me equipment from some vendor makes it hard to catalog the user terminal power levels, then we can monitor the power-control messages going out on the forward channels. If they are consistently more often up than down, then there are user termi- nals that have reached their limits. This is not a gentle cry for help; this is a CDMA system demanding capacity relief right now. There is an easier measurement we can make of reverse congestion on the CDMA carrier. We can look at the total received power at the base station over time. The curve Fig. 28.1 shows us how the received power starts to climb as we get close to CDMA capacity. We can monitor the base station receiver power level over time to determine reverse-link performance. So what levels do we use? In the forward direction, we could relate the transmit power to the maximum amplifier power, and in the reverse direction, we can compare the received signal to the receiver noise. If the total signal power is close to the noise level, then we are getting close to overload. And when the signal power is two or three times the noise level, we are critically close. The carrier is getting overloaded when the total signal of all the channels rises to the point of being just equal to the noise level. This sounds a bit counterintuitive. Shouldn't we be happy when our signal-to-noise (S/N) ratio is high? The answer is that the CDMA channel is designed to operate with signal less than noise, typically far less than noise. CDMA gets high S/N ratios through processing gain, but the physical channel is mostly noise. This is true even when there are many users, many channels on the same carrier. 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. Measuring System Performance for Growth Increasing Capacity The fraction of the time the base station receiver is getting more CDMA signal power than noise is an app