Although I am quite interested in RRC measurements I have never seen the A6 event in action. Rather the eNB vendors have implemented carrier aggregation in a way that the UE provides its capabilities and according to the this the maximum possible numbers of component carriers is added to the connection. There is no RRC measurement report before adding secondary LTE cells to the connection. So what is the A6 event good for and is it used at all?
Surprisingly I needed only 2 attempts to find an example of using the A6 event in a live network configuration. It is used when more component carriers are available than the UE can simultaneously handle. E.g. if there are 4 or more cells with different carrier frequencies available in the same antenna sector the A6 event ensures after the initial CA configuration that the cells with the best radio conditions are selected as secondary cells.
Let’s have a look at this scenario in detail. Figure 1 shows the report configuration for the A6 event. Keep the reportConfigId = 3 in mind.
|Figure 1: Report Configuration for Event A6|
The next step is the configuration of the Measurement ID as shown in figure 2. Here the reportConfigId is combined with a measObjectId that represents the carrier frequency of the potential SCell.
|Figure 2: Measurement ID for Event A6|
Now, if the event A6 is triggered in the UE a RRC Measurement Report with this measId = 3 is sent to the eNB as shown in Figure 3.
|Figure 3: RRC Measurement Report for Event A6|
|Figure 4: Change of SCell|