rrier. An MS operating in multi-carrier aggregation mode is assigned the same STID for the primary and secondary carriers. The MS in this mode of operation is required to monitor all active carriers and to follow resource allocations. The data communication in the carrier aggregation mode may be continued during the handover procedure. As shown in Figure 11-10, the primary carrier of an MS can be changed without changing the MAC layer security and mobility contexts. A multi-carrier-enabled MS is required to support the primary carrier change. The BS may instruct the MS, through the AAI_CM-CMD message on the current primary carrier, to change its primary carrier to one of the assigned fully-configured carriers as a result of load balancing or varying channel conditions. For an MS supporting basic multi-carrier mode, the target primary carrier may be one of the available fully-configured carriers within the same cell. When an MS receives the AAI_CM-CMD MAC control message, it transmits the AAI_MSG-ACK message, and subsequently disconnects from the present carrier and switches to the target fully-configured carrier within the action time specified by the BS. If the MS supports carrier aggregation mode and the target carrier is one of its active secondary carriers, it may receive data and control signals on the target carrier immediately after switching; otherwise, the MS first reconfigures its hardware setting and then switches to the target carrier. The MS may be required to perform periodic ranging with the target carrier. Given that a common MAC entity manages both serving and target primary carriers, network re-entry with the target primary carrier is not required. For a multi-carrier-enabled MS, the physical carrier indices of the current and target primary carriers are swapped after the primary carrier change process. The MS may spontaneously or by instruction scan other assigned carriers which are not serving it. In that case, the MS reports the scanning results to the serving BS, which may be used by the BS 