Abstract:
A communication apparatus and method for an upper node in a communication system. The communication method includes assigning a Connection IDentifier (CID) set to a Relay Station (RS), generating a CID assignment message including information on the assigned CID set, and transmitting the CID assignment message to the RS.

Description:
PRIORITY 
   This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Oct. 11, 2005 and allocated Serial No. 2005-95453, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a multi-hop relay cellular system, and more particularly, to an apparatus and method for managing connection identifiers (CIDs) in a multi-hop relay based Broadband Wireless Access (BWA) system. 
   2. Description of the Related Art 
   Extensive research is being conducted to provide various Quality of Service (QoS) features with a data rate of about 100 Mbps in the advanced fourth-generation (4G) communication system. The 4G communication system is evolving to provide mobility, high data rate transmission, and high QoS in a BWA system such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN) system. Typical examples of the above system are based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16d and IEEE 802.16e. 
   The IEEE 802.16d based system and the BWA system use an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The IEEE 802.16d system considers only a fixed Subscriber Station (SS) and a single cell structure (i.e., the mobility of an SS is not considered). On the other hand, the IEEE 802.16e based system considers the mobility of an SS. When the mobility of an SS is considered, the SS will be referred to as a mobile station (MS). 
     FIG. 1  is a block diagram of a conventional BWA system. 
   Referring to  FIG. 1 , the BWA system has a multi-cell structure. The BWA system includes a cell  100 , a cell  150 , a base station (BS)  110  managing the cell  100 , a BS  140  managing the cell  150 , and a plurality of MSs  111 ,  113 ,  130 ,  151  and  153 . The signal exchange between the BSs  110  and  140  and the MSs  111 ,  113 ,  130 ,  151  and  153  is performed using an OFDM/OFDMA scheme. The MS  130  is located in a boundary region (i.e., a handover region) between the cells  100  and  150 . When the MS  130  moves from the cell  100  of the BS  110  into the cell  150  of the BS  140  while communicating with the BS  110 , the serving BS of the MS  130  is changed from the BS  110  to the BS  140 . 
   In such a BWA system (IEEE 802.16 system), an Uplink (UL) random access channel is used as a ranging channel. Initial ranging, periodic ranging, and bandwidth request ranging are performed using the ranging channel. When an MS enters a network (network entry procedure) or loses its system (network) information, the MS performs initial ranging to obtain UL synchronization. In an initial ranging operation, a BS determines the exact time of arrival of a ranging signal from an MS, calculates a round trip delay between the BS and MS, and informs the MS of a timing offset corresponding to the round trip delay. Further, in the initial ranging operation, the BS allocates the MS a primary management CID and a basic CID that are required for MS to enter the network and receive a control message. 
     FIG. 2  is a diagram for explaining an initial ranging process in a conventional BWA system. 
   Referring to  FIG. 2 , when powered on, an MS  201  receives a Downlink (DL) preamble from a serving BS  203  for synchronization with the BS  203 . In step  204  the MS  201  receives a DL-MAP message and a Downlink Channel Descriptor (DCD) message from the BS  203  and acquires information about a DL channel from the received messages. Next, the MS  201  receives an UL-MAP message and an Uplink Channel Descriptor (UCD) message from the BS  203  and acquires information about an initial ranging opportunity period, an UL channel, and initial ranging parameters. 
   In this way, the MS  201  acquires minimal information about the UL and DL channels, and then the MS  201  performs a basic access procedure using the acquired information to communicate with the BS  203 . This basic access procedure is called an initial ranging procedure. 
   In detail, the MS  201  sends a ranging request (RNG-REQ) message to the BS  203  in step  205 . In response to the RNG-REQ message, the BS  203  sends a ranging response (RNG-RSP) message to the MS  201  in step  207 . In this way, an initial ranging procedure is performed. 
   Since the initial ranging procedure is performed before the MS  201  registers with the BS  203 , both the MS  201  and the BS  203  do not have information about their connection. The MS  201  uses an initial ranging CID as a CID. The initial ranging CID has a predetermined value (e.g., 0×000). Since the initial ranging CID is commonly used for all MSs, the initial ranging CID is not individually handled. 
   In a BWA system, a plurality of different CIDs other than the initial ranging CID is used as shown in Table 1 below. 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               CID 
               Value 
             
             
                 
                 
             
           
           
             
                 
               Initial Ranging CID 
               0x0000 
             
             
                 
               Basic CID 
               0x0001~m 
             
             
                 
               Primary management CID 
               m + 1~2m 
             
             
                 
               Transport CID 
               2m + 1~FE9F 
             
             
                 
               Multicast CID 
               0xFEA0~0x FEFE 
             
             
                 
                 
             
           
        
       
     
   
   Referring to Table 1, an initial ranging CID is used in an initial ranging procedure where an MS initially tries to access a BS as described above. 
   A Basic CID is a unique CID assigned to an MS by a BS. The basic CID can be used in place of a Media Access Control (MAC) address of the MS while the MS and the BS are connected. After an initial ranging procedure, the MS and the BS transmit and receive a control message using the basic CID. 
   A primary management CID is used mainly in a network entry process. The network entry process starts with an initial ranging procedure and includes a sequence of procedures, such as MS registration, service negotiation, and IP address allocation. The network entry process is a process required for an MS to register with a BS. In the network entry process, the primary management CID is mainly used for the BS to identify the MS. Further, the primary management CID is used for the BS and MS to transmit and receive an important control message. The primary CID is assigned to the MS by the BS and maintained while the BS and the MS are connected like in the case of the basic CID. 
   A transport CID is used for transmitting service data. When an MS requests a BS for a service after a network entry process, the BS assigns the MS a transport CID so as to transmit data corresponding to the requested service. Until the requested service is completely provided, the assigned transport CID is maintained to identify a connection established for the service. Each time the MS requests a service, the BS assigns the MS a transport CID. Therefore, when the MS simultaneously requests a plurality of services such as a voice call, an image service, and an Internet service, the MS can be assigned a plurality of transport CIDs. Unlike the transport CID, the primary management CID and the basic CID are assigned to an MS only once. 
   A multicast CID is used when the same data is simultaneously transmitted to a plurality of MSs. When a BS intends to transmit data using the multicast CID, the BS simultaneously assigns a multicast CID to a plurality of MSs and transmits the same data to the plurality of MSs. Then, each of the MSs determines that the data is destined for itself and receives the data. 
   Messages used for an initial ranging procedure will now be described. 
   Table 2 below shows syntax of an RNG-REQ message transmitted from an MS to a BS. 
   
     
       
             
             
             
           
             
           
             
             
             
           
             
             
             
             
           
             
             
           
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Syntax 
               Size 
               Note 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               RNG-REQ_Message_Format( ) { 
             
           
        
         
             
                 
               Management Message Type = 4 
               8 bits 
             
             
                 
               Downlink Channel ID 
             
           
        
         
             
                 
               TLV encoded Information { 
               variable 
               TLV specific 
             
           
        
         
             
                 
               SS MAC Address 
             
             
                 
               Requested Downlink Burst Profile 
             
             
                 
               MAC Version 
             
             
                 
               Ranging Anomalies 
             
             
                 
               AAS broadcast capability 
             
             
                 
               } 
             
           
        
         
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   Referring to Table 2, the RNG-REQ message includes a plurality of information entries. “Management Message Type” is equal to 4 and indicates the identity of the RNG-REQ message. “Downlink Channel ID” indicates a DL channel through which an UCD message including UL channel information is received. A TLV(Type/Lengthlvalue) encoded information field indicates encoded information including an SS Medium Access Control (MAC) Address, a Requested Downlink Burst Profile, an MAC Version, Ranging Anomalies and AAS(Adaptive Antenna System) Broadcast Capability. “SS MAC Address” is a MAC layer address of the MS and is used as an identifier of the MS. “Requested Downlink Burst Profile” is divided into a 0-3 bit section and a 4-7 bit section. In the 0-3 bit section, a Downlink Interval usage code (DIUC) is recorded for requesting formats required to receive and transmit physical channel signals (e.g., a modulation format and a error correcting format). The 4-7 bit section is a section for recording least significant bits (LSBs, 4 bits) of a Configuration Change Court field of the UCD message used for requesting ranging. The BS transmits a predetermined physical channel signal to the MS with reference to the information stored in the 0-3 bit section. “MAC version” indicates the version of a MC layer used by the MS. “Ranging Anomalies” includes information about whether the MS tries to access the BS at a maximum transmission (TX) power or a minimum TX power. When the BS instructs the MS to increase or decrease TX power to compensate for the TX power, time information, etc., the MS can use the Ranging Anomalies in response to the instruction of the BS. “AAS broadcast capability” indicates whether the MS is capable of receiving a broadcast message. 
   Table 3 below shows syntax of an RNG-RSP message transmitted from a BS to an MS. 
   
     
       
             
             
             
           
             
           
             
             
             
           
             
             
             
             
           
             
             
             
           
             
             
             
             
           
             
             
           
             
             
             
           
             
             
             
             
           
             
             
             
           
             
             
           
             
             
           
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               Syntax 
               Size 
               Note 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               RNG-RSP_Message_Format( ) { 
             
           
        
         
             
                 
               Management Message Type = 5 
               8 bits 
             
             
                 
               Uplink Channel ID 
             
           
        
         
             
                 
               TLV encoded Information { 
               variable 
               TLV specific 
             
           
        
         
             
                 
               SS MAC Address 
               6 
             
             
                 
               Downlink Operational 
               2 
             
             
                 
               Burst Profile 
             
             
                 
               Primary Management CID 
               2 
             
             
                 
               Basic CID 
               2 
             
           
        
         
             
                 
               Ranging Status 
               4 
               1 = continue 
             
           
        
         
             
                 
               2 = abort 
             
             
                 
               3 = success 
             
             
                 
               4 = rerange 
             
           
        
         
             
                 
               Timing adjust 
               4 
             
             
                 
               Power level adjust 
               1 
             
           
        
         
             
                 
               Downlink frequency 
               4 
               Center Frequency 
             
           
        
         
             
                 
               override 
               (kHz) allowing an SS 
             
           
        
         
             
                 
               to perform an initial 
             
             
                 
               Ranging Request again 
             
           
        
         
             
                 
               } 
             
           
        
         
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   Referring to Table 3, the RNG-RSP message includes various information. “Management Message Type field” has a value of ‘5’ to indicate that the present message is a Ranging Response message. “SS MAC Address field” contains a MAC layer address of the MS that will receive the RNG-RSP message. “Downlink Operational Burst Profile” is used as a response to the Requested Downlink Burst Profile of the RNG-REQ message from the MS and indicates a DIUC number that will be used by the BS. “Uplink Channel ID” indicates a UL channel for the MS. “Primary Management CID and a Basic CID are CIDs that are assigned to the MS by the BS in order to manage the connection between the BS and the MS while the MS receives a-service from the BS after a ranging procedure. “Ranging Status” indicates a response of the BS to a ranging request of the MS. When the Ranging Status has a value of ‘1’, it indicates the need to continue the ranging process. When the Ranging Status field has a value of ‘2’, it indicates the need to abort (stop) the ranging process. When the Ranging Status field has a value of ‘3’, it indicates the success of the ranging process. When the Ranging Status field has a value of ‘4’, it indicates the need to perform the ranging request again. “Timing Adjust” contains information that enables the MS to correct incorrect time information. “Power Level Adjust” contains information that enables the MS to adjust its TX/receiving (RX) power. “Downlink Frequency Override” is used to inform the MS of a frequency value of another channel, so that the MS can again perform an initial ranging request with another frequency when the Ranging Status is set to ‘2’ for indicating the need to abort the ranging process. 
   As shown in Table 3, in an initial ranging process, an MS is assigned a Primary Management CID and a Basic CID. 
   In an IEEE 802.16e system such as the conventional BWA system illustrated in  FIG. 1 , a fixed BS communicates directly with an MS and thus a reliable wireless communication link can be easily established between the BS and the MS. Since the BS is fixed in the IEEE 802.16e system, it is disadvantageous to construct a flexible wireless communication network. Communication services are not efficiently provided when traffic and call requests change. 
   To address this problem, a multi-hop relay scheme is used in the IEEE 802.16e system such as a cellular wireless communication system. In the multi-hop relay scheme, data is relayed from an origin to a destination through a fixed relay station (RS), a mobile RS, or an MS. A multi-hop relay wireless communication system can immediately reconstruct a network in response to a change in a communication environment and can use wireless network resources more efficiently. For example, a cell service coverage and a system capacity can be increased in the multi-hop relay wireless communication system. When channel conditions are not good between a BS and an MS, an RS can be disposed between the BS and the MS to establish a multi-hop relay path, thereby providing better wireless channels to the MS. Further, the multi-hop relay scheme can be used in a boundary region between cells where channel conditions are poor in order to provide high data rate channels and expand a cell service coverage area. 
     FIG. 3  is a diagram illustrating a BWA system using a multi-hop relay scheme to expand a BS service coverage area. 
   Referring to  FIG. 3 , the multi-hop relay BWA system has a multi-cell structure. The multi-hop relay BWA system includes a cell  300 , a cell  340 , a BS  310  managing the cell  300 , a BS  350  managing the cell  340 , a plurality of MSs  311  and  313  located within the cell  300 , a plurality of MSs  321  and  323  located in a region  330  outside the cell  300  of the BS  310  and communicating with the BS  310 , an RS  320  providing a multi-hop relay path between the BS  310  and the MSs  321  and  323  located in the region  330 , a plurality of MSs  351 ,  353  and  355  located in the cell  340 , a plurality of MSs  361  and  363  located in a region  370  outside the cell  340  of the BS  350  and communicating with the BS  350 , and an RS  360  providing a multi-hop relay path between the BS  350  and the MSs  361  and  363  located in the region  370 . An OFDM/OFDMA scheme is used for communication among the BS  310  and  350 , the RS  320  and  360 , and the MSs  311 ,  313 ,  321 ,  323 ,  351 ,  353 ,  355 ,  361 , and  363 . 
   Although the MSs  311  and  313  located in the cell  300  and the RS  320  can directly communicate with the BS  310 , the MSs  321  and  323  located in the region  330  cannot directly communicate with the BS  310 . Therefore, the RS  320  covers the region  330  to relay signals between the BS  310  and the MSs  321  and  323 . That is, the MSs  321  and  323  can communicate with the BS  310  through the RS  320 . Further, the RS  360  and the MSs  351 ,  353 , and  355  located in the cell  340  can directly communicate with the BS  350 , the MSs  361  and  363  located in the region  370  cannot directly communicate with the BS  350 . Therefore, the RS  360  covers the region  370  to relay signals between the BS  350  and the MSs  361  and  363 . That is, the MSs  361  and  363  can communicate with the BS  350  through the RS  360 . 
   Since the RSs  320  and  360  are additionally used in the multi-hop relay BWA system shown in  FIG. 3 , the conventional initial ranging process shown in  FIG. 2  cannot be used. For example, an RNG-REQ message of an MS located outside a cell coverage area may be delivered to a BS through an RS, and an RNG-RSP message of the BS may be delivered to the MS through the RS. 
   Fields of the RNG-REQ and RNG-RSP messages (refer to Tables 2 and 3) other than the Basic CID field and the Primary Management CID field cannot be processed by a BS communicating with an MS using a multi-hop relay scheme. For example, fields of the messages related to power adjustment and burst profiles used for transmitting and receiving physical channel signals are the most representative fields that cannot be processed by the BS. Since such fields should provide information about actual physical channels between an MS and an RS, the BS indirectly connected with the MS cannot process such fields. 
   For this reason, it can be proposed that an RS processes some fields of an RNG-REQ message of an MS that are related to the RS and then transmits only the remaining fields of the RNG-REQ message to a BS. For example, an RS can transmit only the SS MAC address of an RNG-REQ message of an MS to a BS and receives an assigned CID from the BS. Then, the RS can construct an RNG-RSP message using the CID received from the BS and information processed by the RS in order to send the RNG-RSP message to the MS. 
   However, this proposal disadvantageously requires an additional procedure in which the RS requests from the BS CID assignment and receives the assigned CID, thereby causing a time delay. Moreover, the time delay increases as the number of hops between the BS and the MS increases (in  FIG. 3 , a two-hop relay path is shown) 
   Therefore, there is a need for an apparatus and method for rapidly processing a ranging request of an MS communicating with an RS. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an aspect of the present invention is to provide an apparatus and method for rapidly processing an RNG-REQ message transmitted from an MS to an RS in a multi-hop relay BWA system. 
   Another aspect of the present invention is to provide an apparatus method for an RS to manage a plurality of CIDs assigned from a BS in a multi-hop relay BWA system. 
   A further aspect of the present invention is to provide an apparatus and method for assigning a CID to an MS accessing an RS in a multi-hop relay BWA system. 
   According to one aspect of the present invention, there is provided an apparatus for a Relay Station (RS) in a multi-hop relay cellular communication system, the apparatus including a database for managing a Connection Identifier (CID) set assigned to the RS by a Base Station (BS); a controller for accessing the database to assign one CID of the CID set to a Mobile Station (MS) when receiving a predetermined control message from the MS; and a transmitter constructing a response message including the CID assigned to the MS and transmitting the response message to the MS. 
   According to another aspect of the present invention, there is provided an apparatus for managing CIDs in a multi-hop relay cellular communication system, the apparatus including an RS; a BS for assigning a CID set to the RS when the RS accesses the BS; and an MS for transmitting an RNG-REQ message to the RS and receiving a ranging response (RNG-RSP) message from the RS, wherein the RS manages the CID set assigned by the BS, assigns the MS at least one CID of the CID set when receiving the RNG-REQ message from the MS, and transmits the RNG-RSP message to the MS, the RNG-RSP message including the CID assigned to the MS. 
   According to a further aspect of the present invention, there is provided a communication method for an RS in a multi-hop relay cellular communication system, the communication method including recording a CID set assigned by a BS in a database; when receiving a predetermined control message from an MS, accessing the database to assign one CID of the CID set to the MS; and constructing a response message including the CID assigned to the MS so as to transmit the response message to the MS. 
   According to a still another aspect of the present invention, there is provided a method for managing CIDs in a multi-hop relay cellular communication system, the method including operating a BS to assign a CID set to an RS accessing the BS; operating an MS to transmit an RNG-REQ message to the RS; and operating the RS to manage the CID set assigned by the BS, assign the MS at least one CID of the CID set when the RS receives the RNG-REQ message from the MS, and transmit an RNG-RSP message to the MS, the RNG-RSP message including the CID assigned to the MS. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a block diagram of a conventional Broadband Wireless Access (BWA) system; 
       FIG. 2  is a diagram for explaining an initial ranging process in a conventional BWA system; 
       FIG. 3  is a diagram illustrating a BWA system using a multi-hop relay scheme to expand a BS service coverage area; 
       FIG. 4  is a diagram for explaining an assignment process of Connection Identifier (CID) sets to Relay Stations (RSs) in a multi-hop relay BWA system according to the present invention; 
       FIG. 5  is a diagram for explaining a CID assignment process in which a Base Station (BS) assigns an RS a CID set and the RS assigns CIDs respectively to Mobile Stations (MSs) in a multi-hop relay BWA system according to the present invention; 
       FIG. 6  is a flowchart for explaining a communication process of an RS in a multi-hop relay BWA system according to the present invention; and 
       FIG. 7  is a block diagram illustrating a structure of an RS in a multi-hop relay BWA system according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Also, the terms used herein are defined according to the functions of the present invention. Thus, the terms may vary depending on user&#39;s or operator&#39;s intension and usage. That is, the terms used herein must be understood based on the descriptions made herein. 
   Hereinafter, an apparatus and method for an RS to assign a CID to an MS in a multi-hop relay BWA system will be described in detail. 
   The multi-hop relay BWA system uses an OFDM scheme or an OFDMA scheme, for example. The multi-hop relay BWA system can transmit physical channel signals using a plurality of subcarriers, thereby enabling high-rate data transmission. The multi-hop relay BWA system supports a multi-cell structure, thereby supporting the mobility of an MS. 
   In the following description, a multi-hop relay BWA system is used for explaining the present invention. However, the present invention can be applied to any cellular communication system using a multi-hop relay scheme. 
     FIG. 4  is a diagram for explaining an assignment process of CID sets to RSs in a multi-hop relay BWA system according to the present invention. A serving BS  405  manages a first RS  401 , a second RS  403 , a third RS  407 , and a fourth RS  409 . 
   Referring to  FIG. 4 , in step  411 , the serving BS  405  assigns the third RS  407  a predetermined CID set. For example, the CID set can be assigned when the third RS  407  initially accesses the serving BS  405 . Alternatively, the CID set can be assigned in a separate signaling procedure after the third RS  407  registers with the serving BS  405 . In the step  411 , the serving BS  405  assigns the third RS  407  the CID set that will be managed by the third RS  407 , and the CID set includes a Basic CID set 1  and a Primary management CID set 1 . 
   As shown in  FIG. 4 , the Basic CID set 1  assigned to the third RS  407  includes a plurality of CIDs ranging from x+1 to y, and the Primary management CID set 1  includes a plurality of CIDs ranging from a+1 to b. Each of the Basic CID set 1  and the Primary management CID set 1  includes successive CIDs. The Basic CID set 1  and the Primary management CID set 1  can include non-successive CIDs. 
   In step  413 , the serving BS  405  assigns the second RS  403  a Basic CID set 2  and a Primary management CID set 2 . In step  415 , the serving BS  405  assigns the fourth RS  409  a Basic CID set 3  and a Primary management CID set 3 . 
   In step  417 , the serving BS  405  assigns the first RS  401  a Basic CID set 4  and a Primary management CID set 4 . 
   The RSs  401 ,  403 ,  407 , and  409  may be infrastructure RSs that are installed by a service provider and are managed by the BS  405  knowing the existence of the RSs  401 ,  403 ,  407 , and  409 . The RSs  401 ,  403 ,  407 , and  409  can be client RSs that operate as subscriber terminals (SSs or MSs) or RSs according to environments. The RSs  401 ,  403 ,  407 , and  409  may be fixed RSs, nomadic RSs having nomadic characteristics such as notebook computers, or mobile RSs having mobility such as MSs. 
   After all the RSs  401 ,  403 ,  407 ,  409  are assigned the CID sets, the serving BS  405  records information about the CID set assignment into a CID assignment table (database). 
   Table 4 below shows an example of the CID assignment table of the BS  405 . 
   
     
       
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
                 
                 
               Primary management 
                 
             
             
               Basic CID set 
               RS 
               CID set 
               RS 
             
             
                 
             
           
           
             
               0x0001~x 
               RS2 
               m + 1~a 
               RS2 
             
             
               x + 1~y 
               RS3 
               a + 1~b 
               RS3 
             
             
               y + 1~z 
               RS4 
               b + 1~c 
               RS4 
             
             
               . . . 
               not assigned 
               . . . 
               not assigned 
             
             
               ~m 
               RS1 
               ~2m 
               RS1 
             
             
                 
             
           
        
       
     
   
   The serving BS  405  can effectively manage the Basic CID sets and the Primary management CID sets assigned to the RSs  401 ,  403 ,  407 , and  409  by using the CID assignment table. 
   Each of the RSs  401 ,  403 ,  407 , and  409  re-assigns the CID set to an MS that requests ranging. 
     FIG. 5  is a diagram for explaining a CID assignment process in which an RS assigns CIDs to MSs in a multi-hop relay BWA system according to the present invention. An RS receives RNG-REQ messages from two MSs. 
   Referring to  FIG. 5 , in step  511 , a serving BS  507  assigns an RS  505  a CID set 1 . For example, the CID set 1  may include a Basic CID set 1  used for transmitting control signals and a Primary management CID set 1  used for a network entry procedure. 
   In step  513 , the RS  505  receives an RNG-REQ message from a first MS  503 . In step  515 , the RS  505  assigns the first MS  503  one CID of the CID set 1  previously received from the serving BS  507 , constructs an RNG-RSP message including the CID assigned to the first MS  503 , and sends the RNG-RSP message to the first MS  503 . The CID assigned to the first MS  503  is used before actual data transmission and may include a Primary management CID and a Basic CID. In step  517 , the RS  505  informs the serving BS  507  of the CID assigned to the first MS  503 . Then, the serving BS  507  manages CIDs assigned to each MS using a CID assignment table such as shown in Table 5 below. 
   In step  519 , the RS  505  receives an RNG-REQ message from a second MS  501 . In step  521 , the RS  505  assigns the second MS  501  one CID of the CID set 1  previously received from the serving BS  507 , constructs an RNG-RSP message including the CID assigned to the second MS  501 , and sends the RNG-RSP message to the second MS  501 . The CID assigned to the second MS  501  is used before actual data transmission and may include a Primary management CID and a Basic CID. In step  523 , the RS  505  informs the serving BS  507  of the CID assigned to the second MS  501 . Then, the serving BS  507  manages CIDs assigned to each MS using the CID assignment table such as shown in Table 5 below. 
   
     
       
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
                 
                 
               Primary management 
                 
             
             
               Basic CID set 
               RS 
               CID set 
               RS 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               0x0001~x 
               RS2 
               0x0001 
               MS5 
               m + 1~a 
               RS2 
               m + 1 
               MS5 
             
             
                 
                 
               0x0002 
               . . . 
                 
                 
               m + 2 
               . . . 
             
             
                 
                 
               . . . 
               . . . 
                 
                 
               . . . 
               . . . 
             
             
                 
                 
               x 
               . . . 
                 
                 
               a 
               . . . 
             
             
               x + 1~y 
               RS3 
               x + 1 
                 
               a + 1~b 
               RS3 
             
             
                 
                 
               x + 2 
             
             
                 
                 
               . . . 
             
             
                 
                 
               y 
             
             
               y + 1~z 
               RS4 
               y + 1 
                 
               b + 1~c 
               RS4 
             
             
                 
                 
               y + 2 
             
             
                 
                 
               . . . 
             
             
                 
                 
               z 
             
             
               . . . 
               not 
                 
                 
               . . . 
               not 
             
             
                 
               assigned 
                 
                 
                 
               assigned 
             
             
               ~m 
               RS1 
                 
                 
               ~2m 
               RS1 
             
             
                 
             
           
        
       
     
   
   The serving BS  507  preassigns the RS  505  a plurality of CIDs. Therefore, when an RNG-REQ message is received from the MS  501  or  503 , the RS  505  can assign the MS  501  or  503  one of the CIDs. That is, an MS can be assigned a CID directly by an RS. 
   The steps  517  and  523  in which the RS  505  informs the serving BS  409  of the CIDs assigned to the first and the second MSs  501  and  503  can be omitted. In this case, when the BS  507  receives a message from the first MS  503  (or the second MS  501 ), the BS  507  can determine that which CID is assigned to the first MS  503  since the message includes the CID assigned to the first MS  503 . Therefore, although the RS  505  does not inform the BS  507  of the CIDs assigned to the MSs  501  and  503 , the BS  507  can construct the CID assignment table such as Table 5 using messages from the MSs  501  and  503 . 
     FIG. 6  is a flowchart for explaining a communication process of an RS in a multi-hop relay BWA system according to the present invention.  FIG. 6  describes a communication process of an RS after the RS is assigned a CID set from a BS through an initial access procedure or an additional signaling procedure. 
   Referring to  FIG. 6 , in step  601 , an RS determines if an RNG-REQ message is received from an MS. If so, the RS retrieve a MAC address of the MS from the RNG-REQ message in step  603 . 
   In step  605 , the RS accesses a CID table where CIDs assigned by a serving BS are stored. In step  607 , the RS determines if the CID table contains a CID that can be assigned to the MS. If not, the RS goes to step  617  where the RS performs a predetermined procedure. For example, in step  617 , the RS may reject the RNG-REQ of the MS or request the serving BS to send a CID set. 
   If the CID table contains a CID that can be assigned to the MS; the RS goes to step  609  where the RS assigns the CID (including a Primary management CID and a Basic CID) to the MS and updates the CID table using the CID assigned to the MS and the MAC address of the MS. 
   In step  611 , the RS processes the RNG-REQ message of the MS except for the MAC address field that is already processed. In step  613 , the RS generates an RNG-RSP message using the results obtained from the processing of the RNG-REQ message. The RNG-RSP message includes the Primary management CID and the Basic CID that are assigned to the MS. In step  615 , the RS transmit the RNG-RSP message to the MS. 
     FIG. 7  is a block diagram illustrating a structure of an RS in a multi-hop relay BWA system according to the present invention. An RS using a Time Division Duplex (TDD)-OFDMA scheme is described, concentrating on the control message processing of the RS. 
   Referring to  FIG. 7 , the RS includes a radio frequency (RF) processor  701 , an analog-to-digital converter (ADC)  703 , an OFDM demodulator  705 , a decoder  707 , a message processor  709 , a controller  711 , a CID table  713 , a message generator  715 , an encoder  717 , an OFDM modulator  719 , a digital-to-analog converter (DAC)  721 , an RF processor  723 , a switch  725 , and a time controller  727 . 
   The time controller  727  controls a switching operation of the switch  725  based on frame synchronization. For example, when in an RX section of a frame, the time controller  727  controls the switch  725  so that an antenna is connected to the RF processor  701  to receive a signal from an MS or a BS. When in a TX section of the frame, the time controller  727  controls the switch  725  so that the antenna is connected to the RF processor  723  to transmit a signal to the MS or the RS. 
   In the RX section of the frame, the RF processor  701  converts an RF signal received through the antenna into a baseband analog signal. The ADC  703  samples the analog signal received from the RF processor  701  so as to convert the analog signal into a sampled data (digital signal). The OFDM demodulator  705  performs Fast Fourier Transform (FFT) on the sampled data to output frequency-domain data. 
   The decoder  707  selects data of desired subcarriers from the frequency-domain data. Then, the decoder  707  demodulates and decodes the selected data depending on a predetermined modulation and coding scheme (MCS) level. 
   The message processor  709  processes a control message from the decoder  707  and sends the results of the processing to the controller  711 . When the RS receives an RNG-REQ message, the message processor  709  extracts various information from the RNG-REQ message and provides the extracted information to the controller  711 . 
   The controller  711  performs processing according to the information received from the message processor  709  and provides the results of the processing to the message generator  715 . The CID table  713  is a database for managing CIDs assigned from a serving BS. For example, the CID table  713  stores a Primary management CID set and a Basic CID set that are assigned from a serving BS. 
   When an RNG-REQ message is received from an MS, the controller  711  accesses the CID table  713  to determine whether an available CID is in the CID table  713 . If so, the controller  711  update the CID table  713  by mapping a MAC address of the MS included in the RNG-REG message into the available CID. The controller  711  informs the message generator  715  of the CID assigned to the MS. 
   The message generator  715  generates a message using information received from the controller  711  and provides the message to the encoder  717 . The message generator  715  generates an RNG-RSP message including the received information regarding the CID assigned to the MS and sends the RNG-RSP message to the encoder  717 . 
   The encoder  717  encodes and modulates the RNG-RSP message received from the message generator  715  according to a predetermined MCS level. The OFDM modulator  719  processes the data (RNG-RSP message) received from the encoder  717  by Inverse Fast Fourier Transform (IFFT), obtaining sampled data (OFDM symbols). The DAC  721  converts the sampled data into an analog signal. The RF processor  723  converts the analog signal received from the DAC convert  721  into an RF signal and transmits the RF signal through the antenna. 
   In the configuration of the RS shown in  FIG. 7 , the controller  711  controls the message processor  709 , the message generator  715 , and use of the CID table  713 , as a protocol controller. The controller  711  can perform the functions of the message processor  709 , the message generator  715 , and the CID table  713 . Although separate units are provided for respective functions of the controller  711  in the present embodiment, the controller  711 can perform all or some of the functions instead of such separate units. 
   A CID is assigned to an MS during an initial ranging procedure. However, if necessary, the CID assignment can be applied to other signaling procedure. Although a Primary management CID and a Basic CID are assigned to an MS in the current embodiment, other CIDs can be assigned to the MS in the same way. 
   As described above, in a multi-hop relay BWA system, a serving BS assigns an RS a CID set, and the RS manages the assigned CID set. When the RS receives an RNG-REQ message from an MS, the RS can assign the MS a CID (one of the CID set previously received from the BS) without communication with the serving BS. Unnecessary communication between the serving BS and the RS can be removed in an initial ranging procedure, thereby providing fast initial ranging and saving wireless resources between the serving BS and the RS. Furthermore, even when the RS does not inform the serving BS of the CID assigned to the MS, the serving BS can get information-about the CID assignment since a message transmitted by the MS includes the CID assigned to the MS. 
   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 as defined by the appended claims.