e height hm is 1 to 10 m. The distance r is 1 to 20 km. These ranges are neither guarantees of validity nor absolute boundaries. The model will be valid to the extent that your neighborhood looks like the parts of Tokyo where these measurements were taken. And these predictions are not instantly useless at 1501 MHz. Using this model at 1800 MHz, for example, will be less valid than using it at 900 MHz, but it may be better than a simple inverse fourth law rule. This is a statistical-fit model. The various terms and coefficients chosen here do not represent components of radio signal path reality. We tend to prefer models where each term represents some physical aspect or relationship, but the Hata-Okumura model is not such a formula. It represents a good mathematical estimate of a great deal of dili- gent measurement effort and should be taken and used in this light. For example, we would be pleasantly surprised if the term log2 (f/28) represents some component of the radio signal path process. It is simply a term in a statistical formula fit. 5The astute reader will notice that there is a gap in the large-city model from 200 to 400 MHz. This is not a typographic error; rather, this model "does not know what to do" around 300 MHz in large cities. 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 47.3.2 The Walfisch-Ikegami model The next level of radio signal path model sophistication is the Walfisch-Ikegami model, often called the COST 231 model. 6 This model takes into account several factors not in- cluded in the Hata-Okumura model. The path loss is broken up into three components: Lf = the free-space loss in decibels Lrts = the rooftop-to-street diffraction and scatter loss in decibels Lms = multiscreen loss in decibels. The average path loss in decibels L is the total in Eq. (47.4): (47.4) L = Lf The f