4G – Carrier aggregation (CA)

Carrier aggregation (CA) is a feature to increase the bandwidth by combining multiple carriers (intra‐band or inter‐band), and therefore increase peak bitrates.

Max Bandwidth possible in LTE using CA- Each aggregated carrier is referred to as a component carrier, CC. The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated, hence the maximum aggregated bandwidth is 100 MHz in LTE.

How CA works – In CA, there is a Primary cell (Pcell) and one or more Secondary cell(s) (Scell). UE always uses Pcell to monitor system information, maintain RRC connection, monitor RLF, random access, and so on. All security input and NAS mobility information is communicated using Pcell. Scell operates on a secondary frequency (inter or intra band) and can be configured once the RRC connection is established. UL/DL carrier corresponding to Pcell is primary component carrier (PCC) and to Scell is secondary component carrier (SCC). The Separate UL PC (Power control) is required for different CC due to difference in propagation condition in non‐contiguous CA and for different interference conditions in UL CC in contiguous CA. The SCCs are added and removed as required, while the PCC is only changed at handover.

During attach, RRC re-establishment or incoming handover, eNB always check CA license, CA neighbor cell configuration (i.e., SCell candidate) and UE capability. SCell selection is performed by RRC connection reconfiguration in attach procedure itself. In CA activated state, the UE receives data transfer on the SCell (DL assignments sent on SCell, HARQ ACKs are sent to eNB via PCell) and report the SCell’s CSI (Channel State information) to eNB via PCell.

eNB will not deconfigure but deactivate the SCell when UE goes out of SCell coverage. PCell always changes due to handover. In the new PCell, old SCell is removed and new SCell is configured. All of this is done in the same RRC reconfiguration message as the handover itself.

Prerequisite for CA – Similar Pcell and Scell coverage/quality (defined by A3 offset) is the prerequisite to provide CA capability to UEs. Therefore, it is important to ensure that Pcell and Scell(s) have similar antenna models and physical parameters (such as azimuth, tilt, height etc.).

CA configuration/deconfiguration is performed on RRC level whereas CA activation/ deactivation is performed on MAC layer.

Layer level functionality in CA – As CA is performed at MAC layer, PDCP and RLC are not aware whether a certain packet will be transmitted in the PCell or in the SCell. All counters for MAC and physical layer are registered on the cell where the data is transmitted (as shown below).

LTE protocols for the radio interface, with main changes due to introduction of CA

CA will increase PDCP throughput in the network as PDCP packets will be sent using multiple cells in parallel.

Activate CA in an eNB- CA feature needs to be enabled first at eNB level.