5G-NR: Carrier Aggregation (CA)

5G-NR: Carrier Aggregation (CA)


        when there is no CA in the picture,  UE will receive and transmit data on a single carrier, this carrier is called primary component carrier and the corresponding cell is called a primary serving cell. In case of carrier aggregation, one or more component carriers are aggregated with the primary component carrier in order to support wider transmission bandwidth. 

Carrier Aggregation:

       Carrier Aggregation feature is introduced in the initial version of Release-15 of 3GPP Specifications. 5G New Radio uses carrier aggregation of multiple Component Carriers (CCs) to achieve high-bandwidth transmission (and hence high data rate). 

In LTE, you can aggregate a maximum up to five carriers that is one primary component carrier and four secondary component carriers. But in 5G NR supports aggregation of up to 16 components carriers.

    Carrier aggregation is designed to support aggregation of a variety of different arrangements of CCs, including CCs of the same or different bandwidths, adjacent or non-adjacent CCs in the same frequency band, including CCs of the same or different numerologies and CCs in different frequency bands. Each CC can take any of the transmission bandwidths, namely (5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 90, 100) MHz for FR1 & (50, 100, 200, 400) MHz for FR2 respectively.

 A UE that is configured for carrier aggregation connects to one Primary Serving Cell (known as the ‘PCell’ in MCG or ‘PSCell’ in SCG) and one or more Secondary Serving Cell (known as ‘SCell’).
All RRC connections and Broadcast signalings are handled by the Primary serving cell. The primary Serving cell is the master of the whole procedure. Primary serving cell decides that which serving cell need to be aggregated or added and deleted from the Aggregation.

Now we will look into the role of Primary serving cell and secondary serving cell in terms of carrier aggregation.

1- Role of Primary serving cell: followings are the role of primary serving cell.

=> Dynamically add or remove the secondary component carriers.
=> Dynamically activate and deactivate the secondary cell.
=> Handle all RRC(Radio resource control) and NAS(non-access stratum) procedures.
=> Receive measurement reports and control mobility of UE.

Note: Primary serving cell can be changed only at the time of handover.

2- role of Secondary serving cell: followings are the role of secondary serving cell.

=> An UE can aggregate maximum up to 16 component carrier where 1 is primary component carrier and 15 are secondary component carrier. (In case of LTE it is 1PCC and 4 SCC).
=>Actual number of secondary serving cell that can be allocated to UE is dependents on UE capability.

Note: It is not possible to configure an UE with more UL CCs than DL CCs, while revere of this can be possible.

There are mainly three ways by which component carriers can be allocated.

1- Intra Band Contiguous:
 In this Primary component carrier and secondary component carrier is configured with same band but they are contiguous.

2- Intra Band Non-Contiguous:

   In this Primary component carrier and secondary component carrier is configured with same band but they are not contiguous.

3-inter band Contiguous:

 In this Primary component carrier and secondary component carrier  are allocated on two frequency band.

By using the above configuration, infinite combinations are possible. But 3GPP has defined allowed combinations

Denoting Band combination:
        Denotes intra band contiguous CA
        e.g CA_10(band)

        Denotes intra band non-contiguous CA
        e.g CA_10-10

        Denotes inter band contiguous CA
        e.g CA_10-20

Precondition for CA:
     UE can be configured CA only when it is capable to support CA. UE informs its  capability to the network during registration procedure in "UE capability information" message to network.

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