51,269 research outputs found
Remote Antenna Unit Selection Assisted Seamless Handover for High-Speed Railway Communications with Distributed Antennas
To attain seamless handover and reduce the han- dover failure probability for
high-speed railway (HSR) com- munication systems, this paper proposes a remote
antenna unit (RAU) selection assisted handover scheme where two antennas are
installed on high speed train (HST) and distributed antenna system (DAS) cell
architecture on ground is adopted. The RAU selection is used to provide high
quality received signals for trains moving in DAS cells and the two HST
antennas are employed on trains to realize seamless handover. Moreover, to
efficiently evaluate the system performance, a new met- ric termed as handover
occurrence probability is defined for describing the relation between handover
occurrence position and handover failure probability. We then analyze the
received signal strength, the handover trigger probability, the handover
occurrence probability, the handover failure probability and the communication
interruption probability. Numerical results are provided to compare our
proposed scheme with the current existing ones. It is shown that our proposed
scheme achieves better performances in terms of handover failure probability
and communication interruption probability.Comment: 7 figures, accepted by IEEE VTC-Spring, 201
Engineering Photon Delocalization in a Rabi Dimer with a Dissipative Bath
A Rabi dimer is used to model a recently reported circuit quantum
electrodynamics system composed of two coupled transmission-line resonators
with each coupled to one qubit. In this study, a phonon bath is adopted to
mimic the multimode micromechanical resonators and is coupled to the qubits in
the Rabi dimer. The dynamical behavior of the composite system is studied by
the Dirac-Frenkel time-dependent variational principle combined with the
multiple Davydov D ans\"{a}tze. Initially all the photons are pumped into
the left resonator, and the two qubits are in the down state coupled with the
phonon vacuum. In the strong qubit-photon coupling regime, the photon dynamics
can be engineered by tuning the qubit-bath coupling strength and
photon delocalization is achieved by increasing . In the absence of
dissipation, photons are localized in the initial resonator. Nevertheless, with
moderate qubit-bath coupling, photons are delocalized with quasiequilibration
of the photon population in two resonators at long times. In this case, high
frequency bath modes are activated by interacting with depolarized qubits. For
strong dissipation, photon delocalization is achieved via frequent
photon-hopping within two resonators and the qubits are suppressed in their
initial down state.Comment: 11 pages, 11 figure
Sub-Ohmic spin-boson model with off-diagonal coupling: Ground state properties
We have carried out analytical and numerical studies of the spin-boson model
in the sub-ohmic regime with the influence of both the diagonal and
off-diagonal coupling accounted for via the Davydov D1 variational ansatz.
While a second-order phase transition is known to be exhibited by this model in
the presence of diagonal coupling only, we demonstrate the emergence of a
discontinuous first order phase transition upon incorporation of the
off-diagonal coupling. A plot of the ground state energy versus magnetization
highlights the discontinuous nature of the transition between the isotropic
(zero magnetization) state and nematic (finite magnetization) phases. We have
also calculated the entanglement entropy and a discontinuity found at a
critical coupling strength further supports the discontinuous crossover in the
spin-boson model in the presence of off-diagonal coupling. It is further
revealed via a canonical transformation approach that for the special case of
identical exponents for the spectral densities of the diagonal and the
off-diagonal coupling, there exists a continuous crossover from a single
localized phase to doubly degenerate localized phase with differing
magnetizations.Comment: 11 pages, 7 figure
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