LTE: Carrier Aggregation - Activation and Deactivation of Secondary Cells

To enable reasonable UE battery consumption when CA is configured, an activation/deactivation mechanism of SCells is supported.

If the UE is configured with one or more SCells, the eNodeB may activate and deactivate the configured SCells. Activation/Deactivation does not apply to PCell.

After Configuring/Adding the SCell, as explained here, the SCell is in deactivated state. When the SCell is modified, the UE does not change the activation status.

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.

There is no explicit activation/deactivation of uplink component carriers whenever a downlink component carrier is activated/deactivated, the corresponding uplink component carrier is also activated/deactivated.

In order for the UE to receive data (PDSCH/PDCCH) on the SCell, it has to be activated which is different from configuring the SCell. If the SCell is deactivated, the UE maintains the configuration provided by RRC but it is not possible to receive either PDCCH or PDSCH.

Activation/Deactivation of Secondary Cell

A typical use case of Activation of the SCell would that the network configures the UE with one or more component carriers (CCs) but deactivate all of them except the Primary CC. When there is a need of more data throughput (for example, there is a huge amount of data to delivered to the UE in the downlink), the network can activate several secondary CCs to maximize downlink throughput.

Another use case could be that if the PCell is fully loaded, the SCell can be activated and the data transfer can be scheduled only on the SCell (Load balancing).

The network can deactivate the SCell when there is no more data to be delivered to the UE or the channel quality of the SCell turning to be bad.

The UE (and the network) can deactivate the activated SCell without explicit signaling which is based on sCellDeactivationTimer. This is the amount of time (in radio frames) the UE has not received any data on the SCell.

SCell Activation is done via MAC Control Element whereas the deactivation mechanism is either by using MAC control element or by the expiry of the sCellDeactivationTimer.

The Activation/Deactivation MAC control element is identified by a MAC PDU subheader with unique LCID: 11011. It has a fixed size and consists of a single octet containing seven C-fields and one R-field (Reserved bit and is set to “0”). Each C-field represents an SCell with SCellIndex which ranges from 1-7.

The MAC control element carries a bitmap for the activation and deactivation of SCells: set to 1 denotes activation of the corresponding SCell, while a bit set to 0 denotes deactivation.

With the bitmap, SCells can be activated and deactivated individually, and a single activation/deactivation command can activate/deactivate a subset of the SCells.

The Activation/Deactivation MAC CE and its subheader are illustrated in the below Figure.

As discussed before, there is another mechanism than MAC CE to deactivate the activated SCell which is the expiry of sCellDeactivationTimer.

RRC may configure the UE with sCellDeactivationTimer which is located in mac-MainConfig-v1020. sCellDeactivationTimer can take values starting from 20ms to 1280ms (20, 40, 80, 160, 320, 640, and 1280ms)

E-UTRAN configures sCellDeactivationTimer only if the UE is configured with one or more SCells. If the field is absent, the UE shall delete any existing value for this field and assume the value to be set to infinity.

UE maintains a sCellDeactivationTimer (as configured by the RRC) timer per configured SCell and deactivates the associated SCell upon its expiry. The same initial timer value applies to each instance of the sCellDeactivationTimer.

If the UE receives an Activation/Deactivation MAC CE in subframe #n activating the SCell, the UE shall start (SCell Activation) or restart (SCell Reactivation) the sCellDeactivationTimer associated with the SCell in subframe #n+8.

If the UE receives PDCCH on the activated SCell indicating an uplink grant or a downlink assignment it shall restart the sCellDeactivationTimer associated with the SCell.

Similarly, if the UE receives PDCCH on the serving cell scheduling the activated SCell (Cross Carrier Scheduling) indicating an uplink grant or a downlink assignment, the UE shall restart the sCellDeactivationTimer associated with the SCell.

If the UE receives an Activation/Deactivation MAC CE in subframe #n deactivating the SCell or if the sCellDeactivationTimer associated with the activated SCell expires in subframe #n, the UE shall deactivate the SCell no later than in subframe #n+8.

If the UE receives an Activation/Deactivation MAC CE in subframe #n activating the SCell, the UE shall apply the below defined actions no earlier than subframe #n+8 and no later than subframe #n+24 or subframe #n+34 (as defined in section 7.7.2 of 36.133)

― Transmit SRS on the SCell in case if UL CA and SRS on the SCell is configured;
― PDCCH monitoring on the SCell;
― PDCCH monitoring for the SCell (Cross Carrier Scheduling);
― CSI reporting for the SCell. The UE should start transmitting valid CSI report no later than subframe #n+24 or #n+34 (as defined in section 7.7.2 of 36.133) but it can report out of range (CQI index = 0) values from subframe #n+8, otherwise the eNodeB have to perform blind decoding of PUSCH (if scheduled) from subframe #n+8 till subframe #n+24 or #n+34

If the SCell is deactivated, the UE shall apply the following activations:

― SRS shall not be transmitted in case if UL CA and SRS on the SCell is configured
― The UE shall not transmit UL-SCH on the SCell (UL CA)
― PDCCH on/for the SCell shall not be monitored.
― The UE shall flush all HARQ buffers associated with the SCell.
― The UE shall not transmit RACH on the SCell (Introduced in Release-11, and the RA procedure on an SCell shall only be initiated by a PDCCH order). When SCell is deactivated, the ongoing RA procedure on the SCell, if any, is aborted

― The UE shall stop reporting CSI from subframe #n+8 if the sCellDeactivationTimer associated with the SCell expires in subframe #n or if the UE receives an Activation/Deactivation MAC CE in subframe #n deactivating the SCell

I have created a tool to display timings of different actions defined above based on the activation or deactivation SFN/subframe. Try this timing calculator here. 

Watch the following deck for more details and a demo of the timing calculator

Reference: 3GPP TS 36.321, 36.213, 36.331, 36.133, and 36.300