JP5021517B2 - OFDM receiving apparatus and OFDM relay apparatus - Google Patents

Description

  The present invention relates to an OFDM receiver and the like that receives data modulated by an orthogonal frequency division multiplexing (OFDM) modulation method using a plurality of transmission parameters that can be selected.

  An orthogonal frequency division multiplex modulation system (hereinafter referred to as OFDM system) is adopted in the Japanese terrestrial digital broadcasting system (hereinafter referred to as ISDB-T). In ISDB-T, a plurality of FFT point numbers (FFT sizes) and guard interval ratios are standardized, and broadcasting is performed using these transmission parameters properly. Accordingly, it is necessary for a receiver, a repeater, and the like to automatically detect transmission parameters and perform demodulation processing based on the detected parameters.

The following Patent Documents 1 and 2 are known as methods for detecting the transmission path parameters described above.
These parameter detection algorithms perform a correlation operation between the received OFDM signal and a signal obtained by delaying the received signal by an effective symbol length, extract a correlation component of the guard interval signal included in the OFDM signal, and determine whether the parameter is present or not. Is used.

Japanese Patent No. 2879034 Japanese Patent No. 3959488

  However, the methods such as Patent Documents 1 and 2 have a problem that the parameter detection accuracy deteriorates in an environment where there are a plurality of multipaths, their levels are large, and the delay time is long.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an OFDM receiver capable of correctly detecting and receiving a transmission parameter without being affected by a multipath signal.

In the OFDM receiver of the present invention, pilot carriers whose amplitude and phase are known are arranged in the time direction with L (L is an integer) symbol period, and the FFT point (effective symbol length) and guard interval length are variable. In an OFDM receiver that receives a signal modulated by an OFDM modulation scheme having a plurality of transmission parameters,
A delay means for performing a delay process of an L symbol time in which the received signal is set in accordance with each FFT point and guard interval ratio;
Correlation calculating means for calculating a correlation between the received signal and each L symbol delayed signal;
A (A is an integer of 2 or more) types of the delay means and the correlation calculation means,
Means for comparing the A-type correlation calculation result with a threshold;
And means for discriminating transmission parameters based on the comparison result.

In the OFDM receiver of the present invention, pilot carriers having known amplitudes and phases are arranged in the time direction with L (L is an integer) symbol period, and the FFT point (effective symbol length) and guard interval length are variable. In an OFDM receiver that receives a signal modulated by an OFDM modulation scheme having a plurality of transmission parameters,
A delay means for performing a delay process of L symbol ± α sample time in which the received signal is set in accordance with each FFT point and guard interval ratio; a means for performing a correlation operation within the L symbol ± α sample range for each of the received signal;
Means for adding correlation calculation results within a range of ± α;
A (A is an integer of 2 or more) types of the delay means, the correlation calculation means, and the addition means,
Means for comparing the A type of addition result with a threshold;
And means for discriminating transmission parameters based on the comparison result.

An OFDM relay apparatus comprising the above OFDM receiver and means for retransmitting the OFDM signal received by the OFDM receiver can also be included in the present invention.

In the conventional method using the correlation of the guard interval, since the correlation occurs only in the guard interval section, the correlation value fluctuates every time due to fading or the like when it is obtained. It is a time-averaged correlation between pilots and is stable. Further, the level of the multipath delayed by 4 symbols is very small, and there is almost no influence on the correlation value.

According to the present invention, the received signal and the received signal are subjected to correlation calculation with a signal subjected to a delay of 4 symbols according to the respective transmission parameters, the autocorrelation of the SP signal included in the OFDM signal is calculated, Since each correlation calculation result is compared with the threshold value to determine the transmission parameter on the transmission side, the transmission parameter can be detected with high accuracy without being affected by the multipath signal.
Also, as an example, if the above delay time is set to 4 symbols ± α, the respective correlation operations are performed, and the result obtained by adding them is used, even if the frequency deviation of the VCO occurs, the transmission parameter Can be detected.

Hereinafter, a receiving apparatus of an OFDM transmission apparatus according to the present invention will be described in detail with reference to the first embodiment shown in FIG.
A signal received by the tuner 11 is obtained through the A / D 12 and the quadrature detection unit 1B to obtain a received signal r (t). The reception signal r (t) from the quadrature detection unit 1B is connected to the FFT unit 17 and input to the correlation detection units 13-15.

The structure of the correlation detection parts 13-15 is shown in FIG. The correlation detection units 13 to 15 are configured by a delay unit 21, a complex multiplier 22, an integrator 23, and an absolute value unit 24. The received signal r (t) is connected to the delay unit 21 and the complex multiplier 22, and the delay unit 21 performs a delay process of 4 × T _s . Here, the delay time of the delay device 21 means the baseband clock (sample) and a unit, T _s is the effective symbol length and a guard interval total symbols length length (in samples).

  Correlation detection units 13 to 15 are prepared for the number of parameters to be detected, and each correlation detection unit corresponds to a parameter to be detected. For example, in ISDB-T, 2048, 4096, and 8192 are standardized as FFT points, that is, effective symbol lengths. For each FFT point, 1/4, 1/8, 1 and 1 of the effective symbol length are standardized. A guard interval having a length of / 16 or 1/32 is added.

  There are twelve combinations of all of these, and when detecting all these parameters, it is necessary to prepare 12 patterns of correlation detection units. However, in the ISDB-T operation regulations, only parameters with a guard interval length of 512 samples or more are defined, and the parameters actually used are the guard interval length at FFT point = 8192 (defined as mode 3 in ISDB-T). Are 1/16, 1/8, 1/4, FFT point = 4096 (defined as mode 2 in ISDB-T), there are only 5 patterns with a guard interval length of 1/8, 1/4. Accordingly, in the following description, five patterns according to the operation rules will be described, but the combination of parameters is not limited to this description.

From the above, Table 1 shows the symbol length and the delay time 4 × T _s of the delay unit 21 with the five patterns of operating rules.

  The output signal of the delay unit 21 is input to the complex multiplier 22, and the reception signal r (t) is connected to the other input of the complex multiplier 22. The complex multiplier 22 performs complex multiplication of the received signal r (t) and the signal from the delay unit 21 with a complex conjugate value. The signal from the complex multiplier 22 is input to the integrator 23, and the integrator 23 performs integration for a predetermined period. Further, the integrator 23 may be a moving average process for a predetermined period or a filter process for removing a high frequency signal.

These processes mean that a correlation operation is performed between the received signal r (t) and a signal obtained by delaying the received signal by 4 × T _s sample time. In this example, parameter detection is performed based on the magnitude of the signal level obtained by this correlation calculation. Further, as an advantage of this example, there is a feature that it does not depend on a frequency deviation or a multipath environment.

ここで、パイロット信号sp(t)は4シンボル周期の信号であるので、

の関係がある。この送信信号o(t)がマルチパス伝送路やチューナ１１を経由して受信信号r(t)が得られる。マルチパス伝送路でのマルチパス遅延時間をτ、チューナ１１での周波数偏差をΔωとすると、受信信号r(t)は、

となる。（係数１／√２はAGCにより電力を一定にするために設けた係数） Next, the operation principle of this example will be described below.
In ISDB-T, as shown in FIG. 3, a pilot carrier (Scattered Pilot: hereinafter referred to as SP) having a known amplitude and phase is inserted every 12 carriers in a 4-symbol period. If the transmission signal is o (t), the signal obtained by inverse Fourier transform of only the pilot carrier is sp (t), and the signal obtained by inverse Fourier transform of only the data carrier is d (t), then the transmission signal o (t) is It is represented by 1).

Here, since the pilot signal sp (t) is a signal having a 4-symbol period,

There is a relationship. A reception signal r (t) is obtained from the transmission signal o (t) via the multipath transmission line or the tuner 11. When the multipath delay time in the multipath transmission path is τ and the frequency deviation in the tuner 11 is Δω, the received signal r (t) is

It becomes. (The coefficient 1 / √2 is a coefficient provided to make the power constant by AGC)

となる。 The correlation output signal C from the integrator 23 that has performed correlation calculation on the received signal r (t) is

It becomes.

Here, attention is focused on Sigma _t {o (t) · o (t−4T _s ) ^* } of the first item in the equation (4).
As described above, this calculation indicates a correlation calculation between a transmission signal o (t) and a signal obtained by delaying the transmission signal o (t) by 4 symbols. Further, since the transmission signal o (t) is expressed by the equation (1), the correlation calculation results are the data signal and the delayed data signal, the data signal and the delayed SP signal, the delayed data signal and the SP signal, and the SP signal and the delayed SP signal. Can be divided into respective correlation operations. Among these correlation calculations, the combination having correlation is only the SP signal and the delayed SP signal as can be seen from the equation (2), and the other combinations are uncorrelated. Therefore, the calculation of the first item is Equation (5).

  Therefore, when the correlation calculation shown in Equation (4) is considered again, the first item and the second item have high correlation, and the third item and the fourth item become uncorrelated and become almost zero. In particular, the fact that the second item always has a high correlation irrespective of τ means that a high correlation value is stably output regardless of the multipath delay time and level.

式(6)においてｅ^jΔω4Tsは時間tに依存しない単なる位相項であり、大きさは変わらない。そのため、図２のように積分器２３の出力を絶対値器２４にて絶対値化を行い、相関値Ｃの大きさを算出する。 Therefore, equation (4) can be approximated to equation (6).

In equation (6), ^ejΔω4Ts is a mere phase term that does not depend on time t, and its magnitude does not change. Therefore, as shown in FIG. 2, the output of the integrator 23 is converted to an absolute value by the absolute value unit 24, and the magnitude of the correlation value C is calculated.

As described above, if the symbol length on the transmission side matches the symbol length T _s set by the delay unit 21, a large level of correlation value C can be obtained. As shown in FIG. 4, the autocorrelation waveform of the sp signal is Sharply, the correlation value C when the symbol length on the transmission side does not match the symbol length T _s set by the delay unit 21 is extremely small.

Accordingly, the correlation detection units 13 to 15 set the symbol delay length T _s for each parameter, and the output signals from the respective correlation detection units 13 to 15 are input to the parameter determination unit 16. The parameter discriminating unit 16 determines the level of the correlation value C output from each of the correlation detection units 13 to 15, and sets a parameter corresponding to the correlation value having a large level as a parameter detection result.

  As a criterion for determining the presence or absence of correlation in correlation value C, a value set based on the power of received signal r (t) is used as a threshold value. If correlation value C is equal to or greater than the threshold value, it is recognized as correlated, and the parameter is Judge that they match. However, in a low C / N environment, the noise power included in the received signal r (t) is large and the signal power is small, so the level of the correlation value C is also small in proportion to the signal power. Therefore, the threshold value may be a threshold value that is proportional to the power of the signal obtained by removing the noise component from the received signal r (t). For example, from the power of the received signal r (t), (as a function of the received signal power). It is possible to use a value obtained by subtracting the noise power specific to the tuner, or the original signal power estimated from the symbol power after the symbol determination by the demodulator 18. In this way, the presence or absence of correlation can be detected by comparing the correlation value C.

However, in the detection of mode 2 (FFT point is 4096) and mode 3 (FFT point is 8192), when the same guard interval ratio is obtained, as shown in Table 1, the 4-symbol delay length in mode 3 is The length of 8 symbols in mode 2 (double the SP period) is exactly the same. For this reason, even if mode 3 is transmitted on the transmission side, an SP signal with a 4-symbol period has a correlation even with an 8-symbol delay, and therefore, the correlation value increases with the parameter of the same guard interval ratio in mode 2. This relationship is shown in Table 2. The circles in Table 2 indicate that a large correlation value is output.

  Therefore, the parameter determination unit 16 detects the presence or absence of correlation by the above-described threshold comparison, and uses the correlation pattern of Table 2 as a discrimination between mode 3 and mode 2, and the correlation value equal to or greater than the threshold is shown in Table 2. If it matches the pattern shown, the corresponding reception detection parameter is output as a detection result. For example, when correlation is detected in 1/4 of mode 3 and 1/4 of mode 2, the parameter is determined as 1/4 of mode 2, and when correlated is detected only in 1/4 of mode 3. Determines the parameter as 1/4 of mode 3.

The parameters determined by the parameter determining unit 16 are input to the FFT unit 17, the demodulating unit 18, and the synchronization processing unit 19, and each function unit performs an operation corresponding to the mode and the guard interval length.
The synchronization processing unit 19 performs clock control based on the detected parameter, and controls so that the oscillation frequency of the VCO 1A is synchronized with the clock frequency on the transmission side.

  From the above, this example can detect transmission parameters with high accuracy without depending on multipath delay time (multipath delay time may be equal to or longer than the guard interval), DU ratio, or the like.

Next, a second embodiment according to the present invention will be described in detail with reference to FIG.
FIG. 5 shows a configuration in which the correlation detectors 13 to 15 are replaced with clock deviation corresponding correlation detectors 51 to 53 in the first embodiment shown in FIG. 1, and the other configuration is the same as FIG.

  In the first embodiment, it is difficult for the synchronization processing unit 19 to operate normally in a state where the parameter cannot be detected. In such a case, the frequency control of the VCO 1A is set to the midpoint of the frequency control range, and the frequency control is not normally performed. However, the oscillation frequency of the VCO 1A fluctuates due to the surrounding environment and aging.

  In general, the frequency of the VCO varies in the range of ± several tens to hundreds of ppm. In such a state with a frequency deviation, even if the delay time setting in the sample unit of the delay unit 21 matches the setting on the transmission side, the actual delay time varies depending on the frequency fluctuation, and thus a correct correlation value is obtained. There is no possibility. For example, when there is a frequency deviation of about 100 ppm, ISDB-T causes a deviation of about 5 clocks with respect to a 4-symbol delay length.

As shown in FIG. 4, the autocorrelation waveform of the SP signal is steep, and a high correlation value cannot be obtained when a time shift of 5 clocks occurs. In the present embodiment, the correlation detectors 13 to 15 are replaced with clock deviation corresponding correlation detectors 51 to 53 so as to cope with clock deviation.

The configuration of the clock deviation corresponding correlation detectors 51 to 53 is shown in FIG. In the clock deviation corresponding correlation detectors 51 to 53, the delay units 61 to 63 perform delay processing on the received signal r (t). In the delay units 61 to 63, the delay time of the 4 symbol length (4T _s ) set by the delay unit 21 is set to be shifted by several samples before and after. For example, when the VCO 1A having the above-described frequency deviation is used, a delay of ± 5 samples is set, and 11 types of delay times from 4 symbols-5 samples to 4 symbols + 5 samples are provided as delay devices.

  The output signals from the delay units 61 to 63 are respectively connected to the complex multiplier 22 as in FIG. This is because, for example, when there is a frequency deviation of VCO1A corresponding to a deviation of +3 samples, the received signal r (t) and the signal obtained by delaying the received signal r (t) by 4 symbols + 3 samples have high correlation. Become. In this case, the samples before and after the delay device corresponding to +3 samples have a slight correlation, but the correlation values of the other delay devices are almost zero. This can be understood from the autocorrelation waveform of the SP signal shown in FIG.

  Accordingly, the complex multiplication results of the received signal and each delay unit are added by the adder 64, and then the integration operation is performed by the integrator 23 in the same manner as in FIG. 2, so that the frequency deviation of the VCO 1A exists in an environment. Even if it exists, the effect similar to a 1st Example can be acquired.

  Further, in the configuration of the correlation detector for clock deviation shown in FIG. 6, the delay device is redundant, and many necessary complex multipliers are required. Therefore, as shown in FIG. 7, the shared portion of the delay device is configured by the common delay device 21, and the delay time shift in units of samples is realized by flip-flops 71 to 73 connected to the output of the delay device 21. These output signals are input to the selector 74. The selector 74 performs selection processing of these signals in units of high-speed clocks, the complex multiplier 22 also performs calculation at the same clock speed, and the adder 64 cumulatively adds them. As a result, the number of complex multipliers 22 is reduced.

  Furthermore, in the above description, the number of parameters for detecting the correlation detection unit is installed. However, in order to reduce the circuit scale, only a smaller number than the parameters to be detected are installed, and these circuits are operated in a time-sharing manner. By doing so, the circuit scale can also be reduced.

  Although the above description is about the OFDM receiving apparatus, it can also be applied to a relay apparatus that receives an OFDM signal and performs transmission again.

1 is a configuration diagram of an OFDM receiver according to a first embodiment of the present invention. Configuration diagram of the correlation detector 13 and the like of the first embodiment SP carrier layout SP signal autocorrelation waveform Configuration diagram of an OFDM receiver according to a second embodiment of the present invention Configuration diagram of the correlation detector 51 and the like corresponding to the clock deviation of the second embodiment Configuration diagram of correlation detector for clock deviation with reduced circuit scale

Explanation of symbols

11: tuner, 12: A / D, 13-15: correlation detection unit, 16: parameter discrimination unit, 17: FFT unit, 18: demodulation unit, 19: synchronization processing unit, 1A: VCO, 1B: quadrature detector , 21: delay unit, 22: complex multiplier, 23: integrator, 24: absolute value unit, 51-53: correlation detector for clock deviation, 61-63: delay unit, 64: adder, 71-73 : Flip-flop, 74: selector.

Claims (3)

An OFDM having a plurality of transmission parameters in which pilot carriers whose amplitude and phase are known are arranged in the time direction with L (L is an integer) symbol period, and the FFT point (effective symbol length) and guard interval length are variable. In an OFDM receiver that receives a signal modulated by a modulation method,
A delay means for performing a delay process of an L symbol time in which the received signal is set in accordance with each FFT point and guard interval ratio;
Correlation calculating means for calculating a correlation between the received signal and each L symbol delayed signal;
A (A is an integer of 2 or more) types of the delay means and the correlation calculation means,
Means for comparing the A-type correlation calculation result with a threshold;
Means for discriminating transmission parameters based on a comparison result, and an OFDM receiving apparatus. An OFDM having a plurality of transmission parameters in which pilot carriers whose amplitude and phase are known are arranged in the time direction with L (L is an integer) symbol period, and the FFT point (effective symbol length) and guard interval length are variable. In an OFDM receiver that receives a signal modulated by a modulation method,
A delay means for performing a delay process of L symbol ± α sample time in which the received signal is set in accordance with each FFT point and guard interval ratio; a means for performing a correlation operation within the L symbol ± α sample range for each of the received signal;
Means for adding correlation calculation results within a range of ± α;
A (A is an integer of 2 or more) types of the delay means, the correlation calculation means, and the addition means,
Means for comparing the A type of addition result with a threshold;
Means for discriminating transmission parameters based on a comparison result, and an OFDM receiving apparatus. 3. An OFDM relay apparatus comprising: the OFDM receiving apparatus according to claim 1; and means for retransmitting an OFDM signal received by the OFDM receiving apparatus.

Images