270 research outputs found
Detection and accurate localization of harmonic chipless tags
We investigate the detection and localization properties of harmonic tags working at microwave frequencies. A two-tone interrogation signal and a dedicated signal processing scheme at the receiver are proposed to eliminate phase ambiguities caused by the short signal wavelength and to provide accurate distance/position estimation even in the presence of clutter and multipath. The theoretical limits on tag detection and localization accuracy are investigated starting from a concise characterization of harmonic backscattered signals. Numerical results show that accuracies in the order of centimeters are feasible within an operational range of a few meters in the RFID UHF band
Communicating with Large Intelligent Surfaces: Fundamental Limits and Models
This paper analyzes the optimal communication involving large intelligent
surfaces (LIS) starting from electromagnetic arguments. Since the numerical
solution of the corresponding eigenfunctions problem is in general
computationally prohibitive, simple but accurate analytical expressions for the
link gain and available spatial degrees-of-freedom (DoF) are derived. It is
shown that the achievable DoF and gain offered by the wireless link are
determined only by geometric factors, and that the classical Friis' formula is
no longer valid in this scenario where the transmitter and receiver could
operate in the near-field regime. Furthermore, results indicate that,
contrarily to classical MIMO systems, when using LIS-based antennas DoF larger
than 1 can be exploited even in strong line-of-sight (LOS) channel conditions,
which corresponds to a significant increase in spatial capacity density,
especially when working at millimeter waves.Comment: Presented in part at thge IEEE International Conference on
Communications (ICC), 2020. In publication on IEEE Journal on Selected Areas
in Communications, Special issue on Wireless Networks Empowered by
Reconfigurable Intelligent Surfaces, 2020 (IEEE JSAC, Nov 2020
Statistics of the MLE and Approximate Upper and Lower Bounds - Part 2: Threshold Computation and Optimal Signal Design
Threshold and ambiguity phenomena are studied in Part 1 of this work where
approximations for the mean-squared-error (MSE) of the maximum likelihood
estimator are proposed using the method of interval estimation (MIE), and where
approximate upper and lower bounds are derived. In this part we consider
time-of-arrival estimation and we employ the MIE to derive closed-form
expressions of the begin-ambiguity, end-ambiguity and asymptotic
signal-to-noise ratio (SNR) thresholds with respect to some features of the
transmitted signal. Both baseband and passband pulses are considered. We prove
that the begin-ambiguity threshold depends only on the shape of the envelope of
the ACR, whereas the end-ambiguity and asymptotic thresholds only on the shape
of the ACR. We exploit the results on the begin-ambiguity and asymptotic
thresholds to optimize, with respect to the available SNR, the pulse that
achieves the minimum attainable MSE. The results of this paper are valid for
various estimation problems
Time Domain Measurements of Signals Backscattered by Wideband RFID Tags
Passive wideband RFID is increasing interest for its capability of providing high-accuracy tag localization in addition to identification and tag-reader communication. The measurement of backscattering capabilities of wideband antennas is usually conducted in the frequency domain by using network analyzers, which does not allow for the extraction of the antenna mode component of the backscattered signal when the antenna load is time variant. To overcome this issue, in this paper we present a novel setup for time domain measurements of signals backscattered by wideband RFID tags. Experimental evaluations are presented for comparing different wideband antennas and show the effects of the setup characteristics and of the processing schemes on the achievable measurement results
Direct position estimation from wavefront curvature with single antenna array
In this paper we investigate the possibility to perform direct positioning by retrieving information from the wavefront curvature. Despite such an approach has been considered in the past at microwave and acoustic frequencies using extremely large antennas, it is of interest to investigate its potential exploitation at mm-wave with practical size antennas in the context of next 5G systems. Thus, here we first consider a dedicated model to gather the source position information from the wavefront curvature for different array architectures, i.e., traditional and lens-based arrays, and successively we derive the maximum likelihood estimator to investigate the attainable performance. Results, obtained for different number of antennas, i.e., for different array apertures, confirm the possibility to achieve interesting positioning performance using a single antenna array with limited dimensions
Holographic Communication using Intelligent Surfaces
Holographic communication is intended as an holistic way to manipulate with
unprecedented flexibility the electromagnetic field generated or sensed by an
antenna. This is of particular interest when using large antennas at high
frequency (e.g., the millimeter wave or terahertz), whose operating condition
may easily fall in the Fresnel propagation region (radiating near-field), where
the classical plane wave propagation assumption is no longer valid. This paper
analyzes the optimal communication involving large intelligent surfaces,
realized for example with metamaterials as possible enabling technology for
holographic communication. It is shown that traditional propagation models must
be revised and that, when exploiting spherical wave propagation in the Fresnel
region with large surfaces, new opportunities are opened, for example, in terms
of the number of orthogonal communication channels.Comment: Submitted to IEEE Comm. Magazin
Statistics of the MLE and Approximate Upper and Lower Bounds - Part 1: Application to TOA Estimation
In nonlinear deterministic parameter estimation, the maximum likelihood
estimator (MLE) is unable to attain the Cramer-Rao lower bound at low and
medium signal-to-noise ratios (SNR) due the threshold and ambiguity phenomena.
In order to evaluate the achieved mean-squared-error (MSE) at those SNR levels,
we propose new MSE approximations (MSEA) and an approximate upper bound by
using the method of interval estimation (MIE). The mean and the distribution of
the MLE are approximated as well. The MIE consists in splitting the a priori
domain of the unknown parameter into intervals and computing the statistics of
the estimator in each interval. Also, we derive an approximate lower bound
(ALB) based on the Taylor series expansion of noise and an ALB family by
employing the binary detection principle. The accurateness of the proposed
MSEAs and the tightness of the derived approximate bounds are validated by
considering the example of time-of-arrival estimation
Single-Anchor Localization and Orientation Performance Limits Using Massive Arrays: MIMO vs. Beamforming
open3noIn the next generation of cellular networks, it is
desirable to use single access points both for communication and
localization. This could be made possible thanks to the combination
of femtocells, mm-wave technology and massive antenna
arrays, and would overcome the problem of having an over-sized
infrastructure for positioning which is, nowadays, the bottleneck
for the widespread diffusion of indoor localization systems. In
this context, our paper aims at investigating the localization
and orientation performance limits employing massive arrays
both at the access point and mobile side. To this end, we first
asymptotically demonstrate the tightness of the Cramér-Rao
bound (CRB) in the massive array regime and that the effect
of multipath can be made negligible even for practical values
of SNR levels. Successively, we propose a comparison between
two different transmitter configurations, namely multiple-input
multiple-output (MIMO), where orthogonal waveforms are sent,
and beamforming, which takes advantage of highly correlated
waveforms and directive array patterns. We also consider random
weighting as a trade-off between the diversity gain of MIMO and
the high directivity guaranteed by the beamforming. CRB results
show the interplay between diversity and beamforming gain as
well as the benefits achievable by varying the number of antennas
in terms of localization accuracy and multipath mitigation.embargoed_20181201Guerra, Anna; Guidi, Francesco; Dardari, DavideGuerra, Anna; Guidi, Francesco; Dardari, David
NLOS Localization Exploiting Frequency-selective Metasurfaces
This paper introduces a new approach to localize user devices located in
non-line-of-sight (NLOS) areas using a passive, non-reconfigurable, and
frequency-selective metasurface called metaprism. By analyzing the spatial
filtering of subcarriers in the orthogonal frequency division multiplexing
(OFDM) signal transmitted by each user device, the base station can estimate
the device's angle of view, distance, and subsequently its position. Two
different criteria are proposed for designing the frequency response of the
metaprism, depending on whether the users operate in the far-field or
near-field region of the metaprism. Simulation results in the millimeter-wave
band demonstrate that the system can achieve an accuracy of less than 2 degrees
in angle estimation and in the order of decimeters in position estimation
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