addition to the altitude. Once we have the bins laid out, we estab- lish the locations of the base stations we are studying. We need not only their (x,y) ordinates on the grid but also their heights, so we have three-dimensional (x, y, z) co- ordinates for each base station antenna set. The combination of (x, y) bins and base station locations is shown in Fig. 47.3. The terrain database contains a lot of information. Smaller bins give greater preci- sion in propagation studies, but a smaller bin size means a lot more bin data. Consider a service area that is 100 km by 100 km, a typical U.S. urban area and its suburbs. we use a fairly course bin size of 200 m, then we have to keep track of 250,000 eleva- tion values. This is not a large database by computer memory standards, but it can be a large amount of elevation data to obtain. If we move to a finer grid of 50 m, then we have 4,000,000 bins. Elevation data are comparatively easy to get in the United States, where we have the U.S. National Geological Survey (USGS). The USGS has a complete digitized record elevation data for the entire United States. Worldwide, terrain elevation data can be harder to get. Some governments are concerned about letting private companies have Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Propagation Models Modeling for CDMA Figure 47.3 Dividing the service area into (x, y) bins. access to their (x, y) altitude data for reasons of national security. For those countries, we sometimes have to resort to satellite imagery to construct a three-dimensional pic- ture of a wireless telephone service area. For each (x, y) bin, we generate a path to each base station, as shown in Fig. 47.4. Each of these paths represents a radio path between a base station and a bin. Once we under- stand the radio conditions from every base station to every bin, we can begin to analyze 