nce of 6 km. 4The same caveat applies here. Fading is not a true autoregressive process in frequency or space, but it is close enough for us to treat it like one. This has nothing to do with how long it takes the signal to travel from transmitter to receiver, only to the variation in that travel time. 6Video is a different story, and many readers may remember an urban childhood where broad- cast channels had ghosts in the picture from multipath reception. The left-to-right time in Amer- ican NTSC television is 64 us, quite short enough for a typical 1-us delay to show up as a second image, usually fainter, on our television sets. 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. Radio Signal Quality Radio Signal Quality The high speed that makes CDMA attractive makes it vulnerable to multipath delay spread. If we consider a wideband CDMA system with an internal bit rate of 5 megabits per second (Mbps), then each digital bit is only 200 ns. Smearing the reception over 1 us means that each digital bit is received over five time periods. When the delay spread is greater than the length of time we use to transmit a single symbol, then delayed transmission of one symbol can interfere with reception of the lowing symbols. We have traded signal path fading for intersymbol interference when the delay spread is long enough and our symbol rate is high enough that alternative paths of our own symbols interfere with each other. In CDMA, we have specialized rake filters (described in Sec. 8.4) to deal with this problem. These adaptive matched filters create an inverse image of the signal delay characteristics. The rake filter allows a CDMA receiver to resolve symbols shorter than the delay spread. 2.1.5 Doppler shift Any source of radiated energy that is moving toward or away from the receiver creates a frequency change called the Doppler effect. This is e