249 research outputs found
Globally Intelligent Adaptive Finite-/Fixed- Time Tracking Control for Strict-Feedback Nonlinear Systems via Composite Learning Approaches
This article focuses on the globally composite adaptive law-based intelligent
finite-/fixed- time (FnT/FxT) tracking control issue for a family of uncertain
strict-feedback nonlinear systems. First, intelligent approximators with new
composite updating laws are developed to model uncertain nonlinear terms, which
encompass prediction errors to enhance intelligent approximators' learning
behaviors and fewer online learning parameters to diminish computational
burden. Then, a novel smooth switching function coupled with robust controllers
is designed to pull system states back when the transients are out of the
approximators' active domain. After that, a modified FnT/FxT backstepping
technique is constructed to render output to follow the reference trajectory,
and an adaptive law is employed to alleviate the impact of external
disturbances. It is theoretically confirmed that the proposed control
strategies ensure globally FnT/FxT boundedness of all the closed-loop
variables. Finally, the validity of theoretical results is testified via a
simulation case.Comment: 6 pages,12 figure
Prescribed Performance Adaptive Fixed-Time Attitude Tracking Control of a 3-DOF Helicopter with Small Overshoot
In this article, a novel prescribed performance adaptive fixed-time
backstepping control strategy is investigated for the attitude tracking of a
3-DOF helicopter. First, a new unified barrier function (UBF) is designed to
convert the prescribed performance constrained system into an unconstrained
one. Then, a fixed-time (FxT) backstepping control framework is established to
achieve the attitude tracking. By virtual of a newly proposed inequality, a
non-singular virtual control law is constructed. In addition, a FxT
differentiator with a compensation mechanism is employed to overcome the matter
of "explosion of complexity". Moreover, a modified adaptive law is developed to
approximate the upper bound of the disturbances. To obtain a less conservative
and more accurate approximation of the settling time, an improved FxT stability
theorem is proposed. Based on this theorem, it is proved that all signals of
the system are FxT bounded, and the tracking error converges to a preset domain
with small overshoot in a user-defined time. Finally, the feasibility and
effectiveness of the presented control strategy are confirmed by numerical
simulations.Comment: 6 pages, 4 figure
Preparation and Numerical Modelling of Ceramic Foam Insulation for Energy Saving in Buildings
For the purpose of energy saving in buildings, a foam ceramic insulation (FCI) was prepared by using fly ash (FA) and ceramic waste (CW) as the main raw materials for its matrix part and foam part, respectively. The effects of the sintering temperature and the additive agent on the macroscopic performances were systematically measured and investigated. The experiment results indicate that for the matrix sample 5% quartz addition makes the rupture modulus at 1200°C reach high to 34.28 MPa, while the corresponding water absorption capacity is only 0.83%. In addition, for the foam sample with 1 wt% silicon carbide, the lowest measured bulk density and thermal conductivity at 1200°C are 0.471 g/cm3 and 0.1184 W/(m•K), respectively. Furthermore, the proposed simulation model predicts that the effective thermal conductivity of FCI decreases with the decrease of the bulk density. Moreover, the simulation results calculated by EnergyPlus software indicate that the synthetic FCI can efficiently reduce the building’s heating and cooling loads and exerts excellent energy conservation effect
STARS Enabled Integrated Sensing and Communications
A simultaneously transmitting and reflecting intelligent surface (STARS)
enabled integrated sensing and communications (ISAC) framework is proposed,
where the whole space is divided by STARS into a sensing space and a
communication space. A novel sensing-at-STARS structure, where dedicated
sensors are installed at the STARS, is proposed to address the significant path
loss and clutter interference for sensing. The Cramer-Rao bound (CRB) of the
2-dimension (2D) direction-of-arrivals (DOAs) estimation of the sensing target
is derived, which is then minimized subject to the minimum communication
requirement. A novel approach is proposed to transform the complicated CRB
minimization problem into a trackable modified Fisher information matrix (FIM)
optimization problem. Both independent and coupled phase-shift models of STARS
are investigated: 1) For the independent phase-shift model, to address the
coupling of ISAC waveform and STARS coefficient in the modified FIM, an
efficient double-loop iterative algorithm based on the penalty dual
decomposition (PDD) framework is conceived; 2) For the coupled phase-shift
model, based on the PDD framework, a low complexity alternating optimization
algorithm is proposed to tackle coupled phase-shift constants by alternatively
optimizing amplitude and phase-shift coefficients in closed-form. Finally, the
numerical results demonstrate that: 1) STARS significantly outperforms the
conventional RIS in CRB under the communication constraints; 2) The coupled
phase-shift model achieves comparable performance to the independent one for
low communication requirements or sufficient STARS elements; 3) It is more
efficient to increase the number of passive elements of STARS rather than the
active elements of the sensor; 4) High sensing accuracy can be achieved by
STARS using the practical 2D maximum likelihood estimator compared with the
conventional RIS.Comment: 30 pages, 8 figure
Near-Field Integrated Sensing and Communications
A near-field integrated sensing and communications (ISAC) framework is
proposed, which introduces an additional distance dimension for both sensing
and communications compared to the conventional far-field system. In
particular, the Cramer-Rao bound for the near-field joint distance and angle
sensing is derived, which is minimized subject to the minimum communication
rate requirement of each user. Both fully digital antennas and hybrid digital
and analog antennas are investigated. For fully digital antennas, a globally
optimal solution of the ISAC waveform is obtained via semidefinite relaxation.
For hybrid antennas, a high-quality solution is obtained through two-stage
optimization. Numerical results demonstrate the performance gain introduced by
the additional distance dimension of the near-field ISAC over the far-field
ISAC.Comment: 5 pages, 4 figure
Beamfocusing Optimization for Near-Field Wideband Multi-User Communications
A near-field wideband communication system is studied, wherein a base station
(BS) employs an extremely large-scale antenna array (ELAA) to serve multiple
users situated within its near-field region. To facilitate the near-field
beamfocusing and mitigate the wideband beam split, true-time delayer
(TTD)-based hybrid beamforming architectures are employed at the BS. Apart from
the fully-connected TTD-based architecture, a new sub-connected TTD-based
architecture is proposed for enhancing energy efficiency. Three wideband
beamfocusing optimization approaches are proposed to maximize spectral
efficiency for both architectures. 1) Fully-digital approximation (FDA)
approach: In this approach, the TTD-based hybrid beamformers are optimized to
approximate the optimal fully-digital beamformers using block coordinate
descent. 2) Penalty-based FDA approach: In this approach, the penalty method is
leveraged in the FDA approach to guarantee the convergence to a stationary
point of the spectral maximization problem. 3) Heuristic two-stage (HTS)
approach: In this approach, the closed-form TTD-based analog beamformers are
first designed based on the outcomes of near-field beam training and the
piecewise-near-field approximation. Subsequently, the low-dimensional digital
beamformer is optimized using knowledge of the low-dimensional equivalent
channels, resulting in reduced computational complexity and channel estimation
complexity. Our numerical results unveil that 1) the proposed approaches
effectively eliminate the near-field beam split effect, and 2) compared to the
fully-connected architecture, the proposed sub-connected architecture exhibits
higher energy efficiency and imposes fewer hardware limitations on TTDs and
system bandwidth.Comment: 30 pages, 11 figure
TTD Configurations for Near-Field Beamforming: Parallel, Serial, or Hybrid?
True-time delayers (TTDs) are popular components for hybrid beamforming
architectures to combat the spatial-wideband effect in wideband near-field
communications. A serial and a hybrid serial-parallel TTD configuration are
investigated for hybrid beamforming architectures. Compared to the conventional
parallel configuration, the serial configuration exhibits a cumulative time
delay through multiple TTDs, which potentially alleviates the maximum delay
requirements on the TTDs. However, independent control of individual TTDs
becomes impossible in the serial configuration. In this context, a hybrid TTD
configuration is proposed as a compromise solution. Furthermore, a power
equalization approach is proposed to address the cumulative insertion loss of
the serial and hybrid TTD configurations. Moreover, the wideband near-field
beamforming design for different configurations is studied for maximizing the
spectral efficiency in both single-user and multiple-user systems. 1) For
single-user systems, a closed-form solution for the beamforming design is
derived. The preferred user locations and the required maximum time delay of
each TTD configuration are characterized. 2) For multi-user systems, a
penalty-based iterative algorithm is developed to obtain a stationary point of
the spectral efficiency maximization problem for each TTD configuration. In
addition, a mixed-forward-and-backward (MFB) implementation is proposed to
enhance the performance of the serial configuration. Our numerical results
confirm the effectiveness of the proposed designs and unveil that i) compared
to the conventional parallel configuration, both the serial and hybrid
configurations can significantly reduce the maximum time delays required for
the TTDs and ii) the hybrid configuration excels in single-user systems, while
the serial configuration is preferred in multi-user systems.Comment: 13 pages, 8 figure
Simultaneously Transmitting and Reflecting Surface (STARS) for Terahertz Communications
A simultaneously transmitting and reflecting surface (STARS) aided terahertz
(THz) communication system is proposed. A novel power consumption model is
proposed that depends on the type and resolution of the STARS elements. The
spectral efficiency (SE) and energy efficiency (EE) are maximized in both
narrowband and wideband THz systems by jointly optimizing the hybrid
beamforming at the base station (BS) and the passive beamforming at the STARS.
1) For narrowband systems, independent phase-shift STARSs are investigated
first. The resulting complex joint optimization problem is decoupled into a
series of subproblems using penalty dual decomposition. Low-complexity
element-wise algorithms are proposed to optimize the analog beamforming at the
BS and the passive beamforming at the STARS. The proposed algorithm is then
extended to the case of coupled phase-shift STARS. 2) For wideband systems, the
spatial wideband effect at the BS and STARS leads to significant performance
degradation due to the beam split issue. To address this, true time delayers
(TTDs) are introduced into the conventional hybrid beamforming structure for
facilitating wideband beamforming. An iterative algorithm based on the
quasi-Newton method is proposed to design the coefficients of the TTDs.
Finally, our numerical results confirm the superiority of the STARS over the
conventional reconfigurable intelligent surface (RIS). It is also revealed that
i) there is only a slight performance loss in terms of SE and EE caused by
coupled phase shifts of the STARS in both narrowband and wideband systems, and
ii) the conventional hybrid beamforming achieves comparable SE performance and
much higher EE performance compared with the full-digital beamforming in
narrowband systems but not in wideband systems, where the TTD-based hybrid
beamforming is required for mitigating wideband beam split.Comment: 17 pages, 12 figure
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