272 research outputs found
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
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
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
Active-Sensing-Based Beam Alignment for Near Field MIMO Communications
An active-sensing-based learning algorithm is proposed to solve the
near-field beam alignment problem with the aid of wavenumber-domain transform
matrices (WTMs). Specifically, WTMs can transform the antenna-domain channel
into a sparse representation in the wavenumber domain. The dimensions of WTMs
can be further reduced by exploiting the dominance of line-of-sight (LoS)
links. By employing these lower-dimensional WTMs as mapping functions, the
active-sensing-based algorithm is executed in the wavenumber domain, resulting
in an acceleration of convergence. Compared with the codebook-based beam
alignment methods, the proposed method finds the optimal beam pair in a
ping-pong fashion, thus avoiding high training overheads caused by beam
sweeping. Finally, the numerical results validate the effectiveness of the
proposed method
Electrochemical Insertion/extraction of Lithium in Multiwall Carbon Nanotube/Sb and SnSbâ‚€.â‚… Nanocomposites
Multiwall carbon nanotubes (CNTs) were synthesized by catalytic chemical vapor deposition of acetylene and used as templates to prepare CNT-Sb and CNT-SnSb₀.₅ nanocomposites via the chemical reduction of SnCl₂ and SbCl₃ precursors. SEM and TEM imagings show that the Sb and SnSb₀.₅ particles were uniformly dispersed in the CNT web and on the outside surface of CNTs. These CNT-metal composites are active anode materials for lithium ion batteries, showing improved cyclability compared to unsupported Sb and SnSb particles; and higher reversible specific capacities than CNTs. The improvement in cyclability may be attributed to the nanoscale dimensions of the metal particles and CNT’s role as a buffer in containing the mechanical stress arising from the volume changes in electrochemical lithium insertion and extraction reactions.Singapore-MIT Alliance (SMA
Coupled Phase-Shift STAR-RISs: A General Optimization Framework
A general optimization framework is proposed for simultaneously transmitting
and reflecting reconfigurable surfaces (STAR-RISs) with coupled phase shifts,
which converges to the Karush-Kuhn-Tucker (KKT) optimal solution under some
mild conditions. More particularly, the amplitude and phase-shift coefficients
of STAR-RISs are optimized alternatively in closed form. To demonstrate the
effectiveness of the proposed optimization framework, the throughput
maximization problem is considered in a case study. It is rigorously proved
that the KKT optimal solution is obtained. Numerical results confirm the
effectiveness of the proposed optimization framework compared to baseline
schemes.Comment: 15 pages, 3 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
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
Near-Field Communications: What Will Be Different?
The design dilemma of "What will be different between near-field
communications (NFC) and far-field communications (FFC)?" is addressed from
four perspectives. 1) From the channel modelling perspective, the differences
between near-field and far-field channel models are discussed. A novel Green's
function-based channel model is proposed for continuous-aperture antennas,
which is contrasted to conventional channel models tailored for
spatially-discrete antennas. 2) From the performance analysis perspective,
analytical results for characterizing the degrees of freedom and the power
scaling laws in the near-field region are provided for both spatially-discrete
and continuous-aperture antennas. 3) From the beamforming perspective,
far-field beamforming is analogous to a "flashlight" that enables beamsteering,
while near-field beamforming can be likened to a "spotlight" that facilitates
beamfocusing. As a further advance, a couple of new beamforming structures are
proposed for exploiting the new characteristics of NFC. 4) From the application
perspective, new designs are discussed in the context of promising
next-generation technologies in NFC, where our preliminary numerical results
demonstrate that distance-aware target sensing and enhanced physical layer
security can be realized in NFC. Finally, several future research directions of
NFC are discussed.Comment: 9 pages, 5 figures, submit to possible IEEE journa
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