Method of controlling handover in uplink synchronous transmission scheme

Abstract: Disclosed a method of controlling a handover using both a soft handover and a hard handover in an uplink synchronous transmission scheme to synchronize reception timing between mobile stations in an area of a base station. In the present invention, a uplink synchronous timing controlled based on a base station can be adjusted to a uplink syncluonous timing of the other base station after re-setting a radio link with the information of uplink synchronous transmission. Accordingly, the performance or the receptible capacity and the cell coverage area can be increased by serving the hard handover with the uplink synchronous transmission scheme.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile communication system, and more particularly to a method of controlling a handover by combining both a soft handover and a hard handover in an uplink synchronous transmission scheme (USTS) to synchronize reception timing between mobile stations in an area of a base station. And, the present invention further relates to a method of controlling a handover which can convert a mode of the uplink synchronous transmission scheme by using the current interface protocols in the event that a mobile station moves from a currently serving source base station to a target base station, as controlling a transmission signal timing of the mobile station in the uplink synchronous transmission scheme applied to the mobile communication system. 2. Description of the Related Art Generally, an uplink synchronous transmission scheme (USTS) is used to reduce multiple access interferences using orthogonality of orthogonal codes by controlling reception timing between mobile stations in a base station in using a closed loop timing control manner. A related soft handover manner is a technique that a mobile station (e.g., a user equipment or a mobile terminal) communicates with a plurality of base stations, and continuously maintains a communication channel during the time of serving the handover. Here, the soft handover manner can be applied to a softer handover manner serving the handover between sectors. The handover manner can generally be divided into: (1) a hard handover that terminates a communication channel of a present source base station before connecting the communication channel to an target base station; (2) a soft handover that terminates the communication channel of the source base station after connecting the communication channel to the target base station; and (3) a softer handover that performs the soft handover function between sectors in the same base station divided into at least a sector. At this time, the soft handover allows to continuously maintain the communication channel without disconnecting the channel and to concurrently connect to be overlapped for the same communication channel to two base stations serving the handover during a constant time. While, the hard handover causes discontinuity for the communication channel, a new communication channel is connected after terminating the present communication channel. Fig. 1 is a schematic block diagram of a related mobile communication system for illustrating a handover function between two base stations in an uplink channel. Referring to Fig. 1, the related mobile communication system includes a radio network controller 101 for selecting an optimized frame on the basis of a received radio frame, a mobile station 102, and more than one base station 103 or 104. The radio network controller 101 includes a selector 101 a for selecting the optimized radio frame. A transmission signal of the mobile station 102 is concurrently received at the first base station 103 and the second base station 104 in the uplink channel. If the handover is performed in the related mobile communication system, i.e., the mobile station 102 is moved from a cell area of the first base station 103 to a cell area of the second base station 104. The first base station 103 and the second base station 104 demodulate the transmission signal received from the mobile station 102, and then forward it to the radio network controller 101 with the radio frame, respectively. Then, the radio network controller 101 can select the optimized transmission signal on the basis of each transmission signal. Accordingly, the radio network controller 101 can be connected to the mobile station 102 via the communication channel of the base station corresponding to the optimized transmission signal. Meanwhile, the softer handover serves the same operation as the soft handover function described as above, if the mobile station 102 is moved from a sector to other sector in the same area of base station. That is, in case of the softer handover, the two signals received from the mobile station 102 are demodulated in the first base station 103 or the second base station 104, and then one of two demodulated transmission signals can forward to the base station subsystem 101. Fig. 2 is a block diagram of a related mobile communication system illustrating the soft handover function between two base stations controlled by the same radio network controller. Referring to Fig.2, the related mobile communication system includes a core network 201, a mobile station 220, and a UMTS radio connection network 210 connected between the core network 201 and the mobile station 220. The UMTS radio access network 210 is a generic term for the related radio network controller(s) and base station(s). The UMTS radio access network in Fig. 2 includes a serving radio network controller (SRNC) 211 and at lease a base station 212 or 213 connected with the SRNC 211. The SRNC 211 manages dedicated radio resources allocated to the mobile station 220. At this time, if the mobile station 220 is moved from a source base station 212, which refers to the base station that the mobile station is camp on to, in an area of the SRNC 211 to an target base station 213, the source base station 212 and the target base station 213 demodulate the transmission signal received from the mobile station 220 and then transmit to the SRNC 211 with the radio frame form, respectively. The SRNC 211 selects the optimized transmission signal on the basis of received transmission signal, and thereby continuously maintaining the communication by concurrently connecting the communication channel for the mobile station 220 to two base stations 212 and 213 in the area where the cell areas of the source base station 212 and the target base station 213 overlap. Fig. 3 is a block diagram of the related mobile communication system serving the soft handover between two base stations when radio network controllers controlling at least two more base stations are different each other. Referring to Fig. 3, although the mobile communication system is very similar to the system shown in Fig. 2, a drift radio network controller (DRNC) 312 has been added in the UMTS radio access network 210. Also, is it noted that the mobile communication system includes at least two radio network controllers 312 and 316 which control the transmission signal received from the mobile station 220, and different from each other. The SRNC 316 and the DRNC 312 are linked to at least a base stations 313 or 317 that can be controlled, respectively. Fig. 3 shows the related mobile communication system serving the soft handover function from the cell area of the source base station 317 controlled by the SRNC 316 to the cell area of the target base station 313 controlled by the DRNC 312. Accordingly, if the mobile station 220 moves away from the cell area of the source base station 317 and to the cell area of the target base station 313, the mobile station 220 can concurrently maintain the communication channel for each base station 313 or 317 as a result of a communication channel between the SRNC 316 and the DRNC 312. At this time, the SRNC 316 manages the dedicated radio resources allocated to the mobile station 316, and the DRNC 312 can provide the radio resources to the mobile station 220 when the mobile station 220 is moved from the cell area of the first base station 317 to the cell area of the second base station 313. Fig. 4 is a flow chart illustrating the handover control procedure for the related soft handover. Referring to Fig. 4, the method of controlling the handover can be applied to the case of controlling the handover when the mobile station 102 is moved from the source base station 103 to the target base station 104 as shown in Fig.1. Also, the method can be applied to the case in which the radio network controller(s) controlling each base station is/are the same or different each other. First, the communication channel of the mobile station is connected only through the source base station, when the mobile station is exist on the area of the source base station. But, if the mobile station is entered into the area overlapped by the cell areas of the source base station and the target base station, and thereby serving the handover (step 401), the mobile station connects the communication channel not only to the source base station, but also to the target base station. As described above, the mobile station periodically measures the strength of a pilot signal received from at least one base station, and transmits the measured value to the radio network controller via the base station. Then, the radio network controller selects an optimized pilot strength on the basis of the measured values for at lease one base station. And, the radio network controller can connect to the communication channel with the base station in which the optimized pilot strength has been measured by the mobile station. Accordingly, the mobile station measures the pilot strengths of the source base station and the target base station (step 403), and determines whether the pilot strength measured in the target base station is over than a predetermined pilot reference value. If the measured pilot strength was over than the pilot reference value, then the mobile station transmits the each measured result of the source base station and the target base station to the radio network controller and list up the target base station onto a candidate list for serving the handover (step 405). The radio network controller determines whether to control the handover based on the measurement of the pilot strength of the serving and the target base stations. If it is determined that the pilot strength of the target base station is sufficient, then the radio network controller transmits the first handover message to the mobile station to set up the communication channel with the target base station. That is, if the radio network controller transmits the first handover message to the mobile station, the mobile station starts communicating with the target base station on the basis of the first handover message via a new communication channel. Here, the first handover message includes a PN offset of the target base station and newly allocated Walsh codes. Accordingly, the mobile station transfer the target base station from the candidate list onto an actual communication list, and transmits a handover completion message to the radio network controller after obtaining a synchronization of a downlink communication channel defined in the first handover message. Thus, the base station currently communicating with the mobile station belong to both the source base station and the target base station (steps 407 and 409). According to the description as above, the communication channel is established between the mobile station and the target base station by the first handover message of the radio network controller, when the mobile station enters to the cell area of the target base station. Meanwhile, the mobile station periodically measures the pilot strengths of the source base station and the target base station (step 411 step), and determines whether the pilot strength of the source base station is enough or not (step 413). That is, the mobile station determines whether the pilot strength of the source base station is less than a pilot extracting reference value. And, the mobile station starts measuring for the extraction time from a point that the pilot strength of the source base station is less than the pilot extraction reference value. If the extraction time reaches the predetermined extracting threshold value, the mobile station transmits the pilot strength-measuring message to the radio network controller. So, the mobile station receives the second handover message from the radio network controller (step 415). The mobile station deletes the source base station from the actual communication list on the basis of the received second handover message, and transmits the handover completion message to the radio network controller (step 417). Here, the second handover message only includes a PN offset for the target base station, and does not include the PN offset for the source base station. As described above, the method of releasing the communication channel of the source base station is explained in case where the pilot strength of the source base station is less than the pilot extracting reference value. Fig. 5 is a time sequential chart illustrating the data flow procedure fur protocol entities of each communication element when the related soft handover is performed. Fig. 5 illustrates the data flow procedure in case where the different radio network controllers for controlling at least one base station in operation, and also the same procedure can be applied to the case where more than one base station is controlled by the same radio network controller. That is, the description for the data flow procedure of Fig. 5 can be clearly understood with reference to Fig. 3. Referring to Fig. 5, the data flow procedure can be classified into a radio link adding procedure (step 420) and a radio link deleting procedure (step 433), and each procedure will be explained in detail below. Here, step 420 explains about establishing the communication channel through the radio link between the target base station 327 and the mobile station 329 when the handover is performed, while the step 433 explains for releasing the radio link established between the source base station (SRNS-base station) 325 and the mobile station 329. In step 420, if the mobile station 329 is entered into a handover area, the serving radio network controller (SRNC) 321 decides whether to set up a new link between the mobile station 329 and a target base station (DRNS-base station) 327 (step 419). Here, the handover area means an area overlapped by cell areas of the source base station 325 and the target base station 327 as described above. Of course, the SRNC 321 has to be conducted for receiving the resulting measurement of the pilot strength of each the source base station 325 and the target base station 327 before performing step 419. Accordingly, the SRNC 321 can decide to set up the new link on the basis of the measured results. If it is necessary to establish the new link between the mobile station 329 and the DRNS-base station 327 as a result of the decision, the SRNC 321 transmits a radio link setup request message to the DRNC 323 by using a Radio Network Subsystem Application Part (RNSAP) which is an interfacing protocol between the radio network controllers (step 423). The radio link setup request message includes a command for establishing the new radio link. The DRNC 323 transmits the radio link setup request message to the DRNS-base station 327 by using a Node B Application Part (NBAP) that is a protocol between the base station and the radio network controller (step 423). Where, the "Node B" represents a base station. The DRNS-base station 327 transmits a radio link setup response message to the DRNC 323 by using the NBAP protocol after successfully establishing the radio link with the mobile station 329 on the basis of the radio link setup request message (step 425). Here, the radio link setup response message includes a report of successfully establishing the radio link between the mobile station 329 and the DRNS-base station 327. The DRNC 323 transmits radio link setup response message to the SRNC 321 by using RNSAP protocol (427 step). Subsequently, the SRNC 321 transmits an active set update command message to the mobile station 329 by using the radio resource control (RRC) which is an interface protocol performing between the mobile station and the radio network controller. Then, the mobile station 329 can add the DRNS-base station 327 on an active set on the basis of the active set update command message (step 429). Here, the active set means a group of base stations that are communicating with the mobile station through the same downlink communication channel. The mobile station 329, added the DRNS-base station 327 on the active set, transmits an active set update complete message to the SRNC 321 by using the RRC protocol (step 431). Next, in step 433, if the mobile station 329 moves away from the handover area and enters a cell area of the DRNS-base station 327, the SRNC 321 decides whether to remove a predetermined radio link (step 435). Here, the predetermined radio link means a communication channel between the SRNS-base station 325 and the mobile station 329. Of course, the measured value of the pilot for the SRNS-base station 325 that is measured at the mobile station 329 have to be transmitted to SRNC 321 before performing step 433. Accordingly, the SRNC 321 can decide whether to remove the radio link established at the SRNS-base station 325 based on the measured value of the SRNS-base station 325. As a result, in the case of removing the radio link established on the SRNS-base station 325, the SRNC 321 transmits the active set update command message to the mobile station 329 by using the RRC protocol (step 437). The mobile station removes the radio link established on the present SRNS-base station from the active set on the basis of the active set update command message received from the SRNC 321, and transmits an active set update complete message to the SRNC 321 by using the RRC protocol (step 439). The SRNC 321 transmits a radio link deletion request message to the currently serving SRNS-base station 325 by using the NBAP protocol (step 441). Then, the serving SRNS-base station 325 releases the radio link between the serving SRNS-base station 325 and the mobile station 329 according the radio link deletion request message, and transmits a radio link deletion response message as a result of releasing to the SRNC 321 by using the NBAP protocol (step 443). It is noted that the remained steps excluding 421 to 427 steps may be. same as the steps of Fig. 5, if more than one base station is controlled by the same radio network controller. That is, the command of the SRNC is directly transmitted to the SRNS―base station 325 without passing through the DRNS-base station as shown in steps 421 to 427, since the DRNS-base station 327 does not exist in the case where more than one base station is controlled by the same radio network controller. As described above, the conventional method for controlling the handover does not consider a transmission scheme that is provided to improve performances of the uplink in the base station, i.e., an uplink synchronous transmission scheme for synchronizing the reception timing between the mobile stations. That is, the related handover is served without considering the transmission scheme for controlling the reception timing of the mobile stations by using the closed loop timing control scheme in the base station. Accordingly, in order to improve the method of controlling the handover, a hard handover means of the uplink synchronous timing essentially required in the uplink synchronous transmission scheme should be added together with the conventional soft handover method. In the present invention, it should be noted that the hard handover of the uplink syuchronous timing is used instead of the soft handover, since only one of the base station is selected because synchronizing the uplink synchronous timing for the source base station and the target base station cannot be occurred at the same time. If the hard handover of the uplink synchronous timing is not considered, there are problems of not utilizing the uplink synchronous transmission scheme function or decreasing the receptible capacity and cell coverage areas due to failure in maintaining the uplink synchronous transmission gain by moving the mobile station between the sectors or base stations. In the long run, it is expected that a new method of controlling the handover, that the hard handover manner controlling the synchronous timing is added with considering the related soft handover manner, is strongly required in the field. The uplink synchronous transmission scheme (USTS) has been discussed in a document from TSG-RAN Working Group 1 Meeting #14, July 4-7, 2000, Oulu, Finland (TSGR1 #14 (00) 0903). According to this document, in a USTS mode, time alignment is required to preserve orthoganility between channelisation codes from user equipment and also to properly despread the cell-specific long scrambling code. The transmission time control is carried out by two steps. The first step is Initial synchronization and the second is tracking. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to solve the aforementioned problems in the related art by providing a method of controlling a handover using an uplink synchronous transmission scheme while using the conventional mobile communication system. It is still another object of the present invention to provide a method of controlling a handover for an uplink synchronous transmission scheme which can control an uplink synchronous timing by combining the present soft handover manner for NON-USTS link addition and/or deletion with the hard handover manner for controlling uplink synchronizing timing. It is further object of the present invention to provide a method of controlling a handover for an uplink synchronous transmission scheme which can define a data flow between physical layers of each communication element when the handover is served. To achieve the above object, there is provided a method of controlling a handover using an uplink synchronous transmission scheme in a mobile communication system in which a first and a second base stations are controlled by a radio network controller and a mobile station concurrently to transmit and receive a communication data of an identical signal, the method comprising the steps of transmitting an information for the uplink synchronous transmission to the first and the second base stations; resetting a radio link on the basis of the information for the uplink synchronous transmission; and adjusting an uplink synchronous timing controlled by a base station on the basis of the reset radio link to the uplink synchronous timing of another base station. It is desired that the information for the uplink synchronous transmission is an indicator of the uplink synchronous transmission scheme, a scramble code of the uplink synchronous transmission scheme, or a channelizing code number of the uplink synchronous transmission scheme. However, if the base stations are controlled by different radio network controllers, the method further comprising the steps of transmitting the information on the uplink synchronous transmission from a radio network controller to another radio network controller, resetting the radio link on the basis of the information for the uplink synchronous transmission transmitted from the radio network controller or the other radio network controller; and adjusting an uplink synchronous timing of the mobile station which is controlled by the base station under the control of the radio network controller on the basis of the reset radio link with the uplink synchronous timing of the base station controlled by the cther radio network controller. Also, to achieve the above object, there is provided a method of changing a base time for uplink synchronous transmission scheme in a mobile station of a mobile communication system in which a first base station and a second base station which are controlled by a radio network controller and receive communication data of an identical signal from a mobile station simultaneously, the method comprising the steps of transmitting communication data to the first base station; checking pilot signals from the second base station; transmitting communication data to the second base station; checking pilot signals from the first and second base station; and changing the base time for uplink synchronous transmission scheme to the base time of the second base station. BRIEF DESCRIPTION OF THE DRAWINGS The above objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic block diagram of a related mobile communication system for illustrating a handover function between two base stations in a uplink channel; Fig. 2 is a block diagram of a related mobile communication system illustrating a soft handover function between two base stations in a radio network controller controlling at least two more base stations; Fig. 3 is a block diagram of a related mobile communication system illustrating a soft handover function between two base stations when radio network controllers controlling at least two more base stations are different each other; Fig. 4 is a flow chart illustrating the handover control procedure for a related soft handover; Fig. 5 is a time sequential chart illustrating the data flow procedure for protocols of each communication element when a related soft handover is performed; Fig. 6 is a flow chart illustrating the procedure of controlling a signal timing of a mobile station when the soft handover is performed according to a preferred embodiment of the present invention; Fig. 7 is a time sequential chart illustrating the data flow procedure for protocols of each communication element for controlling a signal timing of a mobile station when a radio network controller controlling at least two more base stations performs soft handover according to a preferred embodiment of the present invention; and Fig. 8 is a time sequential chart illustrating the data flow procedure for physical layers each communication element for controlling a signal timing of a mobile station when different radio network controllers controlling at least two more base stations performs soft handover according to a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. In the following description, same drawing reference numerals are used for the same elements even in different drawings. The matters defined in the description such as a detailed construction and elements of a circuit are nothing but the ones provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention can be carried out without those defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Fig. 6 is a flow chart illustrating the procedure of controlling the signal timing of the mobile station when the soft handover is performed according to a preferred embodiment of the present invention. Fig. 6 further illustrates a method of controlling the handover by combining a soft handover with a hard handover for controlling a transmission signal timing of a mobile station in an uplink. Also, the method of the present invention can be applied in the case where the radio network controller for controlling each base station is either same or different from each other. The following detailed explanation will be emphasized more on the method of controlling the hard handover since establishing or releasing the radio link is the same as the operation of the related art as shown in Fig. 4, and it is different only when additionally serve to the hard handover. First, the communication channel of the mobile station is connected only through the currently located base station, if the mobile station situated in an area of the source base station. However, if the mobile station is entered into an area overlapped by the cell areas of the source base station and the target base station, and thereby serving the handover (step 451), the mobile station measures strengths of pilot signals received from both the source base station and the target base station (step 453). And, the mobile station judges whether the pilot strength measured in the target base station is greater than a predetermined pilot reference value (step 455). The mobile station then transmits the each resulting measurement of the source base station and the target base station to the radio network controller and register the target base station onto a candidate list for serving the handover according to the resulting measurement. And if the intensity of the pilot signal form the target base station exceeds the threshold based on the predetermined pilot reference value, then the communication data are transmitted to the target base station. Meanwhile, the target base station receives the first handover message from the radio network controller (step 457), and set up the communication channel with the mobile station according to the first handover message (step 459). Accordingly, detailed descriptions from step 451 to step 459 will be omitted since they are almost the same as the method of controlling the handover of the related art as shown in Fig. 4. Consequently, the base station that has established the communication channel with the mobile station can be both the currently source base station and the target base station. That is, the source base station is set up as a USTS mode, while the target base station is set up with a NON-USTS mode. Therefore, the source base station maintains the timing between transmission signals of the mobile station through the USTS mode in the uplink synchronous transmission scheme, while the target base station establishes the timing between transmission signals of the mobile station in the NON-USTS mode. At this time, the radio link is established either between the mobile station and the source base station, or between the mobile station and the target base station. Meanwhile, the radio network controller compares the radio link quality transmitted from the mobile station, and determines whether to perform the timing adjustment and the reconfiguration operations using the target base station based on the comparison result (step 461). The determination process can be performed by using the link quality or/and the numbers of the mobile stations. In case of using the link quality, the radio network controller compares the link quality of the currently source base station with the link quality of the target base station. If the link quality of the target base station is better than the source base station, then the radio network controller transmits the handover control command to the mobile station and the target base station, respectively. While, in case of using the number of the mobile stations, the radio network controller compares the number of the mobile stations transmitting in the uplink synchronous transmission for the source base station with the number of the mobile stations transmitting in the uplink synchronous transmission for the target base station. If the number of the mobile stations transmitting are more than the number of the mobile stations transmitting in the uplink synchronous transmission for the source base station in the uplink synchronous transmission for the target base station, the base station transmits the handover control command to the mobile station and the target base station, respectively. However, it is preferable that both the link quality and the quantity of the mobile stations are used for the above determination. If the target base station has selected for performing the timing adjustment and the reconfiguration operations as a result of step 461, the target base station performs the operation using the first handover message after receiving the first handover message from the radio network controller (step 463). Also, the radio network controller transmits the second handover message to the source base station, and thereby the source base station performs the timing adjustment and the reconfiguration based on the second handover message (step 465). It should be noted that the transmission signal of the mobile station is synchronized at the target base station as a result of steps 463 and 465, not synchronizing by the reception timing at the source base station. Consequently, the source base station is converted into the NON-USTS mode, and the target base station converted into the USTS mode, respectively. Accordingly, the interference between the received signals is removed at the target base station receiving the transmission signal of the mobile station transmitted by the non-synchronous type hard handover of the uplink synchronous timing, thereby improving the reception performances of the base station. Meanwhile, the mobile station periodically measures the pilot strength of the currently source base station (step 467), and determines whether the pilot strength of the source base station is enough or not (step 469). The mobile station transmits the pilot strength measurement message to the radio network controller according to the results. Thereafter, the mobile station receives the second handover message from the radio network controller (step 471), and then the mobile station deletes the source base station from the actual communication list according to the second handover message and transmits the handover completion message to the radio network controller (step 473). When the mobile station moves from the cell area of the source base station to the target base stations, whether to establish a new radio link with the target base station depends on the results of the pilot strengths measured for both the source and the target base stations. Further, determining whether to synchronize the transmission signal of the mobile station by the synchronous timing of the target base station depends on the link quality or/and the number of the mobile stations. And the hard handover message is transmitted to the mobile station and the target base station to synchronize with the target base station when the synchronous timing of the target base station has been selected. Fig. 7 is a time sequential chart illustrating the data flow procedure for protocols of each communication element for controlling the signal timing of the mobile station when a radio network controller controlling at least two more base stations performs soft handover according to a preferred embodiment of the present invention. Referring to Fig. 7, the mobile communication system includes a mobile station 337, the first base station 335 as a source base station, the second base station 333 as a target base station, and a serving RNC (SRNC) 331 as a radio network controller. Fig. 7 illustrates forwarding orders of control commands between the communication elements in the case of serving the handover by using USTS mode. The data flow shown in Fig.7 can be classified into 1) a soft handover procedure for setting the radio link in the target base station 333 as shown in step 501, 2) a hard handover procedure for converting the synchronous mode and 3) a soft handover procedure for releasing the radio link of the source base station 335 as shown in step 539. First, the soft handover procedure for setting the radio link in the target base station 333 will be explained. If the mobile station 337 starts entering into a handover area, i.e., entering from the cell area of the current source base station 335 to the target base stations 333, the soft handover is served as illustrated in step 420 shown in Fig. 5, thereby setting the radio link of the target base station 333 (step 501). Here, the setting of the radio link is achieved as the radio network controller 331 requests for setting up the radio link to the target base station 333. And when the message indicating that the radio link setup has been successively accomplished is received from the target base station 333, the radio network controller 331 requests for an update of an active setup to the mobile station 337, and the process is completed when a response indicating that the target base station 333 has been added onto the active set by the mobile station 337 is received by the radio network controller 331. Second, the hard handover procedure for converting the synchronous mode will be explained. The radio network controller 331 decides whether to start the mode change operation after adding the radio link of the target base station 333 (step 503). Here, the mode change operation means to convert the transmission signal of the mobile station 337 into the USTS mode or the NON-USTS mode, i.e., the synchronous mode of the source base station 335 or the target base station 333 is couverted thereof. For example, the base station is set by the USTS mode in the case of the synchronous mode, while the base station is set by the NON-USTS mode in the case of the non-synchronous mode. In the case of starting the mode change procedure, the hard handover procedure includes: (1) measuring procedure (step 500); (2) a mode change procedure for each base station (step 510); (3) a radio link activation procedure (step 520); and (4) a synchronization and reconfiguration procedure (step 530). First, referring to the measuring procedure (step 500), the radio network controller 331 decides whether to perform the hard handover with the synchronous timing of the target base station, as measuring and comparing the signal quality of the mobile station for the up and down links, and then obtains the synchronous timing of the target base station through a dedicated measurement procedure. Accordingly, the radio network controller 331 transmits a dedicated measurement initiation request message to the target base station 333 by using the NBAP protocol (step 505). Here, the dedicated measurement initiation request message includes a dedicated measurement type element (e.g., Round Trip Time (RTT) parameter) and report characteristics element (e.g., On Demand parameter). The On Demand parameter is used for performing and reporting the measurement as soon as the base station receives the dedicated measurement initiation request message, while the RTT parameter is used to represent the time during transmitting the downlink dedicated physical channel (DPCH) frame from the base station to the mobile station and receiving the uplink DPCH frame from the mobile station. Here, the measured value by the RTT parameter is for synchronizing the uplink in the event that the USTS is used later. The target base station 333 performs the RTT measurement by a parameter which is included in the dedicated measurement initiation request message, and then transmits the measurement results to the radio network controller 331 by using the NBAP protocol (step 507). Second, the mode change procedure (step 510) of each base station will now be explained. The mode change procedure is that the radio network controller 331 reconfigures the radio link between the target base station 333 and the mobile station 337 into the USTS mode by using a new timing and codes based on the timing information measured by the measuring procedure (step 500), and to reconfigure the radio link between the source base station 335 and the mobile station 337 into the NON-USTS mode. In the communication system applied to the USTS, operational modes are classified into (1) a normal mode used in the mobile station and the base station in which the USTS is not served, (2) the USTS mode controlling the transmission timing by the USTS due to serve the USTS and (3) the NON-USTS mode not controlling the transmission timing by the USTS, but serving the USTS. Accordingly, the present invention serves both the USTS and the NON-USTS modes, since the base station and mobile station serving with the USTS are used. That is, one base station is operated with the USTS mode and the other base station is operated with the NON-USTS in the case where at least one mobile station exists in a soft handover area overlapped by the cell areas of at least two base stations, and the two base stations serve the USTS. The reason is one mobile station cannot operate with two base stations in the USTS mode at the same time, and the uplink synchronous transmission timing is synchronized with only one base station. Therefore, the communication between the mobile station and the base station can be established in the USTS mode. Even if the USTS is used between the mobile station and the base station, in the NON-USTS mode the uplink synchronization could not be performed. First, the radio network controller 331 transmits a radio link reconfiguration prepare message to the target base station 333 by using the NBAP protocol (step 511). Here, the radio link reconfiguration prepare message includes a USTS indicator, a USTS scrambling code and a USTS channelisation code number. The target base station 333 reconfigures the radio link on the basis of the radio link reconfiguration prepare message, transmits a radio link reconfiguration ready message to the radio network controller 331 by using the NBAP protocol when the reconfiguration of the radio link is completed (step 513). The radio link reconfiguration ready message includes the USTS indicator. At this time, the synchronous mode of the radio link of the target base station is selected by the USTS mode. And, the radio network controller 331 transmits the radio link reconfiguration ready message to the source base station 335 by using the NBAP protocol (step 515). Here, the radio link reconfiguration ready message includes the USTS indicator, the USTS scrambling code, and a USTS channelisation code number. Then, the source base station 335 reconfigures the radio link on the basis of the radio link reconfiguration ready message, and transmits the radio link reconfiguration ready message to the radio network controller 331 by using the NBAP protocol when the reconfiguration of the radio link is completed (step 517). The source base station 335 has been communicating with the mobile station 337 by the USTS mode. But, the present USTS mode is converted into the NON-USTS mode by the radio link reconfiguration command. Second, the radio link activation procedure (step 520) will be explained. The radio network controller 331 transmits a radio link reconfiguration commit message to the target base station 333 by using the NBAP protocol, and the target base station 333 activates the radio link of the USTS mode on the basis of the radio link reconfiguration commit message (step 523). Also, the radio network controller 331 transmits the radio link reconfiguration commit message to the source base station 335 by using the NBAP protocol, the source base station 335 activates the radio link of the NON-USTS mode on the basis of the radio link reconfiguration commit message (step 525). If the radio link of each station is activated, the synchronization and reconfiguration process (step 530) will be performed. That is, the radio network controller 331 transmits a physical channel reconfiguration message to the mobile station 337 by using the RRC protocol (step 533), the mobile station 337 performs the mode change and actualizing modification by transferring the physical channel reconfiguration message to the physical channel (step 535). Here, the physical channel reconfiguration message includes the scramble code, the channelisation code number and a timing adjustment value. If the physical channel is set completely, the mobile station 337 transmits a physical channel reconfiguration complete message to the radio network controller 331 by using the RRC protocol (step 537). Accordingly, the present radio link can be reconfigured with the corresponding radio link at the target base station 333 in USTS mode as shown in step 535. Next, if the mobile station 337 moves out of the handover area, the radio link release procedure of releasing the radio link for the source base station 335 is performed (step 539). Here, the detailed description of step 539 will be omitted hereinafter since it is the same step as step 433 shown in Fig. 5. Fig. 8 is a time sequential chart illustrating the data flow procedure between protocol entities for each communication element for controlling a signal timing of a mobile station when different radio network controllers controlling at least two more base stations perform soft handover according to a preferred embodiment of the present invention. Referring to Fig.8, the mobile communication system includes a mobile station 349, the first base station (DRNS base station) 347 as a target base station, the second base station (SRNS base station) 345 as a source base station, a drift radio network controller (DRNC) 343, and a serving radio network controller (SRNC) 341. Fig. 8 also illustrates forwarding orders of control commands between the communication elements in case of serving the handover by using USTS mode. The data flow shown in Fig.8 can be classified into (1) a soft handover procedure for setting the radio link in the target base station, (2) a hard handover procedure for converting the synchronous mode, and (3) a soft handover procedure for releasing the radio link of the source base station same as Fig. 7. But, the data flow of Fig. 8 differs from the detailed procedure due to add the drifting radio network controller 343 and the different serving radio network controller base station 341. Hereinafter, the soft handover procedure (step 540) for setting the radio link in the target base station and the soft handover procedure (step 599) for releasing the radio !ink of the source base station same as Fig. 7 will be omitted to avoid the repetition of description. The hard handover procedure includes (1) measuring procedure (step 550), (2) a mode change procedure for each base station (step 560), (3) a radio link activation procedure (step 580), and (4) a synchronization and reconfiguration procedure (step 590). First, referring to the measuring procedure (step 550), the serving radio network controller 341 transmits a dedicated measurement initiation request message to the target base station 343 by using the RNSAP protocol to obtain a synchronous timing of the target base station 347 (step 553). Here, the dedicated measurement initiation request message includes a dedicated measurement type element (i.e., Round Trip Time (RTT) parameter) and report characteristics element (i.e., On Demand parameter). The drift radio network controller 343 transmits the dedicated measurement initiation request message tc the target base station 347 by using the NBAP protocol (step 555). And, the target base station 347 performs the measurement by the RTT parameter included in the dedicated measurement initiation request message, and then transmits the measurement results to the drift radio network controller 343 by using the NBAP protocol (step 557). The drift radio network controller 343 transmits the measurement results to the serving radio network controller 341 by using the RNSAP protocol (step 559) Second, the mode change procedure (step 510) of each base station will be explained. The serving radio network controller 341 transmits a radio link reconfiguration prepare message to the drift radio network controller 343 by using the RNSAP protocol (step 563). Here, the radio link reconfiguration prepare message includes a USTS indicator. The drift radio network controller 343 transmits a radio link reconfiguration ready message to the target base station 347 by using the NBAP protocol (step 565). Here, the radio link reconfiguration ready message includes the USTS indicator, the USTS scrambling code, and a USTS channelisation code number. The target base station 347 reconfigures the radio link on the basis of the radio link reconfiguration prepare message, and transmits the radio link reconfiguration ready message to the drift radio network controller 343 by using the NBAP protocol when the reconfiguration of the radio link is completed (step 567). The radio link reconfiguration ready message includes the USTS indicator. At this time, the target base station 347 is set up with the radio link of the USTS mode in step 567. While, the drift radio network controller 343 transmits the radio link reconfiguration ready message to the serving radio network controller 341 by using the RNSAP protocol (step 569). Here, the radio link reconfiguration ready message includes Tref, the USTS indicator, the USTS scrambling code, a USTS channelisation code number, and a USTS scramble code offset. Where, the Tref refers to the reference timing between the target base station and the serving radio network controller. Meanwhile, if the serving radio network controller 341 transmits the radio link reconfiguration prepare message to the source base station 345 by using the NBAP protocol (step 571), the source base station 345 converts the USTS mode into the NON-USTS mode on the basis of the radio link reconfiguration prepare message. Here, the radio link reconfiguration prepare message includes the Tref, the USTS indicator, the USTS scrambling code, and the USTS channelisation code number. The source base station 345 transmits a radio link reconfiguration ready message to the serving radio network controller 341 by using the NBAP protocol when the reconfiguration of the radio link is completed (step 573 step). Second, the radio link activation procedure (step 580) will be explained. If the serving radio network controller 341 transmits a radio link reconfiguration commit message to the drift radio network controller 343 by using the RNSAP protocol (step 583), and the drift radio network controller 343 transmits the radio link reconfiguration commit message to the target base station 347 by using the NBAP protocol (step 585). Then, the target base station 347 activates the radio link of the USTS mode on the basis of the radio link reconfiguration commit message. Also, if the serving radio network controller 341 transmits the radio link reconfiguration commit message to the source base station 345 by using the NBAP protocol (step 587), the source base station 345 activates the radio link of the NON-USTS mode by the radio link reconfiguration commit message. If the radio link of each station is activated by steps 583 and 585, the serving radio network controller 341 transmits a physical channel reconfiguration message to the mobile station 349 by using the RRC protocol, and the mobile station 349 reconfigures the present link with the radio link of the USTS synchronization in the target base station (step 593). That is, mobiie station 349 performs the mode change and timing adjustment by transferring the physical channel reconfiguration message including the scrambling code, and the channelisation code, and a correct transmission timing adjusting value and so on (step 595). If the mode change and timing adjustment is completed, the mobile station 349 transmits a physical channel reconfiguration complete message to the serving radio network controller 341 by using the RRC protocol (step 597). Next, if the mobile station 349 moves out of the handover area, the radio link release procedure releasing the radio link for the source base station 345 is performed (step 599). As described above, the present invention provides the method of controlling the soft handover in the uplink synchronous transmission scheme by serving the soft handover adding the new radio link on the basis of the pilot strength measured from the source base station and the target base station when the mobile station is entered into the handover area, and serving the hard handover for converting each radio link mode for the source base station and the target base station, and then releasing the radio link of the source base station. Also, the detailed data flow procedure between the protocol entities for each communication element can be provided when the hard handover for converting the mode of the uplink synchronous transmission scheme is converted in case of serving the handover. According to the method of controlling the handover of the present invention, the handover combined with the soft handover for NON-USTS link addition and/or deletion and the hard handover for controlling uplink synchronization timing is performed without adding separate elements onto the related mobile communication system. According to the method of controlling the handover of the present invention, the complicated communication elements for controlling the signal transmission timing using the uplink synchronous transmission scheme do not exist additionally, and the synchronization timing for serving the handover can be controlled by the control parameters. According to the method of controlling the handover of the present invention, the performance or the receptible capacity and the cell coverage area can be increased by serving the hard handover with the uplink synchronous transmission scheme. According to the method of controlling the handover of the present invention, the handover of the mobile station can be performed efficiently by defining the data flow between the protocol entities for each communication element. While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A method of controlling a handover of a communication link using an uplink synchronous transmission scheme in a mobile communication system in which a first and a second base stations (333, 335) controlled by a radio network controller (331) transmit and receive communication data of an identical signal to and from a mobile station (337) concurrently, the method comprising the steps of: transmitting an information for the uplink synchronous transmission from the radio network controller (331) to the first and the second base stations (333, 335) (step 510); resetting up a radio link between at least one of said base stations (333, 335) and said mobile station (337) based on the information for the uplink synchronous transmission (step 520): and adjusting a base time for uplink synchronous transmission of said communication data synchronized to a base station (335) to another base station (333) (step 530). The method of claim 1, further comprising the step of transmitting a measurement control command for measuring the uplink synchronous timing (step 500). The method of claim 2, wherein the measurement control command includes a measurement type and a report characteristic. The method of claim 1, wherein the radio link is reset by converting a mode of the uplink synchronous transmission scheme (steps 523, 525). The method of claim 1. further comprising the steps of: transmitting the information on the uplink synchronous transmission from a radio network controller (341), if the base stations (345, 347) are controlled by different radio network controllers (341, 343) (step 563); resetting up the radio link based on the information for the uplink synchronous transmission transmitted from the radio network controller (341) or the other radio network controller (343) (steps 585, 587); and adjusting an uplink synchronous timing of the mobile station (349) which is controlled by the base station (345, 347) under the control of the radio network controller (341, 343) on the basis of the reset radio link with the uplink synchronous timing of the base station (345, 347) controlled by the other radio network controller (341, 343). The method of any one of the claims 1 to 5, wherein the information for the uplink synchronous transmission is an indicator of the uplink synchronous transmission scheme. The method of any one of claims 1 to 5, wherein the information for the uplink synchronous transmission is a scramble code of the uplink synchronous transmission scheme. The method of any one of claims 1 to 5, wherein the information for the uplink synchronous transmission is a channelizing code number of the uplink synchronous transmission scheme. A method of changing a base time for uplink synchronous transmission scheme in a mobile station of a mobile communication system in which a first base station (335) and a second base station (333) both receive communication data of an identical signal from a mobile station (337) simultaneously, the method comprising the steps of: transmitting communication data to the first base station (335); checking pilot signals from the second base station (333) (step 453); transmitting communication data to the second base station (333); checking pilot signals from the first and second base stations (335, 333) (step 469); and changing the base time for uplink synchronous transmission scheme to the base time of the second base station (333) (step 463). The method of claim 9, wherein the step of transmitting the communication data to the second base station (333) is performed when the intensity of the pilot signals from the second base station (333) exceeds a threshold value (steps 455, 457, 459). The method of claim 9, wherein the step of changing the base time for uplink synchronous transmission scheme is done in accordance with the quality of the radio link between the base station (333) and the mobile station (337) (steps 461, 463, 465). The method of claim 11, wherein the radio link is the link between the second base station (333) and the mobile station (337). The method of any one of the claims 9 to 12, wherein the step of changing the base time for the uplink synchronous transmission scheme is conducted in accordance with the number of mobile stations linked to the base station (333, 335) (steps 461, 463, 465). The method of claim 13, wherein the step of changing the base time for uplink synchronous scheme is conducted after comparing the number of mobile stations linked to the second base station (333) with the number of the mobile stations linked to the first base station (335).

Inventors: HWANG SEUNG HOON, KIM EUN JUNG, LEE SO YOUNG, PARK JIN YOUNG, HWANG, SEUNG HOON, KIM, EUN JUNG, LEE, SO YOUNG, PARK, JIN YOUNG
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 2001
Main classification: H04Q7
IPC codes: H04W36