JP2009049792A - Receiver and propagation path estimating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce distortion that is generated at a point in the center of a band or around a null carrier when using DFT. <P>SOLUTION: A receiver for receiving an OFDM signal includes: a first DFT unit 104 for transforming a predetermined band into a signal represented on a frequency axis; a complex dividing unit 106 for performing complex division, on the signal transformed by the first DFT unit 104, using a code used in transmission; a carrier interpolation unit 107 for replacing a value of the point corresponding to at least one null carrier on the frequency axis with a value calculated using at least one point other than the above point; an IDFT unit 108 for transforming an output signal of the carrier interpolation unit 107 into a signal represented on a time base; a time filter 109 for attenuating or deleting part of an output signal of the IDFT unit 108; and a second DFT unit 110 for transforming an output signal of the time filter unit 109 into a signal represented on the frequency axis. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a receiver and a propagation path estimation method applied to an OFDM communication system.

  Conventionally, an OFDM communication system is known. FIG. 21 is a block diagram showing a schematic configuration of a general transmitter of OFDM. It is assumed that the transmission signal is of a general type in a wireless LAN or the like, in which pilot symbols are arranged first, followed by data symbols. Initially, the signal from pilot symbol code generation section 1301 is converted into a time-axis signal by IFFT (Inverse Fast Fourier Transform) section 1303 by input switching section 1302. Then, after adding a guard interval by a GI (guard interval) adding unit 1304, it is converted into an analog signal by a D / A (digital / analog) converting unit 1305, and is subjected to frequency conversion and power amplification by a radio transmitting unit 1306 for transmission. Is done.

  After the pilot symbol transmission, the subsequent data symbols are transmitted by switching the input switching unit 1302 to the modulation unit 1307 side. For the conversion between the time axis signal and the frequency axis signal, DFT / IDFT (Discrete Fourier Transform / Inverse Discrete Fourier Transform) can be used, but FFT / IFFT is used to reduce the amount of computation.

  Next, the receiver will be described. FIG. 22A is a block diagram showing a schematic configuration of a general OFDM receiver. A signal received by the wireless reception unit 1311 is converted into a baseband signal, and is converted into a digital signal by an A / D (analog / digital) conversion unit 1312. The synchronization / GI removal unit 1313 performs symbol synchronization using a pilot symbol and card interval removal, and an FFT (Fast Fourier Transform) unit 1314 converts the signal into a signal on the frequency axis. Output switching section 1315 sends pilot symbols to propagation path estimation section 1316 and data symbols to 1317.

  The pilot symbols are converted into propagation path information by a propagation path estimation unit 1316 and used by a propagation path correction unit 1317 to correct data symbols. The demodulated unit 1318 performs demodulation processing on the corrected data symbol.

  The propagation path correction at the time of reception is important. If the propagation path information referred to here is incorrect, it cannot be demodulated normally. In particular, in the case of a modulation scheme such as QAM (Quadrature Amplitude Modulation) that uses both phase information and amplitude information for modulation, the influence of this error is significant. Therefore, it is desirable that the propagation path information can be estimated as accurately as possible.

  Next, a basic propagation path estimation method will be described. The simplest propagation path estimation method is that a pilot symbol multiplied by a known pilot code is transmitted, and the propagation path information can be extracted by complex division of the pilot symbol received on the receiving side by the code used at the time of transmission. FIG.22 (b) is a block diagram which shows schematic structure of a propagation path estimation part. The propagation path estimation unit includes a pilot code generation unit 1321 that generates the same code as the pilot code used during transmission, and a complex division unit 1322 that performs complex division on the received pilot symbols.

  However, this method has a problem that the noise component included in reception is included in the channel estimation result as it is. As a method for reducing this problem, there is a method using a time filter called a DFT (Discrete Fourier Transform) method or a time window method. In this DFT method, propagation path information obtained by complex division of received pilot symbols, that is, a frequency response of the propagation path is converted into an impulse response by IDFT (Inverse Discrete Fourier Transform), and an effective delay wave is included. This is a method of converting to a frequency response by DFT after deleting a region other than the time domain by a time filter. According to this method, it is possible to reduce noise corresponding to energy that is deleted from the output frequency response by the time filter. Since DFT / IDFT has a large amount of computation, FFT / IFFT is often used to reduce the amount of computation.

  FIG. 23 is a block diagram illustrating a schematic configuration of the propagation path estimation unit. Here, an example in which FFT / IFFT is used instead of DFT / IDFT will be described. Pilot code generation section 1401 generates the same code as that used at the time of pilot symbol transmission on the transmission side. Complex division section 1402 performs complex division on the input pilot symbols. The IFFT unit 1403 converts the input signal on the frequency axis into a time axis signal. The time filter unit 1404 attenuates or deletes part of the input time axis signal. The FFT unit 1405 converts the input time axis signal into a signal on the frequency axis.

The output obtained by complex-dividing the received pilot symbol by the pilot symbol code in the complex division unit 1402 is a frequency response including noise as described above. FIG. 24A shows a frequency response. FIG. 24B is a diagram illustrating a state after this signal is converted into an impulse response by the IFFT unit 1403. The converted signal includes a portion 1406 where effective delay waves are concentrated and a noise portion 1407 not including effective delay waves. If the noise portion 1407 is deleted by the time filter unit 1404, only the noise component is deleted without affecting the effective delayed wave portion 1406. FIG. 24C schematically shows how the time filter unit 1404 removes noise. By performing FFT on the signal after the noise component is deleted by the time filter unit 1404, a frequency response with a reduced noise component is obtained. An example of an outline of the final frequency response is shown in FIG.
"A Study on Distortion in Propagation Path Estimation by Time Window Method", 2006 IEICE General Conference, B-5-93 “An OFDM Channel Estimation Method Using Virtual Waveform Addition”, 2006 IEICE General Conference, B-5-94 J. et al. Seo, et al. , “An enhanced DFT-Based Channel Estimating Using Virtual Information with Guard Bands Prediction for OFDM,” in Proc. IEEE PIMRC 2006, Helsinki, Finland, Sep. 2006 Alcatel-Lucent, “Conditionations on the EUTRA DL reference signal structure and the DC sub-carrier”, 3GPP TSG RAN WG1 # 48, R1-07688, St. 2007 "A Study on Propagation Path Estimation by Time Window Method in MIMO-OFDM", 2006 IEICE Society Conference, B-5-52

  In an actual system, the number of IFFT and FFT processing points used at the time of transmission and reception and the number of subcarriers used are not the same due to problems with the characteristics of the analog filter used in the radio unit. In such a system, since the impulse response is widened when performing propagation path estimation by the DFT method, the power of the delayed wave that is effective when the noise part is deleted by the time filter is slightly deleted, and the propagation path estimation result Distortion occurs. FIG. 25A is a diagram illustrating an outline of a portion to which a time filter is applied during the DFT method. Reference numeral 1501 denotes an impulse response spread because the number of IFFT processing points and the number of subcarriers to be used are different.

  If this widened impulse response 1501 is deleted 1502 by the time filter, a part 1503 of the widened impulse response is deleted. An example of the result of FFT of the impulse response after deletion is shown in FIG. As shown in this figure, distortion occurs in a part 1504 of the signal band of the propagation path estimation result. As a technique for reducing this distortion, there is a method of applying a time filter after inserting a virtual carrier in a null carrier portion outside the band. An outline of inserting a virtual carrier before applying a time filter is shown in FIG. Virtual carriers 1602 are inserted in the null carrier portions at both ends of the signal band 1601, and then IFFT and time filter processing are performed. Various methods of inserting virtual carriers have been proposed.

  However, since these proposals are only extrapolating null carriers outside the signal band, that is, virtual carrier extrapolation to the so-called guard band region, it solves another problem, the generation of distortion due to null carriers in the signal band and DC offset. do not do. When actually making a receiver, a DC offset is generated in the analog unit, and this DC offset is superimposed on the baseband signal as it is. When the baseband signal is DFT with this DC offset superimposed, the DC offset is output at the center point of the band. Therefore, compared with the propagation path estimation result in an ideal state where there is no DC offset, the value of the point at the center of the band is a value shifted by the DC offset. If this value is used as it is, the demodulation performance will drop, so most systems use this point as a null carrier. An example in which a point at the center of the band is a null carrier is shown in FIG. In order to reduce the influence of DC offset, a method has been proposed in which pilot subcarriers are inserted on both sides of a subcarrier at the center of the band and the propagation path information of the subcarrier at the center of the band is obtained from the values of points at both ends.

  However, in a receiver that receives a part of the system band, particularly an OFDM / FDMA system (hereinafter referred to as an OFDMA system) that receives only some subchannels, the center point in the reception band is a null carrier. Not always. In addition, if a null carrier is inserted into a subchannel for a receiver that receives only some bands and subchannels, a null carrier is inserted around the reception band when receiving all bands and all subchannels. It becomes a state. Also, when a subcarrier adaptive modulation technique is used for OFDM, null carriers may be randomly inserted due to propagation path fluctuations. When the DFT method is performed in a state where the subcarrier value is different from the original value due to the influence of the DC offset or a null carrier is present in the reception band, the estimation is performed around the point representing the DC component and the null carrier. Distortion occurs in the result. This is shown in FIG. FIG. 28 shows the case where a null carrier is inserted at the center point of the band, but it can be seen that distortion occurs around this point 1801.

  The present invention has been made in view of such circumstances, and provides a receiver and a propagation path estimation method capable of reducing distortion generated around a center point of a band and a null carrier when using DFT. The purpose is to do.

  (1) In order to achieve the above object, the present invention takes the following measures. That is, the receiver of the present invention is a receiver that receives an OFDM signal, and converts the predetermined band including the signal band of the received signal into a signal represented on the frequency axis; A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission, and a value of a point corresponding to at least one null carrier on the frequency axis, other than the point A carrier interpolating unit that replaces a value calculated using at least one point, an IDFT unit that converts an output signal of the carrier interpolating unit into a signal represented on a time axis, and a part of an output signal of the IDFT unit And a second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis, and the second DFT unit. And characterized by performing propagation path compensation by using the output signal.

  In this way, the value of the point corresponding to at least one null carrier on the frequency axis is replaced with the value calculated using at least one point other than that point, so that it occurs at both ends of the null carrier when using DFT. It is possible to reduce distortion.

  (2) Further, the receiver of the present invention is a receiver that receives an OFDM signal, a frequency converting unit that converts the frequency of the received signal, a frequency filter unit that extracts an arbitrary band in the signal band, A first DFT unit that converts an output signal of the frequency conversion unit into a signal represented on the frequency axis, and a complex division by a code used at the time of transmission for the signal converted by the first DFT unit A complex division unit to perform, a carrier interpolation unit that replaces the value of the central point of the arbitrary band with a value calculated using at least one point other than the point, and the output signal of the carrier interpolation unit on the time axis The IDFT unit for converting to a signal represented by: a time filter unit for attenuating or deleting part of the output signal of the IDFT unit; and the output signal of the time filter unit represented on the frequency axis Comprising a second DFT section for converting a signal, a, is characterized by performing propagation path compensation by using the output signal of said second DFT unit.

  Thus, frequency conversion is performed to extract an arbitrary band of the received signal, an arbitrary band in the signal band is extracted, and the value of the center point of the arbitrary band is set to at least one point other than that point. Therefore, when using DFT, it is possible to reduce distortion that occurs around an arbitrary band center point.

  (3) Further, in the receiver of the present invention, an offset detection unit that detects a DC offset amount at the center of the arbitrary band based on the output signal of the complex division unit and the output signal of the second DFT unit Is further provided.

  Thus, since the DC offset amount at the center of an arbitrary band is detected, it is possible to improve the propagation path estimation accuracy by the time filter while reducing the influence of the offset.

  (4) The receiver of the present invention is a receiver that receives an OFDM signal, and converts a predetermined band including a signal band of the received signal into a signal represented on the frequency axis. A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission, and at least one null carrier dynamically allocated according to a propagation path condition A carrier interpolation unit that replaces the value of the point corresponding to the above with a value calculated using at least one point other than that point, and an IDFT unit that converts the output signal of the carrier interpolation unit into a signal represented on the time axis A time filter unit that attenuates or deletes part of the output signal of the IDFT unit, and a second DFT unit that converts the output signal of the time filter unit into a signal represented on the frequency axis, The provided, is characterized by performing propagation path compensation by using the output signal of said second DFT unit.

  In this way, the value of the point corresponding to at least one null carrier dynamically allocated according to the propagation path condition is replaced with the value calculated using at least one point other than that point, so that adaptive modulation is performed. When the normal frame is demodulated, it is possible to reduce the influence of the null carrier on the channel estimation.

  (5) Moreover, in the receiver of the present invention, the carrier interpolation unit, when there is a point corresponding to a known null carrier at a point other than the center point of the arbitrary band, And the value of the central point of the arbitrary band and the value of the point corresponding to the known null carrier are replaced with a value calculated using at least one point other than the point corresponding to the known null carrier and the point corresponding to the known null carrier It is characterized by that.

  With this configuration, even when there is a point corresponding to a known null carrier, it is possible to reduce distortion generated around that point when using DFT.

  (6) In the receiver according to the present invention, the carrier interpolation unit may convert a known null carrier to a point other than a point corresponding to at least one null carrier dynamically allocated according to the propagation path condition. When a corresponding point exists, calculation is performed using at least one point other than the point corresponding to at least one null carrier dynamically allocated according to the propagation path condition and the point corresponding to the known null carrier. In this case, the value of the point corresponding to at least one null carrier dynamically allocated according to the propagation path condition and the value of the point corresponding to the known null carrier are replaced.

  With this configuration, even when there is a point corresponding to a known null carrier, it is possible to reduce the influence of the null carrier on channel estimation when demodulating an adaptively modulated normal frame.

  (7) Further, the receiver of the present invention is a receiver for receiving an OFDM signal, and a predetermined band including a signal band of a received signal in which pilot symbols are CI-multiplexed is a signal represented on the frequency axis. A first DFT unit to be converted, a complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission, and an output signal of the complex division unit are transmitted. A carrier separation unit that performs grouping according to a combination of phase rotation codes used when performing CI multiplexing on the side, and any group divided by the carrier separation unit includes a center point of the signal band A carrier interpolation unit that replaces the value of the point with a value calculated using at least one point other than the point included in the same group; A carrier synthesizer for rearranging a plurality of output signals of the interpolation unit to a state before being input to the carrier separation unit, and an IDFT unit for converting the output signal of the carrier synthesizer into a signal represented on the time axis A time filter unit that attenuates or deletes a part of the output signal of the IDFT unit, and a second DFT unit that converts the output signal of the time filter unit into a signal represented on the frequency axis, And propagation path correction is performed using the output signal of the second DFT unit.

  In this way, if any group divided by the carrier separation unit includes a central point in the signal band, the value of that point is used using at least one point other than that point included in the same group. Since the calculated value is replaced, even when adaptive modulation is performed by MIMO, subcarriers at the center of the band can be interpolated.

  (8) Further, the receiver of the present invention is a receiver for receiving an OFDM signal, and a predetermined band including a signal band of a reception signal in which pilot symbols are CI-multiplexed is a signal represented on the frequency axis. A first DFT unit to be converted, a complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission, and an output signal of the complex division unit are transmitted. A carrier separation unit that performs grouping according to the combination of phase rotation codes used for CI multiplexing on the side, and at least one group divided by the carrier separation unit corresponds to a null carrier in the signal band If a point is included, the value of that point is replaced with a value calculated using at least one point other than that point in the same group. The rear interpolation unit, the carrier synthesis unit for rearranging the plurality of output signals of the carrier interpolation unit to the state before being input to the carrier separation unit, and the output signal of the carrier synthesis unit are represented on the time axis An IDFT unit for converting to a signal, a time filter unit for attenuating or deleting a part of the output signal of the IDFT unit, and a second for converting the output signal of the time filter unit to a signal represented on the frequency axis The DFT unit is provided, and propagation path correction is performed using the output signal of the second DFT unit.

  As described above, when the point corresponding to the null carrier in the signal band is included in at least one group divided by the carrier separation unit, the value of the point is set to at least one other than the point included in the same group. Since it is replaced with a value calculated using points, null carrier interpolation can be performed even when adaptive modulation is performed by MIMO.

  (9) Further, the receiver of the present invention is a receiver that receives an OFDMA signal, a frequency converting unit that converts the frequency of the received signal, a frequency filter unit that extracts an arbitrary subchannel in the signal band, A first DFT unit for converting an output signal of the frequency conversion unit into a signal represented on a frequency axis; and a complex division by a code used at the time of transmission for the signal converted by the first DFT unit A complex division unit that performs the processing, a carrier interpolation unit that replaces the value of the center or both ends of the arbitrary subchannel with a value calculated using at least one point other than the point, and an output signal of the carrier interpolation unit Of the IDFT unit that converts the signal into a signal represented on the time axis, a time filter unit that attenuates or deletes a part of the output signal of the IDFT unit, and the time filter unit Comprising a second DFT section for converting the force signal to a signal represented on the frequency axis, and is characterized by performing propagation path compensation by using the output signal of said second DFT unit.

  Thus, frequency conversion is performed to extract an arbitrary band of the received signal, an arbitrary band in the signal band is extracted, and the value of the point at the center or both ends of the arbitrary subchannel is set to at least other than the point. Since replacement is performed with a value calculated using one point, it is possible to reduce distortion generated around the center or both ends of an arbitrary subchannel when using DFT.

  (10) The propagation path estimation method of the present invention is a propagation path estimation method for estimating a propagation path from a received OFDM signal, and a predetermined band including a signal band of the received signal is represented on the frequency axis. A second step of performing complex division on the output signal after the first step with a code used at the time of transmission, and an output signal after the second step. A third step of replacing a value of a point corresponding to at least one null carrier on the frequency axis with a value calculated using at least one point other than that point; and an output signal after the third step on the time axis A fourth step for converting the output signal into a signal represented by the following: a fifth step for attenuating or deleting a part of the output signal after the fourth step; and an output signal after the fifth step. It is characterized in that it comprises a sixth step of converting the signal represented on several axes, at least.

  In this way, the value of the point corresponding to at least one null carrier on the frequency axis is replaced with the value calculated using at least one point other than that point, so that it occurs at both ends of the null carrier when using DFT. It is possible to reduce distortion.

  According to the present invention, since the value of the point corresponding to at least one null carrier on the frequency axis is replaced with the value calculated using at least one point other than that point, both ends of the null carrier are used when using DFT. Can be reduced.

  Next, embodiments of the present invention will be described with reference to the drawings. First, a communication frame structure used in the following embodiments will be described. For ease of explanation, each embodiment uses a fixed-length frame. However, the present invention is not only applied to a fixed-length frame, but can also be applied to a variable-length frame or variable-length asynchronous packet communication. It is. The frames used below are communicated by repeatedly transmitting the frames with a fixed length in the time and frequency directions.

  FIG. 1A is a diagram showing a communication frame structure in the present embodiment. A frame is composed of OFDM symbol groups. A synchronization symbol is arranged at the head of the frame, followed by a pilot symbol for propagation path estimation, and then a data symbol. In FIG. 1A, 201 represents the entire frame, and this frame 201 is repeatedly transmitted. A synchronization symbol 202 is arranged at the head of the frame 201. This synchronization symbol is used to identify the beginning of the frame at the time of reception. In many cases, a signal having characteristics in the time axis direction is used. Since signals other than OFDM signals can also be used, details are omitted. As an example of using an OFDM signal, a signal in which a null carrier is inserted for each subcarrier may be used so that the same signal is repeated on the time axis.

  Subsequently, pilot symbols 203 are arranged. Subsequently, a data symbol group 204 is arranged. Since the present invention is not related to the contents of the data symbol group, details of the data symbol group will not be described. In general, control data for indicating the structure in the frame and subsequent frames and data actually used for communication are included.

  FIG. 1B is a diagram showing a configuration within one frame. A guard interval 205 is added to each OFDM symbol in the frame 201. The guard interval is added to absorb the influence of the delayed wave, and has no direct relationship with the present invention. As an example, a method of adding a part of the OFDM symbol as a cyclic prefix is used.

(First embodiment)
FIG. 2 is a block diagram illustrating a schematic configuration of the receiver according to the first embodiment. In the following embodiments, FFT / IFFT is used as DFT / IDFT. The wireless reception unit 101 receives radio waves and converts them into baseband signals. The A / D converter 102 converts an analog baseband signal into a digital signal. Synchronization / GI removal section 103 applies frame synchronization using the synchronization symbol at the head of the frame and removes the guard interval from the subsequent received OFDM symbols. The first FFT unit 104 converts the received symbol from which the guard interval has been removed into a frequency-axis signal by fast Fourier transform. Switching section 105 switches the output to complex division section 106 when the received symbol subjected to the FFT is a pilot symbol, and to propagation path correction section 111 when the received symbol is a data symbol.

  The complex division unit 106 performs complex division on the received pilot symbol by the code used at the time of transmission. The carrier interpolation / extrapolation unit 107 performs interpolation / extrapolation processing on the null carrier. The IFFT unit 108 converts a signal on the frequency axis into a signal on the time axis. The time filter unit 109 attenuates and deletes part of the signal on the time axis. The second FFT unit 110 converts the signal on the time axis into a signal on the frequency axis. The propagation path correction unit 111 corrects the received data symbol according to the propagation path information output from the second FFT unit 110. Demodulation section 112 demodulates the corrected data symbol in accordance with information from control section 113. The control unit 113 controls reception of the entire frame based on information from each block.

  FIG. 3 is a block diagram illustrating a schematic configuration of the transmitter according to the first embodiment. Pilot code generating section 121 generates a code used for pilot symbols. The synchronization code generator 130 generates a code for the synchronization symbol. In response to an instruction from the control unit 129, the input switching unit 122 switches the output signal to one of a synchronization code, a pilot code, and modulation data. The null carrier insertion unit 123 sets a part of the signal input by the instruction from the control unit 129 as a null carrier. The IFFT unit 124 converts the input signal on the frequency axis into a signal on the time axis.

  The GI adding unit 125 adds a part of the input signal as a guard interval. The D / A converter 126 converts the input digital signal into an analog signal. The radio transmission unit 127 performs transmission after converting and amplifying the input signal as a baseband signal to a frequency necessary for transmission. Modulation section 128 performs modulation processing on the input transmission data. The control unit 129 controls each block according to the input transmission control data.

  In the present embodiment, the number of FFT points used for generating a transmission signal is 32, three guard band null carriers are provided at both ends of the signal band, and one null carrier is provided at the center of the signal band. To do.

  Next, a procedure for transmitting a signal having this spectrum by the transmitter shown in FIG. 3 will be described. The control unit 129 determines a location where a null carrier is inserted based on the transmission control data, and instructs the null carrier insertion unit 123 to set the corresponding carrier as a null carrier. In addition, the control unit 129 monitors the frame transmission start timing, and switches the input destination of the input switching unit 122 to the synchronization code generation unit 130 at the transmission timing of the head of the frame.

  The output of the input switching unit 122 is inserted into the null carrier by the null carrier insertion unit 123, and then converted into a time axis signal by the IFFT unit 124, and a guard interval is added by the GI addition unit 125. After being converted into an analog signal by the conversion unit 126, it is transmitted from the wireless transmission unit 127. Control section 129 switches the input destination of input switching section 122 to pilot code generation section 121 at the processing timing of the next OFDM symbol. Thereafter, as in the case of the code for synchronization, after the null carrier is inserted by the null carrier insertion unit 123, it is transmitted through the subsequent block. Hereinafter, the control unit 129 switches the input destination of the input switching unit 122 to the modulation unit 128 and continues to transmit transmission data until the end of the frame. When the next frame start time comes, the switching of the input destination of the input switching unit 122 to the synchronization code generating unit 130 is repeated to transmit the frame having the structure shown in FIG.

  Next, a procedure for receiving the signal transmitted in this way by the receiver shown in FIG. 2 will be described. First, a necessary signal is extracted from the signal received by the wireless reception unit 101 and converted into a baseband signal. Next, the A / D conversion unit 102 converts the signal into a digital signal and inputs the digital signal to the synchronization / GI removal unit 103. The synchronization / GI removal unit first detects the synchronization symbol at the head of the frame. Since any detection method may be used, details are omitted. As long as it is possible to accurately detect the reception timing of the synchronization symbol, for example, a method of measuring the correlation of the time axis with the reference symbol and detecting the correlation peak can be considered. After the head of the frame is detected, the part from which the guard interval is removed in accordance with the subsequent OFDM symbol reception timing is continuously sent to the FFT unit 104 in the subsequent stage. When the head of the frame is detected, the timing is notified to the control unit 113, and the operation of other blocks is triggered.

  The FFT unit 104 performs FFT every time data is input from the synchronization / GI removal unit 103, and converts the OFDM symbol into data on the frequency axis. Switching section 105 outputs the data after FFT to complex division section 106 when the OFDM symbol received by the instruction of control section 113 is a pilot symbol, and to propagation path correction section 111 when the data symbol is a data symbol. The control unit 113 receives the notification of the frame head timing from the synchronization / GI removal unit, and determines the reception timing of subsequent received symbols. Complex division section 106 performs complex division with the code output from pilot code generation section 121 during transmission of the data after FFT. The data after performing this complex division is propagation path information in a state in which noise is included and a null carrier is inserted.

  FIG. 4A is a diagram illustrating an example of propagation path information. In FIG. 4A, 301 is an FFT processing band, 302 is a null carrier inserted outside the signal band, and 303 is a null carrier inserted in the center of the signal band. For this data, the carrier interpolation / extrapolation unit 107 interpolates the data at the location of the null carrier.

  FIG. 4B is a diagram illustrating an example of data complementation. As the interpolation 304 outside the signal band, the method described in the conventional example can be used. Hereinafter, an interpolation method for the null carrier 303 at the center of the signal band will be described. For the null carrier 303 at the center of the band, the vector average of the subcarriers 305 at both ends is inserted as an estimated value (306). The method for obtaining the estimated value does not necessarily need to be this method, and any method may be used as long as it can be estimated with high accuracy. In the situation where the maximum delay dispersion of the propagation path is known in advance, the correlation coefficient between the subcarriers is also known in the same manner, so the number of subcarriers with relatively high correlation around the null carrier 303 is used instead of simple vector averaging. A method of using a low-pass filter or a method of using one of the adjacent subcarriers 305 of the null carrier 303 as an estimated value may be used when it is known that the situation is close to flat fading. . Information on the null carrier to be complemented is obtained from the control unit 113.

  The data after the null carrier interpolation is converted into a time axis signal by the IFFT unit 108. The converted time axis signal is deleted by the time filter unit 109 other than the effective time region including the delayed wave. Considering that the effective signal component spreads to some extent at this time, distortion is reduced by deleting the guard interval section including the delayed wave and several points before and after that. How much before and after the guard interval section is left depends on the number of FFT / IFFT processing points and the allowable amount of distortion generated in the signal after deletion. As an example, when the number of FFT points is 1024, about 30 points are left. There is a way to set.

  The signal from which the extra signal is deleted by the time filter unit 109 is converted into a signal on the frequency axis by the second FFT unit 110, and becomes a frequency response of the propagation path with reduced noise components. The data symbol received by the propagation path correction unit 111 is corrected by the frequency response output from the second FFT unit 110, the data symbol is demodulated by the demodulation unit 112, and the received data is extracted.

  As described above, it is possible to reduce distortion generated at both ends of the null carrier during propagation path estimation by performing the time filter process after supplementing the null carrier.

  In the present embodiment, the carrier at the center of the signal band is a null carrier. However, in the case where the carrier is not a null carrier, the same processing can be performed, and in this case as well, the occurrence of distortion due to the DC offset and the time filter can be suppressed. Is possible. Further, in this embodiment, the code for pilot symbols is fixed, but the present invention can also be applied to a case where the code changes as long as the codes used on both the transmission side and the reception side are synchronized. is there.

(Second Embodiment)
In this embodiment, a case where the present invention is applied to OFDMA will be described as an example when using some subcarriers in an OFDM signal. OFMDA is also referred to as OFDM / FDMA, and is a method of dividing the OFDM signal band into a plurality of frequency bands called subchannels.

  FIG. 5 is a diagram showing an outline of the OFDMA scheme. In this example, the signal band 402 in the FFT processing band 401 is divided into 12 subchannels 403. Null carriers 404 are arranged at both ends of the signal band, and a null carrier 405 is also arranged at the center of the signal band. The number of FFT processing points is 1024, and the number of subcarriers per subchannel is 64. The transmitter can use the configuration shown in FIG. 3 as it is, and it is sufficient to prepare data multiplexed in advance as transmission data.

  The receiver determines which subchannel is received based on information in the received control data. There are various methods for notifying the receiver of which subchannel is received, and since there is no direct relationship with the present invention, detailed description thereof will not be given. As an example, an example in which a super frame that requires reception of all subchannels and a normal frame in which subchannel allocation is performed will be briefly described. FIG. 6 is a diagram illustrating an example in which a superframe is used. The transmitter transmits a super frame 601 that receives all subchannels at regular intervals.

  FIG. 6 shows an example in which a super frame is transmitted every four frames. This super frame 601 is transmitted including control data and broadcast data. In this control data, subchannel allocation information in the subsequent normal frame is included. A normal frame 602 is assigned between the two superframes 601, and a subchannel 603 to be received by each terminal is assigned therein. The assignment may be different for each superframe. If the allocation information may increase, the allocation may be changed for each normal frame.

  The receiver first searches for the superframe 601 and reads the control information in the superframe 601, thereby receiving the assigned subchannel in the subsequent normal frame 602. If the receiver always demodulates signals in the entire signal band when receiving a specific subchannel in the frame, the inserted null carrier is used by using the interpolation method shown in the first embodiment. The influence of can be suppressed. However, when only some subchannels, for example, subchannel 3 are allocated, it is inefficient to operate all of the FFT / IFFT. In the case of FFT / IFFT in which the number of processing points is a power of 2, the calculation amount of N-point FFT / IFFT is given by the following equation.

このため、１サブチャネルのみを受信するためにＦＦＴ／ＩＦＦＴ処理ポイント数を１０２４から１２８と変更した場合、ＦＦＴ／ＩＦＦＴ部の演算量は１／１２となる。つまり、ＦＦＴ／ＩＦＦＴ部で使用する電力が大幅に少なくなることを意味する。なお、６４サブキャリアのサブチャネルを復調するために１２８ポイントのＦＦＴ／ＩＦＦＴを使用するのは、後述するフィルタ／周波数変換部５０２のフィルタの通過特性が理想的でないため、余分な帯域を用意する必要があるためである。

Therefore, when the number of FFT / IFFT processing points is changed from 1024 to 128 in order to receive only one subchannel, the calculation amount of the FFT / IFFT unit is 1/12. That is, it means that the power used in the FFT / IFFT unit is significantly reduced. Note that the use of 128-point FFT / IFFT to demodulate a subchannel of 64 subcarriers is not ideal because the pass characteristics of the filter of the filter / frequency conversion unit 502 described later are used, so an extra band is prepared. This is necessary.

  A problem in making such a receiver is the DC offset of the radio receiver. When a DC offset is superimposed on the baseband signal output from the wireless reception unit, the DC offset amount remains at the point representing the DC component in the data after the FFT, or in the case of normal FFT processing, at the center point of the processing band. Superposed and output. When processing all the normal bands, the center of the band is a null carrier and is not affected by this DC offset. The present embodiment solves this problem.

  FIG. 7 is a block diagram illustrating a schematic configuration of a receiver according to the second embodiment. In FIG. 7, the same numbers are assigned to portions that perform the same functions as those in the first embodiment. The wireless reception unit 514 receives a wireless signal and converts it into a first intermediate frequency signal. First frequency conversion unit 515 performs frequency conversion of a received signal band in accordance with an instruction from control unit 512. Specifically, the frequency conversion is performed so that the center frequency of all the subchannels or one specific subchannel to be received is the center of the second intermediate frequency signal. The frequency filter unit 516 switches the passband to all subchannels or one subchannel according to an instruction from the control unit 512. Here, it is assumed that the center frequency of the pass band of the filter is equal to the center frequency of the second intermediate frequency signal. The second frequency conversion unit 517 converts the second intermediate frequency signal output from the frequency filter unit 516 into a baseband signal in response to an instruction from the control unit 512. At this time, by changing the shift amount at the time of conversion depending on whether all subchannels are received or only one subchannel is received, either when receiving all subchannels or when receiving one subchannel Even in such a case, only a necessary band is correctly converted into a baseband signal. The A / D conversion unit 503 performs analog-digital conversion at a sampling rate corresponding to the band extracted by the filter / frequency conversion unit 502 in accordance with an instruction from the control unit 512. The synchronization / GI removal unit 504 performs frame synchronization using the synchronization symbol at the head of the frame and removes the guard interval from the subsequent received OFDM symbols.

  The first FFT unit 505 can change the number of points to be processed in accordance with an instruction from the control unit 512, and converts a signal on the time axis into a signal on the frequency axis. The complex division unit 506 extracts the portion corresponding to the processing band from the code generated by the pilot code generation unit 121 on the transmission side from the data input by the instruction from the control unit 512, and divides the input data. The IFFT unit 507 can change the number of processing points by an instruction from the control unit 512, and converts a signal on the frequency axis into a signal on the time axis. The time filter unit 508 can control the number of processing points and the data deletion point according to an instruction from the control unit 512, and attenuates / deletes part of the signal on the time axis.

  The second FFT unit 509 performs FFT at the processing point instructed by the control unit 512. The propagation path correction unit 510 performs propagation path correction on a necessary band of received data according to an instruction from the control unit 512. Demodulation section 511 performs demodulation processing on the band designated by control section 512. The control unit 512 determines the reception band based on the control data in the received data after demodulation together with the control of each block shown in the first embodiment, and controls each block according to the reception band. The offset detection unit 513 detects the DC offset by comparing the output of the complex division unit 506 and the output of the second FFT unit 509.

  Hereinafter, a series of operations in these blocks will be described in detail. The case of receiving only subchannel 4 (hereinafter referred to as “SCH4”) among the subchannels shown in FIG. 5 will be taken up. FIG. 8A shows SCH4. The received signal is once converted into an intermediate frequency signal by the wireless reception unit 501, and only the signal of the frequency corresponding to the subchannel (here, SCH 4) instructed from the control unit 512 by the filter / frequency conversion unit 502 is filtered. Extract and then convert to baseband signal.

  FIG. 8B is a diagram showing an outline of the characteristics of the filter used. Here, it is assumed that a band necessary for 128-point FFT processing is allowed to pass, the signal passing characteristic of the target subchannel (here, SCH4) is sufficiently flat, and does not cause aliasing in the A / D converter 503 at the subsequent stage. It is necessary to have a steep band pass characteristic. The signal converted into the baseband signal is converted into a digital signal at a sampling rate corresponding to the band extracted by the A / D converter 503. Thereafter, the synchronization / GI removal unit 504 performs frame synchronization, and cuts out the OFDM symbol from which the guard interval is removed. Any method may be used for this synchronization processing. However, when using the correlation in the time axis direction shown as an example in the first embodiment, it is necessary to examine the correlation after thinning the reference signal in accordance with the sampling rate. is there.

  FIG. 9A is a diagram showing an outline of the spectrum of the cut out OFDM signal. Subsequently, the first FFT section 505 performs 128-point FFT processing, and the complex division section 506 performs complex division with the code used at the time of transmission in SCH4. FIG. 9B is a diagram illustrating an outline of a signal after complex division. In FIG. 9B, the SCH4 band is indicated by 802, and the subcarrier that is the center of the processing band during FFT processing is indicated by 801. Since the subcarrier at the center of this processing band contains a DC offset, if time filtering is performed in the subsequent stage as it is, distortion will occur around this subcarrier.

  For this reason, using the method shown in the first embodiment, the carrier interpolation / extrapolation unit 107 inserts a value estimated using peripheral subcarriers instead of the subcarriers including this DC offset. . Here, as an example, the vector average of the subcarriers 803 at both ends of the subcarrier to be inserted is used. FIG. 9C is a diagram showing an outline of the insertion method. As in the first embodiment, a value obtained using another method may be inserted.

  Thereafter, the noise is reduced through the IFFT unit 507, the time filter unit 508, and the second FFT unit 509. FIG. 9D is a diagram illustrating an outline of the frequency response output from the second FFT unit 509. The offset detection unit 513 estimates the DC offset amount using this frequency response and the output of the complex division unit 506. The estimation of the DC offset amount uses the center carrier in the processing band. A difference 807 between the center carrier 805 output from the second FFT unit 509 and the center carrier 806 output from the complex division unit 506 is defined as a DC offset amount. The center carrier 806 after being output from the complex division unit 506 still contains noise, but since the influence of the DC offset is larger during demodulation, there is no significant influence even if this value is used as it is.

  Thereafter, using the obtained frequency response and the DC offset amount, the propagation path correction unit 510 corrects the data symbol, and the demodulation unit 511 demodulates the data. The DC offset is corrected for the subcarrier at the center of the processing band, and the phase and amplitude are corrected assuming that the received phase point is shifted by the DC offset. Control unit 512 first sets each block to receive all subchannels, and detects a superframe from the output of demodulation unit 511. Thereafter, each block is set based on the control information in the superframe, and necessary subchannels in the normal frame are demodulated.

  As described above, when FFT / IFFT processing is performed on only the subchannel, it is possible to improve the propagation path estimation accuracy by the time filter while reducing the influence of the DC offset.

  In this embodiment, the case of demodulating only one subchannel has been described. However, the case where a plurality of subchannels are demodulated can also be handled by changing the processing band. When demodulating an even number of subchannels, a subcarrier at the center of the processing band is arranged at either end of the subchannel, and when demodulating an odd number of subchannels, a subcarrier at the center of the processing band is located at the center of the subchannel. Since carriers are arranged, it is only necessary to perform processing on a point that is a subcarrier at the center of the processing band.

(Third embodiment)
In the second embodiment, the method of reducing the influence of the DC offset, which becomes a problem when demodulating subchannels in some bands when no null carrier is arranged in the OFDMA subchannel, has been described. In the third embodiment, a method of reducing the influence of null carriers in subchannels when all subchannels are demodulated at once when null carriers are contained in the center subcarriers of all subchannels will be described. To do.

  FIG. 10 is a diagram illustrating an example in which the center carrier of each subchannel of OFDMA is a null carrier. The same functions as those used in the second embodiment are given the same numbers. In FIG. 10, reference numeral 901 denotes a center carrier in the subchannel, and a null carrier is inserted. In this case, since there is no DC component subcarrier that becomes a problem when only one subchannel is demodulated, it is not necessary to consider the influence of the DC offset. However, when all subchannels are received at once, if DFT-based propagation path estimation is performed, there is a problem that distortion occurs at both ends of the null carrier included in each subchannel. The present embodiment solves this problem.

  The transmitter and receiver configurations shown in FIGS. 2 and 3 can be used as they are. Only the operations of the control unit 113 and the carrier interpolation / extrapolation unit 107 of the receiver are changed, and the other components are the same. The control unit 113 sets the carrier interpolation / extrapolation unit 107 so as to interpolate subcarriers for all the null carriers 901 in each subchannel including the null carrier 405 at the center of the signal band shown in FIG. The other operations are performed as shown in the first embodiment. As a result, it is possible to reduce the influence of the null carrier when demodulating all OFDMA subchannels.

  Note that FIG. 10 shows the case where all subchannels are configured with the same number of subcarriers, but the same applies even when the number of subcarriers differs for each subchannel and the position of the null carrier in the center of the subchannel changes. It is applicable to. Further, when demodulating a plurality of subchannels at a time, it is necessary to interpolate the subcarriers at the center of the processing band as shown in the second embodiment.

(Fourth embodiment)
In the present embodiment, an example in which the present invention is applied to subcarrier adaptive modulation will be described. The subcarrier adaptive modulation technique reports the reception quality of each subcarrier from the receiver side to the sequential transmission side, and the transmitter performs the fastest modulation that satisfies the required error rate based on the reported quality information of each subcarrier. Is applied to each subcarrier. In this subcarrier adaptive modulation technique, a subcarrier that does not satisfy the required error rate may be reduced as a null carrier.

  FIG. 11 is a diagram illustrating the relationship between subcarriers and reception quality when subcarrier adaptive modulation is performed. In the example shown in FIG. 11, it is assumed that four stages from 16QAM to a null carrier that is not modulated are assigned to each subcarrier in accordance with the reception quality. A subcarrier to which a null carrier is assigned is also assumed to be a null carrier for the corresponding subcarrier 1001 in the pilot symbol.

  Various methods can be used as the adaptive modulation control procedure. FIG. 12 is a diagram illustrating a state in which control is performed using a superframe between a base station and a terminal. In FIG. 12, communication from the base station to the terminal is assumed to be downlink, and communication from the terminal to the base station is assumed to be uplink. The base station transmits a super frame 1101 at regular intervals in downlink communication, here once every four frames. The superframe uses all subcarriers and is transmitted with a low modulation degree so that it can be received by any terminal, for example, all subcarriers are modulated with BPSK. This includes pilot symbols that also serve for CQI (channel quality information) measurement, information indicating to which terminal the subsequent normal frame 1102 is allocated, and information on the modulation scheme of each subcarrier of the subsequent normal frame. Including control information and broadcast information.

  The subsequent normal frame 1102 is modulated and transmitted in accordance with the content of the control information in the superframe. The terminal receives pilot symbols in the superframe 1101 and measures the quality of each subcarrier. The quality information may be anything as long as it can be used to determine the modulation method, and SINR (signal to interference noise ratio), SNR (signal to noise ratio), etc. are often used. Sometimes. In this embodiment, the power level of each carrier of the received pilot symbol is used.

  The power for each subcarrier of the received pilot symbol is transmitted as CQI information 1103 using uplink communication. The base station that has received the CQI information determines what modulation method to use until the next superframe, and how to allocate to the normal frames after the next superframe, and frames below the superframe. Send. By repeating the above operation, subcarrier adaptive modulation becomes possible. Here, a method has been described in which the terminal device sends a CQI to the base station device and the base station device determines the modulation scheme, but there is also a method in which the terminal device determines the modulation scheme. At this time, the terminal device may transmit modulation information MLI (Modulation Level Information) instead of transmitting the CQI, and the base station device may perform the modulation according to the MLI transmitted from the terminal device.

  However, as a result of performing subcarrier adaptive modulation, when a null carrier is inserted and a null carrier is included in a pilot symbol as shown by 1001 in FIG. 11, there is a problem that distortion occurs at both ends of the null carrier. is there. The present embodiment solves this problem.

  FIG. 13 is a block diagram illustrating a schematic configuration of a receiver according to the fourth embodiment. The upstream transmission unit 1201 is added to the receiver of the first embodiment. The blocks having the same functions as those in the first embodiment are given the same numbers. The uplink transmission unit 1201 can transmit CQI information from the control unit 1203 of the downlink reception unit 1202 in addition to the normal uplink communication transmission data. The uplink communication method may be any method, for example, a method used in PDC or WCDMA can be used. The control unit 1203 controls each unit and creates CQI information based on the propagation path information.

  Hereinafter, how these blocks operate will be described. First, the control unit 1203 performs frame detection by setting each block to receive all subcarriers, and receives a superframe. The carrier interpolation / extrapolation unit 107 is notified so as to complement the subcarrier at the center of the band. If a super frame is not received, the processing from frame detection is repeated until a super frame is received. After the super frame is received, the control unit 1203 reads the control information in the super frame, determines whether the terminal receives the subsequent normal frame from the allocation information of the subsequent normal frame, and receives it. Notifies the demodulator 112 of the modulation information of each subcarrier from the modulation information of each subcarrier so that the demodulation operation is normally performed.

  At the same time, the carrier interpolation / extrapolation unit 107 is notified so as to complement the subcarriers that have become null carriers by adaptive modulation together with null carriers outside the signal band. In addition, the sub-carrier at the center of the band is notified to be supplemented. Also, CQI information is created using the propagation path information used when demodulating the superframe output from the second FFT unit 110, and the CQI information is notified to the base station using the uplink transmission unit 1201. Do. This CQI information is used for adaptive modulation control of subsequent frames in the base station.

  By operating as described above, it is possible to reduce the influence of the null carrier on the propagation path estimation when demodulating the adaptively modulated normal frame. It is also possible to add an OFDMA subchannel arrangement to the target of adaptive modulation. In this case, as shown in the third embodiment, a null carrier and a band depending on the position of the subcarrier used as a subchannel and the number of subcarriers. Since the position of the center subcarrier changes, the subcarriers corresponding to these may be interpolated in the same manner as the null carrier generated by adaptive modulation.

(Fifth embodiment)
In the present embodiment, a case where the present invention is applied to a scattered pilot will be described. FIG. 14A is a diagram showing consecutive pilot subcarriers. Up to this point, the case where pilot subcarriers are arranged continuously to the FFT / IFFT processing point 1901 has been described as shown in FIG. Scattered pilot is a method of disposing pilot subcarriers discontinuously. Normally, pilot subcarriers are disposed at regular intervals, and null carriers and data carriers are disposed at processing points where pilot carriers are not disposed.

  FIG.14 (b) is a figure which shows an example which arrange | positions a pilot subcarrier every other processing point, and makes processing points other than a pilot subcarrier null carrier. The pilot subcarrier is indicated by a solid line as 1902 and the null carrier is indicated by a circle as 1902. Normally, pilot subcarriers are arranged so that processing point 1903 at the center of the band becomes a null carrier. As shown in FIG. 14B, when the pilot subcarriers are arranged every other subcarrier and the DCT method is applied, the obtained impulse response is a delay having the same shape as 1904 and 1905 in FIG. A waveform in which the profile is repeated twice is obtained. If only one of them 1904 is cut out by the time filter to obtain the frequency response, the frequency response of the null carrier as shown in FIG. 14D can be obtained.

  However, in order to obtain propagation path information from a plurality of transmission antennas using only one pilot symbol, when pilot symbols are multiplexed, or in a cellular system, allocation is performed so that pilot symbol carriers between adjacent cells do not interfere with each other. Alternatively, when a part of the band is received, a pilot subcarrier may be assigned to the processing point 2001 at the center of the band as shown in FIG. In this case, the DC offset 2002 is superimposed at the time of reception. In this case, if a time filter is used as shown in the above embodiment, distortion occurs. Even in such a case, the present invention can be applied as in the embodiment described above. This outline is shown in FIG. By generating a value 2004 to be interpolated using pilot subcarriers 2003 before and after the point 2001 representing the DC component and inserting the value, it is possible to reduce distortion generated by the time filter.

(Sixth embodiment)
In the present embodiment, a case will be described in which the present invention is applied to a receiver of CI (Carrier Interferometry) multiplexed pilot symbols. A method for improving channel estimation accuracy using virtual subcarriers when a pilot symbol using CI multiplexing is estimated using DFT is described in the following references. However, the method described in this reference also does not consider the influence of the null carrier or the DC offset.

[References]
"A Study on Channel Estimation by Time Window Method in MIMO-OFDM", 2006 IEICE Society Conference, B-5-52
The outline of the technique described in the reference document will be described below. The CI signal is a signal having a uniform phase difference between adjacent carriers in the IDFT processing band when the signal is generated by IDFT. FIG. 16A is a diagram illustrating a signal having a uniform phase difference between adjacent carriers in the IDFT processing band. Here, an example is shown in which a phase difference is provided so that a total of 8π phases rotate within the DFT processing band 2301. If the CI signal has such a phase difference, a cyclic delay occurs in the symbol. This amount of delay is exactly one point at the output of IDFT when a total of 2π phase rotation occurs within the IDFT processing band.

  In the case of FIG. 16A, since the total is 8π, a cyclic delay of 4 points occurs in the IDFT output. FIG. 16B is a diagram illustrating a state in which a cyclic delay has occurred. In FIG. 16B, 2302 corresponds to the cyclic delay amount. This means that a plurality of signals can be multiplexed on the time axis when a certain amount of phase rotation is applied. In other words, when transmitting a pilot symbol having the same code from a plurality of transmission antennas, if a specific phase rotation is given to each transmission antenna, the pilot symbol arriving from each antenna can be separated by a time filter on the reception side. It can be done.

  FIG. 16C is a diagram illustrating an example of an impulse response after performing IDFT after complex division by the pilot symbol code on the receiver side when the number of transmission antennas is two. A pilot symbol that does not give phase rotation from the transmitting antenna 1 is phase-shifted from the transmitting antenna 2 by a half of the IDFT point, that is, a phase rotation within the IDFT processing band (number of IDFT processing points × π). A pilot symbol set to be generated is transmitted. The impulse response from the transmission antenna 1 appears in the first half 2303 of the impulse response in FIG. 16C, and the impulse response from the transmission antenna 2 appears in the second half of the impulse response 2304. The impulse response from each antenna spreads here as much as the spread of the delayed wave, and has no other effect. That is, an impulse response from a specific antenna can be extracted from the multiplexed pilot symbols by extracting only necessary signals with a time filter, and a frequency response can be obtained by DFT of this impulse response.

  However, since the processing using the DFT / IDFT and the time filter is the DFT method itself, if the number of DFT / IDFT processing points and the number of subcarriers used are different, distortion is generated in the obtained propagation path information. . Usually, it is difficult to insert a desired virtual subcarrier into a pilot signal transmitted from a plurality of transmission antennas and multiplexed. This is because it is necessary to estimate a frequency component for which no signal is transmitted before obtaining propagation path information. When the pilot signal is CI-multiplexed, the subcarrier phase rotation information (hereinafter referred to as phase rotation code) of each transmit antenna is periodic in the subcarrier direction. Have. In the reference literature, this is used to insert virtual subcarriers.

  FIG. 17 is a diagram illustrating an outline of a method of inserting virtual subcarriers. Here, there are two transmitting antennas, antenna 1 has no phase rotation, and antenna 2 is multiplied by a phase difference that causes phase rotation within the IDFT processing band (number of IDFT processing points × π). To do. 2401 represents a part of the band edge of the subcarrier group used as a signal band, and 2402 represents an extrapolated area of the virtual subcarrier. Reference numeral 2403 denotes a phase rotation code in the antenna 1, and 2404 denotes a phase rotation code in the antenna 2. When the phase rotation is set as described above, the phase rotation code in the antenna 1 becomes 1 as shown here, and the phase rotation code in the antenna 2 repeats 1 and -1.

  When a signal from each transmitting antenna set with such a phase rotation code is received by one receiving antenna via a separate propagation path, the phase point moves greatly for each subcarrier more than the delay spread of the propagation path. Received wave is obtained. This situation is shown in 2405. Although the reception points of the subcarriers appear to be different at first glance, it can be seen that the combination of the same phase rotation codes between the antennas used during transmission is a smooth reception point corresponding to the delay spread of the propagation path. That is, when a virtual subcarrier is inserted into the extrapolation region 2402 of the virtual subcarrier, a subcarrier using a combination of phase rotation codes at the time of transmission that should be used by the subcarrier is searched for from the signal band 2401. It may be estimated. Here, since the code combination changes every two subcarriers, the subcarriers of the signal band 2401 are referred to every other subcarrier, and extrapolation by linear interpolation is performed on the extrapolation region 2402 of the virtual subcarrier from the value. Show the case.

  This extrapolation method is applied to the present invention. FIG. 18 is a diagram showing an outline of the interpolation method according to the sixth embodiment. Reference numeral 2501 denotes a subcarrier to be interpolated. The combination of the phase rotation code at the time of transmission in this subcarrier is {1, 1}. Rather than using the subcarriers on both sides of the subcarrier to be interpolated as in the previous embodiment, the subcarrier that complements the most is the same subcarrier that is complemented by the combination of phase rotation codes at the time of transmission Subcarriers are complemented using a vector average using two subcarriers (2502, 2503) close to. Even when the vector average of two points is not used, complementation is performed using subcarriers in which the combination of phase rotation codes during transmission in subcarriers is {1, 1}. In the case of 0th-order hold, either the value 2502 or 2503 may be used.

  FIG. 19 is a block diagram illustrating a schematic configuration of a receiver that implements the complementing method according to the sixth embodiment. This receiver is a MIMO (Multi Input Multi Output) OFDM receiver that receives a signal to which two CI-multiplexed pilot symbols are added with one antenna and demodulates the signal by means of MLD (Maximum Likelihood Detection). As the phase rotation code used in the transmission antenna at the time of CI multiplexing, the above-described one is used. The same numbers are used for blocks having the same functions as those of the receiver shown in FIG. 2, and description of blocks having no special meaning is omitted. In this embodiment, it is assumed that adaptive modulation shown in the fourth embodiment is also performed.

  Carrier separation section 2101 separates subcarrier groups for each combination of phase rotation codes used during transmission. In the present embodiment, it is assumed that a set of {1,1} and {1, -1} is separated. First carrier interpolation / extrapolation section 2102 performs subcarrier interpolation / extrapolation on subcarrier groups whose phase rotation code combination at the time of transmission is {1, 1}. Second carrier interpolation / extrapolation section 2103 performs subcarrier interpolation / extrapolation on the subcarrier group whose combination of phase rotation codes during transmission is {1, -1}. Contrary to the operation of the carrier separation unit 2101, the carrier combining unit 2104 reorders the outputs from the first carrier interpolation / extrapolation unit 2102 and the second carrier interpolation / extrapolation unit 2103 together. The (first) time filter unit 109 and the second FFT unit 110 have the same configuration as that of the first embodiment, and extract an impulse response generated from the pilot signal from the transmission antenna 1 corresponding to 2303 in FIG. The frequency response is obtained.

  The second time filter unit 2105 is a time filter for extracting an impulse response generated from a pilot signal from the transmission antenna 2 corresponding to 2304 in FIG. The phase rotation unit 2106 cancels the phase rotation process performed at the time of transmission. The third FFT unit 2107 converts the output signal of the second time filter unit 2105 into a frequency response. Demodulation section 2108 performs MLD demodulation from the two frequency responses output from second FFT section 110 and third FFT section 2107 and the data symbol output from switching section 105. The control unit 2109 controls the entire block.

  Next, how the above block operates as described above will be described in detail. First, a signal received by the wireless reception unit 101 is converted into a baseband signal and converted into a digital signal by the A / D conversion unit 102. Thereafter, the synchronization / GI removal unit 103 performs frame synchronization. When the frame synchronization is established, the subsequent received symbols are output to the first FFT unit 104 and the control unit 2109 is notified of the timing at which the frame synchronization is successful. The control unit 2109 notified of the frame synchronization operates the switching unit 105 so as to output the received symbol immediately after the frame synchronization to the complex division unit 106 and the subsequent received symbols to the demodulation unit 2108.

  The signal input to complex division section 106 is complex-divided by the code used for the pilot symbol at the time of transmission and input to carrier separation section 2101. In carrier separation section 2101, the phase rotation code at the time of transmission is separated into a subcarrier group with {1,1} and a subcarrier group with {1, -1}, and a subcarrier group with a combination of {1,1} Are input to the first carrier interpolation / extrapolation unit 2102, and the subcarrier group of the combination of {1, -1} is input to the second carrier interpolation / extrapolation unit 2103. The first carrier interpolation / extrapolation unit 2102 and the second carrier interpolation / extrapolation unit 2103 use the subcarrier to supplement the subcarrier, which is a null carrier or a DC carrier, from the control unit 2109. The carrier interpolation / extrapolation processing is performed in response to the instruction on whether or not to perform. When adaptive modulation is performed, control unit 2109 determines which subcarrier group the null carrier is included in and gives an instruction.

  The signals that have been subjected to the interpolation processing such as null carrier in the first and second carrier interpolation / extrapolation units 2102 and 2103 are rearranged in the arrangement before separation by the carrier separation unit 2101 in the carrier synthesis unit 2104. Thereafter, the IFFT unit 108 converts it into an impulse response. As shown in FIG. 16C, the converted impulse response has a waveform in which two impulse responses are overlapped at different times.

  The impulse response by the pilot symbol transmitted from the transmitting antenna 1 is extracted by a time filter similar to the time filter unit 109 shown in the first embodiment. Further, the impulse response by the pilot symbol transmitted from the transmission antenna 2 is extracted by the second time filter unit 2105 whose transit time is shifted by half of the OFDM symbol. Since the pilot symbol transmitted from the transmission antenna 2 is subjected to phase rotation processing as will be described later, the phase rotation unit 2106 performs phase rotation processing of each subcarrier in order to restore this phase rotation. This phase rotation processing may be performed after the FFT unit 2107, but is realized by performing a cyclic delay on the time axis in order to reduce the processing amount. By this phase rotation processing, the impulse response becomes a signal based on time 0 similar to the impulse response on the antenna 1 side.

  The two impulse responses are converted into frequency responses by the second FFT unit 110 and the third FFT unit 2107 and input to the demodulation unit 2108. Demodulation section 2108 uses MLD based on the data symbol input from switching section 105 based on the frequency response input from first FFT section 110 and second FFT section 2107 and the modulation information of the data symbol input from control section 2109. Demodulate. The control unit 2109 sets the demodulation unit 2108 and the first and second carrier interpolation / extrapolation units 2102 and 2103 so that the superframe can be received first.

  As described in the fourth embodiment, a super frame is modulated by BPSK having a low modulation degree so as to be received by any terminal using all subcarriers, and transmitted to all terminals. The super frame includes information indicating which terminal the subsequent normal frame is assigned to, and control information including information on the modulation scheme of each subcarrier of the subsequent normal frame. When the subcarrier corresponding to the DC component is included in the subcarrier group separated by the carrier separation unit 2101 before separation, the first carrier interpolation / extrapolation unit 2102 or the second carrier interpolation / extrapolation unit 2103 The subcarrier is set to be interpolated by an instruction from the control unit 2109. The subcarrier group that has been subjected to the subcarrier interpolation after being separated is combined by being returned by the carrier combining unit 2104 before being separated by the carrier separating unit 2101.

  Thereafter, the IFFT unit 108 converts it into an impulse response. As described above, this impulse response is a combination of two impulse responses. The first time filter unit 109 is used to extract the impulse response from the transmission antenna 1, and the second time filter unit 2105 is used to extract the impulse response from the transmission antenna 2. Since the impulse response from the transmission antenna 1 appears at the same point as in the first embodiment, it is converted into a frequency response by the second FFT unit 110 as in the first embodiment.

  As will be described later, the impulse response from the transmission antenna 2 is subjected to phase rotation processing at the time of transmission. Therefore, the phase rotation unit 2106 performs processing to cancel the phase rotation processing performed at the time of transmission. In this phase rotation process, the reverse phase rotation process may be performed on each subcarrier after the FFT, but here the phase rotation process is performed using a cyclic delay on the time axis before the FFT. In this embodiment, the phase is adjusted so as to rotate in the FFT band (number of FFT processing points × π) at the time of transmission, so that the same effect is obtained by cyclically delaying by half of the processing points for this inverse transformation.

  The signal after this phase rotation processing is converted into a frequency response by third FFT section 2107 and input to demodulation section 2108. The demodulation unit 2108 is instructed by the control unit 2109 to modulate each subcarrier, and uses the frequency responses from the two transmission antennas input from the second FFT unit 110 and the second FFT unit 2107 to transmit data. Demodulate symbols by MLD. The control unit 2109 checks the output of the demodulation unit 2108 and determines whether the currently received data is a super frame. If the data is not a super frame, the control unit 2109 repeats the above operation until the super frame is received. If it is a super frame, the control information in the super frame is extracted, and it is determined whether the subsequent normal frame is received. As a result of the determination, when receiving, the first and second carrier interpolation / extrapolation units 2102 and 2103 and the demodulation unit 2108 are set in accordance with the used subcarrier and modulation method of the normal frame. Thereafter, the received normal frame can be demodulated using the set value.

  By operating as described above, it is possible to interpolate a null carrier or a sub-carrier at the center of the band even when adaptive modulation is performed by MIMO.

  Next, a transmitter that transmits a signal to the receiver will be described. FIG. 20 is a block diagram illustrating a schematic configuration of a transmitter according to the sixth embodiment. This transmitter has two transmission antennas and can transmit separate data symbols from each antenna. The configuration consists of the two transmitters used in the first embodiment as they are, the transmission data 1 has exactly the same configuration as in the first embodiment, and the transmission data 2 is phase-rotated for CI multiplexing as a pilot symbol. The pilot symbols for which are performed are used. Other than that, the configuration is the same. The same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  The phase rotation unit 2201 rotates the phase of each subcarrier in order to CI multiplex pilot symbols. In this embodiment, the phase rotation unit 2201 sets the phase so as to rotate by a total (IFFT point × π) within the IFFT processing band. That is, the phase is set so that the phase difference between adjacent points is multiplied by π, that is, -1. Other than this, the same functional block as the system on the transmission data 1 side is used. The input switching unit 2202 performs the same function as the input switching unit 122, the modulation unit 2203 performs the same function as the modulation unit 128, and the null carrier insertion unit 2204 performs the same function as the null carrier insertion unit 123. . The IFFT unit 2205 performs the same function as the IFFT unit 124, and the GI adding unit 2206 performs the same function as the GI adding unit 125. Further, the D / A conversion unit 2207 performs the same function as the D / A conversion unit 126, and the wireless transmission unit 2208 performs the same function as the wireless transmission unit 127.

  The control unit 2209 controls each block according to the transmission control data. Unlike the control unit 129 of the first embodiment, the control unit 2209 also controls the block for the transmission data 2 according to the transmission control data. The control unit 2209 controls the two input switching units in synchronization so that the OFDM symbols are transmitted from the two radio transmission units 127 and 2208 in synchronization. As a result, two signals with different pilot signals can be transmitted at the same frame timing. The signal transmitted from the transmitter having such a configuration can be received by the receiver having the above-described configuration.

(A) is a figure which shows the frame structure for communication in this embodiment. (B) is a figure which shows the structure in 1 frame. It is a block diagram which shows schematic structure of the receiver which concerns on 1st Embodiment. It is a block diagram which shows schematic structure of the transmitting apparatus which concerns on 1st Embodiment. (A) is a figure showing an example of propagation path information. (B) is a figure which shows the example of a complement of data. It is a figure which shows the outline of an OFDMA system. It is a figure which shows the example which uses a super frame. It is a block diagram which shows schematic structure of the receiver which concerns on 2nd Embodiment. (A) is a figure which shows SCH4. (B) is a figure which shows the outline | summary of the characteristic of the filter to be used. (A) is a figure which shows the outline of the spectrum of the cut-out OFDM signal. (B) is a figure which shows the outline of the signal after complex division. (C) is a figure which shows the outline | summary of the insertion method. (D) is a figure which shows the outline of the frequency response output from the 2nd FFT part 509. FIG. It is a figure which shows an example which makes the center carrier of each subchannel of OFDMA the null carrier. It is a figure which shows the relationship between a subcarrier when receiving subcarrier adaptive modulation, and reception quality. It is a figure which shows a mode that it controls using a superframe between a base station and a terminal. It is a block diagram which shows schematic structure of the receiver which concerns on 4th Embodiment. (A) is a figure which shows the continuous pilot subcarrier. (B) is a figure which shows an example which arrange | positions a pilot subcarrier every other processing point, and makes processing points other than a pilot subcarrier null carrier. (C) is a figure which shows an impulse response. (D) is a figure which shows a frequency response. (A) is a figure which shows a mode that the pilot subcarrier was allocated to the processing point of the center of a zone | band. (B) is a figure which shows a mode that it interpolates using the pilot subcarrier before and behind the point showing DC component. (A) is a figure which shows the signal which gave the uniform phase difference between adjacent carriers within an IDFT processing zone | band. (B) is a figure which shows a mode that the cyclic delay generate | occur | produced. (C) is a figure which shows an example of the impulse response after IDFT, after carrying out complex division by the code | symbol of a pilot symbol at the receiver side when there are two transmission antennas. It is a figure which shows the outline | summary of the insertion method of a virtual subcarrier. It is a figure which shows the outline | summary of the interpolation method which concerns on 6th Embodiment. It is a block diagram which shows schematic structure of the receiver which implement | achieves the complementation method which concerns on 6th Embodiment. It is a block diagram which shows schematic structure of the transmitting apparatus which concerns on 6th Embodiment. It is a block diagram which shows schematic structure of the general transmitter of OFDM. (A) is a block diagram showing a schematic configuration of a general receiver of OFDM. (B) is a block diagram showing a schematic configuration of a propagation path estimation unit. It is a block diagram which shows schematic structure of a propagation path estimation part. (A) is a figure which shows a frequency response. (B) is a figure which shows an impulse response. (C) is a figure which shows the outline which deletes noise by a time filter part. (D) is a figure which shows the outline of a final frequency response. (A) is a figure explaining the outline of the part which applies a time filter on the way when performing a DFT method. (B) is a figure which shows the impulse response after deleting a part of spread impulse response. It is a figure which shows the outline which inserts a virtual carrier before time filter application. It is a figure which shows an example which makes the center of a zone | band the null carrier. It is a figure which shows a mode that distortion generate | occur | produced in the estimation result centering on the point showing a DC component, and a null carrier.

Explanation of symbols

Reference Signs List 101 wireless receiver 102 A / D converter 103 synchronization / GI removal unit 104 first FFT unit 105 switching unit 106 complex division unit 107 carrier interpolation / extrapolation unit 108 IFFT unit 109 (first) time filter unit 110 second FFT unit 111 Propagation path correction unit 112 Demodulation unit 113 Control unit 121 Pilot code generation unit 122 Input switching unit 123 Null carrier insertion unit 124 IFFT unit 125 GI addition unit 126 D / A conversion unit 127 Radio transmission unit 128 Modulation unit 129 Control unit 130 For synchronization Code generation unit 201 Frame 202 Symbol for synchronization 203 Pilot symbol 204 Data symbol group 205 Guard interval 303 Null carrier 501 Radio reception unit 502 Filter / frequency conversion unit 503 A / D conversion unit 504 Synchronization / GI removal unit 505 First FFT unit 506 Division unit 507 IFFT unit 508 Time filter unit 509 Second FFT unit 510 Propagation path correction unit 511 Demodulation unit 512 Control unit 513 Offset detection unit 514 Radio reception unit 515 First frequency conversion unit 516 Frequency filter unit 517 Second frequency conversion unit 1201 Uplink Transmission unit 1202 Downlink reception unit 1203 Control unit 2101 Carrier separation unit 2102 First carrier interpolation / extrapolation unit 2103 Second carrier interpolation / extrapolation unit 2104 Carrier synthesis unit 2105 Second time filter unit 2106 Phase rotation unit 2107 Third FFT unit 2108 Demodulation section 2109 Control section 2201 Phase rotation section 2202 Input switching section 2203 Modulation section 2204 Null carrier insertion section 2205 IFFT section 2206 GI addition section 2207 D / A conversion section 2208 Radio transmission section 2209 Control section

Claims (10)

A receiver for receiving an OFDM signal,
A first DFT unit that converts a predetermined band including a signal band of a received signal into a signal represented on the frequency axis;
A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission;
A carrier interpolation unit that replaces a value of a point corresponding to at least one null carrier on the frequency axis with a value calculated using at least one point other than the point;
An IDFT unit that converts an output signal of the carrier interpolation unit into a signal represented on a time axis;
A time filter unit for attenuating or deleting part of the output signal of the IDFT unit;
A second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis,
A receiver for performing propagation path correction using an output signal of the second DFT unit. A receiver for receiving an OFDM signal,
A frequency converter for converting the frequency of the received signal;
A frequency filter unit that extracts an arbitrary band in the signal band; and
A first DFT unit that converts an output signal of the frequency conversion unit into a signal represented on a frequency axis;
A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission;
A carrier interpolation unit that replaces the value of the center point of the arbitrary band with a value calculated using at least one point other than the point;
An IDFT unit that converts an output signal of the carrier interpolation unit into a signal represented on a time axis;
A time filter unit for attenuating or deleting part of the output signal of the IDFT unit;
A second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis,
A receiver for performing propagation path correction using an output signal of the second DFT unit.   The offset detection unit for detecting a DC offset amount at the center of the arbitrary band based on an output signal of the complex division unit and an output signal of the second DFT unit. Receiver. A receiver for receiving an OFDM signal,
A first DFT unit that converts a predetermined band including a signal band of a received signal into a signal represented on the frequency axis;
A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission;
A carrier interpolation unit that replaces a value of a point corresponding to at least one null carrier dynamically allocated according to a propagation path condition with a value calculated using at least one point other than the point;
An IDFT unit that converts an output signal of the carrier interpolation unit into a signal represented on a time axis;
A time filter unit for attenuating or deleting part of the output signal of the IDFT unit;
A second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis,
A receiver for performing propagation path correction using an output signal of the second DFT unit.   When there is a point corresponding to a known null carrier at a point other than the central point of the arbitrary band, the carrier interpolation unit, the point corresponding to the central point of the arbitrary band and the known null carrier 3. The receiver according to claim 2, wherein a value calculated using at least one point other than the above replaces the value of the central point of the arbitrary band and the value of the point corresponding to the known null carrier. .   The carrier interpolation unit, when there is a point corresponding to a known null carrier at a point other than the point corresponding to at least one null carrier dynamically allocated according to the propagation path state, And a value calculated using at least one point other than the point corresponding to at least one null carrier and the point corresponding to the known null carrier dynamically allocated according to the 5. The receiver according to claim 4, wherein the value of the point corresponding to at least one null carrier assigned to the target and the value of the point corresponding to the known null carrier are replaced. A receiver for receiving an OFDM signal,
A first DFT unit that converts a predetermined band including a signal band of a reception signal in which pilot symbols are CI-multiplexed into a signal represented on the frequency axis;
A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission;
A carrier separation unit that performs grouping according to a combination of phase rotation codes used when CI multiplexing is performed on the output side of the complex division unit;
If any group divided by the carrier separation unit includes the central point of the signal band, the value of the point is calculated using at least one point other than the point included in the same group A carrier interpolation unit to replace with a value;
A carrier synthesizing unit that rearranges the plurality of output signals of the carrier interpolation unit into a state before being input to the carrier separation unit;
An IDFT unit that converts an output signal of the carrier combining unit into a signal represented on a time axis;
A time filter unit for attenuating or deleting part of the output signal of the IDFT unit;
A second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis,
A receiver for performing propagation path correction using an output signal of the second DFT unit. A receiver for receiving an OFDM signal,
A first DFT unit that converts a predetermined band including a signal band of a reception signal in which pilot symbols are CI-multiplexed into a signal represented on the frequency axis;
A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission;
A carrier separation unit that performs grouping according to a combination of phase rotation codes used when CI multiplexing is performed on the output side of the complex division unit;
When the point corresponding to the null carrier in the signal band is included in at least one group divided by the carrier separation unit, the value of the point is set to at least one point other than the point included in the same group. A carrier interpolation unit that replaces the calculated value using
A carrier synthesizing unit that rearranges the plurality of output signals of the carrier interpolation unit into a state before being input to the carrier separation unit;
An IDFT unit that converts an output signal of the carrier combining unit into a signal represented on a time axis;
A time filter unit for attenuating or deleting part of the output signal of the IDFT unit;
A second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis,
A receiver for performing propagation path correction using an output signal of the second DFT unit. A receiver for receiving an OFDMA signal,
A frequency converter for converting the frequency of the received signal;
A frequency filter unit that extracts an arbitrary subchannel in the signal band; and
A first DFT unit that converts an output signal of the frequency conversion unit into a signal represented on a frequency axis;
A complex division unit that performs complex division on the signal converted by the first DFT unit using a code used at the time of transmission;
A carrier interpolation unit that replaces the value of the center or both ends of the arbitrary subchannel with a value calculated using at least one point other than the point;
An IDFT unit that converts an output signal of the carrier interpolation unit into a signal represented on a time axis;
A time filter unit for attenuating or deleting part of the output signal of the IDFT unit;
A second DFT unit for converting the output signal of the time filter unit into a signal represented on the frequency axis,
A receiver for performing propagation path correction using an output signal of the second DFT unit. A channel estimation method for estimating a channel from a received OFDM signal,
A first step of converting a predetermined band including a signal band of the received signal into a signal represented on the frequency axis;
A second step of performing complex division on the output signal after the first step by a code used at the time of transmission;
A third step of replacing the value of the point corresponding to at least one null carrier on the frequency axis with the value calculated using at least one point other than the point for the output signal after the second step;
A fourth step of converting the output signal after the third step into a signal represented on a time axis;
A fifth step of attenuating or deleting part of the output signal after the fourth step;
And a sixth step of converting the output signal after the fifth step into a signal represented on the frequency axis.

Images