System and method for performing handover operation in broadband wireless access communication system

System and method for performing handover operation in broadband wireless access communication systemTechnical field

[0001] The present invention relates to a broadband wireless access communication system, and in particular, to a method used in a BWA (Broadband Wireless Access) communication system using an OFDM (Orthogonal Frequency Division Multiplexing) scheme, A system and method for performing a handover operation after receiving an SS (subscriber station) request.

Background technique

[0002] Intensive research is being directed to 4G (fourth generation) communication systems, which is one of the next generation communication systems for providing specific users with various QoS (Quality of Service) at a transmission rate of about 100 Mbps. service. At present, the 3G (third generation) communication system provides a transmission rate of about 384kbps in an outdoor channel environment with a relatively poor channel environment, and provides a maximum transmission rate of about 2Mbps in an indoor channel environment with a relatively good channel environment. A wireless local area network (LAN) system and a wireless metropolitan area network (MAN) system have been designed to provide a transmission rate of 20-50Mbps. 4G communication systems provide wireless LAN and MAN systems with relatively high transmission rates, mobility and QoS, and many secondary developers are conducting concentrated research on the high-speed services provided by 4G communication systems.

[0003] However, the wireless MAN system is suitable for high-speed communication services because it has a wide coverage area and supports high-speed transmission rates. However, it does not consider the mobility of the subscriber station (SS) at all. Handover operation caused by movement (ie, cell selection operation). The communication system currently considered in the IEEE (Institute of Electrical and Electronics Engineers) 802.16a specification is used as a specific communication system that performs ranging operations between an SS and a base station (BS). The communication system considered in the IEEE 802.16a specification will be described below with reference to FIG. 1.

[0004] FIG. 1 is a block diagram showing a BWA communication system using an OFDM/OFDMA (Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access) scheme. In more detail, Figure 1 depicts the IEEE 802.16a communication system.

[0005] Compared with the wireless LAN, the wireless MAN system used as the BWA communication system has a much wider coverage area and a much higher transmission rate. In the case of applying the OFDM scheme and the OFDMA scheme to the physical channel of the wireless MAN system to provide the wireless MAN system with a broadband transmission network, the application system is called an IEEE 802.16a communication system. The IEEE 802.16a communication system applies the OFDM/FODMA scheme to the wireless MAN system so that it uses multiple subcarriers to transmit physical channel signals, resulting in high-speed data transmission. The IEEE 802.16e communication system has been designed to consider the mobility of the SS in the IEEE 802.16a communication system. There is no detailed specification for the IEEE 802.16e communication system.

[0006] Referring to FIG. 1, the IEEE 802.16a communication system has a single-cell structure and is composed of a BS 100 and multiple SSs 110, 120, and 130 managed by the BS 100. The signal transmission/reception between the BS 100 and the SS 110, 120 and 130 can be established using the OFDM/OFDMA scheme. The downlink frame structure used in IEEE 802.16a will be described below in conjunction with FIG. 2.

[0007] FIG. 2 is a conceptual diagram showing a downlink frame structure used in a BWA communication system using the OFDM/OFDMA scheme. In more detail, FIG. 2 describes the downlink frame structure used in the IEEE 802.16a communication system.

[0008] Referring to FIG. 2, a downlink frame includes a preamble field 200, a broadcast control field 210, and a plurality of TDM (Time Division Multiplexing) fields 220 and 230. Sent after the preamble field 200

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A synchronization signal (ie, preamble sequence) used to obtain synchronization between BS and SS. The broadcast control field 210 is composed of a DL (downlink)_MAP field 211 and a UL (uplink)_MAP field 213. The DL_MAP field 211 is used to send a DL_MAP message. Table 1 below shows multiple IEs (information elements) included in the DL_MAP message:

[0009] Table 1:

[0010]

<td>syntax</td><td>size</td><td>Description</td>

<td>DLMAPMessageFormat(){</td><td></td><td></td>

<td>Management Message Type = 2</td><td>8-bit</td><td>See appropriate PHY description</td>

<td>PHY Synchronization Field</td><td>variable</td><td></td>

<td>DCD Count</td><td>8-bit</td><td></td>

<td>Base Station ID</td><td>48 bits</td><td></td>

<td>Number of DLMAP Element n</td><td>16 bits</td><td></td>

<td>Begin PHY Specific section{</td><td></td><td>See the PHY part of the application</td>

<td>for(i = 1, i &lt;= n; i++)</td><td></td><td>For each DL MAP element 1 to η</td>

<td>DL MAP Information Element ()</td><td>variable</td><td>See the corresponding PHY description</td>

<td>If! (Byte boundary){</td><td></td><td></td>

<td>Padding Nibble</td><td>4</td><td>Padding reaches byte boundary</td>

<td>}</td><td></td><td></td>

<td>}</td><td></td><td></td>

<td>}</td><td></td><td></td>

<td>}</td><td></td><td></td>

[0011] Referring to the above Table 1, the DL_MAP message includes: a Management Message Type (Management Message Type) field representing multiple IEs (ie, message type information to be sent); established in response to a modulation or demodulation scheme applied to a physical channel PHY (Physical) synchronization field (PhysicalSynchronization Field), used to perform synchronization acquisition; DCD Count (DCD Count) field, which indicates that the response to the DCD (downlink channel) containing the downlink burst profile (burst profile) Descriptor) message configuration change count information; the base station ID (Base Station ID) field indicates the base station identifier; and the number of DL_MAP elements n (Number of DL_MAP El emen tn), which indicates the number of elements found after the base station ID . In particular, the DL_MAP message (not shown in Table 1) includes information related to the ranging codes assigned to each ranging process (described later).

[0012] The UL_MAP field 213 is used to send the UL_MAP message, and multiple IEs included in the UL_MAP message are shown in Table 2 below:

[0013] Table 2:

[0014]

<td>syntax</td><td>size</td>

<td>ULMAPMessageFormat(){</td><td></td>

<td>Management Message Type = 3</td><td>8-bit</td>

<td>Uplike Channel ID</td><td>8-bit</td>

<td>UCD Count</td><td>8-bit</td>

<td>Number of ULMAP Element n</td><td>16 bits</td>

<td>Allocation Start Time</td><td>32 bit</td>

<td>Begin PHY Specific section{</td><td></td>

<td>For(i = 1; i &lt;n; i++)</td><td></td>

<td>UL-MAP InformationElement()</td><td>variable</td>

<td>Connection ID</td><td></td>

<td>UIUC</td><td></td>

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<td>Offset</td><td></td>

<td>}</td><td></td>

<td>}</td><td></td>

<td>}</td><td></td>

<td>}</td><td></td>

[0015] Referring to Table 2, the UL_MAP message includes a management message type (ManagementMessage Type) field (ie, sending message type information) representing multiple IEs; and an uplink channel ID (Uplike Channel ID) representing the uplink channel ID used Field; UCD Count (UCD Count) field, which indicates count information in response to changes in the configuration of the UCD (Uplink Channel Descriptor) message containing the uplink burst profile; and the number of UL_MAP elements n (Number of UL_MAP Element n) field, which represents the number of elements found after the UCD count field. In this case, the uplink channel ID can only be allocated to the medium access control (MAC) sublayer.

[0016] The UIUC (Uplink Interval Usage Code, _h uplink interval usage code) area record indicates the usage of the offset recorded in the offset area. For example, if 2 is recorded in the UIUC area, then the start offset used in the initial ranging process is recorded in the offset area. If 3 is recorded in the UIUC area, then in the bandwidth request measurement The starting offset used in the range or maintenance ranging process is recorded in the offset area. The offset area records the initial offset value used in the initial ranging process or the maintenance ranging process based on the information recorded in the UIUC area. The physical channel characteristic information to be transmitted from the UIUC area is recorded in the UCD.

[0017] If the SS causes the ranging failure, a predetermined back-off value is set to increase the probability of success in the next attempt, and the ranging process is re-executed after the predetermined time corresponding to the back-off time has passed. In this case, the information required to determine the yield value is included in the UCD message. The aforementioned UCD message configuration is shown in Table 3 below:

[0018]

<td>syntax</td><td>size</td><td>Description</td>

<td>UCDMessageFomat()}</td><td></td><td></td>

<td>Management Message Type = 0</td><td>8-bit</td><td></td>

<td>Uplink channel ID</td><td>8-bit</td><td></td>

<td>Configuration Change Count</td><td>8-bit</td><td></td>

<td>Mini-slot size</td><td>8-bit</td><td></td>

<td>Ranging Backoff Start</td><td>8-bit</td><td></td>

<td>Ranging Backoff End</td><td>8-bit</td><td></td>

<td>Request Backoff Start</td><td>8-bit</td><td></td>

<td>Request Backoff End</td><td>8-bit</td><td></td>

<td>TLV Encoded Information for the overall channel</td><td>variable</td><td></td>

<td>Begin PHY Specific Section{</td><td></td><td></td>

<td>for(i = 1; i &lt;n; i++)</td><td></td><td></td>

<td>Uplink Burst Descriptor</td><td>variable</td><td></td>

<td>}</td><td></td><td></td>

<td>}</td><td></td><td></td>

<td>}</td><td></td><td></td>

[0019] Referring to Table 3, the UCD message includes: a management message type representing multiple IEs (ManagementMessage

Type) field (ie, send message type information); indicates the uplink channel of the uplink channel identifier used

ID (Uplink channel ID) field; Configuration Change Count (Configuration Change Count) field counted by the BS; Mini-slot size (Mini-slot size) field indicating the number of uplink physical channel mini-slots;

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Range Backoff Start field at the backoff start point of the initial ranging process (ie the initial backoff window size of the initial ranging process); indicates the backoff end point used for the initial ranging process (ie the size of the last backoff window) Range Backoff End (Ranging Backoff End) field; Represents the Request Backoff Start field used to establish contention data and the requested backoff start point (ie the initial backoff window size); and Represents the Request Backoff Start field used to establish contention The Request Backoff Start field of the data and request backoff end point (that is, the size of the last backoff window). In this case, the yield value represents a waiting time, which is the duration between the start of the access failure of the SS and the re-access time of the SS. If the SS fails to perform the initial ranging, the BS must send a backoff value indicating the waiting time information, and the SS must wait for this period of time to arrive at the next ranging process for the SS. For example, assuming that the Ranging BackoffStart and Ranging Backoff End fields shown in Table 3 are determined to be a specific number of 10, then SS must exceed 2¹⁰Executable access opportunities (ie, 1024 executable access opportunities), and then execute the next ranging process according to the Truncated Binary Exponential Backoff Algorithm (Truncated Binary Exponential Backoff Algorithm).

[0020] The TDM fields 220 and 230 indicate fields corresponding to time slots allocated using the TDM/TDMA (Time Division Multiplexing/Time Division Multiple Access) scheme. The BS uses a predetermined center carrier to transmit the broadcast information to be broadcast on the DL_MAP field 211 to the SS managed by the BS. After receiving the power-on signal, the SS monitors all the frequency bands previously allocated to each SS, so that they detect the pilot channel signal with the highest signal strength, that is, the highest pilot CINR (Carrier to Interference and Noise Ratio). It is determined that the SS belongs to a specific BS that has transmitted a pilot channel signal with the highest pilot CINR. The SS checks the DL_MAP field 211 and UL_MAP field 213 of the downlink frame transmitted from the BS so that they recognize their own uplink and downlink control information and specific information indicating the actual data transmission/reception position.

[0021] With reference to FIG. 2, a downlink frame structure used in the IEEE 802.16a communication system is disclosed. The uplink frame structure used in the IEEE 802.16a communication system will be described below in conjunction with FIG. 3.

[0022] FIG. 3 is a conceptual diagram showing the structure of an uplink frame used in a BWA communication system using the OFDM/FODMA scheme. In more detail, FIG. 3 describes the structure of an uplink frame used in the IEEE 802.16a communication system.

[0023] Before describing the uplink frame structure shown in FIG. 3, the following describes in detail the three ranging processing used in the IEEE 802.16a communication system, namely: initial ranging processing, maintenance ranging processing (also called For periodic ranging processing) and bandwidth request ranging processing.

[0024] First, the initial ranging process will be described in detail.

[0025] The initial ranging process used to establish synchronization acquisition between the BS and the SS establishes the correct time offset between the SS and the BS, and is used to control the transmission power (also referred to as the transmission power). In more detail, the SS is powered on and receives the DL_MAP message, UL_MAP message, and UCD message to establish synchronization with the BS, thereby performing initial ranging processing to control the transmission power between the BS and the time offset. In this case, the IEEE 802.16a communication system uses the OFDM/OFDMA scheme, so that the ranging process requires multiple ranging sub-channels and multiple ranging codes. The BS allocates available ranging codes to the SS according to the target of the ranging process (that is, the ranging processing type information). This operation will be explained in detail below.

[0026] By changing the length to 2¹⁵The -1 bit PN (pseudo-random noise) sequence is segmented into multiple predetermined units to establish a ranging code. Typically, a ranging channel consists of two ranging sub-channels, each of which has a length of 53 bits.

PN code segmentation is performed on the 106-bit ranging channel to establish a ranging code. Up to 48 ranging codes RC#1~RC#48 can be assigned to SS. More than two ranging codes for each SS are applied as default values to three ranging processes with different targets, namely, initial ranging processing, periodic ranging processing, and bandwidth request ranging processing. In this way, the ranging code is rooted

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According to the three types of ranging processing, each target is assigned to the SS differently. For example, as indicated by the prescribed item N RC (Ranging Codes) for Initial Ranging (N ranging codes for initial ranging), N ranging codes are assigned to the SS used for initial ranging processing; As indicated by the prescribed item M RCs for maintenance ranging (M RCs for maintenance ranging), M ranging codes are assigned to the SS used for periodic ranging processing; and as in the prescribed item "L RCs for BW-request rangin' (L RCs used for bandwidth request ranging), L ranging codes are assigned to the SS used for bandwidth request ranging processing. The DL_MAP message is used to send the specified ranging code to the SS, The SS uses the ranging code contained in the DL_MAP message to perform necessary ranging processing.

[0027] Next, the periodic ranging processing will be described in detail.

[0028] The bandwidth request ranging process is performed periodically so that the SS that has controlled the time offset and the transmission power between the SS and the BS in the initial ranging process can control the channel state associated with the BS. The SS uses the ranging code assigned to the periodic ranging process to perform the periodic ranging process.

[0029] Third, the bandwidth request ranging process will be explained in detail.

[0030] The bandwidth request ranging process is used to enable the SS that has controlled the time offset and transmission power between the SS and the BS in the initial ranging process to request bandwidth allocation from the BS, so that the SS can communicate with the BS.

[0031] Referring to FIG. 3, the uplink frame includes an initial maintenance opportunity field 300 using initial and periodic ranging processing, and a request contention opportunity field 310 using bandwidth request ranging processing. , And an SS scheduled data (SS scheduled data) field 320 composed of uplink data of multiple SSs. The initial maintenance opportunity field 300 includes multiple access burst fields each of which has actual initial and periodic ranging processing, and conflicts in which there is a conflict between the access burst flow fields ( collision) field. The request contention opportunity field 310 includes a plurality of bandwidth request fields each of which has a real bandwidth request ranging process, and a contention field in which there is contention between the bandwidth request ranging fields. Each of the SS scheduling data fields 320 is composed of multiple SS scheduling data fields (ie, SS 1 scheduling data field~SS N scheduling data field). The SS transition gap (transition gap) is located between the SS scheduling data field (that is, the SS 1 scheduling data field˜SS N scheduling data field).

[0032] FIG. 3 discloses the structure of an uplink frame used in the IEEE 802.16a communication system. The ranging process of the IEEE 802.16a communication system using the OFDM scheme will be described below in conjunction with FIG. 4.

[0033] FIG. 4 is a flowchart showing a ranging process between the SS and the BS in the BWA communication system using the OFDM scheme.

[0034] Referring to FIG. 4, the SS 400 monitors all its own predetermined frequencies after receiving the power-on signal, so that it detects the pilot channel with the highest signal strength (ie the highest pilot CINR (Carrier to Interference and Noise Ratio)) signal. It is determined that the SS 400 belongs to the specific BS 420 that has transmitted the highest pilot CINR. The SS 400 receives the preamble of the downlink frame from the BS 420, thereby acquiring system synchronization with the BS 420.

[0035] After synchronization is established between the SS 400 and the BS 420, the BS 420 sends a DL_MAP message and a UL_MAP message to the SS 400 in steps 411 and 413, respectively. As shown in the previous Table 1, the DL_MAP message includes various information, for example, necessary information for establishing synchronization between SS 400 and BS 420 in the downlink direction and establishing the ability to be received and sent on the downlink channel. Configuration information of the physical channels of various messages of multiple SS 400. As shown in Table 2, the UL_MAP message notifies the SS 400 of SS scheduling interval information and physical channel configuration information.

[0036] The DL_MAP message is periodically broadcast from the BS to all SSs. In the case where the SS 400 can continuously receive periodically broadcast DL_MAP messages, it is assumed that the SS is synchronized with the BS. The SS receiving the DL_MAP message can receive the process

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All messages sent on the downlink channel.

[0037] As shown in the foregoing Table 3, if the SS causes an access failure, the BS sends a UCD message containing indication information of the available backoff value to the SS.

[0038] In the case of performing the above-mentioned ranging processing, the SS 400 sends an RNG_REQ (Ranging Request) message to the BS 420 in step 415. The BS 420 that has received the RNG_REQ message transmits an RNG_RSP (Ranging Response) message containing information for controlling various factors (for example, frequency, time, and transmission power) to the SS 400 in step 417.

[0039] The configuration of the RNG_REQ message is shown in Table 4:

[0040] Table 4:

[0041]

<td>syntax</td><td>size</td><td>Description</td>

<td>RNG-REQ Message Format() {</td><td></td><td></td>

<td>Management Message Type = 4</td><td>8-bit</td><td></td>

<td>Downlink Channel ID</td><td>8-bit</td><td></td>

<td>Pending Until Complete</td><td>8-bit</td><td></td>

<td>TLV Encoded Information</td><td>variable</td><td>Specific to TLV</td>

<td></td><td></td><td></td>

[0042] Referring to Table 4, the Downlink Channel ID (Downlink Channel ID) field indicates the downlink channel ID included in the RNG_REQ message received via UCD in the SS. The Pending Until Complete field indicates the priority information for sending the ranging response. Specifically, if the Suspend Until Completion field is set to "0", the preceding ranging response has priority. Conversely, if the pending-until-completed field is not set to "0", the current sending of the ranging response has priority.

[0043] Table 5 shows the detailed configuration of the RNG_RSP message of the RNG_REQ message shown in Table 4.

[0044] Table 5:

[0045]

<td>syntax</td><td>size</td><td>Description</td>

<td>RNGRSPMessageFormat(){</td><td></td><td></td>

<td>Management Message Type = 5</td><td>8-bit</td><td></td>

<td>Uplink Channel ID</td><td>8-bit</td><td></td>

<td>TLV Encoded Information</td><td>variable</td><td>Specific to TLV</td>

<td></td><td></td><td></td>

[0046] Referring to Table 5, the Uplink Channel ID (Uplink Channel ID) field indicates the uplink channel ID included in the RNG_REQ message.

[0047] FIG. 4 discloses the ranging process when the IEEE 802.16a communication system uses the OFDM scheme. The following describes the ranging process of the IEEE 802.16a communication system using the OFDMA scheme with reference to FIG. 5. In this case, the IEEE 802.16a communication system includes a dedicated ranging interval, so that the IEEE 802.16a communication system can perform ranging processing using the OFDMA scheme more effectively, so that it can perform ranging processing based on the dedicated ranging interval. The ranging code sending scheme sends Ranging-Code (Ranging Code) instead of RNG_REQ message.

[0048] Referring to FIG. 5, the BS 520 sends a DL-MAP message and a UL_MAP message to the SS 500 in steps 511 and 513, respectively. The specific operations of steps 511 and 513 are the same as steps 411 and 413. The communication system using the OFDMA scheme shown in FIG. 5 transmits a ranging code instead of the RNG_REQ message described in FIG. 4 in step 515. BS that received the ranging code

520 sends an RNG_RSP message to SS 500 in step 517.

[0049] New information must be added to the RNG_RSP message so that the information corresponding to the ranging code sent to the BS can be

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Enough to be recorded in the RNG_RSP message. The aforementioned new information to be added to the RNG_RSP message consists of the ranging code (ie, the received ranging CDMA code), the ranging symbol (ie, the OFDM symbol in the received ranging CDMA code), and the permutation code. The channel (that is, the permutation sub-channel in the received permutation CDMA code) and the ranging frame number (that is, the frame number in the received ranging CDMA code) are composed.

[0050] As described above, the IEEE 802.16a communication system works on the basis of the fixed state of the current SS (that is, without considering the mobility of the SS) and the single-cell structure. However, the IEEE 802.16e communication system has been defined as a system that considers the mobility of the SS in the IEEE 802.16a communication system. Therefore, the IEEE 802.16e communication system must consider the mobility of the SS in a multi-cell environment. In order to provide the mobility of the SS in a multi-cell environment, it is necessary to switch the respective operation modes of the SS and the BS. However, the IEEE 805.16e communication system has not yet proposed a new method for the mobility of SS in a multi-cell environment. In short, it is necessary to develop a handoff system that takes into account the idle state and communication service execution mode to provide mobility to the SS of the IEEE 802.16e communication system.

SUMMARY OF THE INVENTION

[0052] Therefore, the present invention has been made from the perspective of the above-mentioned problems, and an object of the present invention is to provide a system and method for performing a handover operation in a BWA communication system.

[0053] Another object of the present invention is to provide a system and method for performing a handover operation after receiving an SS request signal in a BWA communication system.

[0054] According to an aspect of the present invention, there is provided a broadband wireless access BWA communication system composed of a serving base station BS and a plurality of neighbor base stations adjacent to the serving BS, when receiving from a subscriber station SS After the handover request signal is reached, the SS handover method includes the following steps: a) receiving information related to multiple neighbor BSs from the serving BS; b) after receiving the information related to the neighbor BS, measuring The carrier-to-interference and noise ratio CINR of the pilot signal sent from the neighbor BS; c) the handover request signal and the pilot signal CINR information of the neighbor BS are sent to the serving BS; d) The serving BS receives information from a target BS among the neighbor BSs; and e) performs a handover function from the serving BS to the target BS.

[0055] According to another aspect of the present invention, there is provided a broadband wireless access BWA communication system composed of a serving BS and a plurality of neighbor BSs adjacent to the serving BS. After the zone handover request signal, the serving BS cross-zone handover method includes the following steps: a) sending information related to the neighbor BS to the SS; b) receiving a pilot signal containing the neighbor BS from the SS The handover request signal of the signal carrier-to-interference and noise ratio CINR information; c) Determine whether the neighbor BS included in the handover request signal can support the function of the SS handover, and can never support Select a target BS among the neighbor BSs of the handover function of the SS as the handover target of the SS; and d) send the response signal related to the handover request signal of the SS and the target together BS information, and notify the target BS of the handover ready status of the SS.

[0056] According to another aspect of the present invention, there is provided a broadband wireless access BWA communication system composed of a serving base station BS and a plurality of neighbor BSs adjacent to the serving BS. After the handover request signal, the SS handover method includes the following steps: a) Receive information related to multiple neighbor BSs and cross-area handover condition information from the serving BS; b) After receiving the neighbor BS information, measure The carrier-to-interference and noise ratio CINR of the pilot signal transmitted from the neighbor BS; c) Select multiple candidate BSs corresponding to the handover condition information from the neighbor BS, and combine the handover request signal with the The pilot signal CINR information of the candidate BS is sent to the serving BS together; d) after receiving the handover request signal, the serving BS receives information from the target BS among the candidate BSs; and e) Perform a handover function from the serving BS to the target BS.

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[0057] According to another aspect of the present invention, there is provided a broadband wireless access BWA communication system composed of a serving base station BS and a plurality of base stations adjacent to the serving BS. After reaching the handover request signal, the handover method includes the following steps: a) controlling the serving BS to send neighbor BS information and handover condition information to the SS; b) according to the neighbor BS Information, controlling the SS to measure the carrier-to-interference and noise ratio CINR of the pilot signal transmitted from the neighbor BS; c) controlling the SS to determine multiple candidate BSs corresponding to the handover condition information from the neighbor BS, and Send the handover request signal together with the pilot CINR information of the candidate BS to the serving BS; d) If the serving BS receives the handover request information from the SS, it is determined to be included in the handover request signal Whether the neighbor BS can support the handover function for the SS, and select the target BS from the candidate BSs that can support the handover function for the SS as the handover target of the SS; e) control the service The BS sends the response signal related to the handover request signal to the SS together with the target BS information, and informs the target BS of the handover ready state of the SS; and f) according to the handover The target BS signal in the request response signal Information, controlling the SS to perform a handover operation from the serving BS to the target BS.

[0058] According to another aspect of the present invention, there is provided a broadband wireless access BWA communication system consisting of a serving base station BS and a plurality of neighbor BSs adjacent to the serving BS. After the handover request signal, the handover device includes: a serving BS, the serving BS sends neighbor BS information and handover condition information to the SS, and determines after receiving the handover request signal from the SS Whether the neighbor BS included in the handover request signal can support the handover function for the SS, and select the target BS from the candidate BSs that can support the handover function for the SS as the SS A handover target, sending a response signal related to the handover request signal to the SS together with information of the target BS, and notifying the target BS of the handover ready status of the SS; And SS, the SS measures the carrier-to-interference and noise ratio CINR of the pilot signal transmitted from the neighbor BS according to the information of the neighbor BS, selects multiple candidate BSs corresponding to the handover condition information from the neighbor BS, and will The zone switching request signal is sent to the serving BS together with the pilot signal CINR information of the candidate BS, and the zone switching from the serving BS to the target BS is performed according to the target BS information contained in the handover request response signal Switch operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Through the following detailed description in conjunction with the accompanying drawings, the above and other objectives, characteristics and advantages of the present invention will become easier to understand, in which:

[0061] FIG. 1 is a block diagram showing a conventional BWA (Broadband Wireless Access) communication system using the OFDM/OFDMA scheme;

[0062] FIG. 2 is a conceptual diagram showing a conventional downlink frame structure used in a BWA communication system using the OFDM/OFDMA scheme;

[0063] FIG. 3 is a conceptual diagram showing a conventional uplink frame structure used in a BWA communication system using the OFDM/OFDMA scheme;

[0064] The flowchart of FIG. 4 shows the conventional ranging process between the SS and the BS in the BWA communication system using the OFDM scheme;

[0065] FIG. 5 is a flowchart showing a conventional ranging process between the SS and the BS in the BWA communication system using the OFDMA scheme;

[0066] FIG. 6 is a block diagram showing a BWA communication system that uses an OFDM/OFDMA scheme to perform various functions according to a preferred embodiment of the present invention;

[0067] FIG. 7 is a flowchart showing a BWA communication system using an OFDM scheme according to a preferred embodiment of the present invention.

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Handover processing in the system after receiving a handover request from the SS;

[0068] FIG. 8 is a flowchart showing a handover process on the basis of receiving a handover request from an SS in a BWA communication system using an OFDMA scheme according to a preferred embodiment of the present invention;

[0069] FIG. 9 is a block diagram showing the internal configuration of the SS for performing inventive functions according to a preferred embodiment of the present invention;

[0070] FIG. 10 is a flowchart showing the operation of the SS according to a preferred embodiment of the present invention; and

[0071] FIG. 11 is a flowchart showing the operation of a serving BS according to a preferred embodiment of the present invention.

Detailed ways

[0072] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same or similar elements are denoted by the same reference numerals, even if they are described in different drawings. In the following description, when the description of known functions and configurations introduced here makes the subject of the present invention obscure, the description will be omitted.

[0073] FIG. 6 is a block diagram showing a BWA communication system that uses an OFDM/OFDMA scheme to perform various functions according to a preferred embodiment of the present invention.

[0074] Before describing the BWA communication system shown in FIG. 6, it should be noted that the present invention adopts the IEEE 802.16e communication system using the OFDM/OFDMA scheme as a representative example. It should also be noted that the IEEE 802.16e communication system, which is a communication system in which SS mobility is considered in the IEEE 802.16a communication system, has not been developed yet. Assuming that the mobility of the SS is taken into account in the IEEE 802.16a communication system, then the multi-cell structure and the handover operation of the SS between multiple cells (that is, the cell selection operation) can be considered. Therefore, the present invention provides the IEEE 802.16e communication system shown in FIG. 6.

[0075] Referring to FIG. 6, the IEEE 802.16e communication system includes a multi-cell structure, that is, multiple cells 600 and 650. More specifically, the IEEE 802.16e communication system includes a first BS 610 for the management unit 600, a second BS 640 for the management unit 650, and a plurality of SS 611, 613, 630, 651, and 653. Use the OFDM/OFDMA scheme to establish signal transmission/reception between BS 610 and 640 and SS 611, 613, 630, 651 and 653. Among the user stations 611, 613, 630, 651, and 653, the user station 630 is located in the boundary area or the handover area between the cell 600 and the cell 650. Therefore, the cross-zone handover of the user station 630 must be supported in order to support the mobility of the user station 630.

[0076] Generally and according to an embodiment of the present invention, the SS used in the BWA communication system receives multiple signals from multiple BSs. The SS detects each CINR (Carrier to Interference and Noise Ratio) of the received pilot signal. The SS selects a specific BS that has transmitted the pilot channel with the highest CINR among the CINRs of the pilot signals, and determines the selected BS as the serving BS (that is, the active BS) to which the SS belongs. In more detail, the SS selects a BS with the best reception status from a plurality of BSs that send pilot signals, and recognizes the selected BS as its own serving BSo, for ease of description, it can be used for illustrative purposes. The term "active BS" or "serving BS" is used in the present invention.

[0077] The SS that has selected the active BS receives a downlink frame and an uplink frame from the active BS. The detailed structure of the downlink frame and the uplink frame received from the active BS has been disclosed in the prior art, and therefore, for the convenience of description, their description is omitted here. The present invention must add a new IE (information element) to the DL_MAP message provided by the IEEE802.16a/IEEE802.16e communication system, so as to support the handover operation after receiving the request from the SS. Table 6 below shows the detailed configuration of the DL_MAP message used to support the handover operation after receiving the request of the SS:

[0078] Table 6:

[0082]

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<td>MAC management message</td><td>parameter</td><td>Number (mu 11 i)</td><td>.content</td>

<td rowspan="10">DL.MAP7-1</td><td>Neighbor list BSNum</td><td>Share</td><td>Number of neighbor list BS</td>

<td>Neighbor listInfo</td><td>1 to Neighbor list BS Num</td><td></td>

<td>Neighbor list BSID</td><td></td><td>ID of the neighbor list BS</td>

<td>NeighborFrequency</td><td></td><td>Frequency of neighbor BS</td>

<td>NeighborFrequencyOffset</td><td></td><td>Frequency offset of neighbor BS</td>

<td>Neighbor FrameOffset</td><td></td><td>Frequency offset of neighbor list BS</td>

<td>MeasurementInfo</td><td>Share</td><td></td>

<td>Pilot tnin CINR</td><td></td><td>The smallest possible value contained in the neighbor list</td>

<td>MAX-T</td><td></td><td>The maximum time that the BS included in the neighbor list stays below the Pilot min CINR</td>

<td>MIN-T</td><td></td><td>The minimum time that the BS with the highest pilot CINR among the BSs in the neighbor list remains at a value higher than the pilot CINR of the active BS</td>

[0079] Refer to Table 6 above, (by^uNeighbor list BS Num indicates the number of neighbor BSs that are included in the neighbor list. The neighbor list field refers to a list of neighbor BSs where the BS appears. "Neighbor list Info" means the neighbor BS included in the neighbor list, that is, multiple neighbor BSs from the first neighbor BS to the last neighbor BS. The neighbor list information indicated by "Neighbor listinfo" includes the neighbor list

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A BS ID (Neighbor list BS ID) field, a Neighbor Frequency (Neighbor Frequency) field, a Neighbor Frequency Offset (Neighbor Frequency Offset) field, and a Neighbor Frame Offset (Neighbor Frame Offset) field. The Neighbor list BS ID field indicates the ID of each BS included in the neighbor list. The NeighborFrequency field indicates the center frequency of the corresponding neighbor BS. The Neighbor Frequency Offset field indicates the frequency offset of the corresponding neighbor BS. The Neighbor Frame Offset field indicates the frame offset of the corresponding neighbor BS. The Measurement Info field includes a minimum pilot CINR (Potmin CINR) field, a maximum time (MAX_T) field, and a minimum time (MIN_T) field. The POT minCINR field is used as a reference from IEEE 802. The reference of the neighbor BS that can be included in the Neighbor list Info field is selected among multiple BSs of the 16e communication system. In more detail, only BSs that transmit pilot signals each higher than Pot min CINR can be used as neighbor BSs included in the neighbor list. The MAX_T field indicates the maximum time that each neighbor BS is allowed to have a pilot signal lower than Pot min CINR. In more detail, the neighbor BS must send a pilot signal higher than Pot min CINR. If a pilot signal lower than Piolot min CINR is sent during the MAX_T time period, the corresponding neighbor BS is deleted from the neighbor list. In this way, if the neighbor BS that can transmit the pilot signal with Pot min CINR within the MAX_T time period has been deleted from the neighbor list, then the SS does not need to measure the unnecessary pilot CINRo MIN_T time The minimum time that the highest pilot CINR among the multiple pilot CINRs from the neighbor BS must be higher than the pilot CINR of the active BS. In more detail, in order to control the handover of the SS from the active BS to a specific neighbor BS (ie, the target BS), the CINR of the pilot signal sent from the target BS must be high for more than MIN_T time. For other pilot signals sent from the active BS CINR to avoid the ping-pong phenomenon. In the ping-pong phenomenon, whenever the CINR of the received pilot channel is higher than the CINR of the active base station, the user station sends a cross-area handover request to the base station. In this case, it should be noted that the MAX" time and the MIN_T time can be determined differently according to the status and channel condition information of each BS in the IEEE 802.16e communication system.

[0080] The SS that has received the DL_MAP message and the UL_MAP message requests a ranging request step for the current BS for ranging processing, and the ranging response step for sending a response signal to the ranging correction request to the SS that has sent the ranging request is the same as These steps in the prior art are the same, therefore, for the convenience of description, the related description is omitted here. Successfully established wireless communication between the SS for ranging processing and the current BS. The handover process in response to the SS request signal during the wireless access communication time period between the SS and the active BS will be described below with reference to FIG. 7.

[0081] FIG. 7 is a flowchart showing a handover process after a handover request is received from an SS in an IEEE 802.16e communication system using an OFDM scheme according to a preferred embodiment of the present invention.

[0082] Referring to FIG. 7, after the system synchronization is established between the SS 701 and the serving BS (also referred to as the active BS) 702, the serving BS 702 sends a DL_MAP message and a UL_MAP message to the SS 701 in steps 711 and 712, respectively. The DL_MAP message format and the UL_MAP message format are disclosed in Table 6 and Table 2, respectively, so detailed descriptions thereof will be omitted here. The SS 701 receives the DL_MAP message to detect the information of the neighbor BS of the SS 701, so that the SS 701 can measure the CINRo of the pilot signal received from the neighbor BS in step 731

[0083] The SS 701 suspends receiving the transmission data of the serving BS 702. In detail, the SS 701 measures the CINR of the pilot signal received from the neighbor BS during the interruption of the reception of the data transmitted by the serving BS702_OIn this case, the SS 701 does not measure the CINR of each pilot signal received from all neighbor BSs included in the neighbor list of the DL_MAP message, and is only used to measure the pilot signal received by the neighbor BS that exceeds the MIN_T time. CINR of frequency channel signal_OThe method of measuring the CINR of the pilot signal received from the neighbor BS will be described in detail below.

[0084] The SS 701 uses various information of the neighbor BS contained in the neighbor list of the DL_MAP message to establish a relationship with each

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Synchronization of neighbor BS. After establishing synchronization with the neighbor BS, the SS 701 measures the CINRo of the pilot signal received from the neighbor BS

[0085] The SS 701, which has measured the CINR of the pilot signal received from the neighbor BS, determines whether it needs to handover to another BS instead of handover to the serving BS 702. The handover steps of SS 701 will be described in detail below. At least one of the CINRs of the pilot signals received from the neighbor BS must satisfy the MAX_T condition. Assuming that in a period longer than MAX_T, the CINR of the pilot signal received from the neighbor BS included in the neighbor list is less than the minimum pilot CINR indicated by "Pot min CINR", then the corresponding pilot The signal will be deleted from the neighbor list, so the MAX_T condition must be met. In this case, each neighbor BS that has met the MAX_T condition is called a candidate BS (ie, target BS). If the MAX_T condition has been met, the SS 701 determines whether the CINR of the pilot signal transmitted from the neighbor BS is greater than the CINR of the pilot signal received from the serving BS 702. If it is determined that the CINR of the pilot signal received from the serving BS 702 is greater than the respective CINRs of the pilot signals received from the neighbor BS, the SS702 does not request the serving BS 702 for the handover operation. Conversely, if it is determined that the CINR of the pilot signal received from the serving BS is less than the CINR of the pilot signal received from the neighboring BS, then the SS 701 reports to the serving BS 702 requests a handover operation. Needless to say, the SS 701 requests the serving BS 702 for a handover operation only when it finds a neighbor BS that meets the MIN_T condition. The reason why the SS 701 requests a handover operation from the serving BS 702 only when the MIN_T time condition is met is to avoid the aforementioned ping-pong phenomenon.

[0086] If the handover operation is determined, the SS 701 sends a handover request (HO_REQ) message to the serving BS 702 in step 713. Table 7 shows the format of the H0_REQ message:

[0087] Table 7:

[0088]

<td>MAC management message</td><td>parameter</td><td>Number (multi)</td><td>content</td>

<td rowspan="5">HO-REQ</td><td>Uplink Channel ID</td><td>Share</td><td>Uplink channel ID included in the message</td>

<td>Neighbor list BS carrier frequency</td><td>1 To Neighbor list BS Num</td><td>Carrier frequency of neighbor BS</td>

<td>CNIR of Neighbor list BS</td><td></td><td>Pilot CINR of the neighbor list BS</td>

<td>QoS</td><td>Share</td><td>MS service category information</td>

<td>BW request</td><td>Share</td><td>MS's BW information</td>

[0089] Before describing Table 7, it should be noted that after receiving the handover request from the SS 701, the neighbor BS acts as the target BS. Therefore, after the SS has generated the handover request, they are the same as the target BS. Referring to FIG. 7, the Neighbor list BS carrier frequency (Neighbor list BS carrier frequency) field indicates the carrier frequency of the neighbor BS (ie, the target BS) that has received the handover request from the SS 701. CNIR of Neighbor List BS (CNIR of Neighbor

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The list BS) field indicates that the CINRo QoS field of the pilot signal sent from the neighbor BS indicates the quality of service (QoS) expected by the SS 701. The BW request (BW request) field indicates the requested bandwidth in response to the QoS expected by the SS 701. In this case, QoS consists of various service levels, namely, Unsoliciated Grant Sercice (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS) and best effort service (Best Effort). Service, BE)_oThe serving BS 702 that has received the handover request message from the SS 701 arranges the neighbor BSs included in the handover request message. There are various possible ways to arrange neighbor BSs. As described above, the CINR of the neighbor BS that stops using the SS 701 to measure the CINR of the pilot signal is set to "0". The serving BS 702 may configure the information of the arranged neighbor BSs in the form of a list, and may store the information in the list format.

[0090] The serving BS 702 that arranges the neighbor BSs sequentially sends HO_CONECTION_REQ (cross-zone handover connection request) messages to the corresponding neighbor BS (ie, the target BS) in steps 714 and 716 according to the order of the neighbor BSs. The format of the H0_ CONNECTI0N_REQ message is shown in Table 8 below:

[0091] Table 8:

[0092]

<td>MAC management message</td><td>parameter</td><td>Number (multi)</td><td>content</td>

<td rowspan="4">HO-CONNECTION-REQ</td><td>Target BS ID</td><td>Share</td><td>ID of the target BS</td>

<td>CID</td><td>Share</td><td>CID of terminal in serving BS</td>

<td>QoS</td><td>Share</td><td>MS service category information</td>

<td>BW request</td><td>Share</td><td>BW that will be assigned to MS</td>

[0093] Referring to Table 8, the target BS ID (Target BS ID) represents ID information of the target BS. CID (Connection ID) is a connection ID assigned from the serving BS 702 to the SS 701. The QoS field indicates the quality of service (QoS) expected by the SS 701. The BW request field indicates the requested bandwidth in response to the QoS expected by the SS 701. The reason why the serving BS 702 transmits the H0_CONECTION_REQ message to the target BS (ie, the target BS1 703 and the target BS2 704) is to determine whether the QoS and bandwidth related to the service expected by the SS 701 can be satisfied.

[0094] If the target BSs 703 and 704 receive the HO_CONNECTION_REQ message, they send HO_CONNECTION_RSP (Handover Connection Response) messages to the serving BS702 in steps 715 and 717. Table 9 below shows an example of the H0_CONNECT10N_RSP message format:

[0095] Table 9:

[0096]

<td>MAC management message</td><td>parameter</td><td>Multi</td><td>content</td>

<td rowspan="3">H0_C0NNECTI0N_RSP</td><td>Target BS ID</td><td>Share</td><td>ID of the target BS</td>

<td>CID</td><td>Share</td><td>CID of terminal in serving BS</td>

<td>ACK/NACK</td><td>Share</td><td>H0 approved or disapproved</td>

[0097] Referring to Table 9, the target BS ID (Target BS ID) field indicates the target of sending H0_CONNECTION_RSP

ID information of the BS, CID is the connection ID of the serving BS 702 for the SS 701. The ACK/NACK field indicates the SS 701

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Η0 (cross-zone handover) Approval or disapproval of information. In more detail, in the case where the ACK message is included in the HO_CONNECTION_RSP message, the corresponding target BS can provide the SS 701 with the handover function. In the case where the NACK message is included in the HO_CONNECTION_RSP message, the corresponding target BS cannot provide the SS 701 handover function. As shown in FIG. 7, the target BS 703 sends a HO_CONNECTION_RSP message including a NACK field indicating that the handover of the SS 701 cannot be supported. It is assumed that the target BS2 704 sends a HO_CONNECTION_RSP message including an ACK field indicating that the handover of the SS 701 can support the state.

[0098] The H0_CONNECTION_REQ message and the H0_CONNECTION_RSP message are sent sequentially as shown in FIG. 7, but they can also be sent at the same time when needed.

[0099] After receiving the HO_CONECTION_RSP message from the target BS, the serving BS 702 detects the ACK/NACK field contained in the received HO_CONECTION_RSP message, thereby sending the target BS that has sent the HO_CONNECTION_RSP message containing the ACK field (ie, The target BS2 704) is set as the last target BS to which the SS 701 will be handed over. Thereafter, the serving BS 702 sends a HO_RSP (Handover Response) message containing the information of the last target BS to the SS 701 in step 718. The HO_RSP message includes the information of the last target BS to which the SS 701 will be handed over. Table 10 shows an example of the HO_RSP message:

[0100] Table 10:

[0101]

<td>MAC management message</td><td>parameter</td><td>Multi</td><td>content</td>

<td rowspan="2">HO-RSP</td><td>Target BS ID</td><td>Share</td><td>ID of the target BS</td>

<td>Target BS carrier frequency</td><td>Share</td><td>Carrier frequency of target BS</td>

[01021 Referring to Table 10, the Target BS ID (Target BS ID) field indicates the ID information of the last target BS to which the SS 701 will handover. Target BS carrier frequency (Target BS carrierfrequency) represents the carrier frequency of the last target BS. The serving BS 702 that has sent the HO_RSP message sends a HO_CONNECTION_CFM (Handover Connection Confirmation) message to the last target BS (ie, target BS 2704) in step 719. In this case, a HO_CONNECTION_CFM message indicating that the SS 701 will be handed over to the last target BS 704 is sent. Table 11 shows an example of the H0_CONNECT I ON_CFM message:

[0103] Table 11:

[0104]

<td>MAC management message</td><td>parameter</td><td>Multi</td><td>content</td>

<td rowspan="2">HO-CONNECTION.CFM</td><td>Target BS ID</td><td>Share</td><td>ID of the target BS</td>

<td>CID</td><td>Share</td><td>Terminal ID in the serving BS</td>

[0105] Referring to Table 11, the target BS ID (Target BS ID) field indicates the ID information of the neighbor BS (that is, the last target BS) that receives the HO_CONNECTION_CFM message. The CID represents the connection ID allocated from the serving BS 702 to the SS 701. The serving BS 702 that has sent the HO_CONNECTION_CFM message to the target BS2 704 releases the connection to the

SS 701 link.

[0106] The target BS2 704 sends the DL_MAP message and the UL_MAP message to the SS 70L in steps 720 and 721, respectively.

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The DL_MAP message and the UL_MAP message contain the update message of the SS 701. The SS 701 that has received the DL_MAP message and the UL_MAP message sends an RNG_REQ (Ranging Request) message to the target BS2 704 in step 722. The target BS2 704 that has received the RNG_REQ message sends the RNG_RSP message as a response message related to the RNG_REQ message to the SS 701 in step 723. The detailed processing performed in steps 720 to 723 is the same as steps 411 to 417 shown in FIG. 4. Therefore, a detailed description of these steps will not be given here.

[0107] FIG. 7 discloses a handover process performed after receiving a handover request from the SS in the IEEE 802.16e communication system using the OFDM scheme. The flowchart of FIG. 8 shows the handover process performed after receiving the handover request from the SS in the IEEE 802.16e communication system using the OFDMA scheme according to a preferred embodiment of the present invention.

[0108] Before describing FIG. 8 in detail, it should be noted that steps 811 to 821 and steps 831 to 834 shown in FIG. 8 are respectively the same as steps 711 to 721 and steps 731 to 734 shown in FIG. , Its detailed description will be omitted. Since the OFDMA scheme is applied to FIG. 8, the SS 801 transmits a ranging code instead of the RNG_REQ message to the last target BS (ie, target BS2 804) in step 822. The target BS2 804 that has received the ranging code transmits an RNG_RSP message as a response related to the ranging code to the SS 801 in step 823. More specifically, the handover procedure of FIG. 7 is basically the same as the handover procedure of FIG. 8, but the IEEE 802.16e communication system transmits the RNG_REQ message according to the OFDM scheme of FIG. 7, and transmits the RNG_REQ message according to the OFDMA scheme of FIG. 8. Ranging code. FIG. 8 discloses the handover process performed after receiving a handover request from the SS in the IEEE 802.16e communication system using the OFDMA scheme. The internal configuration of the SS used to implement the present invention will be described below with reference to FIG. 9.

[0109] FIG. 9 is a block diagram showing the internal configuration of an SS for performing the functions of the present invention according to a preferred embodiment of the present invention. Referring to FIG. 9, the internal configuration of the SS includes a matched filter 900, a received power measurement unit 910, a received power comparator 920, a controller 930, and a transmitter 940. The matched filter 900 determines whether the PN (pseudo-noise) code used for synchronization detection is synchronized, and outputs a predetermined energy value according to the result of the determination. In this case, the correlator may be used as the matched filter 900. The matched filter 900 compares the received synchronization detection PN code with the unique PN code of the SS, and outputs the energy value associated with the same PN code. Specifically, the matched filter 900 sequentially puts the received signals of the SS in a predetermined window, and performs a predetermined bit operation in parallel with the unique PN code value, thereby generating a sum of bit operation values. Therefore, if the received signal of the SS is equal to the value of the unique PN code stored in the SS, this state is called an auto-correlation state, resulting in the maximum energy value. If the SS received signal is different from the value of the unique PN code, this state is called an inconsistent synchronization state, resulting in a relatively low energy value. Generally, the output energy value is compared with a predetermined threshold value in order to determine whether an autocorrelation state is established. In short, the output value of the matched filter 900 is compared with the predetermined threshold value, so that the presence or absence of the autocorrelation state can be determined.

[0110] If the received power measurement unit 910 is synchronized with the pilot signal received from the neighbor BS through the matched filter, the received power measurement unit 910 measures the received power of the received pilot signal. The received power measurement unit 910 measures the CINR of the received pilot signal, and sends the measured CINR of the pilot signal to the received power comparator 920. After receiving the CINR of the pilot signal from the received power measurement unit 910, the received power comparator 920 compares the received CINR of the pilot signal of the neighbor BS with a predetermined threshold (ie, Pot min CINR). If at least one pilot signal CINR among the pilot signals CINR transmitted from the neighbor BS is greater than the threshold, the received power comparator 920 proceeds to the next step. More specifically, it is determined whether the CINR of at least one pilot signal among the CINRs of the pilot signals of the neighbor BS is greater than the CINR of the pilot signal of the serving BS_OIf it is determined that the neighbor BS whose pilot signal CINR is greater than the CINR of the serving BS's pilot signal is determined, then the CINR of its pilot signal is greater than that of the serving BS's pilot signal.

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The information of the neighbor BS of the CINR will be sent to the controller 930. The controller 930 determines whether the SS sends its own handover request to the serving BSo according to the information received from the received power comparator 920

[0111] More specifically, if the CINR of the pilot signal transmitted from the neighbor BS is greater than the CINR of other pilot signals transmitted from the serving BS, the controller 930 generates the handover request signal of the SS. In this case, although it is not described in the drawings, it should be noted that in the handover process, the MAX_T and MIN_T conditions must be satisfied. The controller 930 generates a handover request (H0_REQ) message by controlling the transmitter 940, and sends the generated handover request message to the serving BSo. The transmitter 940 generates a H0_REQ message after receiving the control command from the controller 930, and sends The H0_REQ message is sent to the serving BS.

[0112] FIG. 9 discloses the internal configuration of the SS according to the present invention. The operation of the SS will be described below with reference to FIG. 10. Fig. 10 is a flowchart showing the operation of the SS according to a preferred embodiment of the present invention. 10, the SS reads the DL_MAP message received from the serving BS in step 1000, reads the UL_MAP message received from the serving BS in step 1002, and reads the neighbor BS list information contained in the DL_MAP message in step 1004. If the SS reads the neighbor BS information in step 1004, it sets the ID "i" representing the number of neighbor BSs to "0" (ie, i=0) in step 1004.

[0113] The SS adds the number 1 to the variable "i" (ie, i=i+1), measures the CINR of the pilot signal sent from the i-th neighbor BS in step 1006, and proceeds to step 1008. In this case, it is assumed that the number of neighbor BSs transmitted from the serving BS is in the range from "BS_1" to U "MAX_BS_NUM", therefore, the SS measures the CINR of the pilot signal of the "BS_1" neighbor BS in step 1006 , And it is determined in step 1008 whether the number of neighbor BSs whose pilot signal CINR has been measured is equal to or greater than "MAX_BS_NUM". If it is determined in step 1008 that the number of measured neighbor BSs is equal to or greater than "MAX_BS_NUM", the SS proceeds to step 1010. Conversely, if it is determined in step 1008 that the number of neighbor BSs to be measured is less than "MAX_BS_NUM", the SS returns to step 1006.

[0114] If it is determined that the value of MAX_Neighbor BS CINR is equal to or less than the CINR of the pilot signal of the serving BS, the SS returns to step 1000. On the contrary, if it is determined that the value of MAX_Neighbor BS CINR is greater than the CINR of the pilot signal of the serving BS, then SS determines its own handover request, and proceeds to step 1012ο

[0115] The SS sends the HO_REQ message to the serving BS in step 1012. The detailed configuration of the H0_REQ message is disclosed in Table 7 above. The SS receives the HO_RSP message related to the HO_REQ message from the serving BS in step 1014, and then proceeds to step 1016. The detailed configuration of the HO_RSP message is disclosed in Table 10 above. The SS reads the ID and carrier frequency information of the last target BS from the H0_RSP message in step 1016, and proceeds to step 1018. In this case, the last target BS is the specific BSo that the SS is handed over to.

[0116] The SS converts the SS frequency to the target BS frequency, and proceeds to step 1020, thereby interrupting data transmission/reception between the SS and the serving BS, and the SS can communicate with the last target BS. The SS reads the DL_MAP message received from the last target BS in step 1020, and proceeds to step 1022. The SS reads the UL_MAP message received from the last target BS and proceeds to step 1024. In step 1024, the SS communicates with the last target BS and ends its handover operation.

[0117] FIG. 10 discloses the operation of the SS according to the present invention. The operation of the serving BS will be described below with reference to FIG. 11. FIG. 11 is a flowchart showing the operation of a serving BS according to a preferred embodiment of the present invention. Serving BS in steps

1100 sends a DL_MAP message to the SS, and in step 1102 sends a UL_MAP message to the SS.

[0118] The serving BS receives the HO_REQ message from the SS in step 1104, and proceeds to step 1106. In the aforementioned table

The detailed configuration of the H0_REQ message is disclosed in 7. The serving BS arranges the pilot signals of neighboring BSs in order of decreasing amplitude

CINR, and go to step 1108. In this case, the CINR of the pilot signal of the neighbor BS is included in the H0_REQ message

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in. As described above, the neighbor BS used to order the SS to measure the pilot signal CINR acts as a candidate BS that satisfies the MAX_T and MIN_T conditions. Therefore, each candidate BS has a CINR greater than Pot min (ie, the minimum pilot signal CINR) signal of. As previously described in FIG. 10, it is assumed that the number of neighbor BSs of the SS is within the range from BS_1 to MAX_BS_NUM. In addition, in step 1106, the ID "i" representing the number of neighbor BSs is set to "0" (ie, i=0). Determine the IDo representing the number "i" of neighbor BSs according to the order of the pilot CINR

[0119] The serving BS sends the HO_CONNECTION_REQ message to the neighbor BSo with the largest pilot CINR among the arranged neighbor BSs in step 1108. The detailed configuration of the HO_CONNECTION_REQ message is shown in the foregoing Table 8.

[0120] In step 1110, the serving BS receives a H0_CONNECTION_RSP message indicating a response to the H0_CONNECTION_REQ message from the neighbor BS that has sent the HO_CONNECTION_REQ message, and proceeds to step 1112. The foregoing Table 9 discloses the detailed configuration of the H0_CONNECTI0N_RSP message. The serving BS determines whether any neighbor BS among the neighbor BSs that send the SS handover request signal can support the handover function of the SS. More specifically, the serving BS determines whether the ACK message is included in the H0_CONNECTION_RSP message. If it is determined that a specific neighbor BS among the neighbor BSs can support the handover service of the SS, the serving BS proceeds to step 1116. Conversely, if it determines that no neighbor BS can support the handover service of the SS, then the serving BS proceeds to step 1114. The serving BS selects the neighbor BS with the second largest pilot CINR among the neighbor BSs in step 1114. And add 1 to the variable "i" (i.e. i = i+1), and return to step 1108ο

[0121] The serving BS sends the HO_RSP message to the SS in step 1116 and proceeds to step 1118. The foregoing Table 10 has disclosed the detailed configuration of the H0_RSP message. In step 1118, the serving BS sends the H0_CONNECTION_CFM message to the neighbor BS capable of supporting the handover function (ie, the last target BS), and returns to step 1120. The foregoing Table 11 has disclosed the detailed configuration of the H0_CONNECTION_CFM message. Needless to say, the serving BS may first send the H0_CONNECTION_CFM message to the last target BS, or may send the H0_RSP message to the SS. The serving BS releases the link connected to the SS in step 1120 and ends its handover operation.

[0122] It can be clearly seen from the above description that the present invention enables an SS based on a multi-cell structure to perform a handover operation, thereby ensuring the mobility of the SS. More specifically, the traditional SS performs communication functions in a single cell, so when the SS moves to another cell, it must abandon the link connected to the BS currently communicating with the SS. In addition, the traditional SS must re-establish the communication mode with the new BS, and therefore, it must re-execute the initial ranging process. However, for the aforementioned problems of the conventional SS, the present invention performs handover processing without performing initial ranging on the new BS, thereby reducing the interruption time of data communication.

[0123] Although the preferred embodiments of the present invention have been described for the purpose of illustration, it is clear to those skilled in the art that, without departing from the scope and spirit of the present invention defined by the appended claims, various Modifications, additions and replacements are all possible.

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