UNDER WATER ACOUSTIC CHANNEL

PRESENTED BY SAROJ KUMARRIGVENDRA KUMARR VARDHANM.TECH ECEPONDICHERRY UNIVERSITY

Why Underwater?

The Earth is a water planetAbout 2/3 of the Earth covered by oceans•Uninhabited, largely unexplored•A huge amount of (natural) resources to discover

Many potential applicationsLong-term aquatic monitoring•Oceanography, marine biology, deep-sea archaeology, seismic predictions, pollution detection, oil/gas field monitoring …

Short-term aquatic exploration•Underwater natural resource discovery, hurricane disaster recovery, anti-submarine mission, loss treasure discovery …

Underwater acoustic communication is a technique of sending and receiving message below water.

There are several ways of employing such communication but the most common is using hydrophones. 

Under water communication is difficult due to factors like multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges.

 In underwater communication there are low data rates compared to terrestrial communication, since underwater communication uses acoustic waves instead of electromagnetic waves.

underwater video? Real-time

Compression to reduce bit rate needed for video representation

High-level modulation to increase the bit rate supported by acoustic channel

?

 

Underwater image transmission: sequence of images (JPEG) at < 1 frame/secMPEG-4 : 64 kbps (video conferencing)

Can we achieve 100 kbps over an acoustic channel?

depth

tx

distancec

Deep sound channeling:-rays bend repeatedly towards the depth at which the sound speed is minimal-sound can travel over long distances in this manner (no reflection loss).

Deep water: a ray, launched at some angle, bends towards

the region of lower sound speed (Snell’s law). Continuous application of Snell’s law ray diagram

(trace).

Shallow water: reflections at surface have little loss; reflection loss at bottom depends on the type (sand,rock, etc.), angle of incidence, frequency.

Multipath gets attenuated because of repeated reflection loss, increased path length.

tx rx

Length of each path can be calculated from geometry:lp: pth path length

τp= lp /c: pth path delay

Ap=A(lp,f): pth path attenuation

Γp: pth path reflection coefficient Gp= Γp/Ap

1/2: path gain

Mechanisms of multipath formation

SCATTERING

VARIATION IN SPPED OF SOUND

continental shelf (~100 m)

continental slice

continental rise

abyssal

plain

land sea

surf shallow deep depth

c

surface layer (mixing)

const. temperature (except under ice)

main thermocline

temperature decreases rapidly

deep ocean

constant temperature (4 deg. C)

pressure increases

Sound speed increases with temperature, pressure, salinity.

Communication channel / summaryPhysical constraints of acoustic

propagation:• limited, range-dependent bandwidth• time-varying multipath • low speed of sound (1500 m/s)

System constraints:• transducer bandwidth• battery power• half-duplex

Worst of both radio worlds (land mobile / satellite)

tt(1±v/c)ff(1±v/c)

A(d,f)~dka(f)d

N(f)~Kf-b

B>1/Tmp

frequency-selective fading

Underwater applications

• Seismic monitoring.• Pollution monitoring• Ocean currents monitoring• Equipment monitoring and control• Autonomous Underwater Vehicles (AUV)

To make these applications viable, there is a need to enable underwater communications among underwater devices -> Wireless underwater networking

Use sound as the wireless communication medium.