1,841 research outputs found
Two-Way Training for Discriminatory Channel Estimation in Wireless MIMO Systems
This work examines the use of two-way training to efficiently discriminate
the channel estimation performances at a legitimate receiver (LR) and an
unauthorized receiver (UR) in a multiple-input multiple-output (MIMO) wireless
system. This work improves upon the original discriminatory channel estimation
(DCE) scheme proposed by Chang et al where multiple stages of feedback and
retraining were used. While most studies on physical layer secrecy are under
the information-theoretic framework and focus directly on the data transmission
phase, studies on DCE focus on the training phase and aim to provide a
practical signal processing technique to discriminate between the channel
estimation performances at LR and UR. A key feature of DCE designs is the
insertion of artificial noise (AN) in the training signal to degrade the
channel estimation performance at UR. To do so, AN must be placed in a
carefully chosen subspace based on the transmitter's knowledge of LR's channel
in order to minimize its effect on LR. In this paper, we adopt the idea of
two-way training that allows both the transmitter and LR to send training
signals to facilitate channel estimation at both ends. Both reciprocal and
non-reciprocal channels are considered and a two-way DCE scheme is proposed for
each scenario. {For mathematical tractability, we assume that all terminals
employ the linear minimum mean square error criterion for channel estimation.
Based on the mean square error (MSE) of the channel estimates at all
terminals,} we formulate and solve an optimization problem where the optimal
power allocation between the training signal and AN is found by minimizing the
MSE of LR's channel estimate subject to a constraint on the MSE achievable at
UR. Numerical results show that the proposed DCE schemes can effectively
discriminate between the channel estimation and hence the data detection
performances at LR and UR.Comment: 1
Strongly Correlated Two-Photon Transport in One-Dimensional Waveguide Coupled to A Two-Level System
We show that two-photon transport is strongly correlated in one-dimensional
waveguide coupled to a two-level system. The exact S-matrix is constructed
using a generalized Bethe-Ansatz technique. We show that the scattering
eigenstates of this system include a two-photon bound state that passes through
the two-level system as a composite single particle. Also, the two-level system
can induce effective attractive or repulsive interactions in space for photons.
This general procedure can be applied to the Anderson model as well.Comment: 12 pages. 3 figures. Accepted by Physical Review Letter
Coordinated Multicasting with Opportunistic User Selection in Multicell Wireless Systems
Physical layer multicasting with opportunistic user selection (OUS) is
examined for multicell multi-antenna wireless systems. By adopting a two-layer
encoding scheme, a rate-adaptive channel code is applied in each fading block
to enable successful decoding by a chosen subset of users (which varies over
different blocks) and an application layer erasure code is employed across
multiple blocks to ensure that every user is able to recover the message after
decoding successfully in a sufficient number of blocks. The transmit signal and
code-rate in each block determine opportunistically the subset of users that
are able to successfully decode and can be chosen to maximize the long-term
multicast efficiency. The employment of OUS not only helps avoid
rate-limitations caused by the user with the worst channel, but also helps
coordinate interference among different cells and multicast groups. In this
work, efficient algorithms are proposed for the design of the transmit
covariance matrices, the physical layer code-rates, and the target user subsets
in each block. In the single group scenario, the system parameters are
determined by maximizing the group-rate, defined as the physical layer
code-rate times the fraction of users that can successfully decode in each
block. In the multi-group scenario, the system parameters are determined by
considering a group-rate balancing optimization problem, which is solved by a
successive convex approximation (SCA) approach. To further reduce the feedback
overhead, we also consider the case where only part of the users feed back
their channel vectors in each block and propose a design based on the balancing
of the expected group-rates. In addition to SCA, a sample average approximation
technique is also introduced to handle the probabilistic terms arising in this
problem. The effectiveness of the proposed schemes is demonstrated by computer
simulations.Comment: Accepted by IEEE Transactions on Signal Processin
Generation of spiral bevel gears with zero kinematical errors and computer aided tooth contact analysis
Kinematic errors in spiral bevel gears are a major source of noise and vibrations in transmissions. A method for the generation of Gleason's spiral bevel gears which provides conjugated gear tooth surfaces and an improved bearing contact was developed. A computer program for the simulation of meshing, misalignment, and bearing contact was written
Transverse force generated by an electric field and transverse charge imbalance in spin-orbit coupled systems
We use linear response theory to study the transverse force generated by an
external electric field and hence possible charge Hall effect in spin-orbit
coupled systems. In addition to the Lorentz force that is parallel to the
electric field, we find that the transverse force perpendicular to the applied
electric field may not vanish in a system with an anisotropic energy
dispersion. Surprisingly, in contrast to the previous results, the transverse
force generated by the electric field does not depend on the spin current, but
in general, it is related to the second derivative of energy dispersion only.
Furthermore, we find that the transverse force does not vanish in the
Rashba-Dresselhaus system. Therefore, the non-vanishing transverse force acts
as a driving force and results in charge imbalance at the edges of the sample.
The estimated ratio of the Hall voltage to the longitudinal voltage is . The disorder effect is also considered in the study of the
Rashba-Dresselhaus system. We find that the transverse force vanishes in the
presence of impurities in this system because the vertex correction and the
anomalous velocity of the electron accidently cancel each other
Spiral bevel and circular arc helical gears: Tooth contact analysis and the effect of misalignment on circular arc helical gears
A computer aided method for tooth contact analysis was developed and applied. Optimal machine-tool settings for spiral bevel gears are proposed and when applied indicated that kinematic errors can be minimized while maintaining a desirable bearing contact. The effect of misalignment for circular arc helical gears was investigated and the results indicted that directed pinion refinishing can compensate the kinematic errors due to misalignment
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