al is another call's interfer- ence, one mathematical model's subject matter is another model's random variation. A distance- propagation model sees urban effects as "random." 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 CDMA rake filter mitigates much of the multipath damage, but we are considerably better off with only one radio path than with multiple paths and a rake filter, even if the rake filter is doing a perfect job. When the delay-spread picture changes rapidly, the rake filter has to work hard to keep up, and multipath takes a greater toll. Calls inside office buildings aggravate all these factors. Walls and windows create rapid changes in the radio path, both in intensity and in multiplicity. Inadequate facil- ities based on optimistic estimates lead to high blocking, lost calls, and poor voice qual- ity. The mad scramble to repair the damage by adding sectors and cells leads to blame, finger pointing, and recrimination between the engineers and the financial planners. realistic plan will take these factors into account so that there are few surprises later on. Good tools help us form a realistic plan. 47.3 Distance Models Back in Sec. 1.7 we discussed the inverse fourth power propagation rule. Radio in free space decreases in power with the square (second power) of distance, but radio on the surface of the earth decreases at a faster rate than this. 3 Based on measurements made around 1973, Bell Telephone Laboratories estimated an exponent of 3.84, which we feel can be rounded off safely to the fourth power in Eq. (47.1): p = = (47.1) This was for one frequency, 900 MHz, in one city, Philadelphia. This formula is ade- quate for some cellular planning, but the next level of sophistication is specific to fre- quency and environment. 47.3.1 The Hata-Okumura model The next level of sophisticat