UE Radio Capability Signaling Optimization (RACS) in Rel. 16

The data volume of UE Radio Capability Information defined in 3GPP 38.306 is already high and will further increase starting with Rel. 16 due to additional supported bands and other features.

Due to this 3GPP has standardized in Release 16 what is called UE Radio Capability Signaling Optimization (RACS) for both, E-UTRAN/EPS and NG RAN/NGC networks. 

Release 16 RACS does not apply to NB-IoT.

The first key element of this feature set is the introduction of a new UE Radio Capability ID that is structured as defined in 3GPP 23.003 and shown in figure 1 below:

Figure 1: UE Radio Capability ID according to 3GPP 23.003

The components of this new ID are:

•    TF - Type Field (TF): identifies the type of UE radio capability ID.

            Type = 0 -> manufacturer-assigned UE radio capability ID
            Type = 1 -> network-assigned UE radio capability ID

•  The Vendor ID is an identifier of UE manufacturer and only present if TF = 0. The vendor IDs are provided by Internet Assigned Numbers Authority (IANA). The complete list of assigned numbers is publicly available here: https://www.iana.org/assignments/enterprise-numbers/enterprise-numbers

•  The Version ID configured by the UE Capability Management Function (UCMF) that is part of the EPS/5GC. The Version ID value makes it possible to detect whether a UE Radio Capability ID is current or outdated.

·      The Radio Configuration Identifier (RCI) identifies the UE radio configuration.

The PLMN-assigned UE Radio Capability ID is assigned to the UE using the Non-Access Stratum UE Configuration Update Command or Registration Accept message (figure 2).

Figure 2: PLMN-assigned UE Radio Capability Update according to 3GPP 23.743

The new UCMF (UE radio Capability Management Function) stores All UE Radio Capability ID mappings in a PLMN and is responsible for assigning every PLMN-assigned UE Radio Capability ID.

Due to introduction of the UMCM in the core networks the new Nucmf service-based interface is defined for the 5GC and new S17 reference point is defined for the EPS as shown in figure 3.

Figure 3: Network Architecture with UCMF according to 3GPP 21.916

Each UE Radio Capability ID stored in the UCMF can be associated to one or both UE radio capabilities formats specified in 3GPP TS 36.331 [LTE RRC] and 3GPP TS 38.331 [NR RRC]. The AMF must only be able ot handle the NR RRC format while the MME uses the LTE RRC format. Which format is required by the UCMF is configurable.

If at any time the AMF/MME has neither a valid UE Radio Capability ID nor any stored UE radio capabilities for the UE, the AMF/MME may trigger the RAN to provide the UE Radio Capability information and subsequently request the UCMF to allocate a UE Radio Capability ID.

In NG RAN the UE Capability Request can be requested by the AMF as a flag in any NGAP Downlink NAS Transport message or by sending a NGAP UE Radio Capability Check Request (for checking compatibility of IMS voice capabilities). This triggers a NR RRC UE Capability Transfer procedure and subsequently NGAP UE Radio Capability Info Indication or NGAP UE Radio Capability Check Response (for IMS voice support parameters).

Using the NGAP UE Capability ID Mapping procedure the NG RAN node is able to request the most recent UE Capability ID mapping information from the core network functions AMF/UCMF. The same functionality is implemented in S1AP for signaling between eNB and MME/UCMF.

If the volume of the LTE/NR RRC UE Capability to be sent by the UE is larger than the maximum supported size of a PDCP SDU (specified in 3GPP 38.323) then the UE Capability Info can be transported in LTE/NR RRC using a chain of UL Dedicated Message Segment messages.

Figure 4: RRC UL Dedicated Segment Message transporting UE Radio Capability Information according to 3GPP 36.331 and 38.331

Each of these message will have a dedicated segment number and the last one has the rrc-MessageSegmentType =  “lastSegment”, which triggers reassembly of the orignal UE Capabability information in the receiving entity.

Comparing S1AP and NGAP UE Context Release

As an addition to my blog post about the 5G RAN Release procedure I would like to have an in-depth view at the details of NGAP UE Context Release Complete message.

Indeed, the S1AP (known from E-UTRAN) and the NGAP are very similar protocols and when reading the 3GPP specs it is obvious that many message names are identical and the procedures fulfill the same purpose when looking at call scenarios.

However, the difference is visible in the details as one can see when looking at the figure below.

While the S1AP UE Context Release Complete message does not contain any additional information we find in the NGAP UE Context Release Complete the identity of the last serving 5G cell, represented by the NR-CGI, the last visited Tracking Area Identity (TAI) and a list with the IDs of the PDU sessions (E-RABs) that have been terminated when the UE context was released.

This additional information in very valuable for network troubleshooting, since in LTE (S1AP) only the ID (ECGI) of the initial serving cell or a new serving cell ID at inter-node handover was signaled. And if you wanted to know how many E-RABs have been terminated with a S1AP UE Context Release procedure it was necessary to look back into the full sequence of call-related S1AP messages starting with the messages for Initial Context Setup.

All in all, with 5G NGAP trace analysis and the life of RAN engineers becomes easier. Thank you, 3GPP! 

Comparision of S1AP and NGAP UE Context Release Complete Messages

EPS Fallback in 5G Standalone Deployments

It can be expected that later this year some mobile network operators will launch their initial 5G standalone (5G SA) deployments.

Nevertheless there will remain areas with temporary or permanently weak 5G NR coverage. One possible reason might be that even when 5G and LTE antennas are co-located, which means: mounted at the same remote radio head, the footprint of the 5G NR cell is significantly smaller when it uses a higher frequency band than LTE - see figure 1.

Figure 1: Smaller footprint of co-located 5G NR cell with higher frequency

Especially UEs making Voice over New Radio (VoNR) calls from the 5G cell edge have a high risk of experiencing bad call quality, in worst case a call drop. To prevent this the UE is forced  during the voice call setup towards 5G core network (5GC) to switch to a LTE/EPS connection where the radio conditions are better for the voice service.

The same procedure for which the term "EPS Fallback" was coined by 3GPP also applies when the UE is served by a 5G cell that is not configured/not optimized for VoNR calls or when the UE does not have all needed VoNR capabilities.

Figure 2: Two options for EPS fallback

When looking at the RAN there are two options for executing the EPS Fallback as shown in figure 2.

In option A the 5G radio connection is released after the initial call attempt is successfully finished and with the 5G RRC Release the UE is ordered to reselect to a 4G cell where a new radio connection is started for the VoLTE call. In this case the UE context is transferred from the AMF to the MME over the N26 interface. 3GPP seems to use also the term "RAT fallback" for this option.

Option B is to perform a 5G-4G inter-RAT handover. Here the session management and user plane tunnels in the core network are handed over from SMF/UPF to MME/S-GW in addition. This is realized with the GTPv2 Forward Relocation procedure on N26 interface.

All in all the EPS fallback is expected to cause an additional call setup delay of approximately 2 seconds.

For the inter-RAT handover case it is easy to detect from signaling information that an EPS fallback was triggered. In the source-eNodeB-to-target-eNodeB-transparent-container sent by the gNB to the eNB a boolean "IMS voice EPS fallback from 5G" indicator will be found that is set to "true". This container is named according to the receiving entity and will be carried by the NGAP Handover Preparation, GTPv2 Forward Relocation Request and the S1AP Handover Request messages.

If a redirection for Voice EPS Fallback is possible or not is indicated in the NGAP Initial Context Setup Request, Handover Request (during 5G intra-system handover) and Path Switch Request Acknowledge (after Xn handover) messages, all sent by the AMF to the gNB.

Further the NGAP protocol provides the cause value "IMS voice EPS fallback or RAT fallback triggered" in the PDU Session Resource Modify Response message indicating that a requested VoNR session cannot be established.  

An excellent, very detailed description of N26 interface functionality and testing ia available here.