Interview Question & Answer series: Carrier Aggregation (CA) in LTE

Hi here we will go with Interview Question on Carrier Aggregation in LTE

Question 1: What is Carrier Aggregation?

Answer:  Carrier Aggregation is a feature which came into 3GPP Release 10. It is part of the feature of LTE- Advanced which is a common term used to describe the improvements in LTE. The focus of Release 10 was to advance LTE towards providing better user experience by providing higher data rates in cost-efficient way and in the same way maintaining backward compatibility.

The two main functionalities that are brought to achieve this criterion for LTE-A are Carrier Aggregation and Multi Antenna techniques.

Carrier Aggregation is a feature in LTE for providing very high data volumes and data rates by the process of combining contiguous and non- contiguous spectrum bands. This can be explained in a simple way. For Ex: A Mobile operator can have 10MHZ in L1800 Band and 10 MHZ in L900 band. So, by using the functionality of CA we can combine these two bands to provide 20MHZ of Carrier Channel BW.

Figure shows an example of how CA can be used to combine five 20 MHz carriers to create a virtual bandwidth of 100 MHz..

Few Pointers about CA:

1.CA is a key feature which allows operators to create virtual carrier bandwidth

2. It can help operators to increase individual carrier bandwidths for different layers by combining them by implementing the CA functionality

3. CA enables combinations of up to 5 Carrier components. Multiple LTE carriers each with Bandwidth of 20MHZ can be transmitted in parallel to and from from the same UE.

4. As of now CA can provides bandwidth up to 100 MHZ (20 MHZ * 5 Component carrier)

5. Cross Component Carrier Scheduling is supported by CA, where we can use the control channel of one carrier (PCC) to allocate resources (scheduling functionality) of another carrier (SCC) 

Question 2: What are the benefits of Carrier Aggregation?

Answer: Carrier Aggregation has several benefits, including:

• Increased data rates
• Improved network performance
• Reduced interference
• Improved resource utilization
• Better coverage

Question 3:  What are the types of Carrier Aggregation?

Answer:   There are two main types of Carrier Aggregation (CA) in LTE:

• Intra-band Contiguous: his Configuration refers to contiguous carriers aggregated in the same operating band. In this type of CA single Band is used (For Ex: L1800 or L900). The spacing between the centre frequencies of the component carriers (continuously aggregated) is multiple of 300KHZ. In this case only one trans-receiver is required at UE end and the UE should be compatible to                                            operate on a larger

• Intraband Non-Contiguous: Here Carriers from the same band is used, but they are not continuous. So, the multi carrier signal can not be considered as a single signal. For this reason, the design and implementation are a bit complex especially from the UE perspective. Unlike Intraband Contagious where only a single Transceiver is needed, noncontiguous requires two transreceivers.

• Interband Noncontiguous: Here different band of frequency is used. The UE is this case needs to have Transreceivers of multiple bands. This method is important in ensuring mobility integrity using the propagation characteristics of multiple bands.

Question 4: What are the different types of cells in Carrier Aggregation?

Answer: There are two types of cells in CA:

Primary Cell (PCell): The cell that is operating on the primary frequency and carries both signaling and data traffic.

Primary Cell ( P-Cell ) :

The Primary cell is the Cell which is selected by the UE during cell search and used for RRC connection establishment. The measurement and mobility procedures are based on P-Cell. The P-Cell can never be deactivated. There is only one P-Cell per mobile device.

• The RRC Connection is only handled by the Primary serving cell, served by the Primary component carrier(DL and UL PCC).it is also on the DL PCC that the UE receives NAS information, such as security parameters. In idle mode the UE listens to system information on the DL PCC. On the UL PCC PUCCH is sent.
• Random access procedure is performed over PCell.
• PDCCH/PDSCH/PUCCH/PUSCH can be transmitted.
• Measurements and mobility procedure are based on PCell
• Cannot be deactivated.

Secondary Cell (SCell): A cell that operates on a secondary frequency and carries only user data.

• The Secondary /Serving cells are those cells which are selected by the Network based on the UE capability and the position/Location of the UE which can serve the UE simultaneously along with the Primary Cell.
• The Secondary cells are Activated /De-activated by MAC Layer and get assigned to the mobile device by higher layers. There can be more than one S-Cell per mobile device.
• The SCCs are added and removed as required, while the PCC is only changed at handover.

Question 5: What are the pre conditions for CA?

Answer:  The pre condition of CA are:

• Access Stratum Release (minimum of R10) must.
• UE Category (at least Category-6)
• Supported Band Combinations must be followed.
• CA Bandwidth Class supported.
• Bandwidth Combination Set

Question 6: How does the RRC layer handle CA configuration and deconfiguration?

Answer: The RRC layer is responsible for adding, removing, and reconfiguring SCells. This is done using dedicated RRC signalling.

Question 7: What is the purpose of the prohibit timer?

Answer: The prohibit timer prevents the UE from activating or deactivating the SCell for a certain period of time. This is to prevent the UE from constantly switching between cells.

Question 8: What are the three scenarios for SCell activation?

Answer: SCell activation can occur in three scenarios:

• Need-based: The SCell is activated based on the amount of data transmission in the RLC buffer of the UE.
• Coverage-based: The SCell is activated if the coverage of the current SCell is poor.
• Prohibit timer: The SCell can be activated if the prohibit timer is not running.

Question 9: What are the conditions for SCell deactivation?

Answer: SCell deactivation can occur in three scenarios:

• RBS buffer: The amount of data in the RBS buffer for a UE is less than the threshold value.
• Channel quality: The channel quality of the SCell is less than the threshold value.
• Prohibit timer: The prohibit timer is running.

Question 10: What is the purpose of Event A6 measurement reports?

Answer: Event A6 measurement reports help to determine the best candidate among SCells on the same frequency band.

Question 11: What is the difference between uplink CA and downlink CA?

Answer: Uplink CA is the aggregation of component carriers for uplink transmissions. Downlink CA is the aggregation of component carriers for downlink transmissions.

Question 12: What are the limitations of uplink CA?

Answer: Uplink CA has the following limitations:

• There can be a maximum of 1 SCell configured for UL CA.
• UL CA is not supported in Intra-Band noncontiguous CA.
• FDD+TDD UL CA is not supported.
• The UL CC and DL CC must be on the same band.

Question 13: What is the difference between FDD-FDD CA and FDD-TDD CA?

Answer: FDD-FDD CA is the aggregation of component carriers between two FDD cells. FDD-TDD CA is the aggregation of component carriers between an FDD cell and a TDD cell.

Question 14: What is the future of CA?

Answer: CA is being extended to support up to 7 DL component carriers.

 Question 15: Provide Some examples of CA Configurations?

Answer:

• CA_25A-25A

two non-contiguous carriers from band 25

• CA_25A-41A

one carrier from band 25 and another carrier from band 41

• CA_23B

two contiguous carriers from band 23

aggregated B/W 25 RBs to 100 RBs

• CA_2C

two contiguous carriers from band 2

aggregated B/W 100 RBs to 200 RBs

• CA_25A-41C

one carrier from band 1 and two contiguous carrier from band 41

Question 15: Explain the CA flow?

Answer:

• Step 1: UE Registration: The user equipment (UE) initiates a registration process with the eNodeB (eNB) to obtain network access. This involves sending a registration request message to the eNB. The eNB verifies the UE’s identity and capabilities, and responds with a registration response message if the UE is authorized to access the network.

• Step 2: UE Capability Information: The eNB sends a UE Capability Information message to the UE. This message contains information about the UE’s capabilities, including its supported frequency bands, maximum number of component carriers, and support for various CA modes (intra-frequency, inter-frequency, etc.).

• Step 3: Measurement Report: The UE sends measurement reports to the eNB periodically or upon request. These reports provide information about the signal strength and quality of the available cells, including the primary cell (PCell) and secondary cells (SCells). This information is used by the eNB to select the most suitable component carriers for the UE.

• Step 4: RRC Connection Reconfiguration: Based on the UE’s capabilities and the available resources, the eNB triggers an RRC Connection Reconfiguration procedure. This involves sending an RRC Connection Reconfiguration message to the UE. This message includes information about the new component carriers to be aggregated for the UE.

• Step 5: RRC Connection Setup Complete: The UE sends an RRC Connection Setup Complete message to the eNB acknowledging the receipt of the RRC Connection Reconfiguration message. This message also includes information about the UE’s ability to support the new component carriers.

• Step 6: MAC and PDCCH Configuration: The UE and eNB exchange MAC and PDCCH configuration information. MAC (Medium Access Control) layer configuration parameters are used to manage the data transmission channels for the UE, while PDCCH (Physical Downlink Control Channel) configuration determines how data is transmitted on the component carriers.

• Step 7: Data Transmission: Once the CA configuration is complete, the UE starts receiving data on the aggregated component carriers. The UE can receive data on up to 5 component carriers simultaneously, enabling higher data rates and improved network performance.

• Step 8: Network Monitoring: The UE and eNB continuously monitor network conditions to ensure optimal performance. If the network conditions change or the UE moves to a different area, the eNB may need to re-configure the component carriers or trigger a handover to another eNB.

• Step 9: UE Termination: When the UE is finished with the CA connection, it sends an RRC Connection Release message to the eNB. This message signals the end of the CA session, and the eNB releases the resources allocated to the UE.

Question 16: What are the functions of the MAC layer in CA?

Answer:

• Maintains the timer “ScellDeactivationTimer”:

The MAC layer maintains a timer called the “ScellDeactivationTimer” for each configured secondary cell (Scell). This timer is used to deactivate the Scell if the UE does not use it for a certain amount of time. This is done to conserve battery power and reduce network congestion.

When the MAC layer detects that the UE is not using an Scell, it starts the ScellDeactivationTimer. If the timer expires before the UE starts using the Scell again, the MAC layer deactivates the Scell. This means that the MAC layer stops sending and receiving data on the Scell and also removes the Scell from the list of available cells.

The value of the ScellDeactivationTimer is configurable and can be set by the network operator. The operator will typically set the timer to a value that is long enough to ensure that the UE does not experience any disruptions to its service if it moves out of range of the Scell. However, the operator will also want to set the timer to a value that is not too long, so that the network does not waste resources on cells that are not being used.

• Used to monitor data inactivity of the UE data usage:

The MAC layer monitors the data inactivity of the UE to determine whether or not to deactivate a Scell. The MAC layer does this by keeping track of the amount of time that has passed since the UE last sent or received data on the Scell.

If the MAC layer detects that the UE has been inactive on a Scell for a certain amount of time, it will start the ScellDeactivationTimer. If the timer expires before the UE starts using the Scell again, the MAC layer will deactivate the Scell.

The amount of time that the MAC layer considers to be inactive is also configurable and can be set by the network operator. The operator will typically set this value to a value that is long enough to ensure that the UE does not experience any disruptions to its service if it stops using the Scell for a short period of time. However, the operator will also want to set the value to a value that is not too long, so that the network does not waste resources on cells that are not being used.

• Aggregates data from multiple cells:

The MAC layer aggregates data from multiple cells before sending it to the upper layers. This means that the MAC layer combines the data from all of the cells that the UE is connected to into a single stream of data. This is done to improve the efficiency of data transmission and to reduce the amount of overhead required to send the data over the network.

The MAC layer aggregates data using a technique called carrier aggregation (CA). CA allows the MAC layer to combine multiple frequency bands into a single channel. This allows for higher data rates and reduced latency.

The MAC layer also uses a technique called HARQ (Hybrid Automatic Repeat Request) to ensure that data is transmitted reliably. HARQ allows the MAC layer to retransmit data that is not received correctly by the UE.

In summary, the MAC layer plays a critical role in CA by maintaining the ScellDeactivationTimer, monitoring data inactivity, and aggregating data from multiple cells. These functions help to ensure that the UE experiences a seamless and efficient connection when using CA.

Question 17: What is CA bandwidth Class in LTE?

Answer: Carrier Aggregation (CA) bandwidth classes supported by LTE networks. The bandwidth class is a measure of the total bandwidth that can be aggregated using CA. The higher the bandwidth class, the more bandwidth can be aggregated and the higher the theoretical peak data rate that can be achieved.

Question 18: What is Cross-carrier scheduling in LTE?

Answer:

Cross-carrier scheduling is a technique in Carrier Aggregation (CA) that allows the eNodeB (eNB) to schedule data transmissions on different component carriers for the same UE. This is in contrast to self-scheduling, where the eNB schedules data transmissions on the same carrier as the grant is received.

Cross-carrier scheduling is beneficial in a number of scenarios, including:

• Inter-cell interference mitigation: When UEs are located near the edge of a cell, they may experience interference from neighboring cells. This interference can reduce the data rate and increase the latency of the UE’s connection. Cross-carrier scheduling can be used to mitigate this interference by scheduling data transmissions on different carriers for the UE.
• Load balancing: When a large number of UEs are connected to a cell, the eNB may need to schedule data transmissions on multiple carriers to avoid congestion. Cross-carrier scheduling can be used to balance the load across the different carriers and ensure that all UEs receive a fair share of the available bandwidth.
• Frequency diversity: Different frequency bands have different propagation characteristics. For example, lower frequency bands have lower path loss but are also more prone to interference from buildings and other obstacles. Higher frequency bands have higher path loss but are also less prone to interference. Cross-carrier scheduling can be used to take advantage of the different propagation characteristics of different frequency bands to improve the performance of the UE’s connection.

To implement cross-carrier scheduling, the eNB includes a carrier indicator field (CIF) in the PDCCH (Physical Downlink Control Channel) messages. The CIF indicates which carrier the scheduled data transmission is for. The UE then uses the CIF to decode the scheduled data transmission.

Cross-carrier scheduling is a complex technique that requires careful coordination between the eNB and the UE. However, it can provide significant benefits in terms of performance and interference mitigation.

Here are some additional details about cross-carrier scheduling:

• Cross-carrier scheduling can be used with both intra-frequency CA and inter-frequency CA.
• Cross-carrier scheduling can be used for both downlink and uplink transmissions.
• The eNB can configure different cross-carrier scheduling configurations for different UEs.
• The UE can report its cross-carrier scheduling capabilities to the eNB.

Question 19: Can the UE support intra-frequency CA and inter-frequency CA?

Answer:

Yes, the UE can support both intra-frequency CA and inter-frequency CA.

Explanation:

Lest Take example, the two PCCs are using different frequency bands (B66 and B17). This means that the UE must be capable of inter-frequency CA to aggregate the two PCCs. Additionally, the UE could also use intra-frequency CA to aggregate the two SCells, if they are using the same frequency band.

Relevant valid points:

• Intra-frequency CA aggregates component carriers within the same frequency band, while inter-frequency CA aggregates component carriers from different frequency bands.
• Intra-frequency CA is generally less complex and costly to implement than inter-frequency CA.
• Inter-frequency CA can provide higher data rates than intra-frequency CA by aggregating bandwidth from multiple frequency bands.

If the operator is using inter-frequency CA to aggregate the two PCCs from different frequency bands. This is likely because the operator wants to provide users with the highest possible data rates. The operator could also use intra-frequency CA to aggregate the two SCells, but this would only provide a smaller increase in data rates.

Overall, with above example the UE is capable of both intra-frequency CA and inter-frequency CA. The operator can choose to use either type of CA depending on their specific needs and requirements.

Question 20: What is the expected behaviour of the UE when it transitions from PCell to SCell?

Answer:

When the UE transitions from PCell to SCell, it is expected to:

• Maintain its RRC connection with the eNB.
• Continue to receive and transmit data on both the PCell and SCell.
• Seamlessly switch between the PCell and SCell without any noticeable disruption to service.

Explanation:

Let’s take example The UE is configured with two Secondary Cells (SCells). The UE is currently connected to the PCell and is receiving data on both the PCell and SCell. When the UE transitions to the SCell, it is expected to maintain its RRC connection with the eNB and continue to receive and transmit data on both the PCell and SCell. This is known as inter-cell handover.

The UE should be able to seamlessly switch between the PCell and SCell without any noticeable disruption to service. This is achieved through a number of mechanisms, including:

• Cross-carrier scheduling: The eNB can schedule data transmissions on both the PCell and SCell for the same UE. This allows the UE to continue to receive and transmit data on both cells even when it is transitioning between them.
• HARQ retransmissions: The eNB can retransmit HARQ messages on both the PCell and SCell to ensure that the UE receives all of the data that was transmitted.
• Buffering: The UE can buffer data received on the PCell while it is transitioning to the SCell. This ensures that the UE does not experience any loss of data during the handover process.

Relevant valid points:

• Inter-cell handover is a complex process that requires careful coordination between the UE and the eNB.
• Seamless inter-cell handover is essential for providing users with a high-quality mobile experience.
• Carrier Aggregation (CA) can make inter-cell handover more challenging, due to the increased number of cells involved.
• However, there are a number of mechanisms that can be used to ensure that inter-cell handover is seamless even in CA scenarios.

In the context the UE is expected to seamlessly switch from the PCell to the SCell without any noticeable disruption to service. This will be achieved through a combination of cross-carrier scheduling, HARQ retransmissions, and buffering.

For more details on Carrier Aggregation : https://techlteworld.com/carrier-aggregation-in-lte-a-lte-advanced/