Abstract:
Method and apparatus for transmitting/receiving an uplink pilot used for channel estimation and measurement of an uplink in an FDMA system. Pilot symbols are transmitted with different frequency mapping patterns in first and second pilot symbol intervals of one time slot interval including data symbol intervals and the inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals. As a result, interpolation of a frequency domain during channel estimation is not necessary, and can correctly obtain channel-estimated values of a frequency at which data is transmitted, in a fast time-varying channel environment.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a Frequency Division Multiple Access (FDMA) system, and in particular, to a method and apparatus for transmitting/receiving uplink pilots used for channel estimation and measurement of an uplink. 
         [0003]    2. Description of the Related Art 
         [0004]    The next generation mobile communication system uses Distributed Frequency Division Multiple Access (DFDMA) and Localized Frequency Division Multiple Access (LFDMA) as a useful uplink multiple access scheme. In the uplink, an increase in Peak to Average Power Ratio (PAPR) causes performance degradation due to linear characteristics of a power amplifier of a transmitter, or terminal, resulting in a reduction in cell coverage. Compared with the system using multiple carriers, DFDMA and LFDMA, as they both use a single carrier, are advantageous in that they can solve the PAPR problem. 
         [0005]    In the wireless mobile communication system where channel characteristics undergo a change in time and frequency domains, proper pilot signals are transmitted along with data signals to make it possible to perform channel estimation necessary for demodulating the data signals. A system employing DFDMA/LFDMA generally uses a Time Division Multiplexing (TDM) format that distinguishes between data signals and pilot signals in the time domain before transmission, in order to maintain the preferred PAPR characteristic. 
         [0006]      FIG. 1  illustrates typical TDM-formatted data signals and pilot signals. 
         [0007]    Referring to  FIG. 1 , reference numeral  110  represents one time slot including one transmission time interval (TTI) or a plurality of TTIs. As the most general TDM format, a plurality of time symbols  120  having a duration of the same length of time T d  exist in the one time slot  110 , and each of the time symbols  120  is allocated for transmission of a pilot signal or a data signal. To prevent inter-symbol interference, a guard period (or guard interval)  130  having a length of a time interval T g  is inserted between time symbols  120 . 
         [0008]    With reference to  FIGS. 2 and 3 , a brief description will be made of a structure of a transmission apparatus for implementing a DFDMA/LFDMA system that transmits data signals and pilot signals in the stated-above TDM format. 
         [0009]      FIGS. 2A to 2D  illustrate a structure of a transmission apparatus for the typical DFDMA system.  FIG. 2A  and  FIG. 2B  illustrate a spectrum  200  in a frequency domain and a one-symbol signal format  210  in a time domain of the DFDMA system, respectively, and  FIGS. 2C and 2D  illustrate an exemplary transmission apparatus  220  in the time domain and an exemplary transmission apparatus  230  in the frequency domain of the DFDMA system, respectively. 
         [0010]    Referring to  FIG. 2A , the spectrum  200  in the frequency domain of the DFDMA system has a format in which C frequency elements  201  are spaced apart over the full band, and a set of C scattered frequency elements is called a comb  202 , which is a resource allocation unit. If a distance between frequency elements in one comb  202  is defined as the number R of repetitions (hereinafter repetition R)  203 , a value of the R  203  is equal to the total number of combs. If comb indexes of 1˜R are sequentially assigned to the frequency elements beginning at the position of a first frequency element where each comb starts in the full band, the comb  202  is assigned a comb index of 2. 
         [0011]      FIG. 2B  illustrates a signal format  210  of a length-T d  DFDMA data symbol in the time domain. If the basic time element is defined as a sample, a length of a sample interval  211  is T s  and a reciprocal of the sample interval length is a sampling frequency. In a time-domain signal format of the DFDMA system, a block of C data symbols is repeated as many times as the repetition R defined in the spectrum  200 . Because 4 data symbols of a, b, c and d are repeatedly transmitted R times for one data or pilot symbol interval T d    212 , a relationship between the sample interval length T s    211  and the data/pilot symbol interval T d    212  is given as in Equation (1). 
         [0000]        T   d   =C·R·T   S   (1) 
         [0012]    A structure of a transmitter for generating a DFDMA transmission signal having the signal format  210  in the time and frequency domains will be described with reference to  FIGS. 2C and 2D . 
         [0013]    Referring to  FIG. 2C , a transmission apparatus  220  in the time domain receives an input bit stream using a proper bit-to-constellation mapper  221 , and outputs C data symbols. Exemplary bit-to-constellation mapping methods used in the bit-to-constellation mapper  221  include Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. In addition, a pilot sequence generator  222  generates C pilot symbols. 
         [0014]    Output symbols of the bit-to-constellation mapper  221  and the pilot sequence generator  222  are input to a selector  223 , and the selector  223  selects one type of the symbols according to the current time symbol index. An output of the selector  223  is repeatedly output by a repeater  224  as many times as the repetition R, and then phase-shifted by a comb-specific phase shifter  225 . A phase-shifted i th  comb is expressed as in Equation (2). 
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         [0015]    In Equation (2), l denotes a sample index described in  FIG. 2B . 
         [0016]    The signal phase-shifted by the comb-specific phase shifter  225  passes through a guard interval adder  226  to prevent inter-symbol interference, and then is transmitted over a wireless channel. The guard interval adder  226  can use any one or both of a zero-padding technique of transmitting no signal, and a cyclic prefix, as a guard interval. 
         [0017]      FIG. 2D  illustrates a transmission apparatus for implementing DFDMA that transmits data signals and pilot signals by TDM, in the frequency domain. A bit-to-constellation mapper  231 , a pilot sequence generator  232  and a selector  233  of  FIG. 2D  are identical in operation to the bit-to-constellation mapper  221 , the pilot sequence generator  222  and the selector  223  of  FIG. 2C , so a description thereof will be omitted. 
         [0018]    Referring to  FIG. 2D , a pilot or data signal output from the selector  223  according to a symbol index is converted into a frequency-domain signal through a size-C Fast Fourier Transform (FFT) block  236 . An output signal of the FFT block  234  is mapped to a size-C*R IFFT block  236 , and mapping between the output of the FFT block  234  and the input to the IFFT block  236  is achieved by a comb-specific mapper  235 . The comb-specific mapper  235  differentiates a first IFFT input index for each individual comb while maintaining an interval at which the outputs of the FFT block  234  are input to the IFFT block  236  at the above-defined repetition R, thereby mapping the outputs of the FFT block  234  such that they should not overlap for each individual comb. Because mapping between the output of the FFT block  234  and the input to the IFFT block  236  is performed in the frequency domain, it can be noted that signals for each individual comb are coincide with the DFDMA frequency spectrum  200  described in  FIG. 2A . The output of the IFFT block  236 , which is a time-domain signal, passes through a guard interval adder  237 , and then is transmitted over a wireless channel. The guard interval adder  217  adds a guard interval to the output of the IFFT block  236  in the manner of  FIG. 2C  before transmission. 
         [0019]    The frequency spectrum  200  and the signal format  210  in the time-frequency domain and the transmission apparatuses  220  and  230  of the DFDMA system has been described so far with reference to  FIGS. 2A to 2D . With reference to  FIG. 3 , a description will now be made of LFDMA. LFDMA can also be implemented by using the above-described spectrum, signal format and transmission apparatus and properly controlling the repetition R and the comb-specific phase shifting or comb-specific mapping. In the LFDMA system, because resources of the continuous frequency domain are allocated to a terminal, a domain of the continuous frequency elements that the terminal is allocated is defined as Region.  FIG. 3A  illustrates a frequency-domain spectrum  310  of the LFDMA system, and  FIG. 3B  illustrates an LFDMA transmission apparatus  320  in the frequency domain. A bit-to-constellation mapper  321 , a pilot sequence generator  322 , a selector  323 , an FFT block  324 , an IFFT block  326 , and a guard interval adder  327  of  FIG. 3B  are equal in operation to the elements of  FIG. 2C , so a description thereof will be omitted. 
         [0020]    Because the frequency-domain spectrum  310  of the LFDMA system appears in the continuous frequency domain, it can be noted that a value of the repetition R is 1 and Region occupies a frequency range  312  including a set of C adjacent frequency elements. Region allocated to an i th  terminal is distinguished according to initial start frequency Φ(i)  311 . In addition, a range of the total frequency band includes C total    313  frequency elements. 
         [0021]    In the LFDMA transmission apparatus  320  of  FIG. 3B , a comb-specific mapper  325  continuously maps outputs of an FFT block  324  to C input nodes in sequence beginning at a Φ(i) th  input node of an IFFT block  326 . Implementation of the LFDMA transmission apparatus  320  is possible by properly modifying mapping parameters for the transmission apparatus  230  in the frequency domain of the DFDMA system in this way. In this case, a size of the IFFT block  326  is C total  corresponding to the total frequency band. 
         [0022]    Similarly, it is also possible to implement the transmission apparatus of the LFDMA system by using the transmission apparatus  220  in the time domain of the DFDMA system. The apparatus has the structure shown in  FIG. 2C , but the repetition R is 1 and the phase shifted by the comb-specific phase shifter  225  is set in accordance with Equation (3) below. In Equation (3), Φ(i) means an index of a frequency element where an i th  Region starts in the full band. 
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         [0023]    In Equation (3), a sample index l is an integer between 0 and RC−1. 
         [0024]    Although the transmitter structure for one comb or Region has been described above, when a plurality of combs or Regions are used, extension to a transmission apparatus for multiple combs or Regions is possible by summing up the signals generated using a plurality of transmission apparatuses. Because this extension is obvious to those skilled in the art, it will be assumed in the following description that one terminal uses one comb/Region, for convenience. 
         [0025]    The basic TDM pilot signal format means a format in which a plurality of time symbols having a length of the same time duration T d  exist in one time slot as described in  FIG. 1 , and each of the time symbols is allocated for transmission of pilot or data. However, when a channel impulse response characteristic suffers a change even in one time slot due to the high moving velocity of the terminal, simply using one pilot symbol in one time slot is not sufficient to accurately estimate and measure channel characteristics. If several pilot symbols are used to solve this problem, pilot overhead increases resulting in a decrease in data transmission efficiency. Therefore, there is a need for a scheme for efficiently transmitting uplink pilots in a fast-varying channel environment without increasing the pilot overhead. 
       SUMMARY OF THE INVENTION 
       [0026]    Accordingly, to substantially solve the problems of the prior art, the present invention provides an apparatus and method for efficiently transmitting and/or receiving an uplink pilot with less pilot overhead in an Frequency Division Multiple Access (FDMA) system. 
         [0027]    The present invention provides an uplink pilot transmission and reception apparatus and method for using DFDMA and LFDMA in an uplink. 
         [0028]    According to one aspect of the present invention, there is provided a method for transmitting an uplink pilot in a frequency division multiple access system. The method includes transmitting data symbols in data symbol intervals of one time slot including the data symbol intervals and inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals; transmitting pilot symbols through a first set of frequency elements in the first pilot symbol interval; and transmitting the pilot symbols through a second set of frequency elements in the second pilot symbol interval. 
         [0029]    According to another aspect of the present invention, there is provided a method for transmitting an uplink pilot in a frequency division multiple access system. The method includes transmitting data symbols for terminals in data symbol intervals of one time slot including the data symbol intervals and inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals; transmitting first pilot symbols for a first terminal among the terminals through a first set of frequency elements in the first pilot symbol interval; and transmitting second pilot symbols for a second terminal among the terminals through the first set of frequency elements in the second pilot symbol interval. 
         [0030]    According to further another aspect of the present invention, there is provided an apparatus for transmitting an uplink pilot in a frequency division multiple access system. The apparatus includes a bit-to-constellation mapper for generating data symbols; a pilot sequence generator for generating pilot symbols; a selector for selecting the data symbols in data symbol intervals of one time slot including the data symbol intervals and inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals, and selecting the pilot symbols in the first and second pilot symbol intervals; and a mapper for mapping the data symbols to frequency elements given for data transmission in the data symbol interval before transmission, mapping the pilot symbols to a first set of frequency elements in the first pilot symbol interval before transmission, and mapping the pilot symbols to a second set of frequency elements in the second pilot symbol interval before transmission. 
         [0031]    According to yet another aspect of the present invention, there is provided an apparatus for transmitting an uplink pilot in a frequency division multiple access system. The apparatus includes a bit-to-constellation mapper for generating data symbols; a pilot sequence generator for generating pilot symbols; a selector for selecting the data symbols in data symbol intervals of one time slot including the data symbol intervals and inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals, and selecting the pilot symbols in the first and second pilot symbol intervals; and a mapper for mapping the data symbols to frequency elements given for data transmission in the data symbol interval before transmission, mapping the pilot symbols through a first set of frequency elements in the first pilot symbol interval before transmission, and waiting in the second pilot symbol interval without mapping the pilot symbols. 
         [0032]    According to still another aspect of the present invention, there is provided an apparatus for receiving an uplink pilot in a frequency division multiple access system. The apparatus includes a divider for receiving a signal of one time slot including data symbol intervals and inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals, distinguishing the received signal for each frequency element, and dividing the received signal into data symbols of the data symbol intervals and pilot symbols of the first and second pilot symbol intervals; a channel estimator for performing channel estimation using the pilot symbols; an equalizer for channel-compensating the data symbols using channel estimated values from the channel estimator; a demodulator for Orthogonal Frequency Division Multiplexing (OFDM)-demodulating the channel-compensated data symbols; and a constellation-to-bit mapper for converting the demodulated signal into a bit stream. The data symbols are mapped to given frequency elements in the data symbol intervals, the pilot symbols are mapped to a first set of frequency elements in the first pilot symbol interval, and the pilot symbols are mapped to a second set of frequency elements in the second pilot symbol interval. 
         [0033]    According to still another aspect of the present invention, there is provided an apparatus for receiving an uplink pilot in a frequency division multiple access system. The apparatus includes a divider for receiving a signal of one time slot including data symbol intervals and inconsecutive first and second pilot symbol intervals which have a shorter length than the data symbol intervals, distinguishing the received signal for each frequency element, and dividing the received signal into data symbols of the data symbol intervals and pilot symbols of the first and second pilot symbol intervals; a channel estimator for performing channel estimation using the pilot symbols; an equalizer for channel-compensating the data symbols using channel estimated values from the channel estimator; a demodulator for Orthogonal Frequency Division Multiplexing (OFDM)-demodulating the channel-compensated data symbols; and a constellation-to-bit mapper for converting the demodulated signal into a bit stream. The data symbols for terminals are mapped to given frequency elements in the data symbol intervals, first pilot symbols for a first terminal among the pilot symbols are mapped to a first set of frequency elements in the first pilot symbol interval, and second pilot symbols for a second terminal among the pilot symbols are mapped to a second set of frequency elements in the second pilot symbol interval. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    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: 
           [0035]      FIG. 1  is a diagram illustrating typical TDM-formatted data signals and pilot signals; 
           [0036]      FIGS. 2A to 2D  are diagrams illustrating a typical DFDMA transmission apparatus; 
           [0037]      FIGS. 3A and 3B  are diagrams illustrating a typical LFDMA transmission apparatus; 
           [0038]      FIG. 4  is a diagram illustrating an exemplary TDM format in which a data symbol interval and a pilot symbol interval have different lengths; 
           [0039]      FIGS. 5A and 5B  are diagrams illustrating DFDMA/LFDMA transmission apparatuses according to an exemplary embodiment of the present invention; 
           [0040]      FIG. 6  is a diagram illustrating a typical DFDMA/LFDMA reception apparatus; 
           [0041]      FIGS. 7A and 7B  are diagrams illustrating transmission of data signals and pilot signals according to a first embodiment of the present invention; 
           [0042]      FIGS. 8A to 8C  are diagrams illustrating transmission of data signals and pilot signals according to a second embodiment of the present invention; 
           [0043]      FIGS. 9A to 9C  are diagrams illustrating transmission of data signals and pilot signals according to a third embodiment of the present invention; 
           [0044]      FIG. 10  is a flowchart illustrating a pilot transmission operation according to an exemplary embodiment of the present invention; and 
           [0045]      FIG. 11  is a flowchart illustrating a pilot reception operation according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0046]    Exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness. The terms used herein are defined taking into account their functions in the present invention, and they are subject to change according to user, user&#39;s intention, or the usual practice. Therefore, a definition thereof should be given depending on the full text of this specification. 
         [0047]    The present invention provides apparatuses and methods to transmit a pilot and data symbols by TDM without increasing pilot overhead when a moving velocity of a terminal is high. A description will now be made of an uplink pilot transmission/reception technology for a system that can selectively use DFDMA and LFDMA for the full or partial frequency band. 
         [0048]      FIG. 4  illustrates an example of a signal format in which a data symbol interval and a pilot symbol interval have different lengths according to an exemplary embodiment of the present invention. 
         [0049]    As illustrated, at least two pilot symbol time intervals  430  and a plurality of data symbol time intervals  450  exist in one time slot  410 , each of the pilot symbol time intervals  430  has a length of T d1 , and a guard interval T g1    420  is inserted in front of each of the pilot symbol time intervals. In addition, the data symbol time intervals  450  have a length of T d2 , and a guard interval T g2    440  is inserted in front of each of the data symbol time intervals  450 . Herein, the pilot symbol interval length T d1  is different from the data symbol interval length T d2 . Similarly, the lengths T g1  and T g2  of the guard intervals  420  and  440  inserted in front of the pilot symbol and the data symbol are different from each other. 
         [0050]    That is, an exemplary embodiment of the present invention sets the pilot symbol interval length to be shorter than the data symbol interval length and uses a plurality of pilot symbol time intervals in one time slot  410  as shown in  FIG. 4 , so a receiver can rapidly estimate channel variation in the situation where a moving velocity of the terminal is high, with less pilot overhead. 
         [0051]    Because a basic time element constituting the pilot and data symbols is a sample and a sampling frequency in one time interval is constant, a transmitter changes the pilot symbol interval length by changing the number of samples included in one symbol by properly adjusting values of a repetition R and a comb size C. In the following description, pilot-related parameters are denoted by R p  and C p , and data-related parameters are denoted by R d  and C d . For convenience, it is assumed that one terminal uses one comb/Region. 
         [0052]      FIGS. 5A and 5B  illustrate transmission apparatuses of a DFDMA system according to an exemplary embodiment of the present invention.  FIG. 5A  illustrates a transmission apparatus  510  in a time domain, and  FIG. 5B  illustrates a transmission apparatus  520  in a frequency domain. 
         [0053]    Referring first to  FIG. 5A , the transmission apparatus  510  in the time domain will be described. An input bit stream generated through error correction coding and rate matching is input to a bit-to-constellation mapper  511  where it is converted into C d  data symbols according to QPSK or QAM modulation. A pilot sequence generator  512  generates C p  pilot symbols, and in this case, the pilot sequence is not limited to a sequence of a specific pattern. 
         [0054]    The symbols output from the bit-to-constellation mapper  511  and the pilot sequence generator  512  are input to a selector  514 , and the selector  514  selects one of the two inputs every symbol interval according to whether the current desired transmission time symbol is allocated as a pilot or data. A control signal indicating a type for the current time symbol is generated by a controller  513 , and then input in common to the selector  514 , a repeater  515 , a comb-specific phase shifter  516  and a guard interval adder  517 . When the input control signal indicates a data symbol interval, an output of the selector  514  and an output of the bit-to-constellation mapper  511  are both C d  data symbols, and when the input control signal indicates a pilot symbol interval, an output of the selector  514  becomes C p  pilot symbols which are outputs of the pilot sequence generator  512 . 
         [0055]    The repeater  615  repeats the output of the selector  514  R d  or R p  times according to the control signal received from the controller  513 . The comb-specific phase shifter  516  performs phase shifting according to Equation (4) below when the control signal received from the controller  513  indicates the data interval, and performs phase shifting according to Equation (5) below when the control signal indicates the pilot interval. 
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         [0056]    In Equations (4) and (5), i denotes an index of a terminal, and Φ d (i) and Φ p (i) denote a data comb index and a pilot comb index allocated to an i th  terminal, respectively. Each comb index, as described above, means the position where the first frequency element starts. A common parameter l of Equation (4) and Equation (5) denotes a sample index in the time domain. There are R d ·C d  samples in the data interval and there are R p ·C p  samples in the pilot interval. Because the sampling frequency is constant, lengths of the data symbol interval and the pilot symbol interval differ according to R d ·C d  and R p ·C p . 
         [0057]    The guard interval adder  517  inserts a guard interval for preventing inter-symbol interference, to the phase-shifted signal, and transmits the guard interval-inserted signal over a wireless channel. Herein, the guard interval adder  517  uses zero-padding and/or periodic prefix. 
         [0058]    Next, with reference to  FIG. 5B , a description will be made of the transmission apparatus  520  in the frequency domain. A bit-to-constellation mapper  521 , a pilot sequence generator  522 , a controller  523 , a selector  524  and a guard interval adder  528  are equal in operation to their associated entities of  FIG. 5A , so a description thereof will be omitted. 
         [0059]    An output of the selector  524  is input to an FFT block  525  where it is converted into a frequency-domain signal. A size of the FFT block  525  is determined as C d  (for data symbol interval) or C p  (for pilot symbol interval) according to the control signal generated by the controller  523 . An output of the FFT block  525  is mapped to an input of an IFFT block  527  by a comb-specific mapper  526 . In the DFDMA system, outputs of the FFT block  525  are input to the IFFT block  527  at stated intervals, and a detailed mapping method follows Equation (6) for the data symbol interval, and Equation (7) for the pilot symbol interval. 
         [0000]        n=Φ   d ( i )+ m*R   d   ,m= 0 , . . . , C   d −1  (6) 
         [0000]        n=Φ   p ( i )+ m*R   p   ,m= 0 , . . . , C   p −1  (7) 
         [0060]    In Equations (6) and (7), i denotes an index of a terminal, and Φ d (i) and Φ p (i) denote a data comb index and a pilot comb index allocated to an i th  terminal, respectively. In addition, m and n denote an FFT output node index and an IFFT input node index, respectively. That is, according to Equation (6) and Equation (7), an m th  output of the FFT block  525  is mapped to an n th  input of the IFFT block  527 . 
         [0061]    The IFFT block  527  converts an input frequency-domain signal into a time-domain signal, and the guard interval adder  528  adds a guard interval to the time-domain signal and transmits the guard interval-added signal over a wireless channel. 
         [0062]      FIG. 6  illustrates a reception apparatus  600  in a time/frequency domain of a DFDMA/LFDMA system according to an exemplary embodiment of the present invention. 
         [0063]    Referring to  FIG. 6 , a divider  601  receives a signal of one time slot, distinguishes it for each frequency element, and divides the received signal into a pilot signal and a data signal in one time slot according to the current symbol index received as a control signal. The divider  601  separates pilot signals corresponding to at least two pilot symbols according to the number of pilot symbol intervals in one time slot and a pilot symbol interval length, both of which are previously known. In particular, the divider  601  divides the frequency elements to which data symbols and pilot symbols are mapped, into pilot signals and data signals. 
         [0064]    The pilot signal is input to a channel estimator  602  where it is used for estimating all channels, and a data signal is input to an equalizer  603 . The equalizer  603  compensates for distortion of the data signal, occurred due to fading, using a channel estimated value output from the channel estimator  602 . A demodulator  604  OFDM-demodulates the compensated data signal into a time-domain signal, and a constellation-to-bit mapper  605  converts the signal-domain signal into a bit stream. 
         [0065]    A controller  606  controls operation of the divider  601  and the channel estimator  602  according to first, second and third embodiments of the present invention. As a result, the data signal is channel-compensated by the equalizer  603  using the channel estimated value corresponding to the frequency (Regent in LFDMA or comb in DFDMA) at which a desired data signal is transmitted. In this case, if there is no pilot corresponding to the frequency where the data signal is transmitted, the controller  606  performs interpolation in the frequency domain, and channel-compensates the data signal using the channel estimated value obtained through the interpolation. 
         [0066]    The technology proposed in exemplary embodiments of the present invention, in which a data symbol interval and a pilot symbol interval are different in length and a TDM pilot pattern is used, can be implemented with the transmission apparatuses of  FIGS. 5A and 5B , and various embodiments are possible according to R d ·C d , R p ·C p , Φ d (i) and Φ p (i). A description will now be made of spectrums and characteristics of TDM signals according to three exemplary embodiments. 
       First Embodiment 
     R d =4, C d =8, R p =4, C p =4 
       [0067]    For convenience, the first embodiment will be described on the assumption that a pilot symbol interval length is ½ of a data symbol interval length and two pilot symbols are transmitted in one time interval. In this embodiment, R d =4, C d =8, and R d ·C d  (=32) is a double of R p ·C p  (=16). 
         [0068]      FIGS. 7A and 7B  illustrate transmission of data signals and pilot signals according to a first embodiment of the present invention. 
         [0069]      FIG. 7A  illustrates a frequency-domain spectrum  710  where a data signal is transmitted, and 4 terminals allocated  4  combs  711  to  714  are called a terminal  1  through a terminal  4 , respectively. For the terminal  1 , as it uses a first comb  711  of the data signal, its data comb index is Φ d (1)=1. For the terminal  2 , as it uses a second comb  712  of the data signal, its data comb index is Φ d (2)=2. For the terminal  3 , as it uses a third comb  713  of the data signal, its data comb index is Φ d (3)=3. For the terminal  4 , as it uses a fourth comb  714  of the data signal, its data comb index is Φ d (4)=4. 
         [0070]      FIG. 7B  illustrates a spectrum  720  in a common frequency domain, where two pilot symbol signals are transmitted. For a terminal  1 , as it uses a first comb  721  of the pilot signal, its pilot comb index is Φ p (1)=1. For a terminal  2 , as it uses a third comb  723  of the pilot signal, its pilot comb index is Φ p (2)=3. For a terminal  3 , as it uses a second comb  722  of the pilot signal, its pilot comb index is Φ p (3)=2. For a terminal  4 , as it uses a fourth comb  724  of the pilot signal, its pilot comb index is Φ p (4)=4. 
         [0071]    The forgoing description is for the DFDMA system, and for the LFDMA system, the first embodiment can be applied by using R d =1, C d =8, R p =1, C p =4, and C total =32 for the method described in  FIG. 3 . 
         [0072]    The first embodiment has been described so far in which the pilot symbol interval is different in length from the data symbol interval and a plurality of pilot symbol intervals have the same comb indexes in the DFDMA/LFDMA system. Because a plurality of pilot symbols each having a short time interval exist in one time slot, the first embodiment can estimate fast channel variation in the time domain with the pilot overhead similar to the conventional one. 
         [0073]    As to a channel estimation process of a receiver according to the first embodiment, because the pilot combs exist only in ½ of the frequency domain forming the data combs in  FIG. 7 , the channel estimation process performs interpolation of finding a channel estimated value of a corresponding frequency domain using channel estimated values of adjacent pilot combs for data signals of the frequency domain where there is no pilot comb. 
       Second Embodiment 
     R d =4, C d =8, R p =4, C p =4 
       [0074]    The second embodiment uses the same parameter values as those used in the first embodiment. 
         [0075]      FIGS. 8A to 8C  illustrate transmission of data signals and pilot signals according to the second embodiment of the present invention. 
         [0076]      FIG. 8A  illustrates a frequency-domain spectrum  810  of a data signal. Similarly to the frequency-domain spectrum  710  of the data signal shown in  FIG. 7A , there are 4 data combs  811 ,  812 ,  813  and  814  allocated to a terminal  1  through a terminal  4 .  FIGS. 8B and 8C  illustrate frequency-domain spectrums  820  and  830  for two pilot symbol signals, respectively. The second embodiment differentiates a comb index of a terminal, being set in a first pilot symbol, from a comb index of a terminal, being set in a second pilot symbol. That is, the second embodiment uses a pilot pattern differently shifted in the frequency domain for each individual pilot symbol. 
         [0077]    The frequency-domain spectrum  820  of the first pilot signal is identical to the pilot symbol spectrum  720  of  FIG. 7B . A comparison between the frequency-domain spectrum  830  of the second pilot signal and the first spectrum  820  will be made hereinbelow. Similarly to the first embodiment, there are 4 terminals of a terminal  1  through a terminal  4 . 
         [0078]    In the second pilot symbol, for the terminal  1 , as it uses a third comb  823  of a pilot signal, its pilot comb index is Φ p (1)=3. For the terminal  2 , as it uses a first comb  821  of the pilot signal, its pilot comb index is Φ p (2)=1. For the terminal  3 , as it uses a fourth comb  824  of the pilot signal, its pilot comb index is Φ p (3)=4. For the terminal  4 , as it uses a second comb  822  of the pilot signal, its pilot comb index is Φ p (4)=2. 
         [0079]    Taking the embedment of the DFDMA system into consideration, it is possible to apply the second embodiment even to the LFDMA system, using R d =1, C d =8, R p =1, C p =4, and C total =32. 
         [0080]    The second embodiment has been described so far in which the pilot symbol interval is different in length from the data symbol interval and a spectrum of a plurality of pilot symbol intervals uses a different comb index where one terminal shifts in the frequency domain in the DFDMA/LFDMA system. Because a plurality of pilot symbols having a short time interval exist in one time slot, the second embodiment can estimate fast channel variation in the time domain with the pilot overhead similar to the conventional one. 
         [0081]    When different comb indexes are used in two pilot symbol intervals in one time slot as done in the second embodiment, it is also possible to obtain a channel estimated value of the full frequency domain constituting the data combs without interpolation of the frequency domain. Even though the examples described in  FIGS. 8B and 8C  are extended to the case where more than two pilot symbol intervals exist in one time slot, the interpolation in the frequency domain can be unnecessary if a comb index of each individual pilot symbol is differentiated. 
       Third Embodiment 
     R d =4, C d =8, R p =2, C p =8 
       [0082]      FIGS. 9A to 9C  illustrate transmission of data signals and pilot signals according to the third embodiment of the present invention. Similarly to the second embodiment, the third embodiment uses two pilot symbol signals in one time slot.  FIG. 9A  illustrates a frequency-domain spectrum  910  of a data signal, and there are 4 data combs  911 ,  912 ,  913  and  914 , which are allocated to a terminal  1  through a terminal  4 , respectively.  FIGS. 9A and 9B  illustrate frequency-domain spectrums  920  and  930  of two pilot signals. Because R p =2 and C p =8 for the pilot, the third embodiment transmits pilot signals for the full frequency region where two terminals transmit data in each pilot symbol interval. 
         [0083]    In the first pilot symbol, for the terminal  1 , as it uses a first pilot comb  921 , its pilot comb index is Φ p (1)=1. For the terminal  3 , as it uses a second pilot comb  922 , its pilot comb index is Φ p (3)=2. In the second pilot signal, for the terminal  2 , as it uses a first pilot comb  931 , its pilot comb index is Φ p (2)=1. For the terminal  4 , as it uses a second pilot comb  932 , its pilot comb index is Φ p (4)=2. The LFDMA system implementation method using the embodiment of the DFDMA system is also equal to the above-described method. 
         [0084]    With reference to  FIG. 10 , a description will now be made of a pilot transmission operation according to an exemplary embodiment of the present invention. 
         [0085]    Referring to  FIG. 10 , a transmitter generates pilot or data symbols in step  1000 , and repeats the pilot or data symbols or performs FFT thereon in step  1002 . The generation of the pilot or data symbols in step  1000  is achieved by selecting symbols of the type corresponding to the current symbol index according to the signal format shown in  FIG. 4 . The data or pilot signal that underwent repetition or FFT in step  1002  undergoes comb-specific phase shifting or IFFT, so it is mapped to the corresponding allocated resources. The frequency resources (Region in the LFDMA or comb in the DFDMA) mapped through the phase shifting or IFFT are set taking into account any one of the first, second and second embodiments, and the first and second pilot symbol intervals according to the signal format disclosed in  FIG. 4 . In step  1006 , a guard interval is added to the pilot and data signals mapped in steps  1002  and  1004 , and then transmitted in step  1008 . 
         [0086]    With reference to  FIG. 11 , a description will now be made a pilot reception operation according to an exemplary embodiment of the present invention. 
         [0087]    Referring to  FIG. 11 , a receiver determines in step  1100  whether the received signal is a pilot signal or a data signal according to the current symbol index. If the current received signal is a pilot signal, the receiver proceeds to step  1102 . If the current received signal is a data signal, the receiver proceeds to step  1104 . 
         [0088]    In step  1102 , the receiver estimates a channel using the pilot signal. In performing channel estimation, the receiver finds channel estimated values of a frequency (Region in the LFDMA or comb in the DFDMA) where a pilot signal is transmitted, and finds channel estimated values corresponding to the frequency where a desired data signal is transmitted according to any one of the first, second and third embodiments. If there is no pilot signal of the (desired) frequency where the desired data signal is transmitted, the receiver finds the channel estimated values of the desired frequency through frequency interpolation. In step  1104 , the receiver performs channel compensation on the data signal using the channel estimated value. In step  1106 , the receiver demodulates the channel-compensated data signal. In step  1108 , the receiver recovers the transmitted information bits from the demodulated data. 
         [0089]    As can be understood from the foregoing description, the present invention uses a plurality of pilot symbol intervals while differentiating a length of the pilot intervals from a length of the data intervals, making it possible to obtain less overhead and excellent channel estimation performance even in the situation where a channel suffers fast variation due to the high-velocity movement in the LFDMA system. In the DFDMA system, the present invention differently sets the comb indexes that one terminal uses in a plurality of pilot symbol intervals, or allows the terminal to use only one of the plurality of pilot symbol intervals, thereby compensating for degradation of the channel estimation performance due to interpolation in the frequency domain. Therefore, the pilot pattern proposed by the present invention efficiently operates in the LFDMA system even at the high rate, and does not cause the channel estimation performance degradation problem due to the interpolation of the frequency domain in the DFDMA system, contributing to improvement in the channel estimation performance of the entire system in various environments. 
         [0090]    While the present 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 present invention as defined by the appended claims.