Can the Same 5G Cell be used for NSA and SA ?
The Short answer is Yes. The network can configure the same cell as an NSA and SA cell. Initially, the operator might deploy the cell as an NSA Cell and as more and more UE starts supporting Stand Alone (SA), and the operator has the capability to support SA it can change the cell configuration to support SA. UE’s can still be camped on the cell as NSA or in SA.
There is no limitation from 3GPP Specifications that does not allow whether the same cell and be in operating in NSA mode or SA mode, However it increases some complexity in the Core network implementation. The network can still redirect UE camped in NSA mode to the same NR Cell in Stand-Alone mode.
How does the UE know whether the NR Cell is NSA Capable or SA Cable?
The network however indicates to UE whether it is an NSA Capable cell or SA Cabaple cell.
When UE scans for SS-PBCH blocks for a particular GSCN, it also gets Master Information Block (MIB) as part of SSB block decoding. MIB contains information on whether the Remaining system information is being broadcasted for the cell or is omitted. If SIB1 Is being broadcasted then UE knows that this is an SA Capable cell and if it does not broadcast this information then UE can understand RMSI is not available for this cell and it might be used as an NSA cell.
In the case of Stand Alone(SA), the UE determines from MIB that a CORESET for Type0-PDCCH CSS set is present if kSSB ≤ 23 for FR1 or if kSSB ≤ 11 for FR2.
Kssb = [ 0 to 23] for FR1
Kssb = [0 to 11] for FR2
If the UE determines that from MIB that a CORESET for Type0-PDCCH CSS set is present it will know that the NR cell is Stand Alone capable. The UE determines the number of consecutive resource blocks and a number of consecutive symbols for the CORESET of the Type0-PDCCH CSS set from controlResourceSetZero in pdcch-ConfigSIB1.
pdcch-ConfigSIB1 maps to an index in a list of Tables 13-1 through 13-10 in 3GPP TS 38.213.
In the case of Non-Stand Alone(NSA), the UE determines from MIB that a CORESET for Type0-PDCCH CSS set is not present if kSSB >23 for FR1 or if kSSB >11 for FR2;
Kssb = [23 to 29 ] for FR1
Kssb = [12, 13] for FR2
Also If If a UE detects a SS/PBCH block and determines that a CORESET for Type0-PDCCH CSS set is not present, and for Kssb =31 for FR1 or for Kssb=15 for FR2, the UE determines that there is no SS/PBCH block having an associated Type0-PDCCH CSS set.
Let’s take an example for an FR1 TDD Cell with 100 Mhz BW, The cell has below IE set in MIB
subCarrierSpacingCommon = 30 KHz Kssb= 25 controlResourceSetZero = 12 searchSpaceZero = 0
Let’s say UE detects SSB index 0 and gets MIB Information as above.
From the Above information, UE determines that a CORESET for Type0-PDCCH CSS set is not present and if 24<=kSSB<=29 for FR1.
The UE may determine the nearest (in the corresponding frequency direction) global synchronization channel number (GSCN) of a second SS/PBCH block i.e SSB index 1 having a CORESET for an associated Type0-PDCCH CSS as indicated in the below equation.
Now if the UE detects the second SS/PBCH block and the second SS/PBCH block does not provide a CORESET for Type0-PDCCH CSS set then the UE may ignore the information related to GSCN of SS/PBCH block locations for performing cell search.
Let’s take another example for an FR1 TDD Cell with 100 Mhz BW, The cell has below IE set in MIB
subCarrierSpacingCommon = 30 KHz Kssb= 31 controlResourceSetZero = 12 searchSpaceZero = 0
In the case where Kssb=31 for FR1 or Kssb=15 for FR2, 3GPP Spec defines that there is no SS/PBCH block having an associated Type0-PDCCH CSS set. From the Above information, UE determines that a CORESET for Type0-PDCCH CSS set is not present and within a GSCN range as indicated by the below formula.
And if the GSCN range is the same as the first detected SSB index then UE determines that there is no information for a second SS/PBCH block with a CORESET for an associated Type0-PDCCH CSS set on the detected SS/PBCH block.
If the UE does not detect any SS/PBCH block providing a CORESET for Type0-PDCCH CSS set, within a time period determined by the UE, the UE may ignore the information related to GSCN of SS/PBCH locations in performing cell search.
For a serving cell without transmission of SS/PBCH blocks, a UE acquires time and frequency synchronization with the serving cell based on receptions of SS/PBCH blocks on the PCell, or on the PSCell, or on an SCell if applicable
Reference: 3GPP TS 38.213 NR; Physical layer procedures for control