d CDMA (TD-SCDMA), is far more complicated than the frequency division duplex (FDD) used in other CDMA standards. In an FDD system, the interfering paths in the forward di- rection are from other antenna faces to the user terminal, and the interfering paths in the reverse direction are from other user terminals to the antenna face. There are no interfering radio paths from one base station to another base station or from one user terminal to another user terminal. In a TDD system, there can be interference from one base station to another base sta- tion. One antenna face may be transmitting in its forward direction, whereas another antenna face is receiving in its reverse direction. This adds additional paths to the in- terference calculations, but the total number of base-station-to-base-station paths is not very great. The interesting part of TDD interference analysis is when one user terminal is trans- mitting in its reverse direction while another is receiving in its forward direction. Now we have a radio environment where any user terminal can interfere with any other user terminal in a different cell. If the sectors of each cell are not synchronized, then interference may be possible between user terminals in different sectors of the same cell. This is an enormous increase in the amount of calculation required because now any (x, y) bin can interfere with any other (x, y) bin. 48.3 Simulation Models Computer simulations are powerful tools for diagnosing system problems. We break a system down into basic functions and reproduce those functions in a computer pro- gram. The simulation technology allows us two liberties we cannot use in a real setting. We can monitor internal aspects in a computer program that would be hard to measure 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. Subscriber Traffic Modeling Modeling for CDMA in real life