48 research outputs found
Cram\'er-Rao Bounds for Near-Field Sensing with Extremely Large-Scale MIMO
Mobile communication networks were designed to mainly support ubiquitous
wireless communications, yet they are also expected to achieve radio sensing
capabilities in the near future. However, most prior studies on radio sensing
usually rely on far-field assumption with uniform plane wave (UPW) models. With
the ever-increasing antenna size, together with the growing demands to sense
nearby targets, the conventional far-field UPW assumption may become invalid.
Therefore, this paper studies near-field radio sensing with extremely
large-scale (XL) antenna arrays, where the more general uniform spheric wave
(USW) sensing model is considered. Closed-form expressions of the Cram\'er-Rao
Bounds (CRBs) for both angle and range estimations are derived for near-field
XL-MIMO radar mode and XL-phased array radar mode, respectively. Our results
reveal that different from the conventional UPW model where the CRB for angle
decreases unboundedly as the number of antennas increases, for XL-MIMO
radar-based near-field sensing, the CRB decreases with diminishing return and
approaches to a certain limit as the number of antennas increases. Besides,
different from the far-field model where the CRB for range is infinity since it
has no range estimation capability, that for the near-field case is finite.
Furthermore, it is revealed that the commonly used spherical wave model based
on second-order Taylor approximation is insufficient for near-field CRB
analysis. Extensive simulation results are provided to validate our derived
CRBs
An Exact Near-Field Model Based Localization for Bistatic MIMO Radar with COLD arrays
Most existing near-field (NF) source localization algorithms are developed based on the Fresnel approximation model, and assume that the spatial amplitudes of the target at the sensors are equal. Unlike these algorithms, an NF source parameter estimation algorithm is proposed, based on the exact spatial propagation geometry model, for bistatic multiple-input multiple-output (MIMO) radar deployed with a linear concentered orthogonal loop and dipole (COLD) array at both the transmitter and receiver. The proposed method first compresses the output signal of the matched filter at the receiver into a third-order parallel factor (PARAFAC) data model, on which a trilinear decomposition is performed, and subsequently three factor matrices can be obtained. Then, multiple parameters of interest, including direction-of-departure (DOD), direction-of-arrival (DOA), range from transmitter to target (RFTT), range from target to receiver (RFTR), two-dimensional (2-D) transmit polarization angle (TPA) and 2-D receive polarization angle (RPA), are estimated from the spatial amplitude ratio exploiting the rotation invariant property and the Khatri-Rao product. Finally, the phase uncertainties of transmit and receive arrays can be extracted from additional phase items. The proposed algorithm avoids spectrum peak search, and the estimated parameters in closed forms can be automatically matched unambiguously. In addition, it is suitable for non-uniform linear arrays (NLA) with arbitrary array element spacing and phase uncertainty. Advantages of the proposed method are demonstrated by simulation results
Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays
Massive MIMO (multiple-input multiple-output) is no longer a "wild" or
"promising" concept for future cellular networks - in 2018 it became a reality.
Base stations (BSs) with 64 fully digital transceiver chains were commercially
deployed in several countries, the key ingredients of Massive MIMO have made it
into the 5G standard, the signal processing methods required to achieve
unprecedented spectral efficiency have been developed, and the limitation due
to pilot contamination has been resolved. Even the development of fully digital
Massive MIMO arrays for mmWave frequencies - once viewed prohibitively
complicated and costly - is well underway. In a few years, Massive MIMO with
fully digital transceivers will be a mainstream feature at both sub-6 GHz and
mmWave frequencies. In this paper, we explain how the first chapter of the
Massive MIMO research saga has come to an end, while the story has just begun.
The coming wide-scale deployment of BSs with massive antenna arrays opens the
door to a brand new world where spatial processing capabilities are
omnipresent. In addition to mobile broadband services, the antennas can be used
for other communication applications, such as low-power machine-type or
ultra-reliable communications, as well as non-communication applications such
as radar, sensing and positioning. We outline five new Massive MIMO related
research directions: Extremely large aperture arrays, Holographic Massive MIMO,
Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive
MIMO.Comment: 20 pages, 9 figures, submitted to Digital Signal Processin
Emerging Approaches for THz Array Imaging: A Tutorial Review and Software Tool
Accelerated by the increasing attention drawn by 5G, 6G, and Internet of
Things applications, communication and sensing technologies have rapidly
evolved from millimeter-wave (mmWave) to terahertz (THz) in recent years.
Enabled by significant advancements in electromagnetic (EM) hardware, mmWave
and THz frequency regimes spanning 30 GHz to 300 GHz and 300 GHz to 3000 GHz,
respectively, can be employed for a host of applications. The main feature of
THz systems is high-bandwidth transmission, enabling ultra-high-resolution
imaging and high-throughput communications; however, challenges in both the
hardware and algorithmic arenas remain for the ubiquitous adoption of THz
technology. Spectra comprising mmWave and THz frequencies are well-suited for
synthetic aperture radar (SAR) imaging at sub-millimeter resolutions for a wide
spectrum of tasks like material characterization and nondestructive testing
(NDT). This article provides a tutorial review of systems and algorithms for
THz SAR in the near-field with an emphasis on emerging algorithms that combine
signal processing and machine learning techniques. As part of this study, an
overview of classical and data-driven THz SAR algorithms is provided, focusing
on object detection for security applications and SAR image super-resolution.
We also discuss relevant issues, challenges, and future research directions for
emerging algorithms and THz SAR, including standardization of system and
algorithm benchmarking, adoption of state-of-the-art deep learning techniques,
signal processing-optimized machine learning, and hybrid data-driven signal
processing algorithms...Comment: Submitted to Proceedings of IEE
Multi-static Parameter Estimation in the Near/Far Field Beam Space for Integrated Sensing and Communication Applications
This work proposes a maximum likelihood (ML)-based parameter estimation
framework for a millimeter wave (mmWave) integrated sensing and communication
(ISAC) system in a multi-static configuration using energy-efficient hybrid
digital-analog arrays. Due to the typically large arrays deployed in the higher
frequency bands to mitigate isotropic path loss, such arrays may operate in the
near-field regime. The proposed parameter estimation in this work consists of a
two-stage estimation process, where the first stage is based on far-field
assumptions, and is used to obtain a first estimate of the target parameters.
In cases where the target is determined to be in the near-field of the arrays,
a second estimation based on near-field assumptions is carried out to obtain
more accurate estimates. In particular, we select beamfocusing array weights
designed to achieve a constant gain over an extended spatial region and
re-estimate the target parameters at the receivers. We evaluate the
effectiveness of the proposed framework in numerous scenarios through numerical
simulations and demonstrate the impact of the custom-designed flat-gain
beamfocusing codewords in increasing the communication performance of the
system.Comment: 16 page
Reconfigurable and Static EM Skins on Vehicles for Localization
Electromagnetic skins (EMSs) have been recently considered as a booster for
wireless sensing, but their usage on mobile targets is relatively novel and
could be of interest when the target reflectivity can/must be increased to
improve its detection or the estimation of parameters. In particular, when
illuminated by a wide-bandwidth signal (e.g., from a radar operating at
millimeter waves), vehicles behave like \textit{extended targets}, since
multiple parts of the vehicle's body effectively contribute to the
back-scattering. Moreover, in some cases perspective deformations challenge the
correct localization of the vehicle. To address these issues, we propose
lodging EMSs on vehicles' roof to act as high-reflectivity planar
retro-reflectors toward the sensing terminal. The advantage is twofold:
\textit{(i)} by introducing a compact high-reflectivity structure on the
target, we make vehicles behave like \textit{point targets}, avoiding
perspective deformations and related ranging biases and \textit{(ii)} we
increase the reflectivity the vehicle, improving localization performance. We
detail the EMS design from the system-level to the full-wave-level considering
both reconfigurable intelligent surfaces (RIS) and cost-effective static
passive electromagnetic skins (SP-EMSs). Localization performance of the
EMS-aided sensing system is also assessed by Cram\'er-Rao bound analysis in
both narrowband and spatially wideband operating conditions
Near-Field Integrated Sensing and Communication: Performance Analysis and Beamforming Design
This paper explores the potential of near-field beamforming (NFBF) in
integrated sensing and communication (ISAC) systems with extremely large-scale
arrays (XL-arrays). The large-scale antenna arrays increase the possibility of
having communication users and targets of interest in the near field of the
base station (BS). The paper first establishes the models of electromagnetic
(EM) near-field spherical waves and far-field plane waves. With the models, we
analyze the near-field beam focusing ability and the far-field beam steering
ability by finding the gain-loss mathematical expression caused by the
far-field steering vector mismatch in the near-field case. We formulate the
NFBF design problem as minimizing the weighted summation of radar and the
communication beamforming errors under a total power constraint and solve this
quadratically constrained quadratic programming (QCQP) problem using the least
squares (LS) method. Moreover, the Cram\'er-Rao bound (CRB) for target
parameter estimation is derived to verify the performance of NFBF. Furthermore,
we also perform power minimization using convex optimization while ensuring the
required communication and sensing quality-of-service (QoS). The simulation
results show the influence of model mismatch on near-field ISAC and the
performance gain of transmit beamforming from the additional distance dimension
of near-field.Comment: under revie
Novel Hybrid-Learning Algorithms for Improved Millimeter-Wave Imaging Systems
Increasing attention is being paid to millimeter-wave (mmWave), 30 GHz to 300
GHz, and terahertz (THz), 300 GHz to 10 THz, sensing applications including
security sensing, industrial packaging, medical imaging, and non-destructive
testing. Traditional methods for perception and imaging are challenged by novel
data-driven algorithms that offer improved resolution, localization, and
detection rates. Over the past decade, deep learning technology has garnered
substantial popularity, particularly in perception and computer vision
applications. Whereas conventional signal processing techniques are more easily
generalized to various applications, hybrid approaches where signal processing
and learning-based algorithms are interleaved pose a promising compromise
between performance and generalizability. Furthermore, such hybrid algorithms
improve model training by leveraging the known characteristics of radio
frequency (RF) waveforms, thus yielding more efficiently trained deep learning
algorithms and offering higher performance than conventional methods. This
dissertation introduces novel hybrid-learning algorithms for improved mmWave
imaging systems applicable to a host of problems in perception and sensing.
Various problem spaces are explored, including static and dynamic gesture
classification; precise hand localization for human computer interaction;
high-resolution near-field mmWave imaging using forward synthetic aperture
radar (SAR); SAR under irregular scanning geometries; mmWave image
super-resolution using deep neural network (DNN) and Vision Transformer (ViT)
architectures; and data-level multiband radar fusion using a novel
hybrid-learning architecture. Furthermore, we introduce several novel
approaches for deep learning model training and dataset synthesis.Comment: PhD Dissertation Submitted to UTD ECE Departmen
Sonar systems for object recognition
The deep sea exploration and exploitation is one of the biggest challenges
of the next century. Military, oil & gas, o shore wind farming,
underwater mining, oceanography are some of the actors interested
in this eld. The engineering and technical challenges to perform
any tasks underwater are great but the most crucial element in any
underwater systems has to be the sensors. In air numerous sensor
systems have been developed: optic cameras, laser scanner or radar
systems. Unfortunately electro magnetic waves propagate poorly in
water, therefore acoustic sensors are a much preferred tool then optical
ones. This thesis is dedicated to the study of the present and
the future of acoustic sensors for detection, identi cation or survey.
We will explore several sonar con gurations and designs and their
corresponding models for target scattering. We will show that object
echoes can contain essential information concerning its structure
and/or composition