How the Addition of 5G Radio Resources Increases the Complexity of LTE Signaling Procedures

While everybody is excited about the growing number of 5G deployments and speed test results it is easy to forget that a highly reliable LTE core and radio access network is the prerequisite for 5G non-standalone (NSA) data transmission.

Indeed, the 5G radio resources are just added to the ongoing LTE connection to provide higher bandwidth that enables in turn higher throughput. In other words: the current 5G deployments are designed for and limited to the needs of enhanced Mobile Broadband (eMBB) traffic.

To boost the user experience a 4G and a 5G base station cooperate and bundle there joint resources in one radio connection. The whole scenario is known as E-UTRA-NR Dual Connectivity (EN-DC) and as a matter of fact this dual connectivity increases the complexity of the RAN signaling tremendously.

The figure below shows the two base stations involved in the radio connection. On the left side is the Master eNodeB (MeNB) that controls the entire signaling connection. On the right side sits the en-gNB, also called Secondary gNodeB (SgNB). The inconsistency of acronyms originates from 3GPP specs. 3GPP 37.340 "E-UTRA and NR Multi-connectivity" can be seen as an umbrella document that originally coined "MeNB" and "SgNB". However, when standarizing more details these acronyms have been replaced with Master Node (MN) and Secondary Node (SN) and the latter is named "en-gNB" when used in EN-DC scenarios. (Sure this spec has a lot more terms to offer an is a must-read for every acroynm enthusiast.)

However, these naming conventions defined in 3GPP 37.340 have not made it into the protocol specs, especially not into 3GPP 36.423 "X2 Application Part" that names its message set for enabling EN-DC consequently "SgNB ...." - as also shown in the figure.

By the way the SgNB should also not be imagined as a single network element. On the 5G side often a virtual RAN architecture is already deployed. In such a VRAN a gNB central unit (CU) controls several gNB distributed units (DUs) and multiple remote radio heads (RRHs) including the 5G antennas can be connected to each DU.

5G Radio Resource Addition in EN-DC Mode

Before 5G radio resources can be added to the connection a LTE RRC connection and at least a default bearer for the user plane including its GTP/IP-Tunnel between S-GW and eNB must have been successfully established.

The trigger for adding 5G resources to this call is mostly an inter-RAT measurement event B1 (not shown in the figure). However, also blind addition of a 5G cells have been observed in some cases where the 5G cell coverage is expected to overlap exactly the footprint of the LTE master cell. 

All in all, there can be a 1:1 mappig between 4G and 5G cells when antennas are mounted very close to each other and pointing into the same direction. However, it is also possible that several 5G small cells (especially when using FR2 frequency bands) are deployed to cover the footprint of a 4G macro cell. 

The end-to-end signaling that adds 5G resources to the connection starts with the X2AP SgNB Addition Request message (1). It contains information about the active E-RABs of the connection, UE NR capabilities and often the singal strenght of the 5G cell as measured before is included as well. The message triggers allocation of 5G radio resources in the SgNB.

Similar to a X2 handover procedure the X2AP SgNB Addition Request Acknowledge message (2) is used to transport a NR RRC CG-Config message (3) back to the MeNB where it is "translated" into NR RRC Connection Reconfiguration and NR RRC Radio Bearer Config messages that are sent to the UE enclosed in a LTE RRC Connection Reconfiguration message. In these messages beside the Cell Group ID the 5G PCI and the absolute SSB frequency (a synonym for NR ARFCN) are found. Both, 5G PCI and SSB frequency in combination represent the identity of a 5G cell "visible" for the UE on the physical 5G radio interface. 

To keep the figure more simple I have spared the "translation" process in MeNB and show instead as next step the combined LTE/NR RRC Connection Reconfiguration Complete (4) that is send by the UE back to the MeNB to confim activation of the 5G radio link. 

After this the UE and the SgNB are ready to the 5G resources for radio transmission. However, one important component is still missing: a new GTP/IP-Tunnel for transporting the payload from the core network's serving gateway (S-GW) to the SgNB. 

The gNB downlink transport layer address (gNB DL TLA) and its appropriate GTP Tunnel Endpoint Identifier (TEID) have been already to the MeNB in step (2). Indeed, there are some more TLAs and TEIDs found in this X2AP message, especially for data forwarding across the X2 user plane interface (not shown in figure).

The MeNB forwards the gNB DL TLA/TEID to the MME (6) where it is forwarded to the S-GW using GTP-C signaling in case the two core network elements are connected over S11 reference point. The uplink TLA/TEID on the S-GW side remain the same as assigned before during establishement of the E-RAB (not shown in figure). So the new tunnel is now ready to be used (7) and transmission of payload packet starts immediately. 

In step (8) the MME confirms the successful tunnel establishment to the MeNB.

To total duration of the entire procedure from step (1) to (8) sums up to slightly more than 100 ms under lab conditions and typically around 300 ms in the live network. 

This delay does not have a direct impact on user plane latency in the initial 5G setup phase. However, the subscriber experience might be different when it comes to inter-MeNB handover, because there is no direct handover between 5G neighbor cells. 

Changing the MeNB due to subscriber mobility means: release all 5G resources on the source (M)eNB side, perform intra-LTE handover to the target (M)eNB and add new 5G resources after handover is successfully completed.