Page 408 - Integrated Wireless Propagation Models

386 C h a p t e r S i x

6.7. 1.1.3 The Overall Path·Loss Formula The overall path loss can be obtained from
three approaches:
1. The overall path loss is the sum of two sources. One is from the path loss caused
by the atmosphere condition, and the other is the path loss to be predicted from
the causes of terrestrial surroundings:
L overall = L + L terrestrial in dB (6.7. . 6)
1
tot
where L , rrestri l can be predicted based on models described in previous chapters
e
a
of this book.
2. We also can calculate the mean square of the received signal r by taking the
measured data r(t) to process to get the measured local mean m'(t) as follows:
1 r + L
I
m'(t) = 2L r-L r(t) dt (6.7.1.7)

1
L
where the length i s around 40 A (see Sec. . 6.3.1). The path loss Lovemu is obtained
by as subtracting the local mean square from the transmitted ERP, Ps, as
'
.
Lovemll = P, - [m (t)F (6.7 1 .8)
'
3. We can calculate the mean power P , from the measured pdf, p (r), as
2 '
1
� = [ r p (r) dr (6.7. . 9)
Then the overall path loss is

L overall = P - s P r (6.7.1.10)

We have noted that the channel models described in Sec. 6.7 1 . 1 . 2 are not considered
.
.
the atmosphere effects as described in Sec. 6.7 1 . 1 . 1 . Therefore, the signal strength
obtained from the predicted pdf of the models has to add the additional atmosphere
effect to be the overall path loss.
7
6. . 2 Prediction of n derwater Communication Signals42-44
U
The electromagnetic waves propagate over only extremely short distances under water.
The underwater signals that are used to carry digital information through an underwa­
ter channel are not radio signals but acoustic waves, which can propagate over long
distances. However, an underwater acoustic channel presents three distinguishing
characteristics: frequency-dependent propagation loss, severe multipath, and low
speed of sound propagation.
Another unique characteristic is geometric spreading. The spreading of the sound
energy results in the expansion of the wave front. There are two spreadings:
1 . One is cylindrical spreading. It is at horizontal radiation only, which characterizes
shallow-water communication.
2. Another is spherical spreading. It is from an omnidirectional point source,
which characterizes deep-water communication.