Carrier Aggregation (CA)

This post will provide an overview on the following concepts:-

1. What is CA?
2. Why CA?
3. Basic CA Terminology
4. CA use cases

1. What is CA?

In simple terms, CA is basically a feature that allows operators to create a larger virtual bandwidth by using separate chunks of bandwidth allotted to them. These chunks are called Component Carriers(CC).

CCs may be in the same band or different band, they may be contiguous or non-contiguous. Based on this, CA can be classified into following types-

1. Intra-band contiguous spectrum aggregation:

When the component carriers are contiguous in the same frequency band, it is called intra-band contiguous carrier aggregation.

2. Inter-band spectrum aggregation:

When the component carriers lie in different frequency band, it is called inter-band carrier aggregation.

3. Intra-band discontinuous spectrum aggregation:

If the component carriers are in the same frequency band but are separated by a gap the carrier aggregation is called intra-band non-contiguous. This was introduced in release 11.

picture courtesy: Wikipedia

2. Why CA?

Following are the major benefits of  introducing CA :-

1. To Support high data rates per user:

LTE-A aims to support data rates of 1Gbps in DL and 500Mbps in UL. To support this, transmission bandwidth of upto 100 MHz is required. However, spectrum is expensive. Such large portion of contiguous bandwidth cannot be allocated to one operator. Hence, Carrier Aggregation uses multiple chunks of bandwidths allocated to a operator. These multiple frequency blocks are assigned to the same user, thereby increasing it’s data rate.

2. Better network performance:

CA allows operators to make efficient use of fragmented spectrum. This can be done by providing multiple services simultaneously over multiple carriers. For ex, during IPL, users may want to see live streaming of the match (which requires high data rate) and alongwith it, they want usual experience with unicast services like email etc. Now using CA, multiple services can be configured on multiple CC’s. This prevents throttling of any of the services that could occur otherwise. Latency of users also decreases significantly.

3. To reduce interference

The concept of Heterogenous Networks(HetNets) is basically, adding low power base stations like relay nodes, picocells etc to a macrocell area. So, there would be a large cell served by a macro eNb and 1 or more small cell(s) served by low power eNb. Small cells basically provides coverage in spots with high user demands that are not covered by macro eNb (can be indoor or outdoor).

However, in HetNet deployments, small cells are privately owned and deployed without coordination with macro cells. If the frequency used in both the cells is same, it can lead to risk of interference. Transmission on control channels of one cell can interfere with transmissions on control channels of another cell. In Carrier Aggregation, PDCCH interference can be avoided by using separate control channel frequencies for both the cells.

4. Load Balancing:

CA can also be used for balancing the load of users intelligently across multiple carriers.

3. Basic CA Terminology-

• Primary Cell (PCell):

The cell in which UE either performs the initial connection establishment procedure or initiates the connection re-establishment procedure is the primary cell.

• Secondary Cell (SCell):

Once an RRC connection is established with a PCell, another cell/carrier which may be used to provide additional radio resources to the user, is termed as secondary cell.

-PCell and SCell are defined with respect to a UE. PCell for one UE may be an SCell for another.

-A UE can have only one primary cell and multiple secondary cells.

-Transmission of PDCCH, PDSCH and PUSCH (not PUCCH) can be done on SCell.

-CA scheduling can be of two types-

• Cross Carrier Scheduling:

It means PDCCH of PCell will be used for giving grants on PCell as well as on SCell. SCell will not have a separate PDCCH channel of it’s own.

• Own Carrier Scheduling:

SCell will have a separate PDCCH channel of it’s own. If a cell has multiple secondary cells, each CC will have it’s own PDCCH.

• SCell Addition/Deletion vs SCell Activation/Deactivation:

In simple terms, SCell addition refers to keeping a record in the PCell and informing the UE that certain neighbouring cells can serve as secondary cells to this UE in future. SCells are added and removed from the set of serving cells through the RRC Connection Reconfiguration, if UE supports CA (which can be seen from UE capability). Upon the completion of AS security, SCell may be added after receiving measurement report from the UE indicating that the measurements from the cell on the carrier frequency used for the SCell is above a certain threshold.

SCell activation refers to enabling UL/DL transmissions on the SCell that has already been added. This is done by a MAC control element sent in downlink. Similarly, SCell deactivation refers to disabling any further DL/UL transmissions on the added SCell.

• CA Bandwidth Class:

Aggregated Transmission Bandwidth Configuration (ATBC) is the number of the aggregated RBs within the fully allocated Aggregated Channel bandwidth. CA bandwidth class indicates a combination of maximum  ATBC and maximum  number of CCs. In R10 and R11 three classes are defined:

4. CA Use Cases:

Few CA Use cases are explained below:

Let, F1: Frequncey of cell 1

F2: Frequency of cell 2

Use Case 1:

F1 and F2 cells are co-located and overlaid, providing nearly the same coverage. Both layers provide sufficient coverage and mobility can be supported on both layers. This scenario helps in providing higher throughput.

Use case 2:

F1 and F2 cells are co-located and overlaid, but F2 has smaller coverage. Only F1 provides sufficient coverage and F2 is used to improve throughput. Mobility is performed based on F1 coverage.

Use case 3:

F1 and F2 cells are co-located but F2 antennas are directed to the cell boundaries of F1 so that cell edge throughput is increased.

Use Case 4:

HetNets as explained in section 2.