22,828 research outputs found
The Universality and stability for a dilute Bose gas with a Feshbach resonance
We study the bosonic atoms with a wide Feshbach resonance at zero temperature
in terms of the renormalization group. We indicate that this system will always
collapse in the dilute limit. On the side with a positive scattering length,
the atomic superfluid is an unstable local minimum in the dilute limit and it
determines the thermodynamics of this system within its lifetime. We calculate
the equilibrium properties at zero temperature in the unitary regime. They
exhibit universal scaling forms in the dilute limit due to the presence of a
nontrivial zero temperature, zero density fixed point. Moreover, we find that
the T=0 thermodynamics of this system in the unitary limit is exactly identical
to the one for an ideal Fermi gas.Comment: 6 pages, 4 figure
Quantum phases of a Feshbach-resonant atomic Bose gas in one dimension
We study an atomic Bose gas with an s-wave Feshbach resonance in a
one-dimensional optical lattice, with the densities of atoms and molecules
incommensurate with the lattice. At zero temperature, most of the parameter
region is occupied by a phase in which the superfluid fluctuations of atoms and
molecules are the predominant ones, due to the phase fluctuations of atoms and
molecules being locked by a Josephson coupling between them. When the density
difference between atoms and molecules is commensurate with the lattice, two
additional phases may exist: the two component Luttinger liquid where both the
atomic and molecular sectors are gapless, and the inter-channel charge density
wave where the relative density fluctuations between atoms and molecules are
frozen at low energy.Comment: 4+epsilon pages, 3 figures; references adde
Channel Acquisition for Massive MIMO-OFDM with Adjustable Phase Shift Pilots
We propose adjustable phase shift pilots (APSPs) for channel acquisition in
wideband massive multiple-input multiple-output (MIMO) systems employing
orthogonal frequency division multiplexing (OFDM) to reduce the pilot overhead.
Based on a physically motivated channel model, we first establish a
relationship between channel space-frequency correlations and the channel power
angle-delay spectrum in the massive antenna array regime, which reveals the
channel sparsity in massive MIMO-OFDM. With this channel model, we then
investigate channel acquisition, including channel estimation and channel
prediction, for massive MIMO-OFDM with APSPs. We show that channel acquisition
performance in terms of sum mean square error can be minimized if the user
terminals' channel power distributions in the angle-delay domain can be made
non-overlapping with proper phase shift scheduling. A simplified pilot phase
shift scheduling algorithm is developed based on this optimal channel
acquisition condition. The performance of APSPs is investigated for both one
symbol and multiple symbol data models. Simulations demonstrate that the
proposed APSP approach can provide substantial performance gains in terms of
achievable spectral efficiency over the conventional phase shift orthogonal
pilot approach in typical mobility scenarios.Comment: 15 pages, 4 figures, accepted for publication in the IEEE
Transactions on Signal Processin
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