JPH09149092A - Afc equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain stable automatic phase control in which effect of fading is suppressed. SOLUTION: An IF-OFDM(intermediate frequency-orthogonal frequency division multiplex modulation) signal and a local intermediate frequency signal outputted from an IF-LO(intermediate frequency local oscillator) 1 are multiplied by mixers 5, 6, and outputted via A/D converters 9, 10, the resulting signals being output signals from orthogonal demodulation circuits 3, 4 are converted into a frequency region signal by an FFT device 11, the power spectrum level for each frequency band of the frequency region signal is calculated by a power spectrum calculation circuit 12. Then a frequency band component signal whose power spectrum level is a prescribed level or over is given to a feedback data selection circuit 13, where the signal is selectively outputted, a frequency control circuit 14 detects a mean phase difference of a phase error caused in each subcarrier of the IF-OFDM signal is detected from an output signal component of the feedback data selection circuit 13 and the frequency control signal based on the mean phase difference is given to the IF-LO1, which controls the local intermediate frequency signal to decrease the phase error.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM receiver for receiving an OFDM (Orthogonal Frequency Division Multiplex Modulation) signal in which each subcarrier is QPSK (Quaternary Phase Modulation), and the OFDM signal converted to an intermediate frequency. Of the local intermediate wave used for quadrature demodulation of
AFC for controlling the frequency of the local intermediate wave so as to cancel the average phase error in one symbol with the FDM signal
(Automatic frequency control) device

[0002]

2. Description of the Related Art OFDM, as the name implies, is a modulation method that uses a large number of subcarriers whose frequency components are orthogonal to each other, and is a high-speed signal in the frequency domain in which data is set at the frequency position of each subcarrier. Inverse Fast Fourier Transform (hereinafter referred to as IFFT) is performed to obtain an OFDM signal which is a time domain signal composed of each phase-modulated subcarrier.

In this OFDM signal, the phase shift of each subcarrier indicates the data set in that subcarrier.

Then, the above OFDM signal is subjected to a fast Fourier transform (hereinafter referred to as FFT).
By doing so, the data set in each subcarrier can be demodulated.

As a communication system adopting such an OFDM, a digital audio broadcasting (hereinafter referred to as DAB) mainly in Europe is mainly used.
Is raised.

[0006] In OFDM in DAB, each subcarrier is QPSK (Quaternary Phase Modulation). 2 bit encoded data is converted into QPSK data composed of real number data and imaginary number data, and each subcarrier is QPSKed by this QPSK data.

FIG. 5 is a block diagram showing a schematic configuration of a conventional AFC device.

In FIG. 5, 1 is an intermediate wave local oscillator (In
termediate Frequency Local Oscillator, hereafter IF−
2) a phase shifter, 3 a mixer 5 and LP
Quadrature demodulation circuit having F7, 4 mixer 6 and LPF
A quadrature demodulation circuit having 8, an A / D converter 9 and 10, an FFT device 11 and a frequency control circuit 15.

The OFDM receiver includes a receiver, A shown in FIG.
An FC device and an encoded data demodulation unit are provided. The reception unit receives the transmitted OFDM signal, and the received OFDM signal of the transmission frequency is IF-OFDM.
Signal (intermediate frequency OFDM signal)
The gain of the OFDM signal is adjusted and input to the AFC device shown in FIG.

In the quadrature demodulation circuit 3 of FIG.
The DM signal and the local intermediate wave input from the IF-LO1 are multiplied by the mixer 5, and the quadrature demodulation circuit 4 outputs I
The F-OFDM signal and the local intermediate wave input from the phase shifter 2 are multiplied by the mixer 6, and the high frequency components are removed from the multiplied signals by the LPFs 7 and 8 to obtain the IF signal.
-Orthogonal demodulation of OFDM signal, baseband OFDM
The I signal component (imaginary part signal component) and the Q signal component (real part signal component) of the signal are obtained.

The I signal component and the Q signal component are A / D
A / D conversion is performed by the converters 9 and 10, and FFT is performed by the FFT device 11 to demodulate the frequency domain signal in which the QPSK data is set.

However, the phase distortion (phase error) generated in each carrier in the radio transmission channel is caused by the FFT device 11
It is also directly multiplied by the QPSK data demodulated by.

Therefore, the frequency control circuit 15 detects the phase error between the IF-OFDM signal in each band and the local intermediate wave from each QPSK data demodulated by the FFT device 11 and including the phase error, and detects the detected phase error. Based on the above, the average of the phase error over the entire band of the IF-OFDM signal is calculated as the average phase difference, and the frequency of the local intermediate wave is controlled so as to cancel this average phase difference.

As a result, the QPSK data demodulated by the FFT device 11 has the average phase difference canceled, and normal demodulation can be performed.

In the frequency control circuit 15, one Q demodulated from one specific subcarrier is used.
A phase error is detected from the PSK data and used as the average phase difference, or an error phase difference is detected from all demodulated QPSK data, and the average value is used as the average phase difference, or a specific number of lines are detected. The phase error was detected from the QPSK data demodulated from the subcarrier of, and the average value was used as the average phase difference.

[0016]

However, in the above-mentioned conventional AFC device, the QPSK data demodulated from the subcarriers which are significantly fading due to the frequency selective fading during transmission have been used for the calculation of the average phase difference. In this case, there is a possibility that the QPSK data may operate in the direction of increasing the phase error of the other QPSK data.

That is, when only one subcarrier undergoes significant fading, only the amplitude and phase of that subcarrier change significantly, so the phase error obtained from the QPSK data demodulated from this subcarrier is the other subcarrier. The value is different from the phase error obtained from the QPSK data demodulated from.

The present invention is intended to solve such a conventional problem, and an object thereof is to provide an AFC device capable of suppressing the influence of fading and performing stable automatic frequency control.

[0019]

In order to achieve the above object, the AFC device of the present invention comprises a local intermediate wave transmitting means for outputting a local intermediate wave based on a frequency control signal, and a plurality of subcarriers by signal data. The received signal that is modulated,
Multiplying means for multiplying by the local intermediate wave, signal converting means for converting an output signal from the multiplying means into a frequency domain signal, and power spectrum level for each frequency band of the signal converted by the signal converting means A power spectrum level detecting means for detecting, a signal selecting means for selectively outputting a frequency band component signal in which the power spectrum level is equal to or higher than a predetermined reference level, and a transmission channel of the received signal from an output signal component of the signal selecting means. Phase difference detection means for detecting an average of phase errors occurring in each subcarrier as an average phase difference, and the frequency control signal for controlling the frequency of the local intermediate wave so as to reduce the phase error based on the average phase difference. And a frequency control means for giving the local intermediate wave transmitting means to the local intermediate wave transmitting means. That.

An AFC apparatus according to a second aspect of the present invention is characterized in that a power spectrum level determined based on a cumulative distribution of a probability density function by Rayleigh distribution is used as a reference level in the signal selecting means. It is a thing.

Therefore, according to the AFC apparatus of the present invention, the received signal obtained by modulating a plurality of subcarriers with the signal data by the multiplying means and the local intermediate wave output from the local intermediate wave transmitting means are multiplied to obtain a signal. The converting means converts the output signal from the multiplying means into a frequency domain signal, the power spectrum level detecting means detects the power spectrum level for each frequency band of the frequency domain signal, and the signal selecting means determines the power spectrum level. A frequency band component signal having a predetermined level or more is selectively output, and the average of phase errors generated in each subcarrier by the transmission channel of the received signal is detected as an average phase difference from the output signal component of the signal selection unit by the phase difference detection unit. , Frequency control signals based on the average phase difference are transmitted by the frequency control means. Gives the intermediate wave transmitting means, by controlling the frequency of the local intermediate wave to reduce the phase error, received significantly influence of fading Q
Since the average phase difference between the intermediate frequency OFDM signal and the local intermediate wave can be detected by excluding the PSK data, it is possible to suppress the influence of fading and perform stable automatic frequency control.

According to the second aspect of the present invention, the accumulation of the probability density function by Rayleigh distribution indicating the probability that the power spectrum level of certain QPSK data will be below a certain level due to the effect of frequency selective fading. By determining the reference level based on the distribution and using the reference level to select the QPSK data in the signal selecting means, the QPSK data affected by fading can be appropriately excluded.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION In the embodiments of the present invention described below, one subcarrier is allocated to 2-bit encoded data, and each subcarrier is QPSK (Quadrature Phase Modulated) OFDM (orthogonal). Frequency division multiplexing modulation).

On the transmitting side, one set of encoded 2-bit data to be transmitted is set, each 2-bit data is converted into QPSK data, and each QPSK data is assigned to each subcarrier. Then, a QPSK data set signal (this QPSK data set signal is a signal in the frequency domain) in which each QPSK data is set at the frequency position of the corresponding subcarrier is created.

This QPSK data set signal is composed of an I signal component and a Q signal component. The I signal component and the Q signal component are in an orthogonal relationship, and a complex number in which the Q signal component is the real part and the I signal component is the imaginary part is , A complex representation of the above QPSK data set signal.

Assuming that the number of subcarriers is N, the coded data 2 × Nbit to be transmitted is divided into 2bit groups.

In OFDM by QPSK, since the coded data of 2 × Nbit is transmitted all at once,
This transmission unit is referred to as 1 symbol, and 2Nbit encoded data is referred to as 1 symbol data.

D (k) (k =
1, 2, ... N) and QP of 2-bit data D (k)
The SK data is set as Q (k) = A _k + jB _k (k = 1, 2, ... N) (1).

Here, A _k represents real part data of QPSK data, and B _k represents imaginary part data, each of which takes a value of "1" or "-1". Further, j is an imaginary unit.

The conversion of the 2-bit data D (k) into the QPSK data Q (k) is performed as follows, for example.

When D (k) = “00” Q (k) = 1 + j When D (k) = “01” Q (k) = − 1 + j When D (k) = “10” Q (k) = −1−j When D (k) = “11” Q (k) = 1−j Next, the frequency of each subcarrier is f _k (k = 1, 2, ... N).
Then, for example, QPSK is assigned to the subcarrier of frequency f _k.
Allocate data D (k).

In the frequency domain, the real part data A _k of the QPSK data Q (k) is set at the position of the frequency f _k of the Q signal component, and the QPSK data Q (k) is set at the position of the frequency f _k of the I signal component. The imaginary part data B _{k of} is set to create a frequency domain QPSK data set signal.

Next, the I signal component and the Q signal component of the above frequency domain QPSK data set signal are respectively IF
An I signal component and a Q signal component of an OFDM signal composed of N subcarriers that are phase-modulated by FT (Fast Inverse Fourier Transform) (both the I signal component and the Q signal component of this OFDM signal are signals in the time domain). Is obtained, and the carrier wave is quadrature-modulated by the I signal component and the Q signal component to obtain an OFDM signal of a carrier frequency.

In this OFDM signal, the phase shift of each subcarrier is the Q set for that subcarrier.
The PSK data is shown.

Finally, the above-mentioned carrier frequency OFDM signal is converted into a transmission frequency and transmitted.

The OFDM receiver includes a receiver, A shown in FIG.
An FC device and an encoded data demodulation unit are provided. The reception unit receives the transmitted OFDM signal, and the received OFDM signal of the transmission frequency is IF-OFDM.
Signal (intermediate frequency OFDM signal)
The gain of the OFDM signal is adjusted and input to the AFC device shown in FIG.

FIG. 1 is a block diagram showing the configuration of an embodiment of the AFC device of the present invention.

In FIG. 1, 1 is an IF-LO (intermediate wave local oscillator) that outputs a phase-adjusted local intermediate wave, and 2 is I.
The phase of the local intermediate wave input from F-LO1 is -90 °
It is a phase shifter that shifts the phase.

3 has a mixer 5 and an LPF 7, and
-By multiplying the OFDM signal and the local intermediate wave input from IF-LO1 by the mixer 5, and removing the high frequency component of the obtained multiplication signal by the LPF 7, the I signal component (time It is a quadrature demodulation circuit for obtaining a region signal).

4 has a mixer 6 and an LPF 8, and an IF
-By multiplying the OFDM signal and the local intermediate wave input from the phase shifter 2 by the mixer 6 and removing the high frequency component of the obtained multiplication signal by the LPF 8, the Q signal component of the baseband OFDM signal ( It is a quadrature demodulation circuit for obtaining a time domain signal).

The above-mentioned I signal component and Q signal component are in an orthogonal relationship, and a complex number having a Q signal component as a real part and an I signal component as an imaginary part is a complex representation of a baseband OFDM signal (time domain signal). Become.

9 is an OFD input from the orthogonal demodulation circuit 3.
A / D converter for A / D converting the I signal component of the M signal, 1
Reference numeral 0 is an A / D converter for A / D converting the Q signal component of the OFDM signal input from the orthogonal demodulation circuit 4.

Reference numeral 11 indicates each subcarrier by FFT (Fast Fourier Transform) of the I signal component input from the A / D converter 9 and the Q signal component input from the A / D converter 10. QPS at the position of frequency f _k
The I signal component (frequency domain signal) of the QPSK data set signal in which the imaginary part data of the K data is set, and the Q signal component of the QPSK data set signal in which the real part data of the QPSK data is set at the position of the frequency f _k ( The FFT device demodulates a frequency domain signal).

Reference numeral 12 is a power spectrum calculation circuit for calculating the power spectrum of each QPSK data in the QPSK data set signal demodulated by the FFT device 11.

Reference numeral 13 denotes a power spectrum reference level R ₀ which is set in advance, and compares this R ₀ with the power spectrum of each QPSK data by the power spectrum calculation circuit 12 to obtain the power spectrum reference level R ₀ . QP with large power spectrum level
This is a feedback data selection circuit that selects SK data, sets a flag "1" for the selected QPSK data, and sets a flag "0" for the unselected QPSK data.

Reference numeral 14 is an IF-OFDM signal and I from the QPSK data selected by the feedback data selection circuit 13.
Detecting the average phase difference with the local intermediate wave by F-LO1,
The frequency control voltage for canceling this average phase difference is IF
It is a frequency control circuit that controls the frequency of the local intermediate wave by giving it to -LO1.

Next, the operation of the embodiment of the present invention having such a configuration will be described.

In the orthogonal demodulation circuit 3, IF-OFDM
The mixer 5 multiplies the signal and the local intermediate wave input from the IF-LO 1 and removes the high frequency component of the obtained multiplication signal by the LPF 7 to obtain the I signal component (time domain) of the baseband OFDM signal. Signal),
In the quadrature demodulation circuit 4, the IF-OFDM signal,
The mixer 6 multiplies the local intermediate wave input from the phase shifter 2 by the mixer 6, and the high frequency component of the obtained multiplication signal is fed to the LPF 8.
Baseband OFDM by removing by
Obtain the Q signal component of the signal (time domain signal).

Next, let the I signal component of the above OFDM signal be A
A / D converter 9 performs A / D conversion, the above-mentioned Q signal component of the OFDM signal is A / D converted by A / D converter 10, and each is input to FFT device 11 and FFT device 1
By performing FFT by 1, the I signal component (frequency domain signal) of the QPSK data set signal in which the imaginary part data of the QPSK data is set at the position of the frequency f _k indicating each subcarrier, and the position of the frequency f _k QPSK with real part data of QPSK data set
The Q signal component (frequency domain signal) of the data set signal is demodulated.

The QPSK data in the demodulated QPSK data set signal is set as q (f _k ) = a _k + jb _k (k = 1, 2, ... N) (2).

Next, in the power spectrum calculation circuit 12, the power spectrum R (f _k ) (k of each QPSK data in the demodulated QPSK data set signal.
= 1, 2, ... N) is calculated by the following equation.

R (f _k ) = (a _k ) ² + (b _k ) ² (3) FIG. 2 is a circuit configuration diagram of the power spectrum calculation circuit 12.

In FIG. 2, reference numerals 21 and 22 denote squaring circuits for squaring an input signal. The squaring circuit 21 has a QPSK circuit.
The real part data a _k of the data q (f _k ) is input, and the imaginary part data b _k is input to the squaring circuit 22.

An adder 23 adds the input signal from the squaring circuit 21 and the input signal from the squaring circuit 22.

By the way, the OFDM transmitted from the transmitting side
If a signal experiences frequency selective fading during transmission,
Since the amplitude and phase of the subcarrier that has undergone fading change, this amplitude displacement and phase displacement are also included in the demodulated QPSK data q (f _k ).

The amplitude (effective value) of each subcarrier is H
Let _k (k = 1, 2 ... N) and the phase shift between each subcarrier and the local intermediate wave be θ _k (k = 1, 2 ... N), the demodulated QPSK data q (f _k ) is As shown.

Q (f _k ) = H _k exp (−jθ _k ) × Q (f _k ) (4) In the above equation (4), amplitude H _k = 1 at the time of normal transmission, but fading occurs. And the amplitude H _k <1,
The value indicates the amplitude distortion rate due to fading.

In the above equation (4), if θ _k = 0, the IF-OFDM signal is correctly orthogonally demodulated.

FIG. 3 shows the power spectrum characteristic and the phase displacement characteristic of the QPSK data q (f _k ) when the OFDM signal that has undergone frequency selective fading during transmission is demodulated by the AFC device. This is the case where the positions of f _L and f _M are strongly faded.

In FIG. 3, reference numeral 31 indicates a power spectrum characteristic, and dip points are formed at the positions of the frequencies f _L and f _M. Further, 32 indicates a phase displacement characteristic.

Returning to FIG. 1, in the feedback data selection circuit 13, the preset power spectrum reference level R ₀ is used to compare this R ₀ with the power spectrum R (f _k ) according to the equation (3). Then, QPSK data having a power spectrum level of R (f _k )> R ₀ (5) is selected, a flag “1” is set to the selected QPSK data, and a flag “0” is set to the QPSK data not selected. ”
Stand up.

The QPSK data that has not been selected, that is, the QPSK data whose selection flag is "0", is distorted in the amplitude of the allocated subcarrier due to the fading (amplitude H _k <1 in the equation (4)). ),
It can be said that it has a large phase displacement accordingly.

On the contrary, it can be said that the selected QPSK data, that is, the QPSK data whose selection flag is "1", has a small phase displacement θ _k .

Next, in the frequency control circuit 14, the average phase difference between the IF-OFDM signal and the local intermediate wave by IF-LO1 is detected from the QPSK data selected by the feedback data selection circuit 13, and the frequency of the local intermediate wave is detected. The average phase difference is canceled by applying a frequency control voltage for controlling the IF-LO1.

FIG. 4 is a circuit configuration diagram of the frequency control circuit 14.

In the frequency control circuit 14 shown in FIG. 4, the average phase difference is set to θ, and the average phase difference θ is calculated by the following calculation.

First, expanding equation (4), q (f _k ) = H _k exp (−jθ _k ) × Q (f _k ) = H _k (cos θ _k −j sin θ _k ) × (A _k + jB _k ) = H _k (A _k cos θ _k + B _k sin θ _k ) + jH _k (B _k cos θ _k −A _k sin θ _k ) (6) The real part in the formula (6) is a _k in the formula (2),
Since the imaginary part in the equation (6) is b _k in the equation (2), a _k = H _k (A _k cos θ _k + B _k sin θ _k ) (7) b _k = H _k (B _k cos θ _k −A _k sin θ _k ) (8) Next, P _k (k = 1, 2, ... N) shown in the following equation is _obtained .

P _k = (a _k ) ² − (b _k ) ² (9) When the above formula (9) is expanded using the formulas (7) and (8), A _k = ± 1, B _k = ± 1, P _k = 2 (H _k ) ² A _k B _k sin (2θ _k ) (10) Further, Q _k (k = 1, 2, ... N) shown in the following equation is _obtained .

Q _k = a _k × b _k (11) When the above formula (11) is expanded using the formulas (7) and (8), Q _k = 2 (H _k ) ² A _k B _k cos (2θ _k ) (12) Next, P _k shown in equation (10) and Q _k shown in equation (12)
Is used to find S _k shown in the following equation.

S _k = P _k × Q _k = (H _k ) ⁴ sin (4θ _k ) (13) Further, P _k shown in equation (10) and Q _k shown in equation (12).
Is used to determine T _k shown in the following equation.

T _k = (P _k ) ² +4 (Q _k ) ² = 4 (H _k ) ⁴ (14) Next, S _k shown in equation (13) and T _k shown in equation (14).
Is used to calculate V _k shown in the following equation.

V _k = S _k / T _k = sin (4θ _k ) / 4 (15) In equation (15), it is assumed that, for example, (4θ _k ) ≦ 30 °, then sin (4θ _k ) = 4θ _k V _k shown in the equation (15) is V _k = θ _k (16) Next, the QPSK data q in which the selection flag is “1”
The number of (f _k ) is N ′ (N ′ ≦ N), and V _{k based} on QPSK data q (f _k ) whose selection flag is “1” is V
′ _H (h = 1, 2, ... _{N ′} ), the phase displacement θ _k of the QPSK data q (f _k ) whose selection flag is “1” is θ ′ _h, and the average value of V ′ _h is V, θ. When _{'h, V = Σ (V'} h) / N'= Σ (θ'h) / N'(17) above V is selected flag is "1" QPSK data q
Since it is the average value of the phase displacement θ ′ _h of (f _k ), it is the average phase difference θ. That is, V = θ (18) In FIG. 4, 41 is a squaring circuit for squaring the input signal, and the real part data a _{k of} the QPSK data q (f _k ) demodulated by the FFT device 11 is input, a _k )
Outputs ² .

Reference numeral 42 is a squaring circuit for squaring the input signal, which receives the imaginary part data b _{k of} the QPSK data q (f _k ) and outputs (b _k ) ² .

Numeral 43 is a multiplier for multiplying two input signals, and the real part data a _k and the imaginary part data b _{k of} the QPSK data q (f _k ) are inputted to it and a _k × b _k , that is, (11 ) Or _Qk shown in (12) is output.

44 is an input signal from the squaring circuit 41,
Is a subtractor for subtracting the input signal from the squaring circuit 42,
( _Ak ) ^2- ( _bk ) ² , that is, formula (9) or (1
A voltage proportional to P _k shown in equation (0) is output.

Reference numeral 45 denotes a multiplier for multiplying the input signal from the multiplier 43 by the input signal from the subtractor 44, and P _k
Outputs × Q _k , that is, S _k shown in equation (13).

Reference numeral 46 denotes a squaring circuit for squaring the input signal from the multiplier 43 and multiplying the squared signal by four.
(Q _k ) ² is output.

Reference numeral 47 is a squaring circuit for squaring the input signal from the subtractor 44 and outputs (P _k ) ² .

Reference numeral 48 is an adder for adding the input signal from the squaring circuit 46 and the input signal from the squaring circuit 47,
T _k shown in equation (14) is output.

Reference numeral 49 is a divider for dividing the input signal from the multiplier 45 by the input signal from the adder 48.
A signal proportional to V _k shown in equation 6), that is, θ _k is output.

Reference numeral 50 denotes an average value calculation circuit for calculating the average value of the input signal from the divider 49 when the selection flag is "1", which is proportional to V shown in the equation (18), that is, the average phase difference θ. Output voltage.

Reference numeral 51 has a gain K, and in order to control the response characteristic of the feedback system, the input voltage V from the average value calculation circuit is multiplied by K, and this KV is used as the frequency control voltage of the local intermediate wave phase. It is an amplifier that feeds back to IF-LO1 shown in FIG.

Returning to FIG. 1, the IF-LO 1 outputs a local intermediate wave whose frequency is adjusted according to the frequency control voltage KV input from the frequency control circuit 14.

The QPSK data demodulated by the AFC device shown in FIG. 1 is subjected to QPSK demodulation, Vitahi decoding, deinterleaving, etc. in a coded data demodulation section of an OFDM receiver and demodulated into coded data.

The QPSK data having a power spectrum below the power spectrum reference level R ₀ , that is, the QPSK data whose selection flag is “0”, is regarded as erroneous demodulated coded data in the coded data demodulation section. However, this can be compensated by the error correction by the Vitahi compound.

Thus, according to the above embodiment, the IF
-The OFDM signal and the local intermediate wave output from IF-LO1 are multiplied by the mixers 5 and 6 of the orthogonal demodulation circuits 3 and 4, and the orthogonal demodulation circuit 3 is input via the A / D converters 9 and 10. FFT the output signals from 4 and
The device 11 converts the signal into the frequency domain signal, the power spectrum level of each frequency band of the frequency domain signal is detected by the power spectrum calculation circuit 12, and the frequency band component signal whose power spectrum level is equal to or higher than a predetermined level is fed back. The data selection circuit 13 selectively outputs the data, and the frequency control circuit 14 outputs the output signal component of the feedback data selection circuit 13 to the transmission channel I
The average of the phase error generated in each subcarrier of the F-OFDM signal is detected as the average phase difference, and the frequency control signal based on this average phase difference is given to IF-LO1 to reduce the phase error. QPSK significantly affected by fading by controlling the frequency
Since the average phase difference between the intermediate frequency OFDM signal and the local intermediate wave can be detected by excluding the data, it is possible to suppress the influence of fading and perform stable automatic frequency control.

Next, an example of a method of determining the power spectrum reference level R ₀ in the feedback data selection circuit 13 will be described.

In mobile communication, fading in the case of transmitting signals in which the amplitudes are about the same magnitude and the phase of each wave is random is the power spectrum level calculated by the power spectrum calculation circuit 12 according to the Rayleigh distribution law. The probability density function p (R) where R is R can be expressed by the following equation by Rayleigh distribution with σ ² being the variance of the power spectrum level.

P (R) = (R / σ ² ) exp {− (R) ² / (2σ ² )} (19) Next, the probability density function p (R) shown in the equation (19) is changed from 0 to R.
The cumulative distribution obtained by integrating in the range of ′ is
The probability P (R ') that the power spectrum level calculated by the power spectrum calculation circuit 12 is equal to or lower than R'is shown, and this probability P (R') can be expressed by the following equation.

[0090]

(Equation 1) When the power spectrum level calculated by the power spectrum calculation circuit 12 when the normally transmitted OFDM signal is received is R _n , the power spectrum level is R ′ ₍ in the subcarriers whose power spectrum level is R _n. An IF in which subcarriers with R ′ <R _n (subcarriers having amplitude distortion and phase distortion) are mixed in at a ratio indicated by the probability P (R ′) of the equation (20).
-Create an OFDM signal. Each subcarrier of this IF-OFDM signal is QPSKed by QPSK data of known encoded data.

The IF-OFDM signal is demodulated by the AFC device and the coded data demodulation section. At this time, in the AFC device, the average phase difference θ is calculated from all the demodulated QPSK data, and the frequency of the local intermediate wave is controlled.

The power spectrum reference level R ₀ in the feedback data selection circuit 13 is checked by checking whether the encoded data assigned to the subcarrier having the power spectrum level R _n is normally demodulated.
To determine.

For example, when all the encoded data of the subcarriers having the power spectrum level R _n are normally demodulated when R ′ ≧ R ₁ and some are erroneously demodulated when R ′ <R _1. Then, this R ₁ is set as the power spectrum reference level R ₀ .

Thus, the reference level is determined based on the cumulative distribution of the probability density function by Rayleigh distribution, which indicates the probability that the power spectrum level of certain QPSK data will be below a certain level due to the influence of fading.
By selecting the QPSK data in the feedback data selection circuit 13 using this reference level, the QPSK data affected by the fading can be appropriately excluded.

[0095]

As is apparent from the above description, according to the AFC device of the present invention, the received signal obtained by modulating the plurality of subcarriers with the signal data by the multiplying means and the local intermediate wave transmitting means are outputted. Multiply with the local intermediate wave,
The signal converting means converts the output signal from the multiplying means into a frequency domain signal, the power spectrum level detecting means detects the power spectrum level for each frequency band of the frequency domain signal, and the signal selecting means uses the power spectrum level. A frequency band component signal whose level is equal to or higher than a predetermined level is selectively output, and the average of phase errors generated in each subcarrier by the transmission channel of the received signal is detected as an average phase difference by the phase difference detection unit from the output signal component of the signal selection unit. However, by giving a frequency control signal based on the average phase difference to the local intermediate wave transmitting means by the frequency control means and controlling the frequency of the local intermediate wave so as to reduce the phase error, it is significantly affected by fading. Intermediate frequency OFDM signal excluding QPSK data It is possible to detect the average phase difference between the local intermediate wave and suppresses the influence of fading,
This has the effect that stable automatic frequency control can be performed, and therefore the accuracy of demodulated data can be improved.

According to the second aspect of the present invention, the AFC apparatus is based on the cumulative distribution of the probability density function by Rayleigh distribution, which indicates the probability that the power spectrum level of certain QPSK data will be below a certain level due to the influence of fading. By determining the reference level and using the reference level to select the QPSK data in the signal selecting means, the QPSK data affected by the fading can be appropriately excluded, and thus the accuracy of the demodulated data can be further improved. It has an effect that it can be further improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of an AFC device of the present invention.

FIG. 2 is a circuit configuration diagram of a power spectrum calculation circuit in the embodiment of the AFC device of the present invention.

FIG. 3 is a diagram of O which has been subjected to frequency selective fading during transmission.
QPS when FDM signal is demodulated by AFC device
It is a power spectrum characteristic view and phase displacement characteristic view of K data.

FIG. 4 is a circuit configuration diagram of a frequency control circuit in the embodiment of the AFC device of the present invention.

FIG. 5 is a block diagram showing a schematic configuration of a conventional AFC device.

[Explanation of symbols]

 1 IF-LO (Intermediate wave local oscillator) 2 Phase shifter 3, 4 Quadrature demodulation circuit 5, 6 Mixer 5 7, 8 LPF 9, 10 A / D converter 11 FFT device (Fast Fourier transform device) 12 Power spectrum Calculation circuit 13 Feedback data selection circuit 14 Frequency control circuit

Claims (2)

[Claims] 1. A local intermediate wave transmitting means for outputting a local intermediate wave based on a frequency control signal, a multiplying means for multiplying a received signal obtained by modulating a plurality of subcarriers with signal data, and the local intermediate wave. A signal converting means for converting an output signal from the multiplying means into a frequency domain signal; a power spectrum level detecting means for detecting a power spectrum level for each frequency band of the signal converted by the signal converting means; A signal selecting means for selectively outputting a frequency band component signal having a spectrum level equal to or higher than a predetermined reference level, and an average of phase errors generated in each subcarrier by the transmission channel of the received signal from the output signal component of the signal selecting means is averaged. Phase difference detecting means for detecting the phase difference, and reducing the phase error based on the average phase difference. And a frequency control means for giving the frequency control signal for controlling the frequency of the local intermediate wave to the local intermediate wave transmitting means. 2. A power spectrum level determined based on a cumulative distribution of a probability density function by Rayleigh distribution,
The AFC device according to claim 1, wherein the AFC device is used as a reference level in the signal selecting means.