Carrier Aggregation Part-3

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Which layer impact using CA?

CA mandatory requirements impact RRC, MAC, and PHY layer procedures.

Which layer does not impact using CA?

Carrier aggregation does not impact the RLC/PDCP/NAS procedures and protocols.

Does CA activate in RRC idle mode?

There is no carrier aggregation for the UE in RRC Idle mode.

Is there any a change or impact in Camping process due to CA?

No, there is no change in camping process due to CA because CA will activate in connected mode only.

Is there any a change or impact in idle mode due to CA?

No, there is no change in idle mode due to CA because CA will activate in connected mode only.

Is there any a change or impact in registration due to CA?

No, there is no change in registration due to CA because CA will activate in connected mode only.

Does SRB(signaling radio bearer) establish for Scell?

SRB is not established between the UE and SCells

Does C-RNTI is same for Pcell and Scell?

The UEs identity (C-RNTI) is the same in the PCell and all its configured Scells.

How to handle handover with dual carrier?

This can be done by either tricky option or through easy option.
Tricky option would be to handover the two carriers without any modification from one cell (source cell) to another (destination cell). But this can bring lot of issues if not implemented carefully.

For other way that is easy option can be to remove the secondary DL channel before handover and then perform handover and configure the secondary channel in the destination cell. It is probable to take an easy option for initial deployment.

How PCC and SCC effect each layer?

The signaling towards an end user device for CA affects only certain layers, not the entire protocol stack. For example, the device is permanently connected through its PCC to the serving Primary Cell (PCell). 

NAS (Non-Access Stratum) functionality such as security key exchange and mobility information are provided by the PCell. All the secondary component carriers are understood as additional transmission resources. For the PDCP (Packet Data Convergence Protocol) and RLC (Radio Link Control) layer carrier aggregation signaling is transparent. 

A terminal is configured on the RRC (Radio Resource Control) layer, to handle secondary component carriers. Moreover, on RRC (Radio Resource Control) the parameters of the SCell(s) are configured. The MAC layer acts as multiplexing entity for the aggregated component carriers as they are activated or deactivated by MAC control elements. 

In the event of activation in subframe n, than 8 sub frames (8 ms) later the resources are available to the device and it can check for scheduling assignments. Here  the MAC acts as multiplexer, also  each component carrier has its own Physical Layer (PHY) entity, HARQ, providing channel coding, data modulation and resource mapping.

Difference between CA and DC:

Carrier aggregation(CA)
Dual connectivity (DC)

CA allows a UE to simultaneously transmit and receive data on multiple component carriers from a single eNB .
Dual connectivity allows a DUT to simultaneously received and transmit data on multiple component carriers from two cell groups via master eNB  (MN-Master node) and secondary eNB (SN- Secondary node)
CA is for the sensors where the backhaul between nodes is ideal
DC is for non ideal backhaul for instance, relatively large delay between nodes

In CA there is only one common CRNTI across all component carriers, and there is only one PUCCH which is on PCell.
A DC UE has two identities – one CRNTI in MCG (master cell group) and another CRNTI in SCG (secondary cell group). It has an always active cell in MCG and another always active cell in SCG and has separate PUCCH resource in MCG and SCG.
CA improves peak rate as well as user throughput at low load
DC can increase user throughput, provide mobility robustness and support load balancing among eNBs

In CA implementation user traffic is split between carriers in MAC layer
In DC user traffic is split in PDCP layer. In DC the data bearer which is split in PDCP layer is called split bearer.

In Upcoming Part we will learn more about CA and CA interview question and its answer. And for more learning please go through our 5G and LTE page.

Stay tune for more Update…………………

Pinal Dobariya…

- 3GPP TS 36.101(User Equipment radio transmission and reception)
- 3GPP TS 36.300(Overall description; Stage 2)

- 3GPP TS 36.331(Radio Resource Control (RRC)


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