Inter-frequency hard handoff and establishing the hard handoff environment
A method for determining the execution time and handoff direction of a hard handoff by a base station controller in a code division multiple access (CDMA) system. The execution time and handoff direction is determined for a mobile station moving between two base station transceiver having different frequency allocations (Fas). The service region of the CDMA system is divided as individual cells where each cell further comprises one or more sectors (sub-cells). Sub-cells are either common frequency sub-cells or border frequency sub-cells. Common frequency sub-cells are defined as those sub-cells for which the common frequencies of the neighboring sub-cells are all in use. Correspondingly, a border frequency sub-cells defines a sub-cell for which the common frequencies of the neighboring sub-cells are not all in use. The method of the present invention generally comprises the steps of: receiving a pilot strength measurement message (PSMM) from the mobile station by the base station controller (BSC), the PSMM including a plurality of pilot signals each having a pilot signal strength; upon receiving the PSMM by the BSC, determining whether all sub-cells with which the mobile station is currently in communication are border frequency sub-cells; if it least one sub-cell is determined not to be a border frequency sub-cell, executing a general handoff; if all sub-cells are determined to be border frequency sub-cells, periodically ordering the PSMM from all sub-cells with which the mobile station is currently in communication by the BSC; analyzing a received PSMM transmitted from the mobile station in response to the PSMM order issued by the BSC to determine whether the pilot signal strengths received from all sub-cells are lower than a predetermined power level, T_DOWN, and activating a timer set to a predetermined time, T_TDOWN if said pilot signal strengths received from all sub-cells are lower than a predetermined power level; if the timer expires without interruption during the time interval T_TDOWN ordering the mobile station to execute an intra-cell inter-frequency hard handoff to a common frequency that all base transceiver stations service.