nt of maximum power, then there is significant interference coming from the same signal at different times. For a simplified example, suppose that the transmitter is sending a sequence of chips A, B, C, D, Consider three paths of equal strength, 2, 4, and 5 chip periods in length, as shown in Fig. 8.6. For example, when the E chip is transmitted, it arrives three separate times. The first time the E chip arrives at the same time as the second We would like to say, "Consider the typical delay spread graph," but radio paths vary SO much that there really isn't a typical radio environment. 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 Chips Figure 8.5 Delay spread graph with chips shown. path of C and the third path of B, so each received chip is really the sum of three chips sent at three different times, B + C + E. The second time the E chip arrives, the three- chip total received is D + E + G. And the third time the E chip arrives, the three-chip total is E + F + H. A single receiver synchronizes with one of the three paths and sees two-thirds noise no matter which of the three paths it chooses. Suppose that it picks the shortest path and looks for B, C, and D from B + C + E, C + D + F, and D + E + G. The other two paths are time-shifted so that the receiver treats them as uncorrelated noise; they may as well be other CDMA telephone calls. This comes across as a factor of 3 loss of S/N or Eb/No, a 5-dB loss. The best solution is to add the three received signals in reverse order of their delay. We call this reverse-order summation a rake filter or, in more engineering-oriented dis- Received A+B+D B+C+E C+D+F D+E+G E+F+H F+G+I Figure 8.6 Transmit and received signals with three radio paths. Downloaded from Digital Engineering Library @ McGraw-Hil (www.digitalengineeringlibrary.com) Copyright © 2004 The 