s on the map without a con- sistent plan is likely to create the four-cell areas shown in Fig. 32.3. As we said earlier, these four-cell areas need not be exact junctions of cell boundaries, but they are areas where four cells are nearly equal in radio signal path. One way to maintain large fourth-delta values, to avoid valleys, is to maintain the hexagonal layout that we used in the conventional-reuse systems (FDMA and TDMA). We can picture a four-cell zone as two triple-points that have been allowed to drift to- gether by deforming the cell grid. Keeping the grid regular keeps the triple-points from drifting together into four- and five-cell zones, as we saw in Fig. 32.1. A regular hexagonal grid pattern of cell placement not only keeps valleys away, but it also makes handoff lists easier to manage. We have a clear visual picture of the neighbors, and the visual neighbors are likely to be the best handoff neighbors as well. In addition to easing the administrative task of assigning neighbors, the regular cell grid also keeps the neighbor lists shorter. In laying out cells for CDMA, we want to maintain the local character of the hexag- onal cell grid. It is not so important that distant cells follow the local pattern. 1 Thus 1In conventional reuse, the primary interferer is several cells away, and we rely on the grid spacing to maintain D/R, as shown in Fig. 27.7. We do not have that kind of distant reuse issue in CDMA. 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. Planning Locations for Base Stations Planning Locations for Base Stations Figure 32.3 Random cell layout has four-cell areas. the cell grid small, can nearby clusters of cells follow a local grid pattern. An to avoid pilot be modified to follow the contours of geography or subscriber example demand of a as warped long as grid is shown in Fig. 32.4. This warped grid is regular e