JP5344429B2 - FFT window extension generation method - Google Patents

Description

  The present invention is an extended FFT analysis section used when a received signal modulated by an OFDM (Orthogonal Frequency Division Multiplex) modulation method is demodulated by FFT (Fast Fourier Transform). The present invention relates to an FFT window extension generation method for generating an FFT window.

  Conventionally, an OFDM system that transmits a plurality of orthogonal subcarriers (carrier waves) at the same time is a terrestrial digital television broadcasting system (hereinafter simply referred to as “terrestrial digital broadcasting”) as described in Patent Documents 1 and 2, for example. It can be used for various purposes.

  FIG. 7 is a frame configuration diagram of a transmission signal in the conventional OFDM system described in Patent Documents 1 and 2 below. Hereinafter, a conventional OFDM demodulation method will be described with reference to FIG.

  Each transmission symbol SB includes a guard interval (also referred to as a cyclic prefix) GI and an effective OFDM symbol (hereinafter simply referred to as “effective symbol”) S. The guard interval GI is obtained by extracting the rear part Sa of the time waveform of the effective symbol S and copying it to the head.

  In digital transmission using the OFDM method, if distortion or multipath exists in the transmission path, the orthogonality of the received signal is damaged and disturbed, and the demodulated signal is referred to as inter symbol interference (hereinafter referred to as “ISI”). ) And the error rate is deteriorated. In order to solve this, at the expense of a part of transmission energy (transmission power), before the effective symbol S that is originally intended to be transmitted, the rear part of this effective symbol S (the whole tens of tenths to one-fifth Period) The data of Sa is used, and an invalid ISI absorption guard interval GI is provided as the buffer data portion. If such a guard interval GI is provided, even if there is a delayed wave reflected by an obstacle in addition to the direct wave, if this delay amount is shorter than the guard interval GI, it is good without causing ISI. Reception is possible.

  When a transmission signal having such a configuration is sent to the receiving side, the receiving side ignores the information of the guard interval GI, so that even if a delay occurs only in a certain carrier, it can be within this guard interval GI. Since the delay is ignored, it can be received correctly. In particular, since the data of the rear portion Sa of the effective symbol S is copied in the guard interval GI, no information is lost even if a certain carrier shifts.

  Therefore, in the conventional demodulation method, the guard interval removal unit removes the guard interval GI for each transmission symbol SB from the received OFDM transmission signal, and extracts only the effective symbol S. The extracted effective symbol S is demodulated by fast discrete Fourier transform by the FFT unit.

  However, in the conventional OFDM demodulation, since only one period of the OFDM modulated signal is used for demodulation from the modulation waveform with the guard interval GI, the received signal waveform corresponding to the same length as the guard interval GI is not used. For this reason, the power efficiency corresponding to the transmission of the guard interval GI is lowered.

  As a countermeasure, for example, in the techniques described in Patent Documents 1 and 2, in the FFT window extension generation method used for the modulation of the OFDM scheme, for a received signal having a time length equal to or greater than the effective symbol length W that is OFDM-modulated, When the received signal is demodulated by FFT processing using the extended FFT window, the extended FFT window is generated by combining the received signals before or after the effective symbol length W by calculation. ing.

JP 2008-109173 A

Special table 2008-537424 gazette

  However, in the conventional FFT window extension generation methods described in Patent Documents 1 and 2 and the like, since reception signals for the extension window are used, reception characteristics can be improved and fading resistance can be improved. As a result, the power efficiency corresponding to the transmission of the guard interval GI can be improved. However, since the effect of improving the characteristics is small, further improvement of the characteristics has been desired.

The FFT window extension generation method of the present invention uses the extended extension window to demodulate the received signal by FFT processing with respect to the received signal having a time length equal to or longer than the OFDM-modulated effective OFDM symbol length. expansion window, the FFT window expansion method of generating synthesized by calculating the received signal before or after the effective OFDM symbol length, the applied to the received signal in said effective OFDM symbol length when said synthetic In the first coefficient and the second coefficient to be multiplied by the received signal in the extended window, the second window is set to be larger than the first coefficient, and the calculation is performed to generate the extended window. To do.

According to the FFT window extension generation method of the present invention, since the calculation is performed by setting the second coefficient to be larger than the first coefficient and generating the extension window, the dominant term causing the channel quality degradation is inter-code interference. If it is based on phase rotation such as (Inter Carrier Interference, hereinafter referred to as “ICI”), the characteristics can be improved over the conventional method of α + β = 1. Therefore, the reception characteristics and fading resistance can be further improved.

FIG. 1 is a frame configuration diagram of a transmission signal in the OFDM system showing an outline of Embodiment 1 of the present invention. FIG. 2 is a frame configuration diagram showing an example of a transmission signal in the OFFM system of FIG. FIG. 3 is a frame configuration diagram showing another example of a transmission signal in the OFDM system of FIG. FIG. 4 is a schematic configuration diagram of a demodulation device in the OFDM system showing Embodiment 1 of the present invention. FIG. 1 is a frame configuration diagram of a transmission signal in the OFDM system, showing an outline of Embodiment 2 of the present invention. FIG. 6 is a frame configuration diagram showing an example of a transmission signal in the OFDM system of FIG. FIG. 7 is a frame configuration diagram of a transmission signal in the conventional OFDM system.

  Modes for carrying out the present invention will become apparent from the following description of the preferred embodiments when read in light of the accompanying drawings. However, the drawings are only for explanation and do not limit the scope of the present invention.

  FIG. 1 is a frame configuration diagram of a transmission signal in the OFDM system showing an outline of the first embodiment of the present invention. FIG. 2 is a frame configuration diagram illustrating an example of a transmission signal in the OFDM scheme of FIG. 1, and FIG. 3 is a frame configuration diagram illustrating another example of a transmission signal in the OFDM scheme of FIG.

FIG. 1 shows an example of the reference phase θ in the received signal S12. Correspondingly, an example of the reference phase θ according to the conventional method and an example of the reference phase θ according to FIG. 2 of the first embodiment (coefficient β = 1). .1, α = 0) and an example of the reference phase θ (coefficient β = 0.6, α = 0.5) according to FIG.

  The transmission signal frame structure shown in FIGS. 1 to 3 is similar to the conventional frame structure of FIG. 7. For example, a transmission signal (transmission signal) modulated by the OFDM method is a group of a plurality of transmission symbols SB. One frame is configured. A pilot symbol for synchronization detection is inserted at the head of one frame. The pilot symbol is a no-signal period, and a symbol reference signal can be generated on the receiving side using this signal. Each transmission symbol SB includes a guard interval GI and an effective symbol S. The guard interval GI is obtained by extracting the rear part of the time waveform of the effective symbol S and copying it to the head.

  A feature of the first embodiment is that a received signal corresponding to a time length equal to or greater than the effective symbol S is stored in a memory, and from the received signal stored in the memory, the time length of the effective symbol S is increased by an FFT window generation operation For generating an FFT input signal using an extended reception signal (that is, an expanded FFT window 1 in which expansion windows (expansion windows) W1 and W2 are added to the beginning and end of the effective symbol length W0), respectively. This is an FFT window expansion method.

  Hereinafter, an OFDM demodulator using this FFT window expansion method and this demodulation method will be described.

(Demodulation device of Embodiment 1)
4 (a) and 4 (b) are schematic configuration diagrams of a demodulation device in the OFDM system showing Embodiment 1 of the present invention. FIG. 4 (a) is an overall view of the demodulation device, and FIG. b) is a functional block diagram of the memory and the FFT window generation calculation unit in the demodulator.

  The OFDM demodulator shown in FIG. 4A includes a synchronization processing unit 10 that controls the operation timing of the apparatus. The synchronization processing unit 10 converts the frequency of the received input signal Sin and outputs an analog baseband signal S11, and converts the analog baseband signal S11 into a received signal S12 that is a digital baseband signal. An analog / digital (hereinafter referred to as “A / D”) conversion unit 12 for conversion, an FFT processing unit 20 that outputs a reception data S24 composed of complex symbol data by performing a fast discrete Fourier transform process on the reception signal S12, and A decoding unit 25 that decodes the received data S24 and outputs demodulated data Sout is connected.

  The FFT processing unit 20 stores a received signal S12 corresponding to a time length equal to or greater than the effective symbol S, and the time length of the effective symbol S (that is, the effective symbol length) from the received signal S12 stored in the memory 21. ) The same FFT input signal length fft_t_length as W0 (the reception signal S12a corresponding thereto is rx_sig (T)) or more of the received signal [that is, the reception signal S12b for the expansion window W1 is rx_sig (T-fft_t_length) , The received signal S12c for the extended window W2 uses rx_sig (T + fft_t_length)] to generate an FFT window generation calculation unit 22 that generates an FFT input signal fft_in (T) and the FFT input signal fft_in (T) at high speed. An FFT unit 23 that performs Fourier transform and outputs parallel received data; a parallel / serial (hereinafter referred to as “P / S”) converter 24 that converts the parallel received data into serial received data S24; have.

  The memory 21 shown in FIG. 4B receives the received signal S12, and receives the received signal S12a = rx_sig (T) corresponding to the effective symbol length W0 and the extended window W1 extended beyond the time length of the effective symbol S. It has an area for storing the signal S12b = rx_sig (T-fft_t_length) and the received signal S12c = rx_sig (T + fft_t_length) of the extended window W2 extended to the effective symbol length W0 or more.

The FFT window generation calculation unit 22 shown in FIG. 4B includes a multiplication unit 22a that multiplies the first coefficient α that is a ratio to the reception signal S12b of the effective symbol length W0 read from the memory 21, and the memory 21. Multiplication means 22b for multiplying the received signal S12b of the expansion window W1 read from the second coefficient β, which is a ratio, and the coefficient β for the reception signal S12c of the expansion window W2 read from the memory 21. Multiplication means 22c for multiplication. An adding means 22d is connected to the output sides of the multiplying means 22a and 22b, and an adding means 22e is also connected to the output sides of the multiplying means 22a and 22c.

Here, the coefficient α is multiplied by the components of the received signals S12b and S12c included in the extended windows W1 and W2 in FIGS. 2 and 3, respectively (the coefficient β at this time is 0 <β ≦ 1.1). ). Further, the coefficient β is multiplied by the component of the received signal S12a included in the effective symbol length W0 in FIGS. 2 and 3 (the coefficient α at this time is 0 ≦ α ≦ 1). The feature of the first embodiment is that α + β ≠ 1 , and β> α is set. FIG. 2 shows an example in which the coefficient α = 0 and the coefficient β = 1.1 . FIG. 3 shows an example in which the coefficient α = 0.5 and the coefficient β = 0.6 .

  The adding unit 22d adds the output signal S22a = α * rx_sig (T) of the multiplying unit 22a and the output signal S22b = β * rx_sig (T-fft_t_length) of the multiplying unit 22b, and adds the output signal S22d = {α * rx_sig ( T) + β * rx_sig (T-fft_t_length)} is output, and the multiplication means 22f is connected to the output side. The adding means 22e adds the output signal S22c of the multiplying means 22c = β * rx_sig (T + fft_t_length) and the output signal S22a of the multiplying means 22a, and outputs an output signal S22e = {β * rx_sig (T + fft_t_length) + α *. rx_sig (T)} is output, and a multiplier 22g is connected to the output side.

  The multiplying unit 22f multiplies the output signal S22d of the adding unit 22d by a coefficient 1 / (α + β) and outputs an output signal S22f = {α * rx_sig (T) + β * rx_sig (T-fft_t_length)} / (α + β) is output, and the switching means 22h is connected to this output side. The multiplication unit 22g multiplies the output signal S22e of the addition unit 22e by a coefficient 1 / (α + β) and outputs the output signal S22g = {β * rx_sig (T + fft_t_length) + α * rx_sig (T)} / (α + β) is output, and the switching means 22h is connected to this output side.

  The switching unit 22h switches the output signal S22f of the multiplication unit 22f, the reception signal S12a, and the output signal S22g of the multiplication unit 22g, and sequentially outputs them to the FFT unit 23 in the form of an FFT input signal fft_in (T). Is.

(Demodulation method of Embodiment 1)
The demodulating method according to the first embodiment is characterized in that, in the demodulating device of FIG. 4, the received signal components by the extended windows W1 and W2 are used in addition to the effective symbol length W0, thereby obtaining a partial time diversity effect. Is. Hereinafter, this demodulation method will be described.

  When a transmission signal as shown in FIGS. 1 to 3 (for example, the transmission signal in FIG. 3) subjected to OFDM modulation is subjected to signal processing such as filtering and then input as an input signal Sin, the input signal Sin is The frequency converter 11 converts the signal into a corresponding analog baseband signal S11. The converted analog baseband signal S11 is sampled by the A / D converter 12 and converted into a received signal S12 which is a digital baseband signal (I signal and Q signal), and a time length equal to or longer than the converted OFDM symbol. Is received in the memory 21 in the FFT processing unit 20.

  As shown in FIG. 3, the FFT window generation calculation unit 22 in the FFT processing unit 20 includes, in addition to the reception signal S12a corresponding to the effective symbol length W0, the expansion windows W1 and W2 before and after the reception signal S12a. In order to use the received signals S12b and S12c for the FFT input signal fft_in (T), the following FFT window generation calculation is performed.

  The reception signal S12b obtained in the extended window W1 positioned at the head of the reception signal S12 in FIG. 3 is combined with the reception signal S12a in the effective symbol S at the time position that is after the effective symbol length W0. Similarly, the reception signal S12c obtained in the extended window W2 located at the end of the reception signal S12 is combined with the reception signal S12a in the effective symbol S at the time position that is the previous effective symbol length W0. When these are expressed by mathematical formulas, the formula (1) is obtained.

Where T: sampling time of the received signal S12
fft_in (T): FFT input signal
rx_sig (T): received signal S12
fft_t_length: FFT input signal length (same as effective symbol length)
spr_win_size: Extended window length
β : a coefficient to be multiplied by signal components included in the expansion windows W1 and W2 outside the FFT window 1 ( 0 <β ≦ 1.1 )
α : coefficient multiplied by signal component included in effective symbol length W0 (0 ≦ α ≦ 1)
(Note that FIG. 2 shows an example in which α = 0)

  The FFT input signal fft_in (T) obtained by such calculation is subjected to fast discrete Fourier transform by the FFT unit 23 in the FFT processing unit 20 and converted into parallel received data corresponding to each subcarrier. The converted parallel received data is converted by the P / S converter 24 into serial received data (complex symbol data) S24.

  The converted serial reception data S24 is subjected to waveform equalization processing for correcting transmission path characteristics, QAM (Quadrature Amplitude Modulation) mapping processing for detecting amplitude and phase information, trellis decoding processing, and error correction processing by the decoding unit 25. Etc. and the demodulated data Sout is output.

(Effect of Example 1)
According to the first embodiment, there are the following effects (a) to (c).

(A) In the first embodiment, unlike the methods described in the conventional Patent Documents 1 and 2, etc., the signal components included in the received signal S12a in the FFT window 1 and the received signals S12b, The coefficients α and β to be multiplied with the signal components included in S12c are 1/2 and the sum of the coefficients α and β is 1 (α + β = 1). Do not dare to match the signal components to the same. For example, when coefficients β = 0.6 and α = 0.5, the signal components of the extension windows W1 and W2 are larger in power than the signal components included in the effective symbol length W0 in the FFT window 1. . However, as shown in FIG. 1, when phase rotation occurs due to fading or the like, the power ratio of the signal components included in the extended windows W1 and W2 having the largest phase rotation angle is increased, and the time position is increased. It is possible to cancel out the phase rotation component in the FFT window 1 more strongly by combining them with different values.

  In this method of α + β ≠ 1, the continuity of the signal level in the time domain of the desired signal component may be lost and the desired signal may be degraded. However, the dominant term causing the channel quality degradation is due to phase rotation such as ICI. If so, the characteristics are improved compared to the conventional method of α + β = 1 described in Patent Documents 1 and 2 and the like.

  (B) The received signals S12b and S12c of the extended windows W1 and W2 having the same modulation signal component and superposed with uncorrelated noise components have an effect of suppressing the noise components included in the extended windows W1 and W2. The reception characteristics can be further improved.

  (C) The reception signals S12b and S12c of the extended windows W1 and W2 having the same modulation signal component and superposed with an uncorrelated noise component alleviate the phase rotation of the modulation signal component due to fading and reduce the influence of ICI. By doing so, fading tolerance can be improved more.

(Configuration of Example 2)
FIG. 5 is a frame configuration diagram of a transmission signal in the OFDM scheme showing an outline of the second embodiment of the present invention, and FIG. 6 is a frame configuration diagram showing an example of a transmission signal in the OFDM scheme of FIG. Elements common to those shown in FIGS. 1 to 3 are denoted by common reference numerals.

  FIG. 5 shows an example of the reference phase θ in the received signal S12. Correspondingly, an example of the second embodiment which is an application example of the first embodiment to FIG. 2 and FIG. 3 of the first embodiment. Another example of the second embodiment which is an application example is shown. FIG. 6 corresponds to FIG. 5 and shows an example of the second embodiment in which the tilt window function is used with respect to FIG. 2 of the first embodiment and a tilt window with respect to FIG. 3 of the first embodiment. Another example of the second embodiment in the case of using a function is shown.

  The frame configuration of the transmission signal and the configuration of the demodulator in the second embodiment are almost the same as those in the first embodiment.

(Demodulation method and effect of Embodiment 1)
In the second embodiment, similarly to the first embodiment, the reception signal S12 corresponding to a time length equal to or greater than the effective symbol S is stored in the memory 21, and the FFT window generation calculation unit is obtained from the reception signal S12 stored in the memory 21. 22 is an FFT window expansion method for generating the FFT input signal fft_in (T) using the received signals of the expansion windows W1 and W2 that have been expanded beyond the time length of the effective symbol S by the FFT window generation calculation using FIG.

  For example, as shown in FIGS. 5 and 6, the FFT window expansion method of the second embodiment is performed by the FFT window generation operation unit 22 in FIG. 4 in the expansion outside the FFT window 1 in FIG. 2 of the first embodiment. When combining the components of the received signals S12b and S12c of the windows W1 and W2 with the components of the received signal S12a having the effective symbol length W0, the time domain in which the components of the received signals S12b and S12c of the extended windows W1 and W2 are combined (for example, , Reception signal S12b + S12c, reception signal S12c + S12a, reception signal S12a + S12b, reception signal S12b + S12c) and a predetermined window function (for example, slope) in the vicinity of the boundary between the unsynthesized time domain (for example, reception signal S12c, reception signal S12b) Window function).

  In FIG. 3 of the first embodiment, the time domain (for example, received signal S12a + S12b + S12c, received signal S12c + S12a, received signal S12a + S12b, received signal S12a + S12b + S12c) in which the components of the received signals S12b and S12c of the expansion windows W1 and W2 are combined. An inclination window function is applied in the vicinity of the boundary line with the time domain (for example, reception signal S12c, reception signal S12b) that is not combined.

  Therefore, in OFDM with a small number of FFT points, degradation of the desired signal caused by discontinuous points with respect to the total number of FFT input signals fft_in (T) (locations where the reference phase θ shown in FIG. 5 is not connected) is reduced. When there is an unnecessary signal component generated by discontinuous points in the FFT input, the larger the number of FFT points, the smaller the deterioration component ratio due to discontinuous points mixed per point. Therefore, as the number of FFT points is smaller, the reception performance is improved by applying such a gradient window function.

  The FFT input signal fft_in (T) obtained by such an FFT window generation operation is subjected to fast discrete Fourier transform by the FFT unit 23 in the FFT processing unit 20 corresponding to each subcarrier, as in the first embodiment. After being converted into parallel received data, the P / S conversion unit 24 and the decoding unit 25 process the same as in the first embodiment, and the demodulated data Sout is output. Therefore, it is possible to achieve substantially the same effect as in the first embodiment.

(Modification)
The present invention is not limited to the above-described embodiment. For example, the FFT window generation calculation unit 22 shown in FIG. 4 is changed to a functional block having another configuration, or the gradient window function shown in FIGS. 5 and 6 is used. You may make it multiply using another window function. The FFT window extension generation method of the present invention can be applied not only to terrestrial digital broadcasting but also to all those using OFDM modulation, and characteristics improvement is strongly expected for them.

20 FFT processing unit 21 Memory 22 FFT window generation calculation unit 23 FFT unit 24 P / S conversion unit

Claims (5)

When the received signal is demodulated by FFT processing using an extended extension window with respect to a received signal having a time length equal to or longer than an OFDM-modulated effective OFDM symbol length, the extension window is set to the effective OFDM symbol length. In the FFT window extension generation method for generating the reception signal before or after by combining the reception signals by calculation,
In the first coefficient to be applied to the received signal within the effective OFDM symbol length and the second coefficient to be applied to the received signal within the extension window at the time of combining, the second coefficient is set to be greater than the first coefficient. An FFT window extension generation method , wherein the extension window is generated by performing a calculation with a large setting . 2. The FFT window extension generation method according to claim 1, wherein an operation in which a sum of the first coefficient and the second coefficient exceeds 1 is performed. FFT window expansion generating method according to claim 1 or 2, characterized in applying a predetermined window function in joint between the received signal of the reception signal and the extension in the window in said effective OFDM symbol length. The method of claim 3, wherein the predetermined window function is an inclined window function. The expansion window is
5. The received signal having a time length equal to or longer than the effective OFDM symbol length modulated by the OFDM is stored in a memory, and is generated by an FFT window generation calculation means using the stored received signal. The FFT window extension generation method according to any one of the above.

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