me but the phases be opposite, a 180-degree phase shift, then we get subtraction rather than addition of voltage. In the case of equal level and opposite phase, we get complete cancellation, no signal at all. An important exam- ple of such a cancellation is a signal that takes two paths to the same place (described in Sec. 2.1.4), where one path is half a wavelength longer than the other. If the two paths have similar signal path gains, then received signal is dramatically reduced. When we are out of phase, one plus one is zero. Two signals can be arranged to be orthogonal13 so that they are in phase as often as they are out of phase. The result is that a receiver looking for one of these signals sees no net contribution from the interfering signal. The most common U.S. CDMA system, cdmaOne (previously Qualcomm's IS-95 standard), uses orthogonal codes to create 64 signals that, at least in theory, have no interference with each other. For orthogonal signals, one plus one is one. 2This is one occasion where decibel notation obscures rather than illuminates what is hap- pening: -80 dBm plus -79 dBm plus -75 dBm comes out to -72.5 dBm, but that is not terribly intuitive. 13 The term orthogonal is a sophisticated mathematical term, and we are using it casually here. When we design orthogonal codes in CDMA, the orthogonality of those designs is sophisticated and quite mathematical, as we shall see in Sec. 8.3.2. 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 Engineering Concepts Key Radio Concepts When the phase shift is 90 degrees, then the voltage sum is a power sum, and one plus one is two. In Sec. 1.10 we mentioned the in-phase I and quadrature Q components of a modulated signal, and these are 90 degrees out of phase with each other. Thus, while the total power is their sum, the I and Q components are orthogonal to each other, and a re