 power control that is reducing the signal variability accomplishes this by chang- ing the transmit power. This change reduces variability because it is deliberately correlated, negatively, with the radio path gain between the base station and the user 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 Figure 27.11 The three-sector plan for N = 7. terminal. However, the interfering radio path typically is not correlated with the sig- nal radio path, SO power control increases the variability of the interference and changes the distributions from Fig. 27.4 to Fig. 27.12. The initial effect of power control is positive. The reduction in signal variation out- weighs the increase in interference variation and the tenth percentile S/I ratio de- creases. As we increase the transmit power changes to narrow the received signal dis- tribution further, the increase in interference variability does more harm than good, as shown in Fig. 27.13. Under ideal measurement conditions, the optimal power control removes half the decibel variation, SO we should change both forward and reverse transmit power by 4 dB to compensate for an 8-dB change of radio signal path. Since the base station measurement equipment is not perfect, the ratio should be reduced. In a simulation study for AMPS, one of us (Rosenberg) found that the best solution was to change transmit power by 4 dB for a 12-dB change in the radio signal path. We expect the same optimal performance for GSM. 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 Received Power dBm Figure 27.12 Signal and interference with 