Abstract:
The invention proposes a method, in a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, and the group of secondary cells belonging to the at least one group of secondary cells, wherein in the event that the group of primary cells and the at least one group of secondary cells each is configured with a time alignment timer, the method comprises the steps of: terminating the uplink transmission of the user equipment over at least one activated secondary cell in the group of secondary cells when the time alignment timer of the group of primary cells expires; in the event that the group of primary cells and the at least one group of secondary cells are configured with a common time alignment timer, the method comprises the steps of: determining whether a time alignment value of the group of secondary cells is valid when the time alignment timer is in operation; and if not, terminating the uplink transmission of the user equipment over the at least one activated secondary cell in the group of secondary cells and transmitting a notification message to a base station.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates to carrier aggregation-based radio communication and particularly to a method of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a communication system, a user equipment (UE) shall keep uplink synchronization with a base station (e.g., eNodeB) through time alignment (TA), that is, uplink signals transmitted from all user equipments of the base station shall arrive in synchronization at the base station to thereby avoid serious interference with the other user equipments. 
         [0003]    In a Carrier Aggregation (CA) transmission-based radio communication network, a base station can configure a user equipment with a plurality of Component Carriers (CCs) so that the user equipment can communication with the base station over these component carriers, where one of the component carriers is referred to as Primary Cell (Pcell) and the other component carriers are referred to as Secondary Cell (Scell). 
         [0004]    Unlike the 3GPP Release 10 in which respective cells of a user equipment share the same time alignment, the concept of Time Alignment Group (TA Group) has been proposed in the 3GPP Release 11. Respective cells of a user equipment are divided into one or more time alignment groups, and the cells in each time alignment group share the same time alignment. Time alignment groups are further divided into two categories: the Primary Cell Group (Pcell Group) including the primary cell and the Secondary Cell Group (Scell Group) including no primary cell. 
         [0005]    Such a solution has also been proposed in the 3GPP Release 11 that each time alignment group of a user equipment is configured with a Time Alignment Timer (TAT). As for this solution, each time alignment group is configured with a time alignment timer, so such a scenario may arise that the time alignment timer of the primary cell group has expired while the time alignment timer of the secondary cell group is still in operation, and this scenario may require the definition of a new behavior of the user equipment. 
         [0006]    Furthermore such a solution has been proposed in the 3GPP Release 11 that a user equipment is configured with a time alignment timer, that is, all of primary cell group and secondary cell groups of the user equipment share the time alignment timer. As for this solution, the time alignment timer operates based upon the primary cell group to guarantee the validity of time alignment of the primary cell group. Specifically upon reception of a time alignment instruction (e.g., to initially set time alignment or to update time alignment) of the primary cell group, the user equipment adjusts time alignment and restarts the time alignment timer in response to the received instruction; and upon reception of a time alignment instruction of the secondary cell group, the user equipment just adjusts time alignment but will not restart the time alignment timer in response to the received instruction. Since the time alignment timer operates only based upon the primary cell group, such a scenario may arise: although time alignment of the secondary cell group has actually been invalidated because a time alignment instruction of the secondary cell group has not been received for a long period of time, the time alignment timer has not expired because a time alignment instruction of the primary cell group has been received continuously, so that the user equipment considers that time alignment of the secondary cell group is still valid and thus continues sending uplink transmission, thus resulting in serious interference with other user equipments. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the foregoing technical problem, according to an aspect of the invention, in an embodiment, there is provided a method, in a user equipment of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of the user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells each being configured with a time alignment timer, wherein the method comprises the step of: terminating the uplink transmission over at least one activated secondary cell in the group of secondary cells when the time alignment timer of the group of primary cells expires. 
         [0008]    In another embodiment, there is provided a method, in a base station of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells each being configured with a time alignment timer, wherein the method comprises the step of: transmitting a first RRC message to the user equipment when the time alignment timer of the group of primary cells expires, the first RRC message being used for de-configuring configuration of sounding reference signal transmission of the plurality of secondary cells in the group of secondary cells, or transmitting a first MAC CE message to the user equipment, the first MAC CE message being used for deactivating at least one activated secondary cell in the group of secondary cells. 
         [0009]    Furthermore according to another aspect of the invention, in an embodiment, there is provided a method, in a user equipment of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of the user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein the method comprises the steps of: determining whether a time alignment value of the group of secondary cells is valid when the time alignment timer is in operation; and if the time alignment value of the group of secondary cells is invalid, terminating the uplink transmission over at least one activated secondary cell in the group of secondary cells and transmitting a notification message to a base station. 
         [0010]    In another embodiment, there is provided a method, in a base station of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein the method comprises the steps of: determining whether a time alignment value of the group of secondary cells is valid when the time alignment timer is in operation; and if the time alignment value of the group of secondary cells is invalid, transmitting an RRC message to the user equipment, the RRC message being used for de-configuring configuration of sounding reference signal transmission of the plurality of secondary cells in the group of secondary cells, or transmitting an MAC CE message to the user equipment, the MAC CE message being used for deactivating at least one activated secondary cell in the group of secondary cells. 
         [0011]    Furthermore according to a further aspect of the invention, in an embodiment, there is provided a method, in a base station of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein when the time alignment timer expires, the method comprises the steps of: —determining whether a time alignment value of the group of secondary cells is valid; —if the time alignment value of the group of secondary cells is valid, keeping the valid time alignment value of the group of secondary cells; and —if the time alignment value of the group of secondary cells is invalid, discarding the time alignment value of the group of secondary cells. 
         [0012]    Optionally when the time alignment timer restarts counting and the base station keeps the valid time alignment value of the group of secondary cells, the method further comprises the step of: —transmitting the valid time alignment value of the group of secondary cells to the user equipment via a second message. 
         [0013]    In another embodiment, there is provided a method, in a user equipment of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of the user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein when the time alignment timer expires, the method comprises the step of: —discarding a time alignment value of the group of secondary cells. 
         [0014]    Optionally when the time alignment timer restarts counting, the method further comprises the step of: —receiving a second message from a base station, the second message including a valid time alignment value of the group of secondary cells. 
         [0015]    In another embodiment, there is provided a method, in a user equipment of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of the user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein when the time alignment timer expires, the method comprises the step of: —discarding a time alignment value of the group of secondary cells. 
         [0016]    Optionally when the time alignment timer restarts counting, the method further comprises the steps of: —terminating uplink transmission over all secondary cells in the group of secondary cells; —if a secondary cell configured with random access in the group of secondary cells is in an active status, triggering initiatively a random access procedure over the secondary cell configured with the random access, or triggering the random access procedure over the secondary cell configured with the random access in response to a PDCCH order signaling from a base station; and —if the secondary cell configured with the random access in the group of secondary cells is in an inactive status, activating the secondary cell configured with the random access in the group of secondary cells in response to an MAC CE message from the base station and triggering automatically the random access procedure over the secondary cell configured with the random access, or activating the secondary cell configured with the random access in the group of secondary cells in response to the MAC CE message from the base station and triggering the random access procedure over the secondary cell configured with the random access in response to the PDCCH order signaling from the base station. 
         [0017]    In another embodiment, there is provided a method, in a base station of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein when the time alignment timer expires, the method comprises the step of: —discarding a time alignment value of the group of secondary cells. 
         [0018]    Optionally when the time alignment timer restarts counting, the method comprises the steps of: —transmitting an RRC message to the user equipment, the RRC message being used for de-configuring configuration of sounding reference signal transmission of the plurality of secondary cells in the group of secondary cells, or transmitting an MAC CE message to the user equipment, the MAC CE message being used for deactivating at least one activated secondary cell in the group of secondary cells; —if a secondary cell configured with random access in the group of secondary cells is in an active status, transmitting a PDCCH order signaling to the user equipment, the PDCCH order signaling being used for instructing the user equipment to trigger a random access procedure; and —if the secondary cell configured with the random access in the group of secondary cells is in an inactive status, transmitting an MAC CE message to the user equipment, the MAC CE message being used for instructing the user equipment to activate the secondary cell configured with the random access in the group of secondary cells and instructing the user equipment not to perform uplink transmission after  8 ms and for instructing the user equipment to trigger the random access procedure, or transmitting an MAC CE message to the user equipment and then transmitting a PDCCH order signaling to the user equipment, the MAC CE message being used for instructing the user equipment to activate the secondary cell configured with the random access in the group of secondary cells and instructing the user equipment not to perform the uplink transmission after  8 ms, and the PDCCH order signaling being used for instructing the user equipment to trigger the random access procedure. 
         [0019]    In another embodiment, there is provided a method, in a user equipment of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of the user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein when the time alignment timer expires, the method comprises the step of: —keeping a time alignment value of the group of secondary cells. 
         [0020]    Optionally when the time alignment timer restarts counting and the time alignment value of the group of secondary cells kept by the user equipment is valid, the method further comprises the step of: —resuming automatically the uplink transmission with the kept time alignment value until a new time alignment value is received from a base station. 
         [0021]    In another embodiment, there is provided a method, in a base station of a carrier aggregation transmission-based radio communication network, of controlling uplink transmission of a user equipment over a plurality of secondary cells in a group of secondary cells, the user equipment being configured with a group of primary cells and at least one group of secondary cells, the group of secondary cells belonging to the at least one group of secondary cells, and the group of primary cells and the at least one group of secondary cells being configured with a common time alignment timer, wherein when the time alignment timer expires, the method comprises the steps of: —determining whether a time alignment value of the group of secondary cells is valid; —if the time alignment value of the group of secondary cells is valid, keeping the time alignment value of the group of secondary cells; and —if the time alignment value of the group of secondary cells is invalid, discarding the time alignment value of the group of secondary cells. 
         [0022]    Optionally when the time alignment timer restarts counting and the base station discards the time alignment value of the group of secondary cells, the method comprises the steps of: —if configuration of sounding reference signal transmission of at least one activated secondary cell in the group of secondary cells is de-configured, transmitting a PDCCH order signaling to the user equipment to instruct the user equipment to trigger a random access procedure over the group of secondary cells, and then transmitting an RRC message to the user equipment to instruct the user equipment to reconfigure the configuration of the sounding reference signal transmission of the de-configured secondary cells in the group of secondary cells; and —if the at least one activated secondary cell in the group of secondary cells is deactivated, transmitting an MAC CE message to the user equipment to instruct the user equipment to reactivate the deactivated secondary cells in the group of secondary cells and to instruct the user equipment not to perform the uplink transmission, and then transmitting a PDCCH order signaling to instruct the user equipment to trigger the random access procedure over the group of secondary cells. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]    Other features, objects and advantages of the invention will become more apparent upon reading the following detailed description of non-limiting embodiments with reference to the drawings in which: 
           [0024]      FIG. 1  illustrates a flow chart of a method of de-configuring the SRS configuration of the secondary cell according to an embodiment of the invention; 
           [0025]      FIG. 2  illustrates a flow chart of a method of reconfiguring the de-configured SRS configuration of the secondary cell according to an embodiment of the invention; 
           [0026]      FIG. 3  illustrates a flow chart of a method of deactivating the secondary cell according to an embodiment of the invention; and 
           [0027]      FIG. 4  illustrates a flow chart of a method of reactivating the deactivated secondary cell according to an embodiment of the invention. 
       
    
    
       [0028]    Throughout the drawings, identical or like reference numerals denote identical or like reference step features or devices/modules. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0029]    Respective embodiments of the invention will be described below respectively for a scenario where each time alignment group of the user equipment  10  is configured with a time alignment timer and a scenario where the user equipment  10  is configured with a time alignment timer. 
         [0030]    Without loss of generality, the following description will be given taking the user equipment  10  being configured with one primary cell group PG and one secondary cell group SG as an example. It shall be appreciated that the user equipment  10  can be configured with more than one secondary cell group in a practical application, and in the event that the user equipment  10  is configured with a plurality of secondary cell groups, it is necessary to control uplink transmission of the user equipment  10  over secondary cells in each secondary cell group when the time alignment timer of the primary cell group PG expires. The uplink transmission may include Sounding Reference Signal (SRS) transmission. 
         [0031]    In the scenario where each time alignment group of the user equipment  10  is configured with a time alignment timer, each of the primary cell group PG and the secondary cell group SG is configured with a time alignment timer. 
         [0032]    Specifically, in an embodiment, when the time alignment timer of the primary cell group PG expires, the user equipment  10  de-configures automatically the configuration of Sounding Reference Signal (SRS) transmission of a plurality of secondary cells in the secondary cell group SG. The plurality of secondary cells include at least one activated secondary cell in the secondary cell group SG. 
         [0033]    In another embodiment, as illustrated in  FIG. 1 , when the time alignment timer of the primary cell group PG expires, the base station  20  transmits a first RRC message to the user equipment  10  in the step S 11 . The first RRC message is used for de-configuring the SRS configuration of the plurality of secondary cells in the secondary cell group. 
         [0034]    In the step S 12 , the user equipment  10  receives the first RRC message from the base station  20 . Then in the step S 13 , the user equipment  10  de-configures the SRS configuration of the plurality of secondary cells in the secondary cell group SG in response to the received first RRC message. The plurality of secondary cells include at least one activated secondary cell in the secondary cell group SG. 
         [0035]    As for the scenario of de-configuration, when the time alignment timer of the primary cell group PG restarts counting, as illustrated in  FIG. 2 , the base station  20  transmits a second RRC message to the user equipment  10  in the step S 21 . The second RRC message is used for reconfiguring the SRS configuration of the de-configured secondary cells in the secondary cell group SG. 
         [0036]    In the step S 22 , the user equipment  10  receives the second RRC message from the base station  20 . Then in the step S 23 , the user equipment  10  reconfigures the SRS configuration of the de-configured secondary cells in the secondary cell group SG in response to the received second RRC message. 
         [0037]    In a further embodiment, when the time alignment timer of the primary cell group PG expires, the user equipment  10  deactivates automatically at least one activated secondary cell in the secondary cell group SG, for example, the user equipment disables the deactivation timer of the activated secondary cell in the secondary cell group SG. 
         [0038]    In a further embodiment, as illustrated in  FIG. 3 , when the time alignment timer of the primary cell group PG expires, the base station  20  transmits a first MAC CE message to the user equipment in the step S 31 . The first MAC CE message is used for deactivating at least one activated secondary cell in the secondary cell group SG. 
         [0039]    In the step S 32 , the user equipment  10  receives the first MAC CE message from the base station  20 . Then in the step S 33 , the user equipment  10  deactivates at least one activated secondary cell in the secondary cell group SG in response to the received first MAC CE message, for example, disables the deactivation timer of the activated secondary cell in the secondary cell group SG. 
         [0040]    As for the scenario of deactivation, when the time alignment timer of the primary cell group PG restarts counting, the user equipment  10  reactivates automatically the deactivated secondary cell in the secondary cell group SG in an embodiment. 
         [0041]    In another embodiment, when the time alignment timer of the primary cell group PG restarts counting, as illustrated in  FIG. 4 , the base station  20  transmits a second MAC CE message to the user equipment  10  in the step S 41 . The second MAC CE message is used for reactivating the deactivated secondary cell in the secondary cell group SG. 
         [0042]    In the step S 42 , the user equipment  10  receives the second MAC CE message from the base station  20 . Then in the step S 43 , the user equipment  10  reactivates the deactivated secondary cell in the secondary cell group SG in response to the received second MAC CE message. 
         [0043]    In the scenario where the user equipment  10  is configured with a time alignment timer, the primary cell group PG and the secondary cell group SG are configured with a common time alignment timer. 
         [0044]    In this scenario, a new secondary cell group status is defined as a status of virtual-UL-SYN (virtual uplink synchronization). When the time alignment timer is in operation and the time alignment value of the secondary cell group is no longer valid, the secondary cell group is deemed as being in the status of virtual-UL-SYN. 
         [0045]    In an embodiment, the base station  20  determines whether the secondary cell group SG is in the status of virtual-UL-SYN, that is determines whether the time alignment value of the secondary cell group SG is valid when the time alignment timer is still in operation. 
         [0046]    If the time alignment value of the secondary cell group SG is invalid, then the base station  20  transmits an RRC message to the user equipment  10 . The RRC message is used for de-configuring the SRS configuration of a plurality of secondary cells in the secondary cell group SG. 
         [0047]    Upon reception of the RRC message from the base station  20 , the user equipment  10  de-configures the SRS configuration of the plurality of secondary cells in the secondary cell group SG in response to the RRC message. The plurality of secondary cells include at least one activated secondary cell in the secondary cell group SG. 
         [0048]    Alternatively, if the time alignment value of the secondary cell group SG is invalid, then the base station  20  transmits an MAC CE message to the user equipment  10 . The MAC CE message is used for deactivating the activated secondary cell in the secondary cell group SG. 
         [0049]    Upon reception of the MAC CE message from the base station  20 , the user equipment  10  deactivates the activated secondary cell in the secondary cell group SG in response to the MAC CE message, for example, disables the deactivation timer of the activated secondary cell in the secondary cell group SG. 
         [0050]    In another embodiment, the user equipment  10  determines whether the secondary cell group SG is in the status of virtual-UL-SYN, that is determines whether the time alignment value of the secondary cell group SG is valid when the time alignment timer is still in operation. 
         [0051]    If the time alignment value of the secondary cell group SG is invalid, then the user equipment  10  de-configures automatically the SRS configuration of a plurality of secondary cell in the secondary cell group SG and notifies the base station  20  that the SRS configuration of the plurality of secondary cells in the secondary cell group SG is removed. The plurality of secondary cells include at least one activated secondary cell in the secondary cell group SG. 
         [0052]    Alternatively, if the time alignment value of the secondary cell group SG is invalid, then the user equipment  10  deactivates automatically the activated secondary cell in the secondary cell group SG, for example, the user equipment  10  disables the deactivation timer of the activated secondary cell in the secondary cell group SG and notifies the base station  20  that the activated secondary cell in the secondary cell group SG is deactivated. 
         [0053]    In a variant embodiment, the user equipment  10  notifies the base station  20  that the secondary cell group SG is in the status of virtual-UL-SYN status, that is, the time alignment value of the secondary cell group SG is invalid, instead of performing an automatic de-configuration or deactivation operation by itself, upon determining that the secondary cell group SG is in the status of virtual-UL-SYN status. 
         [0054]    Upon reception of the notification message, the base station  20  may transmit an RRC message to the user equipment  10 , the RRC message being used for de-configuring the SRS configuration of a plurality of secondary cells in the secondary cell group SG and the plurality of secondary cells include at least one activated secondary cell in the secondary cell group SG; or transmits an MAC CE message to the user equipment  10 , the MAC CE message being used for deactivating the activated secondary cell in the secondary cell group SG. 
         [0055]    As for the foregoing scenario where the base station  20  determines whether the time alignment value of the secondary cell group SG is valid, only the base station  20  knows whether the time alignment value of the secondary cell group SG is valid, and the user equipment  10  always considers that the time alignment value of the secondary cell group SG is valid. 
         [0056]    In this scenario, when the time alignment timer expires, the following three scenarios may be assumed: 
         [0057]    In a first scenario, the base station  20  keeps the time alignment value of the secondary cell group SG, and the user equipment  10  discards the time alignment value of the secondary cell group SG, such that the base station  20  has the valid time alignment value of the secondary cell group SG and the user equipment  10  does not have the time alignment value of the secondary cell group SG. 
         [0058]    In a second scenario, both the base station  20  and the user equipment  10  discard the time alignment value of the secondary cell group SG regardless of whether the time alignment value of the secondary cell group SG is valid, such that neither the base station  20  nor the user equipment  10  has the time alignment value of the secondary cell group SG. 
         [0059]    In a third scenario, the base station  20  only keeps the valid time alignment value of the secondary cell group SG, and the user equipment  10  always keeps the time alignment value of the secondary cell group SG and considers that it is valid, thus resulting in the following two sub-scenarios: 
         [0060]    In a first sub-scenario, when the time alignment value of the secondary cell group SG is valid, both the base station  20  and the user equipment  10  have the valid time alignment value of the secondary cell group SG; and 
         [0061]    In a second sub-scenario, when the time alignment value of the secondary cell group SG is invalid, the base station does not have the time alignment value of the secondary cell group SG, and the user equipment has the invalid time alignment value of the secondary cell group SG. 
         [0062]    With the first scenario occurring after the time alignment timer expires, when the time alignment timer restarts counting, the user equipment  10  will not perform any random access, and the valid time alignment value of the secondary cell group SG kept by the base station  20  can still be used. In order to resume uplink transmission over the secondary cell group SG, the base station  200  will simply notify the user equipment  10  of the valid time alignment value of the secondary cell group SG. 
         [0063]    Specifically, in an embodiment, the base station  20  transmits the valid time alignment value to the user equipment  10  via an RRC message. 
         [0064]    In another embodiment, the base station  20  transmits the valid time alignment value to the user equipment  10  via an MAC CE message. 
         [0065]    The valid time alignment value transmitted from the base station  20  to the user equipment  10  may be an absolute time alignment value or a relative time alignment value, for example. 
         [0066]    After the user equipment  10  obtains the valid time alignment value, the secondary cell group SG will automatically be changed to an uplink synchronization status and restart uplink transmission. 
         [0067]    With the second scenario occurring after the time alignment timer expires, when the time alignment timer restarts counting, the secondary cell group SG shall be in the status of virtual uplink synchronization and will not allow any uplink transmission regardless of whether the secondary cell in the secondary cell group SG is in an active/inactive status or in a SRS configured/de-configured status. 
         [0068]    Then a random access procedure shall be triggered to regain a valid time alignment value. 
         [0069]    Specifically, if a secondary cell configured with random access in the secondary cell group SG, that is, a secondary cell over which random access is configured to be performed, is in an active status, then the user equipment  10  triggers automatically a random access procedure over the secondary cell configured with random access. 
         [0070]    Alternatively the base station  20  can transmit a PDCCH order signaling to the user equipment  10 , and the PDCCH order signaling is used for instructing the user equipment  10  to trigger a random access procedure. The user equipment  10  triggers the random access procedure over the secondary cell configured with random access in response to the PDCCH order signaling, upon reception of the PDCCH order signaling. 
         [0071]    If a secondary cell configured with random access in the secondary cell group SG, that is, a secondary cell over which random access is configured to be performed, is in an inactive status, then the base station  20  transmits an MAC CE message to the user equipment  10 , and the MAC CE message is used for instructing the user equipment  10  to activate the secondary cell configured with random access in the secondary cell group SG and instructing the user equipment  10  not to perform uplink transmission after 8 ms and further for instructing the user equipment  10  to trigger a random access procedure. The user equipment  10  activates the random access procedure over the secondary cell configured with random access upon reception of the MAC CE message and then triggers automatically the random access procedure over the secondary cell configured with random access. 
         [0072]    Alternatively, the base station  20  firstly transmits an MAC CE message to the user equipment  10 , the MAC CE message being used for instructing the user equipment  10  to activate the secondary cell configured with random access in the secondary cell group SG and instructing the user equipment  10  not to perform uplink transmission after  8 ms, and then transmits a PDCCH order signaling to the user equipment  10 , the PDCCH order signaling being used for instructing the user equipment  10  to trigger a random access procedure. 
         [0073]    The user equipment  10  activates the secondary cell configured with random access in the secondary cell group SG in response to the received MAC CE message and then triggers the random access procedure over the secondary cell configured with random access in response to the PDCCH order signaling. 
         [0074]    With the third scenario occurring after the time alignment timer expires, when the time alignment timer restarts counting, for the first sub-scenario, the user equipment  10  has the valid time alignment value of the secondary cell group SG in the first sub-scenario, so the user equipment  10  can simply resume automatically uplink transmission with the kept time alignment value until a new time alignment value is received from the base station  20  in this sub-scenario. 
         [0075]    In the second sub-scenario, if the SRS configuration of the activated secondary cell in the secondary cell group SG is de-configured, then the base station  20  transmits a PDCCH order signaling to the user equipment  10  to instruct the user equipment  10  to trigger a random access procedure over the secondary cell group SG and then transmits an RRC message to the user equipment  10  to instruct the user equipment  10  to reconfigure the SRS configuration of the de-configured secondary cells in the secondary cell group SG. 
         [0076]    If the activated secondary cell in the secondary cell group SG is deactivated, then the base station  20  transmits an MAC CE message to the user equipment  10  to instruct the user equipment  10  to reactivate the deactivated secondary cell in the secondary cell group SG and instructs the user equipment  10  not to perform uplink transmission and then transmits a PDCCH order signaling to the user equipment  10  to instruct the user equipment  10  to trigger a random access procedure over the secondary cell group SG. 
         [0077]    The embodiments of the invention have been disclosed above. It shall be appreciated that the invention will not be limited to the foregoing embodiments and that those skilled in the art can make various variations and modifications without departing from the scope of the invention.