used in both cells in Fig. 27.20. The large cell on the left has an S/I distribution based on D/R1, whereas the small cell on the right has an more favorable S/I distribution based on D/R2. We can make the two distributions more equitable by re- ducing the power level for that channel on the smaller cell. In doing this, we spend some of the surplus performance on the small cell to improve the performance of the larger cell. The technique of setting average power levels for cochannel cells is called power bal- ancing, and it is useful even when cells are the same size. When one cell is atop a hill Figure 27.20 Different cell sizes using the same channel. 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 Conventional Reuse Principles and another is in a valley, we can have an asymmetric interference environment that power balancing can compensate for. When dozens of cells are using the same channel, the power-balancing problem can be a sophisticated exercise in mathematical optimization. Each channel should be power balanced in a wireless telephone system as different channels are used in dif- ferent cells. If one channel is used in 20 cells and another is used in the same 20 and one more cell, then their power-balancing calculations are different. Usually the usage patterns of several channels are the same, so there may be 10 or 20 separate power- balancing calculations to be done for one system. This power balancing is not the same as the power control described in Sec. 27.6. Power balancing is a one-time setting of system parameters, whereas power control is dynamic, moment-by-moment adjustment of power levels depending on radio condi- tions at the time. Power balancing sets the average power levels to improve the aver- age S/I ratios among cochannel cells, whereas the power-control algorithm improves the low-end