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 (Non-Standalone) and SA (Standalone) cell.

Initially, the operator might deploy the cell as an NSA Cell.
As more UEs start supporting Standalone (SA) and the operator enables SA capability, the cell configuration can be changed to support SA.
UEs can still be camped on the cell as NSA or in SA.

There is no limitation from 3GPP Specifications preventing the same cell from operating in both modes. 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 Standalone mode.

NR Cell Available for both NSA and SA modes

How does the UE know whether the NR Cell is NSA Capable or SA Capable?

The network indicates to the UE whether it is an NSA-capable cell or SA-capable cell via the Master Information Block (MIB).

When a UE scans for SS-PBCH blocks for a particular GSCN, it decodes the MIB. The MIB contains information on whether the Remaining System Information (RMSI) is being broadcasted for the cell or is omitted.

SA Capable: If SIB1 is being broadcasted, the UE knows that this is an SA-capable cell.
NSA Capable: If the cell does not broadcast this information, the UE understands RMSI is not available, and it might be used as an NSA cell.

Detecting Standalone (SA) Mode

In the case of Standalone (SA), the UE determines from MIB that a CORESET for Type0-PDCCH CSS set is present if:

kSSB ≤ 23 for FR1
kSSB ≤ 11 for FR2

Note:

kSSB = [0 ... 23] for FR1

kSSB = [0 ... 11] for FR2

If the UE determines from MIB that a CORESET for Type0-PDCCH CSS set is present, it knows that the NR cell is Standalone capable. The UE determines the number of consecutive resource blocks and 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.

Detecting Non-Standalone (NSA) Mode

In the case of Non-Standalone (NSA), the UE determines from MIB that a CORESET for Type0-PDCCH CSS set is NOT present if:

kSSB > 23 for FR1
kSSB > 11 for FR2

Note:

kSSB = [24 ... 29] for FR1

kSSB = [12, 13] for FR2

Additionally, if a UE detects an 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.

Example 1: Finding the Second SSB

Let’s take an example for an FR1 TDD Cell with 100 MHz BW. The cell has the following 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, the UE determines that a CORESET for Type0-PDCCH CSS set is not present since 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.

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.

Example 2: No Associated Type0-PDCCH

Let’s take another example for an FR1 TDD Cell with 100 MHz BW. The cell has the following 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 Specification 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.

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.

Summary

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.