![](bg1.png)
ULTRASONIC OFDM PULSE FOR BEACON IDENTIFICATION
AND DISTANCE MEASUREMENT IN REVERBERANT
ENVIRONMENTS
Daniel F. Albuquerque, Jos
´
e M. N. Vieira, Carlos A. C. Bastos and Paulo J. S. G. Ferreira
Signal Processing Lab – IEETA/DETI, University of Aveiro, 3810-193 Aveiro, Portugal
Keywords:
Indoor location system, Asynchronous communication, OFDM, Ultrasound, Time-of-flight.
Abstract:
In this work we propose a frame architecture for asynchronous data transmission using ultrasonic OFDM
pulses in reverberant environments. The frame has two different OFDM pulses modulated with BPSK. The
first pulse plays an important role, it is used for time synchronization and to demodulated the unknown data in
the second pulse by a differential demodulation scheme. The proposed frame architecture proved to be robust
to the multipath in different scenarios. Results have demonstrated that it is possible to keep the bit error rate
low in the presence of strong signal echos where other techniques fails, moreover, the simulations show that it
would increase the reliability of ultrasonic indoor location systems.
1 INTRODUCTION
Location is an active area of research in the signal
processing community with a large potential from the
point of view of applications (Sayed et al., 2005; Liu
et al., 2007). The GPS is the most popular system for
outdoor location achieving an accuracy between 20 to
30 m (Sayed et al., 2005).
Recently, there has also been a great interest in us-
ing the existing mobile phone antenna infrastructure
to perform outdoor location without the need of any
additional hardware besides the mobile phones. How-
ever, such systems present in urban areas an accuracy
about 100 m (Lakmali and Dias, 2008; Gustafsson
and Gunnarsson, 2005). This accuracy is not enough
for indoor applications, where the system must pro-
vide the exact position of the object. To perform in-
door location, there are 2 main types of solutions: Ul-
trasonic (US) and Radio Frequency (RF) based sys-
tems. RF based systems are extremely inexpensive
but require the profiling of the entire location scenario
to get a RF fingerprint resulting in an accuracy from
1 to 5 meters approximately (Stuntebeck et al., 2008;
Bahl and Padmanabhan, 2000). On the other hand,
the ultrasound technology is the best suited to achieve
the necessary accuracy level in three dimensions, that
can be less than 1 cm in some cases (Gonzalez and
Bleakley, 2009; Prieto et al., 2007).
1.1 LocUS Location System
LocUS is an ultrasonic based location system in de-
velopment, with the main goal of perform indoor
location using only ultrasonic signals. These ultra-
sonic signals will be used to get distance information,
from time-of-flight (TOF) measurements, and also to
implement data communication. Unfortunately, al-
most all of the known ultrasonic location systems use
an auxiliary RF channel for measuring the propaga-
tion delay from the source to receiver (except the M.
Hazas and A. Hopper’s system (Hazas and Hopper,
2006) that presents an accuracy less than 25 cm in
95% of the cases). Although this auxiliary RF channel
allows very simple clock synchronization and delay
measurement solutions, it also gives away two impor-
tant advantages that US-based systems bear in refer-
ence to RF-based ones: the immunity to RF interfer-
ence, and the ability to safely operate in the presence
of critical electronic instrumentation such as medi-
cal or life-support systems. Therefore, one way to
avoid the use of an auxiliary RF signal to measure the
TOF is to synchronize the clocks of the nodes (Skeie
et al., 2001). To achieve this, the nodes should be
able to send to each other the clock information us-
ing the ultrasonic channel. Due to the reflection of
the ultrasonic signals on the walls the acoustic com-
munication channel presents a strong multipath effect
causing inter-symbolic interference, for that reason,
OFDM (Orthogonal Frequency Division Multiplex-
124
F. Albuquerque D., M. N. Vieira J., A. C. Bastos C. and J. S. G. Ferreira P..
ULTRASONIC OFDM PULSE FOR BEACON IDENTIFICATION AND DISTANCE MEASUREMENT IN REVERBERANT ENVIRONMENTS.
DOI: 10.5220/0003375601240132
In Proceedings of the 1st International Conference on Pervasive and Embedded Computing and Communication Systems (PECCS-2011), pages
124-132
ISBN: 978-989-8425-48-5
Copyright
c
2011 SCITEPRESS (Science and Technology Publications, Lda.)