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Index Terms

1. Introduct

The scienrecognized ocean eas the most importacoustic system peOcean environmentin shallow watersphase fluctuationssignal. These are dutransmitter and ocwaves, winds and csignal fluctuations quantified, modeleddesign of underwate

PROCE

d Spectral Broadening of Aan Underwater Chann

Nimmi R Nair and Roshen J

Naval Physical Oceanographic Laboratorynimmirnair@hotmail.com

ct— Boundaries of oceans viz. surface andnnel for the propagation of underwater soundn in this channel is subjected to random flucpresence of in-homogeneities in water coluwith rough sea surface and sea bed.

ities in the water column are caused by tures and dynamics of the ocean. As a result oagation, underwater sound experiences disand amplitude fluctuations. In this snal signal detection algorithms do not perfos to the development of new signal prto take care of fluctuations in acoustic sign

end is to use channel factors caused by thntal variability to derive optimum perform

One of the factors is the Doppler spread. In thof Doppler spread for a CW signal is attempt

s— Coherence, Doppler spread

tion

ntific community has environmental variability tant factor in underwater erformance degradation. tal variability, especially s, cause amplitude and in received acoustic

ue to motions of receiver, ean medium caused by currents. These observed need to be understood,

d and predicted for better er systems.

As thunderwater chabandwidth of thcomponents ofdifferently. Othin shallow wateand dynamic acthe signal passichannel is disfrequency domacapacity especiacommunicationscompared to elethe low velocitmakes design o

EEDINGS OF SYMPOL-2013

Acoustic Pulse in el

Jacob y, Kochi-682021

d bottom d. Sound ctuations umn and The in-

y density of multi-spersion, scenario, rm well.

rocessing nals. The he ocean mance of his paper, ted.

he frequency response of annel is not flat over the he signal, different spectral f the signal are affected her important characteristics ers are multipath propagation coustic medium. As a result, ing through the underwater storted both in time and ains thereby reducing channel ally in underwater acoustic s [1]. This spread is large ectromagnetic waves, due to ty of acoustic waves. This of underwater systems more

Nimmi R Nair and Roshen Jacob.: Observed Spectral broadening of acoustic pulse in an underwater channel

complex. When the delay spread is large in a communication system, a transmitted symbol may interfere with adjacent symbols at the receiver. This leads to severe inter-symbol interference (ISI), and it becomes difficult to extract the desired symbol [1]. In such cases, near real time environmental variability is to be considered and has advantage in processing. It is also used to select the most appropriate frequency band with minimum power. This is highly useful in a communication link which requires highest possible data rate with less probability of error.

In this paper, estimation of Doppler spread of received CW acoustic signal is presented. This study is carried out by making use of acoustic data collected from an at sea experiment. The advantage of this study is that the methodology used can be used for near real time signal processing.

2. Doppler spread

Doppler spread is a measure of the spectral broadening caused by the temporal rate of change of the underwater acoustic channel [2] while Doppler shift is the frequency shift experienced by a signal when either the transmitter or receiver is in motion. In the case of a static channel, received signal may not have any spectral broadening. However, motion of the ocean medium at the point of scattering of acoustic wave results in frequency spread. This spread depends on the geometry of multipath. The channel spread factor is the product of Doppler and multipath delay spreads which gives a measure of maximum possible bandwidth of a signal, in a channel [3].

Waters off Indian west coast are known for their high temporal and spatial variability

in stratification due to various physical processes and the ocean dynamics [4]. Therefore, significant time and Doppler spreads can be experienced when the sound interacts with moving sea-surface, internal waves and ocean currents.

3. Doppler Spread Estimation

There are many methods [2] to estimate Doppler spread from received signal. In this paper, estimation of Doppler spread based on temporal coherence of the signal [4] is attempted. The signal coherence loss is attributed mainly to sound scattering from moving sea surface and time varying oceanographic processes [6,7].

The acoustic pressure time series of a signal can be represented as …. . … .

where R(t) is the amplitude and φ(t) is

the phase. Temporal coherence (ρ(τ)) of the signal is then

… .

... iii

Where ... iv , denotes the mean intensity and φ’ denotes the mean time derivative of the signal phase. Then the temporal coherence in equation (ii) can be expressed as τ

Nimmi R Nair and Roshen Jacob: Observed Spectral broadening of acoustic pulse in an underwater channel PROCEEDINGS OF SYMPOL-2013

The signal coherence-time is defined as the time by which the signal becomes “uncorrelated” i.e. when the normalized temporal coherence drops to 1/e [8,9].

Therefore

ρ τ 1 gives √2 ... v .

Defining where ν is the Doppler spread. From (v), it can be seen that Doppler spread is inversely proportional to signal coherence time. A large variation in temporal coherence and a good agreement of it with fluctuation index for various frequencies and τ has been reported [10] for western coast of India.

High coherence value of 0.8 is selected for signal coherence-time as the variance of it at 1/e varies significantly. This high coherence value is especially suggested [8] for underwater communication purposes. It is related to Doppler spread by

. .0.11 . … … …

4. Experiment

For the Doppler spread estimation, data recorded during an at sea experiment off the west coast of India are utilized. In this experiment, the acoustic projectors were lowered to a depth of 40m from the ship. The signals were received by a moored standard hydrophone at a depth of 6m from sea surface. The transmitting ship moved along a predetermined track of constant contour of 70m depth. The ship stopped at different ranges and transmitted

CW pulses for 1s. These signals were recorded at 24 k sample rate. In the following section, estimated Doppler spread values for 6 kHz and 9 kHz frequencies are presented.

5. Results and discussions In fig 1, a typical raw time series

(a), filtered time series showing first three pulses (b), zoomed time series of first pulse (c) and spectrum of first pulse showing the frequency spread (d) are presented.

(b)

(a)

(c)

(d)

Nimmi R Nair and Roshen Jacob.: Observed Spectral broadening of acoustic pulse in an underwater channel

Fig 1.Time series of (a) Recorded 6 kHz signal, (b) filtered signal, (c) zoomed first pulse and (d) spectrum of first pulse.

For the Doppler spread estimation, we have used central portion of recorded pulses for a duration of 256 ms. From the estimated coherence, time delays corresponding to a coherence value of 0.8 is picked up. These values are then used to obtain frequency spread using equation (vi). Typical results of estimated Doppler spread for the signals received at 2 km and 10 km ranges are presented in Fig 2.

Fig 2 Estimated Doppler spread at 2 and 10 km ranges as a function of frequency (+ indicates frequency spread and values indicate ranges in Km)

From Fig 2 it can be observed that, for a range increase of 8 km, Doppler spread variation is of the order of 100 Hz for both the frequencies. Pulse distortion and coherence variations are explained as frequency dependent mode stability effects due to channel factors such as the effect of sea bottom and internal waves [11]. In this study, since the source and receiver are

stationary, the spread can be explained in terms of multipath propagation.

To confirm whether the spread is caused by the medium alone, the source level of the transmitted signal was also recorded. It was recorded by lowering a hydrophone at a distance of 1m away from the projector. It is observed that the source spectrum peaked only at the respective frequencies and no broadening is noticed. This confirms that spread at far ranges is solely due to underwater channel. The frequency spreads at various ranges indicate the variability in the medium which depends on respective paths. For practical applications, an average value of the spread can be estimated from a number of received pulses. This estimate then can be used as background information for the selection of processing algorithms.

It may be noted that Doppler spread values are estimated from delay corresponding to high coherence. This type of estimation of signal variation is useful for retrieving signal parameters of interest especially in systems based on coherent processing methods. Conclusion

The propagation of sound in an underwater channel experiences spread in time, frequency, amplitude, pulse shape due to variability and in-homogeneity in the ocean medium observed off the south-west coast of India. The Indian waters are known for their layered structure, unique current pattern and internal waves. The present study carried out using data collected during at sea experiment reveals that the frequency spread caused by the medium is of the order of 100 Hz. Such channel related distortion parameters are required to be considered during selection of transmitting waveform and in adapting processing sequence in a receiver.

In many applications, especially in sensor networks, it is required to estimate

5 6 7 8 9 10Frequency (kHz)

0

50

100

150

200

Dop

pler

Spr

ead

(Hz)

2

2

10

10

10

Nimmi R Nair and Roshen Jacob: Observed Spectral broadening of acoustic pulse in an underwater channel PROCEEDINGS OF SYMPOL-2013

the parameters of multipath components more exactly. With a good propagation model, multipath environment can be simulated. However, to include the effect of medium related parameters for practical applications, choice is always towards the real time monitoring of received signal. The work presented in this paper is a step towards this direction.

ACKNOWLEDGMENT

The authors would like to thank Director, Naval Physical and Oceanographic Laboratory, Kochi, India for the support and guidance.

REFERENCES

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[3] M. Stojanovic, “Acoustic (underwater) communications,” in Encyclopedia of Telecommunications, J. G. Proakis, Ed. John Wiley and Sons, 2003.

[4] Lisan Yu, “Variability of the depth of the 200C isotherm along 60N in the Bay of Bengal: its response to remote and local forcing and its relation to satellite SSH variability”, Deep-Sea Research, 50, 2285, (2003)

[4] T. C. Yang, “Measurements of temporal coherence of sound transmissions through shallow water”, J. Acoust. Soc. Am., 120, 2006

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[9] M. R. Schroeder, ’Frequency- correlation functions of frequency responses in rooms’, J. Acoust. Soc. Am., 34, 1819, 1962

[10] Nimmi R Nair, Roshen Jacob, and Ajaikumar MP; Acoustic Fluctuations in Shallow Water, Proceedings International symposium SYMPOL 2011 , 174

[11] H. A. DeFerrari, J. F. Lynch and A. Newhall,”Temporal coherence of mode arrivals”, J. Acoust. Soc. Am., 124, EL104, 2008

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