5G methods and architectures to determine active SCells and their bandwidth parts (BWP) used in carrier aggregation (CA) are disclosed in which activating a BWP for an active SCell is performed initially according to a default value provided in initial radio resource control (RRC) messaging. After initialization, SCells and the SCell BWPs used by the user equipment (UE) are activated by RRC messaging, downlink control information (DCI) or dedicated medium access control (MAC) control elements (MAC CEs) with the initial BWP used for an activated SCell being a default value provided during initialization.