Page 115 - Integrated Wireless Propagation Models

M a c r o c e l l P r e d i c t i o n M o d e l s - P a r t 1 : A r e a - t o - A r e a M o d e l s 93

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Normalized clearance h!F1
B: theoretical knife-edge loss curve
0: theoretical smooth spherical Earth loss curve,
at 6.5 GHz and k e= 4/3
A d : empirical diffraction loss based on equation
(2) for intermediate terrain
h: amount by which the radio path clears the Earth's
for intermediate terrain
F 1 : radius of the first Fresnel zone
FIGURE 2.17.2.1 Diffraction loss for obstructed LOS radio pathsY

where h is the height difference (m) between most significant path blockage and the
h
path trajectory. The height difference i s negative if the top of the obstruction of interest
is above the virtual line of sight.
F1 is the radius of the first Fresnel ellipsoid given by

F1 = 17.3�d��2 m (2.17.2.2)

where f = frequency (GHz), d p ath length (km), and d1 and d 2 = distances (km) from
=
the two terminals to the path obstruction.
1
A curve, referred to as A d , based on Eq. (2.17.2.1), is also shown in Fig. 2.17.2. . This
curve, strictly valid for losses larger than 15 dB, has been extrapolated up to 6-dB losses.
Figure 2.17.2.1 shows three plots of the expected diffraction loss due to terrain
roughness versus the normalized terrain clearance. Curve B is the theoretical knife­
edge diffraction curve. Curve D is the theoretical smooth-earth loss at 6.5 GHz using a
4/3 earth radius. Curve A d is the ITU terrain loss model over intermediate terrain. Each
of these curves represents the excess terrain loss beyond the free space loss.
The ratio h/F1 is the normalized terrain clearance. When the terrain blocks the line
of sight, h/ F1 < 0. This model is generally considered valid for losses about 15 dB, but it