out of power balance. These are not the measurement errors in the last paragraph but rather the message errors in the power-control messages themselves. Again, large numbers average out the errors, but the power-control process becomes slower when the error rate is high. If the measurement part is perfect-that is, if the receiver always makes the right power change determination-then a high rate of power-control messages is supposed to be more energy efficient. We could send a small number of large-change messages with enough energy per power-control bit to ensure a highly accurate power-control process. Or we could send a large number of small-change messages with less energy per bit. The power-control process would be less accurate bit by bit, but the larger num- ber of attempts more than makes up the difference. It is tempting to visualize a system with a thousand power-control messages per sec- ond maintaining exact power level and tracking the small boosts and fades of a tele- phone moving at 60 mi/h (100 km/h). The power control in CDMA does keep control of power, but this kind of accuracy is not part of the picture. There are significant capac- ity losses due to power-control errors in CDMA. If we found that power control were not a problem, then we would reduce the power allotment for power control and increase the capacity of the system in this way. 8.6 Varying Data Rates The cdmaOne channel has voice activity detection (VAD) and can use lower data rates when there is less activity. The speech coder uses a full 9600 bits per second when re- quired but can back off to 4800, 2400, or even 1200 bits per second when there is less 2All the 3G CDMA systems use this dual-loop fast power-control structure in both forward and reverse directions. cdmaOne uses it in the reverse direction and relies on its orthogonal codes and a slower power-control scheme in the forward direction. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies