Cellular telecommunications handoff between cellular systems

Cellular systems obtain high spectrum efficiency by reusing frequency pairs within the system. This is accomplished through the use of a grid of low-power transceivers physically deployed throughout the system in groups of channels called cells. Since the coverage area of each cell is relatively small, it is often necessary to hand off a call in progress from one cell to another. Present cellular systems have the capability to hand off a call between cells controlled by that system, but do not have the capability to hand off a call to a cell controlled by another system. This capability will become more important as cellular systems proliferate and the need to provide continuous service across system borders increases. Today, the procedure used by cellular systems to accomplish handoff is not standardized, but rather reflects the design approach of each manufacturer. The basic principles are, however, common to all system designs. This paper describes a procedure which embraces the common principles of handoff, adapted for use between systems whose internal procedures differ. The procedure is intended to be as simple as is consistent with effective operation, and can be readily implemented in existing cellular systems. Three basic functions essential to a successful inter-system handoff are described in this paper. First, a simple, architecture independent method is described for requesting and returning location measurement information. Second, procedures are provided for negotiating a voice path between systems for use by a particular handoff event. Third, procedures are provided for coordinating the transfer of the subscriber´s transceiver from the initial cell site to the final cell site.