JP6586057B2 - OFDM signal receiving method and OFDM signal receiver - Google Patents

Description

  The present invention relates to OFDM communication technology.

  With the widespread use of wireless communication systems, a shortage of frequency resources has become apparent, especially in the microwave band, and transmission techniques for achieving high frequency utilization efficiency are required. One approach for improving frequency utilization efficiency is to superimpose a plurality of radio signals at the same time, at the same frequency and at the same point by spatial filtering using an adaptive array antenna.

  There are many different synthesis algorithms in the adaptive array. Among these, it is desirable to use a blind adaptive array algorithm from the viewpoint of reducing overhead for array processing and performing spatial filtering between different wireless communication systems. The blind adaptive array algorithm does not ask the signal type of the interference wave to be spatially filtered. In other words, the blind adaptive array algorithm is an algorithm that does not require a known signal in order to calculate a weighting coefficient in the adaptive array. As such an algorithm, PI (Power Inversion), MRC (Maximum Ratio Combining), etc. are widely known.

  As a frequency band division transmission method as described above, multi-carrier transmission by OFDM (Orthogonal Frequency Domain Multiplexing) has been actively used in recent years due to high frequency utilization efficiency (for example, Non-Patent Document 1 and Non-Patent Document 2). . OFDM is a transmission method that uses FFT (Fast Fourier Transform) / IFFT (Inverse FFT), and CP (Cyclic Prefix) is added to the head of the symbol on the transmitting side, and CP is removed on the receiving side. Since it is possible to prevent intersymbol interference due to the delayed wave, the transmission method is suitable for wireless transmission from the viewpoint of delay wave compatibility.

  However, in order to extract each subcarrier component by performing OFDM demodulation, so-called time synchronization processing in which a temporal symbol head position in symbol transmission is detected, CP is removed, and an FFT interval is set is essential. Since the CP is installed so as to maintain continuity with the data part, if the FFT interval is within the range of the CP, OFDM demodulation is possible even if offset slightly forward, but the inter-symbol interference tolerance There is a trade-off. Also, if the start position of the FFT section is set incorrectly beyond the CP range, OFDM demodulation cannot be performed.

Maruta, Masuno, Sugiyama, “Blind Adaptive Array Using Subcarrier Transmission Power Control”, IEICE Tech. 129-134, November 2013 Fujimoto, “Adaptive Array for OFDM Reception”, Science (B), Vol. J95-B, no. 9, pp. 1015-1024, September 1, 2012

  As described in Non-Patent Document 1, in order to establish OFDM time synchronization, a known signal called a training signal or a preamble signal is added to the head of a packet, and the SIR (signal-to-interference power ratio) is lower than that in the data interval. ) But it is common to be able to synchronize. However, in this case, it is difficult to establish time synchronization in an SIR << 0 dB or SIR≈0 dB environment. When implemented on OFDM transmission in such an environment, first, blind adaptive array processing is performed for each specific frequency band to perform spatial filtering, and after securing sufficient SIR, time synchronization for OFDM demodulation is performed. Therefore, a flow for performing OFDM demodulation is required.

  As described in Non-Patent Document 2, adaptive array processing includes a pre-FFT method performed before the FFT of demodulation and a post-FFT method performed after the FFT. Of these, the post-FFT method has been reported to have a greater effect of improving the communication quality, and has become the mainstream. However, it is assumed that the above time synchronization can be established before the spatial filtering. The specific means for performing OFDM demodulation after the spatial filtering by the blind adaptive array processing is not disclosed. When OFDM transmission is used as a base, it is necessary to divide the reception band of the signal into subcarriers and apply an appropriate adaptive array algorithm to each of them before establishing time synchronization for OFDM demodulation on the receiving side. It is desirable to perform the array process in the frequency domain. However, the symbol head position is unknown at the previous stage where time synchronization for OFDM demodulation is established. For this reason, when the frequency conversion process using FFT is performed, the FFT section may be set so as to straddle the symbols. If the FFT interval is set so as to straddle the symbols, the continuity of the signal at the symbol boundary is lost, and the periodicity of the signal, which is a necessary condition for Fourier transform, cannot be secured.

  In view of the above circumstances, an object of the present invention is to provide a technique capable of detecting the synchronization timing between the head position of the received OFDM symbol and the head position of the FFT interval even when the SIR is low.

  One aspect of the present invention includes a first step of receiving an OFDM signal, a second step of removing a guard interval from the received OFDM signal and extracting a signal in an FFT interval, and a symbol error in the extracted FFT interval. A third step of extracting continuous components in the frequency domain, a fourth step of converting discontinuous components of the FFT interval symbols extracted in the frequency domain into time domain signals, and FFT interval symbols converted to the time domain The fifth step of detecting the timing of the start position of the OFDM symbol and the position of the FFT section based on the discontinuous component of the above, and the timing of the guard interval based on the timing of the start position of the OFDM symbol and the position of the FFT section And a sixth step of shifting the removal timing.

  One aspect of the present invention is the above-described OFDM receiving method, wherein in the third step, a component in a band of a desired wave addressed to the own apparatus is removed from the frequency domain signal of the FFT interval.

  One aspect of the present invention includes a step of receiving OFDM signals from a plurality of antennas, a step of weighting the received OFDM signals, and a step of combining the plurality of weighted OFDM signals. This is an OFDM signal receiving method.

  One aspect of the present invention is the above-described OFDM reception method, wherein error detection of a demodulated signal is performed to determine whether the symbol of the received OFDM signal is synchronized with the extracted FFT interval. Further comprising the step of:

  One aspect of the present invention is the above-described OFDM reception method, further comprising the step of determining whether or not the received OFDM signal is addressed to the own station using a user signal included in the demodulated signal.

  One aspect of the present invention includes an antenna that receives an OFDM signal, a removal unit that removes a guard interval from the received OFDM signal and extracts a signal in an FFT interval, and a discontinuous component of the extracted symbol in the FFT interval. An extraction unit that extracts in the frequency domain, a conversion unit that converts a discontinuous component of the FFT section extracted in the frequency domain into a signal in the time domain, and a discontinuous component of the symbol in the FFT section that is converted into the time domain Based on the timing of the OFDM symbol start position and the position of the FFT section, and the timing of the guard interval removal based on the timing of the OFDM symbol start position and the position of the FFT section. An OFDM signal receiver including a timing shift unit.

  According to the present invention, even when the SIR is low, it is possible to detect the synchronization timing between the head position of the received OFDM symbol and the head position of the FFT interval.

It is a block diagram which shows the structure of the receiver 1 of the 1st Embodiment of this invention. The OFDM signal received with the receiver 1 of the 1st Embodiment of this invention is shown. It is explanatory drawing of the symbol synchronization of an OFDM signal. It is explanatory drawing of the symbol synchronization of an OFDM signal. It is explanatory drawing of extraction of the discontinuous component in the receiver of the 1st Embodiment of this invention. It is a block diagram which shows the structure of the receiver 101 of the 2nd Embodiment of this invention. It is explanatory drawing of the interference wave of an OFDM signal. It is a flowchart which shows operation | movement of the receiver 101 of the 2nd Embodiment of this invention. The simulation result of the 2nd Embodiment of this invention is shown.

<First Embodiment>
FIG. 1 is a block diagram illustrating a configuration of a receiver 1 according to the first embodiment of this invention. As shown in FIG. 1, the receiver 1 according to the first embodiment of the present invention includes an A / D (Analog to Digital) conversion unit 11, a CP (Cyclic Prefix) removal unit 12, an S / P (Serial to Parallel). Conversion unit 13, FFT (Fast Fourier Transform) unit 14, demodulation unit 15, demodulation correctness determination unit 16, desired subcarrier null processing unit 17, IFFT (Inverse FFT) unit 18, P / S (Parallel to Serial) conversion unit 19 The timing detection unit 20 and the CP timing shift unit 21 are provided.

  The A / D converter 11 performs A / D conversion on the received signal from the antenna 10 and converts the analog signal into a digital signal. Although not described here, the OFDM signal normally uses a high-frequency signal such as a microwave band, and a high-frequency amplifier circuit or a frequency conversion circuit may be provided before the A / D conversion.

  The CP removal unit 12 removes the CP (Cyclic Prefix) of the guard interval in the received signal, and extracts the signal in the FFT interval.

  The S / P converter 13 converts the data in the FFT section from serial data to parallel data. The FFT unit 14 performs a Fourier transform of the data in the FFT interval, converts the data in the time domain into data in the frequency domain, and demodulates secondary modulation by OFDM.

  The demodulator 15 demodulates data of primary modulation from the output signal of the FFT unit 14. As the primary modulation method, QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), or the like is used. A demodulated signal is output from the demodulator 15.

  The demodulation correctness determination unit 16 performs error detection of the demodulated signal, and determines whether the symbol of the received OFDM signal and the extracted FFT interval are synchronized according to the error detection result. Then, the demodulation correct / incorrect determination unit 16 sets the operation of the timing detection unit 20 based on the error detection result. Error detection is determined by, for example, a CRC (Cyclic Redundancy Check) code.

  The desired subcarrier null processing unit 17 extracts the discontinuous component of the symbol in the FFT section in the frequency domain by acquiring the component outside the band of the received signal addressed to the own device from the output signal of the FFT unit 14. More specifically, the desired subcarrier null processing unit 17 generates nulls in the subcarriers (FFT points) to which signals are assigned to the output of the FFT unit 14, thereby discontinuous symbols in the FFT interval. Extract components in the frequency domain. The IFFT unit 18 converts the discontinuous component extracted by the desired subcarrier null processing unit 17 from the frequency domain to the time domain. The P / S converter 19 converts the time domain data from the IFFT 18 from parallel data to serial data.

  The timing detection unit 20 extracts the FFT section start position and the OFDM symbol start position based on the discontinuous component of the FFT section symbol extracted by the desired subcarrier null processing unit 17 and converted into the time domain by the IFFT unit 18. And detect the timing. The CP timing shift unit 21 sets the CP removal timing according to the detection output of the timing detection unit 20.

  FIG. 2 shows an OFDM signal received by the receiver 1 according to the first embodiment of the present invention. As shown in FIG. 2 (A), CP (Cyclic Prefix) is added as a guard interval to the head of the OFDM signal, and subsequently, a data section composed of OFDM symbols is provided.

  When receiving the OFDM signal, the CP removal unit 12 removes the CP part of the guard interval and extracts only the data part as shown in FIG. 2B. The extracted data portion becomes the FFT interval, and the OFDM unit 14 performs OFDM demodulation. Since the CP part is installed so as to maintain continuity with the data part, OFDM demodulation is possible even if the FFT interval is within the CP range, even if offset slightly forward. However, if the start position of the FFT interval is set incorrectly beyond the CP range, it includes a discontinuous portion in which the OFDM symbol in the FFT interval includes a non-periodic signal, and OFDM demodulation cannot be performed.

  In the first embodiment of the present invention, it is determined from the output of the demodulation correctness determination unit 16 whether OFDM is correctly demodulated. When the OFDM demodulation cannot be performed correctly, the timing detection unit 20 is operated to detect the timing between the start position of the FFT interval and the start position of the OFDM symbol so as to establish time synchronization. At this time, in this embodiment, discontinuous components of symbols in the FFT interval are extracted by generating nulls in subcarriers to which signals are assigned. Then, the extracted discontinuous component is converted from the frequency domain to the time domain, and this is fed back to the timing of CP removal so as to achieve time synchronization. This will be described below.

  3 and 4 are explanatory diagrams of symbol synchronization of the OFDM signal. FIG. 3 shows a case where time synchronization is established between the start position of the FFT interval and the start position of the OFDM symbol, and an OFDM demodulated output is obtained.

  That is, in FIG. 3A, time ta to tb is a guard interval (GI) section, and time tb to tc is a data section. CP is inserted in the guard interval (GI) section. An OFDM symbol is provided in the data section. As the CP, for example, a copy of the latter half of the OFDM symbol is inserted as data for maintaining the periodicity of the OFDM symbol.

  In this example, times t1 to t2 are extracted as FFT intervals. As shown in FIG. 3A, the offset between the time t1 at the start position of the FFT interval and the time tb at the start position of the OFDM symbol is within the guard interval (GI) of the received OFDM signal. Data for maintaining the periodicity of the OFDM symbol is inserted into the CP of the guard interval (GI). Therefore, if the offset between the time t1 at the start position of the FFT interval and the time tb at the start position of the OFDM symbol is within the guard interval (GI), it has periodicity within the FFT interval, so that the demodulation of the OFDM signal is performed. Is possible.

  When the start position of the FFT interval and the start position of the OFDM symbol are synchronized, the spectrum of the received OFDM signal is as shown in FIG. That is, as shown in FIG. 3B, each subcarrier of the received OFDM signal becomes orthogonal on the frequency axis, and a spectrum appears at a predetermined FFT point. Each subcarrier is null for the FFT point where other subcarriers exist. Also, the spectrum band falls within the subcarrier band to which the signal is assigned.

  FIG. 4 shows a case where the time synchronization between the start position of the FFT interval and the start position of the OFDM symbol is not taken, and an OFDM demodulated output cannot be obtained. That is, in FIG. 4A, time ta to tb is a guard interval (GI) section, and time tb to tc is a data section. In this example, times t11 to t12 are extracted as FFT intervals. As shown in FIG. 4A, the offset between the time t11 at the start position of the FFT interval and the time tb at the start position of the OFDM symbol exceeds the range of the guard interval (GI). In this case, the periodicity of the OFDM symbol cannot be maintained, and the OFDM signal cannot be demodulated.

  When the start position of the FFT interval and the start position of the OFDM symbol are out of synchronization, the spectrum of the received OFDM signal is as shown in FIG. That is, as shown in FIG. 4B, there is an extra frequency component even at an FFT point where each subcarrier of the received OFDM signal has other subcarriers. Also, extra frequency components exist outside the subcarrier band to which the signal is assigned.

  As described above, when time synchronization is established between the extracted FFT section head position and the received OFDM symbol head position, only the component of the subcarrier (FFT point) to which the signal is assigned is output from the FFT. Is done. For this reason, there is a strong signal component in the component of the subcarrier to which the signal is assigned, and a null is formed for the adjacent subcarrier. On the other hand, when time synchronization is not established between the extracted FFT section head position and the received OFDM symbol head position, frequency orthogonality is lost due to the spread of the band, and interference occurs between adjacent subcarriers. . For this reason, components also appear between adjacent subcarriers and outside the band of the own signal. A component appearing between adjacent subcarriers or outside the band of the own signal is data resulting from signal discontinuity in the FFT interval.

  Therefore, in the present embodiment, only the discontinuous component of the symbol in the FFT interval is filtered by performing filtering so as to intentionally generate a null for the subcarrier to which the signal is assigned after the received signal is subjected to FFT conversion processing. Extracted in the frequency domain. By performing IFFT on the signal extracted in this frequency domain, only the discontinuous component of the symbol in the FFT section remains in the time domain signal. By selecting a sampling timing at which the residual component remains strongest, it is possible to detect a deviation from the CP interval. By feeding this back to the CP removal timing, it is possible to match the inter-symbol timing of the OFDM signal.

  That is, FIG. 5 is an explanatory diagram of extraction of discontinuous components in the receiver according to the first embodiment of this invention. Assume that an output as shown in FIG. 5A is obtained when an OFDM received signal is subjected to an FFT conversion process. In the signal shown in FIG. 5A, the subcarrier (FFT point) component is the original signal component, and the other components are unnecessary components caused by aperiodicity due to the discontinuity of symbols in the FFT interval. Therefore, as shown in FIG. 5B, a null is intentionally inserted in the subcarrier portion. The remaining component is a discontinuous component of the symbol in the FFT interval. Thus, when the discontinuous component of the symbol in the FFT section extracted in the frequency domain is converted into the time domain by IFFT, the discontinuous component of the symbol in the FFT section in the time domain as shown in FIG. 5C. Can be extracted. By feeding this back to the CP removal timing, it is possible to match the inter-symbol timing of the OFDM signal.

  In the present embodiment, the desired subcarrier null processing unit 17 generates nulls for the subcarriers to which signals are assigned in response to the output of the FFT unit 14, thereby converting the discontinuous components of the symbols in the FFT section into the frequency domain. Extract with Then, the IFFT unit 18 converts the discontinuous component of the symbol in the FFT interval extracted by the desired subcarrier null processing unit 17 from the frequency domain to the time domain. If it is determined from the determination result of the demodulation correctness determination unit 16 that the time synchronization of the OFDM symbol is not established, the timing detection unit 20 is set to the operating state. Then, the timing detection unit 20 detects an offset between the start position of the FFT interval and the start position of the OFDM symbol based on the discontinuous component of the symbol of the FFT interval in the time domain obtained by the IFFT unit 18. . The CP timing shift unit 21 sets the CP removal timing according to the detection output of the timing detection unit 20. This makes it possible to establish time synchronization of OFDM symbols.

  In this embodiment, since the OFDM symbol is different at the first and last portions of the FFT point, it becomes an aperiodic signal including discontinuous components, and an error occurs at the first and last points of the FFT point and several points before and after. It may be detected. Therefore, processing for excluding the first and last points may be added.

<Second Embodiment>
Next, a second embodiment of the present invention will be described. FIG. 6 is a block diagram illustrating a configuration of the receiver 101 according to the second embodiment of this invention. In the present embodiment, spatial filtering using an adaptive array antenna is performed. In this example, the reception signals of the two antenna elements 110a and 110b of the adaptive antenna array 110 are received by the two receiving units 102a and 102b and synthesized by the adaptive array processing unit 130.

  The reception unit 102a includes an A / D conversion unit 111a, a CP removal unit 112a, an S / P conversion unit 113a, and an FFT unit 114a. The A / D conversion unit 111a, the CP removal unit 112a, the S / P conversion unit 113a, and the FFT unit 114a are respectively the A / D conversion unit 11, the CP removal unit 12, the S / P conversion unit 13 and the first embodiment. The configuration is the same as that of the FFT unit 14.

  The reception unit 102b includes an A / D conversion unit 111b, a CP removal unit 112b, an S / P conversion unit 113b, and an FFT unit 114b. The A / D conversion unit 111b, the CP removal unit 112b, the S / P conversion unit 113b, and the FFT unit 114b are respectively the A / D conversion unit 11, the CP removal unit 12, the S / P conversion unit 13 and the first embodiment. The configuration is the same as that of the FFT unit 14.

  The demodulation unit 115, the demodulation correctness / error determination unit 116, the desired subcarrier null processing unit 117, the IFFT unit 118, the P / S conversion unit 119, the timing detection unit 120, and the CP timing shift unit 121 are each a demodulation unit in the first embodiment. 15, a demodulation correct / incorrect determination unit 16, a desired subcarrier null processing unit 17, an IFFT unit 18, a P / S conversion unit 19, a timing detection unit 20, and a CP timing shift unit 21. In the present embodiment, a user detection unit 131 is further provided.

  In the second embodiment of the present invention, the reception signals of the two antenna elements 110a and 110b of the adaptive antenna array 110 are received by the two receiving units 102a and 102b and combined by the adaptive array processing unit 130. Here, the adaptive array processing unit 130 performs weighted addition on each incoming wave, thereby eliminating unnecessary interference waves and improving the SIR characteristics. For this reason, the second embodiment of the present invention is also effective in an environment where interference waves exist.

  Similar to the first embodiment described above, the desired subcarrier null processing unit 117 performs null processing on the subcarriers to which signals are assigned with respect to the outputs of the FFT units 114a and 114b synthesized by the adaptive array processing unit 130. Generate (filter). Thereby, the discontinuous component of the symbol of an FFT area is extracted in a frequency domain. Then, IFFT section 118 converts the discontinuous component of the symbol in the FFT interval extracted by desired subcarrier null processing section 117 from the frequency domain to the time domain. The timing detection unit 120 detects the optimal timing for removing the CP based on the level of the extracted unnecessary component, and the CP timing shift unit 121 detects the CP according to the detection output of the timing detection unit 20. Set the removal timing.

  The first embodiment described above is assumed to be used in an environment in which no interference wave exists, but in the present embodiment, it is assumed to be used in an environment in which an interference wave exists. When the interference wave is asynchronous and has the same signal format, it is assumed that the timing of the interference wave component is detected as shown in FIG.

  FIG. 7 is an explanatory diagram of an interference wave of an OFDM signal. In FIG. 7, times ta1 to tb1 are guard interval (GI) sections of desired waves, and times tb1 to tc1 are data sections of desired waves. Times tb2 to tb2 are guard interval (GI) sections of interference waves, and times tb2 to tc2 are data sections of interference waves. Assume that the times t21 to t22 are extracted as FFT intervals. In this case, since the offset between the time tb1 at the head position of the OFDM symbol of the desired wave and the time t21 at the head position of the FFT interval exceeds the guard interval (GI), the OFDM signal of the desired wave cannot be demodulated. However, since the offset between the time tb2 at the start position of the OFDM symbol of the interference wave and the time t21 at the start position of the FFT interval is within the guard interval (GI), the interference wave can be demodulated. For this reason, the OFDM symbol and the FFT interval are not synchronized in the desired wave, but the OFDM symbol and the FFT interval are synchronized in the interference wave, and the demodulation correctness determination unit 116 generates an error. It is assumed that it cannot be detected.

  Therefore, in this embodiment, the user detection unit 131 is provided in addition to the demodulation correctness determination unit 116 that performs CRC detection. The user detection unit 131 detects a user signal by, for example, detecting a signal in which a unique preamble pattern is embedded in each user's signal, or a signal in which address information addressed to its own device is embedded in a demodulated packet. Then, it is detected whether or not a desired signal addressed to the own apparatus has been received. By adding the user detection unit 131, it is possible to extract a discontinuous component of a desired signal even in an environment where interference waves between asynchronous systems exist.

  In the present embodiment, the adaptive array processing unit 130 combines the FFT outputs acquired from the two systems of receiving units 102a and 102b. Then, for the combined FFT output, nulls are generated in the subcarriers to which signals are assigned, thereby extracting the symbol discontinuous components in the FFT interval. Since the Post-FFT type adaptive array in this embodiment is intended only for detection of discontinuous positions, if the operation of removing the interference wave component and emphasizing the discontinuous component leaked to the frequency component outside the own signal band is performed. Good. For this reason, it may be possible that the FFT timing of the received signal is unknown and the orthogonality is lost and the operation is not strictly performed. The adaptive array algorithm at this time is adaptively changed according to the demodulation state of the OFDM signal in consideration of the influence of the interference wave.

  That is, in an environment in which an interference wave exists, the timing detection unit 120 cannot detect the discontinuous component of the symbol in the FFT interval, and the CRC determination and demodulation detection unit 116 detects the desired signal. If it cannot be detected, it is determined that the weight calculation algorithm for adaptive array signal processing in the subcarrier into which the null is inserted is incorrect, and another adaptive array algorithm is applied. For example, the algorithm is changed from PI (Power Inversion) to CMA (Constant Modulus Algorithm) or from CMA to PI. As a result, it is possible to extract the discontinuous component of the symbol in the FFT section of the desired signal.

  When introducing this technology in a system that uses multiple users at the same time, such as a base station, the difference in the detected signal timing of each user is fed back to the transmitting station so that each user's signal is synchronized. Timing adjustment may be performed.

  FIG. 8 is a flowchart illustrating the operation of the receiver 101 according to the second embodiment of this invention. In FIG. 8, two systems of receiving units 102a and 102b receive signals received from two antenna elements 110a and 110b of adaptive antenna array 110 (step S101). CP removing sections 112a and 112b remove the CP of the guard interval in the received signal and extract the signal in the FFT interval (step S102). The FFT units 114a and 114b perform Fourier transform on the data in the FFT section, and convert the data in the time domain into the data in the frequency domain (step S103). The adaptive array processing unit 130 weights the outputs of the FFT units 114a and 114b. In this processing, an operation of emphasizing an unnecessary frequency component that leaks out of the own band of the desired signal is performed by weighting the signal that causes interference to be null. The weighting may be performed using, for example, the above-described PI or CMA. After that, the adaptive array processing unit 130 adds and combines the plurality of weighted signals (step S104).

  The error detection and user detection unit 131 by the demodulation correctness determination unit 116 determines whether the OFDM symbol and the FFT interval are synchronized with the desired wave addressed to the own device (step S105). If the desired wave addressed to the device is synchronized with the OFDM symbol and the FFT interval, the process is terminated (step S105: Yes).

  If the OFDM symbol and the FFT interval are not synchronized with the desired wave addressed to the own device (step S105: No), the desired subcarrier null processing unit 117 assigns a signal to the output of the adaptive array processing unit 130. By generating nulls for the subcarriers being processed, the discontinuous components of the symbols in the FFT interval are extracted in the frequency domain (step S106). The IFFT unit 118 converts the extracted discontinuous component from the frequency domain to the time domain (step S107). The timing detection unit 120 detects the optimum timing for removing the CP based on the level of the extracted unnecessary component, and the CP timing shift unit 121 shifts the CP removal timing accordingly (step S108). ), The process returns to step S102.

  FIG. 9 shows the simulation result of the second embodiment of the present invention. FIG. 9A shows a frequency waveform after FFT conversion of the received signal when the OFDM symbol and the FFT interval are synchronized. The simulation conditions are as shown in FIG. As shown in FIG. 9A, when the OFDM symbol and the FFT interval are synchronized, it can be seen that the spectrum band falls within the subcarrier band, and no extra frequency component exists.

  On the other hand, FIG. 9C shows a frequency waveform after FFT conversion of the received signal when the OFDM symbol and the FFT interval are asynchronous. As shown in FIG. 9C, it can be seen that when the OFDM symbol and the FFT interval are asynchronous, an extra frequency band component due to the discontinuous component of the symbol in the FFT interval is widened.

  In FIG. 9 (D), by intentionally generating nulls for the assigned subcarriers, the symbol discontinuous components in the FFT interval are extracted in the time domain, and converted to the time domain by IFFT. Is. As shown in FIG. 9D, it can be seen that the discontinuous components of the symbols in the FFT interval can be extracted by intentionally generating nulls for the assigned subcarriers.

You may make it implement | achieve the function of the receiver in embodiment mentioned above with a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program held for a certain period of time. Further, the program may be a program for realizing a part of the above-described functions, and may be a program capable of realizing the functions described above in combination with a program already recorded in a computer system. You may implement | achieve using programmable logic devices, such as FPGA (Field Programmable Gate Array).
The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Receiver, 10 ... Antenna, 11 ... Conversion part, 12 ... CP removal part, 13 ... A / D conversion part, 14 ... FFT part, 15 ... Demodulation part, 16 ... Demodulation correctness determination part, 17 ... Desired subcarrier Null processing unit, 18 ... IFFT unit, 19 ... P / S conversion unit, 20 ... timing detection unit, 21 ... timing shift unit

Claims (5)

A first step of receiving an OFDM signal;
A second step of performing FFT by removing the guard interval from the received OFDM signal;
A third step of extracting in a frequency domain a discontinuous component of a symbol in an FFT interval in which FFT has been performed;
A fourth step of converting a discontinuous component of the symbol in the FFT interval extracted in the frequency domain into a signal in the time domain;
A fifth step of detecting the timing of the start position of the OFDM symbol and the position of the FFT interval based on the discontinuous component of the symbol of the FFT interval converted to the time domain;
And have groups Dzu the timing of the position of the start position and the FFT interval of the OFDM symbol, OFDM signal receiving method and a sixth step of shifting the removal timing of the guard interval.   2. The OFDM signal receiving method according to claim 1, wherein, in the third step, a component in a band of a desired wave addressed to the own apparatus is removed from the frequency domain signal of the FFT section. By performing the error detection of the demodulated signal, further comprising, an OFDM signal according to claim 1 or 2: determining whether synchronization with the received OFDM signal symbol and extracted FFT interval is taken Reception method. The OFDM signal receiving method according to any one of claims 1 to 3 , further comprising a step of determining whether or not the received OFDM signal is addressed to the own station using a user signal included in the demodulated signal. . An antenna for receiving an OFDM signal;
A removal unit that removes the guard interval from the received OFDM signal and performs FFT;
An extraction unit that extracts, in the frequency domain, the discontinuous components of the symbols in the FFT interval in which the FFT is performed;
A conversion unit that converts a discontinuous component of the symbol in the FFT section extracted in the frequency domain into a signal in the time domain;
A timing detection unit that detects the timing of the start position of the OFDM symbol and the position of the FFT interval based on the discontinuous component of the symbol of the FFT interval converted to the time domain;
And have groups Dzu the timing of the position of the start position and the FFT interval of the OFDM symbol, OFDM signal receiver and a timing shift unit for shifting the removal timing of the guard interval.

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