Page 409 - Integrated Wireless Propagation Models

T h e l e e C o m p r e h e n s i v e M o d e l - I n t e g r a t i o n o f t h e T h r e e l e e M o d e l s 387

These characteristics make underwater wireless communication extremely difficult.
To solve these difficulties, two tools can be used. One is the underwater propagation
path-loss model created by Urick, and another is the propagation speed of the acoustic
signal underwater.

2
6.7. . 1 U r ick Propagation Path-Loss Model
One underwater propagation model, called the Urick propagation path-loss model, is
introduced:
L (d, f ) = � · log (d) + a ( j ) . d + A (6.7.2.1)

where � is the geometric spreading, � = 10 (shallow water); and � = 20 (deep water);
a (j) is medium absorption obtained from the experiments; and A is transmission
anomaly in dB a degradation of acoustic intensity due to multipath refraction, diffrac­

A
tion, and scattering of the sound; = 5 to 10 dB [deep water]; > 10 dB [shallow water].

A
6. 7 .2.2 Propagation Speed of Acoustic Signal U n derwater
The propagation speed of the acoustic signal (5) underwater is a function of three
parameters the temperature T, the salinity (Sa), and the depth of the water (D):
j
5 = ( T, Sa, D) (6.7.2.2)
Because of the three effects, the signals bend toward the region of slower sound speed.
There are four main types of thermal structure: isothermal gradient (uniformed), nega­
tive gradient (sound speed increases as water becomes shallower), positive gradient
(sound speed increases as water becomes deeper), and negative gradient over position
(change speed gradient at a certain depth). The four types of thermal structure are
shown in Fig. 6.7.2.2.1 due to the effects of temperature, salinity , and the depth of the
water, as shown in Eq. (6.7.2.1).
6. 7 .2.3 Shallow-Water Communication-Channel Model
In shallow water, multi path occurs due to signal reflection from the surface and bottom,
.
as illustrated in Fig. 6.7.2.3 1 . Shallow-water communication is greatly affected by the
multipaths due to multiple reflected rays from either the sea bottom or the surface, ray
bending as a result of sound speed variations with respect to depth of the water and
cylindrical spreading to expand the wave front horizontally. The shadow water

Water surface

I
I
(1) I \ I (1 ) Isothermal
.<: / (2) \ (3) / ( 4)
a. \ (2) Negative gradient
Q) I
0 I (
I (3) Positive gradient
I \ \
I \ \ (4) Negative gradient over positive
I \
I \
Seabed \
Sound velocity
FIGURE 6.7.2.2.1 Fo r main types of thermal structure for the sound speed under deep water.
u