Page 111 - Integrated Wireless Propagation Models

I
M a c r o c e I P r e d i c t i o n M o d e I s - P a r t 1 : A r e a - t o - A r e a M o d e I s 89
1 0 t- � � -- � � .� :: -- ==== � � � � -�-� .� .: .� . � .. � .� . � .=�1 � :;;= == � �

�
9 / a = 2
8 �----�--------------��------�
1.5
7
6 1.0
"{ "" 5
4 0.5
3
2 0

5 1 0 1 5 20 25 30 35 40 45 50
N u mber of edges n

FIGURE 2.16.2 Variation of attenuation with number of edges - w = 150 A.

The field A" (a) becomes independent of further increases in n, as can be seen from
Fig. 2.16.2. From the results of numerical simulations Walfisch proposed the approxi­
mate function

for n > > 1 (2.16.6)

where I A I is the attenuation relative to the free space field strength. This function sug­
gests a propagation loss dependence of approximately 38 dB I decade of range, which is
in a close agreement with many reported measurements. This function is I A I is only an
approximate, however, and is not a function of n.
The overall path loss from the flat-edge model is a summation of three losses: the
free space loss, the final building diffraction loss, and the reflections from the buildings
across the street using the Ikegami model, shown as follows:
(2.16.7)
where L is the free space loss, L is given in Eq. (2.16.1), and L"(t) can be found from
F5
E
Eq. (2 1 6.3).
.
This formulation is extremely simple to compute and applies for any values of
angle a, even when the base station antenna height is below the rooftop level. The num­
ber of buildings can be increased to extremely high values without difficulty.

2.17 ITU Model
It is often difficult to evaluate the best model for a given application from the many
described in this chapter. Recommendations produced by the ITU are a good reference
source in this situation, as they summarize in simple form some recommended proce­
dures, as shown in Table 2.17.1.
The ITU's recommendations may not always represent the most accurate model for
a given case, but they have the benefit of being widely accepted and used for coordina­
tion and comparison purposes.
We'll focus on ITU-R P.1546 and ITU-R P.530 in this section. ITU-R P.1141 is more
focused on microcells and will be discussed in Chap. 4.