Handover execution and communication resumption in wireless access system

Handover execution and communication resumption in a wireless access system is provided. Performing a handover includes communicating with a serving base station through a first communication link and receiving from the serving base station, handover information associated with at least one candidate target base station. The method also includes transmitting handover indicator to the serving base station indicating a handover operation to a selected target base station and performing a ranging procedure with the selected target base station to establish a second communication link with the selected target base station while maintaining the first communication link with the serving base station. The method also includes being released from the serving base station in response to a handover complete status message from the selected target base station to the serving base station, or resuming normal communication with the serving base station if the handover fails with the selected target base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Application No. 10-2004-0042642, filed on Jun. 10, 2004, the contents of which are hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to a wireless access system, and more particularly to handover execution in a wireless access system.

BACKGROUND OF THE INVENTION

The IEEE802.16e system is part of a global standardization for broadband wireless access systems. IEEE802.16e does not have a hierarchical structure of HLR, VLR, MSC, BSC, RNC, etc. unlike second and third generation mobile communications systems, but does include a mobile subscriber station (MSS), a base station (BS), and an authentication service authorization server (ASA server). A physical (PHY) layer and a medium access control (MAC) layer are defined between the BS and the MSS.

A handover process in a broadband wireless access system according to a related art is explained as follows.

Pre-processing for handover includes broadcasting associated information from a base station to a neighboring base station to inform all mobile subscriber stations within a respective cell of the associated information (Network Topology Advertisement). The pre-processing also includes measuring a channel quality of the neighboring base station based on the associated information (MSS Scanning of Neighboring BS). The pre-processing also includes matching parameters between the base station and the neighboring base station, such as for example, an initial mobile subscriber station power, a time difference for synchronization, and other parameters (Association Procedures).

a. Network Topology Advertisement

In network topology advertisement, the base station broadcasts information associated with network configuration to all mobile subscriber stations within a cell via a MOB_NBR-ADV MAC message, thereby providing all mobile subscriber stations within the cell with the information pertaining to neighboring base stations.

b. MSS Scanning of Neighboring BS

In MSS scanning of neighboring BS, a mobile subscriber station (MSS) scans a neighboring base station (BS) and requests a scanning interval for scanning the neighboring BS from the BS via a MOB_SCN-REQ MAC message. The BS then transmits a MOB_SCN-RSP MAC message in response to the request to assign an interval for scanning (scanning interval) the neighboring BS. Alternatively, the BS may directly transmit the MOB_SCN_RSP MAC message without a request from the MSS (unsolicited response). In the case of an unsolicited response, the scanning interval and an offset unit for scanning start are assigned on a frame unit basis.

c. Association Procedures

During association procedures, the MSS performs a ranging process with a BS to normally join a cell. In the ranging process, the MSS acquires basic information for communications from the BS. In other words, the MSS performs BS scanning to select a new BS and then performs the association procedures. The MSS and the BS transmit a RNG_REQ MAC message and a RNG_RSP MAC message, respectively, to set values such as for example, a power offset value, a timing offset value, and other values of the MSS to suitable values. Transmission of the RNG_REQ MAC message is called an initial ranging operation and is a basic operation that the MSS may perform in a network entry procedure. When a handover is performed, a target BS accepts a new MSS and transmits items associated with the respective cell of the new MSS to the former serving BS and stores information pertaining to the MSS.

In a handover (HO) process, the MSS initiates handover with the neighboring BS based on the channel quality information acquired in the handover pre-processing.

a. Cell Selection

In a cell selection procedure, a cell is changed to enable the MSS to make a new registration to a BS, allowing for reception of a signal having an SINR (signal to interference noise ratio) superior to that of a signal transmitted from the BS of the current cell, prior to a normal registration of the MSS to a cell. In such case, since the MSS does not perform the registration procedure, the BS is unable to determine a moving situation of the MSS.

In HO initiation, a handover may be initiated by the MSS or the BS. When the BS requests (initiates) a handover, the BS transmits a MOB_BSHO-REQ MAC message to the MSS. On the contrary, when the MSS requests a handover, the MSS transmits a MOB_MSSHO-REQ MAC message to the BS.

If the MSS transmits the MOB_MSSHO-REQ MAC message, the MSS preferentially transmits the SINR of a signal received from a neighboring BS to the BS to enable the MOB_MSSHO-REQ MAC message to be transmitted to a current serving BS. The current serving BS then selects a candidate BS as a target BS for performing the handover.

In a case where the BS receives the MOB_MSSHO-REQ MAC message from the MSS or in a case when the BS has not yet transmitted the MOB_BSHO-REQ MAC message to handover the MSS, the BS may allow the handover of the MSS after confirmation of an acknowledgement (ACK) for handing over a specific MSS from the neighboring BS. After receiving the MOB_BSHO/MSSHO-REQ MAC message, the MSS/BS transmits a MOB_MSSHO/BSHO-RSP MAC message to inform the BS/MSS of a target BS for the handover.

In HO cancellation, after the MSS/BS transmits the MOB_MSSHO/BSHO-REQ MAC message, the MSS may cancel the handover. In canceling the HO, the MSS sets a specific field (HO_Type=01) of a MOB_HO-IND MAC message and transmits the set field to the BS. The BS receives the message and cancels the proceeding handover.

d. Termination with the Serving BS

In termination with the serving BS, the MSS transmits the MOB_HO-IND MAC message to inform the serving BS that the handover is normally completed and then completes the handover operation. In completing the handover operation, the MSS sets a specific field (HO_Type=00) of the MOB_HO-IND MAC message to transmit the information pertaining to the normal handover termination to the serving BS. The serving BS receives the MOB_HO-IND MAC message from the MSS and terminates a MAC state machine allocated to the MSS. The serving BS also terminates an ARQ (automatic repeat request) connection and all connections associated with data transmission.

In HO rejection, the MSS is capable of rejecting a handover recommended by the BS. In rejecting a handover, the MSS sets a specific field (HO_Type=10) of the MOB_HO-IND MAC message and transmits the message to the BS. The BS receives the rejection message from the MSS, re-seeks for a target BS and re-transmits the MOB_BSHO-RSP MAC message to the MSS.

FIG. 1is a diagram illustrating a handover process requested by a mobile subscriber station (MSS).

Referring toFIG. 1, an MSS II performs a scanning procedure to measure qualities of signals transmitted from neighboring BSs (S110). If the MSS II determines that a handover needs to be performed based on results of the neighboring BS scanning procedure, the MSS11transmits a handover request message (MOB-MSS-HO-REQ) to a serving BS12(S111).

The serving BS12receives the handover request message and transmits a handover pre-notification message (HO-pre-notification) including information that the MSS11has requested a handover to a neighboring BS, such as for example, handover target BSs (target BSs)13and14(S112, S113). The handover pre-notification message includes an identifier (MSS identifier) of the MSS11requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs13and14receives the handover pre-notification message and transmits a handover pre-notification response message (HO-pre-notification-response) to the serving BS12in response to the handover pre-notification message (S114, S115). The handover pre-notification response message may include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.

The serving BS12selects one of the neighboring handover target BSs13and14according to the response messages for the handover pre-notification and transmits a handover confirm message (HO-confirm) to the selected target BS13or14(S116). The serving BS12then transmits a handover response message (MOB-BSHO-RSP), including information pertaining to the selected handover target BS, to the MSS11(S117). The MSS11receives the handover response message and transmits a handover indicator message (MOB-HO-IND) to release a connection from the MSS11to the serving BS12(S118). Upon receipt of the handover indicator message from the MSS11, the serving BS12releases its connection to the MSS11(S119).

The MSS11then executes a fast ranging (Fast Ranging_IE), a ranging request (RNG-REQ), and a ranging response (RNG-RSP) with the target BS14(S120, S121, S122). The target BS14becomes the serving BS14and the MSS11completes an initial network entry execution after handover (S123).

FIG. 2is a diagram illustrating a handover process requested by a base station (BS).

Referring toFIG. 2, a serving BS22triggers a procedure (trigger-to-release-MSS) to release a connection from an MSS21according to a corresponding decision (S210).

The serving BS22then transmits a handover pre-notification message (HO-pre-notification) including information required for the handover of the MSS21to neighboring handover target BSs23and24(S211, S212). The handover pre-notification message includes an identifier (MSS identifier) of the MSS21requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs23and24receive the handover pre-notification message and transmit a handover pre-notification response message (HO-pre-notification-response) to the serving BS22in response to the handover pre-notification message (S213, S214). The handover pre-notification response message may include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.

The serving BS22then transmits a handover request message to the MSS21based on the information received via the handover pre-notification response message (S215). The handover request message includes information pertaining to the target BSs23and24available for the handover. The MSS21then performs a neighboring BS scanning procedure to measure quality of signals received by the neighboring target BSs, as recognized via a handover response message (S216). In a case where the target BS24is found for the handover is found as a result of the neighboring BS scanning, the MSS21transmits a handover response message including target BS information for the handover to the serving BS22(S217). The serving BS22then transmits a handover confirm message to the target BS24to which the MSS21intends to perform the handover (S218).

The MSS21then transmits a handover indicator message (MOB-HO-IND) for releasing a connection from the MSS21to the serving BS22(S219). Upon receipt of the handover indicator message, the serving BS22releases its connection to the MSS21(S220).

The MSS21then executes a fast ranging (Fast Ranging_IE), a ranging request (RNG-REQ), and a ranging response (RNG-RSP) (S221, S222, S223) with the serving target BS24for the handover and then completes an initial network entry execution after handover (S224).

FIG. 3is a diagram illustrating a process by which an MSS re-enters a former serving BS, such as in the case of a call drop.

Referring toFIG. 3, an MSS31performs a scanning procedure to measure qualities of signals transmitted from neighboring BSs (S310). If the MSS determines that a handover needs to be performed based on results of the neighboring BS scanning procedure, the MSS31transmits a handover request message (MOB-MSS-HO-REQ) to a serving BS32(S311).

The serving BS32receives the handover request message and transmits a handover pre-notification message (HO-pre-notification) including information that the MSS31has requested a handover to a neighboring BS, such as for example, handover target BSs33and34(S312, S313). The handover pre-notification message includes an identifier (MSS identifier) of the MSS requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs33and34receive the handover pre-notification message and transmit a handover pre-notification response message (HO-pre-notification-response) to the serving BS32in response to the handover pre-notification message (S314, S315). The handover pre-notification response message may include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.

The serving BS32selects one of the neighboring handover target BSs33and34according to the response messages for the handover pre-notification and transmits a handover confirm message (HO-confirm) to the selected target BS33or34(S316). The serving BS32then transmits a handover response message (MOB-BSHO-RSP) including information pertaining to the target BS for the handover to the MSS31(S317). The MSS31receives the handover response message from the serving BS32and transmits a handover indicator message (MOB-HO-IND) to release a connection from the MSS31to the serving BS32(S318). Upon receipt of the handover indicator message, the serving BS32releases its connection to the MSS31(S319).

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to handover execution and communication resumption in a wireless access system that substantially obviates one or more problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a method of efficiently resuming communications in a wireless access system when a mobile subscriber station (MSS) returns to a former serving base station (BS) in case of a handover failure.

To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, in one embodiment, a method of performing handover operation in a wireless communication system includes communicating with a serving base station through a first communication link and receiving from the serving base station, handover information associated with at least one candidate target base station, wherein the handover information comprises at least quality of service capable of being handled by the at least one candidate target base station. The method also includes transmitting a handover indicator to the serving base station indicating a handover operation to a selected target base station, wherein the mobile subscriber station determines whether to perform handover in response to the handover information, and performing a ranging procedure with the selected target base station to establish a second communication link with the selected target base station while maintaining the first communication link with the serving base station, wherein the ranging procedure comprises attempting to establish an uplink synchronization and power adjustment between the mobile subscriber station and the selected target base station. The method also includes being released from the serving base station in response to a handover complete status message from the selected target base station to the serving base station, or resuming normal communication with the serving base station if the handover fails with the selected target base station.

The method may also preferably include receiving a handover response from a serving base station upon the serving base station transmitting a handover confirmation to a handover target base station, wherein the handover response comprises at least quality of service information requested to be handled by the handover target base station. The method may also preferably include, in a mobile subscriber station initiated handover, transmitting a handover request to a serving base station through a first communication link prior to receiving the handover response from the serving base station. The method may also preferably include, in a service base station initiated handover, receiving a handover request from the serving base station prior to transmitting a handover response to the serving base station.

The mobile subscriber station may preferably be released from the serving base station when the handover complete status message indicates successful handover. The mobile subscriber station may preferably resume normal communication with the serving base station when the handover complete status message indicates handover failure.

The step of performing the ranging procedure may preferably include receiving a frame slot allocation from the target base station for a fast ranging, transmitting a ranging request to the target base station using the allotted frame slot, and receiving a ranging response from the target base station.

The handover complete status message from the target base station to the serving base station may preferably be an inter-base station backbone message. The handover indicator may preferably include one of (1) serving base station release, (2) handover cancel, and (3) handover rejection.

In another embodiment, a method of performing handover operation in a wireless communication system includes receiving a handover request from a mobile subscriber station through a first communication link, transmitting a handover pre-notification to a target base station, and transmitting a handover response upon the serving base station receiving a handover confirmation from the target base station. The method also includes receiving a handover indicator from the mobile subscriber station indicating a handover operation to the target base station and performing a setup procedure between the mobile subscriber station and the target base station to establish a second communication link with the target base station while maintaining the first communication link with the serving base station. The method also includes releasing the mobile subscriber station or continuing normal communication with the mobile subscriber station in response to a handover complete status message from the target base station to the serving base station.

In yet another embodiment, a method of performing handover operation in a wireless communication system comprises transmitting a handover pre-notification to a target base station. The method also comprises transmitting a handover response upon the serving base station receiving a handover confirmation from the target base station and transmitting a handover request to a mobile subscriber station that is in communication with the serving base station through a first communication link. The method also comprises receiving a handover response from the mobile subscriber station indicating a handover operation to the target base station. The method also comprises performing a setup procedure between the mobile subscriber station and the target base station to establish a second communication link with the target base station while maintaining the first communication link with the serving base station. The method also comprises releasing the mobile subscriber station or continuing normal communication with the mobile subscriber station in response to a handover complete status message from the target base station to the serving base station.

The handover request may preferably comprise a possible handover target base station. The method may also preferably comprise receiving a handover complete message from the target base station to the serving base station.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4is a diagram illustrating a handover process initiated by a mobile subscriber station (MSS), according to one embodiment of the present invention.

Referring toFIG. 4, an MSS41performs a scanning procedure to measure qualities of signals transmitted from neighboring base stations (BSs) (S410). If the MSS41determines that a handover needs to be performed based on results of the neighboring BS scanning procedure, the MSS41may transmit a handover request message (e.g., MOB-MSS-HO-REQ) to a serving BS42(S411).

The serving BS42receives the handover request message and may transmit a handover pre-notification message (e.g., HO-pre-notification) to a neighboring BS, such as for example, handover target BSs (target BSs)43and44(S412, S413). The handover pre-notification message may preferably include information that the MSS41has requested a handover, as well as an identifier (MSS identifier) of the MSS requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs43and44may receive the handover pre-notification message and may transmit a handover pre-notification response message (e.g., HO-pre-notification-response) to the serving BS42in response to the handover pre-notification message (S414, S415). The handover pre-notification response message may preferably include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.

The serving BS42may select one of the neighboring handover target BSs43and44based on the handover pre-notification response message and may transmit a handover confirm message (e.g., HO-confirm) to the selected target BS43or44(S416). The serving BS42may transmit a handover response message (e.g., MOB-BSHO-RSP), including information pertaining to the possible handover target BS, to the MSS41(S417). The MSS41receives the handover response message and may transmit a handover indicator message (e.g., MOB-HO-IND) to release a connection from the MSS41to the serving BS42(S418).

An exemplary configuration of the handover indicator message is shown in Table 1.

The handover indicator message (MOB_HO_IND) may include information to notify the former serving BS42to resume communication with the MSS41if the handover of the MSS41to the handover target BS fails.

Before the connection to the MSS41to the serving BS42is released, the MSS41may preferably execute a fast ranging (e.g., Fast Ranging_IE), a ranging request (e.g., RNG-REQ), and a ranging response (e.g., RNG-RSP) with the target BS selected for the handover (S419, S420, S421). In the embodiment, the target BS44is selected for the handover. The ranging request may preferably require uplink synchronization between the MSS41and the target BS44. The fast ranging may preferably comprise a frame slot allocation by the target base station44to enable the MSS41to use the allotted frame slot for performance of fast ranging.

In one embodiment, if the handover is successfully performed, the handover target BS44may transmit a handover result to the serving BS42via a handover complete message (e.g., HO-Complete). The former serving BS42may preferably recognize the successful completion of the handover via the handover complete message and may release the serving BS's42connection to the MSS41(S423). Execution of initial network entry may then be completed (S424).

An exemplary configuration of the handover complete message is shown in Table 2.

TABLE 2FieldSizeNotesGlobal Header152bitsMSS unique48bits48-bit unique identifier used by MSSidentifieron initial network entryReason8bits#0: Successful Handover#1: Handover Failure due to xxx#2: Handover failure due to yyy#3~7: reservedSecurity fieldTBDA means to authenticate this message

The handover complete message may preferably be an inter-base station backbone message to provide notification of a handover state of the MSS41from a new serving BS to the former serving BS42. Using the handover complete message, the former serving BS42may determine whether the MSS41has completed the handover to the handover target BS44. In a case where there is a handover failure, the reason for the handover failure (failure reason) may preferably be set to be delivered to the former serving BS42. The failure reason may be, for example, a call drop that causes the MSS41to fail in accessing the handover target BS44.

In the event that the handover process is completed, the former serving BS42may maintain resources and information associated with the MSS41for a predetermined time duration (where there is no instruction from the new serving BS44).

FIG. 5is a diagram illustrating a handover process initiated by a BS, according to one embodiment of the present invention.

Referring toFIG. 5, a serving BS52triggers a procedure (e.g., trigger-to-release-MSS) to release a connection from an MSS51according to a corresponding determination (S510).

The serving BS52may transmit a handover pre-notification message (e.g., HO-pre-notification) including information required for the handover of the MSS51to neighboring handover target BSs53and54(S511, S512). The handover pre-notification message may include an identifier (MSS identifier) of the MSS requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs53and54may receive the handover pre-notification message and may transmit a handover pre-notification response message (e.g., HO-pre-notification-response) to the serving BS52in response to the handover pre-notification message (S513, S514). The handover pre-notification response message may preferably include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.

The serving BS52may preferably transmit a handover response message to the MSS51based on the information received via the handover pre-notification response message (S515). The handover response message may preferably include information pertaining to the target BSs53and54available for the handover. The MSS51may preferably perform a neighboring BS scanning procedure to measure quality of received signals on the neighboring target BSs, recognized via the handover response message (S516).

In another embodiment, in a case where the neighboring BS scanning procedure finds the target BS54for the handover, the MSS51may transmit a handover response message including target BS information for the handover to the serving BS52(S517). The serving BS52also may transmit a handover confirm message to the target BS54, with which the MSS51determines to perform the handover (S518).

The MSS51may transmit a handover indicator message (e.g., MOB-HO-IND) to release a connection from the MSS51to the serving BS52(S519). The handover indicator message may preferably be configured as shown in Table 1.

Before the connection of the MSS51to the serving BS52is released, the MSS51may preferably execute a fast ranging (e.g., Fast Ranging_IE), a ranging request (e.g., RNG-REQ), and a ranging response (e.g., RNG-RSP) with the serving target BS54for the handover (S520, S521, S522).

In yet another embodiment, if the handover is successfully performed, the handover target BS54may transmit a handover result to the serving BS52via a handover complete message (e.g., HO-Complete). The handover complete message may preferably be configured as shown in Table 2. The former serving BS52may preferably recognize successful completion of the handover via the handover complete message and may release its connection to the MSS51(S524). Execution of initial network entry may then be completed (S525).

FIG. 6is a diagram illustrating a process by which an MSS resumes communications with a former serving BS, in case of a handover failure, according to one embodiment the present invention. The handover failure may be due to, for example, a call drop or other failure reason.

Referring toFIG. 6, an MSS61performs a scanning procedure to measure qualities of signals transmitted from neighboring BSs (S10). If the MSS61determines that a handover needs to be performed based on results of the neighboring BS scanning procedure, the MSS61may preferably transmit a handover request message (e.g., MOB-MSS-HO-REQ) to a serving BS62(S611).

The serving BS62receives the handover request message and may transmit a handover pre-notification message (e.g., HO-pre-notification) including information that the MSS61has requested a handover to a neighboring BS, such as for example, handover target BSs63and64(S612, S613). The handover pre-notification message may also include an identifier (MSS identifier) of the MSS requesting the handover, connection parameters, capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs63and64may receive the handover pre-notification message and may transmit a handover pre-notification response message (e.g., HO-pre-notification-response) to the serving BS62in response to the handover pre-notification message (S614, S615). The handover pre-notification response message may preferably include acknowledgement information pertaining to the handover pre-notification message and capability information pertaining to the requested quality of service.

The serving BS62may preferably select one of the neighboring handover target BSs63and64based on the handover pre-notification response message and may transmit a handover confirm message (e.g., HO-confirm) to the selected target BS63or64(S616). The serving BS62may also transmit a handover response message (e.g., MOB-BSHO-RSP), including information pertaining to the handover target BS, to the MSS61(S617). The MSS61receives the handover response message and may transmit a handover indicator message (e.g., MOB-HO-IND) to release a connection from the MSS61to the serving BS62(S618).

In still another embodiment, when the MSS61attempts to connect to the handover target BS64, a handover failure, such as for example a call drop, occurs (S619). The MSS61may preferably recognize the handover failure and may notify the handover target BS64. The MSS61may also cancel the handover from the former serving BS62and may transmit a message to resume communications with the former serving BS62. Because the connection between the serving BS62and the MSS61was not yet released, the MSS61may maintain the connection with the serving BS62without executing a procedure for re-entering the serving BS62. Thus, although the handover attempt failed, the MSS61may maintain the connection with the serving BS62without executing a procedure for re-entering the serving BS62. In order to resume communications with the former serving BS62, the exemplary handover indicator message shown in Table 1 may preferably be used.

In one embodiment, a method of performing handover operation in a wireless communication system includes communicating with a serving base station through a first communication link and receiving from the serving base station, handover information associated with at least one candidate target base station, wherein the handover information comprises at least quality of service capable of being handled by the at least one candidate target base station. The method also includes transmitting a handover indicator to the serving base station indicating a handover operation to a selected target base station, wherein the mobile subscriber station determines whether to perform handover in response to the handover information, and performing a ranging procedure with the selected target base station to establish a second communication link with the selected target base station while maintaining the first communication link with the serving base station, wherein the ranging procedure comprises attempting to establish an uplink synchronization and power adjustment between the mobile subscriber station and the selected target base station. The method also includes being released from the serving base station in response to a handover complete status message from the selected target base station to the serving base station, or resuming normal communication with the serving base station if the handover fails with the selected target base station.

The method may also preferably include receiving a handover response from a serving base station upon the serving base station transmitting a handover confirmation to a handover target base station, wherein the handover response comprises at least quality of service information requested to be handled by the handover target base station. The method may also preferably include, in a mobile subscriber station initiated handover, transmitting a handover request to a serving base station through a first communication link prior to receiving the handover response from the serving base station. The method may also preferably include, in a service base station initiated handover, receiving a handover request from the serving base station prior to transmitting a handover response to the serving base station.

The mobile subscriber station may preferably be released from the serving base station when the handover complete status message indicates successful handover. The mobile subscriber station may preferably resume normal communication with the serving base station when the handover complete status message indicates handover failure.

The step of performing the ranging procedure may preferably include receiving a frame slot allocation from the target base station for a fast ranging, transmitting a ranging request to the target base station using the allotted frame slot, and receiving a ranging response from the target base station.

The handover complete status message from the target base station to the serving base station may preferably be an inter-base station backbone message. The handover indicator may preferably include one of (1) serving base station release, (2) handover cancel, and (3) handover rejection.

In another embodiment, a method of performing handover operation in a wireless communication system includes receiving a handover request from a mobile subscriber station through a first communication link, transmitting a handover pre-notification to a target base station, and transmitting a handover response upon the serving base station receiving a handover confirmation from the target base station. The method also includes receiving a handover indicator from the mobile subscriber station indicating a handover operation to the target base station and performing a setup procedure between the mobile subscriber station and the target base station to establish a second communication link with the target base station while maintaining the first communication link with the serving base station. The method also includes releasing the mobile subscriber station or continuing normal communication with the mobile subscriber station in response to a handover complete status message from the target base station to the serving base station.

In yet another embodiment, a method of performing handover operation in a wireless communication system comprises transmitting a handover pre-notification to a target base station. The method also comprises transmitting a handover response upon the serving base station receiving a handover confirmation from the target base station and transmitting a handover request to a mobile subscriber station that is in communication with the serving base station through a first communication link. The method also comprises receiving a handover response from the mobile subscriber station indicating a handover operation to the target base station. The method also comprises performing a setup procedure between the mobile subscriber station and the target base station to establish a second communication link with the target base station while maintaining the first communication link with the serving base station. The method also comprises releasing the mobile subscriber station or continuing normal communication with the mobile subscriber station in response to a handover complete status message from the target base station to the serving base station.

The handover request may preferably comprise a possible handover target base station. The method may also preferably comprise receiving a handover complete message from the target base station to the serving base station.

Accordingly, the present invention may efficiently execute a handover and/or resume communications in a wireless access system when a mobile subscriber station (MSS) returns to a former serving base station (BS), in a case of a failed handover.