When LTE carrier aggregation is used, it is necessary to be able to schedule the data across the carriers and to inform the terminal of the DCI rates for the different component carriers. This information may be implicit, or it may be explicit dependent upon whether cross carrier scheduling is used.
Enabling of the cross carrier scheduling is achieved individually via the RRC signalling on a per component carrier basis or a per terminal basis.
When no cross carrier scheduling is arranged, the downlink scheduling assignments achieved on a per
carrier basis, i.e. they are valid for the component carrier on which they were transmitted.
For the uplink, an association is created between one downlink component carrier and an uplink component carrier. In this way when uplink grants are sent the terminal or UE will know to which uplink component carrier they apply.
Where cross carrier scheduling is active, the PDSCH on the downlink or the PUSCH on the uplink is transmitted on an associate component carrier other than the PDCCH, the carrier indicator in the PDCCH provides the information about the component carrier used for the PDSCH or PUSCH.
It is necessary to be able to indicate to which component carrier in any aggregation scheme a grant relates. To facilitate this, component carriers are numbered. The primary component carrier is numbered zero, for all instances, and the different secondary component carriers are assigned a unique number through the UE specific RRC signalling. This means that even if the terminal or user equipment and the base station, eNodeB may have different understandings of the component carrier numbering during reconfiguration, transmissions on the primary component carrier can be scheduled.