Title of Invention	A METHOD FOR PERFORMING A SOFT HANDOVER IN A BROADBAND WIRELESS ACCESS COMMUNICATION SYSTEM
Abstract	This invention relates to a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the mobile station, schedule to be performed toward the active non-anchor base station, the method comprising the steps of; receiving, by the mobile station, one of first information, second information, and third information when a determination is made, that the mobile station is to perform the soft handover, wherein the first information received from the anchor base station, includes channel assignment information of the active non-anchor base station, the second information, received from the active non-anchor base station, includes channel assignment information of the anchor base station, the third information, received from the active non-anchor base station, represents a relationship between a connection identifier of the mobile station used in the anchor base station and a connection identifier used in the active non-anchor base station; and receiving a signal corresponding to the received information.

Title of Invention

A METHOD FOR PERFORMING A SOFT HANDOVER IN A BROADBAND WIRELESS ACCESS COMMUNICATION SYSTEM

Abstract

This invention relates to a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the mobile station, schedule to be performed toward the active non-anchor base station, the method comprising the steps of; receiving, by the mobile station, one of first information, second information, and third information when a determination is made, that the mobile station is to perform the soft handover, wherein the first information received from the anchor base station, includes channel assignment information of the active non-anchor base station, the second information, received from the active non-anchor base station, includes channel assignment information of the anchor base station, the third information, received from the active non-anchor base station, represents a relationship between a connection identifier of the mobile station used in the anchor base station and a connection identifier used in the active non-anchor base station; and receiving a signal corresponding to the received information.

Full Text	BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a broadband wireless access (BWA) communication system, and more particularly to a system and a method for performing a soft handover. 2. Description of the Related Art In the 4 generation (4G) communication system, which is the next generation communication system, extensive research is being conducted to provide users with services having various Qualities-of-Service (QoSs) at high speeds. In the current 4G communication system, extensive research is being conducted to develop a new type of communication system ensuring the mobility of a mobile station (MS) and a QoS in a BWA communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system capable of supporting relatively high transmission speeds. Representatives of such communication systems are the Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and the IEEE 802.16e communication system. The IEEE 802.16a/d-based communication system and the IEEE 802.16e-based communication system use an orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA) scheme for a physical channel of the WMAN system in order to support a broadband transmission network. The IEEE 802.16a communication system is a single structure system that does not reflect the mobility of a Subscriber Station (SS) at all, in which the SS is in a stationary state. In contrast, the IEEE 802.16e-based communication system is a system realized by supplementing the IEEE 802.16a-based communication system with the ability to handle the mobility of an SS. Hereinafter, a mobile SS will be referred to as a MS. Accordingly, in the IEEE 802.16e-based communication system, the study of handover according to the mobility of the MS is being activated. Handover refers to an operation enabling smooth provision of services to an MS which is moving between base stations (BSs). Such handover may have two schemes (a hard handover scheme and a soft handover scheme). The hard handover scheme and the soft handover scheme will be described. The hard handover scheme denotes a scheme through which, when an MS moves between base stations, the MS releases a channel setup with an anchor base station (a serving base station currently providing a service to the MS) and sets up a channel with an active non-anchor base station (a target base station) in the shortest period of time, so that the service may be continuously provided. The serving base and the target base station will be referred to as an anchor base station and an active non-anchor base station, respectively, for the purpose of description. The hard handover causes a ping-pong phenomenon of continuously alternating channel release and setup when channel states are continuously inferior, for example, when the MS moves across a cell boundary region. The soft handover scheme denotes a scheme through which, when an MS moves between base stations, the MS has channels set up with both an anchor base station and an active non-anchor base station, is relocated in the non-anchor base station while simultaneously receiving signals from both the anchor base station and the non-anchor base station through the setup channel, and then releases the channel from the anchor base station, so that the service is continuously provided. Accordingly, when the MS performs a handover operation through the soft handover scheme, the MS receives downlink signals from a plurality of base stations (i.e. the anchor base station and the non-anchor base station) and improves quality of the received downlink signals by combining the downlink signals received from the base stations. The signal combining scheme may be classified into a soft combining scheme and a selection diversity scheme. The soft combining scheme is a scheme of demodulating each of the signals received from a plurality of base stations and combining the signals in a front end of a channel decoder. The selection diversity scheme is a scheme of de-modulating and channel-decoding each of the signals received from a plurality of base stations and then selecting a signal with the best quality from among the channel-decoded signals. The MS performing a handover operation through the soft handover scheme may obtain a soft combining gain by simultaneously receiving signals through channels set up with both the anchor base station and the non-anchor base station. In order to obtain the combining gain, the MS must receive information regarding channels (channel allocation information) set up with both the anchor base station and the non-anchor base station and receive signals from the anchor base station and the non-anchor base station using the channel allocation information. However, in the typical broadband wireless access system (e.g., the IEEE 802.16e-based communication system), it is difficult for the MS to receive channel allocation information of an active non-anchor base station because an anchor base station reports only information regarding a channel allocated by the anchor base station. SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve at least the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a system and a method for performing a soft handover in a broadband wireless access communication system. Another object of the present invention is to provide a system and a method for providing channel allocation information used for performing a soft handover in a broadband wireless access communication system. Still another object of the present invention is to provide a system and a method in which an anchor base station informs a mobile station of channel allocation information of an active non-anchor base station in a broadband wireless access communication system. Yet another object of the present invention is to provide a system and a method in which an active non-anchor base station informs a mobile station of channel allocation information of an anchor base station in a broadband wireless access communication system. To accomplish the above objects, there is provided a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the MS scheduled to be performed toward the active non-anchor base station. The method comprises the steps of: receiving first information from the anchor base station or receiving second information considering channel state from the active non- anchor base station when it is determined that the mobile station is to perform the soft handover, the firet information including channel assignment information of the active non-anchor base station, the second information including channel assignment information of the anchor base station; and receiving signals from the anchor base station or the active non-anchor base station corresponding to the first information and the second information. According to another aspect of the present invention, there is provided a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the MS scheduled to be performed toward the active non-anchor base station. The method comprises the steps of: determining if there are signals to be transmitted to the mobile station from the active non-anchor base station when it is determined that the mobile station is to perform a soft handover; and transmitting information including channel assignment information of the active non-anchor base station when it is determined that there are signals to be transmitted to the mobile station from the active non-anchor base station. According to still another aspect of the present invention, there is provided a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the MS scheduled to be performed toward the active non-anchor base station. The method comprises the steps of: determining if there are signals to be transmitted to the mobile station from the anchor base station when it is determined that the mobile station is to perform a soft handover; and transmitting first information including channel assignment information of the anchor base station when it is determined that there are signals to be transmitted to the mobile station from the anchor base station. According to still yet another aspect of the present invention, there is provided a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the MS scheduled to be performed toward the active non-anchor base station. The method comprises the steps of: transmitting by the anchor base station first information including channel assignment information of the active non-anchor base station when it is determined that the mobile station is to perform the soft handover, and when there are signals to be transmitted to the mobile station from the active non- anchor base station; transmitting by the active non-anchor base station second information including channel assignment information of the anchor base station when it is determined that the mobile station is to perform the soft handover, and when there are signals to be transmitted to the mobile station from the anchor base station; and receiving by the mobile station signals from the anchor base station and the active non-anchor base station corresponding to the first information and the second information when it is determined that the mobile station is to perform a soft handover. According to still yet another aspect of the present invention, there is provided a system for performing a soft handover in a broadband wireless access communication system. The system comprises an anchor base station for transmitting first information including channel assignment information of an active non-anchor base station when it is determined that a mobile station is to perform a soft handover, and when there are signals to be transmitted to the mobile station from the active non-anchor base station; an active non-anchor base station for transmitting second information including channel assignment information of the anchor base station when it is determined that the mobile station is to perform a soft handover, and when there are signals to be transmitted to the mobile station from the anchor base station; and a mobile station for receiving signals from the anchor base station and the active non-anchor base station corresponding to the first information and the second information when it is determined that the mobile station is to perform the soft handover. According to still yet another aspect of the present invention, there is provided a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the MS scheduled to be performed toward the active non-anchor base station. The method comprises the steps of receiving a first MAP information which is in the DL- MAP of active non-anchor base station and indicates at least one burst from anchor base station or receiving a second MAP information which is in the DL- MAP of the anchor base station and indicates at least one burst from active non- anchor base station considering downlink channel state. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram illustrating the structure of an IEEE 802.de- based communication system according to an embodiment of the present invention; FIG. 2 is a block diagram illustrating the sector structure in an IEEE 802.16e-based communication system according to an embodiment of the present invention; FIG. 3 is a block diagram illustrating the frame structure in an IEEE 802.16e-based communication system according to an embodiment of the present invention; FIG. 4 illustrates the format of a DL-MAP message according to an embodiment of the present invention; FIG. 5 illustrates the format of a UL-MAP message according to an embodiment of the present invention; FIG. 6 illustrates the format of an HO Anchor Active DL MAP IE according to an embodiment of the present invention; FIG. 7 illustrates the format of an HO DL MAP IE in non-anchor BS according to an embodiment of the present invention; FIG. 8 illustrates the format of an HO CID Translation MAP IE according to an embodiment of the present invention; FIG. 9 illustrates the format of an HO Anchor Active UL MAP IE according to an embodiment of the present invention; FIG. 10 illustrates the format of an HO UL MAP IE in non-anchor BS according to embodiment of the present invention; FIG. 11 is a flowchart illustrating the operation of an MS performing soft handover by receiving channel assignment information from an anchor base station according to an embodiment of the present invention; and FIG. 12 is a flowchart illustrating the operation of an MS performing soft handover by receiving channel assignment information from an active non-anchor base station according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Note that the same or similar components in drawings are designated by the same reference numerals as far as possible although they are shown in different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. The present invention proposes a system and a method for performing a soft handover in a broadband wireless access communication system (BWA) (e.g., an Institute of Electrical and Electronics Engineers (IEEE) 802.16e-based communication system). Particularly, the present invention proposes a system and a method in which an anchor base station (a serving base station) and an active non-anchor base station (a target base station) provide channel allocation information in order to perform a soft handover in the IEEE 802.16e-based communication system. Hereinafter, the serving base station and the target base station will be referred to as an anchor base station and an active non-anchor base station, respectively. According to the present invention, although the IEEE 802.16e-based communication system is described by way of example for the purpose of description, it is natural that other communication systems as well as the IEEE 802.16e-based communication system can be employed. FIG. 1 is a block diagram illustrating a structure of an IEEE 802.de- based communication system according to an embodiment of the present invention. The IEEE 802.16e-based communication system having a multi-cell structure includes a cell 100, a cell 150, a base station 110 controlling the cell 100, a base station 140 controlling the cell 150, and a plurality of MSs 111, 113, 130, 151, and 153. Although one base station may control several cells, it is assumed that one base station controls only one cell for the purpose of description in the embodiment of the present invention. In addition, the base stations 110 and 140 transmit/receive signals to/from the MSs 111, 113, 130, 151, and 153 using the OFDM/OFDMA scheme. Herein, it is assumed that the base stations 110 and 140 have a multi-sector structure, sectors in the same cell use the same center- frequency, and adjacent sectors are assigned different sub-channels. The above description deals with the structure of the IEEE 802.16e-based communication system according to an embodiment of the present invention with reference to FIG. 1, and, hereinafter, a sector structure in the IEEE 802.16e-based communication system according to an embodiment of the present invention will be described with reference to FIG. 2. FIG. 2 is a block diagram illustrating the sector structure in the IEEE 802.16e-based communication system according to an embodiment of the present invention. A cell controlled by a base station is divided into several sectors (e.g., three sectors; a first sector 210, a second sector 220, and a third sector 230). Each of the first sector 210 to the third sector 230 employs the same center-frequency of F1, and different sub-channels are used between them. In other words, the first sector 210 to the third sector 230 use mutually-different sub-channel sets as shown in FIG. 2. The first sector 210, the second sector 220, and the third sector 230 use the first sub-channel set, the second sub-channel set, and the third sub- channel set, respectively. The above description deals with the sector structure in the IEEE 802.16e-based communication system according to an embodiment of the present invention with reference to FIG. 2, and, hereinafter, a frame structure in the IEEE 802.16e-based communication system according to an embodiment of the present invention will be described with reference to FIG. 3. FIG. 3 is a block diagram illustrating the frame structure in the IEEE 802.16e-based communication system according to an embodiment of the present invention. The frame structure in the IEEE 802.16e-based communication system includes a downlink (DL) frame 310 and an uplink (UL) frame 320. The DL frame 310 includes a preamble field 312, a DL-MAP/UL-MAP field 314, a PUSC (partial usage of sub-channels) field 316, and an FUSC (full usage of sub- channels) field 318. The UL frame 320 includes a PUSC field 322. The preamble field 312 is a field used for transmitting a preamble sequence for synchronization between a base station and an MS. The DL- MAP/UL-MAP field 314 is a field used for transmitting a DL-MAP message and an UL-MAP message. The base station transmits the MAP message indicating a channel interval allocated to each MS before the channel interval, and the MS detects information included in the MAP message so as to recognize the channel interval and a coding scheme. Detailed descriptions of the DL-MAP message and the UL-MAP message will be given later. The PUSC field 316 is a field used for transmitting DL data burst through a PUSC scheme, and the FUSC field 318 is a field used for transmitting DL data burst through an FUSC scheme. In addition, the PUSC field 322 is a field used for transmitting UL data burst using a PUSC scheme. . FIG. 4 illustrates the format of the DL-MAP message according to an embodiment of the present invention. The DL-MAP message includes various information elements (IEs), i.e., 'Management Message Type' representing a type of a transmitted message, 'Physical (PHY) Synchronization' set corresponding to a modulation scheme or a de-modulation scheme applied to a physical channel in order to acquire synchronization, 'DCD count' representing a count value corresponding to the configuration change of a downlink channel descriptor message including downlink burst profile, 'Base Station ID' representing a base station identifier, and n number of DL-MAP IEs. The DL-MAP message includes information of raging codes assigned for raging. FIG. 5 illustrates the form of the UL-MAP message according to an embodiment of the present invention. The UL-MAP message includes various IEs, i.e., 'Management Message Type' representing a type of a transmitted message, 'Uplink Channel ID' representing a used uplink channel identifier, 'UCD count' representing a count value corresponding to the configuration change of an uplink channel descript message including uplink burst profile, 'Allocation Start Time' representing information regarding start time to allocate uplink resources, and n number of UL-MAP IEs. The Uplink Channel ID is assigned as a unique value in a Medium Access Control (MAC)-sub layer. A data burst field of the DL-frame 310 shown in FIG. 3 includes time slots assigned according to MSs through a Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) scheme. A base station broadcasts control information to MSs (receiving services from the base station) through the DL-MAP/UP-MAP field 314 by using a preset center-frequency. As each of the MSs is powered on, each MS detects a reference signal having the maximum size (e.g., the maximum Carrier to Interference and Noise Ratio (CINR)) by monitoring all of the frequency bands preset in each MS. Herein, the reference signal may include a pilot signal. Each MS determines a base station having transmitted the reference signal having the highest CINR as a base station controlling the MS, analyzes a DL-MAP message and an UL-MAP message in the DL-MAP/UL-MAP field 314 of the DL frame 310 transmitted from the base station so as to determine channel allocation information indicating the interval at which real data burst and control information controlling the DL and the UL of the MS are transmitted/received, and transmits/receives data burst at the corresponding data burst transmit/receive interval. An IEEE 802.16e communication system in which one base station uses one center frequency and a plurality of sectors within the base station use as many sub-bands as the sectors divided from a frequency band corresponding to the center frequency will now be described. In such an IEEE 802.16e communication system, when an MS moves from a current anchor base station to another base station (an active non-anchor base station), or the MS moves from an anchor sector (a serving sector) to another sector (an active non-anchor sector; a target sector) within the same anchor base station, the MS must perform a handover. In particular, according to an embodiment of the present invention, the MS must perform a soft handover.In order to perform the soft handover described above, an MS must obtain channel allocation information of both an anchor base station (an anchor sector in the case of handover between sectors) and an active non-anchor base station (an active non-anchor sector in the case of handover between sectors). Herein, the channel allocation information is included in MAP messages transmitted to the MS (requesting soft handover) from the anchor base station or the non-anchor base station. In other words, all of the base stations of the IEEE 802.16e communication system broadcast channel allocation information of their neighboring base stations through the MAP messages. In the conventional IEEE 802.16e communication system, an anchor base station inserts the MAP IEs (including channel allocation information for data burst to be transmitted/received to/from an MS) into a MAP message to be transmitted, and the MS transmits/receives data burst through a UL/DL corresponding to the channel allocation information included in the MAP IEs. However, according to an embodiment of the present invention, the MS must receive channel allocation information from the anchor base station and the non- anchor base station so that the MS may simultaneously transmit/receive the same data burst from both the anchor base station and the non-anchor base station in an MS handover area. In particular, according to an embodiment of the present invention, it is possible to obtain a soft combining gain by decoding and buffering data burst from the anchor base station and the non-anchor base station having the same data burst without taking into consideration a soft combining scheme employing the same permulation, the same symbol, and the same sub-channel in a physical layer. Hereinafter, further description about the MAP IEs for the soft handover newly proposed in the present invention will be given. In the conventional IEEE 802.16e communication system, it is impossible for an MS to simultaneously receive the MAP IEs from two or more base stations. Accordingly, in an IEEE 802.16e communication system according to an embodiment of the present invention, an anchor base station and an active non- anchor base station insert the MAP IEs including channel allocation information of both the anchor base station and the non-anchor base station for the MS into their own MAP messages. Then, the MS receives the MAP IEs from one of the anchor base station and the non-anchor base station, obtains channel information regarding both the anchor base station and the non-anchor base station from the MAP IEs, and transmits/receives data burst from/to the anchor base station and the non-anchor base station corresponding to the obtained channel information. Hereinafter, a detailed description about the MAP IEs proposed in the present invention will be given.(l) A Handover Anchor Active MAP IE ('HO Anchor Active MAP IE'). The HO Anchor Active MAP IE is a MAP IE transmitted from an active non-anchor base station that includes channel allocation information of a DL or a UL of an anchor base station and indicates that there is data burst transmitted from an anchor base station. The HO Anchor Active MAP IE including the channel assignment information of the DL of the anchor base station corresponds to a HO Anchor Active DL MAP IE, and the HO Anchor Active MAP IE including the channel assignment information of the UL of the anchor base station corresponds to a HO Anchor Active UL MAP IE.(2) A Handover MAP IE in a non-anchor base station ('HO MAP IE in non-anchor BS'). The HO MAP IE in non-anchor BS is a MAP IE transmitted from an anchor base station that includes the channel allocation information of a DL or a UL of an active non-anchor base station and indicates that there is data burst transmitted from the non-anchor base station. The HO MAP IE in non-anchor BS including the channel assignment information of the DL of the non-anchor base station corresponds to 'HO DL MAP IE in non-anchor BS', and the HO MAP IE in non-anchor BS including the channel assignment information of the UL of the non-anchor base station corresponds to 'HO UL MAP IE in non-anchor BS'. (3) A Handover Connection Identifier Translation MAP IE ('HO CID Translation MAP IE') The HO CID Translation MAP IE is a MAP IE transmitted from an active non-anchor base station and is transmitted in order to map a CID of a MS allocated by an anchor base station with a CID of the MS allocated by an active non-anchor base station. In addition, the HO CID Translation MAP IE represents that there is data burst transmitted from the non-anchor base station. FIG. 6 illustrates the format of the HO Anchor Active DL MAP IE according to an embodiment of the present invention. The HO Anchor Active DL MAP IE includes 'Extended DIUC (Downlink Interval Usage Code) field used for distinguishing a plurality of IEs (i.e., the HO Anchor Active DL MAP IE), 'Length' indicating the length of the HO Anchor Active DL MAP IE, and IEs for each data burst. As shown in FIG. 6, the IEs for each data burst include 'Anchor Preamble' representing a preamble of the anchor base station, 'Anchor CID' representing a basic CID assigned to a corresponding MS in the anchor base station, 'DIUC, 'OFDMA Symbol offset' indicating a starting point of a symbol representing assigned data burst, 'Subchannel offset' indicating a starting point of a sub-channel representing assigned data burst, 'Boosting' representing if transmission power of a sub-carrier for the assigned data burst is boosted, 'No. OFDMA Symbol' representing the number of symbols of the assigned data burst, 'No.Subchannels' representing the number of sub-channels of the assigned data burst, and 'Repetition Coding Indication' representing a repetition code used for the assigned data burst. FIG. 7 illustrates the format of the HO DL MAP IE in non-anchor BS according to an embodiment of the present invention. The HO DL MAP IE in non-anchor BS includes 'Extended DIUC (Downlink Interval Usage Code)' used for distinguishing a plurality of the IEs (i.e., the HO DL MAP IE in non-anchor BS), 'Length' indicating the length of the HO DL MAP IE in non-anchor BS, and IEs for each data burst. As shown in FIG. 7, the IEs for each data burst include 'Non-Anchor Preamble' representing a preamble of the active non-anchor base station, 'Anchor CID' representing a basic CID assigned to a corresponding MS in the anchor base station, 'DIUC, 'OFDMA Symbol offset' indicating a starting point of a symbol representing assigned data burst, 'Subchannel offset' indicating a starting point of a sub- channel used for the assigned data burst, 'Boosting' representing if transmission power of a sub-carrier for the assigned data burst is boosted, 'No. OFDMA Symbol' representing the number of symbols of the assigned data burst, 'No.Subchannels' representing the number of sub-channels of the assigned data burst, and 'Repetition Coding Indication' representing a repetition code used for the assigned data burst. FIG. 8 illustrates a format of the HO CID Translation MAP IE according to an embodiment of the present invention. The HO CID Translation MAP IE includes 'Extended DIUC (Downlink Interval Usage Code)' used for distinguishing a plurality of the IEs (i.e., the HO CID Translation MAP IE), 'Length' indicating the length of the HO CID Translation MAP IE, and IEs for each data burst. As shown in FIG. 8, the IEs for each data burst include 'Anchor Preamble' representing a preamble of the anchor base station, 'Anchor CID' representing a basic CID assigned to a corresponding MS in the anchor base station, and 'Non-Anchor CID' representing a basic CID temporally used to represent the channel assignment information for the data burst to be transmitted to a corresponding MS in a DL or a UL of the active non-anchor base station. In other words, in an active non-anchor base station, an MS that is performing a soft handover confirms the HO CID Translation MAP IE, recognizes the preamble of an anchor base station and 'Non-Anchor CID' mapped with 'Anchor CID', and receives data burst transmitted from the active non- anchor base station using the Non-anchor CID assigned to the MS. It is necessary to insert the Anchor Preamble into the HO CID Translation MAP IE because the MSs performing soft handover in the active non-anchor base station may have different anchor base stations. In addition, the anchor base stations may have the same Anchor CIDs. In this case, the Anchor Preamble may be used to distinguish the MSs connected to different anchor base stations. In addition, the MS performing a soft handover must decode the HO CID Translation MAP IE in every frame in order to transmit/receive data burst to/from the active non-anchor base station because the Non-Anchor CID of the active non-anchor base station may be changed in every frame. FIG. 9 illustrates the format of the HO Anchor Active UL MAP IE according to an embodiment of the present invention.The HO Anchor Active UL MAP IE includes 'Extended UIUC (Uplink Interval Usage Code)' used for distinguishing a plurality of the IEs (i.e., the HO Anchor Active UL MAP IE), 'Length' indicating the length of the HO Anchor Active UL MAP IE, and IEs for each data burst. As shown in FIG. 9, the IEs for each data burst include 'Anchor Preamble' representing a preamble of the anchor base station, 'Anchor CID' representing a basic CID assigned to a corresponding MS in the anchor base station, 'UIUC representing an uplink interval usage code, 'Start Subchannel offset' indicating a starting point of a sub-channel representing assigned data burst, 'Duration' indicating the amount of resource in OFDMA slot, and 'Repetition Coding Indication' representing a repetition code used for the assigned data burst. FIG. 10 illustrates the format of the HO UL MAP IE in non-anchor BS according to an embodiment of the present invention. The HO UL MAP IE in non-anchor BS includes 'Extended UIUC used for distinguishing a plurality of the IEs (i.e., the HO UL MAP IE in non-anchor BS), 'Length' indicating the length of the HO UL MAP IE in non-anchor BS, and IEs for each data burst. As shown in FIG. 10, the IEs for each data burst include 'Non-Anchor Preamble' representing a preamble of the active non-anchor base station, 'Anchor CID' representing a basic CID assigned to a corresponding MS in the anchor base station, 'UIUC, 'Start Subchannel offset' indicating a starting point of a sub-channel representing the assigned data burst, 'Duration' indicating the amount of resource in OFDMA, and 'Repetition Coding Indication' representing a repetition code used for the assigned data burst. FIG. 11 is a flowchart illustrating the operation of the MS performing a soft handover by receiving channel assignment information from an anchor base station according to an embodiment of the present invention. The MS detects a preamble sequence from the anchor base station so as to obtain synchronization with the anchor base station in step 1111. In step 1113, the MS determines if it necessary to perform a soft handover between an anchor base station and another base station (i.e. an active non-anchor base station). If it is not necessary for the MS to perform the soft handover, the MS performs step 1115, wherein the MS demodulates and decodes typical MAP messages (MAP messages used for the conventional IEEE 802.16e communication system) so as to detect DL-MAP IEs and UL-MAP IEs. In step 1117, the MS transmits/receives data burst corresponding to the DL-MAP IE and the UL-MAP IE detected from the anchor base station and then terminates the operation of performing the soft handover. If the MS must perform a soft handover as the determination result in step 1113, the MS performs step 1119, wherein the MS demodulates and decodes MAP messages so as to detect the DL-MAP IE, the UL-MAP IE, the HO DL MAP IE in non-anchor BS, and the HO UL MAP IE in non-anchor BS because the MS must perform the soft handover. In step 1121, the MS detects the Non- Anchor Preamble and the Anchor CID included in the detected HO DL MAP in non-anchor BS and the detected HO UL MAP IE in non-anchor BS. It is necessary to detect the Non-Anchor preamble and the Anchor CID so that the MS may distinguish data burst transmitted/received from/to the active non-anchor base station as described above. In step 1123, the MS transmits/receives data burst from/to the anchor base station and the active non-anchor base station correspondingly to the detected DL-MAP IE, the UL-MAP IE, the HO DL MAP IE in non-anchor BS, and the HO UL MAP in non-anchor BS and then terminates the operation of performing the soft handover. FIG. 12 is a flowchart illustrating the operation of an MS performing soft handover by receiving channel assignment information from an active non-anchor base station according to one embodiment of the present invention. The MS detects a preamble sequence from the active non-anchor base station so as to obtain synchronization with the active non-anchor base station in step 1211. The MS determines if a soft handover operation is currently performed in step 1213. If the soft handover operation is being performed, the MS demodulates and decodes MAP messages so as to detect the HO CID Translation MAP IE, the DL-MAP IE, the UL-MAP IE, the HO Anchor Active DL MAP IE, and the HO Anchor Active UL MAP IE in step 1215. In step 1217, the MS detects 'Anchor Preamble' and 'Anchor CID' included in the HO Anchor Active DL MAP IE and the HO Anchor Active UL MAP IE. It is necessary to detect the Anchor preamble and the Anchor CID so that the MS may distinguish data burst transmitted/received from/to the anchor base station as described above. However, if the active non-anchor base station uses a CID identical to the Anchor CID used in the anchor base station, it is impossible for the MS to transmit/receive data burst from/to the anchor base station because CIDs are intrinsically assigned according to base stations. Accordingly, in step 1219, the MS detects the Non-Anchor CID based on the detected HO CID Translation MAP IE. As a result, the MS detects the Non- Anchor CID from the anchor base station, so that the MS can transmit/receive data burst having Anchor CID corresponding to the Non-Anchor CID. In step 1221, the MS transmits/receives data burst from/to both the anchor base station and the active non-anchor base station correspondingly to the detected HO CID Translation MAP IE, the DL-MAP IE, the UL-MAP IE, the HO Anchor Active DL MAP IE, and the HO Anchor Active UL MAP IE, and then terminates the operation of performing soft handover. As described above, according to the present invention, new MAP IEs are embodied in the IEEE 802.16e-based communication system, thereby enabling soft handover of the MS. As a result, the soft handover of the MS improves signal receiving quality of the MS and removes the ping-pong phenomenon caused in had handover, thereby improving the performance of an entire system. While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. Consequently, the scope of the invention should not be limited to the embodiments,but should be defined by the appended claims and equivalents thereof. WE CLAIM 1. A method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the mobile station, schedule to be performed toward the active non-anchor base station, the method comprising the steps of: receiving, by the mobile station, one of first information, second information, and third information when a determination is made, that the mobile station is to perform the soft handover, wherein the first information received from the anchor base station, includes channel assignment information of the active non-anchor base station, the second information, received from the active non-anchor base station, includes channel assignment information of the anchor base station, the third information, received from the active non-anchor base station, represents a relationship between a connection identifier of the mobile station used in the anchor base station and a connection identifier used in the active non-anchor base station; and receiving a signal corresponding to the received information. 2. The method as claimed in claim 1, wherein receiving the signal corresponding to the received information comprises: receiving the signal from the active non-anchor base station, if the information is the first information; receiving the signal from the anchor base station, if the information is the second information; and receiving the signal from the active non-anchor base station, if the information is the third information. 3. The method as claimed in claim 1, wherein the first information includes reference signal information and a connection identifier used in the anchor base station, the reference signal information indicating a reference signal used in the active non-anchor base station. 4. The method as claimed in claim 1, wherein the second information includes reference signal information and a connection identifier used in the anchor base station, the reference signal information indicating a reference signal used in the anchor base station. 5. The method as claimed in claim 1, wherein the third information includes reference signal information indicating a reference signal used in the anchor base station, a connection identifier used in the anchor base station, and a connection identifier used in the active non-anchor base station. 6. The method as claimed in claim 5, wherein the connection identifier used in the active non-anchor base station is mapped with the connection identifier used in the anchor base station when the connection identifier used in the active non-anchor base station is used in the anchor base station. 7. A method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the mobile station schedule to be performed toward the active non-anchor base station, the method comprising the steps of : determining, by the anchor base station, if there is a signal to be transmitted to the mobile station from the active non-anchor base station when a determination is made that the mobile station is to perform a soft handover; and transmitting, by the anchor base station, information including channel assignment information of the active non-anchor base station to the mobile station when a determination is made that there is signal to be transmitted to the mobile station from the active non-anchor base station. 8. The method as claimed in claim 7, wherein the information includes reference signal information indicating a reference signal used in the active non-anchor base station and a connection identifier used in the anchor base station. ABSTRACT TITLE : 'A METHOD FOR PERFORMING A SOFT HANDOVER IN A BROADBAND WIRELESS ACCESS COMMUNICATION SYSTEM' This invention relates to a method for performing a soft handover in a broadband wireless access communication system which includes a mobile station, an anchor base station providing communication services to the mobile station, and an active non-anchor base station different from the anchor base station, the soft handover of the mobile station, schedule to be performed toward the active non-anchor base station, the method comprising the steps of; receiving, by the mobile station, one of first information, second information, and third information when a determination is made, that the mobile station is to perform the soft handover, wherein the first information received from the anchor base station, includes channel assignment information of the active non-anchor base station, the second information, received from the active non-anchor base station, includes channel assignment information of the anchor base station, the third information, received from the active non-anchor base station, represents a relationship between a connection identifier of the mobile station used in the anchor base station and a connection identifier used in the active non-anchor base station; and receiving a signal corresponding to the received information.

Full Text

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a broadband wireless access (BWA)
communication system, and more particularly to a system and a method for
performing a soft handover.
2. Description of the Related Art
In the 4 generation (4G) communication system, which is the next
generation communication system, extensive research is being conducted to
provide users with services having various Qualities-of-Service (QoSs) at high
speeds. In the current 4G communication system, extensive research is being
conducted to develop a new type of communication system ensuring the mobility
of a mobile station (MS) and a QoS in a BWA communication system such as a
wireless Local Area Network (LAN) system and a wireless Metropolitan Area
Network (MAN) system capable of supporting relatively high transmission
speeds. Representatives of such communication systems are the Institute of
Electrical and Electronics Engineers (IEEE) 802.16a communication system and
the IEEE 802.16e communication system.
The IEEE 802.16a/d-based communication system and the IEEE
802.16e-based communication system use an orthogonal frequency division
multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA)
scheme for a physical channel of the WMAN system in order to support a
broadband transmission network. The IEEE 802.16a communication system is a
single structure system that does not reflect the mobility of a Subscriber Station
(SS) at all, in which the SS is in a stationary state. In contrast, the IEEE
802.16e-based communication system is a system realized by supplementing the
IEEE 802.16a-based communication system with the ability to handle the
mobility of an SS. Hereinafter, a mobile SS will be referred to as a MS.
Accordingly, in the IEEE 802.16e-based communication system, the study of
handover according to the mobility of the MS is being activated.
Handover refers to an operation enabling smooth provision of services to
an MS which is moving between base stations (BSs). Such handover may have
two schemes (a hard handover scheme and a soft handover scheme). The hard
handover scheme and the soft handover scheme will be described.

The hard handover scheme denotes a scheme through which, when an
MS moves between base stations, the MS releases a channel setup with an anchor
base station (a serving base station currently providing a service to the MS) and
sets up a channel with an active non-anchor base station (a target base station) in
the shortest period of time, so that the service may be continuously provided.
The serving base and the target base station will be referred to as an anchor base
station and an active non-anchor base station, respectively, for the purpose of
description. The hard handover causes a ping-pong phenomenon of
continuously alternating channel release and setup when channel states are
continuously inferior, for example, when the MS moves across a cell boundary
region.
The soft handover scheme denotes a scheme through which, when an MS
moves between base stations, the MS has channels set up with both an anchor
base station and an active non-anchor base station, is relocated in the non-anchor
base station while simultaneously receiving signals from both the anchor base
station and the non-anchor base station through the setup channel, and then
releases the channel from the anchor base station, so that the service is
continuously provided. Accordingly, when the MS performs a handover
operation through the soft handover scheme, the MS receives downlink signals
from a plurality of base stations (i.e. the anchor base station and the non-anchor
base station) and improves quality of the received downlink signals by combining
the downlink signals received from the base stations.
The signal combining scheme may be classified into a soft combining
scheme and a selection diversity scheme.
The soft combining scheme is a scheme of demodulating each of the
signals received from a plurality of base stations and combining the signals in a
front end of a channel decoder. The selection diversity scheme is a scheme of
de-modulating and channel-decoding each of the signals received from a plurality
of base stations and then selecting a signal with the best quality from among the
channel-decoded signals.
The MS performing a handover operation through the soft handover
scheme may obtain a soft combining gain by simultaneously receiving signals
through channels set up with both the anchor base station and the non-anchor base
station. In order to obtain the combining gain, the MS must receive information
regarding channels (channel allocation information) set up with both the anchor
base station and the non-anchor base station and receive signals from the anchor
base station and the non-anchor base station using the channel allocation
information.

However, in the typical broadband wireless access system (e.g., the IEEE
802.16e-based communication system), it is difficult for the MS to receive
channel allocation information of an active non-anchor base station because an
anchor base station reports only information regarding a channel allocated by the
anchor base station.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made to solve at least the
above-mentioned problems occurring in the prior art, and an object of the present
invention is to provide a system and a method for performing a soft handover in a
broadband wireless access communication system.
Another object of the present invention is to provide a system and a
method for providing channel allocation information used for performing a soft
handover in a broadband wireless access communication system.
Still another object of the present invention is to provide a system and a
method in which an anchor base station informs a mobile station of channel
allocation information of an active non-anchor base station in a broadband
wireless access communication system.
Yet another object of the present invention is to provide a system and a
method in which an active non-anchor base station informs a mobile station of
channel allocation information of an anchor base station in a broadband wireless
access communication system.
To accomplish the above objects, there is provided a method for
performing a soft handover in a broadband wireless access communication
system which includes a mobile station, an anchor base station providing
communication services to the mobile station, and an active non-anchor base
station different from the anchor base station, the soft handover of the MS
scheduled to be performed toward the active non-anchor base station. The method
comprises the steps of: receiving first information from the anchor base station or
receiving second information considering channel state from the active non-
anchor base station when it is determined that the mobile station is to perform the
soft handover, the firet information including channel assignment information of
the active non-anchor base station, the second information including channel
assignment information of the anchor base station; and receiving signals from the
anchor base station or the active non-anchor base station corresponding to the
first information and the second information.
According to another aspect of the present invention, there is provided a

method for performing a soft handover in a broadband wireless access
communication system which includes a mobile station, an anchor base station
providing communication services to the mobile station, and an active non-anchor
base station different from the anchor base station, the soft handover of the MS
scheduled to be performed toward the active non-anchor base station. The method
comprises the steps of: determining if there are signals to be transmitted to the
mobile station from the active non-anchor base station when it is determined that
the mobile station is to perform a soft handover; and transmitting information
including channel assignment information of the active non-anchor base station
when it is determined that there are signals to be transmitted to the mobile station
from the active non-anchor base station.
According to still another aspect of the present invention, there is
provided a method for performing a soft handover in a broadband wireless access
communication system which includes a mobile station, an anchor base station
providing communication services to the mobile station, and an active non-anchor
base station different from the anchor base station, the soft handover of the MS
scheduled to be performed toward the active non-anchor base station. The method
comprises the steps of: determining if there are signals to be transmitted to the
mobile station from the anchor base station when it is determined that the mobile
station is to perform a soft handover; and transmitting first information including
channel assignment information of the anchor base station when it is determined
that there are signals to be transmitted to the mobile station from the anchor base
station.
According to still yet another aspect of the present invention, there is
provided a method for performing a soft handover in a broadband wireless access
communication system which includes a mobile station, an anchor base station
providing communication services to the mobile station, and an active non-anchor
base station different from the anchor base station, the soft handover of the MS
scheduled to be performed toward the active non-anchor base station. The method
comprises the steps of: transmitting by the anchor base station first information
including channel assignment information of the active non-anchor base station
when it is determined that the mobile station is to perform the soft handover, and
when there are signals to be transmitted to the mobile station from the active non-
anchor base station; transmitting by the active non-anchor base station second
information including channel assignment information of the anchor base station
when it is determined that the mobile station is to perform the soft handover, and
when there are signals to be transmitted to the mobile station from the anchor
base station; and receiving by the mobile station signals from the anchor base

station and the active non-anchor base station corresponding to the first
information and the second information when it is determined that the mobile
station is to perform a soft handover.
According to still yet another aspect of the present invention, there is
provided a system for performing a soft handover in a broadband wireless access
communication system. The system comprises an anchor base station for
transmitting first information including channel assignment information of an
active non-anchor base station when it is determined that a mobile station is to
perform a soft handover, and when there are signals to be transmitted to the
mobile station from the active non-anchor base station; an active non-anchor base
station for transmitting second information including channel assignment
information of the anchor base station when it is determined that the mobile
station is to perform a soft handover, and when there are signals to be transmitted
to the mobile station from the anchor base station; and a mobile station for
receiving signals from the anchor base station and the active non-anchor base
station corresponding to the first information and the second information when it
is determined that the mobile station is to perform the soft handover.
According to still yet another aspect of the present invention, there is
provided a method for performing a soft handover in a broadband wireless access
communication system which includes a mobile station, an anchor base station
providing communication services to the mobile station, and an active non-anchor
base station different from the anchor base station, the soft handover of the MS scheduled to be performed toward the active non-anchor base station. The method comprises the steps of receiving a first MAP information which is in the DL- MAP of active non-anchor base station and indicates at least one burst from
anchor base station or receiving a second MAP information which is in the DL-
MAP of the anchor base station and indicates at least one burst from active non-
anchor base station considering downlink channel state.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above and other objects, features and advantages of the present
invention will be more apparent from the following detailed description taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram illustrating the structure of an IEEE 802.de-
based communication system according to an embodiment of the present
invention;
FIG. 2 is a block diagram illustrating the sector structure in an IEEE

802.16e-based communication system according to an embodiment of the present
invention;
FIG. 3 is a block diagram illustrating the frame structure in an IEEE
802.16e-based communication system according to an embodiment of the present
invention;
FIG. 4 illustrates the format of a DL-MAP message according to an
embodiment of the present invention;
FIG. 5 illustrates the format of a UL-MAP message according to an
embodiment of the present invention;
FIG. 6 illustrates the format of an HO Anchor Active DL MAP IE
according to an embodiment of the present invention;
FIG. 7 illustrates the format of an HO DL MAP IE in non-anchor BS
according to an embodiment of the present invention;
FIG. 8 illustrates the format of an HO CID Translation MAP IE according
to an embodiment of the present invention;
FIG. 9 illustrates the format of an HO Anchor Active UL MAP IE
according to an embodiment of the present invention;
FIG. 10 illustrates the format of an HO UL MAP IE in non-anchor BS
according to embodiment of the present invention;
FIG. 11 is a flowchart illustrating the operation of an MS performing soft
handover by receiving channel assignment information from an anchor base
station according to an embodiment of the present invention; and
FIG. 12 is a flowchart illustrating the operation of an MS performing soft
handover by receiving channel assignment information from an active non-anchor
base station according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Hereinafter, a preferred embodiment of the present invention will be
described in detail with reference to the accompanying drawings. Note that the
same or similar components in drawings are designated by the same reference
numerals as far as possible although they are shown in different drawings. In
the following description of the present invention, a detailed description of known
functions and configurations incorporated herein will be omitted when it may
make the subject matter of the present invention rather unclear.
The present invention proposes a system and a method for performing a
soft handover in a broadband wireless access communication system (BWA) (e.g.,
an Institute of Electrical and Electronics Engineers (IEEE) 802.16e-based

communication system). Particularly, the present invention proposes a system
and a method in which an anchor base station (a serving base station) and an
active non-anchor base station (a target base station) provide channel allocation
information in order to perform a soft handover in the IEEE 802.16e-based
communication system. Hereinafter, the serving base station and the target base
station will be referred to as an anchor base station and an active non-anchor base
station, respectively. According to the present invention, although the IEEE
802.16e-based communication system is described by way of example for the
purpose of description, it is natural that other communication systems as well as
the IEEE 802.16e-based communication system can be employed.
FIG. 1 is a block diagram illustrating a structure of an IEEE 802.de-
based communication system according to an embodiment of the present
invention.
The IEEE 802.16e-based communication system having a multi-cell
structure includes a cell 100, a cell 150, a base station 110 controlling the cell 100,
a base station 140 controlling the cell 150, and a plurality of MSs 111, 113, 130,
151, and 153. Although one base station may control several cells, it is assumed
that one base station controls only one cell for the purpose of description in the
embodiment of the present invention. In addition, the base stations 110 and 140
transmit/receive signals to/from the MSs 111, 113, 130, 151, and 153 using the
OFDM/OFDMA scheme. Herein, it is assumed that the base stations 110 and
140 have a multi-sector structure, sectors in the same cell use the same center-
frequency, and adjacent sectors are assigned different sub-channels.
The above description deals with the structure of the IEEE 802.16e-based
communication system according to an embodiment of the present invention with
reference to FIG. 1, and, hereinafter, a sector structure in the IEEE 802.16e-based
communication system according to an embodiment of the present invention will
be described with reference to FIG. 2.
FIG. 2 is a block diagram illustrating the sector structure in the IEEE
802.16e-based communication system according to an embodiment of the present
invention.
A cell controlled by a base station is divided into several sectors (e.g., three
sectors; a first sector 210, a second sector 220, and a third sector 230). Each of
the first sector 210 to the third sector 230 employs the same center-frequency of
F1, and different sub-channels are used between them. In other words, the first
sector 210 to the third sector 230 use mutually-different sub-channel sets as
shown in FIG. 2. The first sector 210, the second sector 220, and the third sector
230 use the first sub-channel set, the second sub-channel set, and the third sub-

channel set, respectively.
The above description deals with the sector structure in the IEEE
802.16e-based communication system according to an embodiment of the present
invention with reference to FIG. 2, and, hereinafter, a frame structure in the IEEE
802.16e-based communication system according to an embodiment of the present
invention will be described with reference to FIG. 3.
FIG. 3 is a block diagram illustrating the frame structure in the IEEE
802.16e-based communication system according to an embodiment of the present
invention.
The frame structure in the IEEE 802.16e-based communication system
includes a downlink (DL) frame 310 and an uplink (UL) frame 320. The DL
frame 310 includes a preamble field 312, a DL-MAP/UL-MAP field 314, a PUSC
(partial usage of sub-channels) field 316, and an FUSC (full usage of sub-
channels) field 318. The UL frame 320 includes a PUSC field 322.
The preamble field 312 is a field used for transmitting a preamble
sequence for synchronization between a base station and an MS. The DL-
MAP/UL-MAP field 314 is a field used for transmitting a DL-MAP message and
an UL-MAP message. The base station transmits the MAP message indicating a
channel interval allocated to each MS before the channel interval, and the MS
detects information included in the MAP message so as to recognize the channel
interval and a coding scheme. Detailed descriptions of the DL-MAP message
and the UL-MAP message will be given later. The PUSC field 316 is a field
used for transmitting DL data burst through a PUSC scheme, and the FUSC field
318 is a field used for transmitting DL data burst through an FUSC scheme. In
addition, the PUSC field 322 is a field used for transmitting UL data burst using a
PUSC scheme. .
FIG. 4 illustrates the format of the DL-MAP message according to an
embodiment of the present invention.
The DL-MAP message includes various information elements (IEs), i.e.,
'Management Message Type' representing a type of a transmitted message,
'Physical (PHY) Synchronization' set corresponding to a modulation scheme or a
de-modulation scheme applied to a physical channel in order to acquire
synchronization, 'DCD count' representing a count value corresponding to the
configuration change of a downlink channel descriptor message including
downlink burst profile, 'Base Station ID' representing a base station identifier,
and n number of DL-MAP IEs. The DL-MAP message includes information of
raging codes assigned for raging.
FIG. 5 illustrates the form of the UL-MAP message according to an

embodiment of the present invention.
The UL-MAP message includes various IEs, i.e., 'Management Message
Type' representing a type of a transmitted message, 'Uplink Channel ID'
representing a used uplink channel identifier, 'UCD count' representing a count
value corresponding to the configuration change of an uplink channel descript
message including uplink burst profile, 'Allocation Start Time' representing
information regarding start time to allocate uplink resources, and n number of
UL-MAP IEs. The Uplink Channel ID is assigned as a unique value in a
Medium Access Control (MAC)-sub layer.
A data burst field of the DL-frame 310 shown in FIG. 3 includes time
slots assigned according to MSs through a Time Division Multiplexing
(TDM)/Time Division Multiple Access (TDMA) scheme. A base station
broadcasts control information to MSs (receiving services from the base station)
through the DL-MAP/UP-MAP field 314 by using a preset center-frequency.
As each of the MSs is powered on, each MS detects a reference signal
having the maximum size (e.g., the maximum Carrier to Interference and Noise
Ratio (CINR)) by monitoring all of the frequency bands preset in each MS.
Herein, the reference signal may include a pilot signal. Each MS determines a
base station having transmitted the reference signal having the highest CINR as a
base station controlling the MS, analyzes a DL-MAP message and an UL-MAP
message in the DL-MAP/UL-MAP field 314 of the DL frame 310 transmitted
from the base station so as to determine channel allocation information indicating
the interval at which real data burst and control information controlling the DL
and the UL of the MS are transmitted/received, and transmits/receives data burst
at the corresponding data burst transmit/receive interval.
An IEEE 802.16e communication system in which one base station uses
one center frequency and a plurality of sectors within the base station use as many
sub-bands as the sectors divided from a frequency band corresponding to the
center frequency will now be described. In such an IEEE 802.16e
communication system, when an MS moves from a current anchor base station to
another base station (an active non-anchor base station), or the MS moves from
an anchor sector (a serving sector) to another sector (an active non-anchor sector;
a target sector) within the same anchor base station, the MS must perform a
handover. In particular, according to an embodiment of the present invention,
the MS must perform a soft handover.In order to perform the soft handover
described above, an MS must obtain channel allocation information of both an
anchor base station (an anchor sector in the case of handover between sectors)
and an active non-anchor base station (an active non-anchor sector in the case of

handover between sectors). Herein, the channel allocation information is
included in MAP messages transmitted to the MS (requesting soft handover) from
the anchor base station or the non-anchor base station. In other words, all of the
base stations of the IEEE 802.16e communication system broadcast channel
allocation information of their neighboring base stations through the MAP
messages.
In the conventional IEEE 802.16e communication system, an anchor base
station inserts the MAP IEs (including channel allocation information for data
burst to be transmitted/received to/from an MS) into a MAP message to be
transmitted, and the MS transmits/receives data burst through a UL/DL
corresponding to the channel allocation information included in the MAP IEs.
However, according to an embodiment of the present invention, the MS must
receive channel allocation information from the anchor base station and the non-
anchor base station so that the MS may simultaneously transmit/receive the same
data burst from both the anchor base station and the non-anchor base station in an
MS handover area. In particular, according to an embodiment of the present
invention, it is possible to obtain a soft combining gain by decoding and buffering
data burst from the anchor base station and the non-anchor base station having the
same data burst without taking into consideration a soft combining scheme
employing the same permulation, the same symbol, and the same sub-channel in a
physical layer.
Hereinafter, further description about the MAP IEs for the soft handover
newly proposed in the present invention will be given.
In the conventional IEEE 802.16e communication system, it is impossible
for an MS to simultaneously receive the MAP IEs from two or more base stations.
Accordingly, in an IEEE 802.16e communication system according to an
embodiment of the present invention, an anchor base station and an active non-
anchor base station insert the MAP IEs including channel allocation information
of both the anchor base station and the non-anchor base station for the MS into
their own MAP messages. Then, the MS receives the MAP IEs from one of the
anchor base station and the non-anchor base station, obtains channel information
regarding both the anchor base station and the non-anchor base station from the
MAP IEs, and transmits/receives data burst from/to the anchor base station and
the non-anchor base station corresponding to the obtained channel information.
Hereinafter, a detailed description about the MAP IEs proposed in the present
invention will be given.(l) A Handover Anchor Active MAP IE ('HO Anchor
Active MAP IE').
The HO Anchor Active MAP IE is a MAP IE transmitted from an active

non-anchor base station that includes channel allocation information of a DL or a
UL of an anchor base station and indicates that there is data burst transmitted
from an anchor base station. The HO Anchor Active MAP IE including the
channel assignment information of the DL of the anchor base station corresponds
to a HO Anchor Active DL MAP IE, and the HO Anchor Active MAP IE
including the channel assignment information of the UL of the anchor base station
corresponds to a HO Anchor Active UL MAP IE.(2) A Handover MAP IE in a
non-anchor base station ('HO MAP IE in non-anchor BS').
The HO MAP IE in non-anchor BS is a MAP IE transmitted from an
anchor base station that includes the channel allocation information of a DL or a
UL of an active non-anchor base station and indicates that there is data burst
transmitted from the non-anchor base station. The HO MAP IE in non-anchor
BS including the channel assignment information of the DL of the non-anchor
base station corresponds to 'HO DL MAP IE in non-anchor BS', and the HO
MAP IE in non-anchor BS including the channel assignment information of the
UL of the non-anchor base station corresponds to 'HO UL MAP IE in non-anchor
BS'.
(3) A Handover Connection Identifier Translation MAP IE ('HO CID
Translation MAP IE')
The HO CID Translation MAP IE is a MAP IE transmitted from an
active non-anchor base station and is transmitted in order to map a CID of a MS
allocated by an anchor base station with a CID of the MS allocated by an active
non-anchor base station. In addition, the HO CID Translation MAP IE
represents that there is data burst transmitted from the non-anchor base station.
FIG. 6 illustrates the format of the HO Anchor Active DL MAP IE
according to an embodiment of the present invention.
The HO Anchor Active DL MAP IE includes 'Extended DIUC
(Downlink Interval Usage Code) field used for distinguishing a plurality of IEs
(i.e., the HO Anchor Active DL MAP IE), 'Length' indicating the length of the
HO Anchor Active DL MAP IE, and IEs for each data burst. As shown in FIG. 6,
the IEs for each data burst include 'Anchor Preamble' representing a preamble of
the anchor base station, 'Anchor CID' representing a basic CID assigned to a
corresponding MS in the anchor base station, 'DIUC, 'OFDMA Symbol offset'
indicating a starting point of a symbol representing assigned data burst,
'Subchannel offset' indicating a starting point of a sub-channel representing
assigned data burst, 'Boosting' representing if transmission power of a sub-carrier
for the assigned data burst is boosted, 'No. OFDMA Symbol' representing the
number of symbols of the assigned data burst, 'No.Subchannels' representing the

number of sub-channels of the assigned data burst, and 'Repetition Coding
Indication' representing a repetition code used for the assigned data burst.
FIG. 7 illustrates the format of the HO DL MAP IE in non-anchor BS
according to an embodiment of the present invention.
The HO DL MAP IE in non-anchor BS includes 'Extended DIUC
(Downlink Interval Usage Code)' used for distinguishing a plurality of the IEs
(i.e., the HO DL MAP IE in non-anchor BS), 'Length' indicating the length of the
HO DL MAP IE in non-anchor BS, and IEs for each data burst. As shown in
FIG. 7, the IEs for each data burst include 'Non-Anchor Preamble' representing a
preamble of the active non-anchor base station, 'Anchor CID' representing a basic
CID assigned to a corresponding MS in the anchor base station, 'DIUC,
'OFDMA Symbol offset' indicating a starting point of a symbol representing
assigned data burst, 'Subchannel offset' indicating a starting point of a sub-
channel used for the assigned data burst, 'Boosting' representing if transmission
power of a sub-carrier for the assigned data burst is boosted, 'No. OFDMA
Symbol' representing the number of symbols of the assigned data burst,
'No.Subchannels' representing the number of sub-channels of the assigned data
burst, and 'Repetition Coding Indication' representing a repetition code used for
the assigned data burst.
FIG. 8 illustrates a format of the HO CID Translation MAP IE according
to an embodiment of the present invention.
The HO CID Translation MAP IE includes 'Extended DIUC (Downlink
Interval Usage Code)' used for distinguishing a plurality of the IEs (i.e., the HO
CID Translation MAP IE), 'Length' indicating the length of the HO CID
Translation MAP IE, and IEs for each data burst. As shown in FIG. 8, the IEs
for each data burst include 'Anchor Preamble' representing a preamble of the
anchor base station, 'Anchor CID' representing a basic CID assigned to a
corresponding MS in the anchor base station, and 'Non-Anchor CID' representing
a basic CID temporally used to represent the channel assignment information for
the data burst to be transmitted to a corresponding MS in a DL or a UL of the
active non-anchor base station.
In other words, in an active non-anchor base station, an MS that is
performing a soft handover confirms the HO CID Translation MAP IE,
recognizes the preamble of an anchor base station and 'Non-Anchor CID' mapped
with 'Anchor CID', and receives data burst transmitted from the active non-
anchor base station using the Non-anchor CID assigned to the MS. It is
necessary to insert the Anchor Preamble into the HO CID Translation MAP IE
because the MSs performing soft handover in the active non-anchor base station

may have different anchor base stations. In addition, the anchor base stations
may have the same Anchor CIDs. In this case, the Anchor Preamble may be
used to distinguish the MSs connected to different anchor base stations. In
addition, the MS performing a soft handover must decode the HO CID
Translation MAP IE in every frame in order to transmit/receive data burst to/from
the active non-anchor base station because the Non-Anchor CID of the active
non-anchor base station may be changed in every frame.
FIG. 9 illustrates the format of the HO Anchor Active UL MAP IE
according to an embodiment of the present invention.The HO Anchor Active UL
MAP IE includes 'Extended UIUC (Uplink Interval Usage Code)' used for
distinguishing a plurality of the IEs (i.e., the HO Anchor Active UL MAP IE),
'Length' indicating the length of the HO Anchor Active UL MAP IE, and IEs for
each data burst. As shown in FIG. 9, the IEs for each data burst include 'Anchor
Preamble' representing a preamble of the anchor base station, 'Anchor CID'
representing a basic CID assigned to a corresponding MS in the anchor base
station, 'UIUC representing an uplink interval usage code, 'Start Subchannel
offset' indicating a starting point of a sub-channel representing assigned data burst,
'Duration' indicating the amount of resource in OFDMA slot, and 'Repetition
Coding Indication' representing a repetition code used for the assigned data burst.
FIG. 10 illustrates the format of the HO UL MAP IE in non-anchor BS
according to an embodiment of the present invention.
The HO UL MAP IE in non-anchor BS includes 'Extended UIUC used
for distinguishing a plurality of the IEs (i.e., the HO UL MAP IE in non-anchor
BS), 'Length' indicating the length of the HO UL MAP IE in non-anchor BS, and
IEs for each data burst. As shown in FIG. 10, the IEs for each data burst include
'Non-Anchor Preamble' representing a preamble of the active non-anchor base
station, 'Anchor CID' representing a basic CID assigned to a corresponding MS
in the anchor base station, 'UIUC, 'Start Subchannel offset' indicating a starting
point of a sub-channel representing the assigned data burst, 'Duration' indicating
the amount of resource in OFDMA, and 'Repetition Coding Indication'
representing a repetition code used for the assigned data burst.
FIG. 11 is a flowchart illustrating the operation of the MS performing a
soft handover by receiving channel assignment information from an anchor base
station according to an embodiment of the present invention.
The MS detects a preamble sequence from the anchor base station so as
to obtain synchronization with the anchor base station in step 1111. In step 1113,
the MS determines if it necessary to perform a soft handover between an anchor
base station and another base station (i.e. an active non-anchor base station). If

it is not necessary for the MS to perform the soft handover, the MS performs step
1115, wherein the MS demodulates and decodes typical MAP messages (MAP
messages used for the conventional IEEE 802.16e communication system) so as
to detect DL-MAP IEs and UL-MAP IEs. In step 1117, the MS
transmits/receives data burst corresponding to the DL-MAP IE and the UL-MAP
IE detected from the anchor base station and then terminates the operation of
performing the soft handover.
If the MS must perform a soft handover as the determination result in step
1113, the MS performs step 1119, wherein the MS demodulates and decodes
MAP messages so as to detect the DL-MAP IE, the UL-MAP IE, the HO DL
MAP IE in non-anchor BS, and the HO UL MAP IE in non-anchor BS because
the MS must perform the soft handover. In step 1121, the MS detects the Non-
Anchor Preamble and the Anchor CID included in the detected HO DL MAP in
non-anchor BS and the detected HO UL MAP IE in non-anchor BS. It is
necessary to detect the Non-Anchor preamble and the Anchor CID so that the MS
may distinguish data burst transmitted/received from/to the active non-anchor
base station as described above.
In step 1123, the MS transmits/receives data burst from/to the anchor
base station and the active non-anchor base station correspondingly to the
detected DL-MAP IE, the UL-MAP IE, the HO DL MAP IE in non-anchor BS,
and the HO UL MAP in non-anchor BS and then terminates the operation of
performing the soft handover.
FIG. 12 is a flowchart illustrating the operation of an MS performing soft
handover by receiving channel assignment information from an active non-anchor
base station according to one embodiment of the present invention.
The MS detects a preamble sequence from the active non-anchor base
station so as to obtain synchronization with the active non-anchor base station in
step 1211. The MS determines if a soft handover operation is currently
performed in step 1213. If the soft handover operation is being performed, the
MS demodulates and decodes MAP messages so as to detect the HO CID
Translation MAP IE, the DL-MAP IE, the UL-MAP IE, the HO Anchor Active
DL MAP IE, and the HO Anchor Active UL MAP IE in step 1215. In step 1217,
the MS detects 'Anchor Preamble' and 'Anchor CID' included in the HO Anchor
Active DL MAP IE and the HO Anchor Active UL MAP IE. It is necessary to
detect the Anchor preamble and the Anchor CID so that the MS may distinguish
data burst transmitted/received from/to the anchor base station as described above.
However, if the active non-anchor base station uses a CID identical to the Anchor
CID used in the anchor base station, it is impossible for the MS to

transmit/receive data burst from/to the anchor base station because CIDs are
intrinsically assigned according to base stations.
Accordingly, in step 1219, the MS detects the Non-Anchor CID based on
the detected HO CID Translation MAP IE. As a result, the MS detects the Non-
Anchor CID from the anchor base station, so that the MS can transmit/receive
data burst having Anchor CID corresponding to the Non-Anchor CID. In step
1221, the MS transmits/receives data burst from/to both the anchor base station
and the active non-anchor base station correspondingly to the detected HO CID
Translation MAP IE, the DL-MAP IE, the UL-MAP IE, the HO Anchor Active
DL MAP IE, and the HO Anchor Active UL MAP IE, and then terminates the
operation of performing soft handover.
As described above, according to the present invention, new MAP IEs are
embodied in the IEEE 802.16e-based communication system, thereby enabling
soft handover of the MS. As a result, the soft handover of the MS improves
signal receiving quality of the MS and removes the ping-pong phenomenon
caused in had handover, thereby improving the performance of an entire system.
While the invention has been shown and described with reference to
certain preferred embodiments thereof, it will be understood by those skilled in
the art that various changes in form and details may be made therein without
departing from the spirit and scope of the invention. Consequently, the scope of
the invention should not be limited to the embodiments,but should be defined by
the appended claims and equivalents thereof.

WE CLAIM
1. A method for performing a soft handover in a broadband wireless access
communication system which includes a mobile station, an anchor base
station providing communication services to the mobile station, and an
active non-anchor base station different from the anchor base station, the
soft handover of the mobile station, schedule to be performed toward the
active non-anchor base station, the method comprising the steps of:
receiving, by the mobile station, one of first information, second
information, and third information when a determination is made, that the
mobile station is to perform the soft handover, wherein the first
information received from the anchor base station, includes channel
assignment information of the active non-anchor base station, the second
information, received from the active non-anchor base station, includes
channel assignment information of the anchor base station, the third
information, received from the active non-anchor base station, represents
a relationship between a connection identifier of the mobile station used
in the anchor base station and a connection identifier used in the active
non-anchor base station; and
receiving a signal corresponding to the received information.
2. The method as claimed in claim 1, wherein receiving the signal
corresponding to the received information comprises:
receiving the signal from the active non-anchor base station, if the
information is the first information;

receiving the signal from the anchor base station, if the information is the
second information; and
receiving the signal from the active non-anchor base station, if the
information is the third information.
3. The method as claimed in claim 1, wherein the first information includes
reference signal information and a connection identifier used in the anchor
base station, the reference signal information indicating a reference signal
used in the active non-anchor base station.
4. The method as claimed in claim 1, wherein the second information
includes reference signal information and a connection identifier used in
the anchor base station, the reference signal information indicating a
reference signal used in the anchor base station.
5. The method as claimed in claim 1, wherein the third information includes
reference signal information indicating a reference signal used in the
anchor base station, a connection identifier used in the anchor base
station, and a connection identifier used in the active non-anchor base
station.
6. The method as claimed in claim 5, wherein the connection identifier used
in the active non-anchor base station is mapped with the connection
identifier used in the anchor base station when the connection identifier
used in the active non-anchor base station is used in the anchor base
station.

7. A method for performing a soft handover in a broadband wireless access
communication system which includes a mobile station, an anchor base
station providing communication services to the mobile station, and an
active non-anchor base station different from the anchor base station, the
soft handover of the mobile station schedule to be performed toward the
active non-anchor base station, the method comprising the steps of :
determining, by the anchor base station, if there is a signal to be
transmitted to the mobile station from the active non-anchor base station
when a determination is made that the mobile station is to perform a soft
handover; and
transmitting, by the anchor base station, information including channel
assignment information of the active non-anchor base station to the
mobile station when a determination is made that there is signal to be
transmitted to the mobile station from the active non-anchor base station.
8. The method as claimed in claim 7, wherein the information includes
reference signal information indicating a reference signal used in the
active non-anchor base station and a connection identifier used in the
anchor base station.

ABSTRACT

TITLE : 'A METHOD FOR PERFORMING A SOFT HANDOVER IN A
BROADBAND WIRELESS ACCESS COMMUNICATION SYSTEM'
This invention relates to a method for performing a soft handover in a broadband
wireless access communication system which includes a mobile station, an
anchor base station providing communication services to the mobile station, and
an active non-anchor base station different from the anchor base station, the
soft handover of the mobile station, schedule to be performed toward the active
non-anchor base station, the method comprising the steps of; receiving, by the
mobile station, one of first information, second information, and third information
when a determination is made, that the mobile station is to perform the soft
handover, wherein the first information received from the anchor base station,
includes channel assignment information of the active non-anchor base station,
the second information, received from the active non-anchor base station,
includes channel assignment information of the anchor base station, the third
information, received from the active non-anchor base station, represents a
relationship between a connection identifier of the mobile station used in the
anchor base station and a connection identifier used in the active non-anchor
base station; and receiving a signal corresponding to the received information.

A METHOD FOR PERFORMING A SOFT HANDOVER IN A BROADBAND WIRELESS ACCESS COMMUNICATION SYSTEM

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