2,912 research outputs found
Optimal Use of Current and Outdated Channel State Information - Degrees of Freedom of the MISO BC with Mixed CSIT
We consider a multiple-input-single-output (MISO) broadcast channel with
mixed channel state information at the transmitter (CSIT) that consists of
imperfect current CSIT and perfect outdated CSIT. Recent work by Kobayashi et
al. presented a scheme which exploits both imperfect current CSIT and perfect
outdated CSIT and achieves higher degrees of freedom (DoF) than possible with
only imperfect current CSIT or only outdated CSIT individually. In this work,
we further improve the achievable DoF in this setting by incorporating
additional private messages, and provide a tight information theoretic DoF
outer bound, thereby identifying the DoF optimal use of mixed CSIT. The new
result is stronger even in the original setting of only delayed CSIT, because
it allows us to remove the restricting assumption of statistically equivalent
fading for all users
Multiple Unicast Capacity of 2-Source 2-Sink Networks
We study the sum capacity of multiple unicasts in wired and wireless multihop
networks. With 2 source nodes and 2 sink nodes, there are a total of 4
independent unicast sessions (messages), one from each source to each sink node
(this setting is also known as an X network). For wired networks with arbitrary
connectivity, the sum capacity is achieved simply by routing. For wireless
networks, we explore the degrees of freedom (DoF) of multihop X networks with a
layered structure, allowing arbitrary number of hops, and arbitrary
connectivity within each hop. For the case when there are no more than two
relay nodes in each layer, the DoF can only take values 1, 4/3, 3/2 or 2, based
on the connectivity of the network, for almost all values of channel
coefficients. When there are arbitrary number of relays in each layer, the DoF
can also take the value 5/3 . Achievability schemes incorporate linear
forwarding, interference alignment and aligned interference neutralization
principles. Information theoretic converse arguments specialized for the
connectivity of the network are constructed based on the intuition from linear
dimension counting arguments.Comment: 6 pages, 7 figures, submitted to IEEE Globecom 201
Aiming Perfectly in the Dark - Blind Interference Alignment through Staggered Antenna Switching
We propose a blind interference alignment scheme for the vector broadcast
channel where the transmitter is equipped with M antennas and there are K
receivers, each equipped with a reconfigurable antenna capable of switching
among M preset modes. Without any knowledge of the channel coefficient values
at the transmitters and with only mild assumptions on the channel coherence
structure we show that MK/M+K-1 degrees of freedom are achievable. The key to
the blind interference alignment scheme is the ability of the receivers to
switch between reconfigurable antenna modes to create short term channel
fluctuation patterns that are exploited by the transmitter. The achievable
scheme does not require cooperation between transmit antennas and is therefore
applicable to the MxK X network as well. Only finite symbol extensions are
used, and no channel knowledge at the receivers is required to null the
interference.Comment: 27 pages, 15 figure
Bimetric varying speed of light theories and primordial fluctuations
We exhibit a varying speed of light (VSL) theory that implements the recently
proposed decaying speed of sound mechanism for generating density fluctuations.
We avail ourselves of bimetric VSL theories, where the speed of gravity differs
from that of light. We first show that a Dirac-Born-Infeld (DBI) type of
-essence has the necessary speed of sound profile to produce (near)
scale-invariant fluctuations. We then examine the map between bimetric and
-essence models: typically the bi-scalar connecting the two metrics is a
-essence field in one of them. Remarkably, the DBI model is found to
perturbatively represent the minimal bimetric model, where the bi-scalar is
Klein-Gordon in the matter frame. But the full non-perturbative bimetric
structure is even simpler: the bi-scalar dynamics should be simply driven by a
cosmological constant in the matter frame, balanced by an opposite cosmological
constant in the gravity frame. Thus the problem of structure formation receives
an elegant and universal solution within bimetric VSL theories, which are known
to also solve the flatness and entropy problems and evade a plethora of
causality concerns
Superconducting Properties of Copper Oxide High-Temperature Superconductors
The equations for the magnon pairing theory of high-temperature copper-oxide-based superconductors are solved and used to calculate several properties, leading to results for specific heat and critical magnetic fields consistent with experimental results. In addition, the theory suggests an explanation of why there are two sets of transition temperatures (Tcapprox 90 K and Tcapprox 55 K) for the Y1Ba2Cu3O6+x class of superconductors. It also provides an explanation of why La2-xSrxCuO4 is a superconductor for only a small range of x (and suggests an experiment to independently test the theory). These results provide support for the magnon pairing theory of high-temperature superconductors. On the basis of the theory, some suggestions are made for improving these materials. The agreement with experiment for various properties predicted by using the magnon pairing model of superconductivity provides strong support for the validity of this model for the Cu--O systems. All quantities are related to the fundamental parameters of the system (Jdd, JOCu, band structure). Some approximations have been made in the solutions to these equations. Nevertheless, the fundamental parameters are well defined, and hence improved calculational approximations will eventually lead to precise predictions of all properties. In this theory, the superconducting properties are related to fundamental structural, chemical, and physical properties, allowing one to use qualitative reasoning in contemplating how to improve the properties
Post density functional theoretical studies of highly polar semiconductive Pb(TiNi)O solid solutions: The effects of cation arrangement on band gap
We use a combination of conventional density functional theory (DFT) and
post-DFT methods, including the local density approximation plus Hubbard
(LDA+), PBE0, and self-consistent to study the electronic properties of
Ni-substituted PbTiO (Ni-PTO) solid solutions. We find that LDA
calculations yield unreasonable band structures, especially for Ni-PTO solid
solutions that contain an uninterrupted NiO layer. Accurate treatment of
localized states in transition-metal oxides like Ni-PTO requires post-DFT
methods. -site Ni/Ti cation ordering is also investigated. The -site
cation arrangement alters the bonding between Ni and O, and therefore strongly
affects the band gap () of Ni-PTO. We predict that Ni-PTO solid
solutions should have a direct band gap in the visible light energy range, with
polarization similar to the parent PbTiO. This combination of properties
make Ni-PTO solid solutions promising candidate materials for solar energy
conversion devices.Comment: 19 pages, 6 figure
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