JP3130716B2 - OFDM transmitter and OFDM receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[Object of the Invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM transmitting apparatus and an OFDM receiving apparatus which suppress interference with other communications and reduce the dynamic range of a circuit.

[0002]

2. Description of the Related Art In recent years, along with digitization in broadcasting or mobile communication, a digital modulation system has been developed. In particular, orthogonal frequency division multiplexing (hereinafter, OFDM)
(Orthogonal frequency division multiplex)
As described in the literature (published by NHK, VIEW magazine, May 1993, p. 16), modulation is a digital modulation method which is less susceptible to multipath distortion on a transmission line, which is a problem in digital transmission systems. Attention has been paid. OFDM modulation is a method of dispersing and modulating a plurality of carriers (hereinafter, referred to as subcarriers) orthogonal to each other.

FIG. 7 is an explanatory diagram showing a frequency spectrum of an OFDM modulated wave.

FIG. 7 shows an OFD using N subcarriers.
The M modulated wave is shown, and each subcarrier is arranged at equal intervals. By setting the frequency difference ΔF between adjacent subcarriers to an integral multiple of the reciprocal of the length Ts of the signal waveform, the OFDM modulated wave forms an orthogonal function system.
The spectrums of the modulation signals (not shown) corresponding to the respective subcarriers overlap each other. However, since the orthogonal condition is satisfied, it is possible to completely separate each subcarrier on the receiving side.

The orthogonal condition can be considered in the same manner as the Nyquist condition in which there is no intersymbol interference in the time domain. Regarding intersymbol interference, even if the response waveforms of the respective pulses overlap each other, if the Nyquist condition is satisfied, the intersymbol interference is completely eliminated by extracting the signal at an appropriate sampling timing. No reception is possible. What realizes the Nyquist condition in the frequency domain is the orthogonal condition in the OFDM modulation method. That is, O
In the FDM modulation method, unlike the frequency division multiplexing method, it is not necessary to set a guard band for preventing spectrum overlap between the modulated waves, and the frequency use efficiency can be improved.

FIG. 8 is a block diagram showing such a conventional OFDM transmitting / receiving apparatus.

The digital data to be transmitted is, for example, QP
It is an SK- or QAM-modulated signal. This input data is supplied to the error correction coding circuit 4 of the OFDM transmission device 1 via the input terminal 3. The error correction coding circuit 4 adds an error correction code to the input data and supplies the input data to the interleave circuit 5. The interleave circuit 5 rearranges the data and outputs it to the scrambler 6 so that, when a continuous burst error occurs during transmission, the error is converted into a plurality of discontinuous random errors on the receiving side. I do. This facilitates error correction on the receiving side.

[0008] The output of the interleave circuit 5 is randomized by a scrambler 6 so that the data does not have a specific continuous waveform during transmission, and then S / P (serial / serial).
(Parallel) conversion circuit 7. S / P conversion circuit 7
Converts the input data sequence into parallel data corresponding to the number of subcarriers of the OFDM modulation, and outputs the parallel data to the inverse FFT circuit 8. This parallel data is converted by an inverse fast discrete Fourier transform (hereinafter referred to as IFFT) circuit 8 into an IFF
The signal is subjected to T processing and modulated into an OFDM modulated wave that is a complex signal.

It is assumed that an OFDM modulated wave is represented by an I-axis signal corresponding to a real part of a complex expression and a Q-axis signal corresponding to an imaginary part. The I-axis signal from the inverse FFT circuit 8 is D / A
Multiplier 11 after being converted into an analog signal by converter 9
Given to. Further, the Q-axis signal from the inverse FFT circuit 8 is converted to an analog signal by the D / A converter 10 and then supplied to the multiplier 12. The multiplier 11 is supplied with a carrier in the intermediate frequency band from the local oscillator 13, modulates the I-axis signal in phase by multiplication, and outputs an intermediate frequency signal (hereinafter, referred to as an IF signal) to the adder 15. The carrier from the oscillator 13 is phase-shifted by 90 degrees by the phase shifter 14 and supplied to the multiplier 12, which quadrature-modulates the Q-axis signal and outputs the IF signal to the adder 15. The modulation outputs of the I-axis signal and the Q-axis signal are added by an adder 15, band-limited by a BPF 16 and provided to a mixing circuit 17. The mixing circuit 17 frequency-converts the orthogonally modulated OFDM modulated wave by multiplication with the local oscillation output from the local oscillator 18 into a high-frequency signal (hereinafter referred to as an RF signal), and outputs the signal to the output terminal 21 via the BPF 19 and the amplifier 20. Output.

The timing circuit 45 generates various timing signals and clocks by using an input clock input through the input terminal 44 and supplies the signals to the respective sections of the OFDM transmitting apparatus 1.

An RF output from an output terminal 21 is input to an input terminal 23 of the OFDM receiver 2 via a transmission line 22. This input RF signal is applied to a mixing circuit 26 via an amplifier 24 and a BPF 25. The mixing circuit 26 receives a local oscillation output of a predetermined channel from the local oscillator 27 and
Frequency conversion to F signal. The IF signal from the mixing circuit 26 is applied to multipliers 29 and 30 after being band-limited by the BPF 28.

The multiplier 29 receives the reproduced carrier from the local oscillator 31, detects the IF signal in phase, converts the baseband I-axis signal into an A / D converter 33 and a clock recovery circuit 35.
Output to The multiplier 30 receives the reproduced carrier whose phase has been shifted by 90 degrees by the phase shifter 32, performs orthogonal detection on the IF signal, and outputs a baseband Q-axis signal to the A / D converter 34.
The clock recovery circuit 35 generates a recovered clock necessary for the FFT operation or the like from the detection output. This reproduced clock is output from the output terminal 46 as an output lock. A / D converter 3
Numerals 3 and 34 use the reproduced clock reproduced by the clock reproducing circuit 35 to convert the I-axis signal or the Q-axis signal into digital signals and output them to the FFT circuit 36, respectively.

The FFT circuit 36 analyzes a frequency spectrum of the input digital signal and detects the phase and amplitude of each subcarrier to obtain an OFDM demodulated signal.
This OFDM demodulated signal is P / S (parallel / serial)
The data is converted into serial data by the conversion circuit 38 and supplied to the descrambler 39. The descrambler 39 restores the data before the scramble process by the descrambling process, and the deinterleave circuit 40 restores the data array to the original array before the interleave process and outputs it to the error correction decoding circuit 41. The error correction decoding circuit 41 performs error correction using the error correction code, and outputs decoded data via the output terminal 42.

Note that the reproduced carrier in the quadrature detection is F
Reproduction is performed using the output of the FT circuit 36. The carrier reproducing circuit 37 obtains carrier synchronization by generating a synchronous detection control signal for controlling the oscillation output of the local oscillator 31 using the output of the FFT circuit 36. Further, the timing circuit 43 generates a necessary timing signal in each section of the OFDM receiver 2 based on the reproduction clock and the timing reference signal included in the transmission data.

The above-described transmission system using OFDM modulation has high resistance to multipath and narrowband interference, has high frequency use efficiency, and is a promising system for digital transmission. However, there is a fundamental problem that a large amplitude peak may occur in an OFDM modulated wave. FIG. 9 is an explanatory diagram for explaining this problem. FIG. 9 conceptually shows the spectrum of an OFDM modulated wave on complex vector coordinates, and shows an example in which N subcarriers are arranged at equal intervals and the power of each subcarrier is equal.

Each subcarrier of an OFDM modulated wave is modulated by, for example, the phase and amplitude of a QAM signal. FIG. 9A shows a case where the phases of the four subcarriers 1 to 4 of the OFDM modulated wave are at 90-degree intervals. Since the frequencies of the subcarriers are different from each other, the subcarriers are actually rotating at mutually different angular frequencies, and FIG. 9A is an observation at a predetermined time. In this case, the sub-carriers 1 to 4 cancel each other, and the combined vector of the sub-carriers 1 to 4 is canceled to have an amplitude of 0.

On the other hand, FIG. 9B shows four subcarriers 1
4 to 4 are all in phase. In this case, the combined vectors of the subcarriers 1 to 4 are added with the same phase, and the combined vector amplitude is 1
Four times as many as one subcarrier. As described above, depending on the data, the phases of the subcarriers of the OFDM modulated wave match, and a peak having a large amplitude may occur in the OFDM modulated wave.

If the phases of N subcarriers match, the maximum peak value is N times the amplitude of one subcarrier. For example, in OFDM modulation in which N = 1024, the maximum peak amplitude is 2 for one subcarrier.
0 log 1024 = 60 dB. Also, the peak power of the modulated wave is 30 dB larger than the average power (same as digital modulation of a single carrier).

Such a peak having a large amplitude tends to interfere with other signals, and hinders the practical use of the OFDM transmission system, such as restrictions on frequency allocation. Further, since the peak amplitude is large, it is necessary to increase the dynamic range in the modulation / demodulation circuit and the transmission system. For this reason, a transmission system such as a repeater and a circuit of a transmission / reception device become expensive. For example, in order to digitally convert a large amplitude peak value with sufficient bit precision in an A / D converter, the dynamic range must be extremely large, and an expensive circuit with high bit precision must be used.

[0020]

As described above, in the above-mentioned conventional OFDM transmitting and receiving apparatus, since the OFDM modulated wave sometimes has a large amplitude peak, it is easy to cause other communication disturbances. There is a problem that an extremely large dynamic range is required.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and suppresses a peak of an OFDM modulated wave to prevent interference with other communication and to provide a necessary dynamic range. It is an object of the present invention to provide an OFDM transmitting apparatus and an OFDM receiving apparatus capable of reducing the following.

[0022]

According to a first aspect of the present invention, there is provided an OFDM receiving apparatus to which a modulated wave subjected to orthogonal frequency division multiplex modulation is inputted, and the modulated wave is subjected to orthogonal frequency division multiplex demodulation to demodulate symbols. Orthogonal frequency division multiplexing demodulation means for obtaining the modulated wave, an envelope detection means for detecting an envelope of the modulated wave, and a judging means for judging a modulation symbol whose envelope is equal to or higher than a predetermined level as an invalid symbol. And an error correction unit to which a determination result of the determination unit is given and for performing erasure error correction on a demodulated symbol corresponding to the invalid symbol. The OFDM transmission apparatus according to claim 3 of the present invention, A plurality of signal conversion means for converting input data into a plurality of signal formats, and orthogonal frequency division multiplex modulation of the input data to form a modulated wave The orthogonal frequency division multiplexing and modulation means for transmitting, and adaptively selecting one of the outputs of the plurality of signal conversion means so that the peak of the modulated wave is minimized. Selection means for giving.

[0023]

According to the first aspect of the present invention, the peak of the modulated wave from the orthogonal frequency division multiplex modulation means is suppressed at a predetermined level by the peak suppression means. This prevents the transmission system from interfering with other communications.

According to a second aspect of the present invention, the input modulated wave is demodulated by an orthogonal frequency division multiplex demodulation means to obtain a demodulated symbol. The envelope detecting means detects the envelope of the modulated wave, and the judging means detects that the envelope has become larger than a predetermined level. Judge as an invalid symbol. The error correcting means corrects the error even in the peak portion by correcting the erasure error of the invalid symbol from the determination result of the determining means.

According to a fourth aspect of the present invention, the plurality of signal conversion means respectively convert input data into a plurality of signal formats. The selection means selects one of the outputs of the plurality of signal conversion means and supplies it to the orthogonal frequency division multiplex modulation means, thereby minimizing the peak of the modulated wave of the orthogonal frequency division multiplex modulation.

According to a seventh aspect of the present invention, the extracting means extracts the selection information from the demodulated symbols demodulated by the orthogonal frequency division multiplex demodulating means. Based on this selection information, the signal reproducing means returns the demodulated symbols to a signal format based on the signal format of the transmitting side to obtain input data.

[0027]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an OFDM transmitting apparatus and an OFDM receiving apparatus according to the present invention.
In FIG. 1, the same components as those in FIG. 8 are denoted by the same reference numerals.

In FIG. 1, although the RF output from the OFDM transmitter 55 is given to the OFDM receiver 56, a signal from a conventional OFDM transmitter (see FIG. 8) which does not have a limiter 48 described later is provided. RF output OFDM receiver
May be given to 56. That is, the OFDM transmission device 55 and the OFDM
The DM receivers 56 can be used independently.

Digital data to be transmitted is input to the input terminal 3 of the OFDM transmitter 55. This input data is, for example, a QPSK-modulated or QAM-modulated signal. The input data is supplied to an error correction coding circuit 4, which adds an error correction code and outputs it to an interleave circuit 5. The interleaving circuit 5 rearranges the data so that even if a burst error occurs during transmission, the error is randomized by deinterleaving on the receiving side. The output of the interleave circuit 5 is provided to a scrambler 6.
The scrambler 6 converts the data into a random number so that the data does not have a specific continuous waveform, and outputs the randomized data to the S / P conversion circuit 7. The S / P conversion circuit 7 converts the sequence of the input data to O
The data is converted into parallel data corresponding to the number of carriers of the FDM modulation and supplied to the inverse FFT circuit 8.

The inverse FFT circuit 8 modulates a number of subcarriers by, for example, a QAM signal by inverse FFT processing to obtain an OFDM modulated wave. As a result, data of the number of subcarriers is transmitted by one OFDM modulated wave (1 OFDM symbol). The OFDM modulated wave is a complex signal. The I-axis signal is supplied to a D / A converter 9 and the Q-axis signal is supplied to a D / A converter 10. D / A converter 9,
Reference numeral 10 converts the input OFDM modulated wave into an analog signal and outputs the analog signal to multipliers 11 and 12, respectively.

The multipliers 11 and 12, the local oscillator 13, the phase shifter 14, and the adder 15 constitute a quadrature modulator. The local oscillator 13 generates a predetermined carrier and supplies the generated carrier to the multiplier 11 and to the multiplier 12 via the phase shifter 14. Phase shifter 14
Shifts the carrier from the local oscillator 13 by 90 degrees. The multipliers 11 and 12 respectively perform quadrature modulation by multiplying the in-phase axis carrier or the orthogonal axis carrier and the input OFDM modulated wave, convert the signals into IF signals, and output the IF signals to the adder 15. The adder 15 adds and outputs the quadrature modulated waves of the I and Q axis OFDM modulated waves.

In this embodiment, the output of the adder 15 is supplied to the BPF 16 via the limiter 48. The limiter 48 limits the output of the adder 15 at a predetermined amplitude. Since the output of the quadrature modulator is a modulated wave in the intermediate frequency band, the limiter 48 uses a circuit operating in this frequency band. For example, as the limiter 48, an easy configuration using the nonlinear characteristics of a high-frequency diode can be used. BPF16
After the output of the limiter 48 is band-limited, the output of the limiter 48 is output to a frequency conversion circuit constituted by the mixing circuit 17 and the local oscillator 18.

The local oscillator 18 supplies a local oscillation output having a predetermined frequency to the mixing circuit 17, and the mixing circuit 17 converts the frequency of the modulated wave into an RF signal and outputs the RF signal to the BPF 19. BPF19 is RF
The signal is band-limited and applied to the amplifier 20, which amplifies the RF signal and outputs it from the output terminal 21 as an RF output.

In the OFDM transmission apparatus thus configured, the input data input via input terminal 3 is added with an error correction code in error correction coding circuit 4 and the data array is converted in interleave circuit 5. The data is input to the scrambler 6. The scrambler 6 converts the input data into a random number and supplies it to the S / P conversion circuit 7, and the input data converted into parallel data is supplied to the inverse FFT circuit 8.

The inverse FFT circuit 8 performs an inverse FFT process on the input data and outputs an OFDM modulated wave to the D / A converters 9 and 10. The OFDM modulated wave is converted into an analog signal by the D / A converters 9 and 10, and then quadrature-modulated by the quadrature modulator to be converted into an intermediate frequency band signal. The quadrature modulated wave from the adder 15 is supplied to a limiter 48.

In the present embodiment, the limiter 48 limits the amplitude of the quadrature modulated wave and supplies it to the BPF 16. Thus, the signal supplied to the circuits after the limiter 48 does not have a large amplitude peak. Therefore, the dynamic range of the circuit after the limiter 48 can be reduced.

The modulated wave whose amplitude is limited by the limiter 48 is band-limited by the BPF 16, and then converted into an RF signal based on the local oscillation output from the local oscillator 18, and the BPF 16
The signal is output to an output terminal 21 via 19 and an amplifier 20.

As described above, in this embodiment, since the RF output from the output terminal 21 does not include a peak component having a large amplitude, the RF output suppresses interference with communication on other channels. Therefore, practical problems such as frequency allocation are remarkably solved. Also, the dynamic range of a repeater or the like provided in the transmission system may be small,
The transmission system can be configured economically, and practical application is further facilitated.

Since the amplitude of the OFDM modulated wave is limited, information may be lost. As described above, OFDM is a modulation method that multiplexes a large number of subcarriers while maintaining an orthogonal relationship. Therefore, when the phases of the subcarriers are aligned, a peak having a large amplitude occurs. The signal component at the peak position also transmits information, and the information may be lost due to the amplitude limitation.

However, by appropriately setting the clipping level in the amplitude limitation, the probability of missing information can be extremely reduced. For example, the maximum peak of the amplitude of an OFDM modulated wave using QPSK modulation occurs when all subcarrier phases match, but the number of carriers N is 1
In the case of 024, the probability of occurrence of the maximum peak is (1/4) to the 1024th power (≒ 0). Also, OFD
Since the M modulated wave can be regarded as substantially narrow band noise, the peak amplitude of the OFDM modulated wave can be represented by a Rayleigh distribution. That is, the occurrence probability of a peak having an amplitude several times or more than the average amplitude is extremely small. Therefore, by employing error correction coding having a relatively high error correction capability, the effect of information loss due to clipping of a peak can be almost ignored.

Next, an OFDM receiver according to an embodiment of the present invention will be described.

The RF input input from the transmission line 22 via the input terminal 23 may have a peak portion. amplifier
The reference numeral 24 amplifies the RF signal input via the input terminal 23 and supplies the amplified RF signal to the BPF 25. The BPF 25 limits the band of the output of the amplifier 24 and outputs it to the mixing circuit 26. The local oscillator 27 outputs a local oscillation output for selecting a predetermined channel to the mixing circuit 26, and the mixing circuit 26
The F signal is converted to an IF signal and output to the BPF 28. BP
F28 limits the band of the IF signal and outputs it to multipliers 29 and 30.

Multipliers 29, 30, local oscillator 31, and phase shifter 32
Constitutes a quadrature detector. The local oscillator 31 is controlled by a carrier reproducing circuit 37 described later, generates a reproduced carrier, and outputs it to the multiplier 29 and the phase shifter 32. The phase shifter 32 shifts the phase of the reproduction carrier by 90 degrees and supplies the same to the multiplier 30 as a reproduction carrier on the orthogonal axis. Multipliers 29 and 30 are supplied with a reproduction carrier of the in-phase axis or a reproduction carrier of the quadrature axis, respectively. Output to 33 and 34.

The OFDM modulated wave from the multiplier 29 is also supplied to a clock recovery circuit 35. A clock recovery circuit 35 generates a recovered clock from the quadrature detection output and generates an A / D converter 33
To the output terminal 46. The A / D converters 33 and 34 use the recovered clock from the clock recovery circuit 35 to convert the OFDM modulated wave, which is the quadrature detection output, into a digital signal and output it to the FFT circuit 36. In this embodiment, the A / D converters 33 and 34 are set to a dynamic range sufficiently smaller than the maximum peak amplitude of the OFDM modulated wave, and constitute a peak suppressing unit.

The FFT circuit 36 performs the FFT processing by regarding the input OFDM modulated waves of the in-phase axis and the orthogonal axis as the real part and the imaginary part of the complex number, respectively. By this FFT processing, synchronous demodulation is performed on each subcarrier. FFT circuit
The OFDM demodulated signal from 36 is supplied to a P / S conversion circuit 38. The P / S conversion circuit 38 converts the OFDM demodulated signal into serial data and supplies it to a descrambler 39. The descrambler 39 descrambles the randomized OFDM demodulated signal to return to the original data and deinterleaves the signal.
Output to 40. The deinterleave circuit 40 returns the data whose data array has been converted by the interleaving process on the transmission side to the original data array,
Output. The P / S conversion circuit 38
Is also supplied to a timing circuit 43. The timing circuit 43 generates various timing signals in the OFDM receiver 56 by using a timing reference signal included in the OFDM demodulated signal.

In this embodiment, an error correction decoding circuit
In order to improve the error correction capability in 49, the quadrature detection outputs from the A / D converters 33 and 34 are used as envelope detection circuits.
50 to give. Envelope detection circuit 50
Is an erasure determination circuit that detects the envelope of an OFDM modulated wave by calculating the absolute values of the detection outputs of the in-phase axis and the orthogonal axis.
Output to 51. As described above, since the dynamic range of the A / D converters 33 and 34 is sufficiently smaller than the maximum peak amplitude of the OFDM modulated wave, the quadrature detection of the peak position among the detection outputs input to the FFT circuit 36 is performed. The output may have errors. The erasure determination circuit 51 compares the envelope with a predetermined reference amplitude, determines a portion having an amplitude level larger than the reference amplitude as a peak position, and transmits an OFDM symbol at this peak position on a subcarrier in this symbol period. If all the data, that is, the number of subcarriers is N, it is determined that all N demodulated symbols are unreliable, and this lost information is sent to the error correction decoding circuit 49 via the delay 52. Output.

When an amplitude limit for removing a peak is performed on the transmitting side as in the OFDM transmitting apparatus 55, the erasure determination circuit 51 sets the reference amplitude to a value slightly smaller than the clipping level on the transmitting side. Is set.

The error correction decoding circuit 49 is a decoding circuit capable of correcting an erasure error, and can obtain a high error correction capability without deteriorating the coding efficiency. That is, the error correction decoding circuit 49 detects the position of the erroneous OFDM (modulation) symbol based on the erasure information of the erasure determination circuit 51, and corrects the error by detecting an error pattern. Since an error position is given, high error correction capability is provided without increasing the number of parity bits. The error correction decoding circuit 49 corrects the error of the demodulated signal from the deinterleave circuit 40 and outputs the error to the output terminal 42.

Next, the operation of the embodiment configured as described above will be described with reference to FIGS.

The RF signal input via the input terminal 23 may have a large amplitude peak due to the principle of OFDM modulation. This RF signal is supplied to a mixing circuit 26 via an amplifier 24 and a BPF 25, and is mixed with an oscillation output from a local oscillator 27 to be converted into an IF signal. After this IF signal is band-limited by the BPF 28, it is supplied to multipliers 29 and 30. Multipliers 29 and 30 perform quadrature detection by multiplying the in-phase axis reproduction carrier or the quadrature axis reproduction carrier by the IF signal. Thereby, an in-phase axis and quadrature axis OFDM modulated wave is obtained from the IF signal, and the A / D converter 33,
Given to 34.

The A / D converters 33 and 34 are clock recovery circuits.
The detection output is converted into a digital signal using the reproduced clock from 35 and supplied to the FFT circuit 36. FFT circuit 36 is O
The FDM modulated wave is subjected to FFT processing and demodulated, and an OFDM demodulated signal is output to the P / S conversion circuit 38. The OFDM demodulated signal is converted into serial data in a P / S conversion circuit 38, subjected to descrambling processing and deinterleaving processing by a descrambler 39 and a deinterleave circuit 40, respectively, and supplied to an error correction decoding circuit 49.

In this embodiment, the A / D converters 33, 34
Is sufficiently lower than the maximum peak amplitude of the OFDM modulated wave, and there is a possibility that an error occurs in the data of the peak portion. The error correction decoding circuit 49 corrects the error in the peak part using the result of the erasure determination. That is, the in-phase axis and quadrature axis OFDM modulated waves from the A / D converters 33 and 34 are supplied to the envelope detection circuit 50.

FIG. 2 is an explanatory diagram in which OFDM modulated waves from the A / D converters 33 and 34 are represented by vectors using a complex plane having an in-phase axis I and a quadrature axis Q. In the figure, black circles indicate OFDM modulated waves at predetermined timings.

X is the amplitude of the in-phase axis detection output from the A / D converter 33, and y is the amplitude of the quadrature axis detection output from the A / D converter 34. r indicates the amplitude of the OFDM modulated wave. The envelope detection circuit 50 outputs the locus of the black circle in FIG. 2, that is, the value of the amplitude r of the OFDM modulated wave to the erasure determination circuit 51. The erasure determination circuit 51 compares the input envelope with a predetermined reference amplitude. In FIG. 2, the magnitude r 'of the reference amplitude is indicated by a broken line. Loss judgment circuit 51
Is determined to be a peak portion when the envelope is larger than the reference amplitude. That is, the erasure determination circuit 51 determines that a peak has occurred when an amplitude r larger than a circle having a radius r 'indicated by a broken line in FIG. This determination result is supplied to the error correction decoding circuit 49 via the delay 52 as erasure information.

Now, each of the OFDM symbols K, K + 1, K + 2 including N data from the deinterleave circuit 40 is
It is assumed that the demodulated symbols shown in FIG. 3 (a) are sequentially input to the error correction decoding circuit 49. It is assumed that symbol K + 2 among these modulation symbols has a peak. The error correction decoding circuit 46 calculates the symbol K +
When it is determined that an error has occurred in No. 2, the error pattern of this symbol K + 2 is detected and error correction is performed.
Since the position of the erroneous symbol is known from the erasure information, it is not necessary to use a parity bit for error detection, and the erasure error correction capability is improved. The error correction decoding circuit 46 outputs the error-corrected demodulated data via the output terminal 42.

As described above, in this embodiment, the A / D
An OFDM symbol having a peak is detected from the outputs of the converters 33 and 34, and this OFDM symbol is regarded as lost, and the erasure position is given to the error correction decoding circuit 49, thereby improving the error correction capability. Since the dynamic range of the A / D converters 33 and 34 is sufficiently smaller than the maximum peak amplitude, an error occurs in the OFDM symbol having a peak, but the error correction capability of the error correction decoding circuit 49 is improved. Error is corrected. Since the A / D converters 33 and 34 having a relatively small dynamic range can be used, the circuit can be configured at low cost.

FIG. 4 is a block diagram showing another embodiment of the present invention. 4, the same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

This embodiment differs from the embodiment of FIG. 1 in that a limiter 63 is used in place of the limiter 48 on the transmitting side and a limiter 64 is provided on the receiving side. Inverse FF of OFDM transmitter 61
The in-phase axis and orthogonal axis OFDM modulated waves from the T circuit 8 are supplied to D / A converters 9 and 10 via a limiter 63. The limiter 63 limits the amplitude of the OFDM modulated wave.

In the transmission-side device configured as above, the OFDM modulated wave from the inverse FFT circuit 8 is transmitted to the limiter 63.
And digitally amplitude-limited. Since the amplitude is limited by digital processing, the clipping level can be accurately defined. A / D
The dynamic range of converters 9 and 10 can be set to a small value.

In the receiving apparatus, an OFDM receiving apparatus
Quadrature detection output from 62 multipliers 29 and 30 is used as peak suppression means
Are supplied to the A / D converters 33 and 34 via a limiter 64 as. The limiter 64 limits the amplitude of the input quadrature detection output at a clipping level based on the dynamic range of the A / D converters 33 and 34 and supplies the output to the A / D converters 33 and 34.

In the receiving apparatus configured as described above, the quadrature detection output is supplied to the A / D converters 33 and 34 after the amplitude is limited by the limiter 64. The limiter 64 limits the quadrature detection output amplitude at a clipping level based on the dynamic range of the A / D converters 33 and 34. This allows
It is possible to prevent an excessive input from being input to the A / D converters 33 and 34, thereby preventing malfunctions of the A / D converters 33 and 34.

It is apparent that each of the above embodiments can be easily realized with other circuit configurations.

FIG. 5 is a block diagram showing another embodiment of the present invention. 5, the same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

As described above, the peak of the OFDM modulated wave is generated when the subcarrier phases match. Therefore, in the present embodiment, the input data is scrambled so that the phases of the subcarriers do not match. The OFDM transmitting apparatus 71 shown in FIG. 1 is different from the OFD in FIG. 1 in that scramblers S1, S2,...
It is different from the M transmitting device 55. The output of the interleave circuit 5 is given to scramblers S1, S2,... SM. The scramblers S1, S2,...
And performs scramble processing different from each other on the input data to output to the scrambler selection circuit 73.

As described above, since the occurrence of the peak is determined by the phase of the subcarrier, that is, the amplitude and phase of the input data, the occurrence of the peak can be known from the signal pattern before the inverse FFT processing. Scrambler selection circuit 73
Stores data on a specific signal pattern where a peak occurs, selects an output from the descramblers S1 to SM where no peak occurs, and multiplexes the output of the selected scrambler and the scrambler selection information. circuit
Output to 74. The multiplexing circuit 74 multiplexes scrambler selection information with the scrambled input data and outputs the multiplexed data to the S / P conversion circuit 7.

On the other hand, the OFDM receiving apparatus 72 includes a separating circuit 75 and a descrambler DS instead of the descrambler 39 in FIG.
1, DS2,..., DSM and descrambler selection circuit 76
Is different from the embodiment of FIG. The output of P / S conversion circuit 38 is applied to separation circuit 75. Separation circuit 75
Separates the scrambler selection information from the OFDM demodulated signal of the P / S conversion circuit 38 and outputs the same to the descrambler selection circuit 76. The descramblers DS1, DS2,..., DSM perform descramble processing for restoring the signals scrambled by the scramblers S1, S2,. The input OFDM demodulated signals are respectively descrambled and output to the descrambler selection circuit 76. The descrambler selection circuit 76 selects one of the outputs of the descramblers DS1, DS2,..., DSM based on the scrambler selection information and outputs it to the deinterleave circuit 40.

FIG. 6 shows the scramblers S1 to SM in FIG.
FIG. 3 is a circuit diagram showing a specific configuration of a descrambler DS1 to DSM.

The input data input via the input terminal 81 is applied to the exclusive OR circuit 82 of the scramblers S1 to SM. The scramble process for digital data is generally obtained by an exclusive OR operation of PN (random number) and input data. The PN generation circuit 83 generates a predetermined random number and outputs it to the exclusive OR circuit 82. The exclusive OR circuit 82 scrambles the data by an exclusive OR operation of two inputs and outputs the scrambled data to an output terminal 84. By making the random number sequences of the scramblers S1 to SM different, it is possible to perform different scrambling processes among the scramblers S1 to SM.

The scramble output input to the input terminals 85 of the descramblers DS1 to DSM is an exclusive OR circuit.
Given to 86. The PN generation circuit 87 includes a scrambler S1
The random numbers of the same sequence having the same initial value synchronized with the random number sequence of SM are generated and output to the exclusive OR circuit 86. The exclusive OR circuit 86 performs descrambling processing by obtaining an exclusive OR of two inputs, and outputs a descramble output to an output terminal 88. The PN generating circuits 87 of the descramblers DS1 to DSM correspond to the PN generating circuits 83 of the scramblers S1 to SM, respectively.

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

The input data from the interleave circuit 5 is applied to scramblers S1 to SM and subjected to scramble processing different from each other. The scramble output of each of the scramblers S1 to SM is supplied to a scrambler selection circuit 73. The scrambler selecting circuit 73 stores in advance a signal pattern in which a peak is generated by OFDM modulation, selects an output from the scramblers S1 to SM that does not generate a peak, and outputs a multiplexing circuit 74 with the selection information. Output to The multiplexing circuit 74 multiplexes the scrambler selection information on the scramble output and outputs it to the S / P conversion circuit 7. As a result, the OFDM modulated wave output from the OFDM transmitter 71 has no peak.

OFDM input to OFDM receiving apparatus 72
Since the modulated wave has no peak, the dynamic range of the A / D converters 33 and 34 is set sufficiently small. The OFDM demodulated signal from the P / S conversion circuit 38 is supplied to a separation circuit 75, where the scrambler selection information is separated. OFDM demodulated signals are descramblers DS1 to DS
In M, descrambling is performed in the same sequence as the random number sequence of each of the scramblers S1 to SM on the transmission side, and is supplied to the descrambler selection circuit 76. The descrambler selection circuit 76 selects the output of the descrambler corresponding to the scramblers S1 to SM selected on the transmission side based on the scrambler selection information, and outputs it to the deinterleave circuit 40. Thus, data can be reproduced from the OFDM demodulated signal.

As described above, in the present embodiment, the transmitting side selects scrambling processing that does not cause a peak from the data pattern, and the receiving side performs descrambling processing corresponding to the scrambling processing on the transmitting side to perform data scrambling. Are playing. Since the OFDM modulated wave has no peak, the same effect as that of the embodiment of FIG. 1 can be obtained, and the peak is not removed by the amplitude limitation, and no information is lost. There is also an advantage that the generation amount is small.

[0074]

As described above, according to the present invention, O
By suppressing the peak of the FDM modulated wave, it is possible to prevent interference with other communications and to reduce the required dynamic range.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an OFDM transmitting apparatus and an OFDM receiving apparatus according to the present invention.

FIG. 2 is an explanatory diagram for explaining the operation of the embodiment.

FIG. 3 is an explanatory diagram for explaining the operation of the embodiment.

FIG. 4 is a block diagram showing another embodiment of the present invention.

FIG. 5 is a block diagram showing another embodiment of the present invention.

FIG. 6 is a block diagram showing a specific configuration of a scrambler and a descrambler in FIG. 5;

FIG. 7 is an explanatory diagram showing a frequency spectrum of an OFDM modulated wave.

FIG. 8 is a block diagram showing a conventional OFDM transmitting apparatus and OFDM receiving apparatus.

FIG. 9 is an explanatory view conceptually showing the spectrum of an OFDM modulated wave.

[Explanation of symbols]

8 Inverse FFT circuit, 48 Limiter, 33, 34 A / D converter, 36 FFT circuit, 49 Error correction decoding circuit, 50 Envelope detection circuit, 51 Erasure determination circuit

Continued on the front page (72) Inventor Yasushi Sugita 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Pref. Toshiba Corporation Multimedia Research Institute (72) Inventor Shigeru Okita 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Toshiba Multi Corp. JP-A-5-130191 (JP, A) JP-A-61-176221 (JP, A) JP-A-5-175880 (JP, A) JP-A-4-11412 (JP) , A) JP-A-3-175823 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04J 11/00

Claims (6)

(57) [Claims] An orthogonal frequency division multiplex demodulation means for receiving a modulated wave subjected to orthogonal frequency division multiplex modulation and receiving the modulated wave by orthogonal frequency division multiplex demodulation to obtain a demodulated symbol; Envelope detecting means for detecting the envelope of the modulated wave, determining means for determining a modulation symbol whose envelope has reached a predetermined level or more as an invalid symbol, and a determination result of this determining means is given to the invalid symbol. An OFDM receiving apparatus comprising: an error correction unit that corrects an erasure error of a corresponding demodulated symbol. 2. The OFDM receiving apparatus according to claim 1, further comprising a peak suppressing means for suppressing a peak of said modulated wave at a predetermined level and applying the same to said orthogonal frequency division multiplexing demodulation means. A plurality of signal conversion means for converting input data into a plurality of signal formats; an orthogonal frequency division multiplex modulation means for performing orthogonal frequency division multiplex modulation on input data and transmitting a modulated wave; Selecting means for adaptively selecting one of the outputs of the plurality of signal conversion means so as to minimize the peak of the modulated wave and providing the selected output to the orthogonal frequency division multiplexing modulation means. OFDM transmitter. 4. The OFDM transmission apparatus according to claim 3, wherein the plurality of signal conversion units obtain a plurality of signal formats by performing a plurality of types of scrambling processing on input data. 5. The apparatus according to claim 1, wherein said selecting means multiplexes and outputs selection information indicating which one of the plurality of signal converting means has selected an output to the selected output of the signal converting means. 4. The OFDM transmission device according to 3. 6. An orthogonal frequency division multiplex demodulation means for receiving a modulated wave from the orthogonal frequency division multiplex modulation means of the OFDM transmission apparatus according to claim 5 and obtaining demodulated symbols by orthogonal frequency division multiplex demodulation; Extracting means for extracting the selection information from the demodulated symbols; signal reproduction for obtaining the input data by returning the demodulated symbols to a signal format based on the signal format of the signal converting means based on the selection information extracted by the extracting means OFDM characterized by comprising:
Receiver.