Communications, Radar & Electronic Warfare (201.. - Index of

Planning Radio Networks for Coverage 239Flat terrain exampleTerrain limiting exampleFigure 13.8Effects of changing antenna heights in non-terrain limited and terram lImited conditlom ..has been moved to a more central location. However, in practice, such things are rarelyso straightforward and the situation shown where the wanted service area is fairly closeto the coverage area is rare. In general, the design will be far more complex.The effect of antenna tilt is illustrated in Figure 13.10 for a directional antenna on flatterrain. When the antenna tilt is increased, the coverage in the direction of the mainbeam is foreshortened in this case. This will depend on the vertical pattern grazingangle, which must be taken into account allow with the antenna vertical polar diagram.When correctly engineered, the coverage can be adjusted to provide coverage in thewanted area and to limit coverage beyond it. This helps both with interference (andjamming) vulnerability and anti-detection.The effects of adaptive modulation on range are illustrated in Figure 13.11 for aWiMax system, with all data rates representative of typical rates (but not exact). Thekey paral!leter that affects the range performance is the Signal to Noise Ratio (SNR).The relative ranges are also indicative only, but show the principle that accepting alower modulation scheme, with concomitant reduction in required SNR provideslonger coverage at the expense of data rate.The principles of modifying parameters to optimise system performance are fairlystraightforward, but applying them can require either planner experience or carefuldesign of automated tools. However, such optimisation is a necessary part of modemnetwork design or mission planning.13.3 Limiting CoverageWe have identified some methods of reducing coverage, but now we will look at somereasons to do so and in a little more depth at how this can be achieved in practice, with