Method of handling uplink timing and related communication device

A method of handling an uplink (UL) timing of a first timing advance (TA) group of a plurality of TA groups for a mobile device in a wireless communication system, the mobile device being configured with a plurality of secondary cells in the first TA group by a network of the wireless communication system, and the method comprising:

• maintaining the UL timing of the first TA group according to a downlink (DL) timing of a first timing reference cell of the plurality of secondary cells, if a time alignment timer of the first TA group does not expire;
• maintaining the UL timing of the first TA group according to a DL timing of a second timing reference cell of the plurality of secondary cells instead of the first timing reference cell and performing a UL transmission according to the UL timing of the first TA group, when the first timing reference cell is deactivated and the time alignment timer of the first TA group does not expire;
• stopping maintaining the UL timing of the first TA group in response to a time alignment timer of a second TA group of the plurality of TA groups expiring and also the second TA group comprising a primary cell;
• and
• clearing hybrid automatic repeat request (HARQ) buffers corresponding to the primary cell and all the secondary cells configured to the mobile device, when the time alignment timer of the second TA group expires and the second TA group comprises the primary cell.

A communication device of a wireless communication system for an uplink (UL) timing of a first timing advance (TA) group of a plurality of TA groups, the communication device being configured with a plurality of secondary cells in the first TA group by a network of the wireless communication system, the communication device comprising:

• a processor, for executing a program;
• and a storage unit, coupled to the processor for storing the program;
• wherein the program instructs the processor to perform the following steps:
• maintaining the UL timing of the first TA group according to a downlink (DL) timing of a first timing reference cell of the plurality of secondary cells, if a time alignment timer of the first TA group does not expire;
• maintaining the UL timing of the first TA group according to a DL timing of a second timing reference cell of the plurality of secondary cells instead of the first timing reference cell and performing a UL transmission according to the UL timing of the first TA group, when the first timing reference cell is deactivated and the time alignment timer of the first TA group does not expire;
• stopping maintaining the UL timing of the first TA group in response to a time alignment timer of a second TA group expiring and also the second TA group comprising a primary cell;
• and
• clearing hybrid automatic repeat request (HARQ) buffers corresponding to the primary cell and all the secondary cells configured to the mobile device, when the time alignment timer of the second TA group expires and the second TA group comprises the primary cell.