JP3405307B2 - Frequency offset detector for multicarrier transmission - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a frequency offset detecting device for detecting a carrier frequency offset in multicarrier transmission. 2. Description of the Related Art FIG. 1 shows a configuration example of the prior art. The frequency offset detection circuit shown in FIG. 1 converts an in-phase signal (hereinafter, I signal) and a quadrature signal (hereinafter, Q signal) obtained by orthogonally demodulating an orthogonal frequency division multiplex modulation signal (hereinafter, OFDM signal). This is a device for inputting and detecting a carrier frequency offset. FIG. 1 shows a delay circuit 1 for obtaining a signal obtained by delaying an I signal by an effective symbol period (hereinafter referred to as Id signal) and a delay circuit 1 for obtaining a signal obtained by delaying a Q signal by an effective symbol period (hereinafter referred to as Qd signal). The difference between the I signal and the Id signal (hereinafter, D0 signal), the difference between the I signal and the Qd signal (hereinafter, D1 signal), and the difference between the Id signal and the Q signal (hereinafter, the D2 signal) ), An absolute value circuit for obtaining the absolute values of D0, D1 and D2, an integrating circuit for integrating the absolute values of D0, D1 and D2 for the guard interval period, Of the obtained integrated values (ID0, ID1, ID2), I
A subtraction circuit 3 for obtaining a difference (ID1-ID2) between D1 and ID2;
And a gate means for extracting the value of (ID1-ID2) at the obtained peak of the drop. When there is no frequency offset, an I signal and an Id signal obtained by orthogonally demodulating an OFDM signal have the same waveform in a guard interval period. However, when there is a frequency offset, the I signal and the Id signal do not have the same waveform even within the guard interval period. This is because the presence of the frequency offset causes the phases of the OFDM signal and the local oscillator input to the quadrature demodulator to change with time. [0004] For example, when the phase rotation due to the frequency offset is -90 degrees, during the guard interval period, I and
Qd has the same waveform. Similarly, when the phase rotation due to the frequency offset is 90 degrees, Q and Id have the same waveform. FIG.
In the prior art shown in (1), the frequency offset is obtained by calculating the identity of the waveform in the guard interval portion. Specifically, the difference between I and Qd and the difference between Q and Id are calculated, and the differences are integrated for the guard interval period to quantitatively determine the waveform identity. FIG. 2 shows the relationship between the frequency offset and the output signal. From this figure, it can be seen that when the frequency offset is -0.25 to 0.25, that is, between -1/4 and +1/4 of the subcarrier interval of OFDM, the output decreases almost in proportion. Thus, in the prior art, (I
By calculating D1-ID2), a numerical value corresponding to the frequency offset is obtained. FIG. 3 shows the relationship between the frequency offset and the output signal when the strength of the received signal changes in the prior art. As can be seen from FIG.
In the related art, the magnitude of the output signal varies depending on the strength of the received signal even when the frequency offset is constant. This is because the input signals ID1 and ID2 themselves to the subtraction circuit 3 for calculating the frequency offset amount fluctuate with the change in the intensity of the received signal as shown in FIG. [0006] When wireless transmission is performed by a mobile object such as an automobile, when a receiving station or a transmitting station moves, the intensity of a received signal changes with time. In this case, if the characteristic shown in FIG. 3 remains unchanged, the value of the frequency offset cannot be accurately detected. An object of the present invention is to provide a frequency offset detecting apparatus for multicarrier transmission that can always accurately detect a frequency offset without being affected by the strength of a received signal. In order to solve the above-mentioned problems, a frequency offset detecting apparatus for multicarrier transmission according to the present invention comprises a guard symbol having the same waveform as an effective symbol period and a part of the effective symbol period. In a multi-carrier signal receiving apparatus for receiving a multi-carrier signal having an interval period, for an in-phase component and a quadrature component orthogonally demodulated from a received signal, an in-phase component for delaying the in-phase component and the quadrature component by an effective symbol period, respectively Delay means and quadrature component delay means; difference calculation means for obtaining a difference between an undelayed in-phase component and a delayed quadrature component, and a difference between a delayed in-phase component and a non-delayed quadrature component, respectively; Means for calculating the absolute value of the obtained difference, and the obtained absolute value Means for integrating the integral value for the guard interval period, subtraction means for calculating the difference between the integral values of the obtained absolute values, and correction means for correcting the output of the subtraction means using the obtained absolute values. And gate means for extracting the output of the correcting means at the symbol detection time. The magnitude of the output of the integral calculating means is determined by the magnitude of the output of the received signal. It changes in proportion to the change in intensity. In the present invention, the effect of the change in the intensity of the received signal is canceled by correcting the output of the subtraction means using the signal input to the subtraction means. Therefore,
Even if the received signal strength changes, the carrier frequency offset can be accurately detected. As described above, according to the present invention, even when the level (signal strength) of a received signal changes, it is possible to accurately detect the carrier frequency offset that causes the deterioration of transmission characteristics. As a result, the frequency stability of the multicarrier receiving apparatus is improved, and deterioration of reception quality due to the frequency offset can be reduced. An embodiment of the present invention will be described with reference to the drawings. FIG. 5 shows the configuration of the first embodiment of the present invention. The frequency offset detection circuit of the present invention is different from the prior art configuration shown in FIG. 1 in that a maximum value circuit that outputs the larger value of ID1 and ID2 and a ratio between the maximum value circuit and the output of the subtraction circuit 3 are provided. The configuration is such that a required division circuit is added. That is, (ID1-ID2) is output from the subtraction circuit 3, and max (ID1, ID2) is output from the maximum value circuit. Each output signal is input to a division circuit, where the signal is divided and {(ID1-ID2) / max (ID1, ID2)} is output. Further, a signal obtained by extracting the output signal of the division circuit in synchronization with the signal from the peak detection circuit is the output signal of the frequency offset detection device. In the conventional frequency offset detecting device, ID1
And ID2 fluctuate with a change in the strength of the received signal. Therefore, the signal (ID1-ID2) output from the subtraction circuit 3 and the signal max (ID1, ID2) output from the maximum value circuit are also:
It fluctuates with a change in the strength of the received signal. However, even if the frequency offset changes, the value of max (ID1, ID2) does not change, and this value indicates the strength of the received signal. Therefore, (ID1-I
D2) and the ratio of max (ID1, ID2), ie the output of the divider
{(ID1-ID2) / max (ID1, ID2)} is a signal representing only the frequency offset without being affected by the intensity change of the received signal. FIG. 6 shows the relationship between the frequency offset and the output of the first embodiment. FIG. 6 shows an output signal when the intensity of the received signal is changed in three ways. FIG. 6 shows that the frequency offset can be accurately detected regardless of the strength of the received signal. In the prior art, when the transmitting / receiving station moves, the frequency offset cannot be detected correctly because the received signal strength changes. However, according to the present invention, even when the transmitting / receiving station moves, the frequency offset can be accurately detected. Can be detected. Therefore, the frequency stability of the multi-carrier receiving apparatus can be improved even when performing communication using the multi-carrier transmission scheme in a mobile object. FIG. 7 shows the configuration of the second embodiment of the present invention. In the second embodiment, the maximum value circuit of the first embodiment
The configuration is such that it is replaced with an adder circuit for obtaining the sum of 1 and ID2. That is, (ID1-ID2) is output from the subtraction circuit, and (ID1 + ID2) is output from the addition circuit. Each output signal is input to a division circuit, where the signal is divided and {(ID1-ID2) / (ID1 + ID2)} is output. FIG. 8 conceptually shows the relationship between the frequency offset and (ID1 + ID2) output from the adding circuit. (ID1 + ID
2) is maximum when the frequency offset is near 0, ± 0.25
It becomes smaller gradually within the range. The value obtained by dividing (ID1-ID2) by this value, that is, the output characteristic of the second embodiment is as shown in FIG.
Are more linear than the characteristics of the first embodiment shown in FIG. Therefore, the frequency offset amount can be detected more accurately than in the first embodiment. FIG. 10 shows the configuration of the third embodiment of the present invention. The third embodiment has a configuration in which the subtraction circuit 3, the addition circuit, and the division circuit of the first embodiment are replaced with conversion circuits. The conversion circuit outputs frequency offset values corresponding to the input ID1 and ID2. This processing can be easily realized using a ROM or the like in which the conversion table is stored. When the frequency offset value is calculated using the conversion table, the frequency offset becomes more linear in the range of ± 0.25 than the characteristic of the second embodiment shown in FIG. Therefore, the frequency offset amount can be detected more accurately than in the second embodiment. In each of the specific examples shown in FIGS. 5, 7 and 10, the integrating circuit 1 and the integrating circuit 2 perform the same operation even if the position is changed between the absolute value circuit and the gate.
The effect is obtained. For example, as in the configuration of the fourth embodiment shown in FIG. 11, the same operation and effect can be obtained by replacing the subtraction circuit and the integration circuit. It should be noted that the frequency offset detecting device for multicarrier transmission of the present invention is not limited to the illustrated example described above, and it is needless to say that various changes can be made without departing from the gist of the present invention. is there.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a configuration of a conventional technique. FIG. 2 is a graph showing a relationship between a frequency offset and an output signal according to the related art. FIG. 3 is a graph showing a relationship between a frequency offset and an output signal when the strength of a received signal changes in the related art. FIG. 4 is a graph showing a relationship between a received signal strength change and a frequency offset in the related art. FIG. 5 is a block diagram showing a configuration of a first example of the present invention. FIG. 6 is a graph showing a relationship between a frequency offset and an output signal when the intensity of a received signal changes in the first embodiment of the present invention. FIG. 7 is a block diagram showing a configuration of a second example of the present invention. FIG. 8 is a conceptual diagram illustrating a relationship between a frequency offset and an output signal according to a second embodiment of the present invention. FIG. 9 is a graph showing a relationship between a frequency offset and an output signal when the intensity of a received signal changes in the second embodiment of the present invention. FIG. 10 is a block diagram showing a configuration of a third example of the present invention. FIG. 11 is a block diagram showing the configuration of a fourth embodiment of the present invention.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tokuyoshi Suzuki 41-1 Ochimichi, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory Co., Ltd. (56) References JP-A-7-99486 (JP, A) JP-A-9-200176 (JP, A) JP-A-6-244818 (JP, A) JP-A 2001-111520 (JP, A) Mitsuyuki Shibata, Shuro Ito , Yoshitoshi Fujimoto, Hideaki Ito, Tokuyoshi Suzuki, Kazuo Otsuka, Keiya Yamazato, Akira Ogawa, Study of OFDM Synchronization Method Using Difference Operation, IEICE Technical Report, Japan Institute of Electronics, Information and Communication Engineers Vol., October 15, 1999. 99 No. 357, p. 97-101 (58) Field surveyed (Int. Cl. ⁷ , DB name) H04J 11/00

Claims (1)

(1) A multicarrier signal receiving apparatus for receiving a multicarrier signal having an effective symbol period and a guard interval period having the same waveform as a part of the effective symbol period. For the in-phase component and the quadrature component that have been quadrature demodulated from, the in-phase component delay unit and the quadrature component delay unit that delay the in-phase component and the quadrature component by the effective symbol period, respectively, Difference calculating means for calculating the difference between the component and the delayed in-phase component and the difference between the delayed quadrature component, the absolute value calculating means for calculating the absolute value of the obtained difference, and the obtained absolute value Means for integrating the distance by the guard interval period, and the difference between the integrated values of the obtained absolute values Arithmetic means, correction means for correcting the output of the subtraction means using the obtained absolute values, and gateway means for extracting the output of the correction means at the symbol detection time. Frequency offset detector for multi-carrier receiver.