Patent Publication Number: US-2022240140-A1

Title: User equipment, base station and method of performing conditional handover

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to user equipment, base station and method of performing handover, and more particularly to user equipment, base station and a method of performing conditional handover. 
     BACKGROUND OF THE INVENTION 
     In a Long Term Evolution (LTE) system, some handover failures occur when a User Equipment (UE) performs conventional handover from a source base station to a target station. In order to avoid these handover failures, conditional handover is introduced. However, in conditional handover, handover failure could still happen but there is no scheme for handling handover failure for conditional handover. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the present disclosure provides a method performed by a user equipment. The method includes: receiving, by a user equipment, a conditional handover message from a source base station, wherein the conditional handover message includes a trigger condition and a first timer corresponding to a target cell; determining, by the user equipment, that the trigger condition is met; and starting, by the user equipment, the first timer corresponding to the target cell. 
     Another embodiment of the present disclosure provides a method performed by a user equipment. The method includes: determining, by a user equipment, that conditional handover to a target cell fails; selecting, by the user equipment, another target cell with a channel quality; determining, by the user equipment, whether to perform conditional handover to the another target cell when the another target cell is associated with a conditional handover message or to perform reestablishment with the another target cell when the another target cell is not associated with the conditional handover message. 
     Yet another embodiment of the present disclosure provides a method for a base station. The method includes: transmitting, by a base station, a conditional handover message to a user equipment, wherein the conditional handover message includes a trigger condition and a first timer corresponding to a target cell so that the user equipment is capable of determining that the trigger condition is met and starting the first timer corresponding to the target cell. 
     Yet another embodiment of the present disclosure provides a method for a base station. The method includes: transmitting, by a base station, a conditional handover message with an offset to the user equipment, wherein the offset is used for the user equipment to update a channel quality between the user equipment and a target cell. 
     Yet another embodiment of the present disclosure provides a user equipment. The user equipment includes a transceiver and a processor. The processor and the transceiver are electrically coupled. The transceiver receives a conditional handover message from a source base station, wherein the conditional handover message includes a trigger condition and a first timer corresponding to a target cell. The processor: determines that the trigger condition is met; and starts the first timer corresponding to the target cell. 
     Yet another embodiment of the present disclosure provides a user equipment. The user equipment includes a processor for determining that conditional handover to a target cell fails; selecting another target cell with a channel quality; and determining whether to perform conditional handover to the another target cell when the another target cell is associated with a conditional handover message or to perform reestablishment with the another target cell when the another target cell is not associated with the conditional handover message. 
     Yet another embodiment of the present disclosure provides a base station. The base station includes a transceiver for transmitting a conditional handover message to a user equipment, wherein the conditional handover message includes a trigger condition and a first timer corresponding to a target cell so that the user equipment is capable of determining that the trigger condition is met and starting the first timer corresponding to the target cell. 
     Yet another embodiment of the present disclosure provides a base station. The base station includes a transceiver for transmitting a conditional handover message with an offset to the user equipment, wherein the offset is used for the user equipment to update a channel quality between the user equipment and a target cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic view of a network system according to an embodiment of the present disclosure. 
         FIG. 1B  is a block diagram of a user equipment according to an embodiment of the present disclosure. 
         FIG. 1C  is a block diagram of a base station according to an embodiment of the present disclosure. 
         FIG. 2A  is a schematic view of a network system according to an embodiment of the present disclosure. 
         FIG. 2B  is a block diagram of a user equipment according to an embodiment of the present disclosure. 
         FIG. 2C  is a block diagram of a base station according to an embodiment of the present disclosure. 
         FIG. 3A  is a schematic view of a network system according to an embodiment of the present disclosure. 
         FIG. 3B  is a block diagram of a user equipment according to an embodiment of the present disclosure. 
         FIG. 3C  is a block diagram of a base station according to an embodiment of the present disclosure. 
         FIG. 4  is a flowchart diagram according to embodiments of the present disclosure. 
         FIGS. 5A to 5C  are flowchart diagrams according to embodiments of the present disclosure. 
         FIG. 6  is a flowchart diagram according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description of the appended drawings is intended as a description of the currently preferred embodiments of the present invention, and is not intended to represent the only form in which the present invention may be practiced. It should be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the present invention. 
     Please refer to  FIG. 1A  through  FIG. 1C .  FIG. 1A  illustrates a schematic view of a network system  1  according to an embodiment of the present disclosure.  FIG. 1B  is a block diagram of a user equipment  11  according to the embodiment of the present disclosure. The user equipment  11  includes a processor  111  and a transceiver  113 . The processor  111  and the transceiver  113  are coupled through a communication bus  115  allowing the processor  111  to control the transceiver  113  to transmit and/or receive information. 
       FIG. 1C  is a block diagram of a base station  13  according to the embodiment of the present disclosure. Base station  13  includes a processor  131  and a transceiver  133 . The processor  131  and the transceiver  133  are coupled through a communication bus  135  allowing the processor  131  to control the transceiver  133  to transmit and/or receive information. The interactions between the individual elements will be further described hereinafter. 
     In some embodiments, the network system  1  may include the user equipment  11 , the base station  13  with a cell  13 C and a base station  15  with a cell  15 C as shown in  FIG. 1A . The user equipment  11  exchanges data with the base station  13  via a network connection  10 . At a preparing stage of conditional handover, the transceiver  113  of the user equipment  11  regularly transmits a measurement report  110  to the base station  13  for reporting channel status of the user equipment  11 . 
     After the transceiver  133  of the base station  13  receives the measurement report  110 , processor  131  of the base station  13  may identify cell  15 C of base station  15  as a target cell for the user equipment  11  to perform conditional handover to when needed. Therefore, the transceiver  133  of the base station  13  transmits a conditional handover request  130  to the base station  15  to notify the base station  15  of a possible conditional handover, and then receives a conditional handover acknowledgement  150  from the base station  15 . 
     Next, the transceiver  133  of the base station  13  transmits a conditional handover message  132  to the user equipment  11  for notifying the user equipment  11  of the cell  15 C of the base station  15  as the target cell to perform conditional handover to when needed. The conditional handover message  132  includes a trigger condition  132 C and a first timer  132 T corresponding to the cell  15 C of the base station  15 . After the transceiver  113  of the user equipment  11  receives the conditional handover message  132 , preparing stage for conditional handover is finished. 
     Accordingly, when the user equipment  11  is moving away from the base station  13  and toward to the cell  15 C of the base station  15 , the processor  111  of the user equipment  11  determines whether the trigger condition  132 C corresponding to the cell  15 C of the base station  15  is met or not. In detail, when the trigger condition  132 C corresponding to the cell  15 C of the base station 15 is met, it means that the user equipment  11  may perform conditional handover from the base station  13  to the base station  15 . Therefore, the processor  111  of the user equipment  11  starts the first timer  132 T corresponding to the cell  15 C of the base station  15  for controlling the period of performing conditional handover to the cell  15 C of the base station  15 . In particular, if the first timer  132 T expires but the conditional handover has not succeeded yet, the user equipment  11  terminates the conditional handover to the cell  15 C of the base station  15 . 
     When the trigger condition  132 C corresponding to the cell  15 C of the base station  15  is not met, it means that the user equipment  11  may not perform conditional handover from the base station  13  to the cell  15 C of the base station  15 . Therefore, the processor  111  of the user equipment  11  may keep checking to see if the trigger condition  132 C corresponding to the cell  15 C of the base station  15  has been met or not. 
     Please refer to  FIG. 2A  through  FIG. 2C .  FIG. 2A  illustrates a schematic view of a network system  2  according to an embodiment of the present disclosure.  FIG. 2B  is a block diagram of a user equipment  21  according to the embodiment of the present disclosure. The user equipment  21  includes a processor  211  and a transceiver  213 . The processor  211  and the transceiver  213  are coupled through a communication bus  215  allowing the processor  211  to control the transceiver  213  to transmit and/or receive information. 
       FIG. 2C  is a block diagram of a base station  23  according to the embodiment of the present disclosure. Base station  23  includes a processor  231  and a transceiver  233 . The processor  231  and the transceiver  233  are coupled through a communication bus  235  allowing the processor  231  to control the transceiver  233  to transmit and/or receive information. The interactions between the individual elements will be further described hereinafter. 
     In some embodiments, the network system  2  may include a user equipment  21 , a base station  23  with a cell  23 C, a base station  25  with a cell  25 C and a base station  27  with a cell  27 C as shown in  FIG. 2A . The user equipment  21  exchanges data with the base station  23  via a network connection  20 . At a preparing stage of conditional handover, the transceiver  213  of the user equipment  21  regularly transmits a measurement report  210  to the base station  23  for reporting Channel Quality Indicator (CQI) between the user equipment  21  and the base station  23 . 
     After the transceiver  233  of the base station  23  receives the measurement report  210 , the processor  231  of the base station  23  may identify cell  25  of the base station  25  and cell  27  of the base station  27  as target cells for the user equipment  21  to perform conditional handover when needed. Therefore, the transceiver  233  of the base station  23  transmits a conditional handover request  230  to the base station  25  for notifying the base station  25  of possible conditional handover, and then receives a conditional handover acknowledgement  250  from the base station  25 . Similarly, the transceiver  233  of the base station  23  transmits a conditional handover request  230  to the base station  27  for notifying the base station  27  of possible conditional handover, and then receives a conditional handover acknowledgement  270  from the base station  27 . 
     Next, the transceiver  233  of the base station  23  transmits a conditional handover message  232  to the user equipment  21  for notifying the user equipment  21  of the cell  25 C of the base station  25  and of the cell  27 C of the base station  27  as the target cells for performing conditional handover when needed. The conditional handover message  232  includes a trigger condition  232 C and a first timer  232 T 1 . After the transceiver  213  of the user equipment  21  receives the conditional handover message  232 , the preparing stage for conditional handover is finished. 
     It should be noted that, in some embodiments, the conditional handover message  232  may include a Radio Resource Control Connection Reconfiguration message (e.g., RRCconnectionreconfiguration). The trigger condition  232 C of the conditional handover message  232  may be associated with a threshold of channel quality which is used as a benchmark for triggering conditional handover. 
     In some embodiments, when the user equipment  21  is moving away from the base station  23  and toward cells  25 C and  27 C of the base stations  25  and  27 , respectively, conditional handover may be triggered. Accordingly, processor  211  of the user equipment  21  determines whether a trigger condition  232 C is met for cells  25 C and  27 C of the base stations  25  and  27  respectively. 
     In detail, as for the cell  25 C of the base station  25 , the processor  211  of the user equipment  21  measures a first channel quality value (not shown) between the user equipment  21  and the cell  25 C of the base station  25  via transceiver  213 . Then, the processor  211  of the user equipment  21  determines whether the first channel quality value is greater than the channel quality threshold associated with the trigger condition  232 C received as part of the conditional handover message  232 . 
     When the first channel quality value is greater than the channel quality threshold, it means that a channel status between the user equipment  21  and the cell  25 C of the base station  25  may be good enough and the user equipment  21  may perform conditional handover from the base station  23  to the cell  25 C of the base station  25 . Otherwise, when the first channel quality value is not greater than the channel quality threshold, it means that a channel status between the user equipment  21  and the cell  25 C of the base station  25  may not be good enough and the user equipment  21  may not perform conditional handover from the base station  23  to the cell  25 C of the base station  25 . 
     As for the cell  27 C of the base station  27 , the processor  211  of the user equipment  21  measures a second channel quality value (not shown) between the user equipment  21  and the cell  27 C of the base station  27  via the transceiver  213 . Then, the processor  211  of the user equipment  21  determines whether the second channel quality value is greater than the channel quality threshold associated with the trigger condition  232 C of the conditional handover message  232 . 
     When the second channel quality value is greater than the channel quality threshold, it means that a channel status between the user equipment  21  and the cell  27 C of the base station  27  may be good enough and the user equipment  21  may perform conditional handover from the base station  23  to the cell  27 C of the base station  27 . Otherwise, when the second channel quality value is not greater than the channel quality threshold, it means that a channel status between the user equipment  21  and the cell  27 C of the base station  27  may not be good enough and the user equipment  21  may not perform conditional handover from the base station  23  to the cell  27 C of the base station  27 . 
     In some embodiments, the first channel quality value and the second channel quality value are both greater than the channel quality threshold. In other words, cell  25 C of the base station  25  and cell  27 C of the base station  27  are both candidate target cells for the user equipment  21  to perform conditional handover to. 
     In some embodiments, the first channel quality value is greater than the second channel quality value. Therefore, the, processor  211  of the user equipment  21  may select cell  25 C of base station  25  for performing conditional handover. Then, processor  211  of the user equipment  21  may apply a configuration (not shown) of the conditional handover message  232  for performing conditional handover to the cell  25 C of the base station  25 . Afterward, the processor  211  of the user equipment  21  may initiate conditional handover to the cell  25 C of the base station  25 . Then, the processor  211  of the user equipment  21  may initiate Random Access (RA) to the cell  25 C of the base station  25 . 
     In some embodiments, the configuration may be the Radio Resource Control Connection Reconfiguration message (e.g., RRCconnectionreconfiguration) which may include a mobility control information (e.g., mobilityControlInfo). The configuration may be applied to the user equipment  21  for the user equipment  21  to set up related network parameters for conditional handover. 
     As described above, during the conditional handover procedure to cell  25 C of the base station  25 , there may be at least five operations performed by the user equipment  21 , for example: (1) determining that the trigger condition  232 C corresponding to the cell  25 C of the base station  25  is met (i.e., the first channel quality value is greater than the a channel quality threshold); (2) selecting the cell  25 C of the base station  25  for performing conditional handover; (3) applying the configuration of the conditional handover message  232  for performing conditional handover to the cell  25 C of the base station  25 ; (4) initiating conditional handover to the cell  25 C of the base station  25 ; and (5) initiating RA to the cell  25 C of the base station  25 . 
     In some embodiments, the processor  211  of the user equipment  21  may start the first timer  232 T 1  with any one of these operations. In detail, the processor  211  of the user equipment  21  may start the first timer  232 T 1  when any one of these operations is executed. In other words, the start of the first timer  232 T 1  and the execution of any one of these operations may occur at the same time. Accordingly, no matter the first timer  232 T 1  is started with which operation, the user equipment  21  terminates the conditional handover to the cell  25 C of the base station  25  once the first time  232 T 1  expires but the conditional handover has not succeeded yet. Then, the user equipment  21  determines whether the trigger condition  232 C is met for another cell. 
     In some embodiments, the trigger condition  232 C is met for cell  27 C of the base station  27  (i.e., the second channel quality value is greater than the channel quality threshold). Therefore, the processor  211  of the user equipment  21  selects the cell  27 C of the base station  27  for performing conditional handover. Then, the processor  211  of the user equipment  21  may apply a configuration (not shown) of the conditional handover message  232  for performing conditional handover to the cell  27 C of the base station  27 . Afterward, the processor  211  of the user equipment  21  may initiate conditional handover to the cell  27 C of the base station  27 . Then, the processor  211  of the user equipment  21  may initiate RA to the cell  27 C of the base station  27 . 
     Similarly, during the conditional handover procedure for the cell  27 C of the base station  27 , there may be at least five operations performed by the user equipment  21 , for example: (1) determining that the trigger condition  232 C corresponding to the cell  27 C of the base station  27  is met (i.e., the second channel quality value is greater than the threshold of channel quality); (2) selecting the cell  27 C of the base station  27  for performing conditional handover; (3) applying the configuration of the conditional handover message  232  for performing conditional handover to the cell  27 C of the base station  27 ; (4) initiating conditional handover to the cell  27 C of the base station  27 ; and (5) initiating RA to the cell  27 C of the base station  27 . 
     In some embodiments, the processor  211  of the user equipment  21  may reset the first timer  232 T 1  and start the first timer  232 T 1  with any one of these operations. In detail, the processor  211  of the user equipment  21  may start the first timer  232 T 1  when any one of these operations is executed. In other words, the start of the first timer  232 T 1  and the execution of any one of these operations may occur at the same time. Accordingly, no matter the first timer  232 T 1  is started with which operation, the user equipment  21  terminates the conditional handover to the cell  27 C of the base station  27  when the first time  232 T 1  expires but the conditional handover has not succeeded yet. Then, the user equipment  21  may determine whether the trigger condition  232 C is met for another cell again. 
     In some embodiments, the first timer  232 T 1  may be used for controlling the period of performing conditional handover to cell  25 C of the base station  25  and for controlling the period of performing conditional handover to cell  27 C of the base station  27 . In some embodiments, the conditional handover message  232  may further include a second timer  232 T 2  with the second timer  232 T 2  being used (as an example) for controlling the period of performing conditional handover to cell  27 C of the base station  27 . 
     In some embodiments, the conditional handover message  232  may further include a multi-handover timer  232 TM. In detail, the first timer  232 T 1  (or the second timer  232 T 2 ) may be used for controlling the period of performing conditional handover to one cell only, whereas the multi-handover timer  232 TM may be used to control the period of performing conditional handover for all available cells. 
     In some embodiments, the processor  211  of the user equipment  21  starts the multi-handover timer  232 TM with the start of the first timer  232 T 1  associated with the conditional handover to cell  25 C of the base station  25 . When the first timer  232 T 1  expires and the processor  211  of the user equipment  21  restarts the first timer  232 T 1  (or starts the second timer  232 T 2 ) for the cell  27 C of the base station  27 , the multi-handover timer  232 TM continues to run for the cell  27 C of the base station  27 . When the multi-handover timer  232 TM expires, the processor  211  of the user equipment  21  stops any possible conditional handover and performs reestablishment with available cell. 
     In some embodiments, the conditional handover message  232  may further include a maximum number  232 N of handover retires, and the maximum number  232 N of handover reties may be used for controlling the number of conditional handover attempts with different cells. In detail, once the user equipment  21  terminates conditional handover for one cell, the processor  211  of the user equipment  21  adds one to a counter (with initial value “0”) and checks whether the counter reaches the maximum number  232 N. If the counter does not reach the maximum number  232 N, the user equipment  21  determines whether the trigger condition  232 C is met for another cell. If the counter reaches the maximum number  232 N, the processor  211  of the user equipment  21  stops any possible conditional handover attempt and performs reestablishment with available cell. 
     In some embodiments, processor  211  of the user equipment  21  selects a cell among cells meeting a trigger condition  232 C expect for the cell which has been selected for conditional handover. 
     Please refer to  FIG. 3A  through  FIG. 3C .  FIG. 3A  illustrates a schematic view of a network system  3  according to an embodiment of the present disclosure.  FIG. 3B  is a block diagram of a user equipment  31  according to the embodiment of the present disclosure. The user equipment  31  includes a processor  311  and a transceiver  313 . The processor  311  and the transceiver  313  are coupled through a communication bus  315  allowing the processor  311  to control the transceiver  313  to transmit and/or receive information. 
       FIG. 3C  is a block diagram of a base station  33  according to the embodiment of the present disclosure. Base station  33  includes a processor  331  and a transceiver  333 . The processor  331  and the transceiver  333  are coupled through a communication bus  335  allowing the processor  331  to control the transceiver  333  to transmit and/or receive information. The interactions between the individual elements will be further described hereinafter. 
     In some embodiments, the network system  3  may include the user equipment  31 , the base station  33  with a cell  33 C, a base station  35  with a cell  35 C and a base station  37  with a cell  37 C as shown in  FIG. 3A . The user equipment  31  exchanges data with the base station  33  via a network connection  30 . Before conditional handover, the transceiver  333  of the base station  33  transmits a conditional handover message  332  to the user equipment  31 . The conditional handover message  332  includes a trigger condition  332 C. Then, the transceiver  313  of the user equipment  31  receives the conditional handover message  332  for performing conditional handover when needed. 
     In some embodiments, a failure happens when the user equipment  31  is performing conditional handover from base station  33  to the cell  35 C of the base station  35 . Accordingly, the processor  311  of the user equipment  31  may determine that conditional handover to the cell  35 C of the base station  35  fails. Therefore, the processor  311  of the user equipment  31  needs to select another cell. 
     In particular, when there is no cell meeting the trigger condition  332 C for conditional handover, the processor  311  of the user equipment  31  selects a cell with best channel quality for performing conditional handover. In detail, the processor  311  of the user equipment  31  measures a first channel quality value (not show) between the user equipment  31  and the cell  35 C of the base station  35 , and measures a second channel quality value (not shown) between the user equipment  31  and the cell  37 C of the base station  37 . 
     In some embodiments, the second channel quality value is greater than the first channel quality value. Then, the processor  311  of the user equipment  31  ranks the second channel quality value as first priority, and the processor  311  of the user equipment  31  selects the cell  37 C of the base station  37  for performing conditional handover or performing reestablishment. 
     In detail, when cell  37 C of the base station  37  is associated with the conditional handover message  332  received from base station  33 , it means that the cell  37 C of the base station  37  has been selected by the base station  33  for conditional handover and the cell  37 C of the base station  37  is prepared for conditional handover. Therefore, the processor  311  of the user equipment  31  may attempt conditional handover with the cell  37 C of the base station  37 . 
     Otherwise, when cell  37 C of the base station  37  is not associated with the conditional handover message  332  from base station  33 , it means that the cell  37 C of the base station  37  has not been selected by the base station  33  for conditional handover and the cell  37 C of the base station  37  is not prepared for conditional handover. Therefore, the processor  311  of the user equipment  31  performs reestablishment with the cell  37 C of the base station  37 . 
     In some embodiments, to improve the chance for cell  37 C of the base station  37  to be selected, the conditional handover message  332  may include an offset  332 O designated to increase the second channel quality value. In detail, the processor  311  of the user equipment  31  may update the second channel quality value with the offset  332 O. In other words, the second channel quality may be added to the offset  332 O (i.e., the second channel quality may include the offset). Accordingly, the rank of the cell  37 C of the base station  37  for being selected may be raised since second the channel quality value is increased by adding the offset  332 O. 
     In some embodiments, when the cell  37 C of the base station  37  is associated with the conditional handover message  332  from the base station  33 , it means that the cell  37 C of the base station  37  has been selected by the base station  33  for conditional handover and the cell  37 C of the base station  37  is prepared for conditional handover. Therefore, the processor  311  of the user equipment  31  determines whether the second channel quality value meets a threshold. 
     In some embodiments, this threshold may include a cell selection criterion. As Reference Signal Receiving Power (RSRP) for example, the cell selection criterion may include: 
         S   rxlev = Q   rxlevmeas −( Q   rxlevmin+   Q   rxlevminoffset )− P   compensation − Q   offsettemp  
 
     Where S rxlev  represents cell selection RX level value, Q rxlevmeas , represents measured cell receiving level value, Q rxlevmin , in represents minimum required receiving level in the cell, Q rxlevminoffset  represents Offset to the signaled Q rxlevmin  taken into account in the S rxlev  evaluation as a result of a periodic search for a higher priority Public Land Mobile Network (PLMN) while camped normally in a Visited-PLMN, P compensation  represents power compensation value for the user equipment, and Q offsettemp  represents an offset temporarily applied to the cell. 
     When the processor  311  of the user equipment  31  determines that the second channel quality value meets the cell selection criterion, the processor  311  of the user equipment  31  performs conditional handover with the cell  37 C of the base station  37 . 
     It should be noted that the channel quality values and the threshold of the channel quality mentioned above may be related with RSRP or Reference Signal Received Quality (RSRQ). 
     It shall be particularly appreciated that the processors mentioned in the above embodiments may be a central processing unit (CPU), other hardware circuit elements capable of executing relevant instructions, or combination of computing circuits that are well-known by those skilled in the art based on the above disclosures. Moreover, the transceivers mentioned in the above embodiments may be a combination of a network data transmitter and a network data receiver. Further, the bus may a communication interface for transferring data between CPU and transceiver, and may include electrical bus interface, optical bus interface or even wireless bus interface. However, such description is not intended to limit the hardware implementation embodiments of the present disclosure. 
     Some embodiments of the present disclosure include a method, and a flowchart diagram thereof is shown in  FIG. 4 . The method of some embodiments is for use in a network system (e.g., the network system of the aforesaid embodiments), wherein the network system includes a user equipment, a source base station and a target base station with a target cell. Detailed operations of the method are described below. 
     Operation S 401  is executed to transmit, by the user equipment, a measurement report to the source base station. Operation S 402  is executed to receive, by the source base station, the measurement report from the user equipment. Then, the source base station may identify the target cell of the target base station for the user equipment to perform conditional handover to when needed. Operation S 403  is executed to transmit, by the source base station, a conditional handover request to the target base station for notifying the target base station of a possible conditional handover. 
     Operation S 404  is executed to receive, by the target base station, the conditional handover request from the source base station. Operation S 405  is executed to transmit, by the target base station, a conditional handover acknowledge to the source base station. Operation S 406  is executed to receive, by the source base station, the conditional handover acknowledge from the target base station. 
     Operation S 407  is executed to transmit, by the source base station, a conditional handover message to the user equipment for notifying the user equipment of the target cell of the target base station for performing conditional handover to when needed. The conditional handover message includes a trigger condition and a first timer corresponding to the target cell of the target base station. Operation S 408  is executed to receive, by the user equipment, the conditional handover message. 
     Operation S 409  is executed to determine, by the user equipment, whether the trigger condition is met. If the trigger condition is met for the target cell of the target base station, operation S 410  is executed to start, by the user equipment, the first timer for controlling the period of performing conditional handover to the target cell of the target base station. In particular, if the first time expires but the conditional handover has not succeeded yet, the user equipment terminates the conditional handover to the target cell of the target base station. If the trigger condition is not met for the target cell of the target base station, operation S 409  is executed by the user equipment to continue checking to see if the trigger condition for the target cell of the target base station is met or not. 
     In some embodiments, when operation S 409  is executed to determine whether the trigger condition is met, there may be at least five sub-operations performed by the user equipment, for example: (1) determining that the trigger condition corresponding to the target cell of the target base station is met; (2) selecting the target cell of the target base station for performing conditional handover; (3) applying a configuration of the conditional handover message for performing conditional handover to the target cell of the target base station; (4) initiating conditional handover to the target cell of the target base station; and (5) initiating RA to the target cell of the target base station. 
     In some embodiments, operation S 410  is executed with any one of these sub-operations. Accordingly, no matter with which sub-operation the first timer is started, the user equipment terminates the conditional handover to the target cell of the target base station when the first time expires but the conditional handover has not successfully terminated yet. Then, the user equipment determines whether the trigger condition is met for another cell. 
     Some embodiments of the present disclosure include a method, and a flowchart diagram thereof is shown in  FIGS. 5A through 5C . The method of some embodiments is for use in a network system (e.g., the network system of the aforesaid embodiments), wherein the network system includes a user equipment, a source base station, a target base station with a target cell and another target base station with anther target cell. Detailed operations of the method are described below. 
     Operation S 501  is executed to transmit, by the user equipment, measurement report to the source base station. Operation S 502  is executed to receive, by the source base station, the measurement report from the user equipment. Then, the source base station may identify two cells, which are the target cells of the target base stations, respectively, for the user equipment to perform conditional handover to when needed. Operation S 503  is executed to transmit, by the source base station (BS), a conditional handover request to both target base stations, for example the Target BS and the Another Target BS, for notifying both base stations of a possible conditional handover. 
     Operation S 504   a  is executed to receive, by the Target BS, the conditional handover request from the source base station (Source BS). Operation S 504   b  is executed to receive, by another target base station (Another Target BS), the conditional handover request from the source base station (Source BS). Operation S 505   a  is executed to transmit, by the Target BS, a conditional handover acknowledge to the Source BS. Operation S 505   b  is executed to transmit, by Another Target BS, a conditional handover acknowledgement to the Source BS. Operation S 506  is executed to receive, by the Source BS, the conditional handover acknowledges from the Target BS and the Another Target BS. 
     Operation S 507  is executed to transmit, by the source base station, a conditional handover message to the user equipment for notifying the user equipment of the target cell of the target base station and the another cell of the another target base station for performing conditional handover when needed. The conditional handover message includes a trigger condition, a first timer, a multi-handover timer and a maximum number of handover. Operation S 508  is executed to receive, by the user equipment, the conditional handover message. Operation S 509  is executed to determine, by the user equipment, whether the trigger condition has been met. 
     Please refer to  FIG. 5B . In some embodiments, the trigger condition is met for the target cell of the target base station, operation S 510  is executed to start, by the user equipment, the first timer for controlling the period of performing conditional handover to the target cell of the target base station and the multi-handover timer for controlling period of performing conditional handover for all available cells. 
     In some embodiments, when operation S 509  is executed to determine that the trigger condition is met, there may be at least five sub-operations performed by the user equipment, for example: (1) determining that the trigger condition corresponding to the target cell of the target base station is met; (2) selecting the target cell of the target base station for performing conditional handover; (3) applying a configuration of the conditional handover message for performing conditional handover to the target cell of the target base station; (4) initiating conditional handover to the target cell of the target base station; and (5) initiating RA to the target cell of the target base station. 
     In some embodiments, operation S 510  is executed with any one of these sub-operations. Accordingly, no matter the first timer is started with which sub-operation, the user equipment terminates the conditional handover to the target cell of the target base station when the first time expires but the conditional handover has not succeeded yet. Then, the user equipment determines whether the trigger condition is met for connection to another cell. 
     In some embodiments, when the first time expires but the conditional handover has not succeeded yet, the user equipment terminates the conditional handover to the target cell of the target base station. Operation S 511  is executed to determine, by the user equipment, that the trigger condition is met for another target cell of the another target base station. Operation S 512  is executed to reset and start, by the user equipment, the first timer for controlling the period of performing conditional handover to another target cell of the another target base station. It should be noted that, during the operations, the user equipment stops any possible conditional handover and performs reestablishment with available cell when the multi-handover timer expires. 
     Please refer to  FIG. 5C . In some embodiments, the trigger condition is met for the target cell of the target base station, operation S 513  is executed to start, by the user equipment, the first timer for controlling the period of performing conditional handover to the target cell of the target base station. 
     In some embodiments, when operation S 509  is executed to determine that the trigger condition is met, there may be at least five sub-operations performed by the user equipment, for example: (1) determining that the trigger condition corresponding to the target cell of the target base station is met; (2) selecting the target cell of the target base station for performing conditional handover; (3) applying a configuration of the conditional handover message for performing conditional handover to the target cell of the target base station; (4) initiating conditional handover to the target cell of the target base station; and (5) initiating RA to the target cell of the target base station. 
     In some embodiments, operation S 513  is executed with any one of these sub-operations. Accordingly, no matter if the first timer is started with which sub-operation, the user equipment terminates the conditional handover to the target cell of the target base station when the first time expires but the conditional handover has not succeeded yet. Then, the user equipment determines whether the trigger condition is met for another cell. 
     In some embodiments, when the first time expires but the conditional handover has not succeeded yet, the user equipment terminates the conditional handover to the target cell of the target base station. Operation S 514  is executed to add, by the user equipment, one to a counter. Operation S 515  is executed to determine, by the user equipment, that the trigger condition is met for the another target cell of the another target base station. Operation S 513  is executed to restart, by the user equipment, the first timer for controlling the period of performing conditional handover to another target cell of the another target base station. It should be noted that, during the operations, the user equipment stops any possible conditional handover and performs reestablishment with available cell when the counter reaches the maximum number of handover. 
     Some embodiments of the present disclosure include a method, and a flowchart diagram thereof is shown in  FIG. 6 . The method of some embodiments is for use in a network system (e.g., the network system of the aforesaid embodiments), wherein the network system includes a user equipment, a source base station, a target base station with a target cell and another target base station with anther target cell. Detailed operations of the method are described below. 
     Operation S 601  is executed to transmit, by the source base station, a conditional handover message to the user equipment. Operation S 602  is executed to receive, by the user equipment, the conditional handover message from the source base station. The conditional handover message includes a trigger condition. When the trigger condition is met, operation S 603  is executed to perform, by the user equipment, conditional handover to the target cell of the target base station. 
     In some embodiments, operation S 604  is executed to determine, by the user equipment, that conditional handover to the target cell fails. Operation S 605  is executed to select, by the user equipment, another target cell with a channel quality. Operation S 606  is executed to determine, by the user equipment, whether to perform conditional handover to another target cell when the another target cell is associated with the conditional handover message or to perform reestablishment with the another target cell when the another target cell is not associated with the conditional handover message. 
     When another target cell is associated with the conditional handover message, operation S 607  is executed to perform, by the user equipment, conditional handover to another target cell of the another target base station. When another target cell is not associated with the conditional handover message, operation S 608  is executed to perform, by the user equipment, reestablishment to another target cell of the another target base station. 
     In some embodiments, another target cell may be selected because the channel quality of another target cell is the highest channel quality value among the channel quality values of the available target cells. In other words, the priority of selecting the cell depends on the channel quality value. Accordingly, the cell with higher channel quality value has higher priority of being selected. 
     In some embodiments, for raising the priority of being selected of another target cell, the conditional handover message may include an offset designated to increase a channel quality value corresponding to another target cell. In detail, the user equipment may update the channel quality value (which corresponds to another target cell) by applying the offset. In other words, the channel quality value (which corresponds to another target cell) may be added to the offset (i.e., the channel quality value may include the offset). Accordingly, the priority of being selected of another target cell may be raised. 
     In some embodiments, when another target cell of the base station is associated with the conditional handover message, the user equipment further determines whether the channel quality value (which corresponds to the another target cell) meets a threshold. When the user equipment further determines that the channel quality value (which corresponds to another target cell) meets the threshold, the user equipment performs conditional handover to the another target cell of the another target base station. In some embodiments, the threshold may include a cell selection criterion as mentioned in the aforesaid embodiment. 
     It should be noted that the user equipment, which has received a conditional handover message from the source base station, may receive a conventional handover message from the source base station. In some embodiments, the user equipment releases the resources of the conditional handover message when receiving the conventional handover message. 
     In some embodiments, the user equipment does not release the resources of the conditional handover message when receiving the conventional handover message. And the first timer and multi-handover timer mentioned in the aforesaid embodiments may be introduced after conventional handover failure. 
     The method of the present disclosure can be implemented on a programmed processor. However, the controllers, flowcharts, and modules may also be implemented on a general purpose or special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an integrated circuit, a hardware electronic or logic circuit such as a discrete element circuit, a programmable logic device, or the like. In general, any device that hasa finite state machine capable of implementing the flowcharts shown in the figures may be used to implement the processing functions of the present disclosure. 
     While the present disclosure has been described with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. For example, various components of the embodiments may be interchanged, added, or substituted in the other embodiments. Also, all of the elements shown in each figure are not necessary for operation of the disclosed embodiments. For example, one skilled in the art of the disclosed embodiments would be capable of making and using the teachings of the present disclosure by simply employing the elements of the independent claims. Accordingly, the embodiments of the present disclosure as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the present disclosure. 
     In this document, relational terms such as “first,” “second,” and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a,” “an,” or the like does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. Also, the term “another” is defined as at least a second or more. The terms “including,” “having,” and the like, as used herein, are defined as “comprising.”