5,251 research outputs found
Opportunistic Interference Mitigation Achieves Optimal Degrees-of-Freedom in Wireless Multi-cell Uplink Networks
We introduce an opportunistic interference mitigation (OIM) protocol, where a
user scheduling strategy is utilized in -cell uplink networks with
time-invariant channel coefficients and base stations (BSs) having
antennas. Each BS opportunistically selects a set of users who generate the
minimum interference to the other BSs. Two OIM protocols are shown according to
the number of simultaneously transmitting users per cell: opportunistic
interference nulling (OIN) and opportunistic interference alignment (OIA).
Then, their performance is analyzed in terms of degrees-of-freedom (DoFs). As
our main result, it is shown that DoFs are achievable under the OIN
protocol with selected users per cell, if the total number of users in
a cell scales at least as . Similarly, it turns out that
the OIA scheme with () selected users achieves DoFs, if scales
faster than . These results indicate that there exists a
trade-off between the achievable DoFs and the minimum required . By deriving
the corresponding upper bound on the DoFs, it is shown that the OIN scheme is
DoF optimal. Finally, numerical evaluation, a two-step scheduling method, and
the extension to multi-carrier scenarios are shown.Comment: 18 pages, 3 figures, Submitted to IEEE Transactions on Communication
Reconfiguration of quantum states in -symmetric quasi-one dimensional lattices
We demonstrate mesoscopic transport through quantum states in quasi-1D
lattices maintaining the combination of parity and time-reversal symmetries by
controlling energy gain and loss. We investigate the phase diagram of the
non-Hermitian system where transitions take place between unbroken and broken
-symmetric phases via exceptional points. Quantum transport in
the lattice is measured only in the unbroken phases in the energy band-but not
in the broken phases. The broken phase allows for spontaneous symmetry-broken
states where the cross-stitch lattice is separated into two identical single
lattices corresponding to conditionally degenerate eigenstates. These
degeneracies show a lift-up in the complex energy plane, caused by the
non-Hermiticity with -symmetry.Comment: 12 pages, 7 figure
Antiresonance induced by symmetry-broken contacts in quasi-one-dimensional lattices
We report the effect of symmetry-broken contacts on quantum transport in
quasi-one-dimensional lattices. In contrast to 1D chains, transport in
quasi-one-dimensional lattices, which are made up of a finite number of 1D
chain layers, is strongly influenced by contacts. Contact symmetry depends on
whether the contacts maintain or break the parity symmetry between the layers.
With balanced on-site potential, a flat band can be detected by asymmetric
contacts, but not by symmetric contacts. In the case of asymmetric contacts
with imbalanced on-site potential, transmission is suppressed at certain
energies. We elucidate these energies of transmission suppression related to
antiresonance using reduced lattice models and Feynman paths. These results
provide a nondestructive measurement of flat band energy which it is difficult
to detect.Comment: 8 pages, 5 figure
Click-aware purchase prediction with push at the top
Eliciting user preferences from purchase records for performing purchase
prediction is challenging because negative feedback is not explicitly observed,
and because treating all non-purchased items equally as negative feedback is
unrealistic. Therefore, in this study, we present a framework that leverages
the past click records of users to compensate for the missing user-item
interactions of purchase records, i.e., non-purchased items. We begin by
formulating various model assumptions, each one assuming a different order of
user preferences among purchased, clicked-but-not-purchased, and non-clicked
items, to study the usefulness of leveraging click records. We implement the
model assumptions using the Bayesian personalized ranking model, which
maximizes the area under the curve for bipartite ranking. However, we argue
that using click records for bipartite ranking needs a meticulously designed
model because of the relative unreliableness of click records compared with
that of purchase records. Therefore, we ultimately propose a novel
learning-to-rank method, called P3Stop, for performing purchase prediction. The
proposed model is customized to be robust to relatively unreliable click
records by particularly focusing on the accuracy of top-ranked items.
Experimental results on two real-world e-commerce datasets demonstrate that
P3STop considerably outperforms the state-of-the-art implicit-feedback-based
recommendation methods, especially for top-ranked items.Comment: For the final published journal version, see
https://doi.org/10.1016/j.ins.2020.02.06
Flat-band localization and self-collimation of light in photonic crystals
We investigate the optical properties of a photonic crystal composed of a
quasi-one-dimensional flat-band lattice array through finite-difference
time-domain simulations. The photonic bands contain flat bands (FBs) at
specific frequencies, which correspond to compact localized states as a
consequence of destructive interference. The FBs are shown to be nondispersive
along the line, but dispersive along the
line. The FB localization of light in a single direction
only results in a self-collimation of light propagation throughout the photonic
crystal at the FB frequency.Comment: 18 single-column pages, 7 figures including graphical to
Can One Achieve Multiuser Diversity in Uplink Multi-Cell Networks?
We introduce a distributed opportunistic scheduling (DOS) strategy, based on
two pre-determined thresholds, for uplink -cell networks with time-invariant
channel coefficients. Each base station (BS) opportunistically selects a mobile
station (MS) who has a large signal strength of the desired channel link among
a set of MSs generating a sufficiently small interference to other BSs. Then,
performance on the achievable throughput scaling law is analyzed. As our main
result, it is shown that the achievable sum-rate scales as
in a high signal-to-noise ratio (SNR) regime, if the
total number of users in a cell, , scales faster than
for a constant . This
result indicates that the proposed scheme achieves the multiuser diversity gain
as well as the degrees-of-freedom gain even under multi-cell environments.
Simulation results show that the DOS provides a better sum-rate throughput over
conventional schemes.Comment: 11 pages, 3 figures, 2 tables, to appear in IEEE Transactions on
Communication
Emergent localized states at the interface of a twofold -symmetric lattice
We consider the role of non-triviality resulting from a non-Hermitian
Hamiltonian that conserves twofold PT-symmetry assembled by interconnections
between a PT-symmetric lattice and its time reversal partner. Twofold
PT-symmetry in the lattice produces additional surface exceptional points that
play the role of new critical points, along with the bulk exceptional point. We
show that there are two distinct regimes possessing symmetry-protected
localized states, of which localization lengths are robust against external
gain and loss. The states are demonstrated by numerical calculation of a
quasi-1D ladder lattice and a 2D bilayered square lattice.Comment: 10 pages, 7 figure
Corporate Political Strategies and Return Predictability
We assess whether observable corporate political strategies can serve as channels of value relevant political information flow into stock prices and form the basis for profitable return predictability strategies. We document that returns of politically connected firms’ stocks lead those of their non-connected peers, suggesting that information shocks associated with new policies and other political developments become evident first in the stock prices of firms that pursue political strategies and then, with delay, in those of similar non-connected firms
Policy Uncertainty and the Dual Role of Corporate Political Strategies
Firms use active political strategies not only to mitigate uncertainty emanating from legislative activity, but also to enhance their growth opportunities. We find that a firm\u27s systematic risk (beta) can be hedged away by employing various political strategies involving the presence of former politicians on corporate boards of directors, contributions to political campaigns, and corporate lobbying activities. The hedging effect is greater when firms operate in more uncertain industries. In addition, active political strategies are associated with greater firm heterogeneity and make real options more value relevant as potential drivers of competitive advantages in uncertain environments
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