This document discusses underwater acoustic communication and some of the challenges. It describes how about 2/3 of the Earth is covered in oceans, leaving a huge amount of natural resources to potentially discover through underwater exploration and monitoring. However, underwater acoustic communication is difficult due to factors like multipath propagation, time variations of the channel, small available bandwidth, and strong signal attenuation over long ranges. It also provides examples of potential underwater applications that could benefit from improving underwater acoustic communication technologies.

1. UNDER WATER
ACOUSTIC CHANNEL
PRESENTED BY
SAROJ KUMAR
RIGVENDRA KUMARR VARDHAN
M.TECH ECE
PONDICHERRY UNIVERSITY

3. Why Underwater?
The Earth is a water planet
About 2/3 of the Earth covered by oceans
•Uninhabited, largely unexplored
•A huge amount of (natural) resources to discover
Many potential applications
Long-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 …

4. 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.

6. underwater video? Real-time
Underwater image transmission: sequence of images (JPEG) at < 1
frame/sec
MPEG-4 : 64 kbps (video conferencing)
Can we achieve 100 kbps over an acoustic channel?
Compression to
reduce bit rate
needed for video
representation
High-level
modulation to
increase the bit
rate supported by
acoustic channel
?

8. depth
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).
c distance
tx
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).
Shallow water: reflections at surface have little loss;
reflection loss at bottom depends on the type
(sand,rock, etc.), angle of incidence, frequency.
tx rx
Multipath gets attenuated because of
repeated reflection loss, increased path length.
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

9. SCATTERING

10. VARIATION IN SPPED OF SOUND
continental shelf (~100 m)
continental slice
continental rise
abyssal
plain
land sea
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.
depth surf shallow deep

11. Communication channel / summary
Physical constraints of acoustic
propagation:
• limited, range-dependent bandwidth
• time-varying multipath
• low speed of sound (1500 m/s)
Worst of both radio
worlds
(land mobile / satellite)
System constraints:
• transducer bandwidth
• battery power
• half-duplex
A(d,f)~dka(f)d
N(f)~Kf-b
tt(1±v/c)
ff(1±v/c)
B>1/Tmp
frequency-selective
fading

12. 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.