Method and apparatus for coordinating transmission of short messages with hard handoff searches in a wireless communications system

#I. Field of the Invention

#The present invention relates to communications systems. More particularly, the present invention relates to a method and apparatus for performing hard handoff between different wireless communication systems.

#II. Description of the Related Art

#The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

#FIG. 1 is a schematic overview of a spread spectrum CDMA communications system in accordance with the present invention;

#FIG. 2 illustrates the amount of time that the mobile station receiver will be tuned to an alternative frequency;

#The following is a detailed description of a method and apparatus that includes embodiments of the present invention. FIG. 1 depicts a communication system in which mobile station 5 is actively communicating with a fixed communication system on a forward link 12 and reverse link 14 through an “origination” base station 10. Origination base station 10 is part of an “origination” system and transmits and receives information over forward link 12 and reverse link 14, respectively, on a first frequency, ƒ1. Mobile station 5 is shown to be traveling from the origination system into a “destination” system which transmits and receives information on a second frequency, ƒ2. The destination system includes “destination” base stations 20 and 22, which are not in active communication with mobile station 5. However, pilot signals (hereafter referred to simply as “pilots”) from destination base stations 20 and 22 can be received by mobile station 5 if mobile station 5 is tuned to frequency ƒ2. Both the origination and destination systems are part of the fixed communication system that allows the mobile station to communicate with other communication devices, such as conventional telephones wired to the public switch telephone network or other wireless communication devices. It should be understood that the fixed communication system may comprise any device or combination of devices that provides wireless communication between the mobile system and other communication devices.

#In accordance with one example of the method and apparatus disclosed herein, the mobile station is triggered to tune to an alternate frequency. For example, the origination base station 10 may use mobile station 5 to perform a mobile station assisted interfrequency hard handoff. One example of a mobile assisted interfrequency hard handoff is disclosed in U.S. patent application entitled “METHOD AND APPARATUS FOR PERFORMING MOBILE ASSISTED HARD HANDOFF BETWEEN COMMUNICATION SYSTEMS”, Ser. No. 08/816,746, filed Feb. 18, 1997, now U.S. Pat. No. 5,999,816, filed Dec. 7, 1999, assigned to the assignee of the present invention. In such mobile assisted interfrequency hard handoffs, the origination base station 10 transmits a “Tune_Message” to mobile station 5. The Tune Message directs the mobile station to tune to an alternate frequency, ƒ2 in this case, and to search for a set of available pilots, the pilots of destination base stations 20 and 22, for example.

#Alternatively, other events may trigger the mobile station to search for hard handoff candidates. For example, the mobile may detect a signal, such as a beacon signal being transmitted by base stations within another system. Such a beacon may be transmitted within the frequency band being monitored by the mobile station. The beacon indicates to the mobile that a hard handoff candidate may be nearby. In response, the mobile will tune to an alternate frequency that is associated with the detected signal.