LTE: Carrier Aggregation - Secondary Cell Addition/Modification/Release

The Secondary Cell (SCell) Addition, Release, and Reconfiguration (Modification) are RRC’s responsibility. RRC configures SCell for CA capable UEs only. RRC’s functionality pertaining to Carrier Aggregation is discussed in this post.

In order for the UE to receive data (PDSCH/PDCCH) on SCell, it has to be activated which is different from configuring the SCell. This activation is done via MAC Control Element.

After Configuring/Adding the SCell, the SCell is initially in deactivated state. When SCell is Modified, the UE does not change the activation status of the concerned SCell.

During Handover, if the same SCell is in use in the target PCell (i.e., SCell is not released during Handover), the SCell in the target Cell in initially in the deactivated state.

Activation/Deactivation of SCells will be discussed in detail in further posts.

SCell Addition/Modification 

SCells can not be added/configured at the time of RRC Connection establishment i.e., there is no provision for SCell addition in the RRC Connection Setup message.

The addition of SCells is performed only when AS security has been activated.

Upon the completion of AS security, the SCell may be added either blindly or after receiving measurement report from the UE indicating that the cell on the carrier frequency used for the SCell is above certain threshold.

SCell is added to the PCell or to a set of serving cells through ‘RRC Connection Reconfiguration’ procedure.

E-UTRAN uses IE sCellToAddModList in RRCConnectionReconfiguration message to add the SCell.

At the time of SCell addition, the eNodeB sends the following information to the UE via RRCConnectionReconfiguration message, a few of which are optional.

SCellIndex: This is used to identify/address the SCell that is being configured/reconfigured/released. If the received SCellIndex is part of the UE’s configuration already, then the UE considers the procedure as SCell Modification. Else if the received SCellIndex is not part of the UE’s configuration, then the UE should consider the procedure as SCell Addition. Based on the UE’s capability, at most four SCells can be configured but the SCellIndex can take values from 1 to 7.

cellIdentification: This is mandatory at the time of SCell Addition and shouldn’t be present at the time SCell Modification. cellIdentification consists of Physical Cell Identity and Downlink Carrier Frequency (EARFCN).

radioResourceConfigCommonSCell: This is a big structure of IEs which is mandatory at the time of SCell Addition and shouldn’t be present if the concerned SCell is being Modified.

When adding a new SCell, radioResourceConfigCommonSCell is used for sending all required system information of the SCell i.e. while in RRC_CONNECTED mode, UEs need not acquire broadcasted system information directly from the SCells.

radioResourceConfigCommonSCell contains downlink configurations such as Downlink Bandwidth, Number of Antenna Ports, List of MBSFN subframe Configurations, PHICH Configuration, PDSCH common configuration, TDD configuration (in case of TDD) etc..

In case of SCells with configured uplink (UL CA), radioResourceConfigCommonSCell contains uplink information such as Uplink Bandwidth, Uplink Carrier Frequency, Uplink Power Control (Common) Information, common information of physical channels, SRS common information etc…

radioResourceConfigDedicatedSCell: This is another big structure of IEs which is mandatory at the time of SCell Addition and is optional otherwise. It contains UE specific (dedicated) configurations for SCell.

radioResourceConfigDedicatedSCell contains downlink dedicated configurations such as information related to Transmission Mode for the SCell, Cross Carrier Scheduling configuration, SCell CSI reference signal information, SCell PDSCH dedicated configuration etc…

E-UTRAN might configure the UE with uplink dedicated configuration as well. This is mandatory in case of UL CA where as it is optional for DL CA. It contains information such as Uplink Transmission Mode for SCell, Uplink dedicated Power Control information, CQI Reporting configuration for SCell, SCell’s dedicated SRS configuration etc…

See an example RRCConnectionReconfiguration message for SCell Addition below. 

As explained above, for an SCell, EUTRAN provides, via dedicated signalling, all system information relevant for operation in RRC_CONNECTED when adding the SCell (radioResourceConfigCommonSCell).

Upon a change of the relevant system information of an already configured SCell, the E-UTRAN releases and subsequently adds the concerned SCell, which may be done with a single RRCConnectionReconfiguration message (sCellToReleaseList and sCellToAddModList are used).

The SI parameter values configured via dedicated signaling may be different than the ones broadcasted in the SIs of the concerned SCell.

Upon receiving in RRCConnectionReconfiguration message the UE first executes SCell Release command (if included in the message) before executing any SCell Addition Command.

At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage with the target PCell. Some of the possible scenarios (considering only one SCell) are given below.

― SCell is already configured in the source PCell and the SCell is left modified/ unmodified i.e., same SCell is used even in the target PCell.

― SCell is already configured in the source PCell and the SCell is left released during handover. i.e., release of the existing SCell during handover

― SCell1 is configured in the source PCell and during the handover, SCell1 released and a new SCell (SCell2) is configured for use in the target PCell i.e., change of SCell during handover.

― SCell is not already configured in the source PCell and an SCell is configured during handover for use in the target PCell.

― SCell is not already configured in the source PCell and an SCell is added during handover for use in the target PCell.

― SCell is already configured in the source PCell and UE receives handover command to handover to configured SCell i.e., Handover to SCell. In this case, RRCConnectionReconfiguration message contains SCell Release and also mobilityControlInfo containing carrierFreq and phyCellId which are same as that of SCell.

― SCell is already configured in the source PCell and UE receives handover command to handover to configured SCell and a new SCell is added whose frequency is same as that of source PCell i.e., Swapping PCell with SCell and vice versa. In this case, RRCConnectionReconfiguration message contains SCell Release (SCell1) and also mobilityControlInfo containing carrierFreq and phyCellId which are same as that of SCell1, SCell Addition whose carrierFreq and phyCellId are same as that of source PCell.

SCell can also be added during Handover from another RAT (e.g. GERAN or UTRAN) to E-UTRAN.

SCell Release

SCell is removed from PCell or from a set of serving cells through ‘RRC Connection Reconfiguration’ procedure.

E-UTRAN uses IE sCellToReleaseList in RRCConnectionReconfiguration message to remove/release an already configured SCell.

UE should release all the configured SCells in case if, RRC Connection Re-establishment procedure is initiated (obviously on PCell).

Reference: 3GPP TS 36.331 and 36.300