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Radio Parameters Analysis and Modeling

In an initial design phase, a simple one-slope model and low-resolution ter- rain data suffice for a rough estimate to qualify customers. As operations progress, actual measurements should be compared to predictions and the process is refined.
For instance, an initial selection process leads to the chart on Figure 5.2. Actual measurements show the right trend, but some variations are very large (sometimes in excess of 20 dB). Better modeling and drive testing should be considered in this case.^∗
During trials, a received signal strength indicator (RSSI), in dBm, is logged at all customer locations. A plot of RSSI as a function of the logarithm of dis- tance is graphed in Figure 5.3. The logarithmic scale for the distance is simply justified by the fact that a one-slope model will show a linear approxima- tion on the graph. Many propagation studies use this scale since it allows for easy comparison of path loss exponents. The variations in RSSI for a given customer location are represented by error bars at each point. Each error bar represents a standard deviation; that is, the total width of the error bar shows two standard deviations.
The next step in data analysis is a comparison between the data set and typical models. For that comparison, a path loss estimate should be derived from the empirical system. The RSSI measurement provides one term of the path loss. The other is in the transmitted power level, which depends on base


^∗ The linear approximation of scatter plot in Figure 5.2 does not cross the axis at zero; the line is offset by almost 5 dB due to some fixed system differences between actual and measured values. The slope of the line is 1 as it should be.


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Actual RSSI (dBm)
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Predicted RSSI (dBm)

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