ownloaded 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 28.3.4 Multipath Multipath has an amplifying effect on interference from the same radio source. The rake filter allows the receiver to see all the power of the transmitted signal even when the radio path spreads that signal over several chips. The price is time sidelobes, as de- scribed in Sec. 8.4, an extra interfering component generating in the receiver. We will let Y be the multipath factor, the time sidelobe power divided by the desired signal power in the rake filter. Multipath, with the rake filter compensating for it, does in a bad way exactly what orthogonal codes do in a good way. The multiple paths in the forward direction increase the effect of same-sector interferers, whereas orthogonal codes reduce their effect. The effect of multipath in the forward direction is to add Y interference for each ra- dio signal in the same cell sector, so Mbb = a + - (1 + y) In the orthogonal case, the good signals no longer interfere with each other, and we are left with only the mul- tipath interference, so Mbb yob. 28.4 Applying the Equations to the Reverse Direction In cdmaOne, the reverse direction does not have orthogonal codes. In 3G systems that have reverse pilots, orthogonal codes are used, but the system has to support separate pilots for each user terminal. These are disadvantages for the reverse channel as com- pared with the forward channel. However, the reverse channel has advantages as well: soft handoff and the fact that multipath interference is coming from different locations rather than from a source co-located with the desired signal. 28.4.1 Reverse pilots and signaling The reverse pilot is extra radio power for each user terminal using the wireless tele- phone system. This means extra bits, extra informatio