NG-RAN and 5G-NR. Frédéric Launay
Mobility in the RRC_Connected state
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. | ||
1.4. Scheduling and QoS
1.4.1. Scheduling
Scheduling enables us to share radio resources among all connected users. A scheduling task is performed in real time to share radio resources among all mobiles. Scheduling attribution is calculated based on:
1 – the quality of the radio link for each mobile;
2 – requirements in terms of the quality of service expected by each mobile;
3 – the state of the mobile buffer.
The scheduler is performed at the MAC layer of the NG-RAN node and defines the scheduling for downlink and uplink transmissions.
The quality of the radio link is used to define the modulation and coding scheme (MCS) for a given mobile, as well as the transmission power. The MCS is dynamically adjusted according to the HARQ (Hybrid Automatic Repeat Request) retransmission rate.
To meet mobile requirements, several strategies can be defined at the scheduler level:
1 – fairness, a strategy for which each mobile receives the same resource allocation regardless of the modulation scheme;
2 – proportional fairness which allocates more frequency resources for mobiles with less efficient MCS;
3 – round-robin, a strategy which consists of allocating equal resources to all mobiles;
4 – max-CQI, a strategy that aims to maximize cell capacity by prioritizing the allocation of radio resources to mobiles which have the best radio quality.
The scheduler is based on measurement reports:
1 – the state of the buffer BSR (Buffer Status Report);
2 – the quality of the radio link (CSI-RS);
3 – the rising power margin PHR (Power Headroom Reports);
4 – Inter-Cell Interference Coordination (ICIC) between NG-RAN nodes.
From this information, the scheduler defined:
1 – the frequency radio resources to be allocated for each mobile;
2 – the number of spatial layers that can be used, depending on the category of the mobile;
3 – the transmission TTI (Time Transmission Interval) instants.
The mobile listens to the information transmitted over the PDCCH (Physical Downlink Control Channel) logical control channel and decodes the information channel when it detects its radio identifier C-RNTI.
The scheduler makes its decisions at each slot. The duration of the TTI slot depends on the spacing between SCS (SubCarrier Spacing). While the scheduler decision is 1 ms in 4G, it is variable from 1 ms, 500 μs or 250 μs for 5G on the FR1 (Frequency Range 1) band, and can go down to 125 μs or 62.5 μs for the FR2 band.
Transmission on the downlink direction can be pre-empted for critical communication (low latency). The NG-RAN node informs the mobiles by transmitting the INT-RNTI identifier over the PDCCH control channel.
Semi-persistent scheduling allows us to periodically allocate radio resources for a mobile. The periodicity of the messages is transmitted over the RRC layer and the resource allocation is transmitted to the mobile via the CS-RNTI identifier.
1.4.2. Support for quality of service on radio link
The QoS (Quality Of Service) control consists of implementing the maximum quality of service applicable to a data flow.
Like the 4G mobile network, only the core network is aware of the service requirements: QoS management is under the control of the core network (AMF). The NG-RAN has no knowledge of the service to be managed. Thus, when establishing a PDU session, the AMF entity establishes QoS rules between the radio node and one or more UPF entities.
The PDU session carries IP flows with one or more different qualities of service for all flows. Each flow is associated with a QFI flow profile identifier. The flow profile corresponds to a QoS indicator (5QI: 5G QoS Identifier) and an allocation and retention priority (ARP). The QFI flag is unique within a PDU session. The flag is either configured during the PDU session establishment procedure or during the PDU session modification procedure.
The value of a QFI is configured by the AMF during the procedure of session establishment; the AMF querying the unified UDR database to know the user’s authorized QoS. For the establishment of dedicated services, the SMF chooses the QoS characteristics (5QI/ARP) according to the values stored at the SMF or by querying the PCF entity. The 5QI/ARP combination defined by the PCF link in a PDU session is a QoS flow binding.
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