WE CLAIM:
1. A user equipment (2101) performing radio communication with a primary cell
(2502, 2504) and a secondary cell (2503, 2508),
wherein the user equipment (2101) is configured to be informed, from the primary cell (2502, 2504), of a timing advance for use in communicating to the secondary cell (2503, 2508), the timing advance indicating an uplink transmission timing alignment value, and
wherein the user equipment (2101) is configured to set a timing advance for use in communicating to the primary cell (2502, 2504) and the timing advance for use in
communicating to the secondary cell (2503, 2508) and perform radio communication via carrier aggregation simultaneously over the primary cell (2502, 2504) and the secondary cell (2503, 2508) based on the set timing advance for each of the primary cell (2502, 2504) and the secondary cell (2503, 2508),
wherein the uplink transmission timing alignment value indicated by the set timing advance for each of the primary cell (2502, 2504) and the secondary cell (2503, 2508) indicates an added amount of timing advance or delay for the user equipment (2101) to wait before the user equipment (2101) sends a radio frequency signal for communication with each of the primary cell (2502, 2504) and the secondary cell (2503, 2508) respectively.
2. The user equipment (2101) as claimed in Claim 1, wherein the timing advance for
use in communicating to the primary cell (2502, 2504) is different from the timing advance for
use in communicating to the secondary cell (2503, 2508).

3. A primary cell (2502, 2504) used in a communication system comprising the primary cell (2502, 2504), a secondary cell (2503, 2508) and a user equipment (2101) performing radio communication with the primary cell (2502, 2504) and the secondary cell (2503, 2508), the primary cell (2502, 2504) comprising:
a transmitter (906) configured to inform the user equipment (2101) of a timing advance for use in communicating to the secondary cell (2503, 2508), the timing advance indicating an uplink transmission timing alignment value; and
a receiver (909), wherein the transmitter (906) and the receiver (909) are configured to perform communication with the user equipment (2101), such that the user equipment (2101) performs communication via carrier aggregation simultaneously over the primary cell (2502, 2504) and the secondary cell (2503, 2508) based on a timing advance for use in communicating to the primary cell (2502, 2504) and the timing advance for use in communicating to the secondary cell (2503, 2508) informed to the user equipment (2101),
wherein the uplink transmission timing alignment value indicated by the timing advance for each of the primary cell (2502, 2504) and the secondary cell (2503, 2508) indicates an added amount of timing advance or delay for the user equipment (2101) to wait before the user equipment (2101) sends a radio frequency signal for communication with each of the primary cell (2502, 2504) and the secondary cell (2503, 2508) respectively.