 requirement as the number of channels gets larger. The fraction of the time the instantaneous power is much larger than the average power is very small. No matter how many simultaneous channels we are trying to send, a peak-to-average value of 10:1 delivers a clean wave- 99.85 percent of the time.5 5 Being willing to live with 0.15 percent clipping makes the design and cost of linear amplifiers more reasonable. Figure 4.4 illustrates the dis- tortion caused by clipping. We talk of the average power but not the average voltage. In almost any radio or au- dio system, the voltage swings high and low about equally, and the average voltage is zero. However, even the average absolute value of the voltage is not a meaningful mea- sure of anything useful. Because voltage changes as the square root of power, we want the voltage that reflects the average power. This measure is the square root of its average squared value, which we shorten to root mean square (RMS). We speak of peak and average power, and we speak of peak and RMS voltage. 6 5For the mathematically curious, we got this value by assuming that the sum of a large num- ber of radio channels is a normal distribution in voltage and calculated that V10 standard devi- ations are exceeded 0.078 percent of the time on the high side and undercut 0.078 percent of the time on the low side. 6In the world of audio equipment, it is common to see a specification for amplifier power in RMS watts, which is technical sounding but wrong. What they are measuring is the average power in watts that could be derived from RMS voltage. 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 Key Radio Concepts 4.4 The Radio Receiver The radio receiver is typically a separate piece of equipment for each voice channel in the cellular system. These are called voice radios or modems in digital radio. Often t