JP2005260705A - Ofdm modem - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM modem capable of transmitting an OFDM signal for offsetting a frequency error towards a counterpart in order to exclude counterpart's coarse control. <P>SOLUTION: A demodulation portion 2 of a first communication device 1 comprises a configuration in which a phase differential Δθ according to a frequency error Δf of an OFDM signal is computed using an AFC 13 in a frequency error processing circuit 11 and the phase differential Δθ is corrected using a phase rotator 14. On the other hand, at a modulation portion 16 of the first communication device 1, a phase of the OFDM signal is shifted beforehand by the phase differential Δθ calculated by the AFC 13 in the demodulation portion 2 so that a frequency of the OFDM signal is shifted to reverse direction beforehand by the frequency error Δf. Therefore, when a second communication device 31 receives the OFDM signal from the first communication device 1, the frequency error Δf according to a propagation path is offset, and the second communication device can receive the OFDM signal with little impact of the frequency error Δf. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an OFDM modulation / demodulation apparatus used for a wireless local area network (LAN) or the like based on an orthogonal frequency division multiplexing (OFDM) transmission system.

  In general, an OFDM modulation / demodulation apparatus includes a demodulation unit that demodulates an OFDM signal received from a counterpart, and a modulation unit that modulates the OFDM signal and transmits it to the counterpart. Then, as an OFDM signal demodulating unit according to the prior art, an automatic frequency control unit that estimates and corrects a frequency error occurring in the OFDM signal, and for each symbol period in the packet of the OFDM signal corrected by the automatic frequency control unit A configuration including a Fourier transform processing unit that performs Fourier transform processing is known (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 10-75228

  In such a conventional technique, the automatic frequency control unit estimates a frequency error using a known training symbol (known symbol) provided on the head side of the packet (frame) of the OFDM signal, and this frequency error is performed by signal processing. It was the structure which performs the phase rotation according to. At this time, since a short short training symbol and a long long training symbol are provided at the beginning of the packet of the OFDM signal, the automatic frequency control unit corrects a large frequency error using, for example, the short training symbol ( After the coarse adjustment, the fine adjustment for correcting the small residual frequency error was performed using the long training symbol.

  By the way, since the local frequency of OFDM signals propagating in space generally shifts (shifts) in accordance with the characteristics of the propagation path, the conventional OFDM modulation / demodulation device always corrects the frequency error using an automatic frequency control unit. There is a need to. At this time, in order to cope with a case where the frequency shift width of the OFDM signal is large (for example, when the frequency is shifted by about half of the carrier interval), the automatic frequency control unit performs frequency adjustment such as rough adjustment and fine adjustment. The error adjustment process had to be performed twice.

  On the other hand, the short training symbol used for coarse adjustment is also used for various processes such as signal edge detection, reception field strength (RSSI) measurement, automatic gain control, and antenna switching. For this reason, when short training symbols are used for coarse adjustment, short training symbols used for these processes tend to be insufficient, and there is a problem that processing accuracy such as RSSI measurement and automatic gain control is lowered.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide an OFDM modulation / demodulation apparatus capable of transmitting an OFDM signal for canceling a frequency error toward the other party and omitting the other party's coarse adjustment. It is to provide.

  In order to solve the above-mentioned problem, the invention of claim 1 is an OFDM modulation / demodulation apparatus comprising a demodulator that demodulates an OFDM signal received from a counterpart and a modulator that modulates the OFDM signal and transmits the modulated OFDM signal to the counterpart. The demodulator includes a frequency error estimator that estimates a frequency error generated in the OFDM signal, a frequency error corrector that corrects a frequency error estimated value by the frequency error estimator, and an OFDM signal that is corrected by the frequency error corrector. A Fourier transform processing unit that performs a Fourier transform process for each symbol period in the packet, and the modulation unit has a frequency of the demodulation unit so as to cancel a frequency error due to a propagation path with the other party. A frequency shift unit that shifts the frequency of the OFDM signal transmitted toward the other party using the frequency error estimated value by the error estimation unit is provided. It is characterized in that was formed.

  According to a second aspect of the present invention, the frequency error estimation unit includes an automatic frequency controller that estimates a frequency error for each packet using a known symbol provided in each packet in the OFDM signal, and the frequency shift unit Is configured to shift the frequency of the OFDM signal transmitted to the other party for each packet by using the frequency error estimated value by the automatic frequency controller.

  In the invention of claim 3, the OFDM signal has a known symbol comprising a short training symbol of a short time and a long training symbol of a long time provided after the short training symbol for each packet, and the automatic frequency The controller is configured to estimate the frequency error using the long training symbol after estimating the frequency error using the short training symbol.

  In the invention of claim 4, the counterpart is equipped with a demodulator that demodulates the received OFDM signal, and the demodulator of the counterpart does not estimate the frequency error in the short training symbol in the OFDM signal, and uses the long training symbol. This is used to estimate the frequency error.

  According to the invention of claim 1, the modulation unit includes a frequency shift unit that shifts the frequency of the OFDM signal transmitted toward the other party using the frequency error estimation value by the frequency error estimation unit of the demodulation unit. The modulator can shift the frequency of the OFDM signal to cancel out the frequency error due to the propagation path with the other party. As a result, the counterpart can receive an OFDM signal that is less affected by the frequency error due to the propagation path, and therefore, the counterpart demodulator can omit rough adjustment for correcting a large frequency error, for example. For this reason, the time for rough adjustment can be used for processing such as RSSI measurement and automatic gain control, and the processing accuracy can be improved.

  According to the invention of claim 2, the frequency error estimator is constituted by an automatic frequency controller that estimates a frequency error for each packet using a known symbol provided in each packet of the OFDM signal, and a frequency shift unit Is configured to shift the frequency of the OFDM signal transmitted to the other party for each packet using the frequency error estimated value by the automatic frequency controller, so the automatic frequency controller performs the correlation calculation of known symbols, etc. By doing so, the frequency error can be estimated for each packet, and the frequency shift unit can shift in advance the OFDM signal to be transmitted by the frequency error estimated value by the automatic frequency controller. Thereby, since the frequency error due to the propagation path is added to the OFDM signal received by the other party, the other party can receive the OFDM signal in which the frequency error is canceled.

  According to the invention of claim 3, the OFDM signal has a known symbol composed of a short training symbol of a short time and a long training symbol of a long time provided after the short training symbol for each packet, and has an automatic frequency. Since the controller is configured to estimate the frequency error using the long training symbol after estimating the frequency error using the short training symbol, the automatic frequency controller uses the short training symbol to increase the frequency error. Can be performed, and fine adjustment can be performed to estimate a small frequency error using a long training symbol.

  According to the invention of claim 4, the demodulator of the other party is configured to estimate the frequency error using the long training symbol without estimating the frequency error for the short training symbol in the OFDM signal. At this time, since the other party can receive an OFDM signal without a frequency error due to the propagation path, the other party's demodulator can omit the coarse adjustment using the short training symbol, and the fine adjustment using the long training symbol. The frequency error can be sufficiently corrected only by this. Further, since the other demodulator can omit coarse adjustment, the time can be used for processing such as RSSI measurement and automatic gain control, and the processing accuracy can be improved.

  Hereinafter, an OFDM modulation / demodulation device according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  In the figure, reference numeral 1 denotes a first communication device as an OFDM modulation / demodulation device that forms an access point, for example. The communication device 1 demodulates an OFDM signal received from the other party (second communication device 31), as will be described later. The demodulating unit 2 that performs the modulation and the modulation unit 16 that modulates the OFDM signal and transmits the modulated OFDM signal to the other party.

  Reference numeral 2 denotes a demodulator of the communication apparatus 1, and the demodulator 2 includes an IQ signal detector 8, an A / D converter 10, an FFT circuit 15, and the like which will be described later. Here, a low-noise amplifier 4 for amplifying a high-frequency signal (RF signal) received from the antenna 3 is provided on the upstream side of the IQ signal detector 8, and the downstream side of the low-noise amplifier 4 is automatically connected to the mixer 5. It is connected to an IQ signal detector 8 via a gain controller 6 (hereinafter referred to as AGC 6). The mixer 5 down-converts the high frequency signal to an intermediate frequency signal (IF signal) using the local oscillation signal LO output from the local oscillator 7, and the intermediate frequency signal is adjusted to a constant gain by the AGC 6, and then the IQ. It is configured to be input to the signal detector 8.

  The IQ signal detector 8 is connected to the mixer 5 via the AGC 6 at the input side and to the variable frequency oscillator 9. The IQ signal detector 8 uses the transmission signal So from the variable frequency oscillator 9 to generate the intermediate frequency signal IF. Quadrature detection is performed, and a baseband OFDM signal composed of an I signal (in-phase signal) and a Q signal (quadrature signal) is output.

  Reference numeral 10 denotes an A / D converter connected to the output side of the IQ signal detector 8. The A / D converter 10 digitally converts the baseband OFDM signal forming the analog signal output from the IQ signal detector 8. It is converted into a signal.

  Reference numeral 11 denotes a frequency error processing circuit connected to the output side of the A / D converter 10, and the frequency error processing circuit 11 is roughly constituted by a training symbol detector 12, an AFC 13 and a phase rotator 14 which will be described later.

  Reference numeral 12 denotes a training symbol detector for detecting a training symbol provided at the head of each packet in the OFDM signal. The training symbol detector 12 is provided at the head of each packet in the OFDM signal as shown in FIG. In addition, a short training symbol ST and a long training symbol LT as known symbols are detected.

  Reference numeral 13 denotes an automatic frequency controller (hereinafter referred to as AFC 13) as a frequency error estimator connected to the training symbol detector 12. The AFC 13 is a short training symbol ST or a long training symbol extracted by the training symbol detector 12. The phase difference Δθ corresponding to the frequency error Δf is calculated (estimated) using LT.

  Specifically, when, for example, a short training symbol ST predetermined for estimating a frequency error is input to the AFC 13, the AFC 13 stores the short training symbol ST as a known symbol S0 and also stores the short training symbol ST. Next, the next short tracing symbol ST that has been input and delayed the symbol ST itself is recognized as another known symbol S1. Then, the AFC 13 calculates (integral calculation) the autocorrelation of these known symbols S0 and S1, and calculates the phase difference Δθ corresponding to the frequency error Δf using the integrated value, as shown in the following equation (1). .

  Even when the long training symbol LT is input to the AFC 13, the phase difference Δθ corresponding to the frequency error Δf can be calculated by repeating the same procedure as when the short training symbol ST is input.

  When the long training symbol LT is used, for example, the phase difference Δθ calculated by the short training symbol ST is used to correct (shift) the phase of one known symbol S0 and then the phase difference from the other known symbol S1. Δθ may be calculated. Thereby, when using the short training symbol ST for a short time, the phase difference Δθ for coarse adjustment corresponding to the large frequency error Δf is calculated, and when using the long training symbol LT for a long time, the phase difference Δθ for coarse adjustment is calculated. It is possible to calculate the fine adjustment phase difference Δθ that remains with respect to (frequency error Δf). In this case, the AFC 13 adds the coarse adjustment phase difference Δθ and the fine adjustment phase difference Δθ, and outputs the total phase difference Δθ to the phase rotator 14 described later.

  Reference numeral 14 denotes a demodulation side phase rotator as a frequency error correction unit that corrects the phase difference Δθ as a frequency error estimation value by the AFC 13, and the phase rotator 14 is provided for each symbol (data symbol, etc.) of the OFDM signal. The phase θ is shifted (shifted) by the phase difference Δθ and input to the FFT circuit 15 described later.

  Specifically, the phase rotator 14 adds a positive phase difference Δθ to the received OFDM signal in order to compensate for the phase difference Δθ generated in the propagation path (for example, Doppler shift) with the second communication device 31. Is generated, the phase θ of the OFDM signal is subtracted by the phase difference Δθ (θ−Δθ), and when the negative phase difference (−Δθ) occurs in the OFDM signal, the phase θ of the OFDM signal is added by the phase difference Δθ ( θ + Δθ). Thereby, the phase rotator 14 outputs an OFDM signal in which the frequency error Δf is corrected.

  Reference numeral 15 denotes a fast Fourier transform circuit (hereinafter referred to as FFT) which is connected to the output side of the phase rotator 14 and performs a Fourier transform process for each symbol period in the OFDM signal packet corrected by the phase rotator 14. The FFT circuit 15 uses a synchronization signal output from a synchronization processing unit (not shown) while each packet of the OFDM signal includes a plurality of symbol periods (data symbols, etc.). Thus, Fourier transform processing is performed for each symbol period in the baseband OFDM signal packet. Specifically, after synchronization is established at the beginning of the packet by the synchronization processing unit, the timing of the FFT window is determined using a trigger signal generated at a constant interval, and Fourier transform processing is performed for each symbol period. It is.

  The FFT circuit 15 converts the baseband OFDM signal into the frequency domain, and outputs the signal converted into the frequency domain to an equalizer (not shown). At this time, the equalizer corrects (adaptive equalization processing) distortion such as amplitude caused by the propagation path with respect to the OFDM signal converted into the frequency domain. As a result, this corrected signal is detected and demapped according to the modulation scheme of each subcarrier using a detection circuit (not shown) connected to the output side of the equalizer, and restored to a data signal. Is.

  16 is a modulation unit that modulates an OFDM signal and transmits it to the other party (second communication device 31). The modulation unit 16 includes an IFFT circuit 17, a D / A converter 18, a quadrature modulator 19, and the like described later. The phase rotator 23 is used.

  Reference numeral 17 denotes an inverse fast Fourier transform circuit (hereinafter referred to as IFFT circuit 17) as an inverse Fourier transform processing unit that performs inverse Fourier transform processing on the mapped data signal. The IFFT circuit 17 converts the mapped data signal into a data signal. Data symbols of the OFDM signal are generated by inverse Fourier transform, and a guard interval is added between the symbols.

  Reference numeral 18 denotes a D / A converter connected to the output side of the IFFT circuit 17 through a phase rotator 23 on the modulation side which will be described later. The D / A converter 18 converts the digital signal output from the IFFT circuit 17 into a digital signal. The resulting OFDM signal is converted to an analog signal.

  Reference numeral 19 denotes a quadrature modulator connected to the output side of the D / A converter 18. The quadrature modulator 19 converts an OFDM signal composed of an I signal and a Q signal into an intermediate frequency using a transmission signal So from a variable frequency oscillator 9. The signal (IF signal) is orthogonally modulated. Further, the output side of the quadrature modulator 19 is connected to the mixer 20, and the mixer 20 upconverts the intermediate frequency signal into a high frequency signal (RF signal) using the local oscillation signal LO output from the local oscillator 7. The high-frequency signal output from the mixer 20 is amplified using the power amplifier 21 and transmitted to the second communication device 31 via the antenna 22.

  23 of the OFDM signal transmitted to the other party using the frequency error estimated value (phase difference Δθ) by the AFC 13 so as to cancel the frequency error Δf due to the propagation path with the other party (second communication device 31). A modulation-side phase rotator as a frequency shift unit that shifts the frequency f (phase θ). The modulation-side phase rotator 23 has a frequency error Δf for each symbol (data symbol, etc.) of the OFDM signal. The phase θ of the OFDM signal is shifted (shifted) by a phase difference Δθ in the direction opposite to the direction in which it acts, and is input to the D / A converter 18.

  Specifically, the phase rotator 23 transmits when a positive phase difference Δθ occurs in the received OFDM signal in order to cancel out the phase difference Δθ generated in the propagation path with the second communication device 31. The phase θ of the OFDM signal is subtracted by the phase difference Δθ (θ−Δθ), and when a negative phase difference (−Δθ) occurs in the received OFDM signal, the phase θ of the OFDM signal to be transmitted is added by the phase difference Δθ ( θ + Δθ). Since the OFDM signal output from the phase rotator 23 is orthogonally modulated by the orthogonal modulator 19, the transmitted OFDM signal is shifted from the predetermined frequency f by a frequency error Δf corresponding to the phase difference Δθ. Having a frequency (f ± Δf).

  Reference numeral 31 denotes a second communication device as a counterpart to be communicated with the first communication device 1, for example, a station or the like. The second communication device 31 is a demodulator of the first communication device 1. 2 and a demodulator 32 and a modulator 33 which are substantially the same as the modulator 16.

  However, unlike the demodulator 2, the demodulator 32 of the second communication device 31 omits rough adjustment of the frequency error Δf (phase difference Δθ), and the correction of the frequency error Δf is performed only when the long training symbol LT is input. It is configured to do. Also, the modulation unit 33 of the second communication device 31 is different from the modulation unit 16 in that the phase rotator on the modulation side is omitted and the phase shift is not performed on the OFDM signal for transmission.

  The OFDM modulation / demodulation apparatus according to the present embodiment is configured as described above, and the operation thereof will be described next.

  First, the demodulation operation of the first communication device 1 will be described. When the high frequency signal is received from the second communication device 31 using the antenna 3, the demodulator 2 converts the high frequency signal into an intermediate frequency signal using the mixer 5 or the like, and then uses the IQ signal detector 8. Then, the baseband OFDM signal composed of the I signal and the Q signal is demodulated from the intermediate frequency signal. Thereafter, the phase of the OFDM signal converted into a digital signal by the A / D converter 10 is corrected by the phase difference Δθ corresponding to the frequency error Δf caused by the propagation path using the frequency error processing circuit 11 and input to the FFT circuit 15. Is done. As a result, the OFDM signal is subjected to Fourier transform processing for each symbol period using the FFT circuit 15, and is also restored to a data signal by performing equalization processing, demapping, and the like.

  Next, the modulation operation of the first communication device 1 will be described. When a data signal is input, the modulator 16 performs inverse Fourier transform, provision of a guard interval, etc. after mapping the data signal. Thereafter, the OFDM signal converted into the analog signal is subjected to quadrature modulation using the quadrature modulator 19 to generate an intermediate frequency signal, and the mixer 20 is used to upconvert the intermediate frequency signal to a high frequency signal. Transmission is performed toward the second communication device 31 using the antenna 22.

  Here, as shown in FIG. 1, when a positive frequency error (+ Δf) is generated by the propagation path between the first and second communication devices 1 and 31, a demodulation unit is used during demodulation of the first communication device 1. 2 corrects the phase difference (+ Δθ) corresponding to the frequency error (+ Δf) and demodulates the data signal from the OFDM signal.

  On the other hand, at the time of modulation of the first communication device 1, the modulation unit 16 reduces the frequency of the OFDM signal in advance by a frequency difference value (−Δf) so as to cancel out the positive frequency error (+ Δf) due to the propagation path. . Thereby, when the second communication device 31 receives the OFDM signal from the first communication device 1, the frequency of the OFDM signal transmitted from the first communication device 1 is lowered in advance, whereas The frequency of the OFDM signal received by the communication device 31 increases due to the frequency error (+ Δf) of the propagation path, and is close to the predetermined frequency f. For this reason, the second communication device 31 can reliably demodulate the OFDM signal from the first communication device 1 only by correcting the frequency error using the long training symbol.

  Even when a negative frequency error (−Δf) occurs due to the propagation path, the demodulator 2 of the first communication device 1 corrects the frequency error (−Δf) and corrects the data from the OFDM signal as described above. In addition to demodulating the signal, the modulation unit 16 transmits the signal in a state where the frequency of the OFDM signal is increased in advance by a frequency difference value (+ Δf) so as to cancel out the frequency error (−Δf).

  Thus, in the present embodiment, the modulation unit 16 uses the frequency error estimation value (phase difference Δθ) by the AFC 13 of the demodulation unit 2 to shift the frequency of the OFDM signal to be transmitted to the second communication device 31. Since the modulator 23 is included, the modulation unit 16 can shift the frequency of the OFDM signal to cancel the frequency error Δf caused by the propagation path between the second communication device 31 and the second communication device 31. As a result, the second communication device 31 can receive an OFDM signal that is less affected by the frequency error Δf due to the propagation path. Therefore, the demodulator 32 of the second communication device 31 can, for example, coarsely correct a large frequency error. Adjustment can be omitted. For this reason, the time for rough adjustment can be used for processing such as RSSI measurement and automatic gain control, and the processing accuracy can be improved.

  Further, the AFC 13 is used to estimate the frequency error Δf for each packet by using known symbols such as the short training symbol ST and the long training symbol LT provided in each packet in the OFDM signal, and the phase rotator 23 on the modulation side. Is configured to shift the frequency of the OFDM signal transmitted to the second communication device 31 for each packet by using the phase difference Δθ as the frequency error estimation value by the AFC 13. The frequency error Δf can be estimated by performing calculations and the like, and the phase rotator 23 can shift the OFDM signal to be transmitted by the phase difference Δθ by the AFC 13 in advance. Thereby, since the frequency error Δf due to the propagation path acts on the OFDM signal received by the second communication device 31, the second communication device 31 can receive the OFDM signal in which the frequency error Δf is canceled.

  The OFDM signal has a known symbol consisting of a short training symbol ST for a short time and a long training symbol LT for a long time provided after the short training symbol ST for each packet. Since the frequency error Δf is estimated using the long training symbol LT after the frequency error Δf is estimated using the ST, the AFC 13 generates a large frequency error Δf (phase difference Δθ) using the short training symbol ST. The rough adjustment to be estimated can be performed, and the fine adjustment for estimating the small residual frequency error Δf remaining by the rough adjustment using the long training symbol LT can be performed.

  On the other hand, the demodulator 32 of the second communication device 31 is configured to estimate the frequency error Δf using the long training symbol LT without estimating the frequency error Δf for the short training symbol ST in the OFDM signal. At this time, since the second communication device 31 can receive an OFDM signal that is less affected by the frequency error Δf due to the propagation path, the demodulator 32 of the second communication device 31 uses the coarse training symbol ST. Adjustment can be omitted, and the frequency error Δf can be sufficiently corrected only by fine adjustment using the long training symbol LT.

  In the above embodiment, the demodulator 2 and the modulator 16 of the first communication device 1 are configured to use separate antennas 3 and 22, but the demodulator and the modulator are modulated by using, for example, an antenna duplexer. It is good also as a structure which shares a single antenna with a part.

  In the above embodiment, the AFC 13 is used to calculate the phase difference Δθ corresponding to the frequency error Δf, and the phase rotators 14 and 23 are used to shift the phase difference Δθ. However, the present invention is not limited to this. For example, the frequency error Δf may be calculated using the AFC 13 and the frequency of the transmission signal So of the variable frequency oscillator 9 may be shifted (shifted) by the frequency error Δf. Even in this case, the frequency error of the OFDM signal can be corrected and canceled.

  Further, in the above embodiment, the mixers 5 and 20 are used to downconvert and upconvert between the high frequency signal and the intermediate frequency signal. However, the present invention is not limited to this. For example, these mixers may be omitted, and a direct conversion configuration in which a signal received by an antenna is directly quadrature-detected may be employed.

It is a whole block diagram which shows the 1st communication apparatus by embodiment of this invention with the 2nd communication apparatus used as the other party. It is a whole block diagram which shows the 1st communication apparatus in FIG. It is explanatory drawing which shows the structure of the OFDM signal used for the 1st, 2nd communication apparatus in FIG.

Explanation of symbols

1 First communication device (OFDM modem)
2 Demodulator 11 Frequency error processing circuit 12 Training symbol detector 13 AFC (frequency error estimator)
14 Phase rotator (frequency error correction unit)
15 FFT circuit (Fourier transform processing unit)
16 Modulating unit 17 IFFT circuit 23 Phase rotator (frequency shift unit)
31 Second communication device (the other party)
32 Demodulator 33 Modulator

Claims (4)

In an OFDM modulation / demodulation apparatus comprising a demodulator that demodulates an OFDM signal received from a counterpart and a modulator that modulates the OFDM signal and transmits the modulated OFDM signal to the counterpart,
The demodulator includes a frequency error estimator that estimates a frequency error generated in the OFDM signal, a frequency error corrector that corrects a frequency error estimate by the frequency error estimator, and an OFDM corrected by the frequency error corrector. A Fourier transform processing unit that performs Fourier transform processing for each symbol period in the signal packet;
The modulation unit shifts the frequency of the OFDM signal to be transmitted to the other party using the frequency error estimation value by the frequency error estimation unit of the demodulation unit so as to cancel out the frequency error due to the propagation path with the other party. An OFDM modulation / demodulation apparatus comprising a frequency shift unit. The frequency error estimation unit is configured by an automatic frequency controller that estimates a frequency error for each packet using a known symbol provided in each packet of the OFDM signal,
2. The OFDM modulation / demodulation apparatus according to claim 1, wherein the frequency shift unit is configured to shift the frequency of the OFDM signal transmitted to the other party for each packet using the frequency error estimation value by the automatic frequency controller. The OFDM signal has a known symbol consisting of a short training symbol for a short time for each packet and a long training symbol for a long time provided after the short training symbol,
The OFDM modulation / demodulation apparatus according to claim 2, wherein the automatic frequency controller is configured to estimate a frequency error using the long training symbol after estimating a frequency error using the short training symbol.   The counterpart is equipped with a demodulator that demodulates the received OFDM signal, and the counterpart demodulator does not estimate a frequency error with a short training symbol in the OFDM signal, but estimates a frequency error with a long training symbol. The OFDM modulation / demodulation apparatus according to claim 3, which is configured.

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