JP3768108B2 - OFDM receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a clock frequency control method, an FFT (Fast Fourie Transform) time window timing control method, and a transmission device and a reception device used therefor. The present invention relates to a receiving device of a data transmission device.
[0002]
[Prior art]
In recent years, an orthogonal frequency division multiplexing modulation system (hereinafter referred to as the OFDM system) characterized by being resistant to multipath fading and ghost as a modulation system suitable for digital transmission for mobiles and terrestrial digital television broadcasting Is attracting attention. The OFDM system is a kind of multi-carrier modulation system, and is a transmission system in which digital modulation is performed on n (n is several tens to several hundreds) carrier waves (carriers) orthogonal to each other.
As the above-described carrier digital modulation system, a 4-phase differential phase shift keying (DQPSK) is often used, but 16-value quadrature amplitude modulation (16QAM), 64QAM, etc. It is also possible to use a multi-level modulation method.
As shown in FIG. 2, the OFDM signals are added so that the carrier waves are in an orthogonal relationship with each other, and an OFDM time axis waveform is generated. This addition processing can be realized by performing IFFT (Inverse Fast Fourie Transform) processing on each carrier. A processing unit in IFFT processing, that is, the number of FFT samples is generally used as a unit of power of 2 such as 1024 or 8192, and the number of samples of the OFDM signal subjected to time axis conversion becomes equal to the number of FFT samples.
As shown in FIG. 3, the OFDM signal is composed of an effective symbol which is a time axis waveform after the IFFT processing and a guard interval in which a part of the effective symbol is copied and added before the effective symbol. Is done.
In the OFDM scheme, by adding a guard interval, it is possible to avoid deterioration due to inter-symbol interference with respect to a delayed wave having a delay time within the guard interval period. Can have.
By the above processing, the OFDM signal generated in the transmission device is frequency-converted to an intermediate frequency (IF) band and a high frequency (RF) and then transmitted.
[0003]
In the receiving apparatus, the received signal is frequency-converted to the baseband band through the RF band and IF band, and then sampled by the A / D converter.
In the demodulation process for the OFDM signal, the FFT processing is performed on the obtained received sample value series in reverse to the transmission apparatus to convert the time axis signal to the frequency axis signal. In the FFT calculation process, a time window having an effective symbol period length is provided on the obtained received sample sequence, and the FFT calculation process is performed on the sample signal included in the time window.
Based on the amplitude and phase information for each carrier thus obtained, demodulation such as DQPSK and 16QAM is performed to complete OFDM transmission.
Regarding such demodulation processing of the OFDM receiver, the Journal of the Institute of Image Information and Television Engineers vol. 53, no. 11, pp 1538-1549 (1999).
Next, two processes necessary for demodulating an OFDM signal in the receiving apparatus will be described.
First, since the frequency interval between each carrier is narrow in the OFDM system, interference between carriers due to a carrier frequency error between transmission / reception devices and a sampling clock frequency error in the demodulation system is likely to occur, and the reproduction of those frequencies is difficult. High accuracy is required. In order to continue to receive the OFDM signal correctly, it is necessary to perform a sampling clock recovery process in which the sampling clock frequency is always matched with the sampling clock frequency of the transmission signal with high accuracy.
Second, the time window provided on the received sample sequence when performing the FFT calculation processing is arranged at the end of the symbol period in order to reduce the influence of multipaths in the guard interval period as shown in FIG. It is desirable to be done. However, when an FFT window is provided at a position including a signal of an adjacent symbol due to a sampling clock frequency error or multipath, intersymbol interference occurs.
Intersymbol interference due to the OFDM signal is regarded as mixing of Gaussian noise, and as a result, it appears as deterioration of C / N (Carrier / Noise), resulting in deterioration of the code error rate. Therefore, it is necessary to perform FFT time window position control processing in which a symbol transition time point is detected with high accuracy from the received sampling sequence, and an FFT time window is provided so that intersymbol interference does not occur.
[0004]
In order to solve the above two points, the OFDM signal is composed of a plurality of synchronization symbol groups followed by a data symbol group. Examples of the synchronization symbol include a null symbol in a no-signal period and a sweep symbol that sweeps the frequency from the lower limit to the upper limit of the frequency band over the symbol period.
A method of performing the sampling clock recovery process and the FFT time window position control process based on the synchronization symbol is described in Japanese Patent Laid-Open No. 7-321762, and this means can accurately demodulate the OFDM signal. .
Further, OFDM transmission is often used for mobile transmission such as marathon relay because of its method. In the case of outdoor transmission, a multipath communication path is formed in which a reflected wave that is reflected from a building or the like and arrives with a delay time exists in addition to the main wave that directly arrives from the transmitter according to the topography. Furthermore, in mobile transmission, a fading environment may occur in which the levels of the main wave and the reflected wave change from moment to moment. Such a multipath is likely to cause a transmission error, and in a marathon relay, a transmission error may cause an image freeze.
In order to increase the reliability of the relay, it is very effective to observe the propagation path characteristics and perform transmission based on the propagation path characteristics.
As a method most often used as means for observing propagation path characteristics, there is a delay profile for calculating the level and delay time of the main wave and reflected wave. This is because the delay profile can be calculated with high accuracy by performing the cross-correlation operation between the sweep symbol and the received sample sequence described above.
[0005]
[Problems to be solved by the invention]
In the OFDM system in the above prior art, it is necessary to add a synchronization symbol to the OFDM signal in order to accurately demodulate the received signal. However, the addition of a synchronization symbol has the disadvantage that the transmission efficiency is reduced accordingly. For example, when synchronization symbols are added at a rate of 4 symbols to 400 symbols, the transmission efficiency is reduced by 1%.
In addition, when a synchronization symbol is not used, it is difficult to calculate a delay profile with high accuracy, and even if a delay profile is obtained, it is difficult to determine multipaths having close delay times.
The present invention eliminates these drawbacks, and reproduces a reception sampling clock from an OFDM signal having no synchronization symbol, accurately detects a symbol transition point, and sets an FFT time window based on the detection result. The purpose is to provide. It is another object of the present invention to provide an OFDM receiver capable of calculating a delay profile with high accuracy even when no synchronization symbol is used.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a receiving apparatus for receiving an OFDM signal having a guard interval period in which a part of a signal of the effective symbol period is copied in a part of the effective symbol period. A correlation calculation means for performing a cross-correlation calculation for each sample between a received sample sequence signal obtained by sampling the received sample sequence signal and a delayed signal obtained by delaying the received sample sequence signal by an effective symbol period, and filtering the obtained correlation calculation value sequence signal in the symbol direction And a guard interval position detecting means for detecting the guard interval position from the filtered correlation calculation value series signal.
Further, means for controlling the position of the time window of the fast Fourier transform based on the detected guard interval position, and variably controlling the received sampling clock frequency to synchronize with the clock frequency of the transmitting apparatus based on the filtered correlation calculation value series signal. An OFDM receiving apparatus comprising means.
Further, the OFDM receiving apparatus includes means for performing differential processing on the filtered correlation calculation value series signal to generate a delay profile.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a receiver of a digital transmission apparatus according to the present invention will be described in detail with reference to an embodiment shown in FIG.
An OFDM received signal that has arrived at a receiving device from a transmitting device (not shown) via a transmission line is converted from an intermediate frequency (IF) band signal to a baseband frequency band signal by an IF / BB converter 11 as in the conventional technique. Converted to a signal. The output from the IF / BB converter 11 is analog / digital converted by the A / D converter 12 using the reception sampling clock supplied from the VCO 1B.
In order to perform demodulation from the OFDM received signal, as described above, it is necessary to control the reception sampling clock frequency and the FFT time window position with high accuracy.
The present invention relates to these control methods, and these will be described below.
The received sample series S obtained by the A / D converter 12 is input to the correlation calculator 14 and the delay circuit 13, and the output D of the delay circuit 13 is connected to the other input terminal of the correlation calculator 14. The delay circuit 13 delays the effective symbol period with respect to the received sample series, for example, a delay of 1024 sampling clocks.
The correlation calculator 14 performs correlation calculation for each sample of the received sample series S and the delayed signal D as shown in FIG. Here, as described above, the OFDM signal copies the rear end portion (a, b in FIG. 5) of the effective symbol and adds it as a guard interval to the front end portion (a ′, b ′ in FIG. 5) of the effective symbol. This is the signal configuration.
Therefore, as shown in FIG. 5, when the delayed signal D delayed by the effective symbol period is a signal (a ′, b ′) in the guard interval period, the corresponding received sample value series S has the same component ( a, b).
Therefore, the correlation result C for each sample has a large correlation coefficient during the signal (a ′, b ′) period of the delayed signal D in the guard interval. In the signal period other than the guard interval period, the delayed signal D becomes a signal uncorrelated with the received sample value series S, and therefore the correlation coefficient is small.
However, even in the case of a signal in the guard interval period, the amplitude distribution of the OFDM signal has a distribution form close to a Gaussian distribution. Therefore, when the signal level of the received sample value series S is small, the correlation value becomes small and the correlation of one symbol The correlation value C obtained by the calculation varies depending on the level of the OFDM signal.
[0008]
Therefore, in order to suppress variations in the level of the correlation value series, the correlation value series signal C from the correlation calculator 14 is input to the noise removal filter 15 to remove the noise component.
For example, the noise removal filter 15 cyclically feedback-adds the correlation value sequence signal C delayed by (1-1 / α) by one symbol to the correlation value sequence signal C multiplied by (1 / α). It is a well-known IIR filter. The noise removal filter 15 filters the correlation sequence output C from the correlation calculator 14 in the symbol direction, that is, removes the noise component by cyclically feedback-adding the correlation value sequence signal C delayed by one symbol. The correlation value series F is output.
The correlation value series F obtained in this way forms a rectangular wave waveform in which the correlation value level increases only during the guard interval period, as shown in FIG.
The output F from the noise removal filter 15 is input to the symbol timing detector 16, and the symbol timing detector 16 detects the guard interval position from the correlation value series F.
For this detection means, first, a predetermined threshold value is provided for the input correlation value series F, and the magnitude relationship between the correlation value series F and this threshold value is compared.
When the correlation value series is at a level larger than the threshold value, it is determined that the signal is a guard interval signal, and the rising position is detected as the start position of the guard interval.
[0009]
However, even for signals other than the guard interval period, the correlation value may rarely become larger than the threshold value due to the influence of noise or the like. In such a case, in order to prevent the guard interval position from being erroneously detected, a means for accurately determining that it is the guard interval position is required.
For example, when the generation of a correlation value signal larger than the threshold value is stably detected over several symbols as a criterion for specifying the guard interval period by the symbol timing detector 16, the position is guarded. Judged as an interval period.
Further, as a threshold value calculation method provided in the symbol timing detector 16, for example, an average value of a correlation value series is calculated and set to multiply the value. Alternatively, the average value of the received sample value series may be calculated and set to multiply the value.
The detection result of the guard interval signal is obtained as position information of the guard interval, and the symbol timing detector 16 outputs a reset signal RS to the symbol counter 19 as shown in FIG. 6 based on this position information.
The symbol counter 19 resets the counter value based on the reset pulse RS, and counts the symbol period in units of clocks supplied from the VCO 1B.
The symbol synchronization timing SST from the symbol counter 19 governs the timing of the entire OFDM receiver, and the FFT time window is also set based on the symbol synchronization timing.
In the symbol timing detector 16, once the reset pulse RS is output (synchronized), the symbol counter 19 is reset until the synchronization is lost in order to prevent the symbol counter 19 from being reset when the guard interval position is erroneously detected. It operates so as not to output the pulse RS.
Furthermore, the symbol timing detector 16 also has a function of outputting a control signal to the VCO control unit 1A so that the reception sampling clock frequency is synchronized with the transmission clock frequency.
[0010]
Hereinafter, an embodiment of a method for calculating the control signal of the reception sampling clock frequency from the correlation value series F in the symbol timing detector 16 will be described. In the first embodiment, the maximum value is detected from the correlation value series F, and the maximum value position is calculated as a control signal.
Since the level of the guard interval period is large in the correlation value series F, the guard interval position can be detected with an accuracy within the number of samples of the guard interval by detecting the maximum value.
Further, since the OFDM signal is a random signal, the position detected as the maximum value is also a random position within the guard interval period. Thus, the maximum value position is detected for each symbol, and an error between the maximum value position detected in the previous symbol and the maximum value position calculated in the current symbol is output as control information to the VCO 1B. Here, if the clock frequency of the transmission device and the reception device are synchronized, the average value of the position error of this maximum value is 0, but if the reception clock frequency is higher than that of transmission, the average value is It becomes a negative value, and when it is low, it becomes a positive value.
Therefore, if this maximum value position error is used as control information for the VCO 1B and control is performed so that the maximum value position error becomes zero, the reception sampling clock frequency can be synchronized with the transmission clock frequency.
[0011]
In the second embodiment, the level difference between both ends of the correlation value series F having a level larger than the threshold value described above is calculated as a control signal.
As described above, if the clock frequencies of the transmission device and the reception device are synchronized, the average values of the correlation value levels within the guard interval period are equal. However, when the reception clock frequency is higher than that of transmission, the correlation value level is small because the front end sample of the correlation value sequence F includes a signal component that is uncorrelated. On the other hand, when the frequency is low, the correlation value level at the rear end becomes small. Therefore, by controlling the levels of these samples to be equal, it is possible to control the reception sampling clock frequency.
In the embodiment described above, the control information output from the symbol timing detector 16 is input to the VCO control unit 1A.
Since the VCO control unit 1A controls the frequency of the VCO 1B based on the control information, the control information is converted into a frequency control voltage and output.
With the above processing, the reception sampling clock frequency can be synchronized with the transmission clock frequency.
[0012]
Next, an embodiment of a delay profile calculation unit will be described.
The output F of the noise removal filter 15 is input to the differentiator 17. The differentiator 17 calculates the difference between the signal one sample before and the current sample signal with respect to the correlation value series F as shown in FIG. 7A, and obtains a differential coefficient K (FIG. 7B). Is calculated.
Here, when the correlation value series signal is in the guard interval period, the correlation value level suddenly increases, and the differential coefficient K at that time becomes a large value.
Further, as shown in FIG. 8A, the correlation value series F when multipath is mixed is composed of the correlation coefficient of the guard interval signal by the main wave and the correlation coefficient of the guard interval signal by the reflected wave. The differential coefficient K also has a large value ((b) in FIG. 8) at the time of switching between the main wave and the reflected wave.
The output K from the differentiator 17 is input to the comparator 18, and only a signal having a positive value is extracted from the differential coefficient K, and the signal having a negative value is converted into a predetermined value (for example, 0). Output. ((C) in FIGS. 7 and 8)
Since this signal has steep peaks corresponding to the respective levels at the positions of the main wave and the reflected wave, it is also possible to distinguish reflected waves having close delay times.
By calculating the delay profile waveform by the above processing, the propagation path characteristics can be accurately observed.
[0013]
【The invention's effect】
As described above, the receiving apparatus according to the present invention can detect the guard interval signal with high accuracy by performing the cross-correlation operation on the received signal even for the OFDM signal having no synchronization symbol. It is possible to provide an FFT time window without intersymbol interference.
Further, it is possible to detect the clock frequency error between the transmission device and the reception device from the detected guard interval signal, and to perform control so that the reception clock frequency is synchronized with the transmission clock frequency.
Furthermore, it is possible to provide a highly accurate delay profile by differentiating the correlation waveform.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a receiving apparatus according to the present invention. FIG. 2 is a schematic diagram showing a signal form of an OFDM modulated signal. FIG. 3 is a schematic diagram showing an OFDM symbol waveform. FIG. 5 is a schematic diagram for explaining the correlation calculation processing status of the present invention. FIG. 6 is a timing chart for explaining the symbol synchronization status of the present invention. FIG. FIG. 8 is a schematic diagram showing a delay profile when a multipath of the present invention is mixed.
11: IF / BB converter, 12: A / D converter, 13: delay circuit, 14: correlation calculator, 15 noise removal filter, 16: symbol timing detector, 17: differentiator, 18 comparator, 19: Symbol counter, 1A: VCO control unit, 1B: VCO, 1C: FFT operation unit, 1D: demodulation unit

Claims (2)

In a receiving apparatus that receives an orthogonal frequency division multiplex modulation signal (hereinafter referred to as an OFDM signal) having a guard interval period in which a part of the signal of the effective symbol period is copied to a part of the effective symbol period, the received signal A correlation calculation means for performing a cross-correlation calculation for each sample between a received sample sequence signal obtained by sampling the received sample sequence signal and a delayed signal obtained by delaying the received sample sequence signal by an effective symbol period, and a correlation calculation value sequence signal for each obtained sample Means for filtering in the symbol direction; guard interval position detecting means for detecting the guard interval position from the filtered correlation calculation value series signal for each sample; and a time window for fast Fourier transform based on the detected guard interval position Means for controlling the position of the filtered OFDM receiving apparatus is characterized in that comprises means for variably controlled to synchronize the reception sampling clock frequency based on the correlation calculation value sequence signal to the clock frequency of the transmitter. 2. The OFDM receiver according to claim 1, further comprising means for differentiating the filtered correlation value sequence signal for each sample to generate a delay profile .

Images