ion is nearly impossible, each receiver rake fil- ter can take advantage of the different radio paths to distinguish its own signal from the other calls. A reverse pilot has no economy of scale, so cdmaOne does not use one at all, and it uses a noncoherent modulation scheme. The lack of synchronization makes the virtual cancellation of interference using orthogonal codes not practical. However, the same or- thogonal codes can be used as a noncoherent maximal-distance spreader. On the positive side, each path has its own multipath characteristics, its own delay- spread graph. A rake receiver optimized for one path is not going to give interferers the same advantage. This relative advantage is only enough to combat the negative effects of multipath, but this is better than the forward direction. 13 We wouldn't want to sit down and do the analysis of the power-control and rake-filter compo- nents for six simultaneous signals from four base stations. 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 Key Radio Concepts Interferer Effect Chip Displacement Figure 8.9 The advantage of nonsynchronized chips. There is another advantage from the lack of synchronization in the reverse direction. The statistical advantage of processing gain is that nonorthogonal interfering PN codes look like noise. In the synchronized case, where the chips line up perfectly, the inter- fering PN code contributes some virtual noise level. Consider the case where the inter- ferer is offset exactly half a chip. When two consecutive interfering chips are the same, the contribution is no less than the synchronized case, but when consecutive interfer- ing chips are different, the contribution goes to zero. A perfectly staggered interferer loses half its impact, 3 dB, from this statistical effect. Let us assume that this effect is linear in the phase offset so