FDTD Simulations for Ultrasonic Communication System
Employing Metal Plates
Nan Gao 1, Shigeru Shimamoto 111

Graduate School of Global Information and Telecommunication Studies, Waseda University, Japan

Introduction

Nowadays, there have no identified ideas for sand digital
information across conductors material, such as metal, water andso on. But in some cases, it will not be a good idea to makesome holes in such materials. So we need other sensor system
which can send the information directly through them. But
conventional wireless communication system cannot be applied
because of the shielding provided by the conductors material,
however the ultrasound propagates readily through them and it
also can be used to convey information. For example, the use ofultrasonic singaling to transmit digital information acrossmetallic barriers has been demonstrated by several groups[1-5].
Also there are few simulation methods can be used forultrasonic communication. The most common ultrasound system
method today is pulse-echo techniques[6,7], or other ultrasoundsimulation techniques use both the transmitted and reflectedenergy to form a two-dimensional image[8,9]. However, all ofthese techniques do not suit for ultrasound communication
simulation. Because they assume that scattering is weak and do
not account for strong diffraction and refraction. So we need
another method for the ultrasound wave simulation.
In this paper, we choose the Finite Difference Time Domain
method to simulate the ultrasound wave behavior. This method
can be used easily to simulate ultrasound propagation in theconductors material. It can lead to better understanding of theeffect of ultrasound signal. In this work, an implementation of
FDTD into an application for ultrasonic communication isdescribed.

Fig. 2 Gaussian pulse propagate in bar
In figure 2, we can find that, ultrasound wave will reflect
back on the border of metal bar. Because the diameter of metal
is very thin. So as the ultrasound wave travelling along the bar,reflected wave will arrive the opposite side of metal bar andreflect back again. These reflected wave may cause intersymbolinterference with subsequent transmissions.
Then we want to simulate how the echoes affect theultrasonic wave when it carries signal information. In thesimulation, we use a group of BPSK-modulated signal as thesource. And we measure the acoustic force at the receiving
transducer side.
1.2
1.0
0.8
0.6

2

FDTD Simulation for two different cases

0.4

In order to find the best position to send ultrasound signal
throw metal, we want to study ultrasound wave behaviour indifferent metal objects.
Metal BarIn this section, we are going to simulate the ultrasound wave
behaviour in a metal bar. This is a very common structure, justlike a metal pipe. Figure 1 shows the geometry and regions
defined for the FDTD simulation. The width of transducer andthe diameter of metal bar are both 3mm. The analysis grid
spacing is 0.013mm. Each simulation is running for 20 thousand
time steps, for about 20  s

Metal Bar
Transudcer

Transudcer

Fig. 1 The Schematic diagram of metal bar
At first, we simulate when a simple Gaussian source is been
set on the left side of metal bar. So we can observe theultrasound wave behaviour in a metal bar. The simulation resultis showing in figure 2.

0.001230.15

4567Data Number

8910

0.10

normalized amplitude

2.1

0.2

0.05
0.00
-0.05
-0.10
-0.15
0

4000

8000

12000

16000

time step

Fig. 3 Original data and multiplied output wave, metal bar
The received signal is multiplied by a reference frequency
sine wave. If data is "1", the corresponding part of multiplied
output should above zero. And if data is "0", the corresponding
part of multiplied output should below zero. So the multiplied
output can be used to compare with the original data.
The multiplied output on 5MHz is showing in figure 3.
From this we can observe that the multiplied output is quite
different from the original one. So it may be very hard to send
ultrasonic signal throw a thin metal bar.

2.2

Subsection Two
Then we are going to simulate the ultrasound wave
behaviour in a metal bulkhead, the schematic diagram of whichis showing in figure 4.

Transducer

Transducer
BulkheadFig. 4 The Schematic diagram of metal bulkhead
In Figure 5a, we can see the pulse entered the bulkhead.
Because the impedance mismatch between transducer andbulkhead, only part of the incident energy has been coupled intothe bulkhead. Figure 5b shows the ultrasonic pulse within thebulkhead. Figure 5c shows part of the ultrasonic pulse passes
into the transducer on the receive side. Again, a portion has beenreflected back into the bulkhead. At last, in Figure 5d, the pulse
has been reflected form the receive side of the bulkhead. Thispart of pulse will travels back into the receiver side too.

abcdFig.5 Gaussian pulse propagate in bulkhead
Figure 6 shows the result at 5MHz. From this we canobserve that the multiplied output is almost the same with theoriginal one. So a metal bulkhead may be better for sanding
ultrasonic signal.
1.2
1.0
0.8
0.6
0.4
0.2
0.00

12

3

4

5

67

Data Number

8910

Normalized Amplitude

1.0

0.5

0.0

-0.5

-1.005000

10000 15000 20000 25000 30000 35000 40000

Time stepFig.6 Original data and multiplied output wave, metal bulkhead

3Conclusion

Presented evaluation shows that FDTD method is easily
applicable for simulating ultrasonic problems. The simulation
results shows that a metal bulkhead is convenient for ultrasoniccommunication.
Future work will include the use of ultrasonic wave in airfor communication. This will offer advantage especially forsecurity. And the slow propagation speed in air will also allows
the location of sources to be tracked. Nowadays, there are somestudies of the properties of ultrasonic communication systembeen published. For example, ultrasonic wave has been used forlocation tracking in hospitals, where time-of-flight from amoving source to a set of fixed receivers can be used[12]. And
simple methods for designing an ultrasonic PC mouse using
narrow bandwidth piezoelectric transducers[13]. We are going
to design a ultrasonic system to supersede the radio frequency
identification system using radio frequency electromagnetic
wave. So it can offer advantage for security.
References[1]

T. Murphy, "Ultrasonic Digital Communication System for a SteelWall Multipath Channel: Methods and Results," Master's thesis,
Rensselaer Polytechnic Institute, 2006.
[2] D. Shoudy, G. Saulnier, H. Scarton, P. Das, S. Roa-Prada, and J.
Ashdown, "P3f-5 an ultrasonic through-wall communication
system with power harvesting, " in Ultrasonics Symposium, 2007.
IEEE, Oct. 2007, pp. 1848-1853.
[3] G. J. Saulnier, H. A. Scarton, A. J. Gavens, D. A. Shoudy, T. L.
Murphy, and M. Wetzel,"P1g-4 through-wall communication of
low-rate digital data using ultrasound, " in Ultransonics
Symposium, 2006. IEEE, Oct.2006, pp. 1385-1389.
[4] M. Kluge, T Becker, J. Schalk, and T. Otterpohl, "Remote acoustic
powering and data transmission for sensors inside of conductive
envelopes," in Sensors, 2008 IEEE, Oct. 2008, pp. 41-44.
[5] D. Graham, J. Neasham, and B. Sharif, "High bit rate
communication through metallic structures using electromagnetic
acoustic transducers," in OCEANS 2009-EUROPE, 2009.
OCEANS'09. May 2009, pp. 1-6.
[6] Schueler, C. L., Lee, H., and Wade, G., "Fundamentals of digitalultrasonic imaging," IEEE Trans. Sonic Ultrason. 31, 1984, pp.
195-217.
[7] Kelly-Fry, E., "Influences on the Development in the United Statesof Ultrasound Pulse-Echo instrumentation," in UltrasoundMammography, A. P. Haper, Ed, 1985.
[8] Greenleaf, J. F., and Bahn, R. C., "Clinical imaging withtransmissive ultrasonic computerized tomography," IEEE Trans.
Biomed. Eng. BME-28, 1981, pp. 177-185.
[9] Sponheim, N., Gelius, L. J., Johansen I., and Stamnes J. J.,"ultrasonic tomography of biological tissue," Ultrasonic Imaging
16, 1994, pp. 19-32.
[10] K. Yee, "Numerical solution of initial boundary value problem
involving maxwell's equations in isotropic media," Antennas andPropagation, IEEE Transactions on vol. 14, no. 3, May 1966, pp.
302-307[11] Fetter A. J., and Walecka J. D., "Theoretical Mechanics ofParticles and Continua," McGraw-Hill, New York, 1980.
[12] S. Holm, "Airborne ultrasound data communications: The core of
an indoor positioning system," in Proc. IEEE Ultrason. Symp.,
Rotterdam, The Netherlands, vol. 3, Sep. 2005, pp. 1801-1804.
[13] R. A. Zurek, M. L. Charlier, and A. Dietrich, "Omnidirectional
ultrasonic communication system," U.S. Patent 6363139, Mar. 26,
2002