical justification to constrain the
load to some maximum value, as required for practical implementation. Numerical
studies are conducted for both under-loaded and over-loaded networks.Comment: 12 pages, accepted for publication in IEEE Transactions on Wireless
Communication
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