Читать книгу NG-RAN and 5G-NR - Frédéric Launay - Страница 25

When the mobile is in the RRC_CONNECTED state and CM_CONNECTED state, its mobility is controlled by the NG-RAN node via the handover mechanism between a source radio node and a target radio node, or between two beams of the same radio node.

In the case of a handover, the NG-RAN node exchanges RRC signaling with the mobile. There are several types of handover:

1 – on the Xn interface between two neighboring connected nodes;

2 – on the NG interface with the UPF.

In the case of beam-based coverage, the beam selection is carried out at the MAC (Medium Access Control) layer of the mobile from mobile measurements. No RRC signaling occurs when the beam is changing since the mobile was configured at the beginning of the radio connection in terms of the measurements to be carried out (Measurement Objects).

The mobile makes radio measurements on one or more beams of a cell and determines:

1 – the radio quality of the beam by filtering the measurements on the L1 layer;

2 – the quality of the radio link of the cell by averaging the measurement of the different beams at the level of the L3-RRC layer.

The mobile also carries out measurements on the quality of the SSB block (Synchronization Signal and Broadcast) of the beams under the coverage of the cell, and also the quality of the neighboring intra- and inter-band cells. Each SSB measurement is seen by the mobile as a different cell.

Table 1.1 summarizes the different cases.

Table 1.1. RRC mobile states

RRC_IDLE	RRC_INACTIVE	RRC_CONNECTED
Cell selection is controlled by the UE in the function of radio access network parameters (AS: Access Stratum).	UE mobility is known by the core network.The core network knows the identity of the radio node with which the terminal is connected.
The UE listens to broadcast signal.
Paging notifications are initiated by the core network.	Paging notifications are initiated by the NG-RAN node.
The mobile has a temporary identity created by the core network.No UE context is stored on the NG-RAN node.	The NG-RAN node knows the RNA location area on which the mobile is camped
The mobile radio context is stored at the mobile side and the NG-RAN node side. The core network and the NG-RAN node exchange information through NG-C and NG-U interfaces.