METHOD AND APPARATUS FOR EFFECTING HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATIONS SYSTEMS

A method of effecting handoff of a mobile station from a first base station in a first cellular communications system controlled by a first mobile switching control station to a second base station in a second, different cellular system controlled by a second mobile switching control station is described. The method comprises generating for the mobile station a cipher key for use by the mobile station during communication in the second cellular communications system. The cipher key is generated by the mobile station from a private key assigned to the mobile station for the second cellular communications system and from a random number generated by the second cellular communications system. The cipher key is then communicated to the first mobile system and a private long code is generated for use by the mobile station during communication in the first cellular communications system.