152 research outputs found
Energy-Efficiency Maximization for a WPT-D2D Pair in a MISO-NOMA Downlink Network
The combination of non-orthogonal multiple access (NOMA) and wireless power
transfer (WPT) is a promising solution to enhance the energy efficiency of
Device-to-Device (D2D) enabled wireless communication networks. In this paper,
we focus on maximizing the energy efficiency of a WPT-D2D pair in a
multiple-input single-output (MISO)-NOMA downlink network, by alternatively
optimizing the beamforming vectors of the base station (BS) and the time
switching coefficient of the WPT assisted D2D transmitter. The formulated
energy efficiency maximization problem is non-convex due to the highly coupled
variables. To efficiently address the non-convex problem, we first divide it
into two subproblems. Afterwards, an alternating algorithm based on the
Dinkelbach method and quadratic transform is proposed to solve the two
subproblems iteratively. To verify the proposed alternating algorithm's
accuracy, partial exhaustive search algorithm is proposed as a benchmark. We
also utilize a deep reinforcement learning (DRL) method to solve the non-convex
problem and compare it with the proposed algorithm. To demonstrate the
respective superiority of the proposed algorithm and DRL-based method,
simulations are performed for two scenarios of perfect and imperfect channel
state information (CSI). Simulation results are provided to compare NOMA and
orthogonal multiple access (OMA), which demonstrate the superior performance of
energy efficiency of the NOMA scheme
A BAC-NOMA Design for 6Â G umMTC With Hybrid SIC: Convex Optimization or Learning-Based?
This paper presents a new backscattering communication (BackCom)-assisted non-orthogonal multiple access (BAC-NOMA) transmission scheme for device-to-device (D2D) communications. This scheme facilitates energy and spectrum cooperation between BackCom devices and cellular downlink users in 6Â G ultra-massive machine -type communications (umMTC) scenarios. Given its quasi-uplink nature, the hybrid successive interference cancellation (SIC) is applied to further improve performance. The data rate of BackCom devices with high quality of service (QoS) requirements is maximized by jointly optimizing backscatter coefficients and the beamforming vector. The use of hybrid SIC and BackCom yields two non-concave sub-problems involving transcendental functions. To address this problem, this paper designs and compares convex optimization-based and unsupervised deep learning-based algorithms. In the convex optimization, the closed-form backscatter coefficients of the first sub-problem are obtained, and then semi-definite relaxation (SDR) is utilized to design the beamforming vector. On the other hand, the second sub-problem is approximated by using a combination of sequential convex approximation (SCA) and SDR. For unsupervised deep learning-based optimization, a loss function is properly designed to satisfy constraints. Computer simulations show the following instructive results: i) the superiority of the hybrid SIC strategy; ii) the distinct sensitivities and efficacies of these two algorithms in response to varying parameters; iii) the superior robustness of the unsupervised deep learning-based optimization
The Global Landscape of Neural Networks: An Overview
One of the major concerns for neural network training is that the
non-convexity of the associated loss functions may cause bad landscape. The
recent success of neural networks suggests that their loss landscape is not too
bad, but what specific results do we know about the landscape? In this article,
we review recent findings and results on the global landscape of neural
networks. First, we point out that wide neural nets may have sub-optimal local
minima under certain assumptions. Second, we discuss a few rigorous results on
the geometric properties of wide networks such as "no bad basin", and some
modifications that eliminate sub-optimal local minima and/or decreasing paths
to infinity. Third, we discuss visualization and empirical explorations of the
landscape for practical neural nets. Finally, we briefly discuss some
convergence results and their relation to landscape results.Comment: 16 pages. 8 figure
GOATS: Goal Sampling Adaptation for Scooping with Curriculum Reinforcement Learning
In this work, we first formulate the problem of robotic water scooping using
goal-conditioned reinforcement learning. This task is particularly challenging
due to the complex dynamics of fluids and the need to achieve multi-modal
goals. The policy is required to successfully reach both position goals and
water amount goals, which leads to a large convoluted goal state space. To
overcome these challenges, we introduce Goal Sampling Adaptation for Scooping
(GOATS), a curriculum reinforcement learning method that can learn an effective
and generalizable policy for robot scooping tasks. Specifically, we use a
goal-factorized reward formulation and interpolate position goal distributions
and amount goal distributions to create curriculum throughout the learning
process. As a result, our proposed method can outperform the baselines in
simulation and achieves 5.46% and 8.71% amount errors on bowl scooping and
bucket scooping tasks, respectively, under 1000 variations of initial water
states in the tank and a large goal state space. Besides being effective in
simulation environments, our method can efficiently adapt to noisy real-robot
water-scooping scenarios with diverse physical configurations and unseen
settings, demonstrating superior efficacy and generalizability. The videos of
this work are available on our project page:
https://sites.google.com/view/goatscooping
Evidence for Majorana bound state in an iron-based superconductor
The search for Majorana bound state (MBS) has recently emerged as one of the
most active research areas in condensed matter physics, fueled by the prospect
of using its non-Abelian statistics for robust quantum computation. A highly
sought-after platform for MBS is two-dimensional topological superconductors,
where MBS is predicted to exist as a zero-energy mode in the core of a vortex.
A clear observation of MBS, however, is often hindered by the presence of
additional low-lying bound states inside the vortex core. By using scanning
tunneling microscope on the newly discovered superconducting Dirac surface
state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting
transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split
zero-bias peak inside a vortex core. Systematic studies of its evolution under
different magnetic fields, temperatures, and tunneling barriers strongly
suggest that this is the case of tunneling to a nearly pure MBS, separated from
non-topological bound states which is moved away from the zero energy due to
the high ratio between the superconducting gap and the Fermi energy in this
material. This observation offers a new, robust platform for realizing and
manipulating MBSs at a relatively high temperature.Comment: 27 pages, 11 figures, supplementary information include
Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor
Majorana zero-modes (MZMs) are spatially-localized zero-energy fractional
quasiparticles with non-Abelian braiding statistics that hold a great promise
for topological quantum computing. Due to its particle-antiparticle
equivalence, an MZM exhibits robust resonant Andreev reflection and 2e2/h
quantized conductance at low temperature. By utilizing variable-tunnel-coupled
scanning tunneling spectroscopy, we study tunneling conductance of vortex bound
states on FeTe0.55Se0.45 superconductors. We report observations of conductance
plateaus as a function of tunnel coupling for zero-energy vortex bound states
with values close to or even reaching the 2e2/h quantum conductance. In
contrast, no such plateau behaviors were observed on either finite energy
Caroli-de Genne-Matricon bound states or in the continuum of electronic states
outside the superconducting gap. This unique behavior of the zero-mode
conductance reaching a plateau strongly supports the existence of MZMs in this
iron-based superconductor, which serves as a promising single-material platform
for Majorana braiding at a relatively high temperature
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