 the signals together. There is significant loss in a combiner, about 6 dB, SO three-quarters of the amplifier's output goes to heat the combiner. Still, ten 4-W amplifiers may be easier to come by than one 100-W monster. There is another argument in favor of the combiner solution. FM, BPSK, QPSK, and m-ary modulation schemes maintain the same wave amplitude over time; we call these constant-envelope modulations. Building a constant-envelope amplifier is far simpler than building an amplifier that follows varying amplitude. To vary amplitude over time, the amplifier has to provide a proportional or linear amplification of its input sig- nal, whereas a constant-envelope amplifier simply can swing from its minimum to its maximum voltage following the input phase instant by instant. Constant-envelope am- plifiers are not only simpler; they also are more efficient electrically. Losing 6 dB in the combiner mitigates some of this efficiency, but the design efficiency is still a compelling argument in favor of using constant-envelope amplifiers with combiners instead of us- ing linear amplifiers. On the other hand, as linear amplifier technology has improved, we have moved away from the combiner solution. The combiner works best when the signals are narrow bands Since these radio waves typically look like sine waves, there is an extra factor of 2 in the peak- to-average ratio calculation. We are deliberately ignoring the sine wave factor of 2 to make the point that combining signals creates its own peak-to-average ratio problems in power amplifier design. 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 Base Station The Base Station Figure 4.4 Minimal clipping causes minimal distortion. of specified frequencies and does not work SO well for broadband solutions such as CDMA or even TDMA. There is another way to deal with the peak-to-average power