In order to improve the deterioration of communication quality due to drastic increases in mobile traffic in recent years and achieve faster communication, the standardization of Carrier Aggregation (CA) functions that enable a radio terminal (User Equipment (UE)) to communicate with a radio base station (eNode B (eNB)) by using a plurality of cells has been undertaken in the 3GPP Long Term Evolution (LTE). Note that the cells that a UE (User Equipment) can use in CA are limited to a plurality of cells of one eNB (i.e., a plurality of cells served by one eNB).
The cells that are used by a UE in CA are categorized into a Primary Cell (PCell) that has already been used as a serving cell when the CA is started and a Secondary Cell(s) (SCell(s)) that is used in addition to the PCell or in dependence thereon. Each SCell can be used by a UE as the need arises, and the use of them can be stopped. Note that starting the use of an SCell is called “activating” or “activation”. Similarly, stopping the use of an SCell is called “deactivating” or “deactivation”. Non-Access Stratum (NAS) mobility information, security information (security input) and the like are transmitted and received through a PCell during radio connection establishment (RRC connection Establishment/Re-establishment) (see Non-patent Literature 1). A downlink (DL) Carrier and an uplink (UL) Carrier corresponding to a PCell are called “DL Primary Component Carrier (PCC)” and “UL PCC”, respectively. Similarly, a DL Carrier and a UL Carrier corresponding to a SCell are called “DL Secondary Component Carrier (SCC)” and “UL SCC”, respectively.
A downlink data (DL data) transmission operation in CA is explained with reference to FIG. 17 (Non-patent Literature 2). Here, it is assumed that a UE uses a first cell (Cell1) and a second cell (Cell2) served by an eNB as a PCell and an SCell, respectively. In a step S1, the eNB transmits, to the UE, configuration information for the SCell (i.e., the Cell2) (RRC Connection Reconfiguration (SCell configuration)). In a step S2, the eNB transmits to the UE an instruction indicating the activation of the Cell2 (Activation control element (activation of SCell)). In a step S3, the UE starts to use the SCell (SCell activation). In steps S4 and S5, the eNB transmits DL data to the UE by using the PCell and the SCell.
In a step S6, the eNB determines that it no longer needs to use the SCell for the UE and hence transmits an instruction indicating the deactivation of the SCell (Deactivation control element (deactivation of SCell)). In a step S7, the UE suspends the use of the Cell2 (SCell deactivation). In a step S8, the eNB and the UE transmit/receive DL data by using only the PCell.
In a step S9, the eNB determines that it needs to use the SCell for the UE again and hence transmits an instruction indicating the activation of the SCell (Activation control element (activation of SCell)). In a step S10, the UE starts to use the SCell (SCell activation). In steps S11 and S12, the eNB transmits DL data to the UE by using the PCell and the SCell.
As explained above, the eNB can control whether the SCell should be used (activated) or not according to the data amount (also called “traffic amount”) regarding the UE. This makes it possible to improve the throughput for each UE while avoiding the increase in the power consumption which would be otherwise caused by the unnecessary decoding of DL control signals (Physical Downlink Control Channel: PDCCH) performed by the UE.