ers explained three-sector directive cells. They actually defined the cell as the region surrounded by three base stations; these were called corner-excited cells. As HCMTS evolved into AMPS, engineers realized that center-excited cells were an easier way to view the cell sector plan. 11 Since 9 X 42 = 144, this is 21.5 dB. Downloaded from Digital Engineering Library @ 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. Conventional Reuse Principles Capacity and Quality Principles Figure 27.10 The three-sector plan removes two-thirds of interferers. 27.6 Power Control Suppose that we could narrow the received signal distribution, the S curve in Fig. 27.3. Even if the average signal remains unchanged, making the distribution less variable ought to keep more calls above the interference level. We can narrow the signal distri- bution by having the base station measure its received signal to control the transmit power of both the base station and the mobile telephone. The transmit power is boosted when the radio path gain is low and cut back down again when the radio path gain is high. This has the effect of reducing the variability of the received signal. The coverage of a power-controlled system is as good as that of a system running at full power. When the user terminal is in a fringe area with poor radio coverage, the base station detects a weak signal, boosts its own transmit power, and directs the user terminal to do the same. Thus power control gives the benefit of S/I management with- out sacrificing service in poor radio environments with low path gain. If all calls are power-controlled, then the average S/I ratio should stay the same. However, it is not the average that we care about but the tenth percentile, the calls at the bottom of the performance range. The reduced variability of the received signal should keep more calls out of the low S/I ratio zone. And it does, to a point. The