34,699 research outputs found
Fast Non-Parametric Learning to Accelerate Mixed-Integer Programming for Online Hybrid Model Predictive Control
Today's fast linear algebra and numerical optimization tools have pushed the
frontier of model predictive control (MPC) forward, to the efficient control of
highly nonlinear and hybrid systems. The field of hybrid MPC has demonstrated
that exact optimal control law can be computed, e.g., by mixed-integer
programming (MIP) under piecewise-affine (PWA) system models. Despite the
elegant theory, online solving hybrid MPC is still out of reach for many
applications. We aim to speed up MIP by combining geometric insights from
hybrid MPC, a simple-yet-effective learning algorithm, and MIP warm start
techniques. Following a line of work in approximate explicit MPC, the proposed
learning-control algorithm, LNMS, gains computational advantage over MIP at
little cost and is straightforward for practitioners to implement
Effective tuning of exciton polarization splitting in coupled quantum dots
The polarization splitting of the exciton ground state in two laterally
coupled quantum dots under an in-plane electric field is investigated and its
effective tuning is designed. It is found that there are significant Stark
effect and anticrossing in energy levels. Due to coupling between inter- and
intra-dot states, the absolute value of polarization splitting is significantly
reduced, and it could be tuned to zero by the electric field for proper
inter-dot separations. Our scheme is interesting for the research on the
quantum dots-based entangled-photon source.Comment: 4 pages, 2 figures, to appear in Appl. Phys. Let
Macroscopic quantum coherence in antiferromagnetic molecular magnets
The macroscopic quantum coherence in a biaxial antiferromagnetic molecular
magnet in the presence of magnetic field acting parallel to its hard anisotropy
axis is studied within the two-sublattice model. On the basis of instanton
technique in the spin-coherent-state path-integral representation, both the
rigorous Wentzel-Kramers-Brillouin exponent and preexponential factor for the
ground-state tunnel splitting are obtained. We find that the quantum
fluctuations around the classical paths can not only induce a new quantum phase
previously reported by Chiolero and Loss (Phys. Rev. Lett. 80, 169 (1998)), but
also have great influnence on the intensity of the ground-state tunnel
splitting. Those features clearly have no analogue in the ferromagnetic
molecular magnets. We suggest that they may be the universal behaviors in all
antiferromagnetic molecular magnets. The analytical results are complemented by
exact diagonalization calculation.Comment: 6 pages, 1 figur
Effects of arbitrarily directed field on spin phase oscillations in biaxial molecular magnets
Quantum phase interference and spin-parity effects are studied in biaxial
molecular magnets in a magnetic field at an arbitrarily directed angle. The
calculations of the ground-state tunnel splitting are performed on the basis of
the instanton technique in the spin-coherent-state path-integral
representation, and complemented by exactly numerical diagonalization. Both the
Wentzel-Kramers-Brillouin exponent and the preexponential factor are obtained
for the entire region of the direction of the field. Our results show that the
tunnel splitting oscillates with the field for the small field angle, while for
the large field angle the oscillation is completely suppressed. This distinct
angular dependence, together with the dependence of the tunnel splitting on the
field strengh, provide an independent test for spin-parity effects in biaxial
molecular magnets. The analytical results for the molecular Fe magnet,
are found to be in good areement with the numerical simulations, which suggests
that even the molecular magnet with total spin S=10 is large enough to be
treated as a giant spin system.Comment: 19 pages, 5 figure
Energy Efficiency of Network Cooperation for Cellular Uplink Transmissions
There is a growing interest in energy efficient or so-called "green" wireless
communication to reduce the energy consumption in cellular networks. Since
today's wireless terminals are typically equipped with multiple network access
interfaces such as Bluetooth, Wi-Fi, and cellular networks, this paper
investigates user terminals cooperating with each other in transmitting their
data packets to a base station (BS) by exploiting the multiple network access
interfaces, referred to as inter-network cooperation, to improve the energy
efficiency in cellular uplink transmission. Given target outage probability and
data rate requirements, we develop a closed-form expression of energy
efficiency in Bits-per-Joule for the inter-network cooperation by taking into
account the path loss, fading, and thermal noise effects. Numerical results
show that when the cooperating users move towards to each other, the proposed
inter-network cooperation significantly improves the energy efficiency as
compared with the traditional non-cooperation and intra-network cooperation.
This implies that given a certain amount of bits to be transmitted, the
inter-network cooperation requires less energy than the traditional
non-cooperation and intra-network cooperation, showing the energy saving
benefit of inter-network cooperation.Comment: in Proceedings of the 2013 IEEE International Conference on
Communications (IEEE ICC 2013), Budapest, Hungary, June 201
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