In a 3GPP (Third Generation Partnership Project) LTE (Long Term Evolution), as one of the next-generation cellular systems, not only an improvement of peak throughput at a radio terminal but also an improvement of cell edge throughput at a radio terminal are required compared to the conventional system, such as WCDMA (Wideband Code Division Multiple Access).  Thus, the exchange of the load information (Load Information) between radio base stations (eNBs: evolved NodeBs) is supported (Non-Patent Document 1) and a technique for decreasing inter-cell interference, which is using the load information, is now under investigation. 
The load information may include, for example, overload information of uplink interference (Uplink Interference Overload Indication: OI).  This information indicates that uplink interference has become excessive, and may be informed on per a given unit of radio resource, termed a physical resource block (PRB).  The radio base station, thus informed of the OI (Overload Indication), avoids the physical resource block (PRB), reported to exhibit significant interference, as much as possible, in its scheduling, thereby reducing the interference to neighbor cells.  In particular, expectation may be made on improving the throughput in the radio terminal at a cell edge. 
On the other hand, standardization of the LTE-Advanced, in which the functions of LTE are further improved, is currently going on.  Among the functions of the LTE-Advanced, there is a carrier aggregation (Carrier Aggregation: CA) in which a plurality of component carriers (Component Carriers: CCs) are used simultaneously for a data transmission/reception with a radio terminal (user equipment: UE).  The CA represents a function to improve the peak data rate per UE (user equipment).  See Non-Patent Document 2. 
The component carrier (CC) is a basic frequency block necessary to implement communication between the radio base station and the radio terminal in LTE. 
In performing the carrier aggregation (CA), a transport block, which is a data transfer unit from a MAC (Medium Access Control) layer to a PHY (Physical) layer, is transmitted/received using a single component carrier (CC), and signal processing is carried out independently in each of the component carriers (CCs). 
In the carrier aggregation (CA), a plurality of component carriers (CCs), each being of the frequency of, for example, 20 MHz at the maximum, are aggregated together in the PHY layer to implement a broad bandwidth up to, for example, 100 MHz at the maximum, with backward compatibility to LTE being maintained.  In a UE having the LTE function (LTE terminal), each component carrier is a single LTE carrier.  However, in a UE having the LTE-Advanced function (LTE-Advanced terminal), the aggregated bandwidths are used in their entirety.  The information on whether or not the carrier aggregation is to be used, or the information on which component carriers are to be aggregated, is provided as individual control information or as system information to the UE having the LTE-Advanced function.  In the downlink multiple access system, multiple carrier transmission by OFDM (Orthogonal Frequency Division Multiplexing) is performed in each component carrier. 
In the investigation into the carrier aggregation (CA), there have been defined three types of the component carriers (CCs) to be aggregated as follows:
a backward compatible carrier (Backward compatible carrier: abbreviated to ‘BC’;
a non-backward compatible carrier (Non-backward compatible carrier: abbreviated to ‘NBC’; and
an extension carrier (Extension carrier: abbreviated to ‘EC’. 
The BC is a CC that can be accessed by both a UE equipped only with the LTE function and a UE equipped with the LTE-Advanced function. 
The NBC is a CC that cannot be accessed by a UE not equipped with the LTE-Advanced function and that can be accessed only by a UE equipped with the LTE-Advanced function. 
The EC is a CC that can be accessed only by a UE equipped with the LTE-Advanced function and that is under a constraint that it is to be used as part of a plurality of component carriers (CCs) to be aggregated, i.e., that it cannot be used as a stand-alone CC. 
At the present time, discussions are now going on about the basic concept of aggregating (CA aggregating) these different types of the component carriers (CCs). 
In these discussions, it is being investigated that, in aggregating (CA aggregating) these different types of the component carriers (CCs), solely the BC (or BC and NBC) has an individual cell ID (PCI: Physical Cell ID and/or ECGI: EUTRAN (Evolved—UMTS Terrestrial Radio Access Network) Cell Global ID), while the NBC and EC (or only EC) lack in the individual cell ID. 
The reason that not the NBC or EC but the BC has the individual cell ID is that even the UE of LTE that does not aggregate component carriers is enabled to access the BC.  It is noted that there are cases wherein the NBC or EC does not have the individual cell ID, such as PCI, and has allocated thereto a cell ID (PCI) of a BC aggregated as a temporary or pseudo cell ID. 
This approach is thought to be effective from the viewpoint that a plurality of component carriers (CCs) aggregated together when carrier aggregation (CA) is performed can be collectively grasped as a cell and a UE can be managed in association with a single cell ID in the same way as when carrier aggregation (CA) is not performed. 
Non-Patent Document 1:
    3GPP TS36.  423v860    (Internet <URL>http: www.3gpp.org/ftp/Specs/html-info/36423.htm)Non-Patent Document 2:    3GPP TR36.  814v100    (Internet <URL>http: www.3gpp.org/ftp/Specs/html-info/36814.htm)Non-Patent Document 3:    3GPP TS36.  331v860    (Internet <URL>http: www.3gpp.org/ftp/Specs/html-info/36331.htm)