JP2002232389A - Ofdm receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an OFDM(orthogonal frequency division multiplex) receiver which detects a symbol transition point accurately in an OFDM signal having no synchronous symbols, and can set an FFT(fast Fourier transform) time window on the basis of detected results and calculate a delay profile with high precision. SOLUTION: In a receiver which receives an OFDM signal having a guard interval period in which a part of a signal of an effective symbol period is copied in a part of the effective symbol period, the OFDM receiver is equipped with a correlation operating means for performing mutual correlation operation of the respective samples of a received sample series signal and a delayed signal in which the received sample series signal is delayed by an effective symbol period, a means for filtering the obtained correlation operation value series signal toward a symbol direction, and a guard interval position detecting means for detecting a guard interval position from the correlation operation value series signal subjected to filtering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clock frequency control method, a timing control method of an FFT (Fast Fourie Transform) time window, and a transmitter and a receiver used therefor, and more particularly to an orthogonal frequency division multiplexing (OFDM).
l Frequency Divisional Multiplexing) The present invention relates to a receiving device of a data transmission device modulated by a modulation method.

[0002]

2. Description of the Related Art In recent years, orthogonal frequency division multiplexing modulation (hereinafter, referred to as "modulation"), which is characterized by being resistant to multipath fading and ghosts, has been proposed as a modulation method suitable for digital transmission for mobile devices and terrestrial digital television broadcasting.
OFDM method) is receiving attention. OFDM
The system is a kind of multi-carrier modulation system, and is a transmission system in which n (n is several tens to several hundreds) carrier waves orthogonal to each other are digitally modulated. As the above-mentioned carrier digital modulation system, a four-phase differential phase shift keying system (DQPSK: Differential Quadratu
Although re-phase shift keying is often used, 16-level quadrature amplitude modulation (16QAM) is used.
ulation) or 64QAM. As shown in FIG. 2, the OFDM signals are added so that the carrier waves are orthogonal to each other, and OFD signals are added.
An M time axis waveform is generated. This addition process is performed for each carrier using an IFFT (Inverse Fast Fourie Transform).
It can be realized by performing processing. The processing unit in the IFFT processing, that is, the number of FFT samples is generally used as a unit of a power of 2 such as 1024 or 8192,
The number of samples of the time-domain transformed OFDM signal becomes equal to the number of FFT samples. As shown in FIG. 3, the OFDM signal is composed of an effective symbol that is a time-axis waveform after the IFFT processing and a guard interval obtained by copying a part of the effective symbol and adding it before the effective symbol. Is done. In the OFDM system, by adding a guard interval, it is possible to avoid deterioration due to inter-symbol interference with respect to a delay 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 transmitted after being frequency-converted into an intermediate frequency (IF) band and a high frequency (RF).

[0003] In a receiving apparatus, a received signal is frequency-converted into a baseband band via an RF band and an IF band, and then sampled by an A / D converter. OFD
In the demodulation process on the M signal, an FFT operation process is performed on the obtained received sample value sequence in a manner reverse to that of the transmission device, and the time-domain signal is converted into a frequency-axis signal. In the FFT operation processing, a time window having an effective symbol period length is provided on the obtained received sample sequence, and the FFT operation processing is performed on the sample signal included in the time window. Based on the amplitude and phase information of each carrier obtained in this way, DQPSK or 1
Demodulation such as 6QAM is performed, and OFDM transmission is completed. Such demodulation processing of an OFDM receiver is described in Journal of the Institute of Image Information and Television Engineers, vol. 53, No. 11, pp1
538-1549 (1999). Next, two points of processing according to the present invention, which are required when the receiving apparatus demodulates an OFDM signal, will be described.
First, in the OFDM system, since the frequency interval between carriers is narrow, carrier frequency errors between transmitting and receiving apparatuses and interference between carriers due to sampling clock frequency errors in a demodulation system are likely to occur. Requires high accuracy. In order to continue to receive the OFDM signal correctly, it is necessary to perform a sampling clock regeneration process that always keeps the sampling clock frequency consistent 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 operation is located at the end of the symbol period in order to reduce the influence of multipath within the guard interval period as shown in FIG. It is desirable to be done. However, if an FFT window is provided at a position including a signal of an adjacent symbol due to a sampling clock frequency error, multipath, or the like, intersymbol interference occurs. Intersymbol interference due to an OFDM signal is regarded as a mixture of Gaussian noise, and as a result, C / N (carrier / noise ratio: Carrier / Nois).
It appears as the deterioration of e), and the deterioration of the bit error rate occurs. Therefore, there is a need for an FFT time window position control process of detecting a transition point of a symbol from a received sampling sequence with high accuracy and providing an FFT time window so that intersymbol interference does not occur.

[0004] In order to solve the above two points, an OFDM signal is composed of a plurality of synchronization symbol groups followed by a data symbol group to form a frame. Examples of the synchronization symbol include a null symbol in a no-signal period and a sweep symbol for sweeping 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 reproduction processing and the FFT time window position control processing based on the synchronization symbol is disclosed in
The OFDM signal can be accurately demodulated by this means. Also,
OFDM transmission is often used for mobile transmission such as marathon relay due to its system. In outdoor transmission, a multipath communication path is formed in which, in addition to the main wave directly arriving from the transmitter according to the terrain, there is a reflected wave reflected from a building or the like and arriving with a delay time. Further, in mobile transmission, a fading environment may occur in which the levels of the main wave and the reflected wave change every moment. Such a multipath is likely to cause a transmission error, and in a marathon relay, a transmission error may cause an image to 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. A method most often used as a means for observing propagation path characteristics is a delay profile for calculating a level of a main wave or a reflected wave and a delay time. In this case, it is possible to calculate the delay profile with high accuracy by performing the cross-correlation calculation between the sweep symbol and the received sample sequence described above.

[0005]

In the OFDM system according to the above-mentioned prior art, in order to accurately demodulate a received signal,
It was necessary to add a synchronization symbol to the OFDM signal.
However, there is a drawback that the addition of the synchronization symbol lowers the transmission efficiency by that amount. For example, when four synchronization symbols are added to 400 symbols, the transmission efficiency is reduced by 1%. In addition, when no synchronization symbol is 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 a multipath of adjacent delay times. The present invention eliminates these drawbacks, and provides an OFDM receiving apparatus that can reproduce a received sampling clock from an OFDM signal having no synchronization symbol, accurately detect a symbol transition point, and set an FFT time window based on the detection result. The purpose is to provide.
Further, even when a synchronization symbol is not used, an OFDM capable of calculating a delay profile with high accuracy.
An object is to provide a receiving device.

[0006]

In order to achieve the above object, the present invention provides an OFDM signal having a guard interval period in which a part of the signal of the effective symbol period is copied in a part of the effective symbol period. A correlation calculating means for performing, for each sample, a cross-correlation operation between a reception sample sequence signal obtained by sampling the reception signal and a delay signal obtained by delaying the reception sample sequence signal by an effective symbol period; An OFDM receiver comprising: means for filtering an operation value sequence signal in a symbol direction; and guard interval position detection means for detecting the guard interval position from the filtered correlation operation value sequence signal. 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 reception sampling clock frequency to synchronize with the clock frequency of the transmission device based on the filtered correlation operation value sequence signal. This is an OFDM receiver provided with means. Further, the OFD includes means for performing a differentiation process on the filtered correlation operation value sequence signal to generate a delay profile.
M receiving device.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A receiving apparatus of a digital transmission apparatus according to the present invention will be described below in detail with reference to an embodiment shown in FIG. An OFDM reception signal arriving at a receiving device via a transmission path from a transmitting device (not shown) is transmitted to an IF / BB converter 11 at an intermediate frequency in the same manner as in the related art.
The signal in the (IF) band is converted into a signal in the baseband frequency band. The output from the IF / BB converter 11 is A /
The D / A converter 12 performs analog / digital conversion using the reception sampling clock supplied from the VCO 1B. In order to perform demodulation from an OFDM reception 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,
These will be described below. The received sample sequence 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 sample period with respect to the received sample sequence, for example, delays a 1024 sampling clock. The correlation calculator 14 performs a correlation calculation for each sample of the received sample sequence S and the delayed signal D as shown in FIG. Here, as described above, in the OFDM signal, the trailing end (a, b in FIG. 5) of the effective symbol is copied and added as a guard interval to the leading end (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 the signal (a ′, b ′) of the guard interval period, the corresponding received sample value sequence S has the same component ( a,
b). Therefore, in the correlation result C for each sample, the correlation coefficient becomes large when the delay signal D is the signal interval (a ', b') of the guard interval. Then, when the delay signal D is a signal period other than the guard interval period, the signal is uncorrelated with the received sample value sequence S, so that the correlation coefficient is also small. However, even if the signal is in the guard interval period, since the amplitude distribution of the OFDM signal has a distribution form close to the Gaussian distribution, when the signal level of the received sample value sequence S is small, the correlation value is also small, and the correlation of one symbol is small. The correlation value C obtained by the calculation has a variation depending on the level of the OFDM signal.

Therefore, in order to suppress the variation 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 noise components. The noise removal filter 15 is, for example, (1 /
This is a well-known IIR filter that cyclically feedback-adds a correlation value sequence signal C multiplied by (1-1 / α) by one symbol to a correlation value sequence signal C multiplied by α).
The noise removal filter 15 filters the correlation sequence output C from the correlation calculator 14 in the symbol direction, that is, removes noise components by cyclically feedback-adding the correlation value sequence signal C delayed by one symbol. The correlation value series F thus output is output. The correlation value sequence F obtained in this manner forms a rectangular 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
The guard interval position is input to the symbol timing detector 16, and the symbol timing detector 16 detects a guard interval position from the correlation value sequence F. In this detecting means, first, a predetermined threshold value is provided for the input correlation value sequence F, and the magnitude relation between the correlation value sequence F and this threshold value is compared. When the correlation value sequence is at a level higher than the threshold value, it is determined that the signal is a guard interval signal, and the rising position is detected as the guard interval start position.

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, to prevent erroneous detection of the guard interval position,
A means for accurately determining that the position is the guard interval position is required. For example, as a criterion for specifying the guard interval period by the symbol timing detector 16, if the occurrence of a correlation value signal larger than the threshold value is detected stably over several symbols, the position of the signal is determined as the guard position. It is determined that it is an interval period. As a method of calculating a threshold value provided in the symbol timing detector 16, for example, an average value of a correlation value series is calculated, and the average value is set to a multiple of the value. Alternatively, an average value of the received sample value series is calculated, and the average value is set to be a multiple of the value. The detection result of the guard interval signal is obtained as guard interval position information, and the symbol timing detector 16 outputs a reset signal RS to the symbol counter 19 based on the position information, as shown in FIG. The symbol counter 19 resets the counter value based on the reset pulse RS,
The symbol period is counted in clock units supplied from CO1B. The symbol synchronization timing SST from the symbol counter 19 controls the timing of the entire OFDM receiver, and the FFT time window is also set based on the symbol synchronization timing. Once the reset pulse RS is output (synchronization is achieved), the symbol timing detector 16 resets the symbol counter 19 until synchronization is lost in order to prevent the symbol counter 19 from being reset when a guard interval position is erroneously detected. It operates so as not to output the pulse RS. Further, the symbol timing detector 16 has a function of outputting a control signal to the VCO control unit 1A so as to synchronize the reception sampling clock frequency with the transmission clock frequency.

An embodiment of a method for calculating a control signal of the reception sampling clock frequency from the correlation value sequence F in the symbol timing detector 16 will be described below. In the first embodiment, the maximum value is detected from the correlation value sequence F, and the position of the maximum value is calculated as a control signal.
In the correlation value sequence F, since the level of the guard interval period is large, the guard interval position can be detected with an accuracy within the guard interval sample number 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 the error between the maximum value position detected for the previous symbol and the maximum value position calculated for the current symbol is output as control information to the VCO 1B. Here, if the clock frequencies of the transmitting device and the receiving device are synchronized, the average value of the position error of the maximum value is 0, but if the receiving clock frequency is higher than that of the transmission, the average value is Negative value, if low,
On the contrary, it becomes a positive value. Therefore, this maximum value position error is calculated as VC
If the control is performed such that the maximum value position error becomes 0 using the O1B control information, the reception sampling clock frequency can be synchronized with the transmission clock frequency.

In the second embodiment, a level difference between both ends of a correlation value sequence F having a level larger than the above-described threshold value is calculated as a control signal. As described above, if the clock frequencies of the transmitting device and the receiving device are synchronized, the average value of the correlation value levels within the guard interval period becomes equal. However, when the reception clock frequency is higher than that of the transmission, the signal at the front end of the correlation value sequence F contains a signal component that is not correlated, and thus the correlation value level becomes small. Conversely, when the frequency is low, the correlation value level at the rear end becomes small, so that the reception sampling clock frequency can be controlled by controlling the levels of these samples to be equal. In the embodiment described above, the control information output from the symbol timing detector 16 is input to the VCO control unit 1A. V
In order to control the frequency of the VCO 1B based on the control information, the CO control unit 1A converts the control information into a frequency control voltage and outputs it. By the above processing, the reception sampling clock frequency can be synchronized with the transmission clock frequency.

Next, an embodiment of the delay profile calculating means will be described. Output F of noise removal filter 15
Is input to the differentiator 17. In the differentiator 17, FIG.
The difference between the signal one sample before and the current sample signal is calculated with respect to the correlation value series F as shown in FIG. 7A to calculate a differential coefficient K (FIG. 7B). Here, when the correlation value series signal enters the guard interval period, the correlation value level rapidly increases, and the differential coefficient K at that time becomes a large value. Also, the correlation value sequence F when multipath is mixed
Is a waveform in which 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 are combined as shown in FIG. It has a large value ((b) in FIG. 8) when the reflected wave is switched. The output K from the differentiator 17 is input to a comparator 18, and only a signal having a positive value is extracted from the differential coefficient K, and a signal having a negative value is converted into a predetermined value (for example, 0). Output. ((C) in FIGS. 7 and 8) This signal has sharp peaks at the positions of the main wave and the reflected wave corresponding to the respective levels, so that it is possible to distinguish the reflected wave having a close delay time. Becomes By calculating the delay profile waveform by the above processing, it is possible to accurately observe the propagation path characteristics.

[0013]

As described above, the receiving apparatus according to the present invention performs a cross-correlation operation on a received signal even for an OFDM signal having no synchronization symbol.
Guard interval signals can be detected with high accuracy, and an FFT time window free of intersymbol interference can be provided. Further, it is possible to detect a clock frequency error between the transmitting device and the receiving device from the detected guard interval signal and perform control so that the receiving clock frequency is synchronized with the transmitting clock frequency. Further, by differentiating the correlation waveform, a highly accurate delay profile can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a receiving device according to the present invention.

FIG. 2 is a schematic diagram showing a signal form of an OFDM modulation signal.

FIG. 3 is a schematic diagram showing an OFDM symbol waveform.

FIG. 4 is a schematic diagram showing an FFT time window;

FIG. 5 is a schematic diagram for explaining a correlation calculation processing state according to the present invention;

FIG. 6 is a timing chart for explaining a symbol synchronization state according to the present invention;

FIG. 7 is a schematic diagram showing a delay profile when the multipath of the present invention is not mixed.

FIG. 8 is a schematic diagram showing a delay profile when the multipath of the present invention is mixed.

[Explanation of symbols]

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 comparators, 19: symbol counter, 1A: VCO control unit, 1B: VCO, 1C: FFT operation unit, 1D: demodulation unit

Claims (3)

[Claims] 1. An orthogonal frequency division multiplexing modulation signal (hereinafter referred to as O) having a guard interval period in which a part of a signal of the effective symbol period is copied in a part of an effective symbol period.
A receiver for receiving an FDM signal), a correlation operation means for performing a cross-correlation operation for each sample of a reception sample sequence signal obtained by sampling the reception signal and a delay signal obtained by delaying the reception sample sequence signal by an effective symbol period. OFDM, comprising: means for filtering the obtained correlation operation value sequence signal in the symbol direction; and guard interval position detection means for detecting the guard interval position from the filtered correlation operation value sequence signal. Receiver. 2. A transmitting apparatus according to claim 1, wherein said means controls a position of a time window of a fast Fourier transform based on the detected guard interval position, and a receiving sampling clock frequency based on said filtered correlation operation value sequence signal. An OFDM receiving apparatus, comprising: means for variably controlling so as to synchronize with a clock frequency. 3. The OFDM receiver according to claim 1, further comprising means for performing a differentiation process on the filtered correlation operation value sequence signal to generate a delay profile.