Frequency-based SCell Configuration

This section describes the procedures of frequency-based SCell configuration for uplink/ downlink 2CC aggregation, downlink 3CC aggregation, and downlink 4CC aggregation.

NOTE

Currently, downlink 4CC aggregation works only for FDD+TDD CA.

Uplink and Downlink 2CC Aggregation

Uplink 2CC aggregation and downlink 2CC aggregation use the same SCell configuration procedure. With both uplink and downlink 2CC aggregation enabled, in a single procedure, an eNodeB configures an SCell for a CA UE for both uplink and downlink transmission.

Figure 3-27 Frequency-based SCell configuration for uplink/downlink 2CC aggregation

Among the carriers defined in SccFreqCfg MOs, the eNodeB treats those that have not been configured as SCCs for the CA UE as candidate SCCs. The eNodeB then arranges all candidate SCCs in descending order of SCC priority (specified by the

SccFreqCfg.SccPriority parameter) and attempts to select a cell on a candidate SCC as an SCell for the UE. The configuration procedure is as follows:

1. The eNodeB delivers the A4 measurement configuration, instructing the UE to measure the top-priority candidate SCC. In addition, the eNodeB may set up measurement gaps

for the UE, depending on the inter-frequency measurement capability reported by the UE.

– If the UE requires measurement gaps, the eNodeB considers the bearers of the UE. If a bearer with a QCI of 1 has been established for the UE, the SCell configuration procedure ends. If no such bearer has been established, the eNodeB sets up

measurement gaps in the measurement configuration. The configuration includes the EARFCN, frequency-specific offset, measurement bandwidth, and other

measurement-related parameters. For details about the measurement parameters, see the descriptions related to inter-frequency measurement in Intra-RAT Mobility Management in Connected Mode.

NOTE

During initial frequency-based SCell configuration for inter-eNodeB CA for a UE in relaxed-backhaul-based or multi-BBU interconnection scenarios, the control-plane link and user-plane path are set up and checked for the eX2 interface between the eNodeBs and information is exchanged between the PCell and the candidate SCell. This takes more time than intra-eNodeB CA. In addition, the SCell cannot be configured within a single gap. Therefore, the eNodeB sets up two gaps in this case: one for eX2 setup and check as well as information exchange and the other for SCell configuration. The interval between the two gaps is determined by the CaMgtCfg.SccCfgInterval parameter.

After the eX2 interface is set up, the eNodeB sets up only one gap during subsequent SCell configuration procedures.

– If the UE does not require measurement gaps, the eNodeB delivers the A4 measurement configuration in which measurement gaps are not set up. NOTE

l AutoGapSwitch under the ENodeBAlgoSwitch.HoModeSwitch parameter affects the decision process of the inter-frequency measurement gap setup. If this switch is on, the eNodeB determines whether to set up the gaps based on the reported UE capabilities. If this switch is off, the eNodeB sets up the gaps, without considering UE capabilities. It is recommended that this switch be off to ensure that only cells with satisfactory signal quality can be configured as SCells.

l The threshold for event A4 used in the preceding procedure is equal to CaMgtCfg.CarrAggrA4ThdRsrp plus SccFreqCfg.SccA4Offset.

l If multiple candidate SCCs have the same priority, the eNodeB performs this step on all candidates.

2. After receiving an A4 measurement report that contains cells on the candidate SCC, the eNodeB selects the reported cells that belong to the serving PLMN of the UE or an equivalent PLMN as candidate SCells. The eNodeB then arranges the candidate SCells in descending order of RSRP and proceeds as follows:

– If the PCell can set up a data link to the top-priority candidate SCell, the eNodeB sends an RRC Connection Reconfiguration message to configure the candidate cell as an SCell for the UE. If the SCell is configured successfully, the procedure ends. If the SCell fails to be configured, the eNodeB tries the next-priority candidate SCell.

– If the PCell cannot set up a data link to the top-priority candidate SCell, the eNodeB tries the next-priority candidate SCell.

– If none of the candidate SCells can be configured as an SCell for the UE, the eNodeB evaluates the next-priority candidate SCC.

NOTE

l When a bearer for an emergency call or with a QCI of 1 is set up for a CA UE whose SCell has been configured, the eNodeB does not automatically remove the SCell. The eNodeB removes the SCell only when the conditions described in 3.2.7 SCell Removal are met. l After delivering the A4 measurement configuration related to a candidate SCC to the UE, the

eNodeB may receive an A2 measurement report, indicating unsatisfactory signal quality of the PCell, for an inter-frequency or inter-RAT handover. In such a case, the eNodeB will not configure a candidate cell as an SCell for the UE, even if the eNodeB later receives an A4 measurement report that contains the candidate cell.

Downlink 3CC Aggregation

The SCell configuration procedure for downlink 3CC aggregation is similar to that for uplink/ downlink 2CC aggregation. The difference lies in the number of SCells to be configured and the consequences that follow. For downlink 3CC aggregation, the eNodeB selects two candidate SCells and attempts to configure them. The consequences are as follows: l Both cells are configured as SCells, and the procedure ends.

l One cell is configured as an SCell. The CA UE stays in the 2CC aggregation state. When the traffic volume of the CA UE meets the triggering condition again, the eNodeB performs an SCell configuration procedure, in which the eNodeB selects only one candidate SCell at a time. The procedure ends when a total of two SCells are configured for the UE.

l Neither cell is configured as an SCell. The CA UE stays in the single carrier state. When the traffic volume of the CA UE meets the triggering condition again, the eNodeB performs an SCell configuration procedure, in which the eNodeB selects two candidate SCells. The procedure ends when a total of two SCells are configured for the UE. Figure 3-28 shows the SCell configuration procedure for downlink 3CC aggregation.

Downlink 4CC Aggregation

The SCell configuration procedure for downlink 4CC aggregation is similar to that for uplink/ downlink 2CC aggregation. The difference lies in the number of SCells to be configured and the consequences that follow. For downlink 4CC aggregation, the eNodeB selects three candidate SCells and attempts to configure them. The consequences are as follows: l All the three cells are configured as SCells, and the procedure ends.

l Two of the cells are configured as SCells. The CA UE stays in the 3CC aggregation state. When the traffic volume of the CA UE meets the triggering condition again, the eNodeB performs an SCell configuration procedure, in which the eNodeB selects only one candidate SCell at a time. The procedure ends when a total of three SCells are configured for the UE.

l Only one cell is configured as an SCell. The CA UE stays in the 2CC aggregation state. When the traffic volume of the CA UE meets the triggering condition again, the eNodeB performs an SCell configuration procedure, in which the eNodeB selects two candidate SCells. The procedure ends when a total of three SCells are configured for the UE. l None of the cells is configured as an SCell. The CA UE stays in the single carrier state.

When the traffic volume of the CA UE meets the triggering condition again, the eNodeB performs an SCell configuration procedure, in which the eNodeB selects three candidate SCells. The procedure ends when a total of three SCells are configured for the UE. Figure 3-29 shows the SCell configuration procedure for downlink 4CC aggregation.

Figure 3-29 Frequency-based SCell configuration for downlink 4CC aggregation