Ranging Results Using a UWB Platformin an Indoor Environment
Alessio De Angelis, Satyam Dwivedi, Peter HandelACCESS Linnaeus Centre, Signal Processing Lab
KTH Royal Institute of Technology
Stockholm, Sweden
Email: { ales, dwivedi, ph }@kth.se
Antonio Moschitta, Paolo CarboneDIEI, Department of Electronic and Information Engineering
University of Perugia
Perugia, Italy
Email: { moschitta, carbone }@diei.unipg.it
Abstract— This paper presents an impulse-radio UWB experi-mental platform for ranging and positioning in GNSS-challengedenvironments. The platform is based on the two-way time-of-arrival principle of operation, which reduces architecturecomplexity and relaxes the synchronization requirements withrespect to time-of-arrival or time-difference-of-arrival solutions.The modular architecture of the platform is described togetherwith the design and features of its main components, namely the5.6-GHz RF front end and the baseband module for measurementand processing. A set of experimental results obtained using therealized platform in an indoor office environment is presentedand discussed. The platform provides a maximum range of about30 m in line-of-sight conditions with an RMSE of the order of40 cm.
I. INTRODUCTION
The wide deployment of global navigation systems (GNSSs)
has recently provided solutions for relevant issues in many
applications in the vehicular, commercial and defense sectors.
However, typically GNSSs do not provide sufficient coverage
in indoor environments and other challenging scenarios, such
as urban canyons or forests. Furthermore, several positioning
and navigation applications require a higher level of accuracy,
reliability and integrity than that provided by GNSSs [1].
In particular, in the indoor positioning scenario, the dy-
namic nature of the propagation channel, the dense multipath
conditions, and different application requirements make the
development and characterization of radio positioning systems
difficult [2].
These issues have given a considerable impulse to research
activities in the indoor positioning field during the last decade.
Many technologies and systems have been proposed; see [3]
for an extensive survey. The usage of commercial wireless
communication infrastructure has been analyzed and modeled
in the literature, e.g. in [4]. In this context, the advantages
provided by the fine time resolution properties of the impulse-
radio Ultra-Wideband (IR-UWB) technology have been ana-
lyzed from the theoretical and experimental viewpoints [5],
[6]. Due to these properties, UWB is particularly suited for
localization in wireless sensor network applications, [5], and
for the implementation of cooperative localization methods
[7]. Specifically, in [8], a methodology for the development
and analysis of cooperative localization algorithms, denoted
as network experimentation, is introduced. Such methodology,
which is based on jointly processing range measurements
and waveform acquisitions, is then used to compare several
existing methods on data obtained in an extensive indoor
measurement campaign with commercially-available UWB
radios [9]. Several systems have been proposed in the liter-
ature using IR-UWB. The characterization of a set of low-
complexity pulse generator circuits is provided in [10]. In
[11], a noncoherent IR-UWB integrated transceiver operating
in the 3.1 - 4.1 GHz band is presented. The experimental
results provided validate the proposed low-power and low-
complexity receiver architecture, which is based on the energy-
collection principle. In the same paper, the performance of
the maximum-selection algorithm for time-of-arrival (TOA)
estimation is investigated by numerical simulation in the UWB
channel model. The algorithm is shown to be robust in low
signal-to-noise-ratio conditions, achieving 5-ns TOA accuracy.
Moreover, several UWB experimental systems have been
used for proof-of-concept verification of theoretical methods,
and operate with external electronic instrumentation, such as
the systems described in [12], [13]. Furthermore, commercial
UWB positioning systems are currently available for appli-
cations such as commercial asset tracking [14], or personnel
positioning [9].
In this paper, we present and experimentally characterize
a research platform for measuring the distance (ranging)
between active radio transceivers using IR-UWB technology.
The platform has been designed and realized in-house, and it is
intended to be used as a flexible test bed for indoor positioning.
The system integrates the radio-frequency front end in [15] and
the baseband processing system in [16], with the aim to exploit
the complementary characteristics of the two subsystems and
overcome their limitations. Differently from the systems found
in the literature mentioned above, the present platform is self-
contained and does not rely on external instrumentation such
as oscilloscopes or vector network analyzers. Also, differently
from the commercial system [14], it does not rely on fine
synchronization between receiving devices. With respect to
[9], the proposed platform employs a low-complexity off-the-
shelf detection hardware which avoids the requirement of high
sampling rates for waveform acquisition.
The remainder of this paper is structured as follows: the
platform architecture is described in Section II, then the
978-1-4799-0486-0/13/$31.00 ©2013 IEEE
Master
Baseband module
RF front end
TX
RX
PC
Slave
TX
RX
RF front end
Baseband module
UWB pulse
Fig. 1. High-level block diagram of the realized platform. The master devicemeasures the round-trip-time of a UWB pulse propagating between itself anda slave device, functioning as a responder.
procedure and results of experiments to characterize the perfor-
mance are provided in Section III. Finally, a conclusion with
discussions of the results and future developments is presented
in Section IV.
II. PRINCIPLE OF OPERATION AND ARCHITECTURE
The proposed ranging system measures the round-trip-time
(RTT) of UWB pulses propagating between two transceiver
devices, which are denoted as master and slave. The master
is able to interrogate the slave by generating a UWB pulse.
Subsequently, the slave generates another pulse in response
after a fixed and predetermined latency time interval.
Such an approach, also referred to as two-way time-of-
arrival (TW-TOA) in the literature, allows for relaxing the
synchronization requirements with respect to time-of-arrival
or time-difference-of-arrival solutions. On the other hand, it
implies the need for calibrating for deviations of the latency
introduced by the slave device.
In the following subsections, the architecture of the overall
platform and of the two main individual subsystems is pre-
sented.
A. High-level architecture
The platform has been developed following a modular
design approach; a high-level block diagram is shown in Fig. 1.
Such an approach allows for comparing the impact of different
solutions on the performance and the fundamental design
choices for each building block. The hardware implementation
of the master and slave device is identical, and each device can
be configured as master or slave via software. As depicted in
the figure, the platform is composed of two main modules: the
radio-frequency (RF) front end and the baseband processing
module.
The 5.6-GHz RF front end allows for the usage of small
antennas and therefore a compact form factor for mobile
device applications. Also, it allows for an operational range
suitable for most indoor positioning applications with a single
pulse transmission (30 m).
On the other hand the baseband processing section, based on
a field programmable gate array (FPGA), provides flexibility
and reconfigurability. This enables the use of the platform in
the context of infrastructure-free and cooperative positioning
systems. Each device can be dynamically reconfigured as
baseband UWB pulse
BPF
IF
LO
RF
LO
RF IF
TX
RX
downconverted UWB pulse
VCO
PA
LNA
VCO
IF
Fig. 2. Block diagram of the RF front end, based on a homodyne architecture.The two voltage controlled oscillators (VCOs) in the upconverter (top) anddownconverter (bottom) sections are operating at a frequency of approximately5.6 GHz [15].
master or as slave and can be modified to adapt to changes
in the external conditions, e.g., by modifying the threshold to
reduce false detections due to interference.
Overall, the platform exploits the complementary features
and advantages of the two subsystems to provide a com-
plete research platform for ranging and positioning in GNSS-
challenged environments. In the following, individual descrip-
tions of the architecture of the two modules are provided.
B. RF front end
A block diagram illustrating the architecture of the RF front
end is shown in Fig. 2. The front end is based on a homodyne
architecture, where a baseband UWB pulse is upconverted by
using a mixer to modulate a carrier in amplitude [15]. The
baseband UWB pulse at the input of the upconverter section
is generated using commercial AHCMOS logic gates, which
provide approximately 1 ns width, 0.6 ns transition time and
2 V amplitude; see [10] for an extensive characterization of
the baseband pulse generation circuit.
In the front end, two ELITE-2460 broadband antennas
by Green-Wave Scientific are used for the receiving and
transmitting sections, featuring an omnidirectional radiation
pattern in the azimuth section, a wide operating band and a
compact size of less than 6 cm. The realized front end module
has a power consumption of approximately 3 W.
C. Baseband module
The baseband module, depicted in the block diagram of
Fig. 3, uses a threshold-based energy detector, which provides
a low-complexity alternative to the costly and impractical
Nyquist-rate sampling of the analog downconverted UWB
pulse. The threshold of the energy detector is controlled
adaptively by means of a digital-to-analog converter (DAC).
Also, a commercial time-to-digital converter (TDC) integrated
circuit, the TDC-GP2 by Acam, is used to provide the RTT
measurement with an intrinsic resolution of 50 ps. Finally,
FPGA (digital control and processing)
UWB measurement section
DAC
TDC UWB
Transceiver
SPI
SPI
START
CONTROL
STOP
THRESHOLD
THRESHOLD
START
UWB transceiver
Energy Detector
STOP
Pulse trigger
downconverter
upconverter
Fig. 3. Block diagram of the baseband module. The FPGA is interfaced withthe TDC and the DAC using a serial peripheral interface (SPI) protocol. TheUWB transceiver, which is triggered and controlled digitally by the FPGA,provides connections with the downconverter and upconverter sections of theRF front end [16].
the timing, control and digital processing functions are im-
plemented by means of a Virtex 5 FPGA by Xilinx, using
a development board connected to the in-house built UWB
measurement section [16].
In the experimental setup considered in this paper, the
digital baseband control section of the master device has been
configured to trigger a single pulse with a repetition period of
200 μs, thus obtaining a measurement update rate of 5 kHz.
Furthermore, the slave device has been configured to respond
to the master pulse after a fixed delay of 5 μs.
III. EXPERIMENTAL RESULTS
In this section, results from a measurement campaign are
presented and discussed by describing the ranging setup and
the calibration procedure, and by analyzing the experimental
data.
A. Ranging Setup
A picture of the experimental setup is shown in Fig. 4.
During the ranging experiments, the master and slave devices
where placed on carts. The master was kept in a constant
position whereas the slave was placed at a series of 42 different
reference distances, from 0.5 m to 30 m. The reference
distances were measured using a handheld laser distance-
measurement device, therefore providing the ground truth for
error characterization. The experiments were performed in
line-of-sight conditions in an indoor office environment. In
particular, the test environment was a 2-m wide corridor with
plaster walls and wooden doors on each side, and containing
intersections with open-space areas and other corridors.
At each reference distance, a set of 65536 RTT measure-
ments has been acquired. A histogram representation of the
acquired raw data at two different distances is shown in Fig. 5.
Fig. 4. Photo of the measurement setup: the master and slave devices areplaced on carts. A laptop is connected to the master device for data acquisition.
Fig. 5. Histogram of the acquired raw RTT measurements at 5 m, left peak,and 25 m, right peak.
B. Calibration
A calibration procedure has been performed with the goal of
compensating for the sources of non-ideal behavior and error
in the system. Such sources include the responder timing jitter,
the impact of the received signal strength on the threshold-
based energy detector, cf. [16], [17], and propagation effects
such as multipath.
First, a threshold-based outlier rejection procedure has been
executed to eliminate the outliers, which consist of false de-
tections due to interference and receiver noise. Subsequently,
the calibration curve has been computed by cubic fitting
and interpolation of the mode of the experimental RTT data
acquired at a subset of the reference distances, consisting of
12 equally spaced points, as shown in Fig. 6.
Finally, a calibration Look-Up Table (LUT), with 1000
entries, has been calculated by sampling the inverse of the
interpolated calibration curve.
Fig. 7. Box plot for all acquired distances, after the calibration procedure. For each box, the central mark is the median, the edges of the box represent the25th and 75th percentiles, and the top and bottom ends of the dashed lines denote the maximum and minimum values, respectively. The total RMSE is 0.34m and the maximum observed absolute error is 2.72 m.
Fig. 6. Calibration curve obtained by 3rd-order polynomial fitting of themode of the experimental data, at a subset of the reference distances, namelyfrom 2.5 m to 30 m at 2.5-m steps.
C. Validation
In the validation stage, range measurements were obtained
by applying the LUT to the RTT data acquired at all of the 42
reference distances. The validation RTT samples have been
processed by means of a 20-tap moving-average filter. The
role of this filter is to emulate the behavior of the ranging
system in an operational condition, where the average over
consecutive samples would be performed to compensate for
short-term fluctuations.
The results of the validation stage are shown in the box-plot
representation of all acquired data sets of Fig. 7, whereas the
outage probability plot is shown in Fig. 8. In particular, the
outage probability plot shows that the error is less than 0.5 m
in approximately 90% of the cases.
Furthermore, Fig. 7 shows abrupt error transitions at ap-
proximately 11 - 12.5 m and at approximately 17 m. These
Fig. 8. Empirical outage probability, defined as the ratio of the numberof times the error exceeds the value on the x-axis over the total number ofsamples in the experimental data set.
are caused by a change in multipath conditions affecting the
received signal strength at the receiver. The signal strength,
in turn, has an impact on the operation of the threshold-based
energy detector, as observed and characterized also in [16].
In the particular test environment considered in this paper,
there is a junction between the corridor and another hallway
in the region corresponding to 11 - 17 m, where the slave
receiver is farther with respect to walls and other structures
than in the rest of the corridor. In this region the behavior
deviates considerably from the calibration curve, resulting in
larger errors.
A possible strategy to overcome this issue could con-
sist in building multiple LUTs, in the calibration phase, in
different propagation conditions. The baseband processing
module could first detect the propagation conditions, based
on statistical analysis of the measurements, and then select
the corresponding LUT. The investigation of such a strategy
is the object of future work.
IV. CONCLUSION
A UWB ranging platform has been presented and character-
ized by experimental tests in an indoor office environment. The
results demonstrate the capability of sub-meter -level accuracy
in line-of-sight conditions up to a maximum operating range
of approximately 30 m.
The impulse radio UWB platform is working in different
configurations, i.e. directly on base-band (0.3 - 1 GHz) as
studied in [16]–[18], and with different high frequency RF
front-ends. Specifically, a 6 to 7 GHz RF front-end utilizing
diode-based pulse generation and connectorized components
was used in [19]. In the current work, the front-end consists
of separate TX and RX boards, characterized by pulse gener-
ation by a logic gate and using mixers for upconversion and
downconversion with a carrier frequency of 5.6 GHz [15].
Future work will be focused on analyzing the effect of
multipath conditions and environment factors. Other develop-
ments include hardware improvements, such as the use of a
single board and a single antenna for the transmitter and the
receiver sections, by means of an antenna switch. This will
enable a further reduction of the form factor of the system and
allow for a more accurate distance measurement. Finally, the
implementation of signal processing methods for extending the
functionality of the platform to dynamic tracking is envisioned.
ACKNOWLEDGMENT
Parts of this work have been funded by The Swedish Agency
for Innovation Systems (VINNOVA).
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