shrinks because neighboring cells are sharing its load. A neighboring cell is a signifi- cant interferer in a CDMA system. The radio overlap between two cells forms the zone where those two cells interfere most with each other. Where two cells have nearly equal radio paths, calls in either cell consume CDMA radio resources in both, more in the for- ward direction because of soft handoff. 1The grid mentality can become inflexible. A carrier deploying an 8-mile (13-km) cell grid fought a long zoning battle for a site in a resistant neighborhood when there was an industrial park less than 2 km away where a site would have been easy to get. 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. Adding Cells to a CDMA System Adding Cells to a CDMA System As neighboring cells get closer to a base station, we want to reduce the coverage so that there is less overlap with neighboring cells. We want to find a way to keep the ra- dio signal path gain high throughout each cell-sector area and to have it drop off sharply outside the area. There is not much we can do about sector-to-sector interference as a system grows. Even in its large-cell coverage form, a base station has greater capacity if its antenna radiation patterns have well-defined edges SO that the boundaries are sharp between sectors of the same cell. Downtilting the antennas, however, seems like the best way to reduce a cell radius. As shown in Fig. 33.1, downtilting sector antennas simultaneously increases the path gain for near calls and decreases the path gain for more distant calls. Further- more, the radius of the cell is adjustable simply by changing the tilt angle of the antennas. We can evaluate the positive effect of downtilting antennas on CDMA capacity using the predictive planning models described in Sec. 48.1. These computer programs should quantify the positive effects of changing 