12,168 research outputs found
Capacity scaling law by multiuser diversity in cognitive radio systems
This paper analyzes the multiuser diversity gain in a cognitive radio (CR)
system where secondary transmitters opportunistically utilize the spectrum
licensed to primary users only when it is not occupied by the primary users. To
protect the primary users from the interference caused by the missed detection
of primary transmissions in the secondary network, minimum average throughput
of the primary network is guaranteed by transmit power control at the secondary
transmitters. The traffic dynamics of a primary network are also considered in
our analysis. We derive the average achievable capacity of the secondary
network and analyze its asymptotic behaviors to characterize the multiuser
diversity gains in the CR system.Comment: 5 pages, 2 figures, ISIT2010 conferenc
Multiuser Diversity for Secrecy Communications Using Opportunistic Jammer Selection -- Secure DoF and Jammer Scaling Law
In this paper, we propose opportunistic jammer selection in a wireless
security system for increasing the secure degrees of freedom (DoF) between a
transmitter and a legitimate receiver (say, Alice and Bob). There is a jammer
group consisting of jammers among which Bob selects jammers. The
selected jammers transmit independent and identically distributed Gaussian
signals to hinder the eavesdropper (Eve). Since the channels of Bob and Eve are
independent, we can select the jammers whose jamming channels are aligned at
Bob, but not at Eve. As a result, Eve cannot obtain any DoF unless it has more
than receive antennas, where is the number of jammer's transmit
antenna each, and hence can be regarded as defensible dimensions against
Eve. For the jamming signal alignment at Bob, we propose two opportunistic
jammer selection schemes and find the scaling law of the required number of
jammers for target secure DoF by a geometrical interpretation of the received
signals.Comment: Accepted with minor revisions, IEEE Trans. on Signal Processin
Deformations of Annuli on Riemann surfaces with Smallest Mean Distortion
Let and be two circular annuli and let be a radial metric
defined in the annulus . Consider the class of
harmonic mappings between and . It is proved recently by
Iwaniec, Kovalev and Onninen that, if (i.e. if is Euclidean
metric) then is not empty if and only if there holds the
Nitsche condition (and thus is proved the J. C. C. Nitsche conjecture). In this
paper we formulate an condition (which we call Nitsche conjecture) with
corresponds to and define Nitsche harmonic maps. We
determine the extremal mappings with smallest mean distortion for mappings of
annuli w.r. to the metric . As a corollary, we find that Nitsche
harmonic maps are Dirichlet minimizers among all homeomorphisms .
However, outside the -Nitsche condition of the modulus of the annuli,
within the class of homeomorphisms, no such energy minimizers exist. % However,
%outside the Nitsche range of the modulus of the annuli, %within the
class of homeomorphisms, no such energy minimizers exist. This extends some
recent results of Astala, Iwaniec and Martin (ARMA, 2010) where it is
considered the case and .Comment: Some misprints are corrected in this version (see Lemma~5.1
- β¦