Page 406 - Integrated Wireless Propagation Models

384 C h a p t e r S i x

The second approach considers all attenuation effects being partially uncorrelated;
therefore, RMS summing is used for the total path loss:
1
=
L101 [A�, + A�,o + A � + A� + A�L + A�J (6.7.1.2)
A combination method reflects the independence of various attenuation factors and
better considers some of the propagation effects uncorrelated.31 2
•3 The corresponding
ITU-R recommendation adopts the following formula for total path loss:

(6.7. . 3)
1

6.7. 1 . 1.2 Channel Models-Long-Term and Short-term Fading Due to the Effects of
Surroundings The channel models for the satellite communication channel, the
LMS channel (land-mobile-satellite channel), are statistical models, that can be
characterized into two categories: single state and multistate models.33 The single­
state models are described by single statistical distributions and are valid for fixed
satellite scenarios where the channel statistics remain constant over the areas of
interest. The multistate or mixture models are used to demonstrate non-stationary
conditions where channel statistics vary significantly over large areas for particu­
lar time intervals in nonuniform environments.34

1 . Single-State Model Loo Model:35 The Loo model is one of the most primitive statistical
LMS channel models with applications for rural environments specifically with shadow­
ing due to roadside trees. In this model, the shadowing attenuation affecting the LOS
signal due to foliage is characterized by log-normal pdf, and the diffuse multipath com­
ponents are described by Rayleigh pdf. The model illustrates the statistics of the channel
in terms of probability density and cumulative distribution functions under the assump­
tion that foliage not only attenuates but also scatters radio waves. The resulting complex
signal enveloper is the sum of correlated lognormal and Rayleigh processes. The pdf of
the received signal envelope (r) is given by Loo and Butterworth:36
p

p (r) = 1 1 [ (In r -1-lf], for r » j(i;
exp -
r�2na 0 2cr 0 for r « j(i; (6.7.1.4)
r ( , )
2
exp -
ba 2b o
where 11 and cr are the mean and standard deviation, respectively. The parameter b 0
0
denotes the average scattered power due to multipath effects. If attenuation due to
shadowing (lognormal distribution) is kept constant then the pdf in Eq. (6.7.1.4) simply
yields a Rician distribution.
There are other models. The Corraza-Vatalaro modeP7 has modeled effects of
shadowing on both the LOS and diffuse components; the extended Suzuki modeP8 is a
statistical channel model for terrestrial communications characterized by Rayleigh and
lognormal process; the Xie-Fang modeP9 is based on propagation scattering theory
and deals with the statistical modeling of propagation characteristics in low-earth­
orbit and middle-earth-orbit satellites communication systems; and the Abdi modeP3