y @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hil Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Planning Locations for Base Stations Planning Locations for Base Stations Six-Cell Valley Figure 32.2 A cell layout with a six-cell valley. delta is small as well. The pilot-pollution problem is not that the pilots are equal; it is that several radio signal paths are nearly equal; that is, they are within a few decibels of being equal. Thus the valley is a zone of difficult CDMA capacity performance, a problem area even if there is no single point where all the competing base stations are exactly equal in radio path gain. The worst valley is the most likely to occur, a valley with low radio path gain. This means that four or more cells are competing for service not only with each other but also with the thermal noise floor of the receiver. A call from such an area can use the resources of 20 or more calls from near the base station. And the low radio path does not have to be visually obvious. We can picture a subscriber equidistant from five base stations making calls in a wooded hollow (a geologic rather than an electromagnetic valley). However, the same radio phenomenon can happen deep in the twentieth floor of a centrally located office building. While it is important to avoid valleys in the cell layout, it is more important to avoid having subscriber demand in the valleys. If six cells circle a lake, then there is a valley on the map, but only a few boating enthusiasts are likely to be making cellular phone calls from there. It is the combination of radio conditions and user demand that creates CDMA capacity problems. It can be just as fruitful to adjust the cell layout for sparsely populated valleys as it is to eliminate valleys altogether. 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 