JP2004304618A - Ofdm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM receiver capable of smoothly and effectively executing reception processing even under environment in which a preceding ghost can be generated. <P>SOLUTION: An FFT window position detection and decision processing part 44 calculates a delay profile by applying FFT processing to a pilot signal extracted by an equalization part 18. In addition, presence/absence of a preceding ghost is decided and delay time ΔT<SB>n</SB>between the current FFT window position and a window position to be applied to the preceding ghost is calculated based on the delay profile. Furthermore, a correction value for setting the window position to the preceding ghost is calculated from the difference between the delay time ΔT<SB>n</SB>and delay time ΔT<SB>n-1</SB>calculated in the previous time. An FFT window position setting part 40 changes the FFT window position based on the correction value. Furthermore, a phase frequency characteristic inverse calculation part 42 multiplies phase rotation quantity equivalent to the correction value to an FET processed complex OFDM signal. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
[Industrial applications]
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplex) OFDM receiving apparatus that receives a transmission wave using an OFDM (Orthogonal Frequency Division Multiplex) method, and more particularly, to an OFDM receiving apparatus that receives an OFDM digital broadcast and executes an FFT (Fast Fourier Transform) process. It is suitable.
[0002]
[Prior art]
FIG. 1 shows an example of the configuration of an OFDM receiver used in a digital broadcast receiver.
[0003]
The transmission wave received by the OFDM receiver conforms to the domestic terrestrial digital broadcasting standard.
[0004]
Referring to FIG. 1, OFDM receiving apparatus 100 includes A / D converter 2, I / Q separation section 4, narrowband carrier synchronization processing section 6, symbol synchronization processing section 8, clock synchronization processing section 10, FFT processing section 12 , A wideband carrier synchronization processing unit 14, a frame decoding processing unit 16, an equalizing unit 18, a frequency & time deinterleaving processing unit 20, a demapping processing unit 22, a Viterbi decoding processing unit 24, and an RS decoding processing unit 26. .
[0005]
In the OFDM receiving apparatus 100, an RF signal received from an antenna (not shown) is down-converted from an RF frequency band to an IF frequency band by a tuner (not shown) to generate an analog OFDM signal in the IF frequency band. . Then, this analog OFDM signal is converted into a digital OFDM signal in the A / D converter 2 in the figure.
[0006]
Thereafter, the digital OFDM signal is separated into an in-phase signal (I signal) and a quadrature signal (Q signal) by the I / Q separation unit 4, and is converted into a complex OFDM signal. Such a complex OFDM signal is sequentially divided for each transmission symbol period, with the transmission symbol period as one unit. One transmission symbol period is composed of a guard interval period and a subsequent effective symbol period. In the effective symbol period, one unit of an IFFT signal (effective symbol) generated by performing IFFT (inverse Fourier transform) on transmission data on the transmission side is allocated. In the guard interval period, the rear end portion 1 / n of the effective symbol is inserted as it is.
[0007]
The distortion of the frequency of the complex OFDM signal is corrected by the narrowband carrier synchronization processing unit 6 so that the frequency of each subcarrier after the FFT processing by the FFT processing unit 12 becomes an integral multiple of a predetermined frequency. . Here, the frequency distortion on the complex OFDM signal is detected based on the magnitude of the correlation in the symbol synchronization processing unit 8. That is, it is detected by the difference between the correlation value of the in-phase signal (I signal) and the correlation value of the quadrature signal (Q signal) at the position (described later) where the correlation is maximum in the symbol synchronization processing unit 8.
[0008]
The complex OFDM signal whose frequency distortion has been corrected in this way is input to the symbol synchronization processing unit 8 and the FFT processing unit 12, respectively.
[0009]
The symbol synchronization process 8 uses the fact that the guard interval period is a copy of a part of the effective symbol period, and uses the correlation between the output signal of the narrowband carrier synchronization processing unit 6 and the signal obtained by delaying the output signal by the effective symbol period. And the position where the value obtained by adding the correlation value of the in-phase signal (I signal) and the correlation value of the quadrature signal (Q signal) becomes the maximum is defined as the start timing of the effective symbol period. Then, a symbol synchronization pulse is generated at the start timing, and this is output to the clock synchronization processing unit 10 and the FFT processing unit 12.
[0010]
The clock synchronization processing section 10 receives an output from the symbol synchronization processing section 8, generates a synchronization clock, and supplies a clock to each block.
[0011]
The FFT processing unit 12 performs an FFT (Fourier transform) process on the complex OFDM signal whose frequency distortion has been corrected by the narrowband carrier synchronization processing unit 6 based on the symbol synchronization pulse output from the symbol synchronization processing unit 8. Is performed, the transmitting side demodulates the N-series (N is the number of multiplexed channels) complex OFDM signals OFDM-multiplexed on each subcarrier band.
[0012]
That is, the FFT processing unit 12 sets a time window (FFT window) starting from the beginning of the effective symbol period and having a time width of the effective symbol period based on the symbol synchronization pulse, and corresponds to the FFT window. An FFT process is performed on the complex OFDM signal for the period, and an N-sequence complex OFDM signal is demodulated.
[0013]
The wideband carrier synchronization processing unit 14 sets the N-sequence complex OFDM signal and the subcarrier band after demodulation so that the N-sequence complex OFDM signal demodulated by the FFT processing is correctly positioned in the corresponding subcarrier frequency band. Is corrected.
[0014]
Specifically, a pilot signal PC previously allocated to a specific subcarrier on the transmitting side is detected from the demodulated complex OFDM signal, and a subcarrier band in which the pilot signal PC is detected and the pilot signal PC on the transmitting side are detected. By detecting a shift between the subcarrier band to which the signal PC is assigned, a frequency shift between the demodulated N-sequence complex OFDM signal and the subcarrier band is obtained. Then, the frequency band of the complex OFDM signal is shifted by the amount of the shift.
[0015]
The frame decode processing unit 16 forms one block (frame) by storing the demodulated N-sequence complex OFDM signal for a predetermined number of symbols, and sets the TMCC allocated to a predetermined sequence (subcarrier band) among these blocks. (Transmission control signal).
[0016]
The equalizer 18 corrects the distortion of the signal received on the transmission path. That is, a phase shift occurring between an in-phase signal (I signal) and a quadrature signal (Q signal) constituting a complex OFDM signal due to an influence of a ghost wave, a shift of an FFT window position, and the like is detected and detected. to correct.
[0017]
Here, among the N-series carrier bands, pilot signals (SP: Scattered Pilot) are included in the transmitting side in advance in carrier bands (n, 2n, 3n,...) At regular intervals. Based on the phase shift between the in-phase signal (I signal) and the quadrature signal (Q signal) generated in the pilot signal SP, the equalizer 18 controls the in-phase on each carrier band constituting the frame. The phase shift between the signal (I signal) and the quadrature signal (Q signal) is estimated, and the in-phase signal (I signal) and the quadrature signal (Q signal) are subjected to equalization processing so as to eliminate the estimated phase shift. .
[0018]
The equalizer 18 also adjusts the amplitude level of the in-phase signal (I signal) and the quadrature signal (Q signal) on each carrier band based on the amplitude level of the pilot signal.
[0019]
The frequency & time deinterleave processing unit 20 performs interleaving in the frequency direction (rearrangement between data sequences) and interleaving in the time axis direction (rearrangement of data in the time axis direction in each data sequence) performed on the transmission side. To release. The demapping processing unit 22 decodes data arranged according to the modulation scheme. The Viterbi decoding processing unit 24 decodes the data convolutionally encoded on the transmission side. The RS decoding processing unit 26 decodes the Reed-Solomon encoded data on the transmission side and generates an MPEG transport stream packet (TSP).
[0020]
The transport stream packet is decoded by an MPEG decoder (not shown) and displayed on a display device (not shown).
[0021]
In the OFDM receiver 100, the time position of the FFT window has a great effect on subsequent processing. Therefore, in the following Patent Document, the frequency and phase characteristics of the complex OFDM signal subjected to the FFT processing are monitored, the amount of deviation of the FFT window from the effective symbol period is detected from the primary tendency of the frequency and phase characteristics, and based on the detection result, Control is performed so that the position of the FFT window is corrected to a regular time position, that is, a position corresponding to an effective symbol period.
[0022]
[Patent Document 1]
JP 2000-295195 A
[Problems to be solved by the invention]
By the way, when such an OFDM receiver 100 is used in a mobile body or the like, the propagation path of radio waves fluctuates, and a ghost may occur temporally ahead of a transmission wave to be originally received by multipath.
[0024]
In this case, if the FFT window is set to an effective symbol period of a normal transmission wave received after the previous ghost, the adjacent symbol (one symbol after the previous ghost) of the previous ghost enters the FFT window. As a result, the symbol of the legitimate transmission wave is interfered by the adjacent symbol, and it is not possible to execute a proper FFT process.
[0025]
On the other hand, if the FFT window is set not to the effective symbol period of the normal transmission wave but to the effective symbol period of the previous ghost, the period in which the adjacent symbol (the immediately preceding symbol) of the normal transmission wave overlaps the previous symbol. Since it is a ghost guard interval, adjacent symbols do not enter the FFT window set corresponding to the effective symbol period. Therefore, in this case, the symbol of the previous ghost is not interfered by an adjacent symbol, and thus the FFT processing itself for the symbol of the previous ghost can be smoothly performed.
[0026]
However, when the processing for the normal transmission wave is switched to the processing for the front ghost when the front ghost occurs, the phase frequency characteristics of the complex OFDM signal output from the FFT processing unit 12 greatly change. It will be. As a result, it becomes difficult to ensure continuity in the processing after FFT, and in the worst case, frame synchronization may be lost or equalization processing may fail.
[0027]
Note that Patent Literature 1 does not address the problem when the above-mentioned ghost occurs or the problem when an FFT window is set in the before-ghost, and naturally describes any suggestions for solving the problem. Absent.
[0028]
Therefore, a main object of the present invention is to prevent the loss of synchronization when the window position of the FFT processing unit 12 is changed based on the symbol synchronization pulse output from the symbol synchronization 8 when a previous ghost due to multipath occurs. An object of the present invention is to provide an OFDM receiving apparatus which can prevent the processing and execute a more stable processing operation.
[0029]
[Means for solving the problem]
According to the present invention, the presence or absence of the occurrence of a previous ghost is detected, and when the occurrence of the previous ghost is detected, the previous ghost is set as a processing target and an FFT window is set in an effective symbol period of the previous ghost. Then, for the complex OFDM signal obtained by the FFT processing, the characteristic change caused by the change of the window position is back calculated and compensated, thereby ensuring the continuity of the signal after the FFT processing before and after the window position change. .
[0030]
The invention according to each claim is as follows.
[0031]
The invention according to claim 1 relates to an OFDM receiving apparatus for receiving a transmission wave according to the OFDM method, and determines a pre-ghost based on the OFDM signal subjected to the FFT processing to determine whether the OFDM signal includes a pre-ghost. Means for correcting the window position with respect to the received OFDM signal so that the window position of the FFT processing becomes the window position corresponding to the previous ghost when the previous ghost is included, and after the FFT processing. And an OFDM signal correction means for correcting the FFT-processed OFDM signal so that the characteristics of the OFDM signal maintain continuity before and after the window position correction by the FFT window position correction means.
[0032]
According to a second aspect of the present invention, in the OFDM receiving apparatus according to the first aspect, the preceding ghost determination means sets a threshold value according to a level of a main wave, and sets a level of a sub wave preceding the main wave to the level. When the threshold value is exceeded, the sub wave is determined as a previous ghost.
[0033]
According to a third aspect of the present invention, in the OFDM receiving apparatus according to the first or second aspect, the FFT window position correcting means performs a FFT process on a pilot signal included in a predetermined subcarrier among the subcarriers after the FFT process. FFT means for calculating a delay profile, and a correction value calculating means for calculating a correction value according to a delay amount between a current window position and a window position to be applied to a previous ghost based on the calculated delay profile. Window position changing means for changing a window position with respect to the received OFDM signal according to the calculated correction value.
[0034]
According to a fourth aspect of the present invention, in the OFDM receiving apparatus according to any one of the first to third aspects, the OFDM signal correction means changes a phase of the FFT-processed OFDM signal in accordance with a correction amount of the window position. It is characterized by correction.
[0035]
According to a fifth aspect of the present invention, in the OFDM receiving apparatus according to the fourth aspect, the OFDM signal correction unit adds a phase corresponding to a correction amount of a window position by the FFT window position correction unit to the OFFT signal subjected to the FFT processing. A phase of the FDM-processed OFDM signal is corrected by multiplying the rotation amount.
[0036]
The features of the present invention will become more apparent from the following description of embodiments.
[0037]
However, the embodiment described below is merely one embodiment of the present invention, and the meaning of the terms of the respective constituent elements according to the present invention is not limited to those described in the following embodiment. .
[0038]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 2 is a diagram illustrating a configuration of an OFDM receiver according to an embodiment of the present invention. The same blocks as those in the OFDM receiver 100 of FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.
[0039]
In the figure, an FFT window position setting unit 40 determines the generation timing of a symbol synchronization pulse output from the symbol synchronization processing unit 8 based on a window position correction value obtained from an FFT window position detection and determination processing unit 44 described later. And thereby adjust the position of the FFT window. The FFT processing unit 12 performs FFT processing on the complex OFDM signal based on the changed symbol synchronization pulse.
[0040]
The phase frequency characteristic inversion unit 42 multiplies the complex data sequence of the FFT-processed complex OFDM signal by a phase rotation amount corresponding to the window position correction value obtained from the FFT window position detection and determination unit 44.
[0041]
The FFT window position detection & determination processing unit 44 uses the pilot signal SP extracted by the equalization unit 18 or the transmission path characteristics estimated by the equalization unit 18 to determine the current FFT window position and the normal FFT window position. A shift between the effective symbol period of the transmission wave and other ghost waves is detected. Further, it is determined whether or not a received ghost wave that exceeds a predetermined threshold is included in the received wave. If a previous ghost wave that exceeds the threshold is included, the effective symbol period of the previous ghost wave is determined. A correction amount of the FFT window position is calculated based on the deviation amount of the FFT window position.
[0042]
The equalizer 18 extracts the pilot signal SP from the complex data sequence, estimates the transmission path characteristics from the extraction result, and executes the above-described equalization processing. Further, the extracted pilot signal SP is supplied to the FFT window position detection & determination processing unit 44.
[0043]
FIG. 3 shows a specific configuration example of the phase frequency characteristic inversion unit 42. As shown in the figure, the phase frequency characteristic inverse operation 42 is composed of a complex multiplication unit 60 and a window position correction amount conversion unit 62.
[0044]
Here, the FFT window position correction can be performed in units of clocks. For example, if the FFT window position is set M clocks earlier than the current time, the complex data in the n-th carrier output from the N-point FFT is set. In the columns, a phase rotation of -2Mn / N (rad) occurs. Therefore, by correcting the phase corresponding to 2πMn / N (rad), which is the opposite phase, it is possible to reduce a sudden change in the phase frequency characteristic in the complex data sequence before and after the window position is changed.
[0045]
The window position correction amount conversion unit 62 calculates a phase amount 2πMn / N (rad) for the window position correction value from the FFT window position detection & determination 44, and generates a sine wave and a cosine wave corresponding to the phase. The complex multiplication 60 performs a phase rotation according to the window position correction on the complex data sequence by complexly multiplying the complex data sequence by the sine wave and cosine wave obtained by the window position correction amount conversion 62.
[0046]
FIG. 4 shows a specific configuration example of the equalizer 18 and the FFT window position detector & determiner 44.
[0047]
As shown in the figure, the equalization unit 18 includes an SP extraction unit 80, a transmission path characteristic estimation unit 82, and an equalization processing unit 84.
[0048]
The SP extraction unit 80 extracts a pilot signal SP from the frame-decoded complex data sequence. In the transmission path characteristic estimation 82, the transmission path characteristic of each carrier in which data is allocated is estimated using the extracted pilot signal SP.
[0049]
Here, pilot signal SP is arranged at an interval of 12 carriers to each of the in-phase signal (I signal) and the quadrature signal (Q signal), and its amplitude, the in-phase signal (I signal) and the quadrature signal (Q signal) are arranged. Signal) are predetermined. For this reason, it is possible to estimate the transmission path characteristics for each carrier based on the state (phase rotation, amplitude state) of each pilot signal SP extracted by the equalizer 18.
[0050]
The equalization processing unit 84 equalizes the complex data sequence by dividing the complex data sequence of each carrier by the transmission path characteristic estimated by the transmission path characteristic estimation unit 82, that is, the phase distortion and amplitude received on the transmission path. Correct the level of.
[0051]
Further, as shown in FIG. 4, the FFT window position detecting and determining unit 44 includes an FFT processing unit 86, a window position detecting unit 88, a threshold value setting unit 90, and a window position correction amount determining unit 92. I have.
[0052]
The FFT processing unit 86 performs an FFT process on the pilot signal SP extracted by the SP extraction unit 80 for each symbol. By such FFT processing, for example, a delay profile as shown in FIGS. 5 and 6 is obtained.
[0053]
FIG. 5 shows a delay profile in a case where pilot signal SP includes a main wave (a transmission wave at the maximum level) and a sub wave (post-ghost wave) delayed with respect to the main wave. In this case, since t = 0 corresponds to the timing of the current window position, the window position is shifted backward from the present, and the FFT window position is made to coincide with the effective symbol period of the main wave.
[0054]
FIG. 6 shows a delay when pilot signal SP includes a main wave, a sub wave delayed after the main wave (post-ghost wave), and a sub wave preceding the main wave (pre-ghost wave). It shows a profile. In this case, the window position is shifted forward from the present, and the FFT window position is made to coincide with the effective symbol period of the previous ghost wave preceding the main wave.
That is, the window position detection 88 shown in FIG. 4 detects the relationship between the current window position and the main wave and the ghost wave from the delay profile. Threshold setting section 90 sets a threshold for performing ghost wave level determination based on the magnitude of the main wave included in the delay profile. Here, the threshold value is set to a value such that a bit error rate in a subsequent reproduction processing system (not shown) becomes equal to or lower than a predetermined value.
[0055]
The window position correction amount determination unit 92 determines whether or not the level of the previous ghost wave exceeds the set level. If the level exceeds the threshold value, the delay time of the previous ghost wave (ΔT in FIG. 6) is determined in clock cycle units. Ask for. On the other hand, when the preceding ghost wave does not exceed the threshold value or when the preceding ghost wave does not exist, the delay time (ΔT in FIG. 5) of the main wave is obtained in clock cycle units.
[0056]
The window position correction amount determination unit 92 has a built-in memory, and stores the delay time obtained by the previous processing in the built-in memory. Then, the delay time ΔT _n−1 stored in the built-in memory is compared with the delay time ΔT _n obtained in the current processing, and a value T = ΔT _n−1 −ΔT _n according to the difference between the two is set to the window position. Output as a correction value.
[0057]
In the above description, when the delay profile is obtained by the FFT processing section 86, the pilot signal SP extracted by the SP extraction section 80 is used. However, the estimated transmission path characteristic which is the output of the transmission path characteristic estimation section 82 is used. The delay profile may be obtained by switching between the two as appropriate.
[0058]
As described above, in the present embodiment, when the previous ghost is included, the FFT window position is corrected so as to match the previous ghost, so that the FFT processing can be appropriately performed even when the previous ghost occurs. Also, when the FFT window position is corrected to match the previous ghost, the complex OFDM signal after the FFT processing is multiplied by the rotation phase corresponding to the correction amount. Abrupt changes in the phase frequency characteristics of the OFDM signal can be suppressed, thereby eliminating processing failures after FFT processing, such as broadband carrier synchronization processing and frame decoding processing.
[0059]
As described above, according to the present embodiment, the problem of the failure of the FFT processing due to the previous ghost can be solved while the processing after the FFT processing is stably maintained. Therefore, even in an environment where the transmission line characteristics can frequently change, for example, when an OFDM device is mounted on a portable device, it is possible to smoothly and effectively execute reception processing according to the transmission line characteristics at each time. Can be.
[0060]
Note that the present invention is not limited to the above embodiment, and it goes without saying that various other changes are possible.
[0061]
For example, in the above embodiment, as shown in the delay profile of FIG. 5, when the front ghost is not generated, the shift of the window position with respect to the main wave is corrected. When only the processing failure is intended, the window position correction for the main wave may not be performed, and only the window position correction for the previous ghost may be performed.
[0062]
Further, in the above-described embodiment, the present invention is applicable to a receiving apparatus that receives a transmission wave conforming to the domestic terrestrial digital broadcasting standard and receives a transmission wave according to another OFDM method.
[0063]
In addition, the embodiments according to the present invention can be appropriately changed within the technical idea of the present invention.
[0064]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the problem of FFT processing failure by a previous ghost can be solved, maintaining the processing after FFT processing stably. Therefore, even in an environment where the front ghost can frequently change, it is possible to smoothly and effectively execute the receiving process according to the transmission path characteristics at each time.
[Brief description of the drawings]
FIG. 1 is a configuration example of an OFDM receiver used in a digital broadcast receiver.
FIG. 2 is a configuration example of an OFDM receiver according to an embodiment.
FIG. 3 is a configuration example of a phase frequency characteristic inversion unit 42;
FIG. 4 is a configuration example of an equalization unit 18 and an FFT window position detection & determination processing unit 44.
FIG. 5 is an example of a delay profile according to the embodiment;
FIG. 6 is an example of a delay profile according to the embodiment;
[Explanation of symbols]
18 Equalization unit 40 FFT window position setting unit 42 Phase frequency characteristic inversion unit 44 FFT window position detection & judgment unit

Claims (5)

An OFDM receiving apparatus that receives a transmission wave according to the OFDM method,
Pre-ghost determining means for determining whether the OFDM signal includes a previous ghost based on the OFDM signal subjected to the FFT processing;
FFT window position correction means for correcting the window position for the received OFDM signal so that the window position of the FFT processing becomes a window position corresponding to the previous ghost when the previous ghost is included,
OFDM signal correction means for correcting the FFT-processed OFDM signal so that characteristics of the OFDM signal after FFT processing maintain continuity before and after window position correction by the FFT window position correction means,
An OFDM receiver comprising: The preceding ghost determining means sets a threshold value according to the level of the main wave, and determines the sub wave as the previous ghost when the level of the sub wave preceding the main wave exceeds the threshold value. The OFDM receiver according to claim 1, wherein: The FFT window position correcting means calculates a delay profile by performing FFT processing on a pilot signal included in a predetermined subcarrier among the subcarriers after the FFT processing;
Correction value calculation means for calculating a correction value according to the delay amount between the current window position and the window position to be applied to the previous ghost based on the calculated delay profile,
3. The OFDM receiving apparatus according to claim 1, further comprising a window position changing unit that changes a window position with respect to the received OFDM signal according to the calculated correction value. 4. The OFDM receiving apparatus according to claim 1, wherein the OFDM signal correction unit corrects a phase of the FFT-processed OFDM signal according to a correction amount of the window position. The OFDM signal correction unit multiplies the FFT-processed OFDM signal by a phase rotation amount corresponding to a correction amount of a window position by the FFT window position correction unit, thereby changing a phase of the FFT-processed OFDM signal. The OFDM receiving apparatus according to claim 4, wherein the correction is performed.

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