Carrier Aggregation & Dual Connectivity

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What is CA?

Carrier Aggregation is a technology through which, a UE can use multiple component carriers to transmit/receive data to/from the eNodeB.

It was introduced in 3GPP Release 10.

Release 8 and 9 UEs supported only one component carrier.

Release 10 UEs can support up to 5 Component Carriers – 1 Primary Carrier, up to 4 Secondary Carriers.

The main component carrier is called Primary Component Carrier (PCC) and others are called Secondary Component Carriers (SCC).

Thus the maximum bandwidth that can be allocated to a UE is 100 MHz (20+20+20+20+20).

The number of uplink carriers can never be more than the number of downlink carriers, but the reverse is possible and usually the case.

It is supported in both FDD and TDD.

What was the motivation behind introducing CA?

The primary objective of introducing CA was to increase the bandwidth and thus provide high data rates to the end user.

The usable spectrum is a limited and costly resource. Telecom operators have to shell out millions to acquire licenses to operate bands. So they prefer to use it stingily. Although an LTE UE can support bandwidth up to 20 MHz, it is rarely allocated to a single UE in order to save bandwidth.  Thus the UE’s capability to support high bandwidth is not fully utilized.

Also, most operators do not have license to operate 20 MHz, but instead own smaller bandwidths (5, 10 MHz) in different bands. Through Carrier Aggregation, they can also utilize their fragmented resources effectively and efficiently.

Types of Component Carriers

Three types of component carriers are possible in CA:

• Intra-band contiguous: Contiguous carriers belonging to the same frequency band are aggregated.
• Intra-band non-contiguous: Non-contiguous carriers belonging to the same frequency band are aggregated.
• Inter band non-contiguous: Non-contiguous carriers belonging to different frequency bands are aggregated.

In CA, all the RRC layer communication happens between the PCell and the UE. No RRC messages are exchanged between the UE and SCells.

RRC Connection Establishment/Re-establishment is handled by PCell. Whenever there is a handover, RRC Connection Reconfiguration message is sent by PCell.

Measurement Reports are received by PCell. The number of SCells to be added depends on UE capability.

The configuration/reconfiguration, activation/deactivation, addition and removal of SCells is handled by RRC messages at PCell.

Whenever a new SCell is to be added, all the required details of System Information are sent through RRC Connection Reconfiguration message from the PCell. The UE does not have to SIBs from the SCell. During intra-NR handover, RRC can also add, remove, or reconfigure SCells for usage with the target PCell.

What is Dual Connectivity?

Dual Connectivity is a feature which was introduced by 3GPP in Release 12.

Like Carrier Aggregation, it also aims to increase the throughput of the UE by utilizing resources from multiple carriers.

The main point where CA and DC differ is in their application scenarios and implementation.

The differences between CA and DC are as follows:

1.  In CA all component carriers belong to the same eNodeB, but in DC, the aggregated carriers belong to different cells (where usually one cell is macro and the other is small). That’s why it is sometimes also referred to as inter-site carrier aggregation. The two eNodeBs are referred to as Master Cell Group (MCG) and Secondary Cell Group (SCG).

2.     In CA, the user traffic is split at the MAC layer of the eNodeB, whereas in DC, the user traffic gets split at the PDCP later itself. This data bearer is referred to as split bearer.

3.   In CA, only one UE identity is used in all component carriers. But in DC, UE is identified by different C-RNTIs in MCG and SCG.

4.   Only one PUCCH is used for uplink signaling messages across all component carriers in CA, and it is present in the Primary Component Carrier. In DC, however, separate PUCCHs are used in MCG and SCG.

So, based on the differences we’ve seen so far between CA and DC, we can figure out in which scenario, which aggregation technology would be used.

If the backhaul of the network is ideal between the nodes, then CA can be implemented. But if the backhaul is not ideal, for example, if there are large delays between the nodes, then the choice should be DC.

Now the question arises, can CA and DC be implemented simultaneously?

Yes, they can. Let’s say there are two eNodeBs, Master(MeNB) and Secondary(SeNB). Within each eNodeB, there are two carriers allocated a UE. User data coming from the S-GW into the Master eNodeB would get split at the PDCP layer.  The split bearer will go through the Secondary eNodeB. Both the bearers will get further divided at the MA layer into two component carriers.

                                                         Fig. DC

In this way, a single UE can have both CA and DC to enhance its throughput.

In DC, the Master eNodeB and Secondary eNodeB are configured in such a way that they are independent of each other. So, the bandwidths they use, the number of component carriers they allocate to the same UE, or whether they are on FDD or TDD are completely independent.

                                                                   FIG. CA

Another important point is that all RRC layer messages are transmitted to/from the MeNB. RRC entity is not present in the SeNB. If there is any RRC configuration message from the SeNB, it is sent to the MeNB and from there transmitted to the UE.

Reference: 

1.  https://www.3gpp.org/technologies/keywords-acronyms/101-carrier-aggregation-explained 

2. 3GPP 36.300

3.  https://www.linkedin.com/pulse/lte-dual-connectivity-zhou-hongwei/

4. https://cdn.rohdeschwarz.com/pws/dl_downloads/dl_application/application_notes/1ma252/1MA252_2e_LTE_Rel12_technology.pdf

--- Pinal Dobariya