71,790 research outputs found
Joint Secure Beamforming for Cognitive Radio Networks with Untrusted Secondary Users
In this paper, we consider simultaneous wireless information and power
transfer (SWIPT) in orthogonal frequency division multiple access (OFDMA)
systems with the coexistence of information receivers (IRs) and energy
receivers (ERs). The IRs are served with best-effort secrecy data and the ERs
harvest energy with minimum required harvested power. To enhance physical-layer
security and yet satisfy energy harvesting requirements, we introduce a new
frequency-domain artificial noise based approach. We study the optimal resource
allocation for the weighted sum secrecy rate maximization via transmit power
and subcarrier allocation. The considered problem is non-convex, while we
propose an efficient algorithm for solving it based on Lagrange duality method.
Simulation results illustrate the effectiveness of the proposed algorithm as
compared against other heuristic schemes.Comment: To appear in Globecom 201
Effects of dark energy on the efficiency of charged AdS black holes as heat engine
In this paper, we study the heat engine where charged AdS black holes
surrounded by dark energy is the working substance and the mechanical work is
done via term in the first law of black hole thermodynamics in the
extended phase space. We first investigate the effects of a kind of dark energy
(quintessence field in this paper) on the efficiency of the RN-AdS black holes
as heat engine defined as a rectangle closed path in the plane. We get
the exact efficiency formula and find that quintessence field can improve the
heat engine efficiency which will increase as the field density grows.
At some fixed parameters, we find that bigger volume difference between the
smaller black holes() and the bigger black holes( ) will lead to a
lower efficiency, while the bigger pressure difference will make the
efficiency higher but it is always smaller than 1 and will never be beyond
Carnot efficiency which is the maximum value of the efficiency constrained by
thermodynamics laws, this is consistent to the heat engine in traditional
thermodynamics. After making some special choices for thermodynamical
quantities, we find that the increase of electric charge and normalization
factor can also promote heat engine efficiency which would infinitely
approach the Carnot limit when or goes to infinity.Comment: 28 pages, 16 figures, refernces added, discussion and computation
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