Abstract:
An apparatus and method for signal detection in a Broadband Wireless Access (BWA) system are provided, in which in a transmitter, a first preamble sequence generator generates preamble sequences, a low-Peak-to-Average Power Ratio (PAPR) preamble sequence selector selects a first low-PAPR preamble sequence from among the generated preamble sequences, a preamble sequence copier copies the first low-PAPR preamble sequence at least once and outputs the first low-PAPR preamble sequence and the at least one copy, a complex conjugator generates preamble sequences orthogonal to the first low-PAPR preamble sequence and the at least one copy by calculating complex conjugates of the first low-PAPR preamble sequence and the at least one copy, and a second preamble sequence generator generates at least one second low-PAPR preamble sequence using the first low-PAPR preamble sequence.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on May 3, 2006 and assigned Serial No. 2006-39821, 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 Broadband Wireless Access (BWA) system, and in particular, to an apparatus and method for identifying a Relay Station (RS) or a Base Station (BS) by its orthogonal preamble signal with a low Peak-to-Average Power Ratio (PAPR) in a system having 1-hop RSs or multi-hop RSs. The present invention also relates to an apparatus and method for generating another preamble signal using a preamble signal. 
     2. Description of the Related Art 
     Provisioning of services with diverse Quality of Service (QoS) requirements at or above 100 Mbps to users is an active study area for the 4 th  Generation (4G) communication system. Particularly, active research is being conducted on providing high-speed service by ensuring mobility and QoS to a BWA communication system such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN). Such major examples are Institute of Electrical and Electronics Engineers (IEEE) 802.16a and IEEE 802.16e. 
     The IEEE 802.16a and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) for physical channels in order to support a broadband transmission network. IEEE 802.16a considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the SS&#39;s mobility to the IEEE 802.16d communication system. Hereinafter, a mobile SS will be referred to as an MS. 
     In general, since a BS and an MS communicate with each other via a direct link, a highly reliable radio link can easily be established between them in the IEEE 802.16e communication system. However, due to the fixedness of the BSs, the configuration of a wireless network is not flexible, making it difficult to provide an efficient service in a radio environment experiencing a fluctuating traffic distribution and a substantial change in the number of required calls. The above drawback can be overcome by a relay service that delivers data over multiple hops using MSs or fixed or mobile RSs. The use of the multi-hop relay scheme expands cell coverage. 
       FIG. 1  illustrates the configuration of a conventional BWA communication system using RSs. 
     Referring to  FIG. 1 , the BWA communication system is configured in a multi-cell structure. Specifically, it includes cells  100  and  140 , BSs  110  and  150  for managing the respective cells  100  and  140 , a plurality of MSs  111 ,  113 ,  151 ,  153  and  155  within the cells  100  and  140 , a plurality of MSs  121 ,  123 ,  161  and  163  under management of the BSs  110  and  150  but in areas  130  and  170  outside the cells  100  and  140 , and RSs  120  and  160  for providing relay paths between the BSs  110  and  150  and the MSs  121 ,  123 ,  161  and  163  in the areas  130  and  170 . Signaling is carried out in OFDM/OFDMA between the BSs  110  and  150  and the MSs  111 ,  113 ,  121 ,  123 ,  151 ,  153 ,  155 ,  161  and  163 . Although the MSs  111  and  113  and the RS  120  within the cell  100  can communicate directly with the BS  110 , the MSs  121  and  123  in the area  130  cannot communicate directly with the BS  110 . Therefore, the RS  120  covers the area  130  and relays signals between the BS  110  and the MSs  121  and  123 . In other words, the MSs  121  and  123  can send and receive signals to and from the BS  110  via the RS  120 . 
     Although the MSs  151 ,  153  and  155  and the RS  160  within the cell  140  can communicate directly with the BS  150 , the MSs  161  and  163  in the area  170  cannot communicate directly with the BS  150 . Therefore, the RS  160  covers the area  170  and relays signals between the BS  150  and the MSs  161  and  163 . In other words, the MSs  161  and  163  can send and receive signals to and from the BS  150  via the RS  160 . 
     In the OFDM communication system, an MS acquires frequency synchronization and frame synchronization from a BS and an RS and identifies them by their preamble signals. 
     However, since data is sent on a plurality of subcarriers, the amplitude of a final OFDM signal can be expressed as the sum of the amplitudes of the subcarriers. If the subcarriers are at the same phase, the OFDM signal has a very high PAPR. In general, the signal with the very high PAPR is beyond the linear operation range of an amplifier and thus experiences distortion after passing through the amplifier. As a result, the high-PAPR signal not only decreases the efficiency of a linear amplifier but also makes the operation point of a non-linear amplifier enter into a non-linear range, thereby causing inter-modulation and out-of-band spectrum radiation. 
     In contrast, a low-PAPR preamble sequence prevents non-linear distortion in a High Power Amplifier (HPA), thus leading to high-quality signal transmission and highly efficient power amplification. A typical OFDM communication system generates a plurality of low-PAPR preamble sequences, but the number of low-PAPR preamble sequences is limited. 
     Accordingly, there exists a need for an apparatus and method for generating a low-PAPR preamble sequence with orthogonality and generating an additional low-PAPR preamble sequence using the low-PAPR preamble sequence, for use in a BS and an RS. 
     SUMMARY OF THE INVENTION 
     An object 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 object of the present invention is to provide an apparatus and method for identifying a BS and RS by low-PAPR orthogonal preamble signals. 
     Another object of the present invention is to provide an apparatus and method for generating an additional preamble sequence orthogonal to a low-PAPR preamble sequence. 
     In accordance with the present invention, there is provided a transmitter for signal detection in a BWA system, in which a first preamble sequence generator generates preamble sequences, a low-PAPR preamble sequence selector selects a first low-PAPR preamble sequence from among the generated preamble sequences, a preamble sequence copier copies the first low-PAPR preamble sequence at least once and outputs the first low-PAPR preamble sequence and the at least one copy, a complex conjugator generates preamble sequences orthogonal to the first low-PAPR preamble sequence and the at least one copy by calculating complex conjugates of the first low-PAPR preamble sequence and the at least one copy, and a second preamble sequence generator generates at least one second low-PAPR preamble sequence using the first low-PAPR preamble sequence. 
    
    
     
       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  illustrates the configuration of a conventional BWA communication system using RSs; 
         FIG. 2  illustrates a preamble structure and preamble signals according to the present invention; 
         FIG. 3  is a block diagram of a transmitter for generating orthogonal preamble sequences according to the present invention; 
         FIG. 4  is a block diagram of an MS for extracting an orthogonal preamble signal by use of a preamble correlator according to the present invention; 
         FIG. 5  illustrates correlation values output from a first preamble correlator according to the present invention, when a preamble signal is formed with two preamble sequences; 
         FIG. 6  illustrates correlation values output from a second preamble correlator according to the present invention, when a preamble signal is formed with two preamble sequences; 
         FIG. 7  illustrates correlation values output from the second preamble correlator according to the present invention, when a preamble signal is formed with three preamble sequences; 
         FIG. 8  illustrates correlation values output from the second preamble correlator according to the present invention, when a preamble signal is formed with four preamble sequences; 
         FIG. 9  illustrates correlation values output from the second preamble correlator according to the present invention, when a preamble signal is formed with five preamble sequences; 
         FIG. 10  illustrates correlation values output from the second preamble correlator according to the present invention, when a preamble signal is formed with six preamble sequences; 
         FIG. 11  illustrates signal links for MSs within two cells according to the present invention; 
         FIG. 12  illustrates an operation of an MS for processing a signal received from a BS or an RS according to the present invention; and 
         FIG. 13  illustrates an operation of the BS or the RS for generating an orthogonal preamble signal 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 for the sake of clarity and conciseness. 
       FIG. 2  illustrates a preamble structure and preamble signals according to the present invention. 
     Referring to  FIG. 2 , preamble signals  201  and  203  are mutually orthogonal so that they can be distinguished by the orthogonality. If the preamble signals  201  and  203  are allocated to a BS and an RS, respectively, an MS can identify signals from the BS  110  and the RS  120  by the preamble signals  201  and  203 . 
     The BS generates a low-PAPR preamble sequence T 1  and forms a long preamble signal (T 1 , T 1 ), i.e. the preamble signal  201 , and the RS forms a long preamble signal (−T* 1 , T* 1 ) with complex conjugates of the preamble sequence T 1 . Thus, the preamble signals  201  and  203  become orthogonal to each other and the MS can identify the BS and the RS based on the orthogonality. 
     When each preamble signal is formed with two preamble sequences, i.e. in a preamble pattern of 2, these preamble signals  201  and  203  can be expressed in Equation (1) as 
                     preamble   2     =     (           T   1           T   1               -     T   1   *             T   1   *           )             (   1   )               
where T denotes a preamble sequence.
 
     For extended preamble patterns, the preamble signals are given as follows. 
     In a preamble pattern of 3, preamble signals  205  and  207  are expressed in Equation (2) as 
     
       
         
           
             
               
                 
                   
                     preamble 
                     3 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                       
                       
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           0 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In a preamble pattern of 4, preamble signals  209  and  211  are expressed in Equation (3) as 
     
       
         
           
             
               
                 
                   
                     preamble 
                     4 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                       
                       
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In a preamble pattern of 5, preamble signals  213  and  215  are expressed in Equation (4) as 
     
       
         
           
             
               
                 
                   
                     preamble 
                     5 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                       
                       
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           0 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In a preamble pattern of 6, preamble signals  217  and  219  are expressed in Equation (5) as 
     
       
         
           
             
               
                 
                   
                     preamble 
                     6 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                       
                       
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     As noted from Equations (2) to (5), even when the preamble pattern is extended, the orthogonality is still maintained. The orthogonality in the extended preamble patterns is shown in  FIGS. 7 to 10 . 
     The extension of preamble patterns is explained as follows. 
     For a preamble pattern of an even number equal to or greater than 4, i.e. 4 or a greater even number of preamble sequences in a preamble signal, the preamble signals are given in Equation (6) as 
                 If   ⁢           ⁢   A     =     (           T   1           T   1               -     T   1   *             T   1   *           )       ,       preamble   n     =     (     A   ⁢           ⁢   …     ⁢           )             
where T denotes a preamble sequence, n is an even number equal to or greater than 4, and A occurs a half of n times.
 
     For example, for n=4, the preamble pattern in Equation (7) is 
     
       
         
           
             
               
                 
                   
                     preamble 
                     4 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                       
                       
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     For a preamble pattern of an odd number equal to or greater than 5, i.e. 5 or a greater odd number of preamble sequences in a preamble signal, the preamble signals are given in Equation (8) as 
                       If   ⁢           ⁢   A     =     (           T   1           T   1               -     T   1   *             T   1   *           )       ,       preamble   n     =     (     A   ⁢           ⁢   …   ⁢           ⁢           T   1             0           )               (   8   )               
where T denotes a preamble sequence, n is an odd number equal to or greater than 5, and A occurs a half of (n−1) times.
 
     For example, for n=5, the preamble pattern in Equation (9) is 
     
       
         
           
             
               
                 
                   
                     preamble 
                     5 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                         
                           
                             T 
                             1 
                           
                         
                       
                       
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           
                             - 
                             
                               T 
                               1 
                               * 
                             
                           
                         
                         
                           
                             T 
                             1 
                             * 
                           
                         
                         
                           0 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
       FIG. 3  is a block diagram of a transmitter for generating orthogonal preamble sequences according to the present invention. 
     Referring to  FIG. 3 , a first preamble sequence generator  301  generates preamble sequences. A low-PAPR preamble sequence selector  303  selects a low-PAPR preamble sequence among from the preamble sequences. 
     A preamble sequence copier  305  copies the low-PAPR preamble sequence. One of the low-PAPR preamble sequence and its copy is delayed in a buffer  311 . Then the low-PAPR preamble sequence and its copy are used as a long preamble of a downlink frame sent from a BS or an RS. The long preamble can be used for sending node identification, frame synchronization, cell search, frequency offset estimation and channel estimation in the BWA communication system. 
     A second preamble sequence generator  313  generates a preamble sequence orthogonal to the low-PAPR preamble sequence. 
     The second preamble sequence generator  313  includes the preamble sequence copier  305 , switches  306  and  307 , complex conjugators  308  and  309 , a sign converter  310  and the buffer  311 . When the second preamble sequence generator  313  generates the preamble sequence orthogonal to the low-PAPR preamble sequence generated from the first preamble sequence generator  301 , the switches  306  and  307  select a different path from that for the preamble sequence generation of the first preamble sequence generator  301 . 
     The complex conjugators  308  and  309 , the sign converter  310  and the buffer  311  generate orthogonal preamble sequences by complex-conjugation, sign conversion and delaying of the low-PAPR preamble sequence and its copy received from the switches  306  and  307 . 
     The orthogonal preamble sequences can be used as a long preamble signal of a downlink frame sent from the BS or the RS. The long preamble can be used for sending node identification, frame synchronization, cell search, frequency offset estimation and channel estimation in the BWA communication system. 
       FIG. 4  is a block diagram of an MS for extracting an orthogonal preamble signal by use of a preamble correlator according to the present invention. 
     Referring to  FIG. 4 , the MS processes a signal received through a Radio Frequency (RF) module and an Analog-to-Digital Converter (ADC)  409 . 
     The MS acquires a preamble signal from the received signal, identifies whether the sending node of the signal is a BS or an RS, acquires synchronization to the sending node and converts the signal to a baseband signal. 
     A first preamble correlator  403  (preamble correlator 1) acquires synchronization to the BS using a preamble signal received from the BS, and a second preamble correlator  405  (preamble correlator 2) acquires to the RS synchronization using a preamble signal received from the RS. That is, if the received signal is from the BS, the first preamble correlator  403  acquires synchronization to the BS, and if the received signal is from the RS, the second preamble correlator  405  acquires synchronization to the RS. 
     Information about the preamble signals of the BS and the RS are stored in a sequence reference block  407 , for use in signal synchronization to the BS and the RS in the first and second preamble correlators  403  and  405 . 
     The preamble sequence information may be preset or received before the synchronization. 
     A baseband processor  401  downconverts the signals received from the first and second preamble correlators  403  and  405  to baseband signals. Then the baseband signals are subject to demodulation and decoding. 
       FIG. 5  illustrates correlation values output from the first preamble correlator  403  in a preamble pattern of 2 according to the present invention. 
     Referring to  FIG. 5 , when the MS receives a signal from the BS, the first preamble correlator  403  identifies the BS by correlating the signal with the preamble sequence of the BS. When the MS receives a signal from the RS, the correlation between the received signal and the preamble sequence of the BS is 0 in the first preamble correlator  403 . Therefore, even though the MS receives signals from the BS and the RS simultaneously, it can distinguish the signals. 
       FIG. 6  illustrates correlation values output from the second preamble correlator  405  in the preamble pattern of 2 according to the present invention. 
     Referring to  FIG. 6 , when the MS receives a signal from the RS, the second preamble correlator  405  identifies the RS by correlating the signal with the preamble sequence of the RS. When the MS receives a signal from the BS, the correlation between the received signal and the preamble sequence of the RS is 0 in the second preamble correlator  405 . Therefore, even though the MS receives signals from the BS and the RS simultaneously, it can distinguish the signals. 
       FIG. 7  illustrates correlation values output from the second preamble correlator  405  in the preamble pattern of 3 according to the present invention. 
     Referring to  FIG. 7 , the graph illustrates the correlation between a BS signal and the preamble signal of the RS and the correlation between an RS signal and the preamble signal of the RS calculated in the second preamble correlator  405  signal received signal, when the preamble signals are configured according to the preamble pattern of 3, such as the preamble signals  205  and  207  shown in  FIG. 2 . For the BS signal, the correlation output from the second preamble correlator  405  is 0. Therefore, even though the MS receives signals from the BS and the RS simultaneously, it can distinguish the signals. 
       FIG. 8  illustrates correlation values output from the second preamble correlator  405  in the preamble pattern of 4 according to the present invention. 
     Referring to  FIG. 8 , the graph illustrates the correlation between a BS signal and the preamble signal of the RS and the correlation between an RS signal and the preamble signal of the RS calculated in the second preamble correlator  405  signal received signal, when the preamble signals are configured according to the preamble pattern of 4, such as the preamble signals  209  and  211  shown in  FIG. 2 . For the BS signal, the correlation output from the second preamble correlator  405  is 0. Therefore, even though the MS receives signals from the BS and the RS simultaneously, it can distinguish the signals. 
       FIG. 9  illustrates correlation values output from the second preamble correlator  405  in the preamble pattern of 5 according to the present invention. 
     Referring to  FIG. 9 , the graph illustrates the correlation between a BS signal and the preamble signal of the RS and the correlation between an RS signal and the preamble signal of the RS calculated in the second preamble correlator  405  signal received signal, when the preamble signals are configured according to the preamble pattern of 5, such as the preamble signals  213  and  215  shown in  FIG. 2 . For the BS signal, the correlation output from the second preamble correlator  405  is 0. Therefore, even though the MS receives signals from the BS and the RS simultaneously, it can distinguish the signals. 
       FIG. 10  illustrates correlation values output from the second preamble correlator  405  in the preamble pattern of 6 according to the present invention. 
     Referring to  FIG. 10 , the graph illustrates the correlation between a BS signal and the preamble signal of the RS and the correlation between an RS signal and the preamble signal of the RS calculated in the second preamble correlator  405  signal received signal, when the preamble signals are configured according to the preamble pattern of 6, such as the preamble signals  217  and  219  shown in  FIG. 2 . For the BS signal, the correlation output from the second preamble correlator  405  is 0. Therefore, even though the MS receives signals from the BS and the RS simultaneously, it can distinguish the signals. 
       FIGS. 5 to 10  reveal that orthogonality is maintained irrespective of the number of preamble sequences forming a preamble signal. 
       FIG. 11  illustrates signal links for MSs within two cells according to the present invention. 
     Referring to  FIG. 11 , when MSs  1111  and  1113  within two cells  1103  and  1105  receive signals from an RS  1107  and BSs  1101  and  1109 , they can identify the RS  1107  and the BSs  1101  and  1109  through the first and second preamble correlators  403  and  405 . 
     The preamble signals of the BSs  1101  and  1109  are the same or mutually orthogonal. When the MSs  1111  and  1113  send signals on the uplink, they may also send their orthogonal preamble signals. 
       FIG. 12  illustrates an operation of the MS for processing a signal received from the BS or the RS according to the present invention. 
     Referring to  FIG. 12 , the MS receives a signal frame from the BS or the RS in step  1203 . If the BS generates the signal frame, it includes its preamble signal in the signal frame. If the RS generates the signal frame, it includes its preamble signal orthogonal to the preamble signal of the BS in the signal frame. 
     The preamble correlators  403  and  405  of the MS receive preamble sequence information from the sequence reference block  407  in step  1204 . The preamble sequence information may be preset or received and stored before synchronization. 
     If the signal frame includes the preamble signal of the BS in step  1205 , the first preamble correlator  403  acquires system synchronization by correlating the signal frame with the preamble signal of the BS in step  1207 . Then the MS detects the BS signal in step  1209  and recovers the data of the signal frame by baseband signal processing of the detected BS signal through the baseband processor  401  instep  1211 . 
     If the signal frame includes the preamble signal of the RS in step  1205 , the second preamble correlator  405  acquires system synchronization by correlating the signal frame with the preamble signal of the RS in step  1206 . Then the MS detects the RS signal in step  1209  and recovers the data of the signal frame by baseband signal processing of the detected RS signal through the baseband processor  401  in step  1211 . 
     Then, the MS ends the algorithm of the present invention. 
       FIG. 13  illustrates an operation of the BS or the RS for generating an orthogonal preamble signal according to the present invention. 
     Referring to  FIG. 13 , the first preamble sequence generator  301  of the BS or the RS generates preamble sequences in step  1305 . The low-PAPR preamble sequence selector  303  selects a low-PAPR preamble sequence from among the preamble sequences in step  1310 . In step  1315 , the preamble sequence copier  305  copies the low-PAPR preamble sequence. 
     If it is determined not to generate a preamble sequence orthogonal to the low-PAPR preamble sequence in step  1320 , one of the low-PAPR preamble sequence and its copy is delayed in the buffer  311  in step  1325 . 
     If it is determined to generate a preamble sequence orthogonal to the low-PAPR preamble sequence in step  1320 , the orthogonal preamble sequence is generated using the complex conjugators  308  and  309 , the sign converter  310  and the switches  306  and  307  in step  1330 . 
     Then the algorithm of the present invention ends. 
     As is apparent from the above description, the present invention generates an unlimited number of low-PAPR preamble signals in a BWA communication system using RSs. Therefore, orthogonal preamble signals can be allocated to BSs, RSs and MSs. 
     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.