JP2006108764A - OFDM receiving apparatus, OFDM receiving control program, and OFDM receiving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily correct a phase change of an OFDM symbol caused by a sampling frequency shift between transmission and reception in an OFDM receiver.
An OFDM receiving apparatus establishes symbol synchronization from a received OFDM signal and, after the establishment, sets an application timing of an FFT window using a clock generated inside the apparatus. When the difference between the correlation peak of the received signal and the application timing of the FFT window is accumulated and the time corresponding to one sample of the sampling frequency is exceeded, the application timing of the FFT window is tracked and tracking is performed. According to the correction value calculated based on the number of OFDM symbols up to, the phase is corrected for each symbol. Therefore, it is possible to easily correct a phase change caused by an error between the sampling frequency on the transmission side and the sampling frequency in the OFDM receiver 1.
[Selection] Figure 2

Description

  The present invention relates to an OFDM receiver, an OFDM reception control program, and an OFDM reception method that sample a received OFDM signal at a predetermined timing and perform demodulation processing.

In recent years, OFDM (Orthogonal Frequency Division Multiplexing) transmission is used in terrestrial digital broadcasting and the like.
In OFDM transmission, a plurality of subcarriers to which bands are allocated so that the center frequencies are different and are orthogonal to each other are used. Then, the data to be transmitted is divided, and each divided data is put on these subcarriers and transmitted in parallel.

In OFDM transmission, a guard interval is added to an OFDM symbol transmitted in each subcarrier. The guard interval is obtained by adding the end portion of the effective symbol to the beginning of the OFDM symbol, thereby suppressing the occurrence of multipath interference.
In such OFDM transmission, it is possible to separate OFDM symbols (establish symbol synchronization) by taking a correlation peak between a received signal and a signal delayed by one OFDM symbol on the receiving side.

In subsequent processing, FFT (Fast Fourier Transform) is performed on the separated OFDM symbols.
When performing FFT, a window function (FFT window) equal to the effective symbol length is applied to each OFDM symbol.
At this time, if the timing of the FFT window for each OFDM symbol is set based on the symbol synchronization established from the received signal, the phase variation will increase in the signal after the FFT. This is because the correlation peak in each OFDM symbol varies due to transmission path characteristics.

Therefore, once the timing of the FFT window is set from the received signal, the sampling frequency is set by the clock inside the receiving apparatus, and the timing of the FFT window for the subsequent OFDM symbol is determined.
However, the sampling frequency generated inside the receiving apparatus and the sampling frequency of the transmitted OFDM signal are not necessarily matched, and a deviation may occur. When such a shift occurs, the FFT window application timing (FFT window timing) gradually changes, and a phase error occurs in the demodulated signal.

Then, in OFDM transmission, a scattered pilot (SP) signal inserted to estimate the transmission path and correct the phase and amplitude on the receiving side is also demodulated with a phase error. Thus, accurate transmission path estimation based on the SP signal becomes difficult.
In order to prevent such a situation, a technique for eliminating synchronization errors between transmission and reception by appropriately controlling an oscillator that sets a sampling frequency based on synchronization established from a received signal in an OFDM receiver is known. (For example, refer to Patent Document 1).
JP 2003-8544 A

However, in the technique described in Patent Document 1, an oscillator for setting a sampling frequency on the receiving side needs to be a voltage controlled variable oscillator or the like, and a D / A converter for generating a necessary voltage is necessary. For example, the circuit area and power consumption increase.
As described above, it is difficult for the OFDM receiver to easily correct the phase change of the OFDM symbol caused by the sampling frequency shift between transmission and reception.

  An object of the present invention is to simply correct a phase change of an OFDM symbol caused by a sampling frequency shift between transmission and reception in an OFDM receiver.

In order to solve the above problems, the present invention provides:
An OFDM receiving apparatus that samples a received OFDM signal at a predetermined timing and performs demodulation processing, and obtains a correlation value from the received OFDM signal and a signal obtained by shifting the OFDM signal by one OFDM symbol length (For example, the GI correlation unit 15 in FIG. 2) and sampling means for sampling the received OFDM signal at a timing based on a clock generated on the receiving side (for example, the symbol synchronization unit 17 and the FFT unit 18 in FIG. 2). Timing detection means for detecting a difference between the timing based on the correlation value acquired by the correlation value acquisition means and the sampling timing in the sampling means (for example, the symbol synchronization unit 17 in FIG. 2), and the timing detection The timing difference detected by the means When the time corresponding to the predetermined sample in the sampling unit is exceeded, the tracking unit (for example, the symbol synchronization unit 17 in FIG. 2) changes the sampling timing in the sampling unit by a predetermined sample in the direction of reducing the timing deviation. ), Symbol counting means for counting the number of OFDM symbols in the received OFDM signal (for example, the symbol counting section 19 in FIG. 2), and OFDM symbols received at time intervals in which the sampling timing is changed by the tracking means Based on the number and the phase change corresponding to the predetermined sample in the sampling means, the phase correction means for correcting the phase in each symbol extracted by the sampling (for example, the correction value calculation unit 21 in FIG. 2). It is characterized in that it comprises a phase correction portion 22) and the called.

  With such a configuration, the sampling timing is set for the received OFDM signal using a clock generated on the receiving side, such as inside the OFDM receiver. Then, when the difference between the timing indicated by the correlation value of the received signal and the sampling timing is accumulated and the time corresponding to the predetermined sample of the sampling frequency is exceeded, the sampling timing is changed (tracked) by the predetermined sample. At the same time, the phase of each symbol is corrected according to the correction value calculated based on the number of OFDM symbols until the sampling timing is changed.

Therefore, it is possible to easily correct the phase change caused by the difference between the sampling frequency on the transmission side and the sampling frequency in the OFDM receiver.
In addition, the phase correction unit is configured to obtain the phase change by dividing the phase change corresponding to the predetermined sample in the sampling unit by the number of OFDM symbols received in the time interval at which the sampling timing is changed by the tracking unit. The calculation processing for correcting the phase of each symbol extracted by the above is performed.

With such a configuration, the phase change can be easily corrected by digital signal processing.
Further, the correlation value averaging means (for example, the peak interval averaging section 16 in FIG. 2) that averages the timing based on the correlation value acquired by the correlation value acquisition means and outputs the averaged timing to the timing detection means. ) Is further included.

With such a configuration, it is possible to acquire stable timing based on the correlation value without being excessively influenced by temporary fluctuations in the correlation value of the received signal.
In addition, the number of OFDM symbols received in the time interval at which the sampling timing is changed by the tracking unit is averaged and output to the phase correction unit (for example, the symbol number averaging unit in FIG. 2). 20) is further included.

With such a configuration, it is possible to calculate the stable number of OFDM symbols without being excessively influenced by changes in the tracking interval due to temporary fluctuations in the received signal.
The present invention also provides:
An OFDM reception control program for performing control related to an OFDM reception process that samples a received OFDM signal at a predetermined timing and performs a demodulation process, the received OFDM signal, and a signal obtained by shifting the OFDM signal by one OFDM symbol length; A correlation value acquisition function for acquiring a correlation value from a sampling function, a sampling function for sampling the received OFDM signal at a timing based on a clock generated on the receiving side, and a timing based on the correlation value acquired by the correlation value acquisition function And a timing detection function that detects a deviation from the sampling timing in the sampling function, and a timing deviation detected by the timing detection function exceeds a time corresponding to a predetermined sample in the sampling function, A sampling function in the sampling function that changes the sampling timing in a direction that reduces the timing deviation, a symbol count function that counts the number of OFDM symbols in the received OFDM signal, and the tracking function A phase correction function for correcting the phase of each symbol extracted by the sampling based on the number of OFDM symbols received in the time interval at which the timing is changed and the phase change corresponding to the predetermined sample in the sampling function. It is characterized by being realized.

The present invention also provides:
An OFDM reception method for sampling a received OFDM signal at a predetermined timing and performing a demodulation process, wherein a correlation value acquisition step of acquiring a correlation value from the received OFDM signal and a signal obtained by shifting the OFDM signal by one OFDM symbol length A sampling step for sampling the received OFDM signal at a timing based on a clock generated on the receiving side, a timing based on the correlation value acquired in the correlation value acquisition step, and a sampling timing in the sampling step A timing detection step for detecting a deviation between the timing and a timing deviation detected in the timing detection step exceeds a time corresponding to a predetermined sample in the sampling step. A sampling step in which the sampling timing is changed by a predetermined number of samples in a direction to reduce the timing deviation, a symbol counting step for counting the number of OFDM symbols in the received OFDM signal, and the sampling timing in the tracking step. And a phase correction step of correcting the phase in each symbol extracted by the sampling based on the number of OFDM symbols received in the changed time interval and the phase change corresponding to the predetermined sample in the sampling step. It is a feature.

  As described above, according to the present invention, it is possible to easily correct a phase change of an OFDM symbol caused by a sampling frequency shift between transmission and reception in an OFDM receiver.

Embodiments of an OFDM receiving apparatus according to the present invention will be described below with reference to the drawings.
First, a method for correcting a phase change caused by an error between the transmission side sampling frequency and the reception side sampling frequency in the OFDM receiving apparatus according to the present embodiment will be described.
First, when the sampling frequency on the transmission side is “f” and the sampling frequency on the reception side is “f ′”, a predetermined coefficient “a” is used,
f ′ = (1 + a) × f
It can be expressed as.

Then, the error “Δf” between the transmission side sampling frequency and the reception side sampling frequency is
Δf = (1 + a) × f−f = a × f
It becomes.
Here, when the deviation between the sampling time “T” (= 1 / f) on the transmission side and the sampling time “T ′” (= 1 / f ′) on the reception side is obtained as a normalized time difference Δt,
Δt = (T′−T) / T = (1 / f′−1 / f) / (1 / f) = (f / f′−1) = (1 / (1 + a) −1)
It becomes.

Then, as shown in FIG. 1, in the OFDM signal, when the OFDM symbol length is “Ts”, the application timing of the FFT window is shifted by Δt × Ts for each symbol. A phase rotation occurs due to an error in the sampling frequency on the receiving side.
On the other hand, since the application timing of the FFT window is set using the sampling frequency generated on the reception side on the receiving side, the application timing of the FFT window should be set in units of one sample of this sampling frequency. Can do. Then, in the k-th (k is a natural number) subcarrier of the OFDM signal, the phase change “Δφ ₀ (k)” corresponding to one sample can be grasped in advance based on the frequency of the subcarrier.

  Therefore, on the receiving side, when the above-described FFT window timing difference Δt × Ts accumulates over time and exceeds the timing of one sample, the application timing of the FFT window is set to one sample in the direction of decreasing the phase change. Accordingly, it is possible to track the application timing of the FFT window. Thereby, it becomes possible to select an appropriate position in the OFDM symbol and perform FFT.

Therefore, in the present invention, the correlation value output for symbol synchronization of the received signal is monitored on the receiving side. When the sampling time difference Δt × Ts is accumulated and exceeds the time ΔT corresponding to one sample at the sampling frequency on the receiving side, the application timing of the FFT window is tracked for one sample. At the same time, the number of OFDM symbols until this tracking is performed is counted, and the phase change amount “Δφ ₀ (k)” corresponding to one sample of each subcarrier is divided by the counted number of OFDM symbols. Thus, the phase change per OFDM symbol is calculated.

By correcting the phase of each OFDM symbol according to the calculated phase change per symbol, it is possible to easily correct the phase change caused by the sampling frequency error between transmission and reception.
Specifically, the phase change can be corrected by the following procedure.
That is, as a result of monitoring the correlation value as described above, if the FFT window tracking is performed with the Nth symbol (N is a positive integer) of the OFDM symbol in the kth subcarrier in the OFDM signal, The phase change per OFDM symbol is expressed as Δφ ₀ (k) / N.

Then, the data subcarrier “D ′ (l, k)” located in the l-th symbol (l is a positive integer) in this subcarrier is transferred to the received data subcarrier “D (l, k)”. In contrast,
D ′ (l, k) = exp (−Δφ ₀ (k) / N × l) × D (l, k) (1)
By performing this calculation, it is possible to correct the phase change caused by the sampling frequency error between transmission and reception.

Next, an OFDM receiving apparatus to which the present invention is applied will be described.
First, the configuration will be described.
FIG. 2 is a diagram showing an overall configuration of the OFDM receiver 1 according to the present invention.
In FIG. 2, the OFDM receiver 1 includes an antenna 11, a high frequency unit 12, an oscillator 13, an A / D (Analog to Digital) conversion unit 14, a GI correlation unit 15, a peak interval averaging unit 16, A symbol synchronization unit 17, an FFT unit 18, a symbol count unit 19, a symbol number averaging unit 20, a correction value calculation unit 21, and a phase correction unit 22 are configured.

The antenna 11 receives the OFDM signal transmitted from the transmission side and outputs it to the high frequency unit 12.
The high frequency unit 12 performs channel conversion using a band pass filter and down-conversion by mixing at a predetermined frequency on the OFDM signal input from the antenna 11, and outputs the processed signal to the A / D conversion unit 14.

The oscillator 13 generates a signal corresponding to the sampling frequency when the A / D converter 14 converts the received signal into a digital signal.
The A / D conversion unit 14 converts the signal input by the high frequency unit 12 into a digital signal by sampling using the signal generated by the oscillator 13, and the digital signal is converted into the GI correlation unit 15 and the FFT unit 18. Output to.

The GI correlator 15 delays the signal input by the A / D converter 14 by one symbol of the OFDM symbol, and obtains a correlation between the delayed signal and the signal input by the A / D converter 14. Then, the GI correlation unit 15 outputs a signal indicating the result of taking these correlations to the peak interval averaging unit 16.
The peak interval averaging unit 16 averages the correlation peak interval in the received signal over a certain period based on the correlation result input by the GI correlation unit 15. Then, the peak interval averaging unit 16 outputs a signal indicating the timing of the averaged correlation peak interval (hereinafter referred to as “average correlation peak signal”) to the symbol synchronization unit 17.

The peak interval averaging unit 16 can acquire a stable correlation peak interval without being excessively influenced by temporary fluctuations in the correlation peak interval of the received signal.
Note that the peak interval averaging unit 16 is arbitrarily provided, and the output of the GI correlation unit 15 may be directly input to the symbol synchronization unit 17.
The symbol synchronization unit 17 establishes symbol synchronization based on the average correlation peak signal input by the peak interval averaging unit 16. Then, the symbol synchronization unit 17 first sets the application timing of the FFT window for the OFDM symbol in accordance with the established symbol synchronization, and for the subsequent OFDM symbol, the clock generated in the OFDM receiver 1 ( For example, the application timing of the FFT window is set using a clock generated by the oscillator 13 or a system clock of the own apparatus.

At this time, the symbol synchronization unit 17 measures the difference Δt × Ts between the timing indicated by the average correlation peak signal and the application timing of the FFT window, and the accumulated time is a time ΔT corresponding to one sample of the sampling frequency. If it exceeds, the application timing of the FFT window is tracked for one sample in the time direction to reduce these deviations.
Then, the symbol synchronization unit 17 outputs the application timing of the FFT window set in this way to the FFT unit 18.

The symbol synchronization unit 17 also tracks a signal indicating the timing of the established symbol synchronization (hereinafter referred to as “symbol synchronization signal”) and a timing when the application timing of the FFT window is tracked (hereinafter referred to as “tracking timing signal”). Are output to the symbol count unit 19.
The FFT unit 18 applies FFT to each OFDM symbol in the signal input by the A / D conversion unit 14 by applying the FFT window at the application timing of the FFT window input by the symbol synchronization unit 17.

Then, the FFT unit 18 outputs a signal in the frequency domain as a result of the FFT, that is, a signal corresponding to each subcarrier, to the phase correction unit 22.
The symbol count unit 19 counts the number N of OFDM symbols received from the symbol synchronization signal and the tracking timing signal input by the symbol synchronization unit 17 in a time interval in which the FFT window tracking is performed.

Then, the symbol count unit 19 outputs the counted OFDM symbol number N to the symbol number averaging unit 20.
The symbol number averaging unit 20 averages the OFDM symbol number N input by the symbol counting unit 19 over a certain period, and outputs the averaged OFDM symbol number N to the correction value calculating unit 21.

The symbol number averaging unit 20 can calculate the stable number N of OFDM symbols without being excessively influenced by the change in the tracking interval due to the temporary fluctuation of the received signal.

Note that the symbol number averaging unit 20 is arbitrarily provided, and the output of the symbol count unit 19 may be directly input to the correction value calculation unit 21.
The correction value calculation unit 21 calculates and calculates a correction value (Δφ (k) / N × l) per OFDM symbol based on the averaged OFDM symbol number N input by the symbol number averaging unit 20. The corrected value is output to the phase correction unit 22. Note that “l” used for calculating the correction value at this time means that it is the l-th OFDM symbol in each subcarrier from the OFDM symbol in which the application timing of the FFT window is updated.

The phase correction unit 22 uses the phase correction value (Δφ (k) / N × l) calculated by the correction value calculation unit 21 to perform phase conversion on the signal input by the FFT unit 18 according to the equation (1). Perform the correction.
Then, the phase correction unit 22 outputs the phase-corrected signal (D ′ (l, k)) to a circuit that performs a subsequent decoding process.

Next, the operation will be described.
When the OFDM receiver 1 receives an OFDM signal via the antenna 11, the high-frequency unit 12 cuts out a predetermined channel and then down-converts the signal into an intermediate frequency band signal.
Then, the signal in the intermediate frequency band is converted into a digital signal by the A / D conversion unit 14, and the correlation peak interval is averaged by the peak interval averaging unit 16 based on the correlation detected by the GI correlation unit 15. .

Then, the symbol synchronization unit 17 establishes symbol synchronization using the averaged correlation peak, and at a predetermined timing based on the established symbol synchronization and the sampling frequency generated from the clock in the OFDM receiver 1, A / An FFT window is applied to the output signal of the D conversion unit 14.
Then, the FFT unit 18 performs FFT on the portion of the OFDM symbol cut out by the FFT window.

On the other hand, a symbol synchronization signal and a tracking timing signal are output from the symbol synchronization unit 17 to the symbol count unit 19, and the number N of received OFDM symbols is counted in a time interval in which the FFT window tracking is performed.
The number of OFDM symbols N is averaged over a certain period in the symbol number averaging unit 20, and the averaged OFDM symbol number N is output to the correction value calculating unit 21.

Then, the correction value calculation unit 21 calculates a correction value per OFDM symbol, and this correction value is input to the phase correction unit 22.
Then, the phase correction unit 22 performs correction based on the correction value input by the correction value calculation unit 21 on the output signal of the FFT unit 18.
The signal obtained as a result is delivered to the subsequent decoding process.

The OFDM receiver 1 corrects the phase change of each symbol while appropriately tracking the shift of the FFT window by continuing such an operation during the reception period of the OFDM signal.
As described above, the OFDM receiver 1 according to the present embodiment establishes symbol synchronization from the received OFDM signal, and after that, uses the clock generated inside the apparatus to set the application timing of the FFT window. Set. Then, when the difference (Δt × Ts) between the correlation peak of the received signal and the application timing of the FFT window is accumulated and exceeds the time (ΔT) corresponding to one sample of the sampling frequency, the application timing of the FFT window is changed. In addition to tracking, the phase of each symbol is corrected according to a correction value (Δφ ₀ (k) / N × l) calculated based on the number of OFDM symbols until tracking is performed.

Specifically, the phase change can be corrected by performing predetermined digital signal processing (processing based on equation (1)) on the signal after FFT.
Therefore, it is possible to easily correct a phase change caused by an error between the sampling frequency on the transmission side and the sampling frequency in the OFDM receiver 1.
In addition, since the application timing of the FFT window is tracked at an appropriate timing, it is possible to prevent the timing deviation of the FFT window from increasing indefinitely over time. The FFT position is always selected by selecting an appropriate position in the OFDM symbol. Can be applied.

  In the present embodiment, the phase change amount when the FFT window is shifted by one sample is stored, and the phase change caused by the shift of the FFT window detected from the SP signal is the phase change for one sample. Although the case where the FFT window is shifted by one sample when it becomes larger than the amount has been described, the phase change amount may be determined in units of two or more samples and tracking may be performed without being limited to one sample.

It is a schematic diagram which shows a mode that the correlation peak of a received signal and the application timing of a FFT window shift | deviate. It is a figure which shows the whole structure of the OFDM receiver 1 which concerns on this invention.

Explanation of symbols

  1 OFDM receiver, 11 antenna, 12 high frequency unit, 13 oscillator, 14 A / D conversion unit, 15 GI correlation unit, 16 peak interval averaging unit, 17 symbol synchronization unit, 18 FFT unit, 19 symbol count unit, 20 symbols Number averaging unit, 21 correction value calculation unit, 22 phase correction unit

Claims (6)

An OFDM receiver that samples a received OFDM signal at a predetermined timing and performs demodulation processing,
Correlation value acquisition means for acquiring a correlation value from the received OFDM signal and a signal obtained by shifting the OFDM signal by one OFDM symbol length;
Sampling means for sampling the received OFDM signal at a timing based on a clock generated on the receiving side;
Timing detection means for detecting a deviation between timing based on the correlation value acquired by the correlation value acquisition means and sampling timing in the sampling means;
When the timing deviation detected by the timing detection means exceeds the time corresponding to the predetermined sample in the sampling means, the sampling timing in the sampling means is set to a predetermined sample amount in a direction to reduce the timing deviation. Tracking means to change,
Symbol counting means for counting the number of OFDM symbols in the received OFDM signal;
The phase of each symbol extracted by the sampling is corrected based on the number of OFDM symbols received in the time interval when the sampling timing is changed by the tracking means and the phase change corresponding to the predetermined sample in the sampling means. Phase correction means for
An OFDM receiving apparatus comprising:   The phase correction means extracts the phase change corresponding to a predetermined sample in the sampling means by the sampling by a phase change obtained by dividing the phase change by the number of OFDM symbols received in the time interval when the sampling timing is changed by the tracking means. 2. The OFDM receiving apparatus according to claim 1, wherein an arithmetic processing for correcting a phase of each symbol is performed.   The correlation value averaging means for averaging the timings based on the correlation values acquired by the correlation value acquisition means and outputting the averaged timings to the timing detection means. OFDM receiver.   4. The method according to claim 1, further comprising a symbol number averaging means for averaging the number of OFDM symbols received in the time interval when the sampling timing is changed by the tracking means and outputting the average to the phase correcting means. The OFDM receiver according to any one of the above. An OFDM reception control program for performing control related to OFDM reception processing for sampling a received OFDM signal at a predetermined timing and performing demodulation processing,
A correlation value acquisition function for acquiring a correlation value from the received OFDM signal and a signal obtained by shifting the OFDM signal by one OFDM symbol length;
A sampling function for sampling the received OFDM signal at a timing based on a clock generated on the receiving side;
A timing detection function for detecting a difference between a timing based on the correlation value acquired by the correlation value acquisition function and a sampling timing in the sampling function;
When the timing shift detected by the timing detection function exceeds the time corresponding to the predetermined sample in the sampling function, the sampling timing in the sampling function is set to a predetermined sample amount in a direction to reduce the timing shift. Change tracking function,
A symbol counting function for counting the number of OFDM symbols in the received OFDM signal;
The phase of each symbol extracted by the sampling is corrected based on the number of OFDM symbols received in the time interval when the sampling timing is changed by the tracking function and the phase change corresponding to the predetermined sample in the sampling function. Phase correction function to
An OFDM reception control program for causing a computer to realize the above. An OFDM reception method for sampling a received OFDM signal at a predetermined timing and performing demodulation processing,
A correlation value acquisition step of acquiring a correlation value from the received OFDM signal and a signal obtained by shifting the OFDM signal by one OFDM symbol length;
A sampling step of sampling the received OFDM signal at a timing based on a clock generated on the receiving side;
A timing detection step for detecting a difference between a timing based on the correlation value acquired in the correlation value acquisition step and a sampling timing in the sampling step;
When the timing shift detected in the timing detection step exceeds the time corresponding to the predetermined sample in the sampling step, the sampling timing in the sampling step is set to a predetermined sample amount in a direction to reduce the timing shift. A tracking step to change,
A symbol counting step of counting the number of OFDM symbols in the received OFDM signal;
Based on the number of OFDM symbols received in the time interval at which the sampling timing is changed in the tracking step and the phase change corresponding to the predetermined sample in the sampling step, the phase in each symbol extracted by the sampling is corrected. A phase correction step to
An OFDM receiving method comprising:

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