may not be large, but even a few percent improvement in capacity should be modeled. 28.2.5 User terminal movement (Doppler) The motion of the user terminal creates Doppler distortion, as described in Sec. 2.1.5. The Doppler distortion contaminates the signal and adds its own noise factor. Unlike multipath, however, there is no rake filter to screen out the distortion. Furthermore, the motion of a user terminal changes the multipath delay spread, so the rake filter is constantly catching up. The effect of a moving user terminal is to reduce the value of a in both forward and reverse directions. The faster the motion, the greater is the effect on CDMA capacity. The third-generation (3G) CDMA carriers with their higher chip rates are more susceptible to this distortion from pedestrian or vehicular movement. 28.2.6 Time division duplex (TDD) The equations presented earlier are separate for forward and reverse links. The as- sumption throughout is that forward radio paths interfere only with other forward 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. CDMA Principles for Multicellular Systems Capacity and Quality Principles paths and that reverse paths interfere only with other reverse paths. In time division duplex (TDD), base stations and user terminals are broadcasting radio in the same spectrum. This is not a problem in one cell because base stations and user terminals are time-synchronized. If neighboring cells do not follow the exact same pattern of forward and reverse timing, then radiation from one base station during its forward phase will interfere with the reception at another base station during its reverse phase. Similarly, user ter- minals transmitting reverse signals will interfere with other user terminals receiving forward signals. To avoid this cross-interference, all the cells have to be time- synchronized, and all have to use