JP2008042575A - Reception device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM reception device and a receiving method thereof that gives a delay even when there is a delay wave exceeding a guard length. <P>SOLUTION: Inter-symbol interference and an SP interpolating filter are controlled independently of each other to set a window position of FFT, i.e. a symbol segmentation position in an optimum position. In this invention, control is performed so as to minimize deterioration due to the inter-symbol interference and deterioration due to the SP interpolating filter respectively while an error rate is monitored. Consequently, even when there is the unreceivable delay wave exceeding the guard length, the delay wave can be received without increasing the circuit scale, and a service area can be expanded. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an orthogonal frequency division multiplexing (OFDM) signal reception method and reception circuit used for terrestrial digital broadcasting, and more particularly to a technique for improving reception performance in OFDM signal reception.

  The digital terrestrial broadcasting system in Japan and Europe adopts the OFDM (Orthogonal Frequency Division Multiplexing) system. The OFDM scheme is a scheme in which a large number of subcarriers are multiplexed and transmitted within one channel band. The OFDM scheme is known as a scheme resistant to multipath interference because the symbol period length is longer than that of single carrier transmission. Further, by providing a so-called guard period in which a part of the effective symbol is cyclically copied, inter-symbol interference does not occur if the multipath is within the guard period. In addition, if the multipath is within the guard period, the signal component can be used effectively.

By utilizing this feature of OFDM, SFN (Single Frequency Network) that constructs a network with a single frequency is used to effectively use the frequency.
Terrestrial digital broadcasting of the ISDB-T system in Japan and the DVB-T system in Europe are performed using the OFDM system.
Next, a transmission process and a reception process common to the ISDB-T system and the DVB-T system will be specifically described.

  In the ISDB-T system and the DVB-T system, pilot signals having known amplitudes and phases are inserted into subcarriers dispersed in the frequency domain. This is called a distributed pilot signal (hereinafter referred to as SP signal). FIG. 9 shows the arrangement of the SP signals. The SP signal is not transmitted for each subcarrier, but the carrier number k is k = 3 (n mod 4) + 12p (mod is a remainder calculation) for the symbol number n in the frequency direction and the time axis direction. Where p is an integer). That is, as shown in FIG. 9, the arrangement of SP signals is repeated with a period of 4 symbols, and the SP signals are transmitted by shifting 3 carriers for each symbol. This SP signal is modulated into a binary value and transmitted in a specific pattern determined by the carrier position.

The information transmission signal is mapped and transmitted to QPSK, 16QAM, 64QAM, etc. using a carrier to which no SP signal is transmitted.
FIG. 10 shows a general transmission side process of this OFDM modulation system.
The data signal is subjected to error correction coding by the error correction coding 101. The error correction encoded data is mapped by the mapping circuit 102 to QPSK, 16QAM, 64QAM, or the like. The mapped data is subjected to interleaving such as time interleaving and frequency interleaving in the interleave circuit 103 in units of carrier symbols. The interleaved carrier symbol unit data is framed by the frame configuration unit 104 together with the SP signal. The framed data is converted into a time domain signal by the IFFT circuit 105. A guard interval adding unit 106 adds a guard interval (hereinafter referred to as a guard period) to the data converted into the time domain signal. This guard period is obtained by cyclically copying the rear part of the effective symbol to the front part of the symbol. That is, one symbol period is composed of a guard period and a subsequent effective symbol period. . This is shown in FIG.

Next, a receiving apparatus that receives the OFDM signal transmitted in this way will be described.
FIG. 12 is a diagram illustrating a configuration of the entire receiver of ISDB-T. An OFDM signal is input to the tuner unit 201 from the antenna 200. The tuner unit 201 selects a channel, down-converts the selected signal to a predetermined band, and inputs the signal to the A / D converter 202. After A / D conversion by the A / D converter 202, the signal is input to the quadrature detection circuit 203. Quadrature detection is performed by the quadrature detection circuit 203, and the signal is output to the synchronization circuit 205 and the FFT circuit 204. The synchronization circuit 205 performs synchronization processing such as symbol synchronization, sampling frequency synchronization, and frequency synchronization. The synchronization circuit 205 determines the FFT window position.

The FFT circuit 204 performs FFT processing on the time domain signal and converts it to a frequency domain signal. The signal after the FFT is equalized by the equalization circuit 206, and the equalized signal is deinterleaved by the deinterleave circuit 207 and then subjected to error correction processing by the error correction circuit 208.
Next, details of the synchronization circuit 205 will be described. The synchronization circuit 205 takes advantage of the fact that the correlation waveform by the guard correlation becomes very large by taking the correlation between the received signal and the signal delayed by the effective symbol length (hereinafter referred to as guard correlation). Establish frequency synchronization. (For example, refer nonpatent literature 1). This is shown in FIG.

Next, based on the correlation waveform, only the effective symbol period is cut out from the symbol period. Usually, in order to obtain the same phase relationship as that on the transmission side, the guard period is discarded, effective symbols are extracted, and information on the window position for FFT processing is sent to the FFT circuit 204.
Next, details of the equalization circuit 206 will be described with reference to FIG.
The SP extraction circuit 301 extracts the SP signal from the output of the FFT circuit 204. The SP signal generation circuit 302 generates an SP signal on the transmission side. The complex division circuit 303 divides the received SP signal by the SP signal on the transmission side. Thereby, the transmission path characteristic of the SP signal position is estimated. Next, the memory 304 accumulates SP signals only for symbols necessary for time axis interpolation. The symbol interpolation circuit 305 interpolates the transmission path characteristic at the SP signal position in the time axis direction. FIG. 15 shows the SP signal interpolated in the symbol direction.

Thus, since the channel characteristics for every three carriers are estimated, the channel characteristics at all carrier positions are estimated by interpolating the carrier interpolation circuit 306 in the frequency axis direction. The interpolation filter at this time is a low-pass filter.
This is shown in FIG. The delay circuit 300 delays the signal of the FFT circuit 204 so as to be the same as the delay of the transmission path characteristic at each carrier position. Next, the complex division circuit 307 divides the signal after the FFT by the transmission path characteristic of the output of the carrier interpolation circuit 306 to equalize the received data signal.

  As shown in FIG. 17A, when there is a delayed wave with respect to the main wave, the signal can be demodulated without intersymbol interference if the effective symbol period on the transmitting side is extracted and demodulated. As shown in FIG. 17B, when there is a preceding wave preceding the main wave, for example, assuming that the preceding wave is at the position of -Tg when the position of the main wave is 0, the effective symbol period is cut out. When the signal is sent to the FFT circuit, intersymbol interference occurs in the hatched portion in FIG. Accordingly, by extracting a symbol at a position different from the transmission side as shown in FIG. 17C and sending it to the FFT circuit, there is no intersymbol interference within the guard period even if there is a preceding wave. Reception is possible.

  However, in such a case, the signal band is out of the SP interpolation signal band. Therefore, it is necessary to use a complex filter as shown in FIG. 16B instead of a low-pass filter as the SP interpolation filter. As a countermeasure for this, for example, Patent Document 1 proposes a method of cyclically rearranging signals as shown in FIG. In Patent Document 1, when there is a preceding wave of −Tg, a symbol is cut out as shown in FIG. At this time, since it is necessary to use a complex filter as shown in FIG. 16B, after the symbols are cut out, the signals are rearranged cyclically as shown in FIG. Then, since it becomes the same as FIG. 18C, the low pass filter shown in FIG. 16A can be used as the interpolation filter.

Further, as shown in FIG. 19, since the signal component is out of the band of the SP interpolation filter, there is a case where the signal component is shifted fixedly by Tg / 2 so that the signal component enters the center of the band of the SP interpolation filter. .
JP 2003-229831 A

In the above conventional technique, when there are multipaths within the guard period, a low-pass filter can be used as the SP interpolation filter by reordering the received signals.
However, when the network is performed by SFN, a signal from outside the service area may be received as a long delay wave. In this case, the delayed wave does not necessarily fall within the guard interval, and there is a long delayed wave exceeding the guard length.

The above conventional technique shows a countermeasure when there are multipaths within the guard period, but does not show a countermeasure against a delayed wave exceeding the guard length.
An object of the present invention is to control the symbol cutout position and the SP interpolation filter independently, and to reduce the degradation of reception characteristics due to intersymbol interference even when there is a delayed wave exceeding the guard length. .

を乗じて、ＦＦＴ後の波形をキャリア毎に可変に回転させることにより、時間軸領域の信号を可変にシフトする可変シフト手段を有することを特徴とする。ガード期間ＧＩ＝有効シンボル長に対する割合，ＮはＦＦＴ後のキャリア番号：無効キャリアを含む。αはシフト量を与える定数）
また、前記可変シフト手段は、数種類のシフト量を用いてシフトされ、その中でエラーレートが最小になるシフト量に固定することを特徴とする。 In order to achieve the above object, the present invention receives an OFDM (Orthogonal Frequency Division Multiplexing) signal, and the OFDM signal is cyclically copied at the beginning of the effective symbol period at the end of the effective symbol period. An OFDM modulated signal having a guard period, and a receiving apparatus that receives the OFDM signal, for each carrier

And a variable shift means for variably shifting the signal in the time axis region by rotating the waveform after FFT variably for each carrier. Guard period GI = ratio to effective symbol length, N includes carrier number after FFT: invalid carrier. α is a constant that gives the shift amount)
Further, the variable shift means is characterized in that it is shifted by using several kinds of shift amounts, and among them, the shift amount is fixed at a minimum error rate.

As described above, according to the present invention, the SP interpolation filter position can be set independently of intersymbol interference by rotating a signal after FFT even for a delayed wave exceeding the guard interval. Therefore, since a virtual complex filter can be realized using a low-pass filter as an interpolation filter, reception may be possible even when there is a delayed wave that cannot be received at the conventional FFT window position. is there.
According to the present invention, it is possible to provide a receiving method and a receiving apparatus that can easily equalize a multipath exceeding the guard length.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(Embodiment 1)
FIG. 8 shows a configuration of a basic reception block diagram of the present embodiment, and FIG. 1 shows a block diagram of equalization processing. An OFDM signal is input to the tuner unit 201 from the antenna 200. The tuner unit 201 selects a channel, down-converts the selected signal to a predetermined band, and inputs the signal to the A / D converter 202. After A / D conversion by the A / D converter 202, the signal is input to the quadrature detection circuit 203. The quadrature detection circuit 203 performs quadrature detection, and the quadrature detection signal is output to the synchronization circuit 205 and the FFT circuit 204. The synchronization circuit 205 performs synchronization processing such as symbol synchronization, sampling frequency synchronization, and frequency synchronization. The synchronization circuit 205 includes an FFT window position determination circuit, and the FFT window position determination circuit determines the FFT window position. In this embodiment, using the error rate information calculated by the error correction circuit, the FFT window position is shifted, and the window position at which the error rate is finally minimized is determined. This will be described later.

      The FFT circuit 204 performs FFT processing on the time domain signal and converts it to a frequency domain signal. The signal after the FFT is equalized by the equalization circuit 206, and the equalized signal is deinterleaved by the deinterleave circuit 207 and then subjected to error correction processing by the error correction circuit 208. Next, details of the equalization circuit 206 will be described with reference to FIG. The output of the FFT circuit 204 is modulated by being variably rotated by multiplying a predetermined number of rotations by a multiplier 209 for each carrier. Since this means that the shift is equivalent in the time domain, it can be regarded as shifted by the variable shift means 210. Next, the SP signal is extracted from the rotated signal by the SP extraction circuit 301. The SP signal generation circuit 302 generates an SP signal on the transmission side. The complex division circuit 303 divides the received SP signal by the SP signal on the transmission side. Thereby, the transmission path characteristic of the SP signal position is estimated. Next, the memory 304 accumulates SP signals only for symbols necessary for time axis interpolation. The symbol interpolation circuit 305 interpolates the transmission path characteristic at the SP signal position in the time axis direction. FIG. 15 shows the SP signal interpolated in the symbol direction.

Thus, since the channel characteristics for every three carriers are estimated, the channel characteristics at all carrier positions are estimated by interpolating the carrier interpolation circuit 306 in the frequency axis direction. The interpolation filter at this time is a low-pass filter.
Next, the embodiment of the present invention will be described in more detail. As shown in FIG. 13, first, in the time domain, a correlation is obtained with an OFDM signal delayed by an effective symbol length. FIG. 13C shows a correlation waveform, and FIG. 13D shows the result of sliding integration of the guard correlation waveform. As shown in FIG. 13D, when the guard correlation waveform is slide-integrated, a triangular wave having a peak symbol boundary position is obtained. However, in practice, due to the influence of noise and delayed waves, a triangular wave in which a peak is accurately positioned at the symbol boundary position is not necessarily obtained.

When a small delayed wave exists, a triangular wave having a peak is obtained in the vicinity of the symbol boundary. However, the position of the delayed wave is unknown from the guard correlation waveform. Therefore, in order to effectively use the signal component of the delayed wave, it is difficult to determine an accurate symbol cutout position only from the guard correlation waveform.
Therefore, after the symbol is cut out based on the peak position of the guard correlation waveform, the symbol is cut out before the peak position of the guard correlation waveform. Further, symbol extraction is performed after the peak position of the guard correlation waveform. Next, C / N, error rate, etc. are monitored, and the symbol cutout position is fixed at the cutout position where C / N or the error rate is the best.

In this way, the symbol cutout position when there is a delayed wave within the guard interval can be fixed at the optimum position.
However, when a preceding wave exists, as shown in FIG. 16A, in the case of an SP interpolation filter using a normal low-pass filter, the signal component goes out of the band, so the signal component band is SP-interpolated. The filter will be sharpened. Therefore, in order to avoid this, it is necessary to use a complex filter. Alternatively, the signals need to be rearranged cyclically.

In this embodiment, as shown in FIG. 1, by rotating the received signal in the frequency domain, the signal band is shifted so as to fall within the band of the low-pass filter used for SP interpolation. FIG. 1 is the same as FIG. 14 of the conventional example except that there is a rotation process after FFT.
Next, the relationship between intersymbol interference and the SP interpolation filter will be specifically described.
FIG. 2 shows a simulation of changes in the error rate by setting D / U = 0 dB, fixing the position of the main wave, and changing the delay time of the delay wave in a two-wave multipath environment.
The simulation was performed with a guard length Tg = Tu / 8.

  In FIG. 2, the symbol cutout position is set to the head of the effective symbol on the transmission side. Therefore, even if the delay time of the delay wave is changed to t = Tg, no change is seen in the error rate. However, in the case of a delayed wave exceeding t = Tg, as the delay time increases, the deterioration due to intersymbol interference increases and the error rate deteriorates. Since the symbol cut-out position is set at the beginning of the effective symbol on the transmission side, when multipath is a preceding wave, if t = 0 or less, an error due to intersymbol interference occurs as the arrival time of the preceding wave becomes earlier The rate is degraded.

  Next, in the SP interpolation filter, when the symbol cut-out position is set at the head of the effective symbol on the transmission side, the transmission path characteristics are flat, and the band of the interpolation filter has a positional relationship as shown in FIG. The characteristic of the SP interpolation filter is shown in the lower part of FIG. 3, and the graph of the error rate degradation caused by the fact that the upper solid line in FIG. Yes. As is apparent from FIG. 3, when the signal component leaves the band of the interpolation filter, the error rate deteriorates more than the deterioration due to intersymbol interference. In the case of FIG. 3, there is no degradation due to the SP interpolation filter if it is a delayed wave within the guard, but when there is a delayed wave exceeding the guard, the signal component goes out of the filter band rather than the performance degradation due to intersymbol interference. Deterioration due to becomes dominant.

In the case of FIG. 3, when multipath exists, characteristics due to degradation due to inter-symbol interference and degradation due to out of band of the SP interpolation filter become asymmetric between the front ghost and the rear ghost.
Therefore, in the present embodiment, the modulation is fixedly performed by Tg / 2 and the positional relationship as shown in FIG. 4 is obtained. In this case, it can be considered that the reference position is the center of the guard and corresponds to the front ghost of Tg / 2 and the rear ghost of Tg / 2.

を乗じて、ＦＦＴ後の波形を回転させる処理である。これにより、時間領域で見た場合の信号をシフトすることができる。
（ガード期間ＧＩ＝有効シンボル長に対する割合，ＮはＦＦＴ後のキャリア番号：無効キャリアを含む）
本発明は、ガードを超える遅延波が存在する場合、図４に示すシンボル間干渉による劣化と、ＳＰ補間フィルタによる劣化の関係を適応的に制御する方法に関する。本発明のフローチャートを図５に示す。また、これに対応するブロック図を図８に示す。 Note that fixed modulation of Tg / 2 is performed for each carrier after FFT of the received OFDM signal.

Is a process of rotating the waveform after the FFT. This makes it possible to shift the signal when viewed in the time domain.
(Guard period GI = ratio to effective symbol length, N is carrier number after FFT: including invalid carrier)
The present invention relates to a method for adaptively controlling the relationship between deterioration due to intersymbol interference shown in FIG. 4 and deterioration due to an SP interpolation filter when there is a delayed wave exceeding the guard. A flowchart of the present invention is shown in FIG. A block diagram corresponding to this is shown in FIG.

First, the symbol cutout position is provisionally determined from the guard correlation waveform. Next, error rate information is obtained from the error correction circuit. Next, the symbol cutout position is shifted to obtain error rate information. This is repeated as many times as the number of cutout positions determined in advance, and the window position where the error rate is minimized is set as the symbol cutout position.
Next, the shift of the signal component by modulation is repeated as many times as the number of predetermined positions of the signal component to determine the rotation that minimizes the error rate.

のαの値を変化させて行う。本実施の形態では、α＝0, 0.5, 1, 1.5, 2の５種類に変化させて、それぞれの場合のエラーレートを確認した。なお、フローチャートでは、便宜上最初にα＝１、すなわち、Ｔｇ／２のシフトを行ない、それとは別に信号成分のシフトを行うように記載しているが、実際の回路では、図８に示すように、初期設定のＴｇ／２の回路の信号成分のシフトのシフト量を変える事によって実現している。 The signal component shift is

This is done by changing the value of α. In the present embodiment, the error rate in each case was confirmed by changing to α = 0, 0.5, 1, 1.5, and 2. In the flowchart, for convenience, α = 1, that is, a shift of Tg / 2 is performed first, and a signal component is shifted separately, but in an actual circuit, as shown in FIG. This is realized by changing the shift amount of the signal component shift of the initially set Tg / 2 circuit.

In this way, the symbol cutout position and the shift of the signal component are determined, and thereafter, the position is fixed at the position and the normal reception process is started.
FIG. 6 shows the symbol cut-out position when there is a delayed wave outside the guard, the deterioration due to inter-symbol interference as a result of the shift by modulation of the signal component, and the deterioration characteristics due to the SP interpolation filter using a low-pass filter. . When the signal component is shifted as shown in FIG. 6, it is estimated that when there is a delayed wave longer than the guard length, there is no deterioration due to the SP interpolation filter as compared with FIGS. .

FIG. 7 shows the result of actual simulation.
FIG. 7 simulates the maximum multipath level (D / U ratio) that can be received with the delay time by changing the delay time of the multipath delay wave in two-wave multipath.
The simulation conditions are DVB-T, 8k mode, guard 1/8, 64QAM, and CR = 3/4. Under this condition, since the guard length is 112 us, the simulation result also shows that the multipath D / U that can be received is increased due to inter-symbol interference after almost 100 us. In other words, it indicates that reception is not possible unless the multipath is small. The broken line data in FIG. 7 is a case where the signal component is fixedly Tg / 2 modulated, and the solid line data in FIG. 7 is an example in which Tg modulation is performed.

  From this result, in the case of Tg / 2 modulation, deterioration starts when the delay time of the delay wave is around 180 us and the signal component protrudes from the pass band of the SP interpolation filter, whereas in the case of Tg modulation, the delay wave The influence of the SP interpolation filter can be seen around the delay time of 240 us. Also, from this result, when the delay time of the delay wave is 220 us, the Tg / 2 modulated signal can receive only a small delayed wave of D / U = 20 dB, whereas the Tg modulated signal is D / U = Even if there is a 13 dB delayed wave, reception is possible.

Therefore, it can be seen that the performance is improved when the non-guard delay wave is present as compared with the conventional fixed phase relationship processing.
In this embodiment, the error rate from the error correction circuit is used to determine the FFT window position and the signal shift means. However, the C / N and other signals that can be used to determine the quality of the signal after reception processing are used. The FFT window position and the shift amount of the signal, that is, the rotation amount of the signal after the FFT may be determined using the index.

  In this specification, the symbol cut-out position and the FFT window position are used in a substantially similar meaning.

  The shift means and circuit according to the present invention separate the deterioration due to inter-symbol interference based on the FFT window position and the deterioration due to the SP interpolation filter, even when a multipath wave with a delay time difference that greatly exceeds the guard interval arrives. It is possible to set the position. Therefore, even when there is a long-delayed delayed wave that exceeds the guard interval in the SFN environment, it is possible to set the optimum FFT window position and SP interpolation filter application position. Such functions include, for example, various devices having a terrestrial digital broadcast receiving function such as a STB and a DVD recorder as well as a PDP and a liquid crystal television, a terrestrial digital broadcast wave analyzing device, and an integrated circuit mounted on these devices. Can be used.

The block block diagram which shows the process of one Embodiment of this invention Diagram explaining degradation due to intersymbol interference The figure explaining the deterioration by SP interpolation The figure explaining the deterioration by SP interpolation Flow chart showing processing of the present invention The figure explaining the deterioration by SP interpolation The figure which shows the simulation result of embodiment of this invention The block block diagram which shows the process of embodiment of this invention The figure which shows arrangement | positioning of SP signal Block configuration diagram showing processing of a general OFDM transmitter Illustration explaining guard interval Block configuration diagram showing processing of a general OFDM receiver The figure explaining the processing of a general OFDM receiver Block configuration diagram showing processing of a general OFDM receiver The figure explaining the processing of a general OFDM receiver Diagram explaining filter characteristics Diagram explaining intersymbol interference The figure explaining processing of a prior art The figure explaining processing of a prior art

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Error correction encoding part 102 Mapping part 103 Interleaving part 104 Frame structure part 105 IFFT part 106 Guard interval addition part 200 Antenna 201 Tuner part 202 A / D converter 203 Orthogonal detection circuit 204 FFT circuit 205 Synchronization circuit 206 Equalization circuit 207 Deinterleave circuit 208 Error correction circuit 209 Multiplier 210 Variable shift means 300 Delay circuit 301 SP extraction circuit 302 SP generation circuit 303 Complex division circuit 304 Memory 305 Symbol interpolation circuit 306 Carrier interpolation circuit 307 Complex division circuit

Claims (6)

A receiving apparatus for receiving an OFDM (Orthogonal Frequency Division Multiplexing) signal, wherein the OFDM signal is an OFDM modulation signal having a guard period in which a signal after the effective symbol period is cyclically copied at the beginning of an effective symbol period. There,
Symbol synchronization means for establishing symbol synchronization, FFT window position determination means for determining an FFT window position, and FFT means for performing FFT on a signal entering the FFT window position,
In the signal after FFT,
Per carrier

And an OFDM receiver having variable shift means for variably shifting the signal in the time axis region by rotating the waveform after FFT variably for each carrier (guard period GI = ratio to effective symbol length, N is Carrier number after FFT: including invalid carrier, α is a constant giving the shift amount)   The variable shift means calculates the error rate in each case after shifting the signal using several kinds of preset variable amounts (α), and sets the shift amount that minimizes the error rate. The receiving device according to claim 1.   The receiving apparatus according to claim 1, wherein the variable shift means performs an FFT operation after setting the FFT window position at a position where the deterioration due to intersymbol interference is the least, and then performs a variable shift. A receiving apparatus for receiving an OFDM (Orthogonal Frequency Division Multiplexing) signal, wherein the OFDM signal is an OFDM modulation signal having a guard period in which a signal after the effective symbol period is cyclically copied at the beginning of an effective symbol period. There,
After establishing symbol synchronization, establishing symbol synchronization, determining the FFT window position, and performing FFT on the signal entering the determined FFT window position,
In the signal after FFT,
Per carrier

, And an OFDM reception method in which a time-domain signal is variably shifted by rotating the waveform after FFT variably for each carrier (guard period GI = ratio to effective symbol length, N is carrier after FFT) Number: Including invalid carrier, α is a constant that gives the shift amount)   The variable shift process uses several preset variable amounts (α) to shift the signal, calculates the error rate in each case, and sets the shift amount that minimizes the error rate. The receiving method according to claim 4.   5. The receiving method according to claim 4, wherein the variable shift means performs an FFT operation after setting the FFT window position at a position where the degradation due to intersymbol interference is least, and then performs a variable shift.

Images