E2 Service Model

In our other post Open RAN E2 Interface , we discussed that E2 is the interface between the Near RT RIC and Open RAN CU/DUs. This interface uses following two protocols.

• E2 Application Protocol (E2AP)
• E2 Service Model (E2SM)

E2AP Protocol establish the logical connection between to E2AP end-points (Near RT RIC and Open RAN CU/DU) provides services to implement an E2 Service Model – E2SM. 

O-RAN Alliance WG3 has standardized following three service models and these service model are implemented as xApps in the Near-RT RICs.

• E2SM Key Performance Matrix (KPM)
• E2SM Network Interfaces (NI)
• E2SM RAN Control (RC)

In this post, we will discuss about E2SM RAN Control, E2SM RAN Control (RC), introduced in July 2021 in O-RAN Specifications. It implements control functionalities through E2 control procedures within the xApps and keeping the goal to optimize the radio resource management.

E2SM RC Characteristic 

• E2SM RC can control radio bearer, to modify parameters for the bearers of all or a specific UE. e.g. QoS parameters, bearer admission control, split bearer and PDCP duplication
• It can control radio resource allocation of a CU/DU, a cell, a slice or a UE to modify DRX, SPS, for PRBs for a slice
• It can control the connected mode mobility, to initiate a mobility procedure for UEs in RRC connected state, i.e., a handover to a specific cell, or a conditional handover to a set of cells
• It can control radio admission with parameters like random access backoff, cell baring
• With Dual Connectivity, E2SM RC can configure and trigger the handover of a UE to selected target secondary cells, secondary cell updates, or DC release
• It can control Carrier Aggregation, to initiate CA and modify the CCs for a specific UE;
• In idle mobility control, It can modify functions for mobility procedures of UEs in a RRC idle state, including cell re-selection priorities and idle timers.

E2SM RAN Control Procedure

E2SM RC also provides capabilities for UE identification and UE information reporting. The control actions and policies that E2SM specifies relate to specific parameters standardized by the 3GPP, i.e., the control action will usually carry the value for a 3GPP IEs defined in the E2SM specifications. For example, the handover control IE includes a target cell ID for the handover encoded as a 3GPP Target Cell Global ID IE or A3 offset IE etc.

To effectively implement control actions and/or enforce policies, the E2SM leverages a combination of the E2 services like E2 Report, E2 Insert, E2 Control and E2 Policy .

Following figure shows an example E2 messages that an E2 node (CU/DU) and an xApp in the near RT RIC can exchange during a typical RAN Control procedure.

• xApp subscribes to the E2 node, which needs to expose a specific RAN function called RAN control with RIC Subscription Procedure. Here, xApp can specify a triggering event or a timer.
• When the timer expires or the condition defined in the trigger is True, the E2 node sends an E2 Insert message as RIC Indication to the RIC. E.g, this may happen when a metric that the gNB monitors exceeds a certain threshold, like the RSRP for a neighboring cell becomes better with an offset than the current cell and decide to Target Cell for Handover
• The Insert RIC Indication Event can have following outcomes.
  • The near-RT RIC can reply with an E2 control message that either denies the control request or accepts and replies with a control action e.g. a target cell ID selected by the xApp.
  • if E2 node timer expires before the reception of a message from the RIC, the procedure may continue autonomously, or it may be halted, according to the specific procedure and configuration of the E2 node.

The control action sent by the xApp can also be asynchronous, i.e., it does not depend on the reception of an insert message from the E2 node. Additionally, the E2SM can also be used to specify policies, i.e. to change pre-defined behaviors in the E2 nodes. These Policies can have two flavors:

• Flavor#1: Allow the E2 node to perform radio resource management actions without the interaction with the RIC, when certain conditions are satisfied
• Flavor#2: The offset policies, which change 3GPP or OEM defined thresholds by adding or removing offsets and thus modifying how the E2 node performs specific functions.

References

• O-RAN near-real-time RAN intelligent controller E2 service model (E2SM), RAN function network interface (NI) 1.0, ORANWG3.E2SM-NI-v01.00.00 Technical Specification,
• O-RAN near-real-time RAN intelligent controller E2 service model, ran control 1.0, ORAN-WG3.E2SM-RC-v01.00 Technical Specification
• O-RAN Working Group 2, “O-RAN Non-RT RIC Architecture 1.0,” O-RAN.WG2.Non-RT-RIC-ARCH-TS-v01.00 Technical Specification
• 3GPP TS 28.552 – 5G performance measurements,” Technical Specification
• 3GPP TS 32.425 Performance measurements Evolved Universal Terrestrial Radio Access Network (E-UTRAN)

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• Open RAN (O-RAN) Reference Architecture
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