219 research outputs found
Annealing-induced reduction in nanoscale heterogeneity of thermally evaporated amorphous As2S3 films
The morphology and structural order of thermally deposited and annealed amorphous As2S3 films
have been investigated using high resolution transmission electron microscopy. It was found that
both the as-deposited and annealed films contained sparsely distributed nanocrystallites of the
orpiment As2S3 crystalline phase. However, from selected area electron diffraction both films
appeared amorphous. Fluctuation electron microscopy revealed that the as-deposited film contained
greater nanoscale inhomogeneity. Low temperature annealing reduced the nanoscale inhomogeneity
and resulted in a more homogeneous and energetically favorable network. The reduction in
nanoscale inhomogeneity upon low temperature annealing was accompanied by the appearance of
a first sharp diffraction peak in the diffraction pattern. This first-sharp diffraction peak has been
attributed to chemical ordering of interstitial voids. Our measurements suggest that this chemical
short-range ordering is associated with the dissolution of the energetically unfavorable larger
correlated structures that contribute to the inhomogeneity of the as-deposited film
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
Non-Orthogonal Multiple Access For Near-Field Communications
The novel concept of near-field non-orthogonal multiple access (NF-NOMA)
communications is proposed. The near-filed beamfocusing enables NOMA to be
carried out in both angular and distance domains. Two novel frameworks are
proposed, namely, single-location-beamfocusing NF-NOMA (SLB-NF-NOMA) and
multiple-location-beamfocusing NF-NOMA (MLB-NF-NOMA). 1) For SLB-NF-NOMA, two
NOMA users in the same angular direction with distinct quality of service (QoS)
requirements can be grouped into one cluster. The hybrid beamformer design and
power allocation problem is formulated to maximize the sum rate of the users
with higher QoS (H-QoS) requirements. To solve this problem, the analog
beamformer is first designed to focus the energy on the H-QoS users and the
zero-forcing (ZF) digital beamformer is employed. Then, the optimal power
allocation is obtained. 2) For MLB-NF-NOMA, the two NOMA users in the same
cluster can have different angular directions. The analog beamformer is first
designed to focus the energy on both two NOMA users. Then, a singular value
decomposition (SVD) based ZF (SVD-ZF) digital beamformer is designed.
Furthermore, a novel antenna allocation algorithm is proposed. Finally, a
suboptimal power allocation algorithm is proposed. Numerical results
demonstrate that the NF-NOMA can achieve a higher spectral efficiency and
provide a higher flexibility than conventional far-field NOMA
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
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