Patent Publication Number: US-2023156567-A1

Title: Apparatus and method for controlling a terminal accessing a base station in broadband wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation application of prior application Ser. No. 17/321,978, filed on May 17, 2021, which is a continuation application of prior application Ser. No. 16/552,485, filed on Aug. 27, 2019, which has issued as U.S. Pat. No. 11,012,918 on May 18, 2021, which is a continuation application of prior application Ser. No. 15/974,361, filed on May 8, 2018, which has issued as U.S. Pat. No. 10,506,495 on Dec. 10, 2019, which is a continuation application of prior application Ser. No. 12/326,376, filed on Dec. 2, 2008, which has issued as U.S. Pat. No. 9,986,488 on May 29, 2018 and was based on and claimed priority under 35 U.S.C § 119(a) of a Korean patent application number 10-2007-0128178, filed on Dec. 11, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a broadband wireless communication system, and in particular, to an apparatus and a method for controlling a terminal accessing a base station in a broadband wireless communication system. 
     Description of the Related Art 
     In a 4 th  generation (4G) communication system, research for providing services having various Quality of Service (QoS) using transmission speeds of about 100 Mbps is in active progress. Particularly, research for supporting high speed services in the form of guaranteeing mobility and QoS for a broadband wireless access (WBA) communication system, such as short-distance wireless communication network system and wireless metropolitan area network (WMAN) system, is in active progress. As an example, communication systems complying with the Institute of Electrical and Electronics Engineers (IEEE) 802.16 specification is representative of such a system. 
     The IEEE 802.16 system standard defines various procedures, such as the operation of a Media Access Control (MAC) layer. Examples of MAC layer procedures include an initial ranging procedure and a handover procedure. Described below are procedures for initial ranging and handover initiated by a terminal in accordance with the IEEE 802.16 system standard. 
     The initial ranging procedure according to the IEEE 802.16 system standard is performed as follows. To initiate initial ranging, a terminal transmits one of a plurality of prescribed ranging codes to a base station. In response thereto, the base station transmits a response message RaNGing ReSPonse (RNG_RSP) message to the terminal. The RNG_RSP message includes ranging status TypeLengthValue (TLV). If the base station is in a status that does not allow ranging of the terminal (i.e., the entry of the terminal to the base station), the base station sets the ranging status TLV to “abort” to inform the terminal of a ranging-not-allowed status. Accordingly, the terminal detects when its request for entry is not allowed and then scans other base stations for entry. 
     The handover procedure according to the IEEE 802.16 system standard is performed as follows. To initiate handover, a terminal scans neighboring base stations and transmits information of scanned neighboring base stations MOBile_Mobile Station HandOver-REQuest (MOB_MSHO-REQ) message that includes information of target base station candidates to its serving base station. Accordingly, the serving base station confirms whether the target base station candidates included in the MOB_MSHO-REQ message received from the terminal can support service for the terminal. At this point, the serving base station transmits a MOBile_Base Station HandOver-ReSPonse (MOB_BSHO-RSP) message indicating that a target base station has been found to the terminal. The MOB_BSHO-RSP message includes information of the target base station candidates that can provide the requested handover. The terminal that has received the MOB_BSHO-RSP message selects a target base station and transmits a MOBile_HandOver-INDication (MOB_HO-IND) message to inform the serving base station of the selected target base station. The serving base station that has confirmed the target base station selected by the terminal provides information regarding the terminal to the target base station, and the terminal performs an entry procedure to connect to the target base station. 
     As described above, the base station can limit entry of the terminal during the initial ranging procedure. However, the base station cannot prevent the terminal from immediately trying to access the base station again, that is, retrying the initial ranging procedure. When the terminal immediately re-requests the base station for entry, there is a high possibility that the entry request will be denied again since the status of the base station has not yet changed. Therefore, immediately requesting entry again by the terminal causes waste of wireless resources. In addition, a terminal that has performed handover to change the serving base station may initiate handover again to the previous serving base station. Repeatedly performing handover between two base stations also causes waste of wireless resources, and repeatedly attempting handover itself wastes wireless resources. Therefore, there is a need to prevent unnecessary attempts for entry as well as actual execution of entry procedures. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an apparatus and a method to prevent unnecessary attempts for entry and access of a network in a broadband wireless communication system. 
     Another object of the present invention is to provide an apparatus and a method to prohibit a terminal from attempting re-entry to a specific base station in a broadband wireless communication system. 
     Still another object of the present invention is to provide an apparatus and a method for informing an entry prohibition time of a specific terminal with respect to a specific base station in a broadband wireless communication system. 
     According to an aspect of the present invention, a base station apparatus in a broadband wireless communication system includes a controller to determine whether to prohibit an entry attempt of a terminal, a generator to generate a media access control (MAC) management message including a value of a timer, the value representing prohibition of entry attempt, and a transmitter to transmit the MAC management message to the terminal. 
     According to another aspect of the present invention, a terminal apparatus in a broadband wireless communication system includes a receiver to receive a media access control (MAC) management message from a base station, a reader to confirm existence of a value of a timer included in the MAC management message, the value representing prohibition of entry attempt, and a controller to designate the base station as prohibited for entry attempt until expiration of the timer. 
     According to still another aspect of the present invention, a method for operating a base station in a broadband wireless communication system includes determining whether to prohibit an entry attempt of a terminal, generating a media access control (MAC) management message including a value of a timer, the value representing prohibition of entry attempt, and transmitting the MAC management message to the terminal. 
     According to yet another aspect of the present invention, a method for operating a terminal in a broadband wireless communication system includes receiving a media access control (MAC) management message from a base station, confirming existence of a value of a timer included in the MAC management message, the value representing prohibition of entry attempt, and designating the base station as being prohibited from entry attempt until expiration of the timer. 
     According to yet another aspect of the present invention, a circuit includes a controller to determine whether to prohibit entry of an end unit to a base station in a wireless communication network, the controller configured to process a media access control (MAC) management message that includes a timer value, the timer value designating a time period during which entry is prohibited. 
     According to yet another aspect of the present invention, a method includes determining whether to prohibit entry of an end unit to a base station in a wireless communication network and processing a media access control (MAC) management message that includes a timer value, the timer value designating a time period during which entry is prohibited. 
    
    
     
       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 view illustrating an example of signal exchange during an initial ranging procedure in a broadband wireless communication system according to the present invention; 
         FIG.  2    is a view illustrating an example of signal exchange during a handover procedure in a broadband wireless communication system according to the present invention; 
         FIG.  3    is a block diagram illustrating a base station in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIG.  4    is a block diagram illustrating a terminal in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIG.  5    is a view illustrating the operating procedure of a base station in a broadband wireless communication system according to an exemplary embodiment of the present invention; and 
         FIG.  6    is a view illustrating the operating procedure of a terminal in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described herein below with reference to the accompanying drawings. 
     Exemplary embodiments of the present invention provide a system and method to prevent unnecessary attempts for entry and access to a base station by a terminal in a broadband wireless communication system. For purposes of example, the exemplary embodiments of the present invention are illustrated in conjunction with a wireless communication system using an orthogonal frequency division multiple access (OFDMA) scheme. However, it is to be understood that the present invention may be implemented on wireless communication systems using other schemes without departing from the scope of the present invention. 
     To limit attempts of entry by a terminal, the present invention defines a TypeLengthValue (TLV) for exchanging entry limit timer information as illustrated in Table 1. The TLV defined by the present invention may be included in various MAC management messages, such as RaNGing-ReSPonse (RNG-RSP) messages, Base Station HandOver-REQuest (BSHO-REQ) messages, and Base Station HandOver-ReSPonse (BSHO-RSP) messages. However, it is to be understood that the entry limit timer TLV of the present invention may be included in other messages without departing from the scope of the present invention. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Type 
                 Length 
                 Value 
                 Scope 
               
               
                   
                   
               
             
            
               
                   
                 Xx 
                 1 
                 0-255: In units of 100 
                 RNG-RSP 
               
               
                   
                   
                   
                 milliseconds 
                 BSHO-REQ 
               
               
                   
                   
                   
                   
                 BSHO-RSP 
               
               
                   
                   
               
            
           
         
       
     
     As illustrated in Table 1, a TLV for limiting entry of a terminal according to the present invention represents a timer value. In an exemplary embodiment, the length of the TLV is 1 byte, and the value thereof expresses entry limit time in units of 100 milliseconds. When the value is 0, it means that an entry limit time is infinite. However, it is to be understood that other data lengths and units of time represented by its value may be used without departing from the scope of the present invention. For convenience in explanation, the present invention calls the TLV on Table 1 an “entry limit timer TLV.” 
       FIG.  1    illustrates an example of signal exchange during an initial ranging procedure between a terminal and a base station in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     As shown in  FIG.  1   , a terminal  110  transmits a ranging request message (e.g., RNG-REQ) to a base station  120  in order to enter a network (step  101 ). The base station  120  that has received the ranging request message (e.g., RNG-REQ) recognizes that the terminal  110  desires to access the base station  120  and determines whether to allow entry of the terminal  110 . Based on the result of the determination step, the base station  120  may decide that entry to the base station  120  cannot be allowed (step  103 ). For example, the base station  120  may determine that the entry of the terminal  110  cannot be allowed due to reasons such as resource shortage, service unavailable, etc. 
     In such a situation, the base station  120  transmits a ranging response message (e.g., RNG-RSP) that includes an entry limit timer TLV to the terminal  110  (step  105 ). That is, the base station  120  transmits the ranging response message (e.g., RNG-RSP) including ranging status TLV (e.g., ranging status TLV set to “abort”) to inform the terminal of the denial of entry to the base station  120  and an entry limit timer TLV to inform the terminal of the prohibition of entry to the base station  120  for a predetermined time. Here, the entry limit timer TLV includes time information prohibiting the entry. At this point, the base station may also incorporate downlink frequency override TLV into the ranging response message (e.g., RNG_RSP). The downlink frequency override TLV includes frequency allocation (FA) information recommended to a terminal designed to induce the terminal to make an entry request through a different frequency. 
     The terminal  110  that has received the ranging response message (e.g., RNG-RSP) including the ranging status TLV and the entry limit timer TLV recognizes that an entry request by the terminal  110  has been rejected, and simultaneously, confirms the entry limit timer TLV to recognize that attempts of entry to the base station  120  is prohibited for the predetermined time (step  107 ). 
     Accordingly, the terminal  110  does not try entry to the base station  120  for the predetermined time but rather scans other accessible base stations (step  109 ). Further, if the ranging response message (e.g., RNG-RSP) includes the downlink frequency override TLV, the terminal  110  scans for accessible base stations at the different FA designated by the downlink frequency override TLV. If, on the other hand, the ranging response message (e.g., RNG-RSP) does not include the downlink frequency override TLV, the terminal  110  scans for accessible base stations at the same FA. 
     After the scanning step  109 , the terminal  110  performs a network entry procedure again. At this point, even though the base station  120  is determined to be the most appropriate base station to connect to, i.e., has the best channel status, the terminal  110  does not try entry to the base station  120  while the predetermined time confirmed in step  107  has not expired. 
       FIG.  2    illustrates an example of signal exchange during a handover procedure between a terminal and a base station in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     As shown in  FIG.  2   , a terminal  210  transmits a handover request message (e.g., MOB_MSHO-REQ) to a serving base station  220  in order to switch (i.e., handover) to an adjacent base station (step  201 ). Here, the handover request message (e.g., MOB_MSHO-REQ) may include information of a target base station  230 . 
     The serving base station  220  that has received the handover request message (e.g., MOB_MSHO-REQ) transmits a handover pre-notification message (e.g., HO_pre_notification) to the target base station  230  (step  203 ). The handover pre-notification message (e.g., HO_pre_notification) is a message for confirming whether the entry of the terminal  210  can be accepted. The target base station  230  that has received the handover pre-notification message (e.g., HO_pre_notification) determines whether the entry of the terminal  210  may be accepted and transmits a handover pre-notification response message (e.g., HO_pre_notification-RSP) informing the acceptance/denial of the handover request to the serving base station  220  (step  205 ). 
     If the handover request has been accepted by the target base station  230 , the serving base station  220  that has received the handover pre-notification response message (e.g., HO_pre_notification-RSP) transmits a base station handover response message (e.g., MOB_BSHO-RSP) including an entry limit timer TLV to the terminal  210  (step  207 ). That is, the serving base station  220  transmits the base station handover response message (e.g., MOB_BSHO-RSP) to the terminal that includes information of the target base station  230  that has accepted the entry request of the terminal  210  and an entry limit timer TLV to inform the terminal of prohibition of entry to the serving base station  220  for a predetermined time. 
     The terminal  210  that has received the base station handover response message (e.g., MOB_BSHO-RSP) recognizes the target base station  230  that has accepted the entry request of the terminal  210  and simultaneously confirms the entry limit timer TLV to recognize that entry to the serving base station  220  is prohibited for the predetermined time (step  209 ). Thereafter, once the terminal  210  decides to initiate handover to the target base station  230 , the terminal  210  transmits a handover indication message (e.g., MOB_HO-IND) to inform the serving base station  220  that handover to the target base station  230  is to be performed (step  211 ). 
     The serving base station  220  that has received the handover indication message (e.g., MOB_HO-IND) provides information of the terminal  210  to the target base station  230  (step  213 ). For example, the information of the terminal  210  may include information of service in use and ability information of the terminal  210 . Once the target base station  230  receives the information of the terminal  210 , the terminal  210  and the target base station  230  exchange access procedure messages and initiates access (step  215 ). 
     Once the process illustrated in  FIG.  2    have been completed, the terminal  210  does not try entry to the serving base station  220  if the predetermined time confirmed in step  209  has not expired, even if the serving base station  220  has been considered for handover. In other words, when the predetermined time has not lapsed, the terminal  210  suspends handover to the serving base station  220 . Though only one target base station is illustrated as a target base station candidate in  FIG.  2    for convenience, a plurality of target base station candidates may exist. 
       FIG.  3    illustrates a block diagram of a base station in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     As shown in  FIG.  3   , the base station includes an RF receiver  302 , an OFDM demodulator  304 , a subcarrier demapper  306 , a symbol demodulator  308 , a decoder  310 , a ranging code detector  312 , a message reader  314 , an MAC controller  316 , a message generator  318 , an encoder  320 , a symbol modulator  322 , a subcarrier mapper  324 , an OFDM modulator  326 , and an RF transmitter  328 . 
     The RF receiver  302  down-converts a radio frequency (RF) signal received through an antenna into a baseband signal. The OFDM demodulator  304  classifies a signal provided by the RF receiver  302  on an OFDM symbol basis, removes a cyclic prefix (CP), and restores complex symbols mapped to a frequency domain through a fast Fourier transform (FFT) operation. The subcarrier demapper  306  extracts complex symbols mapped to the frequency domain on a process basis. For example, the subcarrier demapper  306  extracts signals mapped to a ranging subcarrier and provides the extracted signals to the ranging code detector  312 . The symbol demodulator  308  demodulates complex symbols and converts the demodulated complex signals into a bit stream. The decoder  310  channel-decodes the bit stream to restore an information bit stream. 
     The ranging code detector  312  detects a ranging code transmitted by a terminal. That is, the ranging code detector  312  detects which ranging code has been transmitted by performing a correlation operation on signals mapped to a ranging subcarrier and ranging codes prescribed in advance. 
     The message reader  314  reads an MAC management message received from a terminal. For example, the message reader  314  reads a ranging request message (e.g., RNG-REQ) from a terminal for entry into a network to confirm the terminal intending to enter the network. Also, the message reader  314  confirms target base station candidates by reading a handover request message (e.g., MOB_MSHO-REQ) from a terminal and confirms a target base station to which information of the terminal should be provided by reading a handover indication message (e.g., MOB_HO-IND) that includes information of the target base station determined by the terminal. 
     The MAC controller  316  controls overall functions of the MAC layer of the base station. For example, when a terminal tries initial ranging, the MAC controller  316  determines whether to allow entry of the terminal on the basis of available wireless resources and available services, for example. Also, when a terminal requests handover, the MAC controller  316  confirms with a target base station as to whether the handover of the terminal is acceptable. 
     Particularly, in accordance with the present invention, the MAC controller  316  determines whether to prohibit entry attempts by the terminal. In this case, entry (i.e., access to a base station) may be by initial ranging or handover. For example, in the case where a terminal is denied entry to the base station through initial ranging, the MAC controller  316  prohibits the denied terminal from trying entry (i.e., initial ranging) to the base station for a predetermined time. In the case where a terminal performs handover to an adjacent base station, the MAC controller  316  prohibits the terminal performing the handover from trying entry (i.e., handover) to the base station for a predetermined time. 
     The message generator  318  generates an MAC management message to be transmitted to a terminal. For example, the message generator  318  generates a ranging response message (e.g., RNG-RSP), which is a response to an entry attempt by the terminal, a base station handover response message (e.g., MOB_BSHO-RSP), which is a response to a handover request from the terminal, and a base station handover request message (e.g., MOB-BSHO-REQ) to request the terminal to perform a handover. Particularly, in accordance with the present invention, the message generator  318  generates a parameter representing prohibition of entry attempts to the base station for a predetermine time, such as an MAC management message including an entry limit timer TLV. For example, the entry limit timer TLV may be included in the ranging response message (e.g., RNG-RSP), the base station handover response message (e.g., MOB_BSHO-RSP), or the base station handover request message (e.g., MOB_BSHO-REQ) depending on entry attempt. The entry limit timer TLV may be configured as shown in Table 1. 
     When the MAC management message, the ranging response message (e.g., RNG-RSP), for example, the ranging response message (e.g., RNG-RSP) may include at least one of a parameter representing denial of entry (e.g., ranging status TLV), a parameter representing prohibited time period for entry (e.g., entry limit timer TLV), and a parameter indicating a different frequency (FA) (e.g., downlink frequency override TLV) through which an entry is recommended. When the MAC management message is the base station handover response message (e.g., MOB_BSHO-RSP) or the base station handover request message (e.g., MOB_BSHO-REQ), the base station handover response/request message (e.g., MOB_BSHO-RSP or MOB_BSHO-REQ) may include at least one of a parameter representing prohibited time period for entry (e.g., entry limit timer TLV) and a parameter identifying target base station candidate(s) that can accept a handover. 
     The encoder  320  channel-codes an information bit stream. The symbol modulator  322  modulates the channel-coded bit stream and converts the modulated bit stream into complex symbols. The subcarrier mapper  324  maps the complex symbols to a frequency domain. The OFDM modulator  326  converts the complex symbols mapped to the frequency domain into signals in a time domain through the inverse fast Fourier transform (IFFT) operation and inserts a CP to form an OFDM symbol. The RF transmitter  328  up-converts a baseband signal into an RF signal and transmits the RF signal through the antenna. 
       FIG.  4    illustrates a block diagram of a terminal in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     As shown in  FIG.  4   , the terminal includes an RF receiver  402 , an OFDM demodulator  404 , a subcarrier demapper  406 , a symbol demodulator  408 , a decoder  410 , a signal strength measuring unit  412 , a message reader  414 , an MAC controller  416 , a message generator  418 , an encoder  420 , a symbol modulator  422 , a subcarrier mapper  424 , an OFDM modulator  426 , and an RF transmitter  428 . 
     The RF receiver  402  down-converts an RF signal received through an antenna into a baseband signal. The OFDM demodulator  404  classifies a signal provided from the RF receiver  402  on an OFDM symbol basis, removes a CP, and restores complex symbols mapped to a frequency domain through the FFT operation. The subcarrier demapper  406  extracts complex symbols mapped to the frequency domain on a process basis. For example, while the terminal scans a base station for network entry or handover, the subcarrier demapper  406  extracts a signal for scanning, e.g., a preamble signal, and provides the preamble signal to the signal strength measuring unit  412 . The symbol demodulator  408  demodulates complex symbols and converts the demodulated complex symbols into a bit stream. The decoder  410  restores an information bit stream by channel-decoding the bit stream. 
     The signal strength measuring unit  412  measures strength of a signal received from a base station. For example, the received signal strength can be expressed by received signal strength indication (RSSI), signal-to-noise ratio (SNR), signal-to-interface-and-noise ratio (SINR), carrier-to-interference-and-noise ratio (CINR), etc. 
     The message reader  414  reads an MAC management message received from a base station. For example, the message reader  414  confirms whether initial ranging is allowed by reading a ranging response message (e.g., RNG-RSP), which is a response to an initial ranging request, confirms a target base station that can accept a handover by reading a base station handover response message (e.g., MOB_BSHO-RSP), which is a response to a handover request, and confirms an acceptable target base station with respect to a handover by reading a base station handover request message (e.g., MOB-BSHO-REQ) to request the terminal to perform a handover. Particularly, in accordance with the present invention, the message reader  414  confirms a parameter representing prohibition of entry attempts to a base station for a predetermined time, that is, the entry limit timer TLV. In this case, entry (i.e., access to a base station) may be by initial ranging or handover. For example, the entry limit timer TLV may be included in the ranging response message (e.g., RNG-RSP), the base station handover response message (e.g., MOB_BSHO-RSP), or the base station handover request message (e.g., MOB_BSHO-REQ) depending on the entry attempt. The entry limit timer TLV may be configured as shown in Table 1. 
     The MAC controller  416  controls overall functions of the MAC layer of the terminal. For example, when the terminal tries initial ranging, the MAC controller  416  controls the terminal to generate and transmit a ranging request message (e.g., RNG-REQ). Also, when the terminal requests a handover, the MAC controller  416  controls the terminal to generate and transmit a handover request message (e.g., MOB_MSHO-REQ) including information of collected target base station candidates. The MAC controller  416  controls the terminal depending on allowance/denial status included in a ranging response message (e.g., RNG-RSP), which is a response to an initial ranging attempt. For example, in the case where an entry to a base station through initial ranging is rejected, the MAC controller  416  controls the terminal to scan for other base stations to enter the network. In the case of handover, the MAC controller  416  controls the operation of the terminal depending on information of target base station candidates that can accept a handover included in a base station handover response message (e.g., MOB_BSHO-RSP), which is a response to a handover request from the terminal. That is, the MAC controller  416  controls the terminal to select one of the target base station candidates that can accept a handover and transmit a handover indication message (e.g., MOB_HO-IND) to inform the selected target base station. 
     Particularly, in accordance with the present invention, when an entry limit timer TLV is detected in a received MAC management message, the MAC controller  416  prohibits entry attempts to the base station that has transmitted the MAC management message for a predetermined time designated by the entry limit timer TLV, that is, until the timer has expired. For example, in the case where the entry limit timer TLV is included in the ranging response message (e.g., RNG-RSP), the MAC controller  416  prohibits entry attempts to the base station that has transmitted the ranging response message (e.g., RNG-RSP) for a predetermined time. That is, when the base station that has transmitted the ranging response message is selected again as a result of scanning for base stations in order to enter a network, the MAC controller  416  controls the terminal to try initial ranging to one of the base stations other than the base station that has transmitted the ranging response message if the predetermined time has not expired. When the entry limit timer TLV is included in the base station handover response message (e.g., MOB_BSHO-RSP) or the base station handover request message (e.g., MOB_BSHO-REQ), the MAC controller  416  prohibits entry attempts, i.e., attempts for handover, to the base station that has transmitted the base station handover response/request message (e.g., MOB_BSHO-RSP or MOS_BSHO-REQ) for a predetermined time. 
     The message generator  418  generates an MAC management message transmitted to a base station. For example, the message generator  418  generates a handover request message (e.g., MOB_MSHO-REQ) to request a handover, a handover indication message (e.g., MOB_HO-IND) that includes information of a selected target base station for handover, and a ranging request message (e.g., RNG-REQ) to initiate initial ranging. 
     The encoder  420  channel-codes an information bit stream. The symbol modulator  422  modulates the channel-coded bit stream and converts the modulated bit stream into complex symbols. The subcarrier mapper  424  maps the complex symbols to a frequency domain. The OFDM modulator  426  converts the complex symbols mapped to the frequency domain into signals in a time domain through the IFFT operation, and inserts a CP to form an OFDM symbol. The RF transmitter  428  up-converts a baseband signal into an RF signal, and transmits the RF signal through the antenna. 
       FIG.  5    illustrates the operating procedure of a base station in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     As shown to  FIG.  5   , in step  501 , the base station confirms whether a terminal performs handover to an adjacent base station, or whether an initial network entry of the terminal is rejected. Specifically, when the terminal performs the handover, the base station confirms whether at least one of target base station candidates can accept the handover of the terminal. Alternatively, when an entry request of the terminal through initial ranging occurs, the base station confirms whether entry is rejected because of resource shortage or service unavailable. 
     In step  503 , once the terminal performs handover to an adjacent base station or the initial network entry of the terminal is rejected, the base station determines to prohibit the terminal from trying entry to the base station for a predetermined time. Here, entry (i.e., access to the base station) may be by initial ranging or handover. It is to be understood that the length of time for prohibition (i.e., the predetermined time) may vary depending on the specific instance without departing from the scope of the present invention. 
     In step  505 , once it has been determined to prohibit the terminal from trying entry, the base station generates a parameter representing prohibition of entry attempts, that is, an MAC management message including an entry limit timer TLV. For example, the entry limit timer TLV may be included in the ranging response message (e.g., RNG-RSP), the base station handover response message (e.g., MOB_BSHO-RSP), or the base station handover request message (e.g., MOB_BSHO-REQ) depending on the attempted entry. The entry limit timer TLV may be configured as shown in Table 1. 
     When the MAC management message is the ranging response message (e.g., RNG-RSP), the ranging response message may include at least one of a parameter representing denial of entry (e.g., ranging status TLV), a parameter representing prohibited time period for entry (e.g., an entry limit timer TLV), and a parameter indicating a different frequency (FA) (e.g., downlink frequency override TLV) through which an entry is recommended. When the MAC management message is the base station handover response message (e.g., MOB_BSHO-RSP) or the base station handover request message (e.g., MOB_BSHO-REQ), the base station response/request message (e.g., MOB_BSHO-RSP or MOB_BSHO-REQ) may include at least one of a parameter representing prohibited time period for entry (e.g., entry limit timer TLV) and a parameter identifying target base station candidate(s) that can accept a handover. 
     In step  507 , the base station transmits the MAC management message generated in step  505  to the appropriate terminal. 
       FIG.  6    illustrates the operating procedure of a terminal in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     As shown to  FIG.  6   , in step  601 , the terminal confirms whether an MAC management message is received from a base station. 
     In step  603 , when the MAC management message is received, the terminal confirms whether a parameter representing prohibition of entry attempts (i.e., an entry limit timer TLV) is included in the MAC management message by reading the MAC management message. Here, entry (i.e., access to a base station) may be by initial ranging or handover. For example, the entry limit timer TLV may be included in a ranging response message (e.g., RNG-RSP), a base station handover response message (e.g., MOB_BSHO-RSP), or a base station handover request message (e.g., MOB_BSHO-REQ) depending on the attempted entry. The entry limit timer TLV may be configured as shown in Table 1. That is, the terminal confirms the existence of entry limit timer TLV by reading the ranging response message (e.g., RNG-RSP), the base station handover response message (e.g., MOB_BSHO-RSP), and the base station handover request message (e.g., MOB_BSHO-REQ). If the entry limit timer TLV is not detected in the MAC management message, the terminal performs step  607  (described below). 
     When the entry limit timer TLV is detected, in step  605 , the terminal sets prohibition of entry attempts to the base station that has transmitted the MAC management message for a predetermined time, that is, until expiration of a timer. Accordingly, the terminal does not try entry to the base station when the predetermined time has not expired even if the base station is selected as an optimum base station after scanning for network entry or for handover. 
     Subsequently, in step  607 , the terminal performs MAC layer operation according to the received MAC management message. For example, when the MAC management message is a ranging response message (e.g., RNG-RSP) and ranging status TLV is set to “abort,” the terminal scans for base stations to enter the network. Alternatively, when the MAC management message is a base station handover response/request message (e.g., MOB_BSHO-RSP or MOB_BSHO-REQ), the terminal selects a target base station to which handover is to be made and transmits information of the selected target base station to a serving base station. 
     Accordingly, unnecessary entry attempts and unnecessary execution of entry procedures by a terminal in a broadband wireless communication system are prevented due to prohibition of re-entry attempts using a parameter representing a time period prohibiting entry attempts when initial ranging is rejected or handover of the terminal. 
     Although the invention has been shown and described with reference to certain exemplary 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. Therefore, the scope of the present invention should not be limited to the above-described embodiments but should be determined by not only the appended claims but also the equivalents thereof.