crowave and satellite engineers worry about echo. 6 In Sec. 8.8 we point out that this synchronization makes it economical to send a strong pilot signal that the receiver can use for coherent demodulation. Figure 8.3 Multiple radio paths. 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 The CDMA Principle Time usec Figure 8.4 Delay spread graph. The nanosecond differences in path length are on the order of meters, a few wave- lengths of microwave radio. The effect of multiple paths this close together is Rayleigh fading, local dead spots about a wavelength in size. Thus there will be a bad call place, about human hand size, usually right where you want to make a call. Spread spectrum gives us significant protection against Rayleigh fading because the dead spots are dif- ferent at different frequencies, and we are broadcasting our signal across a wide fre- quency band. When delay spread is in nanoseconds, our signal is unaffected by chip confusion. Even the highest chipping rates used in third-generation (3G) CDMA are no faster than 5 Mchips per second, and a 5 Mchip per second rate is 200 ns per chip, 60 m. The cdmaOne CDMA standard uses chips five times this long. It is the chip-sized delay differences that concern us in CDMA. Except for the Rayleigh fading, double images a microsecond apart are not a major issue in analog ra- dio, but chip-sized delay spread in CDMA means that a receiver picks up two different chips at the exact same time. Consider the delay spread graph shown in Fig. 8.4. 7 This is a plot of the radio power received as a function of time delay after it is transmitted. If we sent a single, instan- taneous pulse of energy, then this would be the energy over time at the receiver. In Fig. 8.5 we show the same delay spread in the timeline of CDMA chips. If our CDMA re- ceiver locks onto the signal at the mome