Supporting two timing advances in a serving cell of a wireless system

1 . A user equipment (UE), comprising: a transceiver configured to enable wireless communication with a base station in a wireless system; and a processor communicatively coupled to the transceiver and configured to:

determine that a first timing advance (TA) group (TAG) and a second TAG are configured for a serving cell based on a configuration received from the base station;

determine a first TA adjustment value based on a first timing advance command (TAC) and a second TA adjustment value based on a second TAC, wherein the first TAC and the second TAC are received from the base station in a same random access response (RAR) message during a Random Access Channel (RACH) procedure by the UE, the first TAC and the second TAC were determined based on a time of arrival of a random access preamble received from the UE by the base station;

select a TAC from the first TAC or the second TAC to be applied to an uplink transmission;

select, based on the selected TAC, a TAG from the first TAG or the second TAG; and

transmit the uplink transmission according to the selected TAC with the selected TAG.

2 . The UE of claim 1 , wherein a first time alignment timer for the first TAG is different from a second time alignment timer for the second TAG.

3 . The UE of claim 1 , wherein the selected TAC is the first TAC when the UE communicates with a first transmission reception point (TRP) of the wireless system, and the selected TAC is the second TAC when the UE communicates with a second TRP of the wireless system.

4 . The UE of claim 1 , wherein the RAR message includes an indication to indicate the selected TAG for the selected TAC for the uplink transmission scheduled by the RAR message.

5 . The UE of claim 1 , wherein the first TAC is applied to the first TAG that is associated with a Physical Random Access Channel (PRACH) transmission, and the second TAC is applied to the second TAG that is not associated with the PRACH transmission.

6 . The UE of claim 1 , wherein the first TAC and the second TAC have different lengths resulting from differential encoding being applied to the first TAC and the second TAC.

7 . The UE of claim 1 , wherein the RAR message is a message B of a 2 step Random Access Channel (RACH) process or a message 2 of a 4-step RACH process.

8 . The UE of claim 1 , wherein an indication of the first TAC and the second TAC are received from the base station in a medium access control-control elements (MAC-CE) transmitted to the UE when the UE is in a connection mode to the base station.

9 . The UE of claim 8 , wherein the first TAC or the second TAC is an absolute TAC carrying an absolute TA adjustment value or a relative TAC carrying a relative TA adjustment value.

10 . The UE of claim 8 , wherein the MAC-CE further includes a TAG identifier corresponding to the first TAC or the second TAC.

11 . A method for a user equipment (UE), comprising:

determining that a first timing advance (TA) group (TAG) and a second TAG are configured for a serving cell based on a configuration received from a base station;

determining a first TA adjustment value based on a first timing advance command (TAC) and a second TA adjustment value based on a second TAC, wherein the first TAC and the second TAC are received from the base station in a same random access response (RAR) message during a Random Access Channel (RACH) procedure by the UE, the first TAC and the second TAC were determined based on a time of arrival of a random access preamble received from the UE by the base station;

selecting a TAC from the first TAC or the second TAC to be applied to an uplink transmission;

selecting, based on the selected TAC, a TAG from the first TAG or the second TAG; and

transmitting the uplink transmission according to the selected TAC with the selected TAG.

12 . The method of claim 11 , wherein a first time alignment timer for the first TAG is different from a second time alignment timer for the second TAG.

13 . The method of claim 11 , wherein the selected TAC is the first TAC when the UE communicates with a first transmission reception point (TRP) of the wireless system, and the selected TAC is the second TAC when the UE communicates with a second TRP of the wireless system.

14 . The method of claim 11 , wherein the RAR message includes an indication to indicate the selected TAG for the selected TAC for the uplink transmission scheduled by the RAR message.

15 . The method of claim 11 , wherein the first TAC is applied to the first TAG that is associated with a Physical Random Access Channel (PRACH) transmission, and the second TAC is applied to the second TAG that is not associated with the PRACH transmission.

16 . The method of claim 11 , wherein the RAR message is a message B of a 2 step Random Access Channel (RACH) process or a message 2 of a 4-step RACH process.

17 . A non-transitory computer-readable medium storing instructions that, when executed by a processor of a user equipment (UE), cause the UE to perform operations, the operations comprising:

determining that a first timing advance (TA) group (TAG) and a second TAG are configured for a serving cell based on a configuration received from [a base station;

determining a first TA adjustment value based on a first timing advance command (TAC) and a second TA adjustment value based on a second TAC, wherein the first TAC and the second TAC are received from the base station in a same random access response (RAR) message during a Random Access Channel (RACH) procedure by the UE, the first TAC and the second TAC were determined based on a time of arrival of a random access preamble received from the UE by the base station;

selecting a TAC from the first TAC or the second TAC to be applied to an uplink transmission;

selecting, based on the selected TAC, a TAG from the first TAG or the second TAG; and

transmitting the uplink transmission according to the selected TAC with the selected TAG.