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.)
ing) could be a viable solution for location and com-
munication. This technique has already been used in
ultrasonic underwater communications (Mason et al.,
2007; Nakashima et al., 2006). OFDM is a very flexi-
ble modulation technique, robust to multipath and that
simplifies the channel equalization. It is also very sen-
sitive to synchronization, which may be an advantage
when the application requires the measurement of the
TOF (Levanon and Mozeson, 2004).
In this paper it will be presented an asynchronous
data transmission with OFDM pulses (section 2). In
section 3 is presented some concerns in the pulse
choice like the shape, size, etc. Some simulated re-
sults and a comparison with other techniques will be
presented in section 4. At the end it will be presented
brief conclusion.
2 ASYNCHRONOUS DATA
TRANSMISSION WITH OFDM
An architecture for asynchronous data transmission
using OFDM pulses is proposed. There will be used
two pulses, one for time synchronization (e.g. time-
of-flight measurement) and another for some data in-
formation transmission (e.g. source identification).
The group of the synchronization pulse and the data
information pulse is called frame.
2.1 Frame prototype
Figure 1 presents the proposed prototype for the
frame, as mention before, there are two different main
pulses in the frame: The OFDM Sync. and the OFDM
Data. The first pulse is known by the receiver besides
to allow synchronization it will be used to demodulate
the second pulse, the OFDM Data, by a differential
demodulation scheme (Haykin, 2001). This method
was chosen manly because the OFDM pulse is robust
to environments with multipath however it does not
produce a very high resolution in time synchroniza-
tion, how will be seen further. Therefore, the differ-
ential demodulation needs to be robust to this small
jitter.
Guard
FFT
OFDM Sync.
Guard
Time
OFDM Data
Figure 1: Asynchronous data transmission with OFDM,
frame prototype.
The Guard FFT, presented in Figure 1 is a protection
in the demodulation process due to the time synchro-
nization jitter. The Guard Time has the same function
of Guard FFT plus the reduction of the inter-symbolic
interference (Schulze and Luders, 2005).
The frame building for the asynchronous data
transmission with OFDM pulses will be presented us-
ing an example with 17 bit modulated BPSK (Binary
Phase Shift Keying) per pulse. The BPSK is a good
modulation for differential demodulation due to the
phase difference (Haykin, 2001), 17 bits were chosen
because this data may be enough to identification and
some data communication. But the extrapolation for
other data sizes and modulations would not be a prob-
lem.
2.2 OFDM Pulse
In the OFDM the information is divided in N blocks
and each value of this block is sent using a differ-
ent carrier, in order to avoid the ISI (inter-symbolic-
interference) the OFDM uses carriers that are orthog-
onal to each other, reducing this way the total signal
bandwidth (Schulze and Luders, 2005).
Moreover, the FFT (Fast Fourier Transform) and
IFFT (Inverse Fast Fourier Transform) can be used
to improve the efficiency of the system and the im-
plementation simplicity. Figure 2 presents the block
diagram of a possible OFDM communication system
using the FFT.
I
F
F
T
S(k)
S/P
... ...
0
0
...
P/S
...
0
0
Re() DAC
Channel
ADCS/P
...
F
F
T
...... ...
P/S
X(k)
s(n)
x(n)
Figure 2: Block diagram of a OFDM communication sys-
tem.
The system input, S(k), can be symbols from some
classic modulation like PSK (Phase Shift Keying) or
QAM (Quadrature amplitude modulation) (Schulze
and Luders, 2005). Moreover, the system only mod-
ulates the carriers of the pass-band transmission sys-
tem, as shown in Figure 2.
In order to better understand the OFDM pulse a
simple example was chosen, an OFDM pulse with
only three bits, 0 1 0, of information that can be mod-
ulated with BPSK resulting in the symbols 1 -1 1.
Therefore, for this three symbols, carriers 5, 6 and
7 of an IFFT with size 1000 were chosen. Figure 3
ULTRASONIC OFDM PULSE FOR BEACON IDENTIFICATION AND DISTANCE MEASUREMENT IN
REVERBERANT ENVIRONMENTS
125
presents the three individual chosen carriers modu-
lated BPSK. Resulting in the OFDM pulse of Fig-
ure 4.
0 20 40 60 80 100
−0.001
0
0.001
Sample
Amplitu d e
k = 5
k = 6
k = 7
Figure 3: OFDM example:3 carriers coded BPSK.
0 20 40 60 80 100
−0.003
0
0.003
Sample
Amplitu d e
Figure 4: OFDM example: The OFDM pulse.
The different combination of carrier codes cre-
ates different pulse shapes and pulse amplitudes. The
systems are normally limited in amplitude and not
in energy due to the DAC(Digital-to-Analog Con-
verter) and amplifiers limitations. Therefore, it will
be important to maximize the pulse energy for the
same amplitude or, in other words, we need to min-
imize PMEPR (Peak-to-Mean Envelope Power Ra-
tio) (Schulze and Luders, 2005). In Figure 5 we com-
pare two different OFDM pulse envelopes where all
the 17 carriers where BPSK modulated. The first
case (Figure 5(a)) shows an OFDM pulse envelope
with the lowest PMEPR and the second case (Fig-
ure 5(b)) shows an OFDM pulse envelope with the
greatest PMEPR. As can be seen the two examples
are very different and give an idea of how different
the pulses envelope can be.
0 N−1
RMS
1
(a) Best PMEPR value.
0 N−1
RMS
1
(b) Worst PMEPR value.
Figure 5: Greatest and lowest PMEPR for an OFDM pulse
with 17 carriers (RMS - root mean square).
3 OFDM PULSE CHOICE
In order to maximize the OFDM pulse detection the
system must use a matched filter (Levanon and Moze-
son, 2004). Moreover, it is possible to demonstrate
that the probability of pulse detection is proportional
to the pulse energy, (each mens that the most impor-
tant thing for pulse detection will be energy of it and
not the envelope of it) (Levanon and Mozeson, 2004).
Due to the maximum signal amplitude limitation
impose by the hardware (DACs, ADCs, amplifiers,
etc.) the OFDM pulse must have the PMEPR as low
as possible in order to maximize the probability of de-
tection.
However, to have good TOF measurement with
less error as possible the OFDM pulse must have an
autocorrelation function similar to an impulse (Lev-
anon and Mozeson, 2004). Therefore, the mainlobe
heigh (at the origin) and the relationship between the
this lobe and the sidelobes must be as big as possible.
However, the mainlobe heigh have a direct relation
with the energy of the OFDM pulse, but the peak-to-
peak amplitude are limited by the source.
Due to the receiver and/or source movement
Doppler effect will occur (Haykin, 2001), therefore,
the correlation between the received pulse and the
matched filter’s pulse will change with the doppler
shift. As a result of this we must look not only to
the autocorrelation function but also to the output of
the match filter for different frequencies shifts. To the
output of match filter in function of time delay and
doppler shift will be called ambiguity function (Lev-
anon and Mozeson, 2004). Consequently, the ideal
ambiguity function is a function that has a single infi-
nite spike at the origin and is zero elsewhere (Levanon
and Mozeson, 2004).
To explain how to chose an OFDM pulse, a pulse
with 17 carriers modulated BPSK was chosen. Al-
though, the extrapolation for other pulse sizes would
not be a problem, it only take more time to find the
best pulse. These OFDM pulses must have an ambi-
guity function similar to the ideal ambiguity function.
Therefore, the mainlobe heigh (at the origin) and the
relationship between the this lobe and the sidelobes
must be as big as possible (similar to the autocorre-
lation function). However, the mainlobe heigh have a
direct relation with the energy of the OFDM pulse, but
the peak-to-peak amplitude are limited by the source.
Therefore, we must minimize the PMEPR and
maximize the MSR (Mainlobe-to-Sidelobe Ratio).
With these 17 carriers modulated BPSK we only have
131072 possibilities, to test all off them will not be a
big problem. Moreover, we only need to test half of
them because one pulse and its complement will
produce the same ambiguity function. And we do not
PECCS 2011 - International Conference on Pervasive and Embedded Computing and Communication Systems
126
need to test all of the half set because a pulse and it
carriers inverse produce the same pulse envelope and
ambiguity function (i.e. considering one pulse with 3
carriers coded BPSK the pulse [1 1 -1] has the ambi-
guity function of pulse [-1 1 1]).
The values for MSR and PMEPR are shown in
Figure 6 and 7 respectively. Note that, the MSR tests
were only performed for zero Doppler shift.
Figure 6: Mainlobe-to-Sidelobe Ratio for OFDM with 17
carriers coded BPSK.
Figure 7: Peak-to-Mean Envelope Power Ratio for OFDM
with 17 carriers coded BPSK.
The PMEPR values are between 1.73 and 16.82
and the MSR are between 1.97 and 7.08. In order to
chose the best OFDM pulse with a big MSR and a low
PMEPR we sort the MSR and PMEPR values by the
ratio of MSR and PMEPR from the shortest to biggest
(Figure 8) and is represented the last eight values in
Figures 6 and 7 by a grey circle.
0 1 2 3 4 5 6
x 10
4
0.5
1
1.5
2
2.5
Sort by MSR/P ME P R
MS R /PMEPR
Figure 8: Sort of the MSR and PMEPR ratio.
It is logic that the best value is the last one. It has
a bigger MSR (4.95) and a smaller PMEPR (1.91). In
that case, the BPSK codes are [00111100100101010],
[11000011011010101], [10101011011000011] or
[01010100100111100]. How can be seen the codes
are complemented in phase and frequency.
For compare this pulse with others we chose three
more pulses and we normalize all pulses to have the
same maximum amplitude. We chose: the pulse
with the biggest PMEPR [01101010111111100], the
pulse with the biggest MSR [00000101010101010]
and the pulse with the lowest MSR and PMEPR ra-
tio [00000000000000000]. The ambiguity function of
these examples are shown in Figures 9, 10, 11 and 12.
Where N is the size of the pulse and d the distance
between two OFDM adjacent carriers. For a better
comparative we normalize all the ambiguity function
to the maximum value of the ambiguity function with
the biggest PMEPR.
Figure 9: Ambiguity function of the pulse with the lowest
PMEPR.
Figure 10: Ambiguity function of the pulse with the biggest
MSR.
Figure 11: Ambiguity function of the pulse with the biggest
MSR-PMEPR ratio.
3.1 Pulse Size
To choose the best pulse size is necessary to know the
limitations of the system that will be used to transmit
the information, the limitations come mainly from the
channel conditions (air) and the waves propagation
(ultra-sounds).
For detection point of view a pulse must be as big
as possible (for the same amplitude the energy in-
crease with the length of the signal) (Levanon and
ULTRASONIC OFDM PULSE FOR BEACON IDENTIFICATION AND DISTANCE MEASUREMENT IN
REVERBERANT ENVIRONMENTS
127
Figure 12: Ambiguity function of the pulse with the lowest
MSR-PMEPR ratio.
Mozeson, 2004). On the other hand the length of
the signal is inversely proportional to the distance of
two adjacent carriers and consecutively to the Band-
width of the resultant signal. So in the first analysis a
huge pulse will be the best, however it will introduce a
problem, the carriers will be very near and a little rel-
ative speed between the source and the receiver pro-
duce a catastrophic change in the carriers. Therefore
an up and a lower bound to the pulse size is presented:
N
p
B
T
Pulse
c + v
2v f
c
(1)
where T
Pulse
is the time length of the pulse, B is the
maximum bandwidth for resultant pulse, c is the wave
propagation speed, v is the maximum allowed relativ-
ity speed between the source and the receiver and f
c
is the central frequency of the resultant desired pulse.
Note that, in this equation only a maximum Doppler
shift of a half of the distance between the adjacent car-
riers will be allowed, moreover, the resultant pulse has
a very narrow band so the Doppler is approximately
equal for all the carriers.
The Guard Time must be greater or equal than
Guard FFT and greater than the minimum time to
avoid the inter-symbolic interference. Normally the
minimum time to avoid the inter-symbolic interfer-
ence is bigger than the Guard FFT. Mathematically
the lower bound to Guard Time is:
T
G
max
{
T
G
FF T
,T
ISI
min
}
(2)
where the minimum to avoid the inter-symbolic inter-
ference can be approximated, for the omni-directional
transducers, by the solution of the equation:
20log(acT
ISI
min
) + αcT
ISI
min
= log(r) + αr (3)
Where a is the maximum ratio between the reflection
wave amplitude which does not interfere in the direct
wave and the direct wave amplitude. The coefficient
α is the attenuation of wave due to the absorption of
ultra-sound by the air and it is given in dB/m. Fi-
nally, r is the maximum distance between the source
and the receiver.
3.2 Asynchronous Receiver
The block diagram of a possible asynchronous re-
ceiver is shown in Figure 13. The Synchronization
part consist in a simple algorithm: after the system de-
tect the pulse, it chooses the maximum of the output
of the detector on the next N samples (the length of
the pulse plus the Guard FFT time if it exists); After
that the system demodulates the data using the syn-
chronization pulse by a differential comparison.
> °
j j
2
y(n)
maximum
counter
0 to N
Buffer
N samples
de-mod
Stop
ind.max
Start
Data
Start
Data demodulator
Synchronization
x(n)
Figure 13: The block diagram of the asynchronous OFDM
receiver.
4 RESULTS
To test the proposed asynchronous data transmission
with OFDM pulses a practical example was chosen,
the source and the receiver are almost motionless,
therefore, a 0.02 m/s of maximum relativity speed
was chosen. The system works at 40 KHz of cen-
tral frequency with 2 KHz of bandwidth and sampling
rate of 200 KHz. Nevertheless the system must have
at least 10 different channels for communication and
17 bits to transmit. So each channel has a maximum
bandwidth of 200 Hz. Therefore, from the equation
1, we can compute the size of the pulses:
85 ms T
Pulse
107 ms (4)
A T
Pulse
= 100 ms can be chosen, as a result of this
the distance between two adjacent carriers is 10 Hz
(d = 10 Hz) and each pulse has 10000 samples.
Too choose T
G
FF T
the pulse ambiguity function
must be used. It can be seen that the main lobe had
about 6 ms of width. Therefore 3 ms is the distance
in time from the maximum of main lobe to the mini-
mum of it. So we use 5 ms for T
G
FF T
to allow a poor
accuracy in the received instant.
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128
Finally to compute the Guard Time, the maxi-
mum distance between the source and receiver must
be used. It is 5 m (r = 5), therefore the attenuation of
wave during propagation due to the absorption of ul-
trasounds by the air is 1 dB/m (α = 1) and a reflected
wave with amplitude of one fifth (a = 0.2) of the di-
rect wave does not produce a relevant inter-symbolic
interference. With these values the T
ISI
min
is 15 ms so
15 ms can be used for the Guard Time.
One possible frame for the given example is
shown in Figure 14;
0 5 105 120 220
−1
0
1
Time (ms)
Figure 14: Asynchronous data transmission with OFDM,
example of a frame.
To validate the proposed frame we will compare
it with other used method. This method uses a very
common pulse for synchronization, a chirp (Levanon
and Mozeson, 2004), and an usually differential mod-
ulation (DBPSK) to data transmission which allows
some synchronization jitter (Haykin, 2001). As a re-
sult of using DBPSK for transmit the same data we
will need one more extra bit. This extra bit is go-
ing to be used as a reference for differential demod-
ulation. The chirp ambiguity function is presented in
Figure 15 and the block diagram of the frame in Fig-
ure 16.
Figure 15: Ambiguity function of a chirp pulse.
Chirp DBPSK Data
Figure 16: Asynchronous data transmission with chirp and
DBPSK, frame prototype.
The first primordial characteristic to be set is the
size of the frame, therefore both frames must have the
same length (220 ms). The chirp has almost the dou-
ble of energy compared to the chosen OFDM pulse, so
it will produce the double of main peak in the match
filter output. Consequently, in this comparative, we
are going to use a chirp with a half size of the OFDM
pulse (50 ms), as a result we maintain similar capa-
bility of detection for both cases. We do this because
we want give more time (170 ms) to the data in order
to reduce the total bandwidth. But even with this data
length, it is impossible to get the same bandwidth that
we can obtain using the OFDM pulses. Moreover, we
want to transmit 18 bits (17 bits plus one bit for dif-
ferential demodulation) in 170 ms so we need about
212 Hz of bandwidth instead of the 170 Hz in the case
of the OFDM pulse. One possible frame of this exam-
ple is shown in Figure 17;
0 50.08 78.4 106.72 135.04 163.36 191.68 220
−1
0
1
Time (ms)
Figure 17: Asynchronous data transmission with chirp and
DBPSK, example of a frame.
The main goal of the proposed frame is the robust-
ness to multipath that exist in the rooms when we use
ultrasonic waves to communicate (reverberant envi-
ronment). All tests will be performed to demonstrate
how robust the proposed frame is and how better or
worst it is comparatively to other common technique.
To test our system we use two different ap-
proaches. In the first one we use a synthetic impulse
response to simulate the multipath, in the second one
we use an ultrasonic room acoustics simulator.
4.1 Test with a Synthetic Impulse
Response
To simulate the robustness to multipath we implement
a synthetic impulse response. The system begins to
receive the direct pulse at instant 0 and receives three
more attenuated echoes. This impulse response is
shown in Figure 18 where N (µ,σ) represent a Gaus-
sian independent variable with µ mean and σ
2
vari-
ance.
0
N(7.5,1)N(10,1) N(15,1)
N(0.01,10
-4
)
N(0.05,10
-4
)
N(0.4,10
-2
)
1
time (ms)
Amplitude
Figure 18: The synthetic impulse response for test the sys-
tem.
ULTRASONIC OFDM PULSE FOR BEACON IDENTIFICATION AND DISTANCE MEASUREMENT IN
REVERBERANT ENVIRONMENTS
129
The data to send was chosen randomly for OFDM
and Chirp plus DBPSK, in DBPSK the first bit was
set to one in all simulations. Moreover, it was added
white noise to resultant signal and the threshold was
set to have a probability of false alarm of the 10
9
.
The Bit error rate (BER) was computed for each sig-
nal (OFDM or Chirp plus DBPSK) and 1 million sim-
ulations per signal amplitude to noise standard devia-
tion were performed. The result of this test is shown
in Figure 19. How you can be seen the OFDM had
an excellent behavior comparatively to the Chirp plus
DBPSK, which has a BER floor greater than 10
2
.
−40 −30 −20 −10 0 10 20 30 40 50 60
10
−6
10
−4
10
−2
10
0
Signal amplitude to noise standard deviation ratio (dB)
BER
OFDM
Chirp
Figure 19: The Bit error rate in a multipath channel for a
theoretical impulse response.
4.2 Tests in a Room Simulator
To test and compare the two methods we use a simula-
tor implemented by the authors and presented in (Al-
buquerque et al., 2010), it is an acoustic simulator that
assumes specular reflections and aimed to simulate
ultrasonic communications. Moreover, it takes in to
account the attenuation due the air propagation losses
(as a function of the signal frequency, room tempera-
ture, air pressure and humidity), wall reflections and
source and receiver beams. The proposed simulator
was used to model a real room and it closely matched
the experimental observations.
In this simulator two test are performed in the fol-
low room conditions:
The room has 5x5x3m of dimension;
The reflection coefficient of the wall is 0.7;
The reflection coefficient of the floor is 0.5;
The reflection coefficient of the ceil is 0.9;
No noise was added to the incoming signal;
For each position were sent 170 bits (10 frames);
For the source was used an approximated beam
function from the Murata transmitter MA40B4T;
For the receiver was used an approximated beam
function from the Murata receiver MA40B4R;
The source and the receiver were placed at 1m
from the floor;
The room temperature was set to 22
C;
The atmospheric pressure inside the room was set
1 atm;
The relativity humidity inside the room was set to
33%.
The first test was conducted in the room of Figure
20, the source was located at 1cm of the “top” wall
and 10cm of the “left” wall. The main propagation
direction of the source was to “down” and parallel
to “left” wall. The receiver started “walk” at 2 m
from the “top” and “left” wall and stopped at about
27.5 cm from the source. Therefore, the simulator get
the probability of bit error with a “step” resolution of
2.75 mm resulting in 900 probabilities (Figure 21).
How can seen the two frames have a good behavior
(the OFDM has a small advantage) in this test.
1 m
1 m
0.01 m
from the top wall
and 0.1 m
from the left wall
S
R
R
R
Figure 20: First simulator test room.
The second test was conducted in the room of Fig-
ure 22, this room is similar to the room of the first
test, but this one has a wall with a 1 m of width and
20 cm of deep. The source was placed at the centre of
the “top” and at 1 cm from it. The main propagation
direction of the source was set to “down” and paral-
lel to “left” wall. The receiver started “walk” at 1 m
from the “left” and “bottom” wall and stopped at 1 m
from the “right” and “bottom” walls. The simulator
get the probability of bit error with a “step” resolu-
tion of 3 mm resulting in 1000 probabilities (Figure
23). How can seen there will be in these type of cases
that the proposed prototype will present a better re-
sult in comparative to other usual techniques. With an
appropriated code correction it will be possible to re-
duce the probability of bit error to almost zero where
with other techniques it is impossible.
PECCS 2011 - International Conference on Pervasive and Embedded Computing and Communication Systems
130
100 200 300 400 500 600 700 800 900
0
0.2
0.4
OFDM
P. error
Position
100 200 300 400 500 600 700 800 900
0
0.2
0.4
0.6
Chirp+DBPSK
P. error
Position
Figure 21: The probability of error in the first simulator test.
1 m
1 m
S
R R RRR
0.01 m
from the wall
Figure 22: Second simulator test room.
5 CONCLUSIONS
In this paper is proposed an asynchronous data trans-
mission with OFDM pulses which is robust to the
multipath effect. To achieve these goals OFDM with
BPSK modulated carriers was used. It was presented
the OFDM pulse restrictions and it was given some
advices of how to build the proposed asynchronous
data transmission and how to choose the size and the
shape of the OFDM pulse. Therefore, some sim-
ulations to compute the bit error rate of the pro-
posed system were performed. The performance of
the proposed technique, in the presence of multipath,
is undoubtedly better than the use of the chirp with
DBPSK. Moreover, some considerations can be made
100 200 300 400 500 600 700 800 900 1000
0
0.2
0.4
OFDM
P. error
Position
100 200 300 400 500 600 700 800 900 1000
0
0.2
0.4
0.6
Chirp+DBPSK
P. error
Position
Figure 23: The probability of error in the second simulator
test.
about the Doppler effect. The OFDM pulse may be
robust than the chirp pulse. Because the ambiguity
function is more similar to the ideal ambiguity func-
tion which mays allow to recovery the instant and the
speed of the source with a better precision.
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