JP3688697B2 - OFDM transmission method, OFDM transmitter and OFDM receiver - Google Patents

Description

  The present invention relates to an OFDM transmission method, an OFDM transmitter, and an OFDM receiver that improve equalization performance.

  In recent years, high-quality digital modulation with high wave number utilization efficiency has been developed for transmission of video signals or audio signals. In particular, in mobile communications, the adoption of orthogonal frequency division multiplex (hereinafter referred to as OFDM [orthogonal frequency division multiplex]) modulation that is resistant to multipath interference is being studied. In addition, digital television (TV) broadcasting using OFDM has been studied. This OFDM is described in detail in Non-Patent Document 1 and the like.

  OFDM is a system in which transmission digital data is distributed over a large number (approximately 256 to 1024) of carrier waves (hereinafter referred to as subcarriers) orthogonal to each other and modulated. In addition to the feature that OFDM is less susceptible to multipath interference, it has the advantage of high frequency utilization efficiency and is less likely to interfere with others.

  Each subcarrier of OFDM is modulated by symbol data such as QPSK or multilevel QAM. This modulation is performed by an inverse fast Fourier transform (hereinafter referred to as IFFT) circuit, and one symbol (hereinafter also referred to as an OFDM symbol) of an OFDM modulated wave is obtained by performing an IFFT operation on N symbol data for each subcarrier. Created.

  When transmitting an OFDM symbol, a transmission frame is composed of a plurality of OFDM symbols in consideration of error correction on the receiving side, and an OFDM symbol (hereinafter referred to as a reference symbol) that serves as a reference for correcting transmission path characteristics in units of frames. ) Is inserted and transmitted.

  FIG. 8 is an explanatory diagram for explaining the transmission method in units of frames, and is described in Non-Patent Document 2.

  As shown in FIG. 8, an OFDM transmission frame is composed of a set of data in the frequency direction (carrier unit) and the time direction (OFDM symbol unit). Each time slot of the frame represents each OFDM symbol. The number of subcarriers in one OFDM symbol is 448, and the number of OFDM symbols in one frame is 300. Note that the number of subcarriers is determined by the number of points in the IFFT circuit to be used. In FIG. 8, one frame is composed of 448 data in the frequency direction and 300 data in the time direction.

  The OFDM symbol in the first time slot of the frame is a null symbol for reception synchronization, and is created by modulating all 448 subcarriers with symbol data having an amplitude of 0. The second time slot is a reference symbol serving as a phase reference for each subcarrier, and the OFDM symbol in the third time slot is fixed data for transmission control. Information symbol data is transmitted by OFDM symbols (information symbols) in the fourth and subsequent time slots.

  Information symbol data may be interleaved in the frequency direction and the time direction. Interleaving can prevent continuous data errors even when specific frequency slots and time slots are disturbed, and increases the possibility that data can be restored by error correction on the receiving side.

  Next, an OFDM equalization technique will be described.

  In OFDM transmission, a guard period is provided to prevent intersymbol interference due to multipath. FIG. 9 is an explanatory diagram for explaining a guard period of an OFDM signal. In FIG. 9, an OFDM signal with one subcarrier is shown to simplify the description.

  As shown in FIG. 9, a signal of one symbol of OFDM is composed of a guard period and an effective symbol period. The guard period is formed by cyclically copying the signal in the latter half of the effective symbol period. When the delay time of multipath interference is within the guard period, it is possible to prevent intersymbol interference due to delayed adjacent symbols by demodulating only the signal in the effective symbol period at the time of demodulation.

  FIG. 10 is an explanatory diagram for explaining equalization by such a guard period.

  Now, it is assumed that the delay time of the signal waveform of the multipath interference wave is within the guard period as shown in FIG. 10B with respect to the signal waveform of the direct wave of the OFDM symbol shown in FIG. On the receiving side, a sum signal of the OFDM symbol of FIG. 10A and the interference wave of FIG. 10B is obtained (FIG. 10C). In this case, since the delay time of the interference wave is within the guard period, the received signal (sum signal) in the effective symbol period has a phase and amplitude changed at the same frequency as the OFDM symbol. That is, when the OFDM symbol of FIG. 10 (a) is, for example, modulated by the symbol data S0 of FIG. 11, demodulating the received signal, for example, can obtain the symbol data S1 of FIG. .

  That is, by demodulating a reference symbol whose phase and amplitude are known in advance and correcting the demodulated symbol data so as to cancel out the change (offset) of the phase and amplitude between the original symbol data and the demodulated symbol data, Data transmission that is not affected by path interference is possible. Since the influence of multipath interference differs for each subcarrier, the reference symbol is set for each subcarrier. Note that the offset correction can be realized by one complex multiplier by switching the correction coefficient for each subcarrier.

  However, depending on the reception state, there may be a multipath whose delay time exceeds the guard period. If the multipath delay time exceeds the guard period, the demodulated symbols cannot be equalized even if the reference symbols are used. FIG. 12 is an explanatory diagram for explaining this problem.

  As shown in FIGS. 12A and 12B, when the delay time of the multipath interference wave exceeds the guard period, the latter half of the previous OFDM symbol adjacent to the effective symbol period of the predetermined OFDM symbol becomes a multipath. In addition, intersymbol interference occurs in the demodulated output. Therefore, even if the reference symbol is used, the multipath cannot be completely removed. Furthermore, when the multipath delay time exceeds the guard period, the reference symbol for equalization also deteriorates due to the intersymbol interference, so that the deterioration of the demodulated symbol after equalization becomes large. Since the reference symbol is transmitted only once per frame, there is a problem that the demodulated signal is extremely deteriorated when the reference symbol is disturbed.

Although the equalization range can be expanded by setting the guard period longer, there is a drawback that the transmission rate is lowered. In the example of FIG. 12, if one symbol period is 80 μs, the guard period is set to 16 μs.
Mobile digital audio broadcasting using OFDM (issued by NHK, VIEW May 1993) Le Floch et al, "Digital Sound Broadcasting to Mobile Receivers", IEEE Transactios on Consumer Electronics, Vol.35, No.3, pp.493-530 1989.

  Thus, conventionally, when a multipath having a delay time exceeding the guard period occurs, there is a problem that the demodulated signal is significantly deteriorated.

  The present invention has been made in view of such a problem. By transmitting a reference symbol with a waveform of two or more symbols continuous in an OFDM transmission frame, degradation of demodulated symbols after equalization and reference symbols are reduced. An object of the present invention is to provide an OFDM transmission method, an OFDM transmission apparatus, and an OFDM reception apparatus that can suppress adverse effects caused by interference.

The OFDM transmitter according to claim 1 of the present invention is configured to configure the reception synchronization symbol data, reference data, and information symbol data so as to form a transmission frame based on reception synchronization symbol data, reference data, and information symbol data. A transmission frame composed of a synchronization symbol, a reference symbol and an information symbol for reception synchronization is created by performing orthogonal frequency division multiplex modulation on a plurality of subcarriers using data from the arrangement means and data from the arrangement means The reference symbol is equivalently arranged in the transmission frame as one reference symbol by two reference symbols having continuous waveforms,
In the OFDM transmission method according to claim 2 of the present invention, the reception synchronization symbol data, reference data, and information symbol data are configured so as to form a transmission frame based on reception synchronization symbol data, reference data, and information symbol data. And a transmission frame composed of a synchronization symbol for reception synchronization, a reference symbol, and an information symbol by performing orthogonal frequency division multiplexing modulation on a plurality of subcarriers using the data arranged by the arrangement procedure. And the reference symbol is equivalently arranged in the transmission frame as one reference symbol by two reference symbols having a continuous waveform, and
In the OFDM receiver according to claim 3 of the present invention, a synchronization symbol and waveform for reception synchronization are obtained by orthogonal frequency division multiplexing modulation using a plurality of subcarriers for symbol data for reception synchronization, reference data, and information symbol data. A transmission frame composed of a reference symbol and an information symbol that are equivalently arranged as one reference symbol by two consecutive reference symbols is created and transmitted, and this transmission frame is subjected to orthogonal frequency division multiplexing demodulation to obtain information symbol data and Demodulating means for obtaining reference data, detecting means for detecting amplitude and phase shift of each subcarrier using demodulated outputs for the two or more consecutive reference symbols among demodulated outputs of the demodulating means, Corrects the amplitude and phase shift of the information symbol data based on the detection result It is characterized in that it has and a that correction means.

  In the present invention, two or more reference symbols are continuously transmitted in one transmission frame while a waveform is continuous between adjacent reference symbols. Will be arranged. Thereby, even when a multipath having a long delay time occurs, the equalization performance on the reception side can be improved, and the deterioration of the demodulated symbol is suppressed.

  As described above, according to the present invention, when a reference symbol is transmitted by continuing a waveform of two or more symbols in an OFDM transmission frame, the demodulated symbol deteriorates after equalization and the reference symbol is disturbed. It has the effect that the adverse effect of can be suppressed.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view showing an embodiment of the OFDM transmission method according to the present invention. In FIG. 1, each time slot from 1 to M indicates each OFDM symbol.

  The number of subcarriers in one OFDM symbol is N. An OFDM transmission frame is composed of M OFDM symbols. That is, one transmission frame is composed of N data in the frequency direction and M data in the time direction.

  As shown in FIG. 1, the first time slot of a transmission frame is a null symbol for reception synchronization, and symbol data having an amplitude of 0 is assigned to all subcarriers. In the present embodiment, the reference symbol A and the reference symbol B are allocated to the second and third time slots as equalization reference signals for each subcarrier. Information symbols are assigned after the fourth time slot.

  FIG. 2 is an explanatory diagram showing a specific method for transmitting the reference symbols A and B in FIG. In FIG. 2, for simplicity of explanation, only one subcarrier waveform is shown.

  The reference symbol A has a guard period G1 and an effective symbol period R1. The guard period G1 is a copy of the waveform of the latter half of the effective symbol period R1. The reference symbol B has a guard period G2 and an effective symbol period R2. The guard period G2 is a copy of the waveform of the latter half of the effective symbol period R2. In the present embodiment, as shown in FIG. 2, the amplitude and phase of the waveforms of the effective symbol periods R1 and R2 are set so that the waveform of the reference symbol B continues to the waveform of the end of the reference symbol A. ing. Thus, these two reference symbols A and B can be regarded as one reference symbol having an equivalently longer guard period.

  By the way, since each subcarrier satisfies the orthogonality condition, the wave number of each subcarrier becomes an integer in the effective symbol period. For example, the subcarrier shown in FIG. 2 indicates that the wave number of the effective symbol period is 2. Therefore, in order to make the reference symbol B continue to the reference symbol A, it is only necessary to shift the start phases of the reference symbols A and B by the phase corresponding to the length of the guard period. For example, in FIG. 2, since the lengths of the guard periods G1 and G2 are 1/4 of the effective symbol periods R1 and R2, the start phases of the reference symbols A and B may be shifted by 180 degrees.

  Now, as shown in FIG. 3, the carrier numbers k of the N subcarriers of the OFDM symbol are set to −N / 2 to 0 to (N / 2) −1 in order of frequency. Further, the length of the guard period when the length of the effective symbol period is 1 is assumed to be x. Then, the phase difference Δθk for the guard period in the kth subcarrier is expressed by the following equation (1).

Δθk = x · k · 2π (1)
When the phases of the reference symbols A and B in the k-th carrier are θ1k and θ2k, respectively, in order to make the two reference symbols A and B continuous, it is necessary to satisfy the following equation (2).

θ1k = θ2k−Δθk
= Θ2k−x · k · 2π (2)
The amplitudes of the reference symbols A and B are made equal.

  For example, FIG. 4 shows a subcarrier having a wave number of 1 in the effective symbol period. That is, k = 1. In addition, the interval of the guard period is set to 1/4 of the effective symbol period. Therefore, when these conditions are substituted into the above equation (2), the following equation (3) is obtained.

θ1k = θ2k-1 / 4 ・ 1 ・ 2π
= Θ2k−π / 2 (3)
As is apparent from the equation (3), in this case, the phase of the reference symbol B may be advanced by 90 degrees with respect to the reference symbol A (see FIG. 4).

  Next, the operation of the OFDM transmission method of this embodiment will be described.

  One OFDM symbol has N subcarriers. Each subcarrier is modulated by, for example, 0 symbol data, predetermined reference data, or information symbol data. A transmission frame is composed of M OFDM symbols. An OFDM symbol (null symbol) modulated with 0 symbol data is transmitted in the first time slot of the transmission frame. Each time slot is composed of a guard period and an effective symbol period, and the waveform of the guard period is a copy of the latter half of the effective symbol period.

  In the next second time slot, a reference symbol A in which a subcarrier is modulated by predetermined reference data is transmitted. In the next third time slot, the reference symbol B is transmitted. The phase of the reference symbol B satisfies the above expression (3) with respect to the phase of the reference symbol A. As a result, the waveforms of the guard period G2 and effective symbol period R2 of the reference symbol B are transmitted in succession to the waveforms of the guard period G1 and effective symbol period R1 of the reference symbol A for each subcarrier. That is, the guard period of the reference symbol B is equivalently increased by the guard period G1 + the effective symbol period R1 + the guard period G2. In the 4th to Mth time slots, information symbols modulated by the information symbol data are transmitted.

  Assume that the delay time of the multipath interference wave is longer than the guard period. However, even in this case, if the delay time is shorter than the guard period G1 + the effective symbol period R1 + the guard period G2, the multipath interference wave in the effective symbol period R2 of the reference symbol B has the same waveform as the effective symbol period R2. It becomes. Accordingly, the received wave in this case has a phase and amplitude that are changed at the same frequency as that of the reference symbol B. Since the phase and amplitude of the reference symbol B are known, it is possible to reproduce reference data that is not affected by multipath by correcting the demodulated signal by the amount of change.

  Next, the demodulated reference data is used to equalize demodulated symbols of information symbols for each subcarrier. Since the reference signal is not affected by multipath, it is possible to suppress the deterioration of the demodulated symbol.

  Thus, in this embodiment, the reference symbols are transmitted in two consecutive time slots, and the two reference symbols are set to be continuous by adjusting the phase and amplitude of these reference symbols. Equivalently, the guard period can be sufficiently extended. Therefore, the reference symbol can be demodulated without being affected by multipath, and deterioration of the demodulated symbol due to multipath can be suppressed.

  Since the guard period G2 and the effective symbol period R2 of the reference symbol B can be used as the guard period of the reference symbol A, there is an advantage that it is possible to cope with the previous ghost.

  FIG. 5 is a block diagram showing an embodiment of the OFDM transmitter according to the present invention.

  2-bit information data input via the input terminal 1 is applied to the S / P conversion circuit 2. The S / P conversion circuit 2 converts the serial data into parallel data and supplies it to the symbol mapping circuit 3. The symbol mapping circuit 3 converts information data into information symbol data (I data, Q data) by, for example, PSK modulation or QAM modulation. Information symbol data from the symbol mapping circuit 3 is applied to the multiplex circuit 4.

  The multiplex circuit 4 is also provided with the outputs of ROMs 5, 6 and 7. ROMs 5, 6 and 7 store 0 symbol data, reference data A and reference data B, respectively. The reference data A and B are set so that the waveforms of OFDM symbols (reference symbols A and B) created based on the reference data A and B are continuous. The multiplex circuit 4 selects the output of the symbol mapping circuit 3 and the outputs of the ROMs 5 to 7 corresponding to each time slot of the transmission frame and outputs it to the interleave circuit 8. The interleave circuit 8 interleaves the input data and outputs it to the IFFT circuit 9.

  The IFFT circuit 9 performs an IFFT process using the input N pieces of data, thereby creating an OFDM modulated wave (OFDM symbol) and outputting it to the guard period adding circuit 10.

  The guard period addition circuit 10 adds a guard period to the OFDM modulated wave in order to reduce the influence of multipath, and outputs I-axis and Q-axis signals to the D / A converters 11 and 12, respectively. The D / A converters 11 and 12 convert the input digital signals into analog signals and output the analog signals to low-pass filters (hereinafter referred to as LPF) 13 and 14. The LPFs 13 and 14 remove the harmonic components of the input signal output, respectively, and output them to the multipliers 18 and 19 of the quadrature modulation circuit 15.

  The quadrature modulation circuit 15 includes a local oscillator 16, a phase shifter 17, multipliers 18 and 19, and an adder 20. The local oscillator 16 generates a carrier having a predetermined frequency and outputs it to the phase shifter 17 and outputs it to the multiplier 18 as an in-phase axis carrier. The phase shifter 17 shifts the oscillation output of the local oscillator 16 by 90 degrees to create an orthogonal axis carrier and outputs it to the multiplier 19. The multiplier 18 obtains an in-phase axis modulation output by multiplication of the output of the LPF 13 and the in-phase axis carrier, and outputs it to the adder 20. The multiplier 19 obtains an orthogonal axis modulation output by multiplication of the output of the LPF 14 and the orthogonal axis carrier, and outputs it to the adder 20. The adder 20 adds the outputs of the multipliers 18 and 19 and outputs an orthogonal modulation output to the frequency conversion circuit 21.

  The frequency conversion circuit 21 includes a band-pass filter (hereinafter referred to as BPF) 22, an amplifier 23, a multiplier 24, a local oscillator 25, and a BPF 26. The quadrature modulation output is supplied to the BPF 22, and the BPF 22 band-limits the quadrature modulation output to the multiplier 24 via the amplifier 23. The local oscillator 22 gives a local oscillation output of a predetermined frequency for frequency conversion to the intermediate frequency band to the multiplier 24. The multiplier 24 multiplies the output of the amplifier 23 and the local oscillation output to convert the quadrature modulation output of the OFDM modulated wave into a high frequency band and outputs it to the BPF 26. The BPF 26 band-limits the output of the multiplier 24 and outputs it as an RF signal.

  Note that a clock is supplied to the timing circuit 28 via the input terminal 27. The timing circuit 28 generates a clock having a predetermined frequency based on the input clock, and generates various timing signals and supplies them to each circuit.

  Next, the operation of the embodiment configured as described above will be described.

  The information data input through the input terminal 1 is converted into parallel data by the S / P converter 2 and then converted into information symbol data composed of I data and Q data by the symbol mapping circuit 3. Information symbol data from the symbol mapping circuit 3 is supplied to the multiplex circuit 4. The multiplex circuit 4 also receives 0 symbol data, reference data A, and reference data B from the ROMs 5, 6, and 7, respectively. The multiplex circuit 4 receives information corresponding to each time slot of the transmission frame of FIG. Symbol data, 0 symbol data, and reference data A and B are switched and applied to the interleave circuit 8. The interleave circuit 8 interleaves the input data and outputs it to the IFFT circuit 9.

  Note that a clock is input to the input terminal 27, and the timing circuit 28 generates a clock having a predetermined frequency and various timing signals based on this clock and supplies them to each circuit.

  The IFFT circuit 9 performs an IFFT operation using the input N symbols. As a result, OFDM modulation is performed on the N symbols, and the real part Re and the imaginary part Im of the OFDM modulated wave are output to the guard period addition circuit 10.

  When the multiplex circuit 4 selects the ROM 5, a null symbol is output from the IFFT circuit 9. Further, when the multiplex circuit 4 selects the ROMs 6 and 7, the reference symbols A and B are output from the IFFT circuit 9, respectively. Similarly, when the multiplex circuit 4 selects the output of the symbol mapping circuit 3, an information symbol is obtained. When the multiplex circuit 4 performs selection corresponding to the time slot of the transmission frame in FIG. 1, the IFFT circuit 9 outputs the transmission frame shown in FIG.

  The guard period adding circuit 10 adds a guard period for reducing the influence of multipath to the real part Re and the imaginary part Im of the input OFDM modulated wave, and outputs them to the D / A converters 11 and 12. . The reference data A and B are set based on the subcarrier frequency and the length of the guard period, and the guard period and effective symbol period of the reference symbol B are continuous with the waveforms of the guard period and effective symbol period of the reference symbol A. The waveform continues.

  The D / A converters 11 and 12 convert the input signals into analog signals and apply them to the LPFs 13 and 14, respectively. The LPFs 13 and 14 band-limit the OFDM symbols to the multipliers 18 and 19 of the orthogonal modulation circuit 15, respectively. Output. The local oscillator 16 generates a carrier having a predetermined frequency, and the phase shifter 17 shifts the carrier by 90 degrees. The oscillation output from the local oscillator 16 is supplied to the multiplier 18 as an in-phase axis carrier, and the multiplier 18 obtains an in-phase axis modulation output by multiplication of the in-phase axis carrier and the output of the LPF 13 and outputs it to the adder 20. The multiplier 19 obtains an orthogonal axis modulation output by multiplication of the orthogonal axis carrier from the phase shifter 17 and the output of the LPF 14 and outputs it to the adder 20. The outputs of the multipliers 18 and 19 are added by an adder 20 to obtain an orthogonal modulation output.

  The quadrature modulation output of the quadrature modulation circuit 15 is band-limited by the BPF 22 of the frequency conversion circuit 21, amplified by the amplifier 23, and supplied to the multiplier 24. The multiplier 24 is supplied with a local oscillation output from the local oscillator 25, and the multiplier 24 frequency-converts the quadrature modulation output into a high frequency (RF) band signal. The output of the multiplier 24 is band-limited by the BPF 26 and output as an RF signal.

  Thus, in this embodiment, when a transmission frame is configured, the reference symbol A is transmitted in the second time slot, and then the reference symbol B is transmitted in the third time slot. The reference data A and B for modulating the subcarriers of the reference symbols A and B are set based on the subcarrier frequency and the length of the guard period by the guard period addition circuit 10, and the reference symbols A and B are The waveform is continuous for each subcarrier.

  FIG. 6 is a block diagram showing an embodiment of the OFDM receiving apparatus according to the present invention.

  An OFDM modulated wave (OFDM symbol) having N subcarriers is input to the input terminal 31 in units of transmission frames. A null symbol is transmitted in the first time slot of the transmission frame, a reference symbol A is transmitted in the second time slot, and a reference symbol B is transmitted in the third time slot. Information symbols are transmitted in the 4th to Mth time slots. The reference symbols A and B have a continuous waveform. These OFDM symbols are subjected to orthogonal modulation at a predetermined carrier frequency and then frequency-converted to an RF band and input.

  This OFDM symbol in the RF band is supplied to the BPF 32. The BPF 32 removes out-of-band noise and outputs it to the amplifier 33. The amplifier 33 amplifies the output of the BPF 32 and outputs it to the multiplier 34. The local oscillator 35 outputs a local oscillation output for frequency conversion of the RF band signal to the multiplier 34. The multiplier 34 multiplies the output of the amplifier 33 by the local oscillation output and converts the frequency of the OFDM modulated wave to a predetermined frequency and outputs it to the BPF 36. The BPF 36 removes the harmonic component of the OFDM symbol and outputs it to the variable gain amplifier 37.

  The variable gain amplifier 37 is controlled by an envelope detection circuit 50, which will be described later, and converts the output of the BPF 36 into a constant amplitude and outputs it to the quadrature detection circuit 38. The quadrature detection circuit 38 includes multipliers 39 and 40, a local oscillator 41, and a phase shifter 42. The local oscillator 41 is controlled by a carrier recovery circuit 43, which will be described later, to generate a playback carrier having a predetermined frequency and output it to the multiplier 39 and the phase shifter. The output of the local oscillator 41 is supplied to the multiplier 39 as an in-phase axis carrier. The phase shifter 42 shifts the local oscillation output by 90 degrees, regenerates the orthogonal axis carrier, and supplies it to the multiplier 40. The multiplier 39 multiplies the output of the variable gain amplifier 37 by the in-phase axis carrier to obtain the in-phase axis detection output, and the multiplier 40 multiplies the output of the variable gain amplifier 37 by the orthogonal axis carrier to obtain the quadrature axis detection output. The outputs of the multipliers 39 and 40 are supplied to LPFs 44 and 45, respectively.

  The LPFs 44 and 45 limit the band of the real part Re and the imaginary part Im of the baseband OFDM modulated wave, which is the quadrature detection output of the multipliers 39 and 40, respectively, and output them to the A / D converters 46 and 47. The A / D converters 46 and 47 convert the outputs of the LPFs 44 and 45 into digital signals and output them to the guard period removal circuit 48, respectively. The guard period removal circuit 48 removes the guard period signal and outputs it to the FFT circuit 49. The FFT circuit 49 performs OFDM demodulation by performing an FFT process on the OFDM modulated wave from the guard period removal circuit 48, so that 0 symbol data, reference data A and B, and I of the information symbol data are generated from N subcarriers. Get Q data. The I data and Q data from the FFT circuit 49 are output to the equalization circuit 55 and also output to the carrier reproduction circuit 43.

  The carrier recovery circuit 43 is supplied with demodulated I data and Q data, recovers the carrier, and outputs it to the local oscillator 41 of the quadrature detection circuit 38. Thereby, carrier synchronization is obtained based on the oscillation output of the local oscillator 41. The outputs of the A / D converters 46 and 47 are also input to the synchronous reproduction circuit 51. The synchronous reproduction circuit 51 reproduces a signal for frame synchronization and symbol synchronization by detecting a null symbol from the outputs of the A / D converters 46 and 47 and outputs it to the timing circuit 52. The timing circuit 52 generates a clock having a predetermined frequency and a timing signal to each circuit based on the input signal. The outputs of the A / D converters 46 and 47 are also supplied to the envelope detection circuit 50. The envelope detection circuit 50 detects the envelopes of the outputs of the A / D converters 46 and 47 and adjusts the gain of the variable gain amplifier 37 so that a constant amplitude can be obtained.

  The equalization circuit 55 includes a reference data detection circuit 56, an error detection circuit 57, a ROM 58, a complex multiplier 59, a memory 60, and an averaging circuit 61. The demodulated output of the FFT circuit 49 is input to the reference data detection circuit 56. The reference data detection circuit 56 detects the reference data B from the demodulated symbol and outputs it to the error detection circuit 57. Data stored in the ROM 58 is also given to the error detection circuit 57. The ROM 58 stores the same data as the reference data B on the transmission side, and the error detection circuit 57 calculates the amplitude and phase error of the demodulation reference data B due to the amplitude and phase shift of each subcarrier generated by multipath and noise. Detect and output to the averaging circuit 61. The averaging circuit 61 averages the output of the error detection circuit 57 over several frames, thereby removing an error due to noise and improving the S / N, and outputting an error due to multipath to the memory 60 as an error signal. . The memory 60 stores the error signal for each subcarrier and outputs the stored error signal to the complex multiplier 59. The complex multiplier 59 performs complex multiplication of the demodulated symbol from the FFT circuit 49 and the error signal from the memory 60, thereby correcting an error due to multipath and outputting it to the deinterleave circuit 62.

  The deinterleave circuit 62 deinterleaves the equalized demodulated symbols and outputs them to the demultiplex circuit 63. The demultiplexing circuit 63 demultiplexes the input demodulated symbols and outputs information symbol data to the symbol demapping circuit 64. The symbol demapping circuit 64 demodulates the information symbol data and outputs the information data to the P / S converter 65. The P / S converter 65 converts the input parallel data into 2-bit serial data and outputs it. It is like that.

  Next, the operation of the embodiment configured as described above will be described.

  The RF signal input through the input terminal 31 is subjected to removal of out-of-band noise by the BPF 32, amplified by the amplifier 33, and then supplied to the multiplier 34. The multiplier 34 multiplies the local oscillation output from the local oscillator 35 by frequency conversion of the RF band signal to a predetermined frequency, and outputs it to the BPF 36. After the harmonic component is removed by the BPF 36, the output of the multiplier 34 is controlled to have a constant amplitude by the variable gain amplifier 37 and is supplied to the multipliers 39 and 40 of the quadrature detection circuit 38.

  The multiplier 39 obtains the in-phase detection output by multiplication with the in-phase regenerated carrier from the local oscillator 41 and outputs it to the LPF 44, and the multiplier 40 performs quadrature axis detection by the multiplication with the orthogonal regenerative carrier from the phase shifter 42. Obtain output and output to LPF45. Thus, a baseband OFDM modulated wave is obtained from the quadrature detection circuit.

  The in-phase detection axis output and the quadrature detection axis output from the quadrature detection circuit 38 are band-limited by the LPFs 44 and 45, respectively, and then converted into digital signals by the A / D converters 46 and 47, respectively. In the OFDM modulated wave converted into the digital signal, the signal in the guard period is removed by the guard period removing circuit 48, and only the signal in the effective symbol period is given to the FFT circuit 49. The FFT circuit 49 obtains N demodulated symbols by performing FFT processing on the OFDM modulated wave. The demodulated symbol is provided to the equalization circuit 55.

  The outputs of the A / D converters 46 and 47 are also input to the synchronous reproduction circuit 51, and the synchronous reproduction circuit 51 reproduces signals for frame synchronization and symbol synchronization. The output of the synchronous reproduction circuit 51 is supplied to a timing circuit 52. The timing circuit 52 generates a clock having a predetermined frequency and a timing signal to each circuit. The output of the FFT circuit 49 is also given to the carrier recovery circuit 43, and carrier synchronization is obtained by the local oscillator 41 outputting the local oscillation output based on the output of the carrier recovery circuit 43.

  The demodulated symbol from the FFT circuit 49 is supplied to the complex multiplier 59 and the reference data detection circuit 56 of the equalization circuit 55. The reference data detection circuit 56 detects the reference data B in the demodulated symbol and supplies it to the error detection circuit 57. On the other hand, the reference data B stored in advance in the ROM 58 is also input to the error detection circuit 57. The error detection circuit 57 detects an error signal (complex signal) indicating a deviation in amplitude and phase due to the influence of multipath for the reference data B in the demodulated symbol, and outputs it to the averaging circuit 61. The error signal is averaged over several frames by the averaging circuit 61, whereby the S / N of the error signal is improved and supplied to the memory 60.

  The memory 60 stores an error signal for each subcarrier and outputs it to the complex multiplier 59. The complex multiplier 59 complex-multiplies the output of the FFT circuit 49 and the error signal. Accordingly, the phase of the demodulated symbol is rotated by an amount corresponding to the error due to the multipath, the amplitude is corrected, and the demodulated symbol not affected by the multipath is output to the deinterleave circuit 62.

  Since the reference symbols A and B have a continuous waveform on the transmission side, the guard period is equivalently longer for the reference symbol B, and the reference data B is almost certainly not affected by multipath. Demodulated data can be obtained.

  The demodulated symbols from the equalization circuit 55 are deinterleaved by the deinterleave circuit 62, demultiplexed by the demultiplex circuit 63, and information symbol data is input to the symbol demapping circuit 64. The information symbol data is demodulated by the symbol demapping circuit 64, converted to 2-bit serial data by the P / S converter 65, and output as information data.

  In this way, in this embodiment, the reference data B whose guard period is equivalently longer is equalized by the equalization circuit, and the reference data B can be demodulated because it can be almost surely equalized. The effect of symbol multipath can be suppressed.

  FIG. 7 is a block diagram showing another embodiment of the present invention. This embodiment differs from the embodiment of FIG. 6 only in the equalization circuit. In FIG. 7, the same components as those in FIG.

  The demodulated symbol from the FFT circuit is supplied to the complex multiplier 59 and also to the reference data detection circuit 70. The reference data detection circuit 70 detects the reference data A and B from the demodulated symbols and outputs them to the error detection circuit 73 and the power detection circuit 71. In this embodiment, in addition to the ROM 58 for storing the reference data B, a ROM 74 for storing the same data as the reference data A is also provided. The error detection circuit 73 is supplied with reference data A and B from these ROMs 58 and 74.

  The power detection circuit 71 detects the power of all the subcarriers of the reference data A and B detected by the reference data detection circuit 70 and obtains the total. As a result, the power detection circuit 71 detects whether the reference data A and B have deteriorated due to, for example, impulse noise. The output of the power detection circuit 71 is supplied to the reference data selection circuit 72. The reference data selection circuit 72 compares the power detection result with a predetermined reference value to determine which reference data A or B should be used for equalization. A selection signal for selecting the reference data is output to the error detection circuit 73. For example, the reference data selection circuit 72 selects the reference data B when neither of the two reference data is disturbed, and is disturbed when one of the two reference data is disturbed. If no reference data is selected and neither of the two reference data is disturbed, no reference data is selected.

  The error detection circuit 73 reads the reference data designated by the selection signal from the ROMs 58 and 74, obtains an error from the reference data from the reference data detection circuit 70, and outputs it to the averaging circuit 71. If no reference data is selected by the reference data selection circuit 72, the error detection circuit 73 outputs the previous error detection result.

  Next, the operation of the embodiment configured as described above will be described.

  In the embodiment of FIG. 6, the reference data B is used for error detection in order to equalize the reference data even when there is a multipath whose delay time exceeds the guard period. However, if the multipath delay time is within the guard period, the same effect can be obtained even if the reference data A is used. Even if the reference data A is used when the multipath delay time exceeds the guard period, a certain degree of equalization is possible. For this reason, in this embodiment, when the reference data B is deteriorated, for example, when the reference data B is disturbed by impulse noise or the like, the reference data A is used instead of the reference data B, etc. It has become possible to perform.

  The reference data detection circuit 70 detects reference data A and reference data B from the demodulated symbols. The power detection circuit 71 calculates the total power of all subcarriers for the reference data A and B. The power detection result of the power detection circuit 71 is input to the reference data selection circuit 72 and compared with a predetermined reference value.

  Assume that the power detection result of the reference data B is larger than the reference value. In this case, a selection signal for selecting reference data B is output from the reference data selection circuit 72 to the error detection circuit 73. The error detection circuit 73 detects the data read from the ROM 58 and the reference data detection circuit 70. An error with respect to the reference data B is obtained. That is, in this case, the operation is the same as that of the embodiment of FIG.

  On the other hand, when the power detection result of the reference data B is smaller than the reference value and the power detection result of the reference data A is larger than the reference value, the reference data selection circuit 72 generates a selection signal for selecting the reference data A. Output. As a result, the error detection circuit 73 reads the data in the ROM 74 and obtains an error from the reference data A detected by the reference data detection circuit 70.

  Other operations are the same as those of the embodiment of FIG.

  As described above, in this embodiment, when the reference data B is disturbed by impulse noise or the like, equalization is performed using the reference data A, and an error signal is erroneously detected due to the impulse noise or the like. Prevents the equalization performance from deteriorating.

  In each of the above-described embodiments, an example in which two reference symbols are continuously transmitted has been described. However, it is obvious that three or more reference symbols may be continuously transmitted.

Explanatory drawing which shows one Example of the OFDM transmission method which concerns on this invention. Explanatory drawing for demonstrating an Example. Explanatory drawing for demonstrating the conditions on which two reference symbols are made continuous. Explanatory drawing for demonstrating the conditions on which two reference symbols are made continuous. The block diagram which shows one Example of the OFDM transmitter based on this invention. The block diagram which shows one Example of the OFDM receiver which concerns on this invention. The block diagram which shows the other Example of this invention. Explanatory drawing for demonstrating the conventional OFDM transmission method. Explanatory drawing for demonstrating a guard period. Explanatory drawing for demonstrating equalization. Explanatory drawing for demonstrating equalization. Explanatory drawing for demonstrating the problem of a prior art example.

Explanation of symbols

A, B ... Reference symbol Agent Patent attorney Susumu Ito

Claims (3)

Arrangement means for arranging data arrangement of the reception synchronization symbol data, reference data and information symbol data so as to constitute a transmission frame based on reception synchronization symbol data, reference data and information symbol data;
Modulation means for creating a transmission frame composed of a synchronization symbol for reception synchronization, a reference symbol, and an information symbol by performing orthogonal frequency division multiplex modulation on a plurality of subcarriers with data from the arrangement means,
The OFDM transmitter according to claim 1, wherein the reference symbols are equivalently arranged in the transmission frame as one reference symbol by two reference symbols having continuous waveforms. An arrangement procedure for performing data arrangement of the reception synchronization symbol data, reference data and information symbol data so as to constitute a transmission frame based on reception synchronization symbol data, reference data and information symbol data;
A modulation procedure for creating a transmission frame composed of a synchronization symbol for reception synchronization, a reference symbol, and an information symbol by performing orthogonal frequency division multiplexing modulation on a plurality of subcarriers using data arranged by this arrangement procedure. ,
The OFDM transmission method according to claim 1, wherein the reference symbols are equivalently arranged in the transmission frame as one reference symbol by two reference symbols having continuous waveforms. By the orthogonal frequency division multiplex modulation using a plurality of subcarriers for symbol data for reception synchronization, reference data and information symbol data, one reference symbol is equivalently equivalent to two synchronization symbols for reception synchronization and two reference symbols having continuous waveforms. A transmission frame composed of reference symbols and information symbols arranged as is generated and transmitted, and demodulation means for obtaining information symbol data and reference data by orthogonal frequency division multiplexing demodulation of the transmission frame;
Detecting means for detecting an amplitude and a phase shift of each subcarrier using a demodulated output with respect to the two or more consecutive reference symbols among the demodulated outputs of the demodulating means;
An OFDM receiving apparatus, comprising: correction means for correcting an amplitude and a phase shift of the information symbol data based on a detection result of the detection means.

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