System and method to manage stored network slice selection assistance information

1 . A method performed by a user equipment (UE) in a wireless communication system, the method comprising:

receiving, from a first network related to 5th generation core (5GC), a first message including first rejected network slice selection assistance information (NSSAI) including a first single-network slice selection assistance information (S-NSSAI) with rejection cause information indicating that an S-NSSAI is not available due to a maximum number of UEs reached and storing the first S-NSSAI into second rejected NSSAI for a maximum number of UEs reached based on the rejection cause information;

upon the reception of the first message including the first rejected NSSAI including the first S-NSSAI with the rejection cause information, starting a back-off timer;

determining an intersystem change to a second network related to evolved packet core (EPC), while the back-off timer is running;

receiving, from the second network related to the EPC, an activate default evolved packet service (EPS) bearer context request message;

identifying that the first S-NSSAI and a public land mobile network (PLMN) identifier (ID) are provided in at least one protocol configuration option included in the activate default EPS bearer context request message; and

removing the first S-NSSAI from the second rejected NSSAI for the maximum number of UEs reached and stopping the back-off timer, based on the identification.

2 . The method of claim 1 , wherein the first S-NSSAI is mapped for the second rejected NSSAI based on a reject cause indicating that an S-NSSAI is unavailable, and

wherein the reject cause is at least one of an S-NSSAI not available in a current PLMN, an S-NSSAI not available in a current registration area, an S-NSSAI not available due to a failed or revoked network slice-specific authentication and authorization, or an S-NSSAI not available due to a maximum number of UEs reached.

3 . The method of claim 1 , wherein removing the first S-NSSAI comprises:

deleting the second rejected NSSAI in response to a successful completion of a tracking area update (TAU) procedure with the second network.

4 . The method of claim 1 , further comprising:

continuing to run the back-off timer in response to an intersystem change from N1 mode to S1 mode of the UE.

5 . The method of claim 1 , wherein the first message is one of:

a registration accept message,

a registration reject message,

a deregistration request message, or

a configuration update command message.

6 . The method of claim 1 , wherein the back off timer is received with the first message, and

wherein the back off timer is initiated for a predefined time period.

7 . A user equipment (UE) in a wireless communication system, the UE comprising:

a transceiver; and

at least one processor coupled to the transceiver, the at least one processor configured to:

receive, from a first network related to 5th generation core (5GC), a first message including first rejected network slice selection assistance information (NSSAI) including a first single-network slice selection assistance information (S-NSSAI) with rejection cause information indicating that an S-NSSAI is not available due to a maximum number of UEs reached and storing the first S-NSSAI into second rejected NSSAI for a maximum number of UEs reached based on the rejection cause information,

upon the reception of the first message including the first rejected NSSAI including the first S-NSSAI with the rejection cause information, start a back-off timer,

determine an intersystem change to a second network related to evolved packet core (EPC), while the back-off timer is running,

receive, from the second network related to the EPC, an activate default evolved packet service (EPS) bearer context request message,

identify that the first S-NSSAI and a public land mobile network (PLMN) identifier (ID) are provided in at least one protocol configuration option included in the activate default EPS bearer context request message, and

remove the first S-NSSAI from the second rejected NSSAI for the maximum number of UEs reached and stop the back-off timer, based on the identification.

8 . The UE of claim 7 , wherein the first S-NSSAI is mapped for the second rejected NSSAI based on a reject cause indicating that an S-NSSAI is unavailable, and wherein the reject cause is at least one of an S-NSSAI not available in a current PLMN, an S-NSSAI not available in a current registration area, an S-NSSAI not available due to a failed or revoked network slice-specific authentication and authorization, or an S-NSSAI not available due to a maximum number of UEs reached.

9 . The UE of claim 7 , wherein the at least one processor is further configured to:

delete the second rejected NSSAI in response to a successful completion of a tracking area update (TAU) procedure with the second network.

10 . The UE of claim 7 , wherein the at least one processor is further configured to:

continue to run the back-off timer in response to an intersystem change from N1 mode to S1 mode of the UE.

11 . The UE of claim 7 , wherein the first message is one of:

a registration accept message,

a registration reject message,

a deregistration request message, or

a configuration update command message.

12 . The UE of claim 7 , wherein the back off timer is received with the first message, and

wherein the back off timer is initiated for a predefined time period.