under water communication using parametric transduction

7/27/2019 under water communication using parametric transduction

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Underwater Acoustic Communication Utilising Parametric Transductionwith M-ary DPSK Modulation

Ming Z heng, Rodney F W Coates, Liansheng Wang, Richard StonerSchool of E lectronic and Electrical Engineering

The University of BirminghamEdgbaston, Birmingham,B 15 2TT, UK

Abstract-A real-tim e M-ary Differe ntialPhase-Shift Keying (MDPSK) communication systemutilising parametric transduction has been constructed.The system employs a 50 kH z primary frequency and a5 1 H z difference frequency. It has been tested in theGulf of Lion and at Cap Ferrat in the MediterraneanSea. Experimental results indicate that the system canbe used in shallow water to realise real-time acousticcomm unications at ranges of tens of kilometres and canachieve data rates of 1, 2, and 3 kb s- fo r 2-, 4-, and 8-DPSK respectively.

I. INTRODUCTION

One of the most important requirements for anunderwater acoustic commu nication system is to achieve ahigh data rate with a reasonable level of reliability.However, this is hindered by multipath induced signalfluctuation and intersymbol interference (ISI). Theapproaches to overcoming multipath-induced problemshave been sum marised in [I ]. The objective of the presentwork is to investigate the possibilities of using parametrictransduction with differential signalling to combat theseproblems.

Parametric transduction offers a solution toISI.Parametric transduction occurs whentwo high intensityprimary signals co-exit in water in front of the transmittransducer. The non-linear behaviour of the water resultsin the generation of a difference frequency component.The difference frequency sound wave may be consideredto be radiated from a line array of acoustic sourcesdistributed continuously through the interaction region.This virtual array is shad ed exponentially in the pro cess ofconversion of the primary frequency waves to thedifference frequency wave with increasing distance fromthe transmitter. The secondary source behaves as a taperend-fire array, its acoustic axis being normal to the face oftransmitter. Its angular response decays monotonicallywith increasing directional angle, thus avoidingundesirable sidelobes. Since the length of the virtual array

spans many hundredsof wavelengths, the beamwidth ofthe array will be much narrower than would be achievby a conventional array o perating directly at the differenfrequency. This property can be employed to combmacromultipath propagation. Furthermore, the wide-bannature of the parametric conversion process enables it be used for high data rate communication or wide-bansignal processing techniques such as spread spectrum .

Differential signalling provides a solution tsignal fluctuation. In a Differential Phase Shift Keyin(DPSK) communication system, the phase of the curresymbol is compared with the phase of the precedin

symbol. The channel characteristics can be considered abeing frozen over a symbol period which is only2 msfor a DPSK system with a 5 kHz carrier and a 1 ksymbs-l data rate. Since the chann el fading rate is much slowthan the signalling rate, the differential phase fluctuatiodue to the channel change in a symbol period is negligiblTherefore, differential signalling can be employed tcombat the signal fluctuation. Furthermore, M-ardifferential signalling has a higher bandwidth efficiencthan M-ary Frequency Shift Keying (MFSK) modulatioand also it can use noncoherent techniques to demodulaa received signal. Differential detection avoids carrirecovery and achieves fast synchronisation and is thusuitable for m ultipath fading channels.

11. THE BASS 50 PARACOM LINK

To achieve a transmission distance of the order otens of kilometres, the secondary frequency should be the low kHz region [2], since signal attenuation loss withen be greatly reduced. BASS50 PARACOM uses a 50kHz primary frequency and a 5 kH z secondary frequenand it employs2-, 4- and 8-DPSK. Its major features arshown in Table 1.

A . TransmitterFor the system to be applicable to variou

situations, both the summing and the multiplyinmethods of param etric transduction have been used.

0-7803-3519-8/96 $5.00 0 1996 IEEE 832


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Table 1 Parameters of BASS 50 PARACOM

f, Higher Primary 42.5 or 52.5 or 5 5 kH zf , Lower Primary 37.5 or47.5 or 50 kH zf d Secondary 5 kH zB Bandwidth 1 kHzR , Signalling Rate 1 ksymbol s-

RI, Data Rate M=2 1 kbs-M=4 2 kb s- lM=8 3 kb s-l

225 dB re lpPal,, , Sll Source Level@ f,, ,

Summing method the transmit system using thismethod is shown in Fig. 1. The system employs twoprimary frequencies. A 50 kHz sinusoid is modulated bythe digital data sequence and is added to a 55 kHzcontinuous sinewave. T he50 kHz and 55 kHz componentsare phase locked together in order to avoid phase shiftbetween them.

As shown in Fig. 1, there aretwo ways by whichthe modulated signal can be radiated into the water. Thefirst method involves directly applying the summed signalc o s ( 2 n ~ t + 8 , )+ cos(2nf2t) to the entire set ofelements in the transmit array. For the second method, themodulated signal cos(27i&t +e,) and unmodulatedsignal cos(2nf2 t )are amplified by two amplifiers andapplied to separate, interleaved staves of array elements.The latter approach is less demanding of power amplifierlinearity.

Multiplying method The transmitter using thismethod is shown inFig. 2. It can be seen that the system

described here is similar to that ofFig. 1. Apart from thedifference of the transmit primary and secondary

frequencies, the only difference is that the presentmodulator uses a coherent local oscillatorcoS(27cfCt)

B. ReceiverAlthough tw o transmitters have been used for

BASS 50 PARACOM system, nevertheless, after theparametric downmixing propagation, both transmittersgenerate the same difference frequency signal. Therefore,a single receiver will be suited to both transmitters. TheBASS 50 P ARACO M receiver is, thus, presented inFig. 3 in a block diagram form. Here, the BPF has a centrefrequency o f 5 kHz, and the LPFs have a cut-off frequencyof 1 kHz that corresponds to a symbol rate, UTs, of 1 kb

- 1s .

C. Transmit ArraysTwo different transmitter arrays were employed

during the two sets of field trials described in Section111.The first parametric source, a circular transducer with a0.8m diameter, was provided by SACLANT ASWResearch Centre, Italy during the Modal Lion projectconducted during the sum mer of 1995. The transducer hada centre frequency of 40 kHz and was capable ofdelivering primary and secondary source levels ofapproximately 230 dB and 190 dB respectively.

The second transmitter array, shown inFig. 4 anddesigned and built at the University of Birmingham, has arectangular shape and consists of 18 staves eachcontaining 6 Tonpilz elements. Each element can deliver20 watts acoustic power. Thus, the array has 2.2 kWacoustic power capacity, which corresponds to a totalprimary so urce level of 231 dB.

EpromStored

o r PC

fi+ tfi Tx

DifferentialCoding t BP F

COS(27tht)

tt PLLymbol

Fig. 1. The block diagram showing DPSK parametric transmit system with primary frequenciesf,=50 kHz,f,= 55 kHz andsymbol duration T,=l ms. (8,: ransmit differential encoded phase)

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EpromStored

vsSampler - ifferential -

Decoding

A Log1c5 kH zCircuit

Parallel

V to Serial

- 7 b -

44

- w 4

I I 10;

PC

-

Tx

Fig. 2. The block diagram showingDPSK parametric transmit system withf,=50 k H z , f ~ 5 Hz andT,=l ms.(8, : transmit differential encode d phase)

Fig. 3. The block diagram showing DPSK receive system with difference frequency f,=5 kH z andT,=l ms

(8, : estimated phase)A

19 0 mm

r d

Fig. 4 The arrangem ent of array elements

111. SEA RIALS

A. Gulfof Lion TrialThe experime ntal arrangement is shown inFig.

The projector was deploye d4 m above the sea bed.

. . . . .

-

2 J m '

,..,

under water communication using parametric transduction

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