Format of a downlink control channel for a user equipment UE is changed in conjunction with switching the UE between being enabled for cross carrier scheduling and being disabled for cross carrier scheduling. The changed format is used on at least one serving cell of the UE's configured set of component carriers CCs. In one embodiment the changed format comprises an explicit carrier indicator field CIF for the case in which the switching is from disabled to enabled; and the changed format has no explicit CIF when the switching is from enabled to disabled. In another embodiment switching from disabled to enabled comprises the UE's configured set of CCs being changed from one to multiple cells; and switching from enabled to disabled comprises the UE's configured set of CCs being changed from multiple cells to one cell. In one embodiment the changed format is implicit.