Comprehensive Introduction to 5G NSA Option 3x Networking Process

Due to the lack of systematic information available online about the NSA Option 3x complete process, many references are scattered and fail to provide a comprehensive introduction to the NSA Option 3x process. Therefore, by comprehensively referencing various materials, I have summarized my personal understanding of the NSA Option 3x process, and I would like to share and discuss it with everyone. If my understanding is incorrect, please send me a message so I can improve it for better communication and learning.

1.5G NSA Option 3x Networking Architecture

From the current networking method of China Mobile, it belongs to the Option 3x type. The architecture diagram provided in the reference material is as follows:

NSA Networking Option 3x

Option 3x: Option 3x deployment architecture (data can be offloaded from NR to LTE); user plane data on the same bearer can be transmitted simultaneously on LTE and NR. The EPC needs to be upgraded to support connection with NR, and the backhaul between LTE and NR must support the transmission rate of LTE.

I have drawn the following architecture diagram based on my understanding and summarized materials (though it looks a bit ugly, it does not affect understanding, please do not criticize!).

According to the summarized materials, I personally feel it is quite good, as follows:

(1) LTE eNB serves as the main node of dual connectivity, MeNB, carrying control and user plane data, while the terminal accesses the core network EPC through LTE eNB, and the NR gNB acts as a secondary node, carrying user plane data.

(2) UE and the main and secondary nodes each have their own RRC connections, independently managing their own resources (RRM), but the UE only has an RRC state facing the main node.

(3) The initial connection establishment of the UE must go through the main node MeNB, with SRB1 and SRB2 established at the main node.

(4) The UE can establish SRB3, for direct RRC PDU transmission with the secondary node SgNB.

(5) The SgNB side needs to broadcast at least the MIB system information. In the EN-DC scenario (for example, when adding SgNB), the SgNB side broadcasts the system information SIB1 through proprietary signaling reconfiguration RRC Connection Reconfiguration message provided to the UE, which is transmitted to the UE through MeNB.

2.5G NSA Option 3x Access Process

The NSA Option 3x access process can be roughly divided into 3 processes:

1) X2 Connection Establishment Process

2) UE LTE Access Process

3) SgNB Addition Process

Below are the respective introductions:

X2 Connection Establishment Process:

(1) SgNB triggers the X2 connection establishment: SgNB sends an X2 Setup Request message to MeNB to request establishing the X2 connection, and MeNB responds with an X2 Setup Response message.

(2) MeNB triggers the X2 connection establishment: MeNB sends an X2 Setup Request message to SgNB to request establishing the X2 connection, and SgNB responds with an X2 Setup Response message.

UE LTE Access Process:

The UE attaches to the main node MeNB network and the core network EPC and establishes service bearer, which can refer to the normal access process of LTE. The process change is not significant, and I will not elaborate here, just need to specifically state that: Under NSA, the UE will report capability information twice. The first report is for EUTRA capability, and the second report is for NR and EUTRA-NR capability (as shown in the figure, 10 and 11 locations, the NSA will perform the capability reporting twice).

LTE attachment process

SgNB Addition Process:

Process Introduction:

(1) MeNB and SgNB establish X2 connection.

(2) UE attaches to the main node MeNB network and the core network EPC and establishes service bearer.

(3) MeNB issues NR measurement configuration to the UE, which can be understood as similar to inter-frequency measurement, including B1 event threshold (the meaning of B1 event threshold is that the neighboring signal of the different system is above a threshold value), and the UE will start measuring the NR according to the rules.

(4) If the B1 event threshold is met, the UE reports the B1 measurement report. MeNB judges that it is for adding SgNB, and sends an Sn Addition Request message to the SN. This Sn Addition Request message mainly carries the E-RABs-ToBeAdded-List and MeNBtoSeNBContainer. Among them, MeNBtoSeNBContainer carries the SCG-ConfigInfo.[t1]

(5) SgNB receives the SgNB Addition Request message, performs PSCell candidate cell selection and admission control. If the admission is successful, it replies to MeNB with an SgNB Addition Request Acknowledge message; if the admission fails, it replies to MeNB with an SgNB Addition Request Reject message.

(6) MeNB receives the SgNB ‘s SgNB Addition Request Acknowledge message, and issues the air interface RRC Connection Reconfiguration message to the UE, carrying the SgNB side’s SCG configuration.

(7) ～9a) UE receives the RRC Connection Reconfiguration message, completes the configuration of SCG, and replies to MeNB with the RRC Connection Reconfiguration Complete message.

UE detects the downlink signal of the PSCell, captures the system broadcast MIB information, and parses the RRC Connection Reconfiguration message carrying the ServingCellConfigCommon information to obtain relevant system broadcast SIB1 parameters.

Note: In the EN-DC scenario (for example, when adding SgNB), the broadcast system information of the SIB1 of the PSCell on the SgNB side is provided to the UE through the proprietary signaling reconfiguration RRC Connection Reconfiguration message, which is transmitted to the UE through MeNB.[t2]

(7) ～9b) UE competes or non-competitively accesses the SgNB cell.

(10) MeNB receives the UE‘s RRC Connection Reconfiguration Complete message, sends the SN Reconfiguration Complete message to SgNB, notifying the SN that the air interface reconfiguration for the UE is complete.

(11) ～12) Only in cross PCE scenarios, MeNB replies to SgNB with the SN Status Transfer message, and data feedback from MeNB to SgNB is performed to avoid service interruption during the dual connectivity activation process.

(13) ～14) Only in cross PCE scenarios, MeNB sends the EPC E-RAB Modification Indication message to notify that the downlink tunnel information of the EPC has changed, and the EPC receives a reply of the E-RAB Modification Confirmation message.

(15) ～16) After completing the addition of the SgNB process, the PSCell on the SgNB side issues a measurement reconfiguration message to the UE, carrying the A2 event threshold. The A2 event threshold means that the service cell signal is below a threshold value.

Note: I would like to clarify my personal understanding.

After powering on, the UE will perform LTE network selection and LTE network attachment. This process is similar to the original LTE process, just that during the network attachment process, the UE will report its capabilities to the network twice.

After the UE attaches to the main node MeNB network, MeNB issues a B1 measurement containing NR measurements. Through the B1 measurement, the UE reports the target SgNB information and then reports it to the SgNB. After communication between MeNB and the target SgNB, the SgNB sends some necessary information to the UE through MeNB, and the UE initiates access to the target SgNB based on this information. After successful access, the UE will follow the established process to switch to the LTE frequency and maintain RRC continuity with LTE to ensure the control plane information is smooth. At this time, the UE will use the B1 event measurement in the RRC reconfiguration message to report the NR information. If necessary to perform an SgNB handover, it will report it to MeNB, who will decide and cooperate with the execution.

If there is data to be transmitted, the actual situation can determine whether to transmit through MeNB on the LTE network or through SgNB on the NR network. If transmitted on the NR network, the SgNB will complete the RRC process (at this point, the SgNB and NR already have the UE‘s RRC state stored and context default bearer).