 power-control measurements. 8.7 Soft and Softer Handoff CDMA uses a different kind of handoff, a soft handoff, where a user terminal is served by two base station antennas at the same time. The need for extremely accurate power control in CDMA requires a more sophisticated coordination among serving base sta- tions than the break-and-make handoffs used in conventional reuse cellular systems. As we will see in Sec. 26.1, it is important that a CDMA call be served by the right cell sector, the one with the highest path gain. When CDMA calls are served by the wrong cell sector, even momentarily, the overall capacity of the system is reduced dra- matically. Radio signal paths vary enough from place to place that a mobile telephone near a cell boundary might change its choice of the right cell dozens of times per minute. Rather than execute dozens of handoffs (a ping-pong handoff situation) when radio signal paths are close, CDMA shares the call with both cell sectors. Unlike AMPS and GSM, cdmaOne CDMA systems let the mobile telephone deter- mine when a handoff should occur. When the mobile telephone detects a pilot from a new antenna face B, it informs its primary server A, and a link is established between A and B. The new antenna face begins transmitting, so A and B are both sending the same data. The base stations and mobile telephone coordinate their PN codes to make this work. Let us take a moment and examine how a two-to-one link can work. Consider two base station antennas A and B serving a single mobile telephone. In the forward direction we have two separate copies of the same message coming from A and B. The mobile telephone resolves the two signals using the rake filter that it uses to resolve multiple radio paths, as described above in Sec. 8.4. When the mobile telephone evaluates the 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. 