Abstract:
A pilot designing method in an uplink OFDMA system is provided. In the uplink OFDMA system, communications are carried out in a frame divided into time-frequency lattices, and each time-frequency lattice includes a plurality of data symbol periods and a plurality of pilot symbol periods intermittently arranged with respect to the data symbol periods. The frame is divided into a plurality of blocks. The blocks are allocated to the terminals. A predetermined allocated pilot time-frequency lattice is shared between adjacent terminals.

Description:
PRIORITY 
   This application claims priority under 35 U.S.C. §119 to an application entitled “Pilot Designing Method in an Uplink OFDMA System” filed in the Korean Intellectual Property Office on Mar. 11, 2004 and Nov. 26, 2004 and assigned Serial Nos. 2004-16552 and 98174-2004, respectively, the contents each of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a mobile communication system, and in particular, to a pilot designing method for channel estimation on the uplink in an orthogonal frequency division multiple access (OFDMA) system. 
   2. Description of the Related Art 
   High-speed, high-quality data transmission is required for next-generation mobile communications to provide a variety of multimedia services with improved quality. 
   Orthogonal Frequency Division Multiplexing (OFDM), on which OFDMA is based, boasts high-speed communication due to low equalization complexity for frequency-selective fading channels. Thus, OFDM is widely used as a physical layer transmission scheme in various wireless communication systems including wireless local access network (WLAN), digital TV broadcasting, and next-generation mobile communication systems. 
   OFDMA is a multiple access scheme of allocating different subcarriers to a plurality of users. On the downlink, all users can use pilot information for channel estimation, which facilitates pilot transmission and channel estimation. On the uplink, however, when a particular use transmits pilot information in a predetermined pilot sample, other users are prohibited from using the same pilot sample. Therefore, many users share a fixed number of pilot samples. The limit of the pilot information which is available to each user degrades channel estimation performance. 
     FIGS. 1A and 1B  are graphs which illustrate subcarrier allocation to users in a known uplink OFDMA system. 
   Referring to  FIGS. 1A and 1B , the uplink OFDMA system allocates resources to users, each on the basis of a transport block  110  within the same frame. The transport block  110  comprises data subcarriers  103  and pilot subcarriers  105 . Only a pilot channel in the transport block  110  is used for channel estimation of the user allocated to the transport block  110 . 
     FIG. 2  is a channel gain graph illustrating interpolation-based channel estimation in the uplink OFDMA system. When user # 1  and user # 2  are allocated to resources within the same frame, pilot subcarriers  205   a  of user # 1  and pilot subcarriers  205   b  of user # 2  are arranged equidistantly on the frequency domain. The channel between the pilot subcarriers  205   a  (or  205   b ) is estimated by interpolating the channel estimates of pilots within the transport block of user #1 (or user # 2 ). However, at the boundary A between the transport blocks of the two users, a pilot subcarrier is nonexistent for user # 1  on the outside of his transport block and thus a reference channel estimate is not available for interpolation. As a result, a channel estimation error increases, which leads to the degradation of the whole data detection performance. To solve this problem, it has been proposed that an additional pilot subcarrier be allocated at the boundary A, or that a pilot subcarrier be disposed at the boundary without increasing the number of pilot subcarriers. Yet, the additional pilot subcarrier allocation decreases band efficiency, and the arrangement of pilot subcarriers at wider intervals without increasing the number of the pilot subcarriers increases a channel variation between pilot subcarriers, thereby increasing a channel estimation error. 
   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 a pilot designing method for maximizing channel estimation efficiency, while maintaining the number of pilot subcarriers in a transport block allocated to each user in an uplink OFDMA frame. 
   Another object of the present invention is to provide a pilot designing method for maximizing channel estimation efficiency by sharing pilot subcarriers between users allocated to adjacent transport blocks in an OFDMA frame. 
   A further object of the present invention is to provide a pilot designing method for maximizing channel estimation efficiency by exchanging pilot symbols between users allocated to adjacent transport blocks in an OFDMA frame. 
   The above objects are achieved by providing a pilot designing method in an uplink OFDMA system. 
   According to one aspect of the present invention, in a pilot designing method in an uplink OFDMA system where communications are carried out in a frame divided into time-frequency lattices, and each time-frequency lattice includes a plurality of data symbol periods and a plurality of pilot symbol periods intermittently arranged with respect to the data symbol periods, the frame is divided into a plurality of blocks, the blocks are allocated to the terminals, and a predetermined allocated pilot time-frequency lattice is shared between adjacent terminals. 
   According to another aspect of the present invention, in a data transmitting method in an uplink OFDMA system where, for communications, a frame with time-frequency lattices, each including a plurality of data symbol periods and a plurality of pilot symbol periods intermittently arranged with respect to the data symbol periods, is divided into a plurality of blocks, and the blocks are allocated to the terminals, data signals and pilot signals of each terminal are arranged in the data symbol periods and the pilot symbol periods of a block allocated to the terminal, a transmission frame is generated by inserting pilot signals of the terminal in predetermined pilot time-frequency lattices of an adjacent block allocated to a different terminal, and the transmission frame is transmitted. 
   According to a further aspect of the present invention, in a data transmitting method in an uplink OFDMA system where, for communications, a frame having time-frequency lattices, each including a plurality of data symbol periods and a plurality of pilot symbol periods intermittently arranged with respect to the data symbol periods, is divided into a plurality of blocks, and the blocks are allocated to the terminals, data signals and pilot signals of each terminal are arranged in the data symbol periods and the pilot symbol periods of a block allocated to the terminal, one of at least two time-frequency lattices of a first pilot symbol period of the terminal is emptied for filling a pilot signal of a different terminal in the empty time-frequency lattice, a transmission frame is generated by inserting a pilot signal of the terminal in an empty pilot time-frequency lattice among at least two pilot time-frequency lattices of an adjacent block allocated to the different terminal, and the transmission frame is transmitted. 
   According to still another aspect of the present invention, in a channel estimation method in an uplink OFDMA system where, for communications, a frame having time-frequency lattices, each including a plurality of data symbol periods and a plurality of pilot symbol periods intermittently arranged with respect to the data symbol periods, is divided into a plurality of blocks, and the blocks are allocated to the terminals, a frame is received which has time-frequency lattices shared between adjacent blocks, and a channel of each of the terminals is estimated using a pilot signal included in a time-frequency lattice shared with an adjacent block allocated to a different terminal. 
   According to yet another aspect of the present invention, in a channel estimation method in an uplink OFDMA system where, for communications, a frame having time-frequency lattices, each including a plurality of data symbol periods and a plurality of pilot symbol periods intermittently arranged with respect to the data symbol periods, is divided into a plurality of blocks, and the blocks are allocated to the terminal, a frame is received which has time-frequency lattices exchanged between adjacent blocks, and a channel of each of the terminals is estimated using a pilot signal included in a time-frequency lattice exchanged from an adjacent block allocated to a different terminal. 
   According to further another aspect of the present invention, in a pilot designing method in an uplink OFDMA system where communications are carried out in a frame divided into time-frequency lattices, each time-frequency lattice being identified by frequency-axis subcarrier indexes and time-axis symbol period indexes and including a plurality of intermittently arranged pilot subcarriers, the frame is divided into transport blocks of a predetermined size and the transport blocks are allocated to terminals. Pilot subcarriers of adjacent transport blocks allocated to different terminals are shared between them in at least one symbol period. 
   A transport block allocated to each terminal is preferably shared by at least two antennas of the terminal. Moreover, these antennas preferably map orthogonal pilot signals to pilot subcarriers in the transport block. 
   Furthermore, a pilot subcarrier is preferably shared by the different terminals during four symbol periods and can be selectively allocated to the two antennas of the terminal for two successive symbol periods. It is also preferable that the pilot subcarrier be allocated to the two antennas of the terminal for the same two successive symbol periods. 
   Moreover, a pilot signal is preferably mapped to pilot subcarriers having different indexes in the transport block for different symbol periods. It is also preferable that the pilot signals mapped to the two antennas of the terminal are orthogonal. The pilot subcarrier is also preferably selectively allocated to the two antennas of the terminal for a different symbol period. 
   According to yet further aspect of the present invention, in a pilot designing method in an uplink OFDMA system where communications are carried out in a frame divided into time-frequency lattices, each time-frequency lattice being identified by frequency-axis subcarrier indexes and time-axis symbol period indexes and including a plurality of intermittently arranged pilot symbol periods, the frame is divided into transport blocks of a predetermined size and the transport blocks are allocated to terminals. Pilot symbol periods of adjacent transport blocks allocated to different terminals are shared between them in at least one subcarrier. 

   
     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. 1A  is a graph which illustrates an example of resources allocation to each user in an uplink OFDMA system; 
       FIG. 1B  is a graph which illustrates another example of resources allocation to each user in the uplink OFDMA system; 
       FIG. 2  is a channel gain graph illustrating the interpolation-based channel estimation of a conventional uplink OFDMA system; 
       FIG. 3  is a conceptual view illustrating a pilot designing method according to an embodiment of the present invention; 
       FIG. 4  is a conceptual view illustrating a pilot designing method according to another embodiment of the present invention; 
       FIG. 5  is a conceptual view illustrating a pilot designing method according to a third embodiment of the present invention; 
       FIG. 6  is a conceptual view illustrating a pilot designing method according to a fourth embodiment of the present invention; 
       FIG. 7  is a conceptual view illustrating a pilot designing method according to a fifth embodiment of the present invention; 
       FIG. 8  is a conceptual view illustrating a pilot designing method according to a sixth embodiment of the present invention; 
       FIG. 9  is a conceptual view illustrating a pilot designing method according to a seventh embodiment of the present invention; 
       FIG. 10  is a conceptual view illustrating a pilot designing method according to a eighth embodiment of the present invention; and 
       FIG. 11  is a conceptual view illustrating a pilot designing method according to a ninth embodiment of 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. 
   The present invention is intended to increase the channel estimation performance of a base station by sharing or exchanging pilot subcarriers between users allocated to adjacent blocks in an uplink OFDMA system. 
   In the pilot sharing method, two users having adjacent transport blocks share the same pilot subcarrier using orthogonal pilot patterns. In the pilot exchanging method, two users having adjacent transport blocks at a particular time exchange at least one of the pilot subcarriers. 
     FIG. 3  is a conceptual view illustrating a pilot designing method according to an embodiment of the present invention. 
   For notational simplicity, it is assumed that there are four user terminals allocated to adjacent transport blocks and each transport block includes 16 subcarriers for two successive symbol periods. 
   Referring to  FIG. 3 , user terminal #a, user terminal #b, user terminal #c, and user terminal #d each is allocated a transport block including data subcarriers  303  which are not overlapped with data subcarriers  303  of the other user terminals in two symbol periods. However, some of pilot subcarriers  305   a ,  305   b ,  305   c  and  305   d  are shared by at least two users at subcarrier indices #k, #k+16, and #k+32. 
   Typically, pilot subcarriers are arranged intermittently and it is preferable to design the pilot subcarriers such that the distance between adjacent pilot subcarriers is not wider than a coherent bandwidth. 
   Taking the transport blocks of user terminal #a and user terminal #b for an example, the first subcarrier (i.e. the subcarrier #k) of the transport block of user terminal #b is a pilot subcarrier. User terminal #b shares the pilot subcarrier #k with user terminal #a. Notably, user terminal #a and user terminal #b use mutually orthogonal pilot patterns [1 1] and [1 −1], respectively. 
   In the pilot design according to the embodiment of the present invention, the maximum number of additional pilot subcarriers available to one user terminal through pilot sharing with its adjacent user terminal is calculated in the following manner. Let the total number of pilot subcarriers in an OFDM symbol be denoted by N Pilot  and the number of multiple access terminals be denoted by N User . Then, the original number (N Original ) of pilot subcarriers equally available to each user is set forth in Equation (1): 
   
     
       
         
           
             
               
                 
                   N 
                   Original 
                 
                 = 
                 
                   
                     N 
                     Pilot 
                   
                   
                     N 
                     User 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
   
   If every two adjacent users share a pilot subcarrier, each user can be additionally allocated as many pilot subcarriers as the number of originally allocated pilot subcarriers. Therefore, the total number of pilot subcarriers (N Total ) allocated to one user is computed as shown in Equation (#2): 
   
     
       
         
           
             
               
                 
                   N 
                   Total 
                 
                 = 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       N 
                       Pilot 
                     
                     
                       N 
                       User 
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
   
   Given N Pilot =16 and N User =4, 8 pilot subcarriers are allocated to each user in the pattern illustrated in  FIG. 3 . 
   When two adjacent users A and B share the subcarrier #k, signals received at the base station for n th  and (n+1) th  symbol periods are, under the assumption that the channels of users A and B minimally changed for the symbol periods, expressed as Equations (3) and (4):
 
 Y   n ( k )= H   A ( k ) X   n   A ( k )+ H   B ( k ) X   n   B ( k )+ W   n ( k )  Equation (3)
 
 Y   n+1 ( k )= H   A ( k ) X   n+1   A ( k )+ H   B ( k ) X   n+1   B +( k )+ W   n+1 ( k )  Equation (4)
 
where X n   A (k) and X n   B (k) are transmission signals on the subcarrier #k of an n th  OFDM symbol from users A and B, H A (k) and H B (k) are the channel coefficients of the subcarrier #k for users A and B, W n (k) is the additive white Gaussian noise (AWGN) of the subcarrier #k in the n th  OFDM symbol, and Y n (k) is the signal received on the subcarrier #k of the n th  OFDM symbol.
 
   The above Equation (3) and Equation (4) are equivalent to the following matrix (5): 
                   [             Y   n     ⁡     (   k   )                   Y     n   +   1       ⁡     (   k   )             ]     =         [             X   n   A     ⁡     (   k   )               X   n   B     ⁡     (   k   )                   X     n   +   1     A     ⁡     (   k   )               X     n   +   1     B     ⁡     (   k   )             ]     ⁡     [             H   A     ⁡     (   k   )                   H   B     ⁡     (   k   )             ]       +     [             W   n     ⁡     (   k   )                   W     n   +   1       ⁡     (   k   )             ]               Equation   ⁢           ⁢     (   5   )                 
which can be further simplified to the following Equation (6):
   y ( k )= X ( k ) h ( k )+ w ( k )  Equation (6) 
   where Y n (k) is a signal received at the n th  period through the k th  subcarrier, X n (k) is a signal transmitted at the n th  period through the k th  subcarrier, H(k) is a channel coefficient of kth channel, W(k) is additive white Gaussian noise (AWGN) and k is the generalized subcarrier index. All of those are defined right above the Equation 5. 
   It is noted from the above equations that an equation for achieving the channel of the pilot subcarrier shared between users A and B is represented as an AWGN-added linear equation. If the inverse of X(k), which is X(k) −1  exists, the maximum likelihood (ML) estimate of the pilot subcarrier channel of users A and B can be expressed as Equation (7):
 
 ĥ ( k ) ML   =X ( k ) −1   y ( k )  Equation (7)
 
   The mean squared error (MSE) of the estimated pilot subcarrier channel is computed as shown in Equation (8):
 
MSE=σ 2   ·tr (( X   H   X ) −1 )  Equation (8)
 
   where σ 2  is the noise variance, and X is a unitary matrix minimizing the MSE and is characterized by
 
 XX   H   =X   H   X=αI   Equation (9)
 
where I is an Identity matrix.
 
   When the users sharing the pilot subcarrier use mutually orthogonal patterns such as [1 1] and [1 −1], the above condition is satisfied. Consequently, optimum channel estimation performance is achieved. 
   The present invention utilizes a polynomial interpolation function to estimate data subcarrier channels. Specifically, three data subcarrier channels between pilot subcarriers are estimated by calculating the coefficient of a third-order polynomial function using two pilot subcarriers on the left and two pilot subcarriers on the right of the data subcarriers, and then using the resulting polynomial interpolation function. 
   Meanwhile, a system in which two user terminals use two or more transmit antennas can be considered. In a multiple-antenna OFDMA system using N TX  transmit antennas, the number of channel parameters to be estimated using a shared or exchanged pilot subchannel is 2N TX . Therefore, at least 2N TX  linear formulas are needed for the estimation. For example, if two transmit antennas are used and a kth pilot subcarrier is shared, linear formulas are required for four received signals and the received signals at the base station for nth to (n+3)th OFDM symbol periods are expressed as defined in Equation (10) below. It is assumed herein that the channels of user A and user B are not changed for the four OFDM symbol periods. 
                   [             Y   k     ⁡     (   n   )                   Y   k     ⁡     (     n   +   1     )                   Y   k     ⁡     (     n   +   2     )                   Y   k     ⁡     (     n   +   3     )             ]     =             [             X     k   ,   1     A     ⁡     (   n   )               X     k   ,   2     A     ⁡     (   n   )               X     k   ,   1     B     ⁡     (   n   )               X     k   ,   2     B     ⁡     (   n   )                   X     k   ,   1     A     ⁡     (     n   +   1     )               X     k   ,   2     A     ⁡     (     n   +   1     )               X     k   ,   1     B     ⁡     (     n   +   1     )               X     k   ,   2     B     ⁡     (     n   +   1     )                   X     k   ,   1     A     ⁡     (     n   +   2     )               X     k   ,   2     A     ⁡     (     n   +   2     )               X     k   ,   1     B     ⁡     (     n   +   2     )               x     K   ,   2     b     ⁡     (     n   +   2     )                   X     k   ,   1     A     ⁡     (     n   +   3     )               X     k   ,   2     A     ⁡     (     n   +   3     )               X     k   ,   1     B     ⁡     (     n   +   3     )               X     k   ,   2     B     ⁡     (     n   +   3     )             ]     ⁢             [           H     k   ,   1     A               H     k   ,   2     A               H     k   ,   1     B               H     k   ,   2     B           ]     +     [             W   k     ⁡     (   n   )                   W   k     ⁡     (     n   +   1     )                   W   k     ⁡     (     n   +   2     )                   W   k     ⁡     (     n   +   3     )             ]                     Equation   ⁢           ⁢     (   10   )                 
where X k,i   A  and X k,i   B  are kth subcarrier signals in an nth OFDM symbol from ith transmit antennas of user A and user B, H k,i   A  and H k,i   B  are channel coefficients from kth transmit antennas of user a and user B in the nth to (n+3)th OFDM symbols, W k (n) is the AWGN of the kth subcarrier in the nth OFDM symbol, and Y k (n) is a signal received on the kth subcarrier in the nth OFDM symbol.
 
   In the embodiment of the present invention, 4 pilot subcarriers are allocated to each user terminal and a total of 8 pilot subcarriers are available to each user terminal through pilot sharing with its adjacent user terminal. 
   Since the same data subcarriers are estimated by using twice as many pilot subcarriers as those originally allocated to a user terminal, a more accurate channel estimation is enabled. 
     FIG. 4  is a conceptual view illustrating a pilot designing method according to a second embodiment of the present invention. 
   As in the first embodiment of the present invention, it is assumed that there are four user terminals allocated to adjacent transport blocks, each transport block includes 16 subcarriers for two successive symbol periods, and the channels do not change for the symbol periods. 
   Referring to  FIG. 4 , user terminal #a, user terminal #b, user terminal #c, and user terminal #d are each allocated a transport block including data subcarriers  403  which are not overlapped with data subcarriers  403  of the other user terminals in two symbol periods. 
   In symbol period # 1 , user terminal #a uses a pilot subcarrier which is located at a pilot subcarrier position #k which is allocated to user terminal #b as its own, and user terminal #c uses a pilot subcarrier which is located at a pilot subcarrier position #(k+32) th  which is allocated to user terminal #d as its own. 
   In symbol period # 2 , user terminal #b uses a pilot subcarrier which is located at a pilot subcarrier position #(k+16) th  which is allocated to user terminal #c as its own, and user terminal #d uses the first pilot subcarrier which is located at a pilot subcarrier position # 0  allocated to user terminal #a as its own. 
   As described above, each user terminal uses a pilot subcarrier which is allocated to its adjacent user terminal in one of two symbol periods assumed that there are no channel variations. Thus, channel estimation is done more accurately. 
   In the second embodiment of the present invention, because adjacent user terminals use the same symbol alternately in different symbol periods, there is no need for maintaining orthogonality between pilot subcarriers. 
     FIG. 5  is a conceptual view illustrating a pilot designing method according to a third embodiment of the present invention. 
   For notational simplicity, it is assumed that there are two user terminals allocated to adjacent transport blocks, and each transport block includes 2 subcarriers for 28 successive symbol periods. 
   Referring to  FIG. 5 , user terminal #a and user terminal #b are allocated pilot symbol periods  505   a  and  505   b  respectively, which are intermittent with respect to data symbol periods  503 . At the end time area of the transport block of user terminal #a, there is no pilot symbol period. Hence, user terminal #a shares a pilot symbol period #k which is allocated to user terminal #b. User terminals #a and #b use orthogonal pilot patterns [1 1] and [1 −1]. 
     FIG. 6  is a conceptual view illustrating a pilot designing method according to a fourth embodiment of the present invention. 
   As in the third embodiment of the present invention, it is assumed that there are two user terminals allocated to adjacent transport blocks, and each transport block includes 2 subcarriers for 28 successive symbol periods. 
   Referring to  FIG. 6 , user terminal #a and user terminal #b are allocated pilot symbol periods  605   a  and  605   b  which are intermittent with respect to data symbol periods  603 . To compensate for the non-existence of a pilot symbol at the end time area of the transport block of user terminal #a, the first pilot symbol period of a first subcarrier of user terminal #b is allocated to user terminal #a in the first subcarrier, and the last pilot symbol period of the first subcarrier of user terminal #a is allocated to user terminal #b in the first subcarrier. 
     FIGS. 7 to 10  are conceptual views illustrating pilot designing methods according to the fifth to eighth embodiments of the present invention. The pilot design in these embodiments is for a system in which a terminal uses two transmit antennas. However, the number of transmit antennas is not limited and thus, for example, the terminal may have three or more transmit antennas. 
   The fifth to eighth embodiments are based on the assumption that two adjacent transport blocks are allocated to two user terminals and each transport block includes 17 subcarriers for 5 symbol periods. 
   In  FIG. 7 , transport blocks  700  and  750  allocated to user terminal #a and user terminal #b include pilot subcarriers  702  to  705 . To compensate for the absence of pilot subcarriers at the frequency-domain start and end of the transport block  700  for user terminal #a, the transport block  700  shares pilot subcarriers  701  and  704  allocated to other user terminals, bordering on the transport block  700 . Similarly, user terminal #b borrows pilot subcarriers  703  and  706  allocated to other terminals outside his transport block  750 . 
   First and second antennas of user terminal #a share the transport block  700  and first and second antennas of user terminal #b share the transport block  750 . 
   In accordance with the fifth embodiment, pilot signals for the first and second antennas, which use the same pilot subcarriers to avoid interference between the antennas in the same terminal, are designed to be orthogonal with each other on the time domain. 
   Therefore, the first antenna of user terminal #a maps a pilot signal [ 1   1   1   1 ] and the second one maps a pilot signal [ 1  − 1   1  − 1 ] for four symbol periods  711  to  714 . The first antenna of user terminal #b maps a pilot signal [ 1   1  − 1  − 1 ] and the second one maps a pilot signal [ 1  − 1  − 1   1 ] for four symbol periods  715  to  718 . 
   Also, orthogonality is maintained between the antennas of the user terminals. That is, the pilot signal [ 1   1   1   1 ] of the first antenna in user terminal #a is orthogonal to the pilot signals [ 1   1  − 1  − 1 ] and [ 1  − 1  − 1   1 ] of the first and second antennas of user terminal #b. 
     FIG. 8  is a conceptual view illustrating a pilot designing method according to the sixth embodiment of the present invention. Referring to  FIG. 8 , as in the fifth embodiment, transport blocks  800  and  850  allocated to user terminal #a and user terminal #b include pilot subcarriers  802  to  805  which are intermittently arranged with respect to data subcarriers. To compensate for the absence of pilot subcarriers at the frequency-domain start and end of the transport block  800  for user terminal #a, the transport block  800  shares pilot subcarriers  801  and  804  allocated to other user terminals, bordering on the transport block  800 . First and second antennas of user terminal #a share the transport block  800  and first and second antennas of user terminal #b share the transport block  850 . 
   In the sixth embodiment of the present invention, the first and second antennas, which use the same pilot subcarriers to avoid interference between the antennas are designed to transmit pilot signals in different symbol periods. 
   Specifically, the first antenna of user terminal #a maps a pilot signal [ 1   1 ] to first and second symbol periods  811  and  812  in the transport block  800  and the second antenna maps the same pilot signal [ 1   1 ] to third and fourth symbol periods  813  and  814 . In the same manner, the first antenna of user terminal #b maps a pilot signal [ 1  − 1 ] to first and second symbol periods  815  and  816  in the transport block  850  and the second antenna maps the same pilot signal [ 1  − 1 ] to third and fourth symbol periods  817  and  818 . 
   Also, pilot signals are mapped to pilot subcarriers having different subcarrier indexes for different two successive symbol periods in the same block. 
   Specifically, the first antenna of user terminal #a allocates the same pilot signal [ 1   1 ] to the first pilot subcarrier  802  in the first and second symbol periods  811  and  812  and to the second pilot subcarrier  803  in the third and fourth symbol periods  813  and  814  in the transport block  800 . The second antenna of user terminal #a allocates the same pilot signal [ 1   1 ] to the first pilot subcarrier  802  in the third and fourth symbol periods  813  and  814  and to the second pilot subcarrier  803  in the first and second symbol periods  811  and  812  in the transport block  800 . 
   The first antenna of user terminal #b allocates the same pilot signal [ 1  − 1 ] to the first pilot subcarrier  804  in the first and second symbol periods  815  and  816  and to the second pilot subcarrier  805  in the third and fourth symbol periods  817  and  818  in the transport block  850 . The second antenna of user terminal #b allocates the same pilot signal [ 1  − 1 ] to the first pilot subcarrier  804  in the third and fourth symbol periods  817  and  818  and to the second pilot subcarrier  805  in the first and second symbol periods  815  and  816  in the transport block  850 . 
     FIG. 9  is a conceptual view illustrating a pilot designing method according to the seventh embodiment of the present invention. Referring to  FIG. 9 , as in the fifth and sixth embodiments, transport blocks  900  and  950  allocated to user terminal #a and user terminal #b include pilot subcarriers  902  to  905  which are intermittently arranged with respect to data subcarriers. To compensate for the absence of pilot subcarriers at the frequency-domain start and end of the transport block  900  for user terminal #a, the transport block  900  shares pilot subcarriers  901  and  904  allocated to other user terminals, bordering on the transport block  900 . First and second antennas of user terminal #a share the transport block  900  and first and second antennas of user terminal #b share the transport block  950 . 
   In the seventh embodiment of the present invention, each of the first and second antennas, which use the same pilot subcarriers to avoid interference between the antennas, map the same pilot signal to different pilot subcarriers for different symbol periods, and the first and second antennas map orthogonal pilot signals in the same symbol period. 
   Therefore, the first and second antennas of user terminal #a allocate pilot signals [ 1   1 ] and [ 1  − 1 ], respectively to the first pilot subcarrier  902  in the first and second symbol periods  911  and  912 . 
   The first and second antennas of user terminal #a allocate the pilot signals [ 1   1 ] and [ 1  − 1 ], respectively to the second pilot subcarrier  904  in the third and fourth symbol periods  913  and  914 . 
   While user terminal #b maps the pilot signals in the same manner as in user terminal #a, it is preferable to map the pilot signals to be orthogonal with pilot signals from the antennas of user terminal #a. 
   To serve the purpose, the first and second antennas of user terminal #b allocate the pilot signals [ 1   1 ] and [ 1  − 1 ], respectively to the first pilot subcarrier  904  in the third and fourth symbol periods  917  and  918  in the transport block  950 . Also, the first and second antennas of user terminal #b allocate the pilot signals [ 1   1 ] and [ 1  − 1 ], respectively to the second pilot subcarrier  905  in the first and second symbol periods  915  and  916  in the transport block  950 . 
   It is noted here that the two antennas of a user terminal map different pilot signals to the same pilot subcarriers for different two successive symbol periods in the allocated transport block. 
     FIG. 10  is a conceptual view illustrating a pilot designing method according to the eighth embodiment of the present invention. Similarly to the fifth, sixth and seventh embodiments discussed above, transport blocks  1000  and  1050  allocated to user terminal #a and user terminal #b include pilot subcarriers  1002  to  1005  which are intermittently arranged with respect to data subcarriers. To compensate for the absence of pilot subcarriers at the frequency-domain start and end of the transport block  1000  for user terminal #a, the transport block  1000  shares pilot subcarriers  1001  and  1004  allocated to other user terminals, bordering on the transport block  1000 . First and second antennas of user terminal #a share the transport block  1000  and first and second antennas of user terminal #b share the transport block  1050 . 
   In the eighth embodiment of the present invention, to avoid interference between the antennas within the same terminal as well as between the terminals, the antennas of the two terminals transmit pilot signals for different symbol periods. 
   Therefore, the first and second antennas of user terminal #a allocate a pilot signal [ 1 ] to the first pilot subcarrier  1002  in the first and second symbol periods  1011  and  1012 , respectively. 
   Also, the first and second antennas of user terminal #a allocate the pilot signal [ 1 ] to the second pilot subcarrier  1003  in the third and fourth symbol periods  1013  and  1014 , respectively. 
   Meanwhile, the first and second antennas of user terminal #b allocate the pilot signal [ 1 ] to the first pilot subcarrier  1004  in the second and fourth symbol periods  1016  and  1018 , respectively. Also, the first and second antennas of user terminal #b allocate the pilot signal [ 1 ] to the second pilot subcarrier  1005  in the fourth and second symbol periods  1018  and  1016 , respectively. 
   The above-described mapping renders pilot signals to be orthogonal between the first and second antennas of the same terminal as well as between the two terminals. Therefore, interference between the terminals, between the antennas of the same terminal, and between channels of the same antenna can be avoided. 
     FIG. 11  is a conceptual view illustrating a pilot designing method according to the ninth embodiment of the present invention. Pilot signals are mapped to subcarriers allocated to the terminals for predetermined symbol periods. The ninth embodiment is similar to the fifth embodiment except for the change of position of the time axis and the frequency axis. 
   Transport blocks  10  and  20  allocated to user terminal #a and user terminal #b include pilot symbol periods  12  to  15  which are intermittently arranged with respect to data symbol periods. To compensate for the absence of pilot symbols at the time-domain start and end of the transport block  10  for user terminal #a, the transport block  10  shares pilot symbol periods  11  and  14  allocated to other user terminals, bordering on the transport block  10 . Similarly, to compensate for the absence of pilot symbols at the time-domain start and end of the transport block  20  for user terminal #b, the transport block  20  shares pilot symbol periods  13  and  16  allocated to other user terminals, bordering on the transport block  20 . 
   First and second antennas of user terminal #a share the transport block  10  and first and second antennas of user terminal #b share the transport block  20 . 
   In this manner, under the assumption that the channel responses of two successive subcarriers are identical in the frequency domain, adjacent user terminals exchange their pilot symbol periods of one of subcarriers allocated to them, thereby enabling more accurate channel estimation. 
   In accordance with the present invention as described above, a predetermined number of pilot subcarriers (or pilot symbol periods) which are allocated to each user terminal, are shared with or exchanged with an adjacent user terminal. Therefore, virtually a greater number of pilot subcarriers (pilot subcarrier periods) than those that are allocated to the user terminal are used. 
   Also, the user terminal utilizes pilot subcarriers of its adjacent user terminal as its own without increasing a pilot subcarrier (pilot symbol period) allocation ratio for the user terminal. Hence, channel estimation performance is improved. 
   Since pilots signals are designed taking into account the use of multiple antennas in user terminals, an antenna diversity gain is achieved. 
   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.