Page 410 - Integrated Wireless Propagation Models

388 C h a p t e r S i x

Water surface

(2)

FIGURE 6.7.2.3.1 Three paths conduct shallow-water communication.

communication is performed through three paths, as shown in Fig. 6.7.2.3.1; (1) direct
path; (2) reflection from the surface, and (3) reflection from the bottom as-well as from
other objects underwater.
The shallow-water communication channel is modeled as a multiray Rayleigh fad­
ing channel. Each of the three paths is modeled as a Rayleigh fading channel. The com­
position of three rays at the receiver forms a shallow water channel model.4 2

6. 7 .2 .4 Deep-Water Communication
In deep water, the bending of acoustic waves occurs, that is, the tendency of acoustic
waves to travel along the axis of lowest sound speed. Figure 6.7.2.4.1 shows an ensem­
ble of channel responses obtained in deep water. The multipath spread, measured
along the delay axis, is on the order of 10 ms in this example. The channel response
varies in time. The deep-water propagation channel is not as affected by multipaths
but due to the spherical spreading of the acoustic signal. The four main types of ther­
mal structure of the sound wave are the unique characteristics for the deep-water com­
munications, as shown in Fig. 6.7.2.2.1. The path of the acoustic signals underwater is
also unique, as shown in Fig. 6.7.2.4.1. The Urick propagation path-loss formula
expressed in Eq. (6.7.2.1) is used to evaluate the communication performance. The
sound signal in deep water also exhibits randomness in the propagation waves and is
also modeled using the Rayleigh fading model.43

6. 7.3 Prediction of Aeronautical Communication Signal
The aeronautical mobile radio channel appears in many applications. Air ground radio
communication between aircraft and ground radio sites is one of the most important
applications. In this section, a stochastic model is proposed.45-5 0
In the aeronautical VHF band, ionospheric effects usually can be neglected. Aero­
nautical communication is a different medium than terrestrial mobile medium. There
are two kinds of interferences. One is interferential propagation due to the direct and
reflected signal, and the other is diffraction and scattering. Sometimes, these two inter­
ferences are mixed.
The transmission is carried by troposcatters; and those small, random irregulari­
ties or fluctuations in the refractive index of the atmosphere dominate. The refrac­
tion index changes with seasons and also varies according to altitude and decreases