All About Carrier Aggregation (CA)

Carrier Aggregation (CA):

- LTE Release 10 Feature.

- Known as LTE Advance.

- DL Speeds upto 1 Gbps and UL Speeds upto 500Mbps.

- Backward Compatibility with Rel 8 and Rel 9.

- Can be used for both FDD and TDD.

- The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz.

- A maximum of 5 Component Carriers(CC) can be aggregated.

- The maximum aggregated bandwidth is 100 MHz.

- Each component carrier is treated as an R8 carrier.

- The spacing between center frequencies of contiguously aggregated component carriers will be a multiple of 300 kHz to be compatible with the 100 kHz frequency raster of Release 8/9 and at the same time preserve orthogonality of the subcarriers, which have 15 kHz spacing.

- For non-contiguous cases the CCs are separated by one, or more, frequency gap(s).

LTE-Advanced Support Three Component Carrier Aggregation Scenarios:

- Intra-Band Contiguous

- Intra-Band Non-contiguous

- Inter-Band Non-contiguous

PCell and SCell :
- The Cell UE selects during initial establishment (RRC Connection Request/RRC Connection Re-establishement Request) will become the PCell.
- eNodeB can add / delete Scell(s) using RRC Connection Reconfiguration message.

PCell Vs SCell:
- PCell always have both Uplink(UL) and Downlink(DL). Scell always have DL (While activated) but may or may not have UL.
- PCell is always activated whereas SCell has to be activated or deactivated using MAC-CE.
- UE does not required to acquire System Information and decode Paging from SCell.
- For Scell SI is passed to UE while adding the Scell. 
- When an Scell is added using RRC Connection Reconfiguration Message it remains in the deactivated state till it is activated using MAC-CE.
- If Scell activation/deactivation MAC-CE is received on Subframe n the Scell is activated/deactivated on Subframe n+24 or n+34.(TS 36.133 Section 7.7.2)
- When sCellDeactivationTimer expires then Scell is deactivated.
- Once Scell is deactivated 
         - PDCCH on Scell and PDCCH for Scell is not monitored. 
         - PUSCH is not transmitted and PDSCH is not received. 
         - The SRS is not transmitted.
         - The CQI/PMI/RI for Scell is not reported.


Activation/Deactivation MAC-CE:
- The MAC-CE can activate and deactivate Scell(s) which is already configured using RRC Connection Reconfiguration Meassage.
- Control Element is identified by a MAC PDU subheader with LCID.
.

Values of LCID for DL-SCH

Index	LCID values
11011	Activation/Deactivation

- fixed size and consists of a single octet containing seven C-fields and one R-field.

Activation/Deactivation MAC control element

- The Ci field is set to "1" to indicate that the SCell with SCellIndex i shall be activated.

- The Ci field is set to "0" to indicate thatthe SCell with SCellIndex i shall be deactivated.

- R: Reserved bit, set to “0”.
- 

Pcell and Scell Concepts:
- Pcell can be changed using RRC Connection Reconfiguration With MobilityControlInfo i.e. Handover.
- Scell can be changed using RRC Connection Reconfiguration message.
- During Radio Link Failure, the Scell is release first before initiating RRC Connection Re-establishment procedure.
- On receiving Handover Command i.e. RRC Connection Reconfiguration With MobilityControlInfo, UE deactivates the Scell, if configured.
- TTI Bundling is not supported when configured with one or more Scell with Configured Uplink.
- The RSRP and RSRQ measurement for Pcell shall follow time domain measurement resource restriction in accordance with measSubframePatternPCell, if configured.

Procedure	Pcell	Scell
Radio Link Monitoring	Y	N
System Information Acquisition	Y	N
Random Access Procedure	Y	N
Time Alignment	Y	N
SPS	Y	N
Connected Mode DRX	Y	Y
Paging	Y	N
NAS Mobility Information	Y	N
Data Transmission	Y	Y
S-measure Criteria	Y	N
Power Control	Y	Y
Link Adaptation	Y	Y
Pathloss Meaurement	Y	Y= when pathloss reference=Scell N= when pathloss reference=Pcell

Channel	Pcell	Scell
PDCCH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PDCCH when CCS but eNodeB might be transmitting PDCCH.)
PDSCH	Y	Y
PUCCH	Y	N
PUSCH	Y	Y
PRACH	Y	N
SRS	Y	Y
PBCH	Y	N - UE does not look at PBCH But eNodeB might be transmitting it.
PSS/SSS	Y	N - UE does not look at PSS/SSS but to get timing information eNodeB might be transmitting PSS/SSS.
PCFICH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PCFICH when CCS but eNodeB might be transmitting PCFICH.)
PHICH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PHICH when CCS but eNodeB might be transmitting PHICH.)

Cross Carrier Scheduling(CCS):
- Downlink Scheduling or Uplink Grant information of One Component Carrier(CC) can be carried by the PDCCH of another Component Carrier(CC).
- 3 bit CIF field indicates target CC.
- Pcell shall always be scheduled by Pcell only.
- Scell can be cross scheduled by Pcell or by other Scell.
- UE indicates whether it supports CCS or not.
- Cross Carrier Scheduling is not applicable for PDCCH order. It is transmitted on Pcell.
- CCS is applicable for aperiodic SRS transmission.

- The cif-Presence-r10 in physicalConfigDedicated indicates whether CIF will be present in PDCCH of Pcell.

- The RadioResourceConfigDedicatedSCell-r10.PhysicalConfigDedicatedSCell-r10.CrossCarrierSchedulingConfig-r10 indicates CCS status of Scell.

- cif-Presence indicates whether carrier indicator field is present (value TRUE) or not (value FALSE) in PDCCH DCI formats.

- pdsch-Start indicates the starting OFDM symbol of PDSCH for the concerned SCell. Values 1, 2, 3 are applicable when dl-Bandwidth for the concerned SCell is greater than 10 resource blocks, values 2, 3, 4 are applicable when dl-Bandwidth for the concerned SCell is less than or equal to 10 resource blocks.

- schedulingCellId Indicates which cell signals the downlink allocations and uplink grants, if applicable, for the concerned SCell.(When Scell cross scheduled  other Scell.)

- The other-r10.schedulingCellId-r10 and cif-Presence-r10 of that cell should be consistent.

Carrier Aggregation and Measurement Events:

- Definition of Serving Cell Measurement is Modified.

- For Event A1 and Event A2 The Carrier Frequency in Measurement Object indicates whether this event is for Pcell or any Scell. 

- The eNodeB shall configure separate A1/A2 events for each serving cell.

- Event A3 - Neighbor becomes offset better than Pcell.

- Event A5 - Pcell becomes worse than theshold1 and neighbour becomes better than threshold2.

        - For Event A3 and Event A5 the frequency mentioned in the associated measObjectEUTRA indicates neighbours.

        - For Event A3 and Event A5 the Scell become neighbouring cell.

- Event B2 - Pcell becomes worse than theshold1 and inter RAT neighbour becomes better than threshold2.

- Event A6 - Intra Frequency Neighbour becomes offset better than Scell.

- No change in the definition of Event A4 and Event B1.

Carrier Aggregation and Periodic Measurement:

- If (Purpose == reportStrongestCells && reportAmount > 1)

      UE initiates a first MR immediately after the quantity to be reported becomes available for the Pcell.

- If (Purpose == reportStrongestCells && reportAmount == 1)

      UE initiates a first MR immediately after the quantity to be reported becomes available for the Pcell and for the strongest cell among the applicable cells.

- If (Purpose == reportStrongestCellsForSON)

      UE initiates a first MR when it has determined the strongest cells on the associated frequency.

Carrier Aggregation and Measurement Gap:

- UE shall be able to carry out Measurement on any serving frequency without measurement gap i.e. intra-frequency measurement for any serving frequency.

- UE may required measurement gap to perform inter-frequency or inter-RAT measurement.

Typical CA Call Flow: