13,370 research outputs found
Invertible condition of quantum Fisher information matrix for a mixed qubit
Estimating multiparamter simultaneously as precise as possible is an
important goal of quantum metrolo- gy. As a first step to this end, here we
give a condition determining whether two arbitrary parameters can be estimated
simultaneously for a qubit in the mixed state. An application of this condition
is shown.Comment: The European Physical Journal D 201
Background field method in the large expansion of scalar QED
Using the background field method, we, in the large approximation,
calculate the beta function of scalar quantum electrodynamics at the first
nontrivial order in by two different ways. In the first way, we get the
result by summing all the graphs contributing directly. In the second way, we
begin with the Borel transform of the related two point Green's function. The
main results are that the beta function is fully determined by a simple
function and can be expressed as an analytic expression with a finite radius of
convergence, and the scheme-dependent renormalized Borel transform of the two
point Green's function suffers from renormalons.Comment: 13 pages, 4 figures, 1 table, to appear in the European Physical
Journal
Distributed Opportunistic Scheduling for MIMO Ad-Hoc Networks
Distributed opportunistic scheduling (DOS) protocols are proposed for
multiple-input multiple-output (MIMO) ad-hoc networks with contention-based
medium access. The proposed scheduling protocols distinguish themselves from
other existing works by their explicit design for system throughput improvement
through exploiting spatial multiplexing and diversity in a {\em distributed}
manner. As a result, multiple links can be scheduled to simultaneously transmit
over the spatial channels formed by transmit/receiver antennas. Taking into
account the tradeoff between feedback requirements and system throughput, we
propose and compare protocols with different levels of feedback information.
Furthermore, in contrast to the conventional random access protocols that
ignore the physical channel conditions of contending links, the proposed
protocols implement a pure threshold policy derived from optimal stopping
theory, i.e. only links with threshold-exceeding channel conditions are allowed
for data transmission. Simulation results confirm that the proposed protocols
can achieve impressive throughput performance by exploiting spatial
multiplexing and diversity.Comment: Proceedings of the 2008 IEEE International Conference on
Communications, Beijing, May 19-23, 200
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