hat the mobile telephone has played no part in the handoff search process. While some of this may be attributed to a "we know best" attitude on the part of base station designers, there is also a more practical issue. The mobile telephone has a two-way radio completely tied up with the duplex transmission of the full-time FM signal. Maybe a third radio circuit could have been designed into the mobile telephone, or maybe it could have been designed to break the audio link for one-tenth of a second (the duration of a blank and burst message) to measure other channels. Whatever could have been done, however, the AMPS system design has the community of base stations making all the power-control and handoff decisions. If there is nowhere to hand the call off and the call is below some yet-lower in- terference protection threshold (as we called it at AT&T), then the call is dropped. This threshold is not designed to protect subscribers against their own bad calls, since we have SAT to do that. The idea of the interference-protection threshold is to protect the rest of the system against a mobile telephone that has wandered so far away from its serving cell that it presents an unacceptable interference hazard to other cells. After all the local measurements, the neighbor cell measurements, the determination that a handoff is appropriate, and the selection of a new radio is made, the base station finally directs the mobile telephone to change frequency. The new base station starts transmitting, the current base station sends a handoff message, and the new base sta- tion looks for SAT. The system tries its handoff message up to three times. Handoffs can get lost in a variety of ways. The mobile telephone may change to a channel and find no signal, SAT can be lost, or the signal level of the new connection may be below the new cell's interference-protection threshold. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved