56 research outputs found
Robust Near-Field 3D Localization of an Unaligned Single-Coil Agent Using Unobtrusive Anchors
The magnetic near-field provides a suitable means for indoor localization,
due to its insensitivity to the environment and strong spatial gradients. We
consider indoor localization setups consisting of flat coils, allowing for
convenient integration of the agent coil into a mobile device (e.g., a smart
phone or wristband) and flush mounting of the anchor coils to walls. In order
to study such setups systematically, we first express the Cram\'er-Rao lower
bound (CRLB) on the position error for unknown orientation and evaluate its
distribution within a square room of variable size, using 15 x 10cm anchor
coils and a commercial NFC antenna at the agent. Thereby, we find cm-accuracy
being achievable in a room of 10 x 10 x 3 meters with 12 flat wall-mounted
anchors and with 10mW used for the generation of magnetic fields. Practically
achieving such estimation performance is, however, difficult because of the
non-convex 5D likelihood function. To that end, we propose a fast and accurate
weighted least squares (WLS) algorithm which is insensitive to initialization.
This is enabled by effectively eliminating the orientation nuisance parameter
in a rigorous fashion and scaling the individual anchor observations, leading
to a smoothed 3D cost function. Using WLS estimates to initialize a
maximum-likelihood (ML) solver yields accuracy near the theoretical limit in up
to 98% of cases, thus enabling robust indoor localization with unobtrusive
infrastructure, with a computational efficiency suitable for real-time
processing.Comment: 7 pages, to be presented at IEEE PIMRC 201
Magneto-inductive Passive Relaying in Arbitrarily Arranged Networks
We consider a wireless sensor network that uses inductive near-field coupling
for wireless powering or communication, or for both. The severely limited range
of an inductively coupled source-destination pair can be improved using
resonant relay devices, which are purely passive in nature. Utilization of such
magneto-inductive relays has only been studied for regular network topologies,
allowing simplified assumptions on the mutual antenna couplings. In this work
we present an analysis of magneto-inductive passive relaying in arbitrarily
arranged networks. We find that the resulting channel has characteristics
similar to multipath fading: the channel power gain is governed by a
non-coherent sum of phasors, resulting in increased frequency selectivity. We
propose and study two strategies to increase the channel power gain of random
relay networks: i) deactivation of individual relays by open-circuit switching
and ii) frequency tuning. The presented results show that both methods improve
the utilization of available passive relays, leading to reliable and
significant performance gains.Comment: 6 pages, 9 figures. To be presented at the IEEE International
Conference on Communications (ICC), Paris, France, May 201
Practical Accuracy Limits of Radiation-Aware Magneto-Inductive 3D Localization
The key motivation for the low-frequency magnetic localization approach is
that magnetic near-fields are well predictable by a free-space model, which
should enable accurate localization. Yet, limited accuracy has been reported
for practical systems and it is unclear whether the inaccuracies are caused by
field distortion due to nearby conductors, unconsidered radiative propagation,
or measurement noise. Hence, we investigate the practical performance limits by
means of a calibrated magnetoinductive system which localizes an active
single-coil agent with arbitrary orientation, using 4 mW transmit power at 500
kHz. The system uses eight single-coil anchors around a 3m x 3m area in an
office room. We base the location estimation on a complex baseband model which
comprises both reactive and radiative propagation. The link coefficients, which
serve as input data for location estimation, are measured with a multiport
network analyzer while the agent is moved with a positioner device. This
establishes a reliable ground truth for calibration and evaluation. The system
achieves a median position error of 3.2 cm and a 90th percentile of 8.3 cm.
After investigating the model error we conjecture that field distortion due to
conducting building structures is the main cause of the performance bottleneck.
The results are complemented with predictions on the achievable accuracy in
more suitable circumstances using the Cram\'er-Rao lower bound.Comment: To appear at the IEEE ICC 2019 Workshops. This work has been
submitted to the IEEE for possible publication. Copyright may be transferred
without notice, after which this version may no longer be accessibl
Pairwise Distance and Position Estimators From Differences in UWB Channels to Observers
We consider the problem of obtaining relative location information between
two wireless nodes from the differences in their ultra-wideband (UWB) channels
to observer nodes. Our approach focuses on the delays of multipath components
(MPCs) extracted from the observed channels. For the two different cases of
known and unknown MPC association between these channels, we present estimators
for the distance and for the relative position vector between the two nodes.
The position estimators require both MPC directions and MPC delays as input.
All presented estimators exhibit very desirable technological properties: they
do not require line-of-sight conditions, precise synchronization, or knowledge
about the observer locations or about the environment. These advantages could
enable low-cost wireless network localization in dynamic multipath
environments. The exposition is complemented by a numerical evaluation of the
estimation accuracy using random sampling, where especially the position
estimators show the potential for great accuracy.Comment: To appear at the IEEE Global Communications Conference (GLOBECOM)
Workshops 2021, Madrid, Spain. This work has been submitted to the IEEE for
publication. Copyright may be transferred without notice. This is the short
conference version of the full paper arXiv:2108.09703. v2 fixes a
copy-and-paste error in (21). arXiv admin note: substantial text overlap with
arXiv:2108.0970
Load Modulation for Backscatter Communication: Channel Capacity and Near-Capacity Schemes
In backscatter communication (BC), a passive tag transmits information by
just affecting an external electromagnetic field through load modulation.
Thereby, the feed current of the excited tag antenna is modulated by adapting
the passive termination load. This paper studies the achievable information
rates with a freely adaptable passive load. As a prerequisite, we unify
monostatic, bistatic, and ambient BC with circuit-based system modeling. We
present the crucial insight that channel capacity is described by existing
results on peak-power-limited quadrature Gaussian channels, because the
steady-state tag current phasor lies on a disk. Consequently, we derive the
channel capacity for the case of an unmodulated external field, for general
passive, purely reactive, or purely resistive tag loads. We find that
modulating both resistance and reactance is important for very high rates. We
discuss the capacity-achieving load statistics, rate asymptotics, technical
conclusions, and rate losses from value-range-constrained loads (which are
found to be small for moderate constraints).
We then demonstrate that near-capacity rates can be attained by more
practical schemes: (i) amplitude-and-phase-shift keying on the reflection
coefficient and (ii) simple load circuits of a few switched resistors and
capacitors.
Finally, we draw conclusions for the ambient BC channel capacity in important
special cases.Comment: This work has been submitted to the IEEE for possible publication.
Copyright may be transferred without notice. Included conference paper:
arXiv:2201.0024
Magneto-Inductive Powering and Uplink of In-Body Microsensors: Feasibility and High-Density Effects
This paper studies magnetic induction for wireless powering and the data
uplink of microsensors, in particular for future medical in-body applications.
We consider an external massive coil array as power source (1 W) and data sink.
For sensor devices at 12 cm distance from the array, e.g. beneath the human
skin, we compute a minimum coil size of 150 um assuming 50 nW required chip
activation power and operation at 750 MHz. A 275 um coil at the sensor allows
for 1 Mbit/s uplink rate. Moreover, we study resonant sensor nodes in dense
swarms, a key aspect of envisioned biomedical applications. In particular, we
investigate the occurring passive relaying effect and cooperative transmit
beamforming in the uplink. We show that the frequency- and location-dependent
signal fluctuations in such swarms allow for significant performance gains when
utilized with adaptive matching, spectrally-aware signaling and node
cooperation. The work is based on a general magneto-inductive MIMO system
model, which is introduced first.Comment: 6 pages, to appear at IEEE WCNC 2019. This work has been submitted to
the IEEE for possible publication. Copyright may be transferred without
notice, after which this version may no longer be accessibl
Inter-Node Distance Estimation from Multipath Delay Differences of Channels to Observer Nodes
We study the estimation of distance d between two wireless nodes by means of
their wideband channels to a third node, called observer. The motivating
principle is that the channel impulse responses are similar for small d and
drift apart when d increases. Following this idea we propose specific distance
estimators based on the differences of path delays of the extractable multipath
components. In particular, we derive such estimators for rich multipath
environments and various important cases: with and without clock
synchronization as well as errors on the extracted path delays (e.g. due to
limited bandwidth). The estimators readily support (and benefit from) the
presence of multiple observers. We present an error analysis and, using ray
tracing in an exemplary indoor environment, show that the estimators perform
well in realistic conditions. We describe possible localization applications of
the proposed scheme and highlight its major advantages: it requires neither
precise synchronization nor line-of-sight connection. This could make wireless
user tracking feasible in dynamic indoor settings.Comment: To appear at IEEE ICC 2019. This work has been submitted to the IEEE
for possible publication. Copyright may be transferred without notice, after
which this version may no longer be accessibl
Cooperative Magneto-Inductive Localization
Wireless localization is a key requirement for many applications. It concerns
position estimation of mobile nodes (agents) relative to fixed nodes (anchors)
from wireless channel measurements. Cooperative localization is an advanced
concept that considers the joint estimation of multiple agent positions based
on channel measurements of all agent-anchor links together with all agent-agent
links. In this paper we present the first study of cooperative localization for
magneto-inductive wireless sensor networks, which are of technological interest
due to good material penetration and channel predictability. We demonstrate
significant accuracy improvements (a factor of 3 for 10 cooperating agents)
over the non-cooperative scheme. The evaluation uses the Cram\'er-Rao lower
bound on the cooperative position estimation error, which is derived herein. To
realize this accuracy, the maximum-likelihood estimate (MLE) must be computed
by solving a high-dimensional least-squares problem, whereby convergence to
local minima proves to be problematic. A proposed cooperative localization
algorithm addresses this issue: first, preliminary estimates of the agent
positions and orientations are computed, which then serve as initial values for
a gradient search. In all our test cases, this method yields the MLE and the
associated high accuracy (comprising the cooperation gain) from a single solver
run. The preliminary estimates use novel closed-form MLE formulas of the
distance, direction and orientation for single links between three-axis coils,
which are given in detail.Comment: To appear at the IEEE PIMRC 2021 conference. This work has been
submitted to the IEEE for publication. Copyright may be transferred without
notic
The Cellular Relay Carpet: Distributed Cooperation with Ubiquitous Relaying
We consider the up- as well as downlink of a cellular network in which base stations (BSs) are supported by a large amount of relays spread over the entire area like a carpet. The BSs only see the static relays as the nodes they communicate with, which enables large antenna arrays at the BSs with sophisticated multi-user MIMO transmission. Together with a simple form of BS cooperation, the communication via the small relay cells allows to improve the data rates by distributed interference management and to reduce the complexity at the terminals. We investigate different types of relays as well as different relaying strategies for this relay carpet and compare them with respect to complexity, required channel state information (CSI), and performance in the interference-limited environment of dense cellular networks. The robustness of the different schemes with respect to channel estimation errors is studied and we conclude that especially relays of very low complexity are not sensitive to CSI imperfections. Relays can thus be applied in large numbers and enable massive MIMO at the BSs. The relay carpet proves thereby to be an efficient approach to enhance future generations of cellular networks significantly
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