Page 268 - Integrated Wireless Propagation Models

246 C h a p t e r F o u r

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measurements carried out in the metropolitan areas of the United States,3 -37 where the
street orientation is very much a grid-like structure.
4.5.4. 1 Propagation Mechanisms in N L OS
In LOS, when the LOS path in a microcell is blocked, signal energy can propagate from
the base to the mobile via mainly three mechanisms:

• Diffraction over building rooftops
• Diffraction around building edges
• Reflection and scattering around walls and the ground

Figure 4.5.4. . 1 shows the plan view of buildings arranged in a regular grid struc­
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ture.38 Path A shows a building edge created by the motion of a mobile across the
shadow boundary. Because this building is isolated, diffraction in the shadow region
takes place, and the signal strength drops very rapidly with increasing distance. The
other path to reach the mobile from the base station is through path B. The diffraction
path propagates through the building roof, no reflected signal exists, and the rooftop­
diffracted signal begins to dominate due to the large number of diffractions and causes
interference between co-channel microcells. Path C describes the reflection path of the
radio wave. In path C, the building is now surrounded by other buildings, which act as
reflecting surfaces. In this case, the reflected ray is much stronger than the diffracted
ray, so the signal strength remains strong over much longer distances.
In Fig. 4.5.4.1.1, there are many different paths. The short paths A and B are single
reflections or diffractions and are main sources of signal strength. The long path C after
four sequential reflections becomes very weak at the spot, and the rooftop-diffracted
path D then dominates. This variation in propagation mechanism with distance is an
additional parameter in the two path loss slopes in the empirical models.
The resultant coverage area in NLOS microcells generally is broadly diamond
shaped, as shown in Fig. 4.5.4. . 2. From the measurement, the curved boundaries of
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signal strength forming the diamonds indicate that the dominant mechanism of propa­
gation into side streets is diffraction rather than reflection.39
In more realistic environments, where buildings are not regular in size, advanced
planning techniques must be applied, particularly when frequencies are shared between
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microcell and macrocell layers.4 4

Diffraction #2 over the top

Diffraction #1 over the edge

FIGURE 4.5.4.1.1 Street geometry where diffraction dominates.