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

12Planning and OptimisingRadio Links12.1 Path Profile PredictionThe most basic tool available to the radio planner is the path profile prediction. Most ofthe more sophisticated tools used by the planner are extensions of this; for example,coverage predictions are only path profile predictions carried out for a large number ofpaths.The basic path profile is shown in Figure 12.1. The terrain is shown. In this case, noclutter is shown on the profile, but some tools will show clutter as a coloured overlay ontop of the terrain. In all cases, the profile should be along the great circle path betweenthe two points and the effective Earth radius bulge must also be included for the pathunder consideration.To this path profile, we can apply a suitable propagation model, in this case ITU-RP.526, to determine the additional loss due to diffraction. This is illustrated inFigure 12.2, where a transmit antenna has been added at the beginning of the link anda receiver added at the end. The black line shows the result of applying string theoryto the path. In this case, there is double diffraction over two separate obstructions.We ned to consider the factors that need to be considered when computing thepower leaving the transmit antenna and arriving at the receiver. A typical process isshown in Table 12.1.An example of a link budget created to determine the minimum working input powerto the antenna is shown in Table 12.2.Without including the additional loss factors, a planner may conclude erroneouslythat it is acceptable to simply subtract the EIRP from the required input power into thereceiver, giving a value of 144 dB maximum allowable path loss. This would of coursebe wildly inaccurate and would lead to major problems on the ground. The differenceCommunications. Radar and Electronic Warfare© 201 1 John Wiley & Sons. LtdAdnan Graham