ute the total amplitude of all the chips. The first 10 One decibel down is 80 percent. 11 1.5 dB is 1.4 and -1.5 dB is 0.71, a 30 percent reduction. 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. The CDMA Principle Key Radio Concepts sum is the estimate of signal plus interference, and the second sum estimates signal af- ter processing gain. If the second divided by the first is low, then we increase trans- mitter power. When the ratio is high, we decrease power. Both these techniques rely on statistical averaging for their success. Under normal conditions, they point in the right direction just over half the time. It might take a hun- dred measurements and messages to get power levels reliably within 1.0 dB of optimal. Whereas AMPS sends power-control messages every 5 seconds, cdmaOne sends almost 1000 power-control messages each second. Even at this high rate, a moving user ter- minal in a fading environment poses a power-control challenge. Power control in CDMA uses all three methods. Open-loop power control is used to set initial power in the reverse direction, and closed-loop power control is used to main- tain optimal levels. Closed-loop power control has an inner loop that measures the Eb/No directly and sends power-change messages in 1-dB steps. There is an outer loop that measures the frame error rate (FER) and adjusts the target Eb/No for the inner loop. 12 There is an interesting tradeoff in power control. The energy efficiency of power con- trol has a significant effect on total signal capacity. The power-control messages have to travel between the system and user terminal somehow, and this takes more power and costs CDMA capacity. We can send nearly perfect power-control messages by de- voting a lot of power to power control. Or we can economize on power-control power, have a lot of power-control errors, and live with a system 