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 PSTN and Telephone Switching The PSTN and Telephone Switching of range of base station A and is served only by base station B, we can say, in casual usage, that a soft handoff has occurred. In soft handoff, there is no change of radio frequency; multiple base stations are serv- ing the call on the same CDMA carrier throughout the process. 11.4.6 Softer handoff Softer handoff is a handoff between faces of a single base station. It is managed entirely at the base station, without the MSC participating at all. The antennas on the two faces deliver their signals to the base station controller (BSC), which puts the two sig- nals through a rake filter to create a combined signal. The rake filtering at the base station does its calculations at the chip level, which is what makes softer handoff different from soft handoff, where the MSC makes its selection frame by frame, not chip by chip. 14 Like soft handoff, softer handoff involves no change of CDMA carrier frequency. 11.4.7 Hard handoff and semisoft handoff In CDMA, hard handoff is used in three situations where soft handoff is impossible. They are When a call leaves an area serviced by CDMA, and the call is switched to another service. When a call goes to a new cell, and the call's carrier frequency is already busy on the new serving base station. When the timing of the two base stations cannot be coordinated. Some CDMA systems use a form of hard handoff called semisoft handoff. 15 In hard handoff, the second link to the MSC is set up after the second receiving base station es- tablishes communication with the user terminal. This creates dead space in the middle of the call. In semisoft handoff, the link between the MSC and the new serving base station is established before the switchover of the radio link service to the new base sta- tion. The switchover 