htly more foresightful reason would be recognition that the traffic is increasing and that there will be congestion if we do not add relief to the existing large cells. A third rea- son might be to relieve other cells in a congested channel set. For example, consider the small cell D in the upper left of the small cell grid. It may be that its large-cell neigh- bors (B, C, and G) are getting too busy, but the addition of cells may be to free up some of the C channels used by the large C cell SO that the small C cell nearby can have more C channels. While it may be that channel set C cells can only use channel set C channels, a grow- ing system does not guarantee that any C channel can be used in any C cell. Failure to meet the D/R requirement will cause unacceptable interference within the same chan- nel when cell sizes mix. Thus each channel set in a growing cellular system becomes a mathematical exercise to figure out how to assign its individual channels. As the demand continues and as the center of town (near the river) continues to be growth area, we continue to add cells. Figure 27.17 shows a system with three cell sizes because the small cells required even smaller cells to relieve their traffic loads. Since the small cells have been referred to as secondary cells, the even-smaller cells are some- times called tertiary cells. 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. Conventional Reuse Principles Capacity and Quality Principles Figure 27.17 A growing N = 7 system with three cell sizes. 27.7.3 Overlaid cells One technique used to increase the efficiency of cell splitting is to use overlaid cells, cells that share the same base station at the center but cover a smaller area. We do this by adding server groups and logical faces (see Sec. 5.2.3) to existing base stations. The large- and small-radius channels use the same antenna fac