1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method of timing reference management in a wireless communication system.
2. Description of the Prior Art
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of Long-Term Evolution (LTE) system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where a user equipment (UE) can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and transmitting. In carrier aggregation, the UE only has one radio resource control (RRC) connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the Non-Access Stratum (NAS) mobility information, and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as a primary cell (PCell). In the downlink, the component carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). In addition, cells other than the PCell are named secondary cell (SCell).
Since the UE may not need to use all of the configured cells (i.e. PCell and one or more SCells), only some SCells are activated, so as to save UE power. Note that, the PCell is always activated. Generally, an evolved Node-B (eNB) activates or deactivates a SCell by sending a signalling (e.g. a medium access control control element (MAC CE), or Activation/Deactivation command) to the UE. In addition, the UE starts a deactivation timer for a SCell when the SCell is activated, wherein the SCell is deactivated when the deactivation timer expires. In other words, the deactivation timer provides a period of time for SCell activation. Please note that, when the SCell is deactivated, the UE does not need to monitor physical downlink control channel (PDCCH) of the deactivated SCell. Further, the UE shall not transmit on UL-SCH on the deactivated SCell.
As abovementioned, it is possible to configure a UE of a PCell and one SCell or more SCells. Therefore, multiple timing advance values, each for synchronization with a serving eNB on uplink timing for preventing signals transmitted from the UE from colliding with those sent from other UEs under the coverage of the eNB, are needed for PCell and SCell or more SCells. Note that, serving cells having uplink to which the same timing advance value applies are grouped in a timing advance group (TAG). Each TAG contains at least one serving cell with configured UL, and the mapping of each serving cell to a TAG is configured by the serving eNB. In order to realize uplink timing alignment, the UE maintains a time alignment timer (TAT) whose running state indicates that uplink transmission is still synchronized. TAT may be applied for a TAG.
In carrier aggregation, a UE may apply information (i.e. parameter settings) to one cell by referring to the information observed from other cell, which is called reference cell in this disclosure. For example, the reference cell may be a timing reference cell. For the timing reference cell, if a UE uses the downlink timing of Cell #x as the timing reference for the Cell #y to derive an uplink transmission timing, then call Cell #x is the timing reference cell of the Cell #y.
Applicant notices a problem associated to timing reference cell change. Although the uplink transmission timing is the same for all cells in a TAG, the downlink timing for each cell in the TAG may be different. A timing reference cell of the TAG whose downlink timing is used to derive the common uplink transmission timing is introduced in the LTE-Advanced system. Based on specification of the LTE-Advanced system, the uplink transmission timing is a downlink timing of the timing reference cell plus an integer value, called timing adjust value NTA. The eNB transmits a timing advance command (TAC) to the UE to adjust the timing adjust value NTA. The TAC includes an integer which is a positive or negative integer to indicate the UE to advance or slow the current uplink timing for the amount of the value. The abovementioned TAT is further used for determining whether timing adjust value NTA is still valid for the UE to derived uplink transmission timing. When the TAT expires, the UE considers timing adjust value NTA is invalid, so there is no uplink transmission timing available.
Note that, in the LTE-Advanced system, it requires that cells in a TAG must have downlink timing difference smaller than 1 step of timing adjust value NTA, so as to avoid timing jump problem, which may impact data decoding in the UE. However, specification of the LTE-Advanced system does not specify how to set timing adjust value NTA when timing reference cell changes from a cell to another. Thus, the UE may not derive correct uplink transmission timing.