nal re- ceiver would allow each transmitter to send half the power, so the total effect would be the same factor of 2 as we have in the reverse direction. If the two soft-handoff signals are no better than two separate calls, then the total effect of having one call on two cells is four times worse than a single call near the base station. The true effect of soft hand- off is somewhere between the ideal and the worst case. When a call is equidistant from three cells, the reverse-direction cost of the call is three times that of a call close to the base station, and the forward cost with soft handoff is somewhere between three and nine times. Still, soft handoff is more efficient than trying to maintain the correct CDMA carrier through a sequence of hard handoffs. Every cell layout has boundaries between cells, so avoiding two-cell coverage zones is impossible. And every cell layout has to have some place where those cell boundaries come together in triple-points, so avoiding three-cell coverage zones is impossible. These two- and three-cell zones are shown in Fig. 32.1. It is the zones with four or more nearly equal cells that we are trying to avoid. In such zones, each CDMA channel is interfering with too many carriers, and user termi- nals are offered too many choices. In the planning stage, we look at the fourth-pilot delta, p1 - P4, the term used for the difference between the strongest pilot P1 and the fourth-strongest pilot P4. When P1 - p4 is low, we have an area where four or more cells are competing for each CDMA channel. The term used for this is pilot pollution, al- though the pollution goes beyond the pilot to the CDMA call channels. A valley is an area covered evenly by many cells, an area with pilot-pollution prob- lems. The ring of six cells in Fig. 32.2 creates a severe valley in the middle, a six-cell coverage zone. Not only is the fourth-pilot delta too small in this area, the sixth-pilot Figure 32.1 Regular cell layout with two- and three-cell regions. Downloaded from Digital Engineering Librar