7.5 Carrier Aggregation

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

When CA is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information (e.g. TAI), and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell). In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC).

Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).

The configured set of serving cells for a UE therefore always consists of one PCell and one or more SCells:

– For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger than or equal to the number of UL SCCs and no SCell can be configured for usage of uplink resources only);

– An SCell may be configured to start in either deactivated, dormant or activated mode;

– From a UE viewpoint, each uplink resource only belongs to one serving cell;

– The number of serving cells that can be configured depends on the aggregation capability of the UE (see clause 5.5);

– PCell can only be changed with handover procedure (i.e. with security key change and, unless RACH-less HO is configured, with RACH procedure);

– PCell is used for transmission of PUCCH;

– If DC is not configured one additional PUCCH can be configured on an SCell, the PUCCH SCell;

– Unlike SCells, PCell cannot be de-activated or be in dormant SCell state (see clause 11.2);

– Re-establishment is triggered when PCell experiences RLF, not when SCells experience RLF;

– NAS information is taken from PCell.

The reconfiguration, addition and removal of SCells can be performed by RRC. During connection resume from suspended RRC connection or from RRC_INACTIVE, the network may decide to keep or release any previously configured SCells from the UE context. At intra-LTE handover and during connection resume from RRC_INACTIVE, the network can also add, remove, or reconfigure SCells for usage with the target PCell. When adding a new SCell, dedicated RRC signalling is used for sending all required system information of the SCell i.e. while in connected mode, UEs need not acquire broadcasted system information directly from the SCells. A common configuration, applicable for multiple SCells, may be provided in addition to dedicated SCell configuration.

When a PUCCH SCell is configured, RRC configures the mapping of each serving cell to Primary PUCCH group or Secondary PUCCH group, i.e., for each SCell whether the PCell or the PUCCH SCell is used for the transmission of ACK/NAKs and CSI reports. A PUCCH SCell cannot be in dormant state.

For LAA SCell the following additional principles are applied:

– The eNB can configure whether the data of a logical channel can be transferred via LAA SCells.