How UE will know which 5G cell to attach in ENDC NSA


How UE will know which 5G cell to attach in ENDC in case of NSA deployment

How UE will know which 5G cell to attach in ENDC NSA

In this blog I will explain how 5G capable UE know that which 5G cell to attach in ENDC in case of NSA deployment. I am assuming you have basic knowledge of 5G NSA if not please read about it here 5g standalone vs non standalone.

Lets discuss How UE will know which 5G cell to attach in ENDC step by step

Note: Most of the steps here are actually from LTE attach procedure, so I will not go into details.

Step 1. Cell selection. UE will select a LTE cell using  PSS and SSS.

Step 2.  UE will start RACH procedure to send RRC message also called msg3.

Step 3. UE will establish RRC connection to send Attach request.

Step 4.  EMM Attach. In this step MME will initiate authentication procedure and NAS level security procedure and will send attach accept message to establish default bearer.

Step 5. The MN-eNB sends the rrc connection re-configuration message to UE which includes the attach accept and Activate Default Bearer Request .

In case of 5G ENDC, this message also includes measurement objects for 5G NR frequencies. It tells the UE what are the 5G channels it should measure and sends the reports.

Step 7. UE after allocating resources, sends the attach complete message to the eNB.

Step 8. There is no signaling here. I have added this step just for understanding.  Now that we have established default bearers, data will start flowing through these default bearers.

Measurement of 5G NR channels

LTE attach is complete now. So, UE will initiate measurement of 5G NR channels which are specified in the Measurement Objects received in the RRC Connection Reconfiguration message. Please remember UE will also measure LTE channels as configured by the network. That’s why 5G cell measurements are scheduled during
measurement gaps.

Step 9. 5G NR PSS: – UE will acquire the 5G NR Primary Synchronization Signal.

Step 10. 5G NR SSS: – UE will also acquire the 5G NR Secondary Synchronization Signal. Using PSS and SSS UE finds the signal strength of 5G cells.

Step 11. 5G signal quality is reported back to LTE eNB which is a MN-eNB(Master eNB).

After step 11 LTE cell(MN-eNB) decides which cell to add as a secondary based on the measurements report reported by UE and start the secondary cell addition process which i will explain in next blog “5G ENDC Signaling Call Flows”.

In 5G, PSS and SSS are part of SS block. To read more about 5G initial access click this link Initial Access and cell selection call flows in 5G NR Network.



2 thoughts on “How UE will know which 5G cell to attach in ENDC NSA

  1. Hi,

    The explanation for NSA call flow is totally wrong. I think you should go for spec again. In NSA mode gNB does not send MIB and SIM for synchronization. all the synchronization related parameters is received via rrc connection reconfiguration message.

    MIB SIB will be in use when gNB deployed as a SA mode. when UE directaly campon to 5g cell.


    1. Yes MIB and system information are part of rr-reconfiguration message but the UE has to acquire MIB(Read the MIB frame number to synchronize downlink framing) . Even though all the RACH information are sent to UE through rr-reconfiguration message but UE still has to perform rach procedure with the 5G cell.

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