Uplink timing maintenance upon time alignment timer expiry

1. A method, performed at a User Equipment (UE) of a wireless communications network, the method comprising:

detecting that a downlink transmission timing has changed by an amount of time;

determining that a time alignment timer (TAT) is not running; and

while the TAT is not running, adjusting a timing advance for uplink transmissions based on the amount of time that the downlink transmission timing has changed, wherein the timing advance for uplink transmissions was stored in memory at expiration of the TAT.

2. The method of claim 1 , further comprising starting the TAT upon application of a timing advance command (TAC) received in a TAC medium access control (MAC) control element (CE).

3. The method of claim 2 , further comprising adjusting the uplink transmission timing to a new uplink transmission timing that is equal to the sum of the maintained timing advance and a timing advance received in the TAC MAC CE.

4. The method of claim 1 , wherein determining that the TAT is not running comprises determining that the TAT has expired.

5. The method of claim 1 , further comprising applying the adjusted timing advance to an uplink transmission timing.

6. A user equipment of a wireless communications network, the user equipment comprising:

an antenna;

a transceiver; and

a processor configured to:

detect a downlink transmission timing has changed by an amount of time;

determine that a time alignment timer (TAT) is not running; and

while the TAT is not running, adjust a timing advance for uplink transmissions based on the amount of time that the downlink transmission timing has changed, wherein the timing advance for uplink transmissions was stored in memory at expiration of the TAT.

7. The UE of claim 6 , wherein the processor is further configured to start the TAT upon application of a timing advance command (TAC) received in the TAC medium access control (MAC) control element (CE).

8. The UE of claim 6 , wherein determining that the TAT is not running comprises determining that the TAT has expired.

9. The UE of claim 6 , further comprising applying the adjusted timing advance to an uplink transmission timing.

10. The UE of claim 7 , further comprising adjusting the uplink transmission timing to a new uplink transmission timing that is equal to the sum of the adjusted timing advance and and a timing advance received in the TAC MAC CE.

11. A non-transitory computer readable medium storing instructions to cause a processor to perform operations comprising:

detecting that a downlink transmission timing has changed by an amount of time;

determining that a time alignment timer (TAT) is not running; and

while the TAT is not running, adjusting a timing advance for uplink transmissions based on the amount of time that the downlink transmission timing has changed, wherein the timing advance for uplink transmissions was stored in memory at expiration of the TAT.

12. The computer readable medium of claim 11 , the instructions further comprising starting the TAT upon application of a timing advance command (TAC) received in a TAC medium access control (MAC) control element (CE).

13. The computer readable medium of claim 12 , the instructions further comprising adjusting the uplink transmission timing to a new uplink transmission timing that is equal to the sum of the maintained timing advance and a timing advance received in the TAC MAC CE.

14. The computer readable medium of claim 11 , wherein the instructions comprising determining that the TAT is not running comprises the instructions further comprising determining that the TAT has expired.

15. The computer readable medium of claim 11 , the instructions further comprising applying the adjusted timing advance to an uplink transmission timing.