For practical implementation of massive multiple-input multiple-output (MIMO) systems, the hybrid processing (precoding/combining) structure is promising to reduce the high implementation cost and ...power consumption rendered by large number of radio frequency (RF) chains of the traditional processing structure. The hybrid processing is realized through low-dimensional digital baseband processing combined with analog RF processing enabled by phase shifters. We propose to design hybrid RF and baseband precoders/combiners for multistream transmission in massive MIMO systems, by directly decomposing the predesigned unconstrained digital precoder/combiner of a large dimension. This approach is fundamental and general in the sense that any conventional full RF chain precoding solution of a MIMO system configuration can be converted to a hybrid processing structure by matrix decomposition. The constant amplitude constraint of analog RF processing results in the matrix decomposition problem nonconvex. Based on an alternate optimization technique, the nonconvex matrix decomposition problem can be decoupled into a series of convex subproblems and effectively solved by restricting the phase increment of each entry in the RF precoder/combiner within a small vicinity of its preceding iterate. A singular value decomposition-based technique is proposed to secure an initial point sufficiently close to the global solution of the original nonconvex problem. Through simulation, the convergence of the alternate optimization for such a matrix decomposition-based hybrid processing (MD-HP) scheme is examined, and the performance of the MD-HP scheme is demonstrated to be near-optimal.
The electrocaloric effect (ECE) in dielectric materials has great potential in realizing solid‐state cooling devices with compact size and high efficiency, which are highly desirable for a broad ...range of applications. This paper presents the general considerations for dielectric materials to achieve large ECE and reviews the experimental efforts investigating ECE in various polar dielectrics. For practical cooling devices, an ECE material must possess a large isothermal entropy change besides a large adiabatic temperature change. We show that polar dielectrics operated at temperatures near order–disorder transition have potential to achieve large ECE due to the possibility of large change in polarization induced by electric field and large entropy change associated with the polarization change. We further show that indeed the ferroelectric poly(vinylidene fluoride–trifluoroethylene)‐based polymers display a large ECE, i.e., an isothermal entropy change of more than 55 J (kgK)−1 and an adiabatic temperature change of more than 12 °C, at temperatures above the order–disorder transition.
Applying an electrical field to a dielectric may induce a large entropy and temperature change which is attractive for solid‐state cooling. We present the general considerations and review the experimental efforts to achieve large electrocaloric effect (ECE) in dielectrics. We show that by operating above the order‐disorder transitions, a large ECE can be achieved in a ferroelectric polymer.
A mass supercritical problem revisited Jeanjean, Louis; Lu, Sheng-Sen
Calculus of variations and partial differential equations,
10/2020, Volume:
59, Issue:
5
Journal Article
Peer reviewed
Open access
In any dimension
N
≥
1
and for given mass
m
>
0
, we revisit the nonlinear scalar field equation with an
L
2
constraint:
-
Δ
u
=
f
(
u
)
-
μ
u
in
R
N
,
‖
u
‖
L
2
(
R
N
)
2
=
m
,
u
∈
H
1
(
R
N
)
,
(
P
...m
)
where
μ
∈
R
will arise as a Lagrange multiplier. Assuming only that the nonlinearity
f
is continuous and satisfies weak mass supercritical conditions, we show the existence of ground states to
(
P
m
)
and reveal the basic behavior of the ground state energy
E
m
as
m
>
0
varies. In particular, to overcome the compactness issue when looking for ground states, we develop robust arguments which we believe will allow treating other
L
2
constrained problems in general mass supercritical settings. Under the same assumptions, we also obtain infinitely many radial solutions for any
N
≥
2
and establish the existence and multiplicity of nonradial sign-changing solutions when
N
≥
4
. Finally we propose two open problems.
This paper deals with the design of a sliding-mode disturbance observer (SDOB) with switching-gain adaptation for a class of nonlinear systems subject to exogenous signals. The proposed SDOB, ...referred to as IMP-SDOB, is based on the internal-model principle and thus achieves robust asymptotic disturbance estimation, whereas the conventional SDOB leads to nonzero estimation error to a time-varying disturbance. Moreover, while the conventional SDOB needs a switching gain greater than the magnitude of an unknown disturbance, the proposed IMP-SDOB design releases this constraint and requires only a small switching gain as compared with a conventional design, further alleviating the chattering problem. The stability of the combined controller-observer system is derived on the basis of a Lyapunov analysis. Moreover, in contrast with the previous switching-gain adaptation law that inevitably yields a nondecreasing switching gain and asymptotic convergence of the switching function, a novel adaptation law is proposed to reduce the switching gain whenever appropriate and to guarantee finite-time convergence of the switching function. The effectiveness of the proposed scheme is demonstrated through experimental studies on the track-following system of an optical disk drive suffering from significant disk runout.
Voting-based methods (e.g., VoteNet) have achieved promising results for 3D object detection. However, the simple voting operation in VoteNet may lead to less accurate voting results that are far ...away from the true object centers. In this work, we propose a simple but effective 3D object detection method called Transformer3D-Det (T3D), in which we additionally introduce a transformer based vote refinement module to refine the voting results of VoteNet and can thus significantly improve the 3D object detection performance. Specifically, our T3D framework consists of three modules: a vote generation module, a vote refinement module, and a bounding box generation module. Given an input point cloud, we first utilize the vote generation module to generate multiple coarse vote clusters. Then, the clustered coarse votes will be refined by using our transformer based vote refinement module to produce more accurate and meaningful votes. Finally, the bounding box generation module takes the refined vote clusters as the input and generates the final detection result for the input point cloud. To alleviate the impact of inaccurate votes, we also propose a new non-vote loss function to train our T3D. As a result, our T3D framework can achieve better 3D object detection performance. Comprehensive experiments on two benchmark datasets ScanNetV2 and SUN RGB-D demonstrate the effectiveness of our T3D framework for 3D object detection.
Hydrogen production from the dehydrogenation of formic acid (FA) is promising. Most of the current catalysts for FA dehydrogenation are effective only in the presence of bases or additives. We report ...here newly developed iridium complexes containing conjugated N,N′‐diimine ligands for FA dehydrogenation in water without the addition of bases or additives. A turnover frequency (TOF) of 487 500 h−1 with Cp*Ir(L1)ClCl (L1=2,2′‐bi‐2‐imidazoline) at 90 °C and a turnover number (TON) of 2 400 000 with in situ prepared catalyst from IrCp*Cl22 and 2,2′‐bi‐1,4,5,6‐tetrahydropyrimidine (L2) at 80 °C were obtained, the highest values reported for FA dehydrogenation to date. A mechanistic study reveals that the formation of Ir‐H intermediate species is the rate‐determining step in the catalytic cycle.
Hydrogen production from the dehydrogenation of formic acid (FA) is a promising process. Most of the current catalysts for FA dehydrogenation are effective only in the presence of bases or additives. Reported here are newly developed iridium complexes containing conjugated N,N′‐diimine ligands for FA dehydrogenation in water without the addition of any bases or additives, giving a turnover frequency (TOF) of up to 487 500 h−1 (catalyst Cp*Ir(L1)ClCl) and a turnover number (TON) of 2 400 000 (in situ prepared catalyst with L2; Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl).
In any dimension
N
≥
1
, for given mass
m
>
0
and for the
C
1
energy functional
I
(
u
)
:
=
1
2
∫
R
N
|
∇
u
|
2
d
x
-
∫
R
N
F
(
u
)
d
x
,
we revisit the classical problem of finding conditions on
F
∈
...C
1
(
R
,
R
)
insuring that
I
admits global minimizers on the mass constraint
S
m
:
=
u
∈
H
1
(
R
N
)
|
‖
u
‖
L
2
(
R
N
)
2
=
m
.
Under assumptions that we believe to be nearly optimal, in particular without assuming that
F
is even, any such global minimizer, called energy ground state, proves to have constant sign and to be radially symmetric monotone with respect to some point in
R
N
. Moreover, we show that any energy ground state is a least action solution of the associated action functional. This last result answers positively, under general assumptions, a long standing issue.
Biomass is the most abundant renewable resource on earth and developing high-performance nonprecious selective hydrogenation (SH) catalysts will enable the use of biomass to replace rapidly ...diminishing fossil resources. This work utilizes ZIF-67-derived nitrogen-doped carbon nanotubes to confine Co nanoparticles (NPs) with Co-N
active sites as a high-performance SH catalyst. The confined Co NPs with Co-N
exhibit excellent catalytic activity, selectivity, and stability toward a wide range of biomass-derived compounds. Such active sites can selectively hydrogenate aldehyde, ketone, carboxyl, and nitro groups of biomass-derived compounds into value-added fine chemicals with 100% selectivity. The reported approach could be adopted to create other forms of catalytically active sites from other nonprecious metals.
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