LTE Event A6 for Carrier Aggregation

When LTE was launched a decade ago, setting up measurements was a simple thing. When a connection is established, the network typically instructs the mobile device to measure the signal strength of the currently serving cell and the signal strength of neighboring cells. The result is either reported periodically or when predefined events occur. When I last looked more closely, most networks were using event A1 for reporting when the serving cell becomes better than a threshold, A2 when the serving cell becomes worse than a threshold and A3 and A4 when a neighbor cell becomes better than the serving or than a threshold. That was nice and easy. And then over time things got way more complicated and I didn’t look more closely anymore. And yes, there is event B1 and B2 for inter-RAT measurements when things get bad and the network needs to consider GSM and UMTS neighbors.

Today, Carrier Aggregation CA is used in most networks and it is not uncommon for high end devices and good networks to bundle 4 different carriers together. And for each carrier, measurements and events are setup up, before and after CA is added. In other words, there’s more than a handful of measurement objects, report configs and measurement ids configured at any one time and things change constantly. Not easy to keep track of all those ids when you see measurement reports in traces. Questions like ‘what has measurement id 7 been set-up for’ and ‘what is measurement object 3 again’ leaves one puzzled and requires careful note taking? Would be nice to see trace tools starting to de-reference these pointers.

Anyway, apart from rising complexity, one event that must be used for quite some time already but which I haven’t really noticed consciously so far is event A6. Interestingly, that’s an event that wasn’t there in the original specifications. It’s name gives a hint why: ‘Neighbor becomes offset better than SCell ‘. An SCell is a secondary cell which is one of the cells that is added as part of carrier aggregation to the initial primary cell (PCell). In other words, event A6 triggers a change of one of the SCells that are part of a Carrier Aggregation configuration!

Another thing I’ve noticed in several networks lately is that the addition of carriers for CA is done in two ways: For carriers in lower frequency bands compared to the PCell, I’ve noticed in different networks that they are added blindly, i.e. without any prior measurement if the device can actually see those layers and cells. Makes sense as it’s very likely that a device can receive all lower bands compared to the PCell as they reach further. For higher frequency bands compared to the PCell, measurements are configured and measurement reports are expected before such bands/cells are added to the Carrier Aggregation set.

Yes, and then there’s B1 for inter-RAT measurements, not only for the ultimate escape to GSM and UMTS but also for finding 5G cells. But that’s another story.