How Does An LTE eNB Signal A Co-Located 5G Cell?

In the previous post I looked at 3GPP TS 23.401 and how the UE, the RAN and the LTE core network signal their 5G capabilities. TS 23.401 states that the LTE eNodeB signals a locally connected 5G gNB to the UE. But how does it actually do that?

Actually it is very nicely hidden in the specs. TS 36.331 on Radio Resource Control (RRC) should actually contain this information. A search for a few keywords, however, brought up nothing. So I had a closer look at the system information messages and in particular which information elements were added in Release 15 (-r15). After a while I stumbled over the following:


Could that be it? And it is indeed as discussion paper R2-1801529 reveals:

A 1 bit NR indicator is added per PLMN. The indicator is added to SIB2. And further from the paper: “[…] this indicator has been captured in 36.331 using upperLayerIndication without explicit description to show the intention of the indicator and this creates ambiguity for the reader”. Bingo!

So the upperLayerIndication which can only be set to true or is missing otherwise indicates to the upper layers of the protocol stack that a 5G cell is co-located!

The paper notes that for clarity reasons the parameter should be renamed, for example to ENDC-Indication and a more detailed description on its purpose should be added. I had a look at the subsequent RAN2 meeting reports but couldn’t find any agreement or CR to change this there. So for the moment the EN-DC indication in SIB-2 is referred to as upperLayerIndication. A very unfortunate choice which really makes me wonder what kind of politics was behind this!?