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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