Experimental results on underwater communication using vector transducersErjian Zhang and Ali Abdi

Citation: Proc. Mtgs. Acoust. 23, 070011 (2015); doi: 10.1121/2.0000195View online: http://dx.doi.org/10.1121/2.0000195View Table of Contents: http://asa.scitation.org/toc/pma/23/1Published by the Acoustical Society of America

Published by the Acoustical Society of America

Volume 23 http://acousticalsociety.org/

169th Meeting of the Acoustical Society of America Pittsburgh, Pennsylvania

18-22 May 2015

Underwater Acoustics: Paper 1aUW1

Experimental results on underwater

communication using vector transducers

Erjian Zhang and Ali Abdi Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, NJ; ez7@njit.edu; ali.abdi@njit.edu

Recent studies show that a vector transducer which excites signals in acoustic particle velocity channels

can serve as a compact multichannel communication transmitter (C. Chen and A. Abdi, “Signal

transmission using underwater acoustic vector transducers,” IEEE Trans. Signal Processing, vol. 61, pp.

3683-3698, 2013). In this paper, performance of frequency shift keying modulation is experimentally

studied for data transmission via underwater particle velocity channels using a vector transducer.

Multiple test environments are considered. Comparisons are also made with systems that utilize scalar

transducers only. System and transducer design considerations, implementation details and experimental

results are presented in the paper as well.

© 2016 Acoustical Society of America [DOI: 10.1121/2.0000195]Received 31 March 2016; Published 20 April 2016Proceedings of Meetings on Acoustics, Vol. 23 070011 (2016) Page 1

I. INTRODUCTION

Communication via acoustic particle velocity channels using vector sensors and projectors has been

recently proposed for underwater systems [1][2]. In addition to the acoustic pressure channel [3][4],

acoustic particle velocity channels serve as extra channels for communication [1][2][5]. Different

characteristics of these channels such as capacity [6][7], delay and Doppler spreads [8], instantaneous

mutual information [9] and possible correlations [10] have been studied to date. In this paper,

transmission of frequency shift keying (FSK) signals using a vector transducer is studied via experiments.

FSK modulation is considered due to its simplicity, to focus more on understanding particle velocity

channels and vector transducers for underwater communication.

The rest of the paper is organized as follows. In Section II a 2×1 multiple-input single-output (MISO)

underwater system is introduced, where two independent data streams are sent thru two particle

velocity channels. Experimental results are presented and analyzed in Section III, to explore the

performance of the proposed vector transmitter and compare it to an array of two scalar transmitters.

Concluding remarks are provided in Section IV.

II. TRANSMISSION OF TWO DATA STREAMS USING A TWO-CHANNEL

VECTOR TRANSDUCER

The top view of a 2×1 MISO acoustic vector system is shown in Fig. 1. There is one vector projector

[1] with four scalar elements at the transmit side, labeled by 1, 2, 3 and 4, with spacing TxL , and a scalar

sensor (hydrophone) at the receive side . The four channels in Fig. 1 are labeled as 1p , 2p , 3p and 4p .

When TxL is small, elements 1 and 2 operate as a dipole in x-direction, and elements 3 and 4 operate as

Fig. 1. Top view of the proposed 2×1 MISO underwater acoustic communication system. The transmiter Tx is a vector projector and the receiver Rx is a scalar sensor.

LTx

21

Rx

y

x

Tx

p1(p

2)

p3

4

3p

4

E. Zhang and A. Abdi Experimental results on underwater communication using vector transducers

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a y-oriented dipole. The pressure-equivalent particle velocity channels are given by [1] [11]

1

2 1

1

4 3

,

,

x

Tx

y

Tx

p jkL p p

p jkL p p

(1)

where 1j , 2 /k is the wavenumber, 0/c f is the wavelength, c is the speed of sound

and 0f is the carrier frequency.

Upon transmitting two signals thru the above two particle velocity channels, the received signal can

be written as

1

2

x ys

r p p ns

. (2)

Here 1s and

2s are two independent FSK modulated signals and n is the acoustic pressure ambient

noise. Note that bit duration Tb is chosen to be long enough, so that the channels in (2) can be

considered to be frequency-flat, causing no intersymbol interference (ISI).

The block diagram of the receiver is shown in Fig. 2, which includes two non-coherent binary FSK

(BFSK) demodulators, to recover the signals 1s and

2s .

III. INITIAL EXPERIMENTAL RESULTS

A. Experiment Setup

In all the experiments, BFSK modulated signals are generated by MATLAB® and then applied to one

vector projector or two scalar projectors using a D/A converter controlled by MATLAB® Data Acquisition

Toolbox™. Received signals are captured by one scalar hydrophone connected to an A/D convertor

Fig. 2. Block diagram of two non-coherent BFSK demodulators at the receive side of the 2×1 MISO underwater acoustic communication system.

E. Zhang and A. Abdi Experimental results on underwater communication using vector transducers

Proceedings of Meetings on Acoustics, Vol. 23 070011 (2016) Page 3

controlled by MATLAB® Data Acquisition Toolbox™. The carrier frequency is chosen to be 0 20.2f kHz.

The bit duration is set at 0.1bT s, to ensure that in one of the experiment setups, lab water tank, we

have rmsbT , where

rms is rms multipath delay spread of the tank [13]. The frequency spacing f

chosen to be 100 Hz, to keep BFSK frequencies orthogonal. The vector projector with four elements acts

as two dipoles, to transmit two independent data streams, simultaneously.

B. Lab Test Results

The MISO system with one vector projector and one scalar sensor receiver is first tested in lab water

tank, as shown in Fig. 3b. As shown in Fig. 3a, a fully-scalar MISO system is also considered for

comparison, which consists of two scalar projectors and one scalar sensor receiver. The transmitters and

receiver in both systems are separated by about 1m in the same lab water tank. Each transmission in the

experiments is 5 minutes long for each MISO system.

(a) Two scalar projectors and one scalar receiver (b) One vector projector and one scalar receiver

Fig. 3. Two MISO systems in lab water tank.

Bit error rates (BERs) of the MISO systems are shown in Table I. With similar received signal-to-noise

ratios (SNRs), the vector system shows zero or very close to zero BER. The smaller size of the vector

projector compared to the two scalar projectors together is another advantage of the proposed vector

system. To better understand the BER results, spectra of the received signals are shown in Fig. 4.

Table I. Experimental BER performance of the MISO systems in the lab

Transmitting System SNR (dB) BER

Two scalar projectors -15.3 0.02

One vector projector -15.2 0

Two scalar projectors -13.7 0

One vector projector -13.6 0.006

E. Zhang and A. Abdi Experimental results on underwater communication using vector transducers

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Unbalanced power distribution is observed in both systems. After inspecting the received signals

and the demodulated bits, we observed that all errors occurred at frequencies which had lower spectral

magnitudes. This means that Fig. 4 explains the BER results in Table I.

(a) SNR is about -15dB (b) SNR is about -13dB

Fig. 4. Spectra of the received signals for both MISO systems, measured in the lab.

C. Pool Test Results

The same MISO system with one vector projector and one scalar sensor receiver is also tested in a

large pool, where the transmitter and receiver are placed at the same depth, about 15 m far apart. The

fully-scalar MISO system with two scalar projectors and one scalar sensor receiver is also deployed in

the pool, for comparison purposes. Other parameters remain the same as those considered in the lab

experiments. BER results and received signal spectra of both MISO systems in the pool are shown in

Table II and Fig. 5, respectively.

Table II. Experimental BER performance of the MISO systems in the pool

Transmitting System SNR (dB) BER

Two scalar projectors -9.6 0.0277

One vector projector -9.7 0

With the same received SNR, the vector system exhibits a lower BER, which may be attributed to the

frequency content of the received signals shown in Fig. 5. The last frequency in the fully-scalar system

spectrum, top panel of Fig. 5, has a very small magnitude, which causes many errors. In addition to

lower BER, compact size of the transmitter is another benefit of the vector system.

E. Zhang and A. Abdi Experimental results on underwater communication using vector transducers

Proceedings of Meetings on Acoustics, Vol. 23 070011 (2016) Page 5

Fig. 5. Spectra of the received signals for both MISO systems, measured in the pool.

IV. CONCLUSION

In this paper, underwater simultaneous transmission of two independent data streams using a two-

channel acoustic vector transducer is considered, where two independent FSK signals are sent thru two

orthogonal particle velocity channels. Lab and pool experiments demonstrate that the proposed vector

system offers zero or very low bit error rate, compared to a fully-scalar system with two individual scalar

transmitters. The compact size of the vector transmitter is another advantage which makes it

particularly suitable for small underwater vehicles and platforms.

V. ACKNOWLEDGMENT

This work is supported in part by the National Science Foundation (NSF), Grants IIP-1340415 and CCF-

0830190.

E. Zhang and A. Abdi Experimental results on underwater communication using vector transducers

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