The explanation for why 5G ANR is not working when SIB1 is disabled in the 5G NSA Network

This article explains why 5G ANR is not triggered and does not work whenever SIB1 is disabled in the 5G Non-Standalone network.

The below figure shows the signaling flow for NR PSCell changes in 5G NSA Network.

1. The UE sends a measurement report for the frequency specified by the gNodeB in the RRC reconfiguration message (Message sent before the MR).
2. MR Report covers information about Serving and NBR PCI, RSRP, and RSRQ.
3. If the serving 5G detects that the reported PCI is missing, it will request the UE to report the CGI information for the target 5G Site.
4. The 4G e-NB will forward the request to the UE through the RRC Reconfiguration message and add DRX configuration. DRX is needed to terminate DL Data reception in the serving cell so that the UE can receive System information from the target cell
5. If SIB1 is not enabled in the target site, the UE will not be able to obtain CGI information from the Target 5G Site, and accordingly 5G Serving site will not be able to add the missing NBRs and ANR will not take effect.
6. If SIB1 is enabled, the UE will decode CGI information and report MR report, which includes CGI information, and the 5G Serving site will proceed with the missing NBR addition.
   

NR Cell Global Identity (NCGI)

Cells are identified at a global level using their NR Cell Global Identity (NCGI)

•The Mobile Country Code (MCC) and Mobile Network Code (MNC) uniquely identify the operator`s network.

•The NR Cell Identity (NCI) uniquely identifies a cell within the operator`s network.

The NCGI and NCI structures are illustrated in the Figure below.

• The NCI has a total size of 36 bits. These 36 bits are shared between the g-NB Identity and the Cell Identity. The g-NB identity can be configured to use between 22 and 32 bits. This leaves between 14 to 3 bits for the Cell Identity.
• Allocating 22 bits for the g-NB identity is likely sufficient for the majority of networks.
• * The number of bits indicates the number of Sites/Cells that can be configured by the network; for example, the number of cells per Base Station can become high when using the Centralized Unit(CU)/ Distribute Unit (DU) split Base Station Architecture.

Side note: SIB1 is not mandatory to be enabled in a 5G NSA Network.

Working Materials:

YouTube Video: