KR20050026193A - Apparatus for fft and ifft in communication system using orthogonal frequency division multiplexing and method thereof - Google Patents

Abstract

An adaptive fast fourier transform and inverse transform apparatus and a method therefor in a communication system of orthogonal frequency division multiplexing are provided to efficiently compensate a channel distortion due to speed of a terminal by adaptively changing the number of points for an IFFT(Inverse Fast Fourier Transform) based on the kind of service and an estimated channel value. A terminal(700) receives a preamble signal in a frame basis from a base station(750)(711). The terminal(700) performs a channel estimation of each sub-carrier through the received preamble signal(713). Upon completing the channel estimation, the terminal(700) transmits the estimated channel information to the base station(750)(715). The base station(750) determines the number of FFT(Fast Fourier Transform)/IFFT points using the received the estimated channel information and service information(717). The base station(750) transmits determined FFT/IFFT point information to the terminal(700)(719). The terminal(700) and the base station(750) share changed FFT/IFFT point information to perform data receiving/transmitting operation(721).

Description

APPARATUS FOR FFT AND IFFT IN COMMUNICATION SYSTEM USING ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING AND METHOD THEREOF

The present invention relates to an adaptive fast Fourier transform / inverse transform apparatus and method in an orthogonal frequency division multiple communication system.

In a data communication scheme based on an Orthogonal Frequency Division Multiplexing (hereinafter referred to as 'OFDM') communication system, data to be transmitted from a base station to a terminal is transmitted in units of blocks in which symbols are collected. That is, the information to be transmitted is N (where N denotes the number of modulation symbols simultaneously input for FFT / IFFT processing) -point Inverse Fast Fourier Transform (hereinafter referred to as 'IFFT'). And a symbol block generated by adding a cyclic prefix to the main symbols having the inverse fast Fourier transform information. As a result, in the conventional OFDM-based data communication scheme, the entire frequency band is divided into N small frequency bands (that is, subcarriers), and then data modulation symbols are mapped and transmitted one by one on each subcarrier.

In a conventional OFDM-based data communication scheme, a base station receives a signal transmitted by a terminal and estimates a user symbol as follows. First, after finding the starting point of each symbol block, the cyclic prefix is removed, and then the N-modulated symbols are N-point fast Fourier transform (hereinafter referred to as 'FFT') to obtain a frequency domain signal. Convert to Then, the data distortion is detected after compensating for the signal distortion generated by the channel at each of the frequencies. Here, the cyclic prefix is added to prevent the interblock interference caused by the channel, and should be longer than the impulse response of the channel.

Next, the transmitter and receiver structures of the OFDM system described above will be described in detail with reference to FIGS. 1 and 2.

First, a transmitter structure of the OFDM system will be described with reference to FIG. 1.

Referring to FIG. 1, the transmitter includes a symbol mapper 101, a serial to parallel converter 103, a pilot symbol inserter 105, and an IFFT unit 107. , Parallel to serial converter (109), guard interval inserter (111), digital to analog converter (113) and radio frequency (hereinafter referred to as 'RF'). Processor 115).

First, when information data bits to be transmitted are generated, the information data bits are input to the symbol mapper 101. The symbol mapper 101 modulates the input information data bits by a predetermined modulation method, converts the symbols, and outputs the converted symbols to the serial / parallel converter 103. Here, the modulation scheme may be a quadrature phase shift keying (QPSK) scheme or a quadrature amplitude modulation (16QAM) scheme.

The serial / parallel converter 103 receives the serial modulation symbols output from the symbol mapper 101 and converts them in parallel and outputs them to the pilot symbol inserter 105. The pilot symbol inserter 105 inserts pilot symbols into the parallel-converted modulated symbols output from the serial / parallel converter 103 and outputs them to the IFFT unit 107.

The IFFT unit 107 inputs the signal output from the pilot symbol inserter 105 to perform an N-point IFFT and then outputs it to the parallel / serial converter 109. The IFFT unit 107 converts a signal in the frequency domain into a signal in the time domain. The detailed operation of the IFFT unit 107 will be described later. The parallel / serial converter 109 inputs the signal output from the IFFT unit 107 and serially converts the signal, and outputs the serial signal to the guard interval inserter 111.

The guard interval inserter 111 receives a signal output from the parallel / serial converter 109, inserts a guard interval signal, and outputs the guard interval signal to the digital / analog converter 113. Here, the guard period is the cyclic prefix described above and when the OFDM symbol is transmitted in the OFDM communication system, the interference between the OFDM symbol transmitted at the previous OFDM symbol time and the current OFDM symbol to be transmitted at the current OFDM symbol time ( It is inserted to remove the interference.

In addition, the guard interval has been proposed in the form of inserting null data of a predetermined interval, but in the form of transmitting null data in the guard interval is the interference between the sub-carriers when the receiver incorrectly estimates the start point of the OFDM symbol There is a disadvantage in that the probability of misjudgment of the received OFDM symbol increases, and thus the 'Cyclic Prefix' method of copying and inserting the last predetermined bits of the OFDM symbol in the time domain into a valid OFDM symbol or the first constant of the OFDM symbol in the time domain It uses 'Cyclic Postfix' to copy bits and insert them into a valid OFDM symbol.

The digital-to-analog converter 113 receives the signal output from the guard interval inserter 111 and converts the signal to the RF processor 115. Here, the RF processor 115 may include components such as a filter and a front end unit, and may transmit a signal output from the digital-to-analog converter 113 in an actual air environment. After RF processing to transmit through a Tx antenna to the air (air). That is, the RF processor 115 low-pass filters the analog-converted signal through a low pass filter (not shown), and has a center frequency f _c through a mixer (not shown). Up converting into a pass band signal. The signal upconverted through the mixer is then amplified through a High Power Amplifier (HPA) (not shown) and then transmitted through the transmit antenna.

In FIG. 1, an OFDM transmitter has been described. Hereinafter, an OFDM receiver will be described with reference to FIG. 2.

2 illustrates a receiver structure of a conventional OFDM communication system.

Referring to FIG. 2, the receiver includes an RF processor 201, an analog / digital converter 203, a guard interval remover 205, a serial / parallel converter 207, and an FFT device. 209, equalizer 215, pilot symbol extractor 211, synchronization & channel estimator 213, parallel / serial converter 217, and symbol demapper demapper;

First, the signal transmitted from the transmitter described above with reference to FIG. 1 is received through a reception antenna of the receiver in the form of a multipath channel and noise added. The signal received through the receive antenna is input to the RF processor 201, and the RF processor 201 down converts the signal received through the receive antennas to an intermediate frequency (IF) band. And then output to the analog-to-digital converter 203. In this case, the RF processor 201 amplifies a signal received from the antenna through a low noise amplifier (not shown), band pass filtering through a band pass filter (not shown), and a mixer (Mixer; not shown) converts the passband signal having a center frequency of f _c into a baseband signal. The received signal converted into the baseband signal is then low pass filtered through a low pass filter (not shown) and output to the analog-to-digital converter 203.

The analog-to-digital converter 203 digitally converts the analog signal output from the RF processor 201 and outputs the digital signal to the guard interval remover 205. The guard interval remover 205 receives the signal output from the analog / digital converter 203, removes the guard interval signal, and outputs the signal to the serial / parallel converter 207. The serial / parallel converter 207 receives a serial signal output from the guard interval remover 205 and converts the serial signal in parallel and outputs the parallel signal to the FFT unit 209.

The FFT unit 209 performs an N-point FFT conversion on the signal output from the serial / parallel converter 207 and outputs the same to the equalizer 215 and the pilot symbol extractor 211. The equalizer 215 receives the signal output from the FFT 209 and compensates for channel equalization, that is, signal distortion caused by the channel, and outputs the signal to the parallel / serial converter 217. The parallel / serial converter 217 receives a parallel signal output from the equalizer 215 and serially converts it to the symbol demapper 219. The symbol demapper 219 receives the signal output from the parallel / serial converter 217 and demodulates the demodulation method corresponding to the modulation scheme applied by the transmitter to output the received information data bits.

On the other hand, the signal output from the FFT 209 is input to the pilot symbol extractor 211, the pilot symbol extractor 211 detects pilot symbols in the signal output from the FFT 209, The detected pilot symbols are output to the sync & channel estimator 213. The sync & channel estimator 213 performs sync and channel estimation using the pilot symbols output from the pilot symbol extractor 211 and outputs the result to the equalizer 215.

Meanwhile, IFFT and FFT used in the transmitter / receiver of the OFDM system are one of the most important algorithms used in the field of digital signal processing (DSP), and implement a Discrete Fourier Transform (DFT). The generic name of the algorithm. The algorithm of the FFT is implemented as one or more integrated circuits for processing signals in real time. In addition, the operation of the fast Fourier transform is performed by a software implemented with a programmable DSP (Digital Signal Processor) or a dedicated fast Fourier transform processor. The basic equation for the N point direct discrete Fourier transform (DFT) is expressed by Equation 1 below.

Where to be. According to Equation 1, data in N time domains is converted into data in N frequency domains. Meanwhile, above Is called a coefficient or a twiddle factor, and the phase rotation values are differentially assigned to each of the N input points according to Equation 1. For the number of operations performed on the given N point input value, complex number multiplication is performed N × N times, and complex addition is performed N × (N−1) times. In this case, the complexity of the operation is O (N × N), which means an operation of an integer multiple of N × N.

The Fast Fourier Transform (FFT) described above as an algorithm for actually implementing the DFT was devised by Cooley & Tukey in the 1960s, and the Radix-2 method of the Fast Fourier Transform. ) Or a radix-4 FFT. The Radix-2 FFT is defined as in Equation 2 below.

Where to be. As can be seen from Equation 2, the Redix-2 FFT is calculated by dividing the even and odd portions of the DFT operation. Hereinafter, with reference to FIG. 3, the basic configuration of the butterfly part shown by generalizing the points performing the Reddick-2 FFT according to Equation 2 will be described. The structure of the butterfly portion indicates a relationship between input values and output values in the FFT transform.

3 is a diagram illustrating a complex operation on each element of a radix-2 structure in a typical FFT operation.

Referring to FIG. 3, as in Equation 2, the output value X [k] reflects the rotation factor, that is, the W _N ^k value, to the input values of x [n] and x [n + N / 2]. Is calculated. The output value X [k + N / 2] is similarly calculated by reflecting the rotation factor, that is, the W _N ^k value, in the input values of x [n] and x [n + N / 2].

The process of performing the actual FFT operation with the Redix-2 structure in FIG. 3 is the same as that of FIGS. 4A and 4B. 4A illustrates an FFT calculation process when an input point is 16 (that is, N = 16), and FIG. 4B illustrates an FFT calculation process when an input point is 8 (ie, N = 8).

First, the signal flow of the processor of the 16-point Redix-2 FFT will be described with reference to FIG. 4A. The butterfly operation of the 16-point FFT is performed by four butterfly stages, each stage consisting of eight butterfly parts. The butterfly operation between each point in each stage is performed by the method of FIG. 3 and Equation 2 as described above.

Meanwhile, as shown in FIG. 4A, according to the Redix-2 FFT, a complex multiplication process is performed. Times, the complex addition process Is performed once. Therefore, it can be seen that the complexity of the Redix-2 FFT is reduced to O (N × log ₂ N) compared to the complexity of the DFT O (N × N) described above. That is, in the case of the 16-point Reddix-2 FFT of FIG. 4A, complex multiplication is performed 16/2 × log ₂ 16 = 32 times, and complex addition is performed 16 × log ₂ 16 = 64 times.

4B is a signal flow diagram of an 8-point Redix-2 FFT processor. The 8-point Redox-2 FFT operation illustrated in FIG. 4A may be performed in the same structure as the 16-point FFT butterfly operation of FIG. 4A. It can be seen that only one stage is reduced as a difference from FIG. 4A. 4A and 4B, it is possible to implement the 8-point FFT structure of FIG. 4B using the 16-point FFT structure of FIG. 4A.

That is, when only x [0] ~ x [7] values are input without using x [8] ~ x [15] (for example, inputting a value of 0) among the inputs of FIG. 4A, X [0] is used at the output terminal. ] ~ X [7] and X [8] ~ X [15] shows the same value repeatedly. That is, when the input of even points in the time domain is 0 due to the characteristics of the FFT, two identical signals are repeatedly output in the frequency domain. As a result, selecting one of the same two output values results in the same result as performing the 8-point FFT operation of FIG. 4B.

The above has described the FFT operation based on the Redix-2 method, and the FFT operation based on the Redyx-4 method will now be described with reference to FIGS. 5 and 6.

The Redox-4 butterfly part structure may be represented by Equation 3 below.

Where p = 0,1,2,3, l = 0,1,2,3, and q = 0,1, ..., N / 4-1.

As can be seen from Equation 3, the Reddix-4 FFT performs the above complex operation on the four result values of the previous stage in the DFT operation. Hereinafter, with reference to FIG. 5, the basic configuration of the butterfly part shown by generalizing the points for performing the Reddicks-4 FFT according to Equation 3 will be described. The structure of the butterfly portion indicates a relationship between input values and output values in the FFT transform.

5 is a diagram illustrating a complex operation on each element of a radix-4 structure in a typical FFT operation.

Referring to FIG. 5, output values X [k], X [k + N / 4], X [k + N / 2] and X [k + 3N / 4] in the Reddix-4 structure are input values. x [n], x [n + N / 4], x [n + N / 2] and x [n + 3N / 4] are calculated by calculating by the calculation method as shown in Equation 3 above.

On the other hand, when comparing the reddix-4 structure of FIG. 5 with the reddix-2 structure of FIG. 3, the reddix-2 structure has two inputs and outputs for one butterfly structure, Since it is four, it can be seen that the larger the redids, the more computation is required. In addition, the structure of the hardware for performing the operation is further complicated. On the other hand, the computation stage that performs the above operation is faster than the Reddix-2 in the calculation speed because the Redix-4 is much smaller. On the other hand, in the case of Redix-2, the calculation stage is log ₂ 16 = 4 when the 16-point FFT, and the calculation stage is log ₄ 16 = 2 when the Redix-4. In conclusion, it can be seen that the more nodes are computed at the same time, the more computational complexity is required, while the processing speed is faster.

6 is a diagram illustrating the operation flow of a typical Redix-4 structure 16 point FFT block.

Referring to FIG. 6, the butterfly calculation of the Redix-4 structure 16 point FFT is performed by two butterfly stages, each stage consisting of four butterfly parts. Here, the operation of the butterfly part is performed by the method of FIG. 5 and Equation 3 as described above.

In the conventional FFT / IFFT structure, the above-described Redix-2 structure can process all inputs composed of 2 ⁿ such as 256, 512, 1024, and 2048, but has a relatively slow processing speed. On the other hand, the Redix-4 structure can process an input composed of 4 ⁿ such as 256 and 1024, but the input mode such as 512 or 2048 cannot be processed because it is not 4 ⁿ . Therefore, in view of this, it is preferable to use a combination of the Reddick-2 structure and the Reddix-4 structure.

Meanwhile, in order to increase transmission efficiency in the aforementioned OFDM-based data communication scheme, the IFFT / FFT point (ie, N value in the above equations) should be made as large as possible. However, if the N value is too large, the size of hardware for implementing FFT / IFFT is relatively increased, which makes it difficult to implement a data communication scheme based on the conventional OFDM. In addition, when the channel changes quickly with time or has a characteristic of frequency selective fading, if the N value is too large, a channel change occurs during one OFDM symbol period, and thus it is not easy to obtain the advantages of the OFDM system. In this case, additional channel estimation is needed to take advantage of OFDM.

In addition, when an adaptive modulation and coding (AMC) technique is used in the OFDM system, modulation and channel encoding may be performed in subcarrier units. Therefore, when it is assumed that each subcarrier undergoes an independent channel in the OFDM system, the maximum data rate can be obtained by applying AMC for each subcarrier. On the other hand, what modulation scheme to use in the AMC or how much the channel coding rate is determined based on feedback information sent from the terminal.

Each terminal (ie, a user) may obtain information about a channel using a pilot channel, a pilot subcarrier, or a preamble signal as a reference channel sent from the base station, and obtain information on the channel. When sent to the base station, the base station determines the modulation scheme and the channel coding rate accordingly to allow data transmission and reception. However, when the above-mentioned FFT / IFFT point is determined when starting data transmission and reception in the OFDM system, the determined FFT / IFFT point is fixedly used. Accordingly, there is a problem in that there is a limit in applying the above-described AMC technique in the fixed FFT point. That is, in an OFDM system having a fixed FFT / IFFT point, data that is constantly transmitted and received is always mapped and transmitted to a certain number of points, thereby preventing effective AMC.

In addition, in a multimedia wireless communication environment where various speeds and characteristics, such as a low speed voice service and a high speed data service, exist, only a limited service may be desired depending on a price or a purpose of a terminal by a user's request. For example, low cost terminals would like to receive only low speed data services, such as voice services, while expensive terminals will want high speed data services and video services.

Therefore, in a multimedia wireless communication environment, the FFT point should be selected for the highest quality than the maximum data rate for the voice service and the low speed terminal, and the FFT point (FFT point) will be focused on the maximum data transmission in a given environment for the high speed data service. That is, the value N in the above) should be selected. However, the conventional OFDM-based data communication method has a limitation in that it cannot effectively solve the above problems with only channel estimation and AMC while fixing the FFT point.

Accordingly, an object of the present invention is to provide a fast Fourier transform / inverse transform apparatus and method for variably applying an FFT / IFFT point according to a channel change in an OFDM communication system.

Another object of the present invention is to provide an apparatus and method for providing a maximum data rate in an OFDM communication system.

Another object of the present invention is to provide an apparatus and method for performing Fast Fourier Transform / Inverse Transform adaptively to a service provided in an OFDM communication system for a multimedia wireless communication environment having various speeds and characteristics.

The method of the present invention for achieving the above objects; In an orthogonal frequency division multiplexing (OFDM) communication system for transmitting predetermined data via fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the fast Fourier transform and inverse transform method includes: The fast Fourier transform and the inverse transform are performed by varying the number of input points in the fast Fourier transform and the inverse transform according to channel estimation information for a wireless environment.

In addition, the method of the present invention for achieving the above objects; In an orthogonal frequency division multiplex (OFDM) communication system in which predetermined data is transmitted through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the data is transmitted by inverse fast Fourier transform (IFFT) from a terminal to a base station. The method of claim 1, further comprising: determining the number of input points in the inverse fast Fourier transform according to channel estimation information on a wireless environment for transmitting the data, and performing inverse fast Fourier transform on the data according to the determined number of input points Characterized in that the transmission process.

In addition, the method of the present invention for achieving the above objects; In an orthogonal frequency division multiplex (OFDM) communication system in which predetermined data is transmitted through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the data is transmitted by inverse fast Fourier transform (IFFT) from a terminal to a base station. The method of claim 1, further comprising: transmitting channel estimation information on the wireless environment for transmitting the data to the base station, and receiving, from the base station, the number of input points in the inverse fast Fourier transform determined according to the channel estimation information. And inversely fast Fourier transforming and transmitting the data according to the determined number of input points received from the base station.

On the other hand, the terminal device of the present invention for achieving the above objects; In an orthogonal frequency division multiplexing (OFDM) communication system that transmits predetermined data through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the terminal device transmits the data by inverse fast Fourier transform (IFFT). An inverse fast Fourier transform (IFFT) point determiner for determining the number of input points in the inverse fast Fourier transform according to channel estimation information for a wireless environment for transmitting the data, and the data according to the determined number of input points And an inverse fast Fourier transform controller for controlling the inverse fast Fourier transform to be inverse fast Fourier transform.

On the other hand, the base station apparatus of the present invention for achieving the above objects; In an Orthogonal Frequency Division Multiplexing (OFDM) communication system that transmits predetermined data through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the base station apparatus transmits the data by inverse fast Fourier transform (IFFT). An inverse fast Fourier transform (IFFT) point for receiving channel estimation information for a wireless environment for transmitting the data from a terminal and for determining the number of input points in the inverse fast Fourier transform determined according to the received channel estimation information And a inverse fast Fourier transform controller for controlling an inverse fast Fourier transform such that the data is inverse fast Fourier transformed according to the determined number of input points.

Hereinafter, with reference to the accompanying drawings in accordance with the present invention will be described in detail. It should be noted that in the following description, only parts necessary for understanding the operation according to the present invention will be described, and descriptions of other parts will be omitted so as not to distract from the gist of the present invention.

The present invention provides an adaptive fast Fourier transform / inverse transform that provides a maximum data rate in an Orthogonal Frequency Division Multiplexing (OFDM) communication system in which the total number of sub-carriers is N. (Fast Fourier Transform / Inverse Fast Fourier Transform; hereinafter referred to as 'FFT / IFFT') An apparatus and method are provided.

That is, the present invention variably controls the operation point of the FFT / IFFT according to the channel environment and the type of service to be provided in order to provide the maximum data rate.

In the OFDM communication system, although the actual number of subcarriers is N, null data, that is, zero data, must be inserted into subcarrier 0, which represents a DC component in the time domain. Since null data is inserted into subcarriers indicating a guard interval, the number of subcarriers used for the actual FFT / IFFT input is N _FFT . Therefore, the FFT / IFFT point available for actual data transmission is the maximum N _FFT . In addition, the channel feedback information used in the adaptive FFT / IFFT block according to the present invention uses the channel estimation value used at the receiving end of the OFDM system, or the existing channel information that can reflect the channel situation experienced by each terminal such as an AMC structure. Can be used.

Meanwhile, the N _FFT used hereinafter means the number of subcarriers used for the actual FFT input as described above, and when the N _FFT is varied according to the present invention, the N _{FFT is applied} to the input of the IFFT to be considered by the transmitter. The number of subcarriers used, i.e., N _IFFT , is preferably equal to the N _FFT .

The structure of the adaptive FFT / IFFT block according to the present invention can be implemented through the structure of a general FFT / IFFT block, and the channel information and the kind of service to be provided exist as a control signal of the FFT / IFFT block. According to the control signal, the number of N _FFTs (that is, the number of input / output points) is changed. In addition, it is preferable that the relationship between the control signal and the N _FFT is tabled and stored in a memory. The N _FFT may be changed according to the relationship, or the channel information value among the control signal values may be compared with a specific threshold. N _FFT can also be adjusted.

On the other hand, the FFT / IFFT block is composed of a structure that can implement a number of N _FFT point FFT / IFFT, such as Red Dix-2 or Red Dix-4, the characteristics of implementing the FFT / IFFT of the desired N _FFT point according to the control signal Has In addition, to perform the FFT change to the same point in the base station according to the N _FFT changed according to the control signal, it is necessary to continuously know the converted N _FFT . Therefore, the terminal preferably transmits the converted N _FFT information to the base station.

Hereinafter, an adaptive FFT / IFFT apparatus and method according to the present invention will be described in more detail.

First, after N _FFT of the FFT / IFFT is determined in consideration of a given bandwidth and a maximum delay spread, a preamble signal is transmitted through a transmitter before the data to be transmitted is actually transmitted and received. The preamble is a reference signal, and includes information on a service provided by a base station to a terminal. The preamble is used for synchronization or channel estimation by the receiver using the received preamble.

At this time, since both the transmitting and receiving end already know the information on the preamble, it is possible to synchronize by using the known preamble signal, it is also possible to estimate the degree of the phase shift of the received signal to estimate the channel. As a signal having the same function as the preamble signal, a pilot signal is used. In the present invention, a channel estimation value that is a result of the basic signal is used regardless of the types of the basic signals. Therefore, it is obvious that the present invention can be applied regardless of whether the basic signal received by the terminal is a preamble signal or a pilot signal.

In a typical communication system, channel distortion experienced by data transmitted and received between a base station and a terminal is compensated through a channel estimation operation of the reference signal. However, this does not take into account the kind of data service (voice, SMS, still / video transmission service, etc.) to be provided, but only passive channel estimation in a given parameter.

Meanwhile, the present invention adaptively determines an N _FFT based on the type of service provided as described above and the channel estimation value described above, and effectively estimates the channel distortion coming from the speed of the terminal. We propose an FFT / IFFT apparatus and method that can provide quality.

Of course, when the first terminal receives the preamble signal, the terminal may check the type of data to be serviced, determine channel information through channel estimation based on the preamble signal, and determine the FFT / IFFT point by itself. However, when the terminal determines the changed FFT / IFFT point, the base station cannot know the changed FFT / IFFT point, so that the base station changes the FFT / IFFT point to inform the terminal of the changed FFT / IFFT information. Do.

Otherwise, when the terminal determines the FFT / IFFT point as described above, the changed point information should be transmitted to the base station so that the base station knows the changed FFT / IFFT point. That is, if the terminal informs the changed information by signaling from the terminal to the base station before transmitting data to the changed FFT / IFFT point, the terminal may change the FFT / IFFT point.

Therefore, as a preferred embodiment of the present invention, the terminal transmits the channel information estimated through the preamble signal to the base station as feedback information, and the base station receives the feedback information and the service type information as a control signal and receives the input. Determine the FFT / IFFT point that meets the condition of the control signal. Thereafter, when transmitting the next preamble, the terminal informs the terminal of the information of the determined FFT / IFFT point. According to the above embodiment, since the base station determines and determines the FFT / IFFT point, the complexity of the terminal algorithm can be reduced. Of course, by adding some processes as described above, the terminal may determine the FFT / IFFT point, and inform the base station of the determined FFT / IFFT point to implement an adaptive FFT / IFFT.

Hereinafter, an adaptive FFT / IFFT point determination procedure in a base station or a terminal according to an embodiment of the present invention will be described with reference to FIGS. 7 and 8.

7 is a flowchart illustrating a procedure of determining an FFT / IFFT point at a base station according to the first embodiment of the present invention.

Referring to FIG. 7, the terminal 700 receives a preamble signal from the base station 750 at a predetermined time interval (for example, in a frame unit) (711). As described above, the terminal 700 performs a channel estimation of each subcarrier through the received preamble signal (713). Meanwhile, the channel estimation may be performed not only by the preamble signal but also by a pilot signal as described above. When the channel estimation is completed, the channel estimation information is transmitted to the base station 750 (715). In this case, in a general OFDM system using AMC, since channel information for the AMC is periodically transmitted through a reverse channel, channel estimation information transmitted to the base station 750 is transmitted through a reverse channel for the AMC. It may be implemented through signaling.

The base station 750 receiving the channel estimation information of the terminal 700 determines the appropriate FFT / IFFT point using the received channel estimation information and the service information of the terminal (717). That is, as described above, when the channel condition of the terminal 700 is good, it is preferable to allocate a lot of the FFT / IFFT points, and when the channel condition is not good, it is preferable to allocate fewer FFT / IFFT points. . In addition, in the case of a service requiring high-speed data transmission according to the type of service requested by the terminal 700, it is preferable to allocate a lot of the FFT / IFFT points. A more specific example of the method of determining the FFT / IFFT point will be described later in the description of FIG. 9.

By varying the number of FFT / IFFT points according to the channel condition or the type of service required as described above, more efficient data transmission is possible and performance of the entire system can be improved.

On the other hand, as described above, in order to be able to transmit and receive through the FFT / IFFT point changed to the new value, the terminal 700 must know the changed FFT / IFFT point value. Accordingly, the base station 700 transmits the determined FFT / IFFT point information to the corresponding terminal 700 (719).

In this case, the changed FFT / IFFT point information may be transmitted by being included in the preamble, which is the first information transmitted from the base station 750 to the terminal 700. Accordingly, the terminal 700 receives the preamble from the base station 750, checks the changed IFFT point, and then changes the processing point of the IFFT unit as the changed IFFT point.

That is, the terminal 700 receiving the newly changed FFT / IFFT point information performs FFT / IFFT conversion according to the changed FFT / IFFT point when transmitting and receiving data. In addition, the base station 750 also performs FFT / IFFT conversion according to the changed FFT / IFFT point. That is, the terminal 700 and the base station 750 share the changed FFT / IFFT point information, thereby performing subsequent data transmission and reception 721 as the changed FFT / IFFT point information.

In the above, the procedure of determining the FFT / IFFT point by the base station has been described. Hereinafter, a procedure of determining, by the terminal, the FFT / IFFT point will be described with reference to FIG. 8.

8 is a flowchart illustrating a procedure of determining an FFT / IFFT point in a terminal according to a second embodiment of the present invention.

Referring to FIG. 8, the terminal 800 receives a preamble signal from the base station 850 at a predetermined time interval (for example, in a frame unit) (811). As described above, the terminal 800 performs channel estimation of each subcarrier through the received preamble signal (813). Meanwhile, the channel estimation may be performed not only by the preamble signal but also by a pilot signal as described above. When the channel estimation is completed, an appropriate FFT / IFFT point is determined using the channel estimation information and the service information of the terminal according to the second embodiment of the present invention (815). The method of determining the FFT / IFFT points is the same as in FIG. 7, and a detailed example thereof will be described later with reference to FIG. 9.

By varying the number of FFT / IFFT points according to the channel condition or the type of service required as described above, more efficient data transmission is possible and performance of the entire system can be improved.

On the other hand, as described above, in order to be able to transmit and receive through the FFT / IFFT point determined and changed to the new value, the base station 850 must know the changed FFT / IFFT point value. Accordingly, the terminal 800 transmits the determined FFT / IFFT point information to the base station 850 (817).

The base station 850 receiving the newly changed FFT / IFFT point information performs FFT / IFFT conversion according to the changed FFT / IFFT point when transmitting and receiving data. In addition, the terminal 800 performs FFT / IFFT conversion according to the changed FFT / IFFT point. That is, the terminal 800 and the base station 850 share the changed FFT / IFFT point information, thereby performing transmission and reception of data 819 as the changed FFT / IFFT point information.

7 and 8 illustrate a process of transmitting and receiving data by determining an FFT / IFFT point according to the present invention. Hereinafter, a method of determining the FFT / IFFT point value in the terminal or the base station will be described with reference to FIG. 9.

9 is a flowchart illustrating a procedure for determining an FFT / IFFT point according to a channel environment according to an embodiment of the present invention.

Before describing FIG. 9, the channel estimation method performed to determine the FFT / IFFT point may be performed in the time domain or may be performed in the frequency domain. When the channel estimation for the FFT / IFFT point determination is performed in the time domain, the number of pilot subcarriers and the channel estimate through each pilot signal are determined. When the channel estimation is performed in the frequency domain, the OFDM is used. The number of symbols and the channel estimate corresponding to each OFDM symbol are determined.

When the channel estimation is performed in the frequency domain, a pilot subcarrier is used as described above, and the pilot subcarriers are appropriately arranged in all bands in order to obtain an optimal channel estimate in a state where resources are minimized. Based on the number of pilot subcarriers and channel information received on each pilot subcarrier, the base station increases, decreases, or maintains an IFFT point.

Meanwhile, when the channel estimation is performed in the time domain, a section smaller than the coherence time is used as a basic unit of channel estimation. The coherence time means a maximum time interval in which the channel condition does not change in a wireless environment. In general, an OFDM system sets one OFDM symbol as a coherence time. Therefore, channel estimation is performed in units of 1 OFDM symbol. However, in order to convert the IFFT point according to the present invention, since the period of one OFDM symbol is too short as described above, the change of the IFFT point is too frequent. Accordingly, when channel estimation is performed in the time domain according to the present invention, the IFFT point is converted using a predetermined number of predetermined OFDM symbols and channel estimates obtained from the plurality of OFDM symbols.

The channel estimate value may be a bit error rate (BER) or a signal to noise ratio (SNR). In other words, any value may be used as long as the channel estimation information reflects the channel state.

In the following description of FIG. 9, a channel estimation method through the frequency domain is described, and it is obvious that the channel estimation in the time domain is equally applicable only to the channel estimation method and the target parameter through the frequency domain.

When the channel estimation is performed in the frequency domain, first, a pilot number N _p and a channel estimate Ch _p corresponding to each pilot subcarrier are received 901. On the other hand, when performing the channel estimation in the time domain as described above, the number of OFDM symbols and the channel estimate corresponding to each OFDM symbol should be used instead of the number of pilots.

Next, a variable Count value for counting the received pilot number is set to 1, and then the count value is increased every time the channel estimate value for each pilot is determined. In addition, N _up and N _down values, which are variables for counting the number of values larger and smaller than a predetermined threshold Ch _th , among the channel estimation values received through the pilot subcarriers, are set to zero.

First, the channel estimate Ch ₁ received on the first pilot subcarrier is compared with the threshold Ch _th (905). At this time, when the channel estimate Ch ₁ is greater than the threshold Ch _th , the N _up value is increased by 1 (907). When the channel estimate Ch ₁ is smaller than the threshold Ch _th , the N _down value is 1. Increase by 909.

Then, the Count value is increased by 1 to determine the channel estimate value for the next pilot subcarrier (913), and the procedure is repeated until the Count value is equal to the total number of pilot subcarriers N _p used. If the channel estimation for all the pilot subcarriers is completed, that is, the count value is the number of pilot subcarriers N _p , the FFT / IFFT is performed using the determined N _up , N _down, and service type information D _type . Determine the point.

Meanwhile, the method of determining the FFT / IFFT point using the determined N _up , N _down, and service type information D _type may be determined by the method shown in Table 1 below.

D _type (N _up , N _down ) N _IFFT default 0 (2, 2) 256 One (2, 2) 512 2 (2, 2) 1024 0 (3, 1) / (4, 0) 512 (1, 3) / (0, 4) 128 One (3, 1) / (4, 0) 1024 (1, 3) / (0, 4) 256 2 (3, 1) / (4, 0) 1024 (1, 3) / (0, 4) 512

In Table 1, the indexes N _up , N _down, and D _type indicate the number of pilots whose channel estimates exceed the threshold, the number of pilots lower than the threshold, and the type of data to be serviced, as described above. The type of data to be serviced may be, for example, a data type such as voice, still image, and video. In Table 1, D _{type is set} to 0, 1, and 2 for the audio, still image, and video for convenience. Let's set it.

Referring to <Table 1>, it is assumed that the default IFFT points are 256, 512 and 1024 according to each service (i.e., voice, still image and video), and the maximum IFFT point that can be provided is 1024. do. In addition, it is assumed that the number of pilot subcarriers transmitting the channel estimate is four. At this time, the N _up and N _down is a relationship of N _up + N _down = 4 is established.

In the case of applying the present invention, when the D _type value is 0 and the number of pilot subcarriers whose channel estimate exceeds the threshold is large (that is, in case of (3, 1) and (4, 0)), the IFFT point is The default value is increased from 256 to 512. On the other hand, if the number of pilot subcarriers whose channel estimate exceeds the threshold is small (that is, in the case of (1, 3) and (0, 4)), the IFFT point is reduced from the default value of 256 to 128.

In addition, when the D _type value is 1 and the number of pilot subcarriers whose channel estimate exceeds a threshold is large (that is, in the case of (3, 1) and (4, 0)), the IFFT point is the default value of 512 to 1024. To increase. On the other hand, if the number of pilot subcarriers whose channel estimate exceeds the threshold is small (that is, in the case of (1, 3) and (0, 4)), the IFFT point is reduced from the default value of 512 to 256.

In addition, when the D _type value is 2 and the number of pilot subcarriers whose channel estimate exceeds the threshold is large (that is, in the case of (3, 1) and (4, 0)), the IFFT point has a default value of 1024. It remains at 1024 points. On the other hand, if the number of pilot subcarriers whose channel estimate exceeds a threshold is small (that is, in the case of (1, 3) and (0, 4)), the IFFT point is reduced from the default value of 1024 to 512.

On the other hand, when the D _type value is 2, the reason why the IFFT point does not increase even though the channel condition is good is that the maximum allowable IFFT point is assumed to be 1024. Table 1 is an example for understanding the present invention, and the number of IFFT points that may increase according to channel conditions may vary depending on the setting. In addition, it is possible to implement a subdivision of the above-described service to be able to adjust the number of IFFT points more optimal and precise.

In the above, the FFT / IFFT point determination procedure according to the present invention has been described. Hereinafter, an embodiment in which the present invention is applied to an OFDM system will be described with reference to FIGS. 10 and 11.

10 is a block diagram showing a transmitter structure of an OFDM system according to an embodiment of the present invention.

Referring to FIG. 10, a transmitter of the OFDM system includes a symbol mapper 1011, a serial to parallel converter 1013, a pilot symbol inserter 1015, and an IFFT device. 1017, parallel to serial converter 1019, guard interval inserter 1021, digital to analog converter 1023, and radio frequency RF ') processor 1025, IFFT point controller 1027 and IFFT point determiner 1029.

Meanwhile, the transmitter structure of the OFDM system according to the embodiment of the present invention is similar to the transmitter structure described above with reference to FIG. 1, and the IFFT point controller 1027 controls to process the IFFT point variably in order to apply the present invention. And the IFFT point determiner 1029. Accordingly, FIG. 10 will be described based on the IFFT point controller 1027 and the IFFT point determiner 1029 related to the present invention.

Detailed functions of the IFFT point determiner 1029 are as described above with reference to FIG. 9. That is, the number of pilots and channel estimate information for each pilot subcarrier are received, and the number of IFFT points is determined according to the channel estimate information and service type.

In this case, when the transmitter is a base station and the base station determines the number of the IFFT points, the determined information should be transmitted to the terminal, and then the IFFT point controller 1027 receives the IFFT point determiner 1019 for data to be transmitted. According to the number of IFFT points determined in the control to be processed in the IFFT unit 1017. In addition, the serial-to-parallel converter 1013 converts the symbols input in series according to the changed IFFT point in parallel in the symbol unit of the number of points.

Meanwhile, when the transmitter is a terminal and the base station determines the number of the IFFT points, the determined information should be transmitted to the terminal, and the IFFT point information received by the terminal is transmitted to the IFFT point controller 1027. For the transmitted data, the IFFT point controller 1027 controls the IFFT device 1017 to process the data according to the determined number of IFFT points.

In the above, the structure of an OFDM transmitter according to an embodiment of the present invention has been described. Hereinafter, the structure of an OFDM receiver according to an embodiment of the present invention will be described with reference to FIG. 11.

11 is a block diagram illustrating a receiver structure of an OFDM system according to an embodiment of the present invention.

Referring to FIG. 11, the receiver includes an RF processor 1101, an analog / digital converter 1103, a guard interval remover 1105, a serial / parallel converter 1107, and an FFT device. (1109), equalizer 1111, pilot symbol extractor 1117, synchronization & channel estimator 1119, parallel / serial converter 1113, symbol demapper demapper 1115, FFT point controller 1121, and FFT point determiner 1123.

Meanwhile, the receiver structure of the OFDM system according to the embodiment of the present invention is similar to the receiver structure described above with reference to FIG. 2, and the FFT point controller 1121 controls to variably process the FFT point in order to apply the present invention. And an FFT point determiner 1123. Accordingly, FIG. 11 will be described based on the FFT point controller 1121 and the FFT point determiner 1123 according to the present invention.

Detailed functions of the FFT point determiner 1123 are as described above with reference to FIG. 9. That is, the number of pilots and channel estimate information for each pilot subcarrier are received from the synchronization and channel estimator 1119, and the number of FFT points is determined by the FFT point determiner 1123 according to the channel estimate information and the service type.

In this case, when the receiver is a base station and the base station determines the number of the IFFT points, the determined information should be transmitted to the terminal, and the FFT point controller 1121 may then transmit the IFFT point determiner 1123 to the data to be transmitted. Control by the FFT unit 1109 according to the number of FFT points determined in the. In addition, the serial-to-parallel converter 1107 converts the symbols input in series according to the changed FFT point in parallel in symbol units of the number of points.

Meanwhile, when the receiver is a terminal and the base station determines the number of FFT points, the determined information should be transmitted to the terminal, and the FFT point information received by the terminal is transmitted to the FFT point controller 1121. For the transmitted data, the FFT point controller 1121 controls the FFT device 1109 to process the data according to the determined number of FFT points. In this case, since the FFT point is determined and received by the base station, the FFT point determiner 1123 only needs to be provided at the base station, and the terminal only needs to include the FFT point controller 1121.

Hereinafter, a method of controlling the IFFT device 1017 using the changed IFFT point information in the IFFT point controller 1017 of FIG. 10 will be described in more detail with reference to FIG. 12.

12 is a flowchart illustrating a procedure of allocating data in an IFFT device using modified FFT / IFFT point information according to an embodiment of the present invention.

Referring to FIG. 12, first, the changed IFFT point information (ie, N _IFFT ) is input (1201), and the changed IFFT point value is compared with an N _max value that is the maximum allowable IFFT point value (1203).

As a result of the comparison, if the number of IFFT points to be changed is not the maximum allowable number of points in the IFFT device (i.e., less than the maximum allowable number of points), as many points as the changed number (i.e., [0 to N _IFFT -1] ) Is inputted to the IFFT process, and 0 is inputted without assigning data to the point where the remaining data is not input (that is, [N _IFFT ~ N _max ]). For example, assuming that the maximum number of IFFT points is 1024 as in the embodiment of Table 1, and if the number of changed IFFT points is 512 points, data is allocated to the changed 512 points, and the data is not allocated. Inserts 0 into the remaining 512 points. In this case, the method of inserting data into the IFFT pointer is inserted according to a predetermined rule as described above with reference to FIGS. 4A and 4B.

As a result of the comparison, if the number of IFFT points to be changed coincides with the maximum allowable number of points in the IFFT device, data is inputted to all IFFT points (that is, [0 to N _IFFT- 1]) 1207 to perform IFFT processing. Data input to the IFFT device is IFFT-processed and output 1209.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the scope of the following claims, but also by the equivalents of the claims.

As described above, the present invention has an advantage of efficiently compensating for channel distortion coming from the speed of a terminal by adaptively changing an IFFT point based on a type of service and a channel estimation value provided by an OFDM communication system. In addition, the FFT / IFFT whose point is changeable has the advantage of improving the overall performance of the OFDM communication system. In addition, when the present invention is applied and used together with the AMC technology, it is possible to implement an optimal data transmission rate, and it is possible to implement the transmission with the best quality according to the type of service provided.

1 illustrates a transmitter structure of a conventional OFDM communication system.

2 shows a receiver structure of a conventional OFDM communication system.

3 illustrates a complex operation on each element of a radix-2 structure in a typical FFT operation.

FIG. 4A shows the flow of computation in a typical Redix-2 structure 16 point FFT block. FIG.

4B shows the flow of computation in a typical Redix-2 structure 8 point FFT block.

5 shows a complex operation on each element of the Radix-4 butterfly portion in a typical FFT operation.

FIG. 6 shows the flow of operations in a typical Redix-4 structure 16 point FFT block. FIG.

7 is a flowchart illustrating a procedure for transmitting and receiving data by determining an FFT / IFFT point in a base station according to the first embodiment of the present invention.

8 is a flowchart illustrating a procedure for transmitting and receiving data by determining an FFT / IFFT point in a terminal according to a second embodiment of the present invention.

9 is a flowchart illustrating a procedure for determining an FFT / IFFT point according to a channel environment according to an embodiment of the present invention.

10 is a block diagram illustrating a transmitter structure of an OFDM system according to an embodiment of the present invention.

11 is a block diagram showing a receiver structure of an OFDM system according to an embodiment of the present invention.

12 is a flowchart illustrating a procedure for allocating data in an FFT / IFFT device using modified FFT / IFFT point information according to an embodiment of the present invention.

Claims (47)

In an orthogonal frequency division multiplexing (OFDM) communication system for transmitting predetermined data via fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the fast Fourier transform and inverse transform method includes: And performing fast fast Fourier transform and inverse transform by varying the number of input points in the fast Fourier transform and inverse transform according to channel estimation information for a wireless environment for transmitting the data. The method of claim 1, And the channel estimation information for the wireless environment is a channel estimate estimated through a pilot signal. The method of claim 2, And the more pilot subcarriers whose channel estimates exceed a threshold increase the number of input points in the fast Fourier transform and the inverse transform. The method of claim 1, The number of input points in the fast Fourier transform and the inverse transform is variable according to the service type of the data to be transmitted. The method of claim 4, wherein The service type increases the number of input points in the fast Fourier transform and the inverse transform as the higher data rate is required. The method of claim 4, wherein The service method may be one or more selected from among a voice call service, a still image transmission service, a video transmission service, and a high speed data transmission service according to a user's application. The method of claim 1, Wherein the number of input points in the variable fast Fourier transform and inverse transform does not exceed the maximum allowable number of points in the fast Fourier transform and inverse transform. In an orthogonal frequency division multiplex (OFDM) communication system in which predetermined data is transmitted through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the data is transmitted by inverse fast Fourier transform (IFFT) from a terminal to a base station. In the way, Determining the number of input points in the inverse fast Fourier transform according to channel estimation information for a wireless environment for transmitting the data; And transmitting the data by inverse fast Fourier transform according to the determined number of input points. The method of claim 8, The channel estimation information is Wherein the method is generated from a predetermined reference signal received from the base station and known to the base station and the terminal. The method of claim 9, The reference signal is characterized in that the preamble signal. The method of claim 10, The preamble signal is a signal which is transmitted first in a predetermined frame for transmitting the data. The method of claim 9, And the channel estimation information for the wireless environment is a channel estimate estimated through a pilot signal. The method of claim 12, And the more pilot subcarriers whose channel estimates exceed a threshold increase the number of input points in the fast Fourier transform and the inverse transform. The method of claim 8, The number of input points in the fast Fourier transform and the inverse transform is variable according to the service type of the data to be transmitted. The method of claim 14, The service type increases the number of input points in the fast Fourier transform and the inverse transform as the higher data rate is required. The method of claim 14, The service method may be one or more selected from among a voice call service, a still image transmission service, a video transmission service, and a high speed data transmission service according to a user's application. The method of claim 8, Wherein the number of input points in the variable fast Fourier transform and inverse transform does not exceed the maximum allowable number of points in the fast Fourier transform and inverse transform. In an orthogonal frequency division multiplex (OFDM) communication system in which predetermined data is transmitted through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the data is transmitted by inverse fast Fourier transform (IFFT) from a terminal to a base station. In the way, Transmitting channel estimation information on a wireless environment for transmitting the data to the base station; Receiving, from the base station, the number of input points in the inverse fast Fourier transform determined according to the channel estimation information; And inversely fast Fourier transforming and transmitting the data according to the determined number of input points received from the base station. The method of claim 18, The channel estimation information is Wherein the method is generated from a predetermined reference signal received from the base station and known to the base station and the terminal. The method of claim 19, The reference signal is characterized in that the preamble signal. The method of claim 20, The preamble signal is a signal which is transmitted first in a predetermined frame for transmitting the data. The method of claim 19, And the channel estimation information for the wireless environment is a channel estimate estimated through a pilot signal. The method of claim 22, And the more pilot subcarriers whose channel estimates exceed a threshold increase the number of input points in the fast Fourier transform and the inverse transform. The method of claim 18, The number of input points in the fast Fourier transform and the inverse transform is variable according to the service type of the data to be transmitted. The method of claim 24, The service type increases the number of input points in the fast Fourier transform and the inverse transform as the higher data rate is required. The method of claim 24, The service method may be one or more selected from among a voice call service, a still image transmission service, a video transmission service, and a high speed data transmission service according to a user's application. The method of claim 18, Wherein the number of input points in the variable fast Fourier transform and inverse transform does not exceed the maximum allowable number of points in the fast Fourier transform and inverse transform. In an orthogonal frequency division multiplexing (OFDM) communication system that transmits predetermined data through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the terminal device transmits the data by inverse fast Fourier transform (IFFT). In An inverse fast Fourier transform (IFFT) point determiner for determining the number of input points in the inverse fast Fourier transform according to channel estimation information for a wireless environment for transmitting the data; And an inverse fast Fourier transform controller for controlling an inverse fast Fourier transform such that the data is inverse fast Fourier transformed according to the determined number of input points. The method of claim 28, The channel estimation information is The apparatus, characterized in that for generating from a predetermined reference signal received from the base station and known between the base station and the terminal. The method of claim 29, And the reference signal is a preamble signal. The method of claim 30, And the preamble signal is a signal transmitted first in a predetermined frame for transmitting the data. The method of claim 29, And the channel estimation information for the wireless environment is a channel estimate estimated through a pilot signal. 33. The method of claim 32, And the more pilot subcarriers the channel estimate exceeds the threshold increase the number of input points in the fast Fourier transform and the inverse transform. The method of claim 28, The number of input points in the fast Fourier transform and the inverse transform is variable according to the service type of the data to be transmitted. The method of claim 34, wherein And the number of input points in the fast Fourier transform and the inverse transform increases as the service type requires a high data rate. The method of claim 34, wherein The apparatus may be one or more selected from among a voice call service, a still image transmission service, a video transmission service, and a high speed data transmission service according to a user's request. The method of claim 28, Wherein the number of input points in the variable fast Fourier transform and inverse transform does not exceed the maximum allowable number of points in the fast Fourier transform and inverse transform. In an Orthogonal Frequency Division Multiplexing (OFDM) communication system that transmits predetermined data through fast Fourier transform and inverse transform (FFT / IFFT) through a plurality of subcarriers, the base station apparatus transmits the data by inverse fast Fourier transform (IFFT). In An inverse fast Fourier transform (IFFT) point determiner for receiving channel estimation information on a wireless environment for transmitting the data from a terminal and determining the number of input points in the inverse fast Fourier transform determined according to the received channel estimation information; , And an inverse fast Fourier transform controller for controlling an inverse fast Fourier transform such that the data is inverse fast Fourier transformed according to the determined number of input points. The method of claim 38, The channel estimation information is And transmit the signal included in a signal for adaptive modulation and coding transmitted from the terminal through a reverse channel. The method of claim 39, The channel estimation information is generated by the terminal through a reference signal known to the terminal and the base station. The method of claim 40, And the reference signal is a preamble signal transmitted first in a predetermined frame for transmitting the data. The method of claim 39, And the channel estimation information for the wireless environment is a channel estimate estimated through a pilot signal. The method of claim 42, wherein And the more pilot subcarriers the channel estimate exceeds the threshold increase the number of input points in the fast Fourier transform and the inverse transform. The method of claim 38, The number of input points in the fast Fourier transform and the inverse transform is variable according to the service type of the data to be transmitted. The method of claim 44, And the number of input points in the fast Fourier transform and the inverse transform increases as the service type requires a high data rate. The method of claim 44, The apparatus may be one or more selected from among a voice call service, a still image transmission service, a video transmission service, and a high speed data transmission service according to a user's request. The method of claim 38, Wherein the number of input points in the variable fast Fourier transform and inverse transform does not exceed the maximum allowable number of points in the fast Fourier transform and inverse transform.