30,410 research outputs found
Tucker Decomposition For Rotated Codebook in 3D MIMO System Under Spatially Correlated Channel
This correspondence proposes a new rotated codebook for three-dimensional
(3D) multi-input-multi-output (MIMO) system under spatially correlated channel.
To avoid the problem of high dimensionality led by large antenna array, the
rotation matrix in the rotated codebook is proposed to be decomposed by Tucker
decomposition into three lowdimensional units, i.e., statistical channel
direction information in horizontal and vertical directions respectively, and
statistical channel power in the joint horizontal and vertical direction. A
closed-form suboptimal solution is provided to reduce the computational
complexity in Tucker decomposition. The proposed codebook has a significant
dimension reduction from conventional rotated codebooks, and is applicable for
3D MIMO system with arbitrary form of antenna array. Simulation results
demonstrate that the proposed codebook works very well for various 3D MIMO
systems.Comment: accepted by IEEE Transactions on Vehicular Technolog
Uniformly expanding vacuum: a possible interpretation of the dark energy
Following the spirit of the equivalence principle, we take a step further to
recognize the free fall of the observer as a method to eliminate causes that
would lead the perceived vacuum to change its original state. Thus, it is
expected that the vacuum should be in a rigid Minkowski state or be uniformly
expanding. By carefully investigating the impact on measurement caused by the
expansion, we clarify the exact meaning of the uniformly expanding vacuum and
find that this proposal may be able to explain the current observations of an
accelerating universe.Comment: 5 pages, accepted by Physics of the Dark Univers
Emergent cosmic space in Rastall theory
Padmanabhan's emergent cosmic space proposal is employed to study the Rastall
theory which involves modifying the conservation law of energy-momentum tensor.
As a necessary element for this approach, we firstly propose a modified Komar
energy which reflects the evolution of the energy-momentum itself in the case
of a perfect fluid. After that, an expansion law is invoked to reobtain the
Friedmann equation in this theory.Comment: 8 pages, no figures, published version in CQ
Generalized Hodge dual for torsion in teleparallel gravity
For teleparallel gravity in four dimensions, Lucas and Pereira have shown
that a generalized Hodge dual for torsion tensor can be defined with
coefficients determined by mathematical consistency. In this paper, we
demonstrate that a direct generalization to other dimensions fails and no new
generalized Hodge dual operator could be given. Furthermore, if one enforces
the definition of a generalized Hodge dual to be consistent with the action of
teleparallel gravity in general dimensions, the basic identity for any sensible
Hodge dual would require an \textit{ad hoc} definition for the second Hodge
dual operation which is totally unexpected. Therefore, we conclude that at
least for the torsion tensor, the observation of Lucas and Pereira only applies
to four dimensions.Comment: 12 pages, corrected typos, rearranged reference
A modified variational principle for gravity in modified Weyl geometry
The usual interpretation of Weyl geometry is modified in two senses. First,
both the additive Weyl connection and its variation are treated as (1, 2)
tensors under the action of Weyl covariant derivative. Second, a modified
covariant derivative operator is introduced which still preserves the tensor
structure of the theory. With its help, the Riemann tensor in Weyl geometry can
be written in a more compact form. We justify this modification in detail from
several aspects and obtain some insights along the way. By introducing some new
transformation rules for the variation of tensors under the action of Weyl
covariant derivative, we find a Weyl version of Palatini identity for Riemann
tensor. To derive the energy-momentum tensor and equations of motion for
gravity in Weyl geometry, one naturally applies this identity at first, and
then converts the variation of additive Weyl connection to those of metric
tensor and Weyl gauge field. We also discuss possible connections to the
current literature on Weyl-invariant extension of massive gravity and the
variational principles in f(R) gravity.Comment: 16 pages. Minor correction
Hidden conformal symmetry of extremal Kaluza-Klein black hole in four dimensions
We study the hidden conformal symmetry of four-dimensional extremal
Kaluza-Klein black hole. The scalar Laplacian corresponding to the radial
equation in the near-region is rewritten in terms of the
quadratic Casimir. Using the first law of black hole thermodynamics, this
symmetry enables us to obtain the conjugate charges for the CFT side. The
real-time correlators are also found to agree with the CFT expectations
A sphere theorem for Bach-flat manifolds with positive constant scalar curvature
We show a closed Bach-flat Riemannian manifold with a fixed positive constant
scalar curvature has to be locally spherical if its Weyl and traceless Ricci
tensors are small in the sense of either or -norm.
Compared with the complete non-compact case done by Kim, we apply a different
method to achieve these results. These results generalize a rigidity theorem of
positive Einstein manifolds due to M.-A.Singer. As an application, we can
partially recover the well-known Chang-Gursky-Yang's -dimensional conformal
sphere theorem.Comment: 11 page
DEBIT: Distributed Energy Beamforming and Information Transfer for Multiway Relay Networks
In this paper, we propose a new distributed energy beamforming and
information transfer (DEBIT) scheme for realizing simultaneous wireless
information and power transfer (SWIPT) in multiway relay networks (MWRNs),
where multiple single-antenna users exchange information via an
energy-constrained single-antenna relay node. We investigate the optimal
transceiver designs to maximize the achievable sum-rate or the harvested power.
The resultant sum-rate maximization problem is non-convex and the global
optimal solution can be obtained through a three-dimensional search in
combination with conventional convex optimization. To reduce the computation
complexity, a suboptimal DEBIT scheme is also proposed, for which the
optimization problem becomes linear programming. The achievable sum-rate
performance is analyzed and a closed-form lower bound is derived for the MWRN
with a large number of users. Furthermore, we consider the harvested-power
maximization problem under a target sum-rate constraint, and derive a lower
bound of the average harvested power for MWRNs with a large number of users.
Numerical results show that the DEBIT scheme significantly outperforms the
conventional SWIPT and the derived lower bounds are tight.Comment: 8 page
A Dynamic Programming Implemented 2x2 non-cooperative Game Theory Model for ESS Analysis
Game Theory has been frequently applied in biological research since 1970s.
While the key idea of Game Theory is Nash Equilibrium, it is critical to
understand and figure out the payoff matrix in order to calculate Nash
Equilibrium. In this paper we present a dynamic programming implemented method
to compute 2x2 non-cooperative finite resource allocation game's payoff matrix.
We assume in one population there exists two types of individuals, aggressive
and non-aggressive and each individual has equal and finite resource. The
strength of individual could be described by a function of resource consumption
in discrete development stages. Each individual undergoes logistic growth hence
we divide the development into three stages: initialization, quasilinear growth
and termination. We first discuss the theoretical frame of how to dynamic
programming to calculate payoff matrix then give three numerical examples
representing three different types of aggressive individuals and calculate the
payoff matrix for each of them respectively. Based on the numerical payoff
matrix we further investigate the evolutionary stable strategies (ESS) of the
games.Comment: 9 pages 3 sub models to illustrate how dynamic programming is
implemented to construct payoff matrix of 2x2 symmetric gam
Equivalence of SLNR Precoder and RZF Precoder in Downlink MU-MIMO Systems
The signal-to-leakage-and-noise ratio (SLNR) precoder is widely used for
MU-MIMO systems in many works, and observed with improved performance from
zeroforcing (ZF) precoder. Our work proofs SLNR precoder is completely
equivalent to conventional regulated ZF (RZF) precoder, which has significant
gain over ZF precoder at low SNRs. Therefore, with our conclusion, the existing
performance analysis about RZF precoder can be readily applicable to SLNR
precoder
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