ies using ray-tracing models, and the computation effort has been enormous. A single wall or floor can have multiple combinations of re- flection and refraction at the two sides where concrete meets air. Radio waves can bounce around corners in many ways, and all of these have to be calculated and added up. Snell's law says that the angle of incidence 0 equals the angle of reflection OR and that the an- gle of transmission OT satisfies the equation sin 0 = VE sin OT, where E is the change in the speed of transmission called the index of refraction. The index of refraction can depend on the frequency of the radio or light wave. The fading characteristic in a wireless system may be different in the forward and reverse di- rections. This is so because the same collection of radio paths may have phase differences that combine at one frequency and cancel at another. However, the combination of multiple paths in the two directions is the same. 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 Propagation Models To conduct a full three-dimensional ray-tracing study of an office building requires a huge database of every floor and wall. To do it right requires having all the internal struc- tures of the building with their three-dimensional coordinates. This includes elevator shafts, metal reinforcements for concrete walls and floors, pipes, and electrical fittings. Because of the complexity of the models and the vast data requirements for accurate prediction of radio propagation, we only use ray tracing for critical evaluations. Micro- cells used in areas with a lot of tall buildings are a good candidate for ray tracing. Picocells delivering cdma2000 1x EV-DO service require high radio path gain with min- imum multipath to achieve very high data rates. It may be worthwhile to do a full ray- tracing analysis to ensure that these rad