# Patent

## Title
Handover method in a wireless communication system

## Abstract

A handover method in a wireless communication system is disclosed. A serving base station transmits a handover request message to a base station controller in response to a handover request of a mobile station. The base station controller determines target base stations to which the mobile station can perform handover among target base stations in a list of neighbor base stations, and transmits a handover response message including the determination result information to the mobile station via the serving base station. The mobile station determines a target base station to which it will perform handover, and transmits a message indicating that it will perform handover, to the base station controller via the serving base station. The base station controller transmits a confirm message indicating that the mobile station will perform handover, to the determined target base station, and establishes a tunnel to the target base station. The mobile station registers itself in the target base station.

## Background
PRIORITY
This application claims the benefit under 35 U.S.C. § 119(a) of an application filed in the Korean Intellectual Property Office on Jan. 31, 2005 and assigned Ser. No. 2005-8845, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a system and method for providing optimized handover in a wireless communication system.
2. Description of the Related Art
Technologies generally used to provide data service to users in the current wireless communication environment are classified into the 2.5th Generation or 3rd Generation cellular mobile communication technology such as Code Division Multiple Access 2000 1x Evolution Data Optimized (CDMA2000 1xEVDO), General Packet Radio Services (GPRS) and Universal Mobile Telecommunication Service (UMTS), and the wireless Local Area Network (LAN) technology such as an Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless LAN.
The local wireless access technologies have been proposed to provide high-speed data service in a wireless environment, replacing wire communication networks such as the cable modem or xDSL (Digital Subscriber Line) in hot spot areas such as public places and schools or in a home network environment.
However, when high-speed data service is provided using the wireless LAN, there are limitations in providing public network service to users due to interference as well as due to limited mobility and narrow service area.
However, the high-speed data service system, for example, IEEE 802.16, does not have a proposed handover method. Accordingly, there is a need for a handover scheme efficient in the high-speed data service system to provide improved services to users.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an efficient handover system and method in a BWA communication system.
It is another object of the present invention to provide an optimized handover system and method capable of efficiently supporting real-time services being susceptible to a delay in a wireless communication system.
It is further another object of the present invention to provide a fast handover system and method for supporting real-time services in a wireless communication system.
According to one aspect of the present invention, there is provided a handover method in a wireless communication system. The handover method includes transmitting, by a serving base station, a handover request message to a base station controller in response to a handover request of a mobile station; determining, by the base station controller, target base stations to which the mobile station can perform handover among target base stations in a list of neighbor base stations, and transmitting a handover response message including the determination result information to the mobile station via the serving base station; determining, by the mobile station, a target base station to which it will perform handover, and transmitting a message indicating that it will perform handover, to the base station controller via the serving base station; transmitting, by the base station controller, a confirm message indicating that the mobile station will perform handover, to the determined target base station, and establishing a tunnel to the target base station; and registering, by the mobile station, itself in the target base station.
According to another aspect of the present invention, there is provided a handover method in a wireless communication system. The handover method includes upon receiving an active set request message, exchanging with target base stations, by a base station controller, information used for determining each of the target base stations whether it can accept handover of a mobile station thereto and connection information for the target base stations; transmitting, by the base station controller, an active set response message including identifiers (IDs) of base stations to which the mobile station can perform handover, to a serving base station; upon receiving a handover indication message from the mobile station, transmitting, by the serving base station, an active set indication message to the base station controller; after exchanging an active set add/drop message with the target base stations, establishing, by the base station controller, a tunnel between the base station controller and the target base station; upon receiving an anchor base station report over a channel quality information channel (CQICH), transmitting by the serving base station an anchor switching indication message to the base station controller; and upon receiving an anchor switch confirm message from the base station controller, performing communication by the target base station without a ranging procedure with the mobile station.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other 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 diagram schematically illustrating a configuration of a wireless communication system according to an embodiment of the present invention;
FIG. 2 is a diagram schematically illustrating an exemplary interface structure in a wireless communication system according to an embodiment of the present invention;
FIG. 3 is a signaling diagram illustrating a hard handover procedure in a wireless communication system according to an embodiment of the present invention; and
FIG. 4 is a signaling diagram illustrating an FBSS handover procedure in a wireless communication system according to an embodiment of the present invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness.
The present invention relates generally to a Broadband Wireless Access communication system. In particular, the present invention provides a handover scheme in the BWA communication system, for example, an IEEE802.16 system.
The present invention provides an optimized handover scheme by reducing a handover time through tunneling between a base station (BS), for example, a radio access station (RAS), and a base station controller (BSC), for example, an access control router (ACR), in performing handover in a wireless communication system.
With reference to the schematic diagram of FIG. 1, a description will now be made of an exemplary configuration of a wireless communication system to which the present invention is applicable.
Referring to FIG. 1, a wireless communication system, for example, a WiBro system, includes an MS 100, a plurality of RASs 120 and 170 for performing wireless communication with the MS 100, and a plurality of ACRs 110 and 160 for controlling functions of the RASs 120 and 170.
The ACRs 110 and 160, which are systems interposed between a core network (CN) and the RASs 120 and 170, perform a Convergence Sublayer (CS) function, an Automatic Repeat reQuest (ARQ) processing function, a handover control function, etc. In addition, the ACRs 110 and 160 provide an interface with the CN.
The RASs 120 and 170, which are systems interposed between the ACRs 110 and 160 and the MS 100, provide a wireless access interface based on the wireless access standard, for example, the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard.
The MS 100, which is an end point of a wireless channel, accesses the RASs 120 and 170 to perform communication with them according to the wireless access standard.
As illustrated in FIG. 1, interfacing between the ACRs 110 and 160 is achieved through interface signaling according to the present invention, for example, interface signaling, and interfacing between the ACR 110 and the RAS 120 is achieved through another interface signaling according to the present invention, for example, interface signaling and traffic.
FIG. 2 is a diagram schematically illustrating an exemplary interface structure in a wireless communication system according to the present invention.
Referring to FIG. 2, the interface defines a signaling and traffic processing scheme between a RAS 210 and an ACR 220 in a wireless communication system.
A signaling plane 230, for example, an interface signaling plane, defines a session control message necessary between the RAS 210 and the ACR 220, and manages a traffic path, i.e., a Generic Routing Encapsulation (GRE) tunnel, using the session control message. Herein, the session control message corresponds to a Medium Access Control (MAC) Management message.
An operation in the signaling plane will now be described. Upon receiving a Ranging Request (RNG-REQ) message with an Initial Ranging Connection ID (CID), the RAS 210 transmits an interface signaling message using a default Internet Protocol (IP) address/port number of the ACR 220.
Upon receipt of the signaling message, the ACR 220 allocates an IP address/port number for each individual Basic Management CID and Primary Management CID to respond to the signaling message from the RAS 210.
Thereafter, upon receiving a MAC Management message with the Basic Management CID and Primary Management CID, the RAS 210 transports the signaling message using an IP address and User Datagram Protocol (UDP) port number for each individual CID of the ACR 220, determined in an initial ranging process.
An operation in a traffic plane 240 will now be described. As described above, the traffic plane 240 transmits control traffic or user traffic between the RAS 210 and the ACR 220. In addition, the traffic plane 240 performs ARQ control and flow control between the RAS 210 and the ACR 220, and defines a sub-header including additional information necessary for the ARQ and flow control.
The GRE tunnel, which is uniquely generated for each individual service flow, has a format shown in Table 1.















TABLE 1





C
R
K
S
s
Recur
Flags
Ver
Protocol Type
















Checksum (optional)
Offset (optional)







Key (optional)


Sequence Number (optional)


Routing: a list of Source Route Entries (optional)









As shown in Table 1, GRE Key is individually allocated for each of an uplink and a downlink, and is transported through an interface between the RAS 210 and the ACR 220, illustrated in FIG. 2. The GRE tunnel is used for generating a virtual link between the RAS 210 and the ACR 220 during handover of an MS 200, and the GRE tunneling protocol has low overhead and has been proven in the existing wireless communication network.
Table 2A and Table 2B show formats of the interface signaling messages transmitted from the signaling plane 230.










TABLE 2A









Message ID (16)
Message Length (16)










ACR Job ID (32)





















TABLE 2B









Message ID (16)
Message Length (16)










BS ID (48)












RAS Job ID (16)










Specifically, Table 2A shows a format of a signaling message for an uplink interface, transmitted from the RAS 210 and the ACR 220, and Table 2B shows a format of a signaling message for a downlink interface, transmitted from the ACR 220 to the RAS 210. Job IDs shown in Table 2A and Table 2B are used in the RAS 210 and the ACR 220 for allocating a unique Job for each MS 200 for session signaling.
The traffic plane 240, for example, an interface traffic plane, transmits control traffic for, for example, Dynamic Host Configuration Protocol (DHCP) and Mobile IP, and user traffic, between the RAS 210 and the ACR 220, performs ARQ control and flow control, and defines a sub-header including additional information necessary for the ARQ control and flow control.
The control traffic for DHCP or Mobile IP is allocated a particular Transport CID, and transported using an IP address and a particular GRE Tunnel Key for control traffic between the RAS 210 and the ACR 220 (hereinafter referred to as a “GRE Tunnel Key for RAS-ACR control traffic”).
The user traffic has a Transport CID, and is transported using an IP address and a GRE Tunnel Key for RAS-ACR user traffic.
A description will now be made of embodiments of a handover scheme of the present invention in the HPi system.
FIG. 3 is a signaling diagram illustrating a handover procedure in a wireless communication system according to the present invention. In particular, with reference to FIG. 3, a description will now be made of a hard handover method.
Referring to FIG. 3, an MS 310 transmits to a serving RAS 320 a Handover Request (HO-REQ) message including a Neighbor BSID and handover-related parameters to request handover in step 301 Then the serving RAS 320 transmits to a current serving ACR (or anchor point) 350 an MS Handover Request message including the parameters of the HO-REQ message received from the MS 310 in step 303.
An exemplary format of the MS Handover Request message according to the present invention is shown in Table 3.









TABLE 3





Name
Size
Description







Message ID
16



Message Length
16


ACR Job ID
32


MSHO-REQ message
variable
Include MSHO-REQ




MAC Management message




(see 6.3.2.3.53 section in




IEEE 802.16e/D12 spec)









As shown in Table 3, the MS Handover Request message according to the present invention includes a Message ID field, a Message Length field, an ACR Job ID field, and an MSHO-REQ message field. The MSHO-REQ message field includes an MSHO-REQ MAC Management message. As described above, the MS Handover Request message has the parameters included in the HO-REQ message transmitted by the MS 310. Therefore, it can be noted that a Neighbor BSID is added to the MS Handover Request message in response to a handover request.
Upon receiving the MS Handover Request message from the serving RAS 320, the ACR 350 checks target RASs, for example, a target RAS#1 330 and a target RAS#2 340, in a Neighbor BS set included in the received MS Handover Request message. Thereafter, the ACR 350 transmits a RAS Capability Request message to each of the target RASs 330 and 340, in steps 305 and 307. An exemplary format of the RAS Capability Request message is shown in Table 4 below.











TABLE 4









Name
Size
Description







Message ID
16
0x8302



Message
16



Length



BS ID
48



RAS Job ID
16
0x0000



MS MAC
48
PSS unique identifier.



Address



Up IP Address
32
Serving ACR IP Address



Up Port
16
Serving ACR Port Number



Number



ACR Job ID
32
Serving ACR Job ID



MS MAC
8



Version



Target BS ID
48



Received
24
The Frame Number in which the RAS received the PSSHO - REQ from PSS



Frame



Estimated HO
8
In frames, Estimated number of frames by PSS starting from the frame



Start

following the reception of the PSSHO - REQ message until the handover may





take place.



Required BW
8
Bandwidth which is required by PSS (to guarantee minimum packet data





transmission)



N_SFID
8

















Type
Length
Value





UL/DL
For(i=0; i<N_SFID; i++){


Service Flow
 SFID
1
4
1 - 4294967295


[145/146]—
 Service Class Name
3
2~128
Null - terminated string of ASCII characters.






The length of the string, including null - terminator may not






exceed 128 bytes



 QoS Parameter Set Type
5
1



 Traffic Priority
6
1



 Maximum Sustained Traffic Rate
7
4
in bits/second



 Maximum Traffic Burst
8
4
in Bytes



 Minimum Reserved Traffic Rate
9
4
in bits/second



 Minimum Tolerable Traffic Rate
10
4
in bits/second



 Service Flow Scheduling Type
11
1



 Request/Transmission Policy
12
1



 Tolerated Jitter
13
4
ms



 Maximum Latency
14
4
ms



 Fixed - length versus
15
1



 Variable - length SDU Indicator



 SDU Size
16
1



 Global Service Class Name
rr
6



 Type of Data Delivery Services
29
1



 SDU Inter - arrival Interval
30
2



 Time Base
31
2



 Paging Preference
32
1



}









As shown in Table 4, the RAS Capability Request message is transported by the ACR 350 to the target RASs 330 and 340 in the list requested by the MS 310, to determine whether handover of a particular MS 310 is acceptable in the target RASs 330 and 340. In Table 4, the RAS Capability Request message includes a Required BW field as a parameter used for requesting acceptability information of the handover.
Then the target RASs 330 and 340 each transmit to the ACR 350 a RAS Capability Response message including the parameter result value requested by the ACR 350 in steps 309 and 311. An exemplary format of the RAS Capability Response message is shown in Table 5 below.









TABLE 5





Name
Size
Description

















Message ID
16
0x8303


Message Length
16


ACR Job ID
32
0x00000000~0xFFFFFFFE


MS MAC Address
48
PSS unique identifier.


Mode

0 = HHO request




1 = SHO/FBSS request: Anchor BS update with CID update




2 = SHO/FBSS request: Anchor BS update without CID update




3 = SHO/FBSS request: Active Set update with CID update




4 = SHO/FBSS request: Active Set update without CID update




5~7 = reserved




8~255 = not used


If (Mode == 3) {

For collecting CID information within Active BS Set


 CID Update


}


Service Level Prediction
8


BW Estimated
8
Bandwidth which is provided by RAS (to guarantee minimum packet data




transmission)


QoS Estimated
8
Quality of Service Level -




UGS, rtPS, nrtPS, BE









As shown in Table 5, the RAS Capability Response message includes, for example, an Estimated BW field and a Service Level Prediction field as the parameter result value requested by the ACR 350.
Next, upon receiving the RAS Capability Response messages from the target RASs 330 and 340, the ACR 350 transmits to the serving RAS 320 in step 313 a Base Station (BS) Handover Response (BS HO Response) message including Recommended Neighbor BSIDs, Handover ID (HO-ID), and such parameters as Estimated BW and Quality-of-Service (QoS) result value. An exemplary format of the BS Handover Response message is shown in Table 6.









TABLE 6





Name
Size
Description







Message ID
16



Message Length
16


BS ID
48
e.g., Operator ID + ACR-ID + RAS-ID


RAS Job ID
16


BSHO-RSP message
variable
Include BSHO-RSP




MAC Management message




(see 6.3.2.3.54 section in




IEEE 802.16e/D12 spec)









As shown in Table 6, the BS Handover Response message according to the present invention includes a Message ID field, a Message Length field, a BSID field, a RAS Job ID field and a BSHO-RSP message field. The BSID includes Operator ID, ACR-ID and RAS-ID. The BSHO-RSP message field includes a BSHO-RSP MAC Management message.
Thereafter, the serving RAS 320 transmits to the MS 310 a BSHO-RSP message including the parameter of the BS Handover Response message received from the ACR 350 in step 315.
The MS 310 transmits to the serving RAS 320 in step 317 a Handover Indication (HO-IND) message including Handover Indication Type (HO-IND Type) and Target BSID, to finally indicate that it performs handover. In response, the serving RAS 320 transmits a HO Indication message with the HO-IND Type and the Target BSID to the ACR 350 in step 319. An exemplary format of the HO Indication message is shown in Table 7.











TABLE 7







Name
Size
Description









Message ID
16




Message Length
16



ACR Job ID
32



HO-IND message
variable
Include HO-IND MAC





Management message





(see 6.3.2.3.55 section in





IEEE 802.16e/D12 spec)










As shown in Table 7, the HO Indication message according to the present invention includes a Message ID field, a Message Length field, an ACR Job ID field and a HO-IND message field. The HO-IND message field includes an HO-IND MAC Management message. The HO Indication message is transported to the ACR 350 to indicate approval/denial for handover of a particular MS 310. If HO-IND Type of the HO Indication message is set to ‘Serving BS release’, the ACR 350 releases MS-related information.
Upon receiving the HO Indication message from the serving RAS 320, the ACR 350 transmits in step 321 a HO Confirm message with an MS MAC address to a target RAS, for example, the target RAS#1 330, included in the HO Indication message. An exemplary format of the HO Confirm message is shown in Table 8 below.









TABLE 8





Name
Size
Description

















Message ID
16
0x8306


Message Length
16


BS ID
48


RAS Job ID
16
0x0000


MS MAC Address
48
PSS unique identifier.


ACR Job ID
32


MS MAC Version
8


Serving RAS - ID
48


Frame Information
32
In frames




#0~#23: the Frame Number in which the




RAS received the PSSHO - REQ from PSS




#24~#31: Estimated number of frame by PSS




starting from the frame following the




reception of the PSSHO - REQ message until




the handover may take place









As shown in Table 8, the HO Confirm message is transported to a target RAS, for example, the target RAS#1 330, by the ACR 350 to indicate approval for the handover of the particular MS 310.
Upon receiving the HO Indication message from the ACR 350, the target RAS#1 330 transmits an Ack message to the ACR 350 in response thereto in step 323.
Upon receiving the Ack message, the ACR 350 transmits to the target RAS#1 330 in step 325 an HO Optimization message including MS information necessary for performing optimized HO, for example, Subscriber Station's Basic Capability Negotiation (SBC) information, Privacy Key Management (PKM) information, Registration (REG) information and Provisioned Service Flow (Provisioned SF) information, and also including an ACR GRE Key used for establishing a GRE tunnel. An exemplary format of the HO Optimization message is shown in Table 9 below.











TABLE 9









Name
Size (bit)
Description







Message ID
16



Message Length
16



BS ID
48



RAS Job ID
16
0x0001 ~0xFFFF



Num_Tunnels

GRE Tunnel for assigned CID



For(I=0;



I<Num_Tunnels;I++) {



 CID



 New GRE Key



 New IP Address



}















Type
Length
Value





SBC Information


SS Management Support
2
1


Bandwidth Allocation Support
1
1


MAC PDU Capabilities
4
1


Subscriber Transition Gaps
2
2


Maximum Transmit Power
3
4


Authorization Policy Support
25
1


PKM Version Support
26
1
Bit#0: PKM Version 1





Bit#1: PKM Version 2





Bit#2~7: reserved, shall be set to zero


Handoff Supported
19
1
Bit #0: SHO/FBSS HO - Single - BS Map Supported





Bit #1: SHO/FBSS HO - Multi - BS MAP Supported





Bit #2-#7: reserved, shall be set to zero


Current Transmit Power
147
1


OFDMA SS FFT Sizes
150
1


OFDMA SS demodulator
151
1


OFDMA SS modulator
152
1


The number of HARQ ACK Channel
153
1


OFDMA SS Permutation support
154
1


OFDMA MAP Capability
155
1


Uplink Control Channel Support
xxx
1


OFDMA SS demodulator for MIMO Support
155
1


OFDMA SS modulator for MIMO Support
156
1


PKM Information


AUTH - Key
7
128
128 byte quantity representing an RSA - encrypted AK


Key Lifetime
9
4
32 - bit quantity representing AK's lifetime





A key lifetime of zero indicates that the





corresponding AK is not valid.


Key Sequence Number
10
1
4 - bit Sequence Number (AK)


For (i=0; i<Num_SA; i++) {


Each compound SA - Descriptor attribute specifies an





SAID and additional properties of the SA.











SA Descriptor
SAID
12
2



23—
SA Type
24
1



Cryptographic Suite
20
3


}


PKM Config Settings
Authorize Wait Timeout
1
4
in seconds


27—
Reauthorize Wait Timeout
2
4
in seconds



Authorization Grace Time
3
4
in seconds



Operational Wait Timeout
4
4
in seconds



Rekey Wait Timeout
5
4
in seconds



TEK Grace Time
6
4
in seconds



Authorize Reject Wait Timeout
7
4
in seconds


REG Information










SS Management Support
2
1



IP Management Mode
3
1


IP Version
4
1


Number of UL CID Support
6
2
Number of Uplink CIDs the SS can support


Number of DL CID Support
X
2
Number of Downlink CIDs the SS can support


Classification/PHS Options and SDU Encapsulation Support
7
2


Maximum Number of Classifier
8
2


PHS Support
9
2


ARQ Support
10
1


DSx Flow Control
11
1


MAC CRC Support
12
1


PKM Flow Control
15
1


Maximum Number of Supported Security Associations
17
1











SA Update
Old SAID
1
2



20—
New SAID
2
2



New SA type
3
1



New Cryptographic suite
4
4










Method for Allocating IP Address
23
1












Mobility Parameters
Mobility Features Supported
18
1
Bit #0: Mobility (handover) support


Support



Bit #1: Sleep - mode support


24—



Bit #2: Idle - mode support



SKIP - ADDR - ACQUISITION
11
1
0: No IP address change






1: Re - acquire IP address










System Resource Retain Time
50
1
multiple of 100 ms





200 msec is recommended as default


SS Mode Selection Feedback Support
20
1


Packing Support
51
1











Vender Specific Information
Vender ID
144
3



143—


Provisioned SF


Information










Old CID


For Provisioned SF


reserved









As shown in Table 9, the HO Optimization message is transmitted to a corresponding target RAS, for example, the target RAS#1 330, along with BSC information, PKM information, REG information and Provisioned SF information for the MS 310, and GRE tunnel information with the target RAS#1 330, all of which are requested in a handover signaling process.
The target RAS#1 330 transmits to the ACR 350 in step 327 an HO Optimization Reply message including New CIDs for New Basic CID & Primary Management CID and Old Provisioned SF and also including a GRE Key used for establishing a corresponding GRE tunnel. An exemplary format of the HO Optimization Reply message is shown in Table 10 below.









TABLE 10





Name
Size (bit)
Description







Message ID
16



Message Length
16


ACR Job ID
32
0x00000000~0xFFFFFFFE


Num_Tunnels

GRE Tunnel for assigned


For(I=0; I<Num_Tunnels;I++) {
CID


 CID


 New GRE Key


 New IP Address


}









After the transmission/reception of the HO Optimization message between the ACR 350 and the target RAS#1 330 is completed, a ORE tunnel is established between the ACR 350 and the target RAS#1 330 in step 329. Specifically, the handover signaling procedure previously generates a GRE tunnel to the target RAS#1 330, so the MS 310 can reduce a delay time required after a ranging procedure to the target RAS#1 330. In this manner, the present invention can provide faster handover. In addition, as the ACR 350 provides the target RAS#1 330 with the SBC information, PKM information, REG information and Provisioned SF information stored in the former serving RAS 320, the MS 310 can omit the additional procedure for SBC, PKM, REG and Dynamic Service Addition (DSA) after the ranging procedure to the target RAS#1 330. This optimized handover technique can meet various requirements for services by establishing a tunnel for each individual service flow.
Thereafter, the target RAS#1 330 transmits an uplink MAP (UL-MAP) message with a fast ranging information element (Fast_Ranging_IE) to the MS 310 in step 331. In response, the MS 310 transmits to the target RAS#1 330 in step 333 a Ranging Request (RNG-REQ) message including MAC address, Serving BSID, HO Indication and HO-ID
In response to the RNG-REQ message, the target RAS#1 330 transmits a Ranging Response (RNG-RSP) message with a HO Process Optimization Flag to the MS 310 in step 335. At this moment, the target RAS#1 330 determines whether to perform the SBC, PKM, REG and DSA processes according to a bit value set in the HO Process Optimization Flag. In addition, the target RAS#1 330 transmits a Registration Response (REG-RSP) message with CID_Update and SAID_Update to the MS 310 in step 337, to reestablish a relation to the former Provisioned SF.
The hard handover method in the wireless communication system according to the present invention has been described thus far. A description will now be made of a Fast BS Switch (FBSS) handover method in a wireless communication system according to the present invention.
FIG. 4 is a signaling diagram illustrating a handover procedure in a wireless communication system according to the present invention. In particular, with reference to FIG. 4, a description will now be made of an FBSS handover method.
Referring to FIG. 4, an MS 410 transmits to a serving RAS 420 a HO-REQ message including a Neighbor BSID and handover-related parameters to request Active Set Update in step 401. Then the serving RAS 420 transmits to a current serving ACR (or anchor point) 450 an Active Set Request message including the parameters of the HO-REQ message received from the MS 410 in step 403.
Although the Active Set Request message according to the present invention is similar in format to the MS Handover Request message shown in Table 3, a set value of an Arrival Time Difference Indication field is subject to change.
Upon receiving the Active Set Request message from the serving RAS 420, the ACR 450 checks target RASs, for example, a target RAS#1 430 and a target RAS#2 440, in a Neighbor BS set included in the received Active Set Request message. Thereafter, the ACR 450 transmits a RAS Capability Request message to each of the target RASs 430 and 440 in step 405 and 407. An exemplary format of the RAS Capability Request message is shown above in Table 4A and Table 4B.
The target RASs 430 and 440 each transmit to the ACR 450 a RAS Capability Response message including a parameter result value requested by the ACR 450 and an HO-ID in steps 409 and 411. An exemplary format of the RAS Capability Response message is shown above in Table 5.
Thereafter, the ACR 450 transmits to each of the target RASs 430 and 440 an HO Optimization message including SBC information, PKM information, REG information and Provisioned SF information necessary for performing Optimized HO and also including an ACR GRE Key used for establishing a GRE tunnel, in steps 413 and 415. An exemplary format of the HO Optimization message is shown above in Table 9.
The target RASs 430 and 440 each transmit to the ACR 450 an HO Optimization Reply message including New CIDs for new Basic CID & Primary Management CID and Old Provisioned SF and also including a GRE Key used for establishing a corresponding GRE tunnel, in steps 417 and 419. An exemplary format of the HO Optimization Reply message is shown above in Table 10.
Upon receiving the HO Optimization Reply messages from the target RASs 430 and 440, the ACR 450 transmits an Active Set Response message including Recommended Neighbor BSIDs, Temp BSIDs and New CIDs to the Serving RAS 420 in step 421. An exemplary format of the Active Set Response message is shown above in Table 6.
The serving RAS 420 transmits a BS Handover Response (BSHO-RSP) message including the parameters of the Active Set Response message received from the ACR 450 to the MS 410 in step 423.
The MS 410 transmits a HO-IND message including FBSS-IND Type and Temp BSID to the serving RAS 420 in step 425, to finally indicate that it will perform Active Set Update. Then the serving RAS 420 transmits an Active Set Indication message including the FBSS-IND Type and Temp BSID to the ACR 450 in step 427. An exemplary format of the Active Set Indication message is shown above in Table 7.
Upon receiving the Active Set Indication message from the serving RAS 420, the ACR 450 transmits an Active Set Add message with an MS MAC address to the target RAS#1 430 in step 429, to add a target RAS, for example, the target RAS#1 430, included in the received Active Set Indication message to its Active Set. An exemplary format of the Active Set Add message is shown in Table 11 below.











TABLE 11







Name
Size
Description









Message ID
16




Message Length
16



ACR Job ID
32
0x00000000~0xFFFFFFFE



MS MAC Address



Reserved










Upon receiving the Active Set Add message from the ACR 450, the target RAS#1 430 transmits an Ack message to the ACR 450 in response thereto in step 431.
Thereafter, the ACR 450 transmits an Active Set Drop message with an MS MAC address to the target RAS#2 440 in step 433, to drop the target RAS#2 440 from the Active Set. An exemplary format of the Active Set Drop message is shown above in Table 10.
Upon receiving the Active Set Drop message from the ACR 450, the target RAS#2 440 transmits an Ack message to the ACR 450 in response thereto in step 435.
After the transmission/reception of the Active Set Add/Drop messages between the ACR 450 and the target RASs 430 and 440 is completed, a GRE tunnel is established between the ACR 450 and the target RAS#1 430 in step 437. That is, the HO signaling procedure according to the present invention previously generates a GRE tunnel to the target RAS#1 430, so the MS 410 can reduce a delay time required for an access to the target RAS#1 430. In this manner, the present invention can provide faster handover. In addition, as the ACR 450 provides the target RAS#1 430 with the SBC information, PKM information, REG information and Provisioned SF information stored in the old serving RAS 420, the MS 410 can omit the additional procedure for SBC, PKM, REG and DSA after the access to the target RAS#1 430. This optimized handover technique can meet various requirements for services by establishing a tunnel for each individual service flow.
Thereafter, the MS 410 sends an Anchor BS Switching request over a Channel Quality Information (CQI) channel (CQICH) in step 439, to request FBSS handover to the target RAS#1 430. Upon receiving an Anchor BS Report (codeword) from the MS 410 over a CQICH, the serving RAS 420 transmits an Anchor Switching Indication message to the ACR 450 in step 441, to request switching to the target RAS#1 430 which is a designated Anchor. As a result, the MS 410 does not perform a separate ranging process. An exemplary format of the Anchor Switching Indication message is shown in Table 12 below.









TABLE 12





Name
Size
Description

















Message ID
16



Message Length
16


ACR Job ID
32
0x00000000~0xFFFFFFFE


MS MAC Address


FBSS_IND_Type
8
0b00: confirm Anchor BS




update




0b01: Anchor BS update




cancel




0b10: Anchor BS update




reject




0b11: reserved


If (FBSS_IND_Type == 0b00){


 Anchor BS - ID
8
TEMP_BS_ID of the Anchor




BS


 Action Time
8
Action time when the Anchor




BS shall be updated


}









As shown in Table 12, the Anchor Switching Indication message is transported from the serving RAS 420 to the ACR 450 to indicate approval/denial for FBSS of a particular MS 410 for Anchor BS Switching.
Upon receiving the Anchor Switching Indication message from the serving RAS 420, the ACR 450 transmits to the target RAS#1 430 in step 443 an Anchor Switch Confirm message indicating success in Anchor Switching to the target RAS#1 430. Then the target RAS#1 430 transmits in step 445 an Ack message to the ACR 450 in response to the Anchor Switch Confirm message received from the ACR 450. An exemplary format of the Anchor Switch Confirm message is shown in Table 13 below.











TABLE 13







Name
Size
Description









Message ID
16




Message Length
16



ACR Job ID
32
0x00000000~0xFFFFFFFE



MS MAC Address



Reserved










As can be understood from the foregoing description, the present invention provides normalized messages for a handover signaling scheme between a RAS (or BS) and an ACR (or BSC) to provide optimized handover in a wireless communication system, and a method for providing the same. In the handover signaling process, once a target RAS is determined, session information is previously transported to the determined target RAS. As a result, in a network re-entry process, an MS can minimize signaling overhead between the RAS and the ACR, and also minimize a handover delay time.
While the invention has been shown and described with reference to a certain preferred embodiment 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 spirit and scope of the invention as defined by the appended claims.

## Claims

1. A handover method in a wireless communication system, comprising the steps of:
transmitting, by a serving base station, a handover request message to a base station controller in response to a handover request of a mobile station;
determining, by the base station controller, target base stations to which the mobile station can perform handover among target base stations in a list of neighbor base stations, and transmitting a handover response message including the determination result information to the mobile station via the serving base station;
determining, by the mobile station, a target base station to which it will perform handover, and transmitting a message indicating that it will perform handover, to the base station controller via the serving base station;
transmitting, by the base station controller, a confirm message indicating that the mobile station will perform handover, to the determined target base station, and establishing a tunnel to the target base station; and
registering, by the mobile station, itself in the target base station.

2. The handover method of claim 1, wherein the handover request message includes message identifier (ID) information, message length information, base station controller's job ID information and handover request information, and the handover request information includes parameters included in the handover request message transmitted by the mobile station and neighbor base station identifier (Neighbor BSID) information determined in response to the handover request.

3. The handover method of claim 1, wherein the step of determining target base stations to which the mobile station can perform handover comprises determining, by the base station controller, whether each of the neighbor base stations can accept handover of the mobile station thereto, and transmitting a handover response message including IDs of the base stations to which the mobile station can perform handover, to the serving base station.

4. The handover method of claim 3, wherein the step of determining whether each of the neighbor base stations can accept handover of the mobile station thereto comprises the step of transmitting, by the base station controller, a base station capability request message to the target base stations and determining whether each of the target base stations can accept handover of the mobile station thereto, based on a base station capability response message received in response to the base station capability request message.

5. The handover method of claim 4, wherein the base station capability request message includes a required bandwidth field used for requesting acceptability information of handover as a message transmitted to target base stations in a neighbor base station list requested by the mobile station, to determine whether handover of a particular mobile station is acceptable.

6. The handover method of claim 4, wherein the base station capability response message includes parameter information requested by the base station controller, and the parameter information includes estimated bandwidth information and service level prediction information.

7. The handover method of claim 1, wherein the step of transmitting by the mobile station a message indicating that it will perform handover comprises transmitting to the serving base station a handover indication message including a handover indication type field used for indicating that the mobile station will perform handover, and an ID of a target base station.

8. The handover method of claim 7, wherein the handover indication message includes message ID information, message length information, base station controller's job ID information and handover indication information, and the handover indication information includes information used for informing the base station controller of approval/denial for handover of the mobile station.

9. The handover method of claim 1, wherein the step of transmitting by the base station controller a confirm message indicating that the mobile station will perform handover comprises, upon receiving a handover indication message from the serving base station, transmitting a handover confirm message to a target base station included in the handover indication message;
wherein the handover confirm message includes information used by the base station controller to inform the target base station that the mobile station will perform handover thereto.

10. The handover method of claim 1, wherein the tunnel establishing step comprises:
upon receiving a response from the target base station, transmitting, by the base station controller, a handover optimization message including information necessary for performing optimized handover and information for tunnel establishment, to the target base station;
transmitting, by the target base station, a handover optimization reply message including a new connection ID (CID) and tunnel key information used for establishing a corresponding tunnel, to the base station controller; and
after transmitting/receiving the handover optimization message and the handover optimization reply message, establishing a tunnel between the base station controller and the base station.

11. The handover method of claim 10, wherein the handover optimization message includes subscriber station's basic capability negotiation (SBC) information, privacy key management (PKM) information, registration (REG) information, provisioned service flow (Provisioned SF) information, and information on a tunnel to the target base station for the mobile station, requested in a handover signaling process.

12. The handover method of claim 1, wherein the registration step comprises:
transmitting, by the target base station, an uplink MAP message including a fast ranging information element (Fast_Ranging_IE) to the mobile station;
transmitting, by the mobile station, a ranging request message to the target base station;
transmitting, by the target base station, a ranging response message including a handover process optimization flag to the mobile station; and
transmitting, by the target base station, a registration response message to the mobile station.

13. A handover method in a wireless communication system, comprising the steps of:
upon receiving an active set request message, exchanging with target base stations, by a base station controller, information used for determining whether each of the target base stations can accept handover of a mobile station thereto and connection information for the target base stations;
transmitting, by the base station controller, an active set response message including identifiers (IDs) of base stations to which the mobile station can perform, to a serving base station;
upon receiving a handover indication message from the mobile station, transmitting, by the serving base station, an active set indication message to the base station controller;
after exchanging an active set add/drop message with the target base stations, establishing, by the base station controller, a tunnel between the base station controller and the target base station;
upon receiving an anchor base station report over a channel quality information channel (CQICH), transmitting by the serving base station an anchor switching indication message to the base station controller; and
upon receiving an anchor switch confirm message from the base station controller, performing communication by the target base station without performing a ranging procedure with the mobile station.

14. The handover method of claim 13, wherein the step of determining whether each of the target base stations can accept handover of the mobile station thereto comprises transmitting a base station capability request message to the target base stations and determining whether each of the target base stations can accept handover of the mobile station thereto, based on a base station capability response message received in response to the base station capability request message.

15. The handover method of claim 13, wherein the step of exchanging connection information with the target base stations comprises:
upon receiving a response from the target base station, transmitting by the base station controller a handover optimization message including information necessary for performing optimized handover and information used for tunnel establishment, to the target base station; and
transmitting by the target base station a handover optimization reply message including a new connection ID (CID) and tunnel key information used for establishing a corresponding tunnel.

16. The handover method of claim 13, wherein the active set indication message includes fast base station switch (FBSS) indication type information and temporary base station ID (Temp BSID) information.

17. The handover method of claim 13, wherein the active set add/drop message includes a mobile station (MS) medium access control (MAC) address to add or drop target base stations included in the active set indication message to/from an active set.

18. The handover method of claim 13, further comprising sending, by the mobile station, a request for anchor base station switching over the CQICH to request FBSS handover to the determined target base station.

19. The handover method of claim 13, wherein the anchor switching indication message includes information used by the serving base station to inform the base station controller that it approves/denies FBSS of the mobile station for anchor base station switching.

20. The handover method of claim 13, further comprising, upon receiving the anchor switching indication message from the serving base station, transmitting by the base station controller an anchor switch confirm message indicating success in anchor switching to the target base station.