KR100266601B1 - Channel interference cancellation method and apparatus for digital television system - Google Patents

Abstract

PURPOSE: A method of removing a channel interference of a digital television system and an apparatus are respectively provided to compensate an OFDM signal received by estimating an interfering signal based on a statistical characteristic of the interfering signal. CONSTITUTION: The apparatus for removing a channel interference of a digital television system includes a transmitting apparatus and a receiving apparatus. The transmitting apparatus includes a QAM mapper(201), a reverse-Fourier converter(202), and a transmission filter. The receiving apparatus includes a matching filter, and a Fourier converter(206), an estimator(210) and an adder(220). The QAM mapper(201) outputs QAM-samples by QAM-mapping an OFDM signal. The reverse-Fourier converter(202) performs a reverse-Fourier transform for the QAM-samples signal to output OFDM samples. The transmission filter transmits the OFDM samples via a transmission channel. The Fourier converter(206) performs a Fourier transform for the signal received through the transmission channel by the matching filter. The adder(220) subtracts an estimated signal generated by the estimator(210) from a Fourier transform signal outputted from the Fourier converter(206).

Description

Method and apparatus for canceling channel interference in digital TV system

The present invention relates to a digital TV system, and more particularly, to a method and apparatus for canceling channel interference in a European terrestrial digital TV system.

Currently, in Europe, Orthogonal Frequency Division Multiplexing (OFDM) transmission is adopted as a standard for terrestrial digital TV broadcasting.

The OFDM transmission method uses one block of information bits on multiple orthogonal subcarriers and superimposes them at a low symbol transmission rate so that all available frequency bands can be used and the effects of multipath distortion and impulsive noise can be reduced. have.

In order to cope with the performance degradation caused by the multipath environment in mobile reception, the research on applying the OFDM transmission scheme to the system is actively conducted. In addition, the concept of guard interval is introduced in the OFDM transmission scheme and error correction coding is performed. A COFDM transmission scheme combining the (Error Correction Coding) has been proposed.

The COFDM transmission method has a very small transmission power of about 10% than the conventional analog transmission power, and has a characteristic of added Gaussian White Noise (AWGN), which is relatively unlikely to affect analog broadcasting. Do.

However, since terrestrial digital TV broadcasting has to share a limited channel with a conventional analog broadcasting channel (for example, NTSC), interference by the existing analog broadcasting channel is inevitably a problem.

That is, as shown in the exemplary diagram of FIG. 1, the terrestrial digital TV broadcast must be compatible until fully digitalized because the channel is allocated to the frequency band of the current analog broadcast. This means that it is affected by the adjacent channel (Adjacent-Channel) or the same channel (Co-Channel).

Accordingly, in order to share the existing TV broadcasting channel and the terrestrial digital TV broadcasting channel, interference by the carrier of the existing analog broadcasting should be minimized.

First, looking at the spectrum of the NTSC signal, as shown in the waveform diagram of FIG. 3, the position of the image carrier is at 1.25 MHz, the color subcarrier is at 3.58 MHz from the image carrier, and the voice carrier is 4.5 MHz away from the image carrier. do.

Also, the image and color signals are offset at half of 15.734KHz respectively.

Therefore, in the characteristics of NTSC system, the carrier spacing of the OFDM system can be selected and '7.867, which is half of 15.734KHz. × Interference can be reduced by placing the OFDM carrier at an integer multiple of 1 / n KHz '(n is a positive integer) to offset the NTSC frequency components.

Alternatively, interference can be reduced by not using the OFDM carriers of such portions by setting a value corresponding to '0' in the array of FFT data at the strong energy band of each carrier position appearing in the NTSC characteristic.

In the present invention, a method of reducing channel interference with an existing NTSC scheme using null data will be described.

In an OFDM system, interference of an NSC signal is caused by a component of each carrier having strong power among NTSC signal components.

In general, the reception waveform of the OFDM signal is shown in the form of additional noise as shown in FIG.

In addition, the waveform of the NTSC signal has the form as shown in FIG. 3 in the same frequency band as the OFDM signal.

For example, when the carrier of the OFDM signal is in the 2K mode, it exists at a frequency interval of about 3.52KHz by '6MHz / 1705'.

In this case, the position where an OFDM symbol appears in each of the carriers of NTSC is located at about 355th in the case of the video carrier '1705 * (1.25MHz / 6MHz)', which is the NTSC signal centered on the 355th carrier of the OFDM subcarriers This means that the interference of the image carrier will occur.

Similarly, when the color subcarrier and the voice carrier are located, they appear at the 1372 and 1634 th positions.

Therefore, when interference is generated by the NTSC signal of the same channel, the received OFDM signal has a large energy overlapping the carrier portion of the NTSC as shown in FIG.

Conventionally, in order to remove the interference generated as shown in FIG. 4, as shown in FIG. 5, the image carrier, color subcarrier, and voice carrier having a strong energy band are transmitted as '0' without transmitting an OFDM signal. Alternatively, it is called a spectral hole.

However, conventionally, when the OFDM FFT mode is 2k, the number of nulls requires a large amount of about 8 to 10% of the OFDM subcarriers, thereby reducing the data transmission amount, resulting in deterioration of image quality.

Therefore, in order to solve the conventional problem, the present invention predicts the statistical characteristics of the interference signal caused by the OFDM signal from the NTSC signal and corrects the received signal based on the null signal given to the OFDM signal to remove the interference of the same channel. An object of the present invention is to provide a method and apparatus for canceling channel interference in a digital TV system, which is designed to minimize the number of nulls to prevent degradation of image quality.

1 is an exemplary diagram showing interference in digital TV broadcasting.

2 is an exemplary view showing a form of an OFDM signal.

3 is an exemplary view showing a spectrum of an NTSC signal.

4 is an exemplary diagram showing a form of an interference OFDM signal.

5 illustrates insertion of a null signal for removal of NTSC interference.

6 is a block diagram illustrating a channel model of an OFDM system.

7 is an exemplary view showing one frame of an OFDM signal.

8 is an exemplary view showing insertion of a prediction signal.

9 is a block diagram showing an embodiment of the present invention.

FIG. 10 is an exemplary view illustrating a prediction range setting in FIG. 9. FIG.

Explanation of symbols on the main parts of the drawings

201: QAM Mapper 202: Inverse Fourier Converter

203: transmission filter 205: matching filter

206: Fourier transformer 210: Interference signal predictor

220: adder

In order to achieve the above object, the present invention provides a method for eliminating interference by receiving a signal in which '0' data is inserted and transmitted in order to remove interference of an analog broadcast in an OFDM system. Storing the received signal for a predetermined time to obtain a time axis average value, predicting the magnitude and shape of the interference according to the average value, obtaining a correction range according to the magnitude of the interference obtained above, Predicting the interference signal using the obtained correction range, reducing the number of nulls by correcting the received signal using the predicted interference signal, and removing the interference signal from the corrected received signal. Characterized by performing the steps.

In this case, the received signal is stored for one frame to obtain a time axis average value.

In addition, the present invention, in order to achieve the above object in the OFDM system for removing interference by receiving and encoding a signal inserted with '0' data in order to remove the interference of the analog broadcast, transmitted to the transmission channel A matched filter for receiving and matching OFDM samples, a Fourier transformer for Fourier transforming the output signal of the matched filter, and an output signal of the Fourier transformer for one frame to obtain a time-base average, The receiver comprises an interference signal predictor for predicting magnitude and shape and generating an estimated interference signal, and an adder for subtracting an output signal of the Fourier transformer to an output signal of the interference signal predictor and outputting an original OFDM transmission signal. It is characterized by.

Hereinafter, the present invention will be described in detail with reference to the drawings.

Fig. 9 is a block diagram showing an embodiment of the present invention, as shown here. The QAM mapper 201 outputs QAM samples by QAM mapping an OFDM signal, and the inverse Fourier transform of the output signal of the QAM mapper 201. A inverse Fourier transformer 202 for extracting OFDM samples and a transmission filter 203 for transmitting the OFDM samples extracted in the inverse Fourier transformer 202 to the transmission channel 204; Fourier transforming the matching filter 205 for transmitting the interference signal r (t) transmitted to the transmission channel 204 and the signal r (t) received by the matching filter 205. Fourier transformer 206 and the output signal of the Fourier transformer 206 are stored for one frame to obtain a time axis average, and then set the magnitude and shape of the prediction signal according to the magnitude and shape of the interference to generate the prediction signal. The OFDM signal corrected by subtracting the output signal of the predictor 210 and the Fourier transformer 206 to the output signal of the interference signal predictor 210 ( The receiving device is configured with an adder 220 that outputs a).

Referring to the operation and effect of the embodiment of the present invention configured as described above are as follows.

The channel model of a typical OFDM system is shown in the block diagram of Figure 6, OFDM transmitter, the inverse Fourier transformer 202 inverse Fourier transform (IFFT) the QAM symbol (B (k)) is an OFDM sample (b (k) ), The transmission filter 203 convolutionally matches the extracted OFDM sample b (k) with the transmission characteristic g (k) and transmits the matched signal s (k) to the transmission channel ( 204, and the OFDM receiver transmits a signal r (t) to which an NTSC interference signal i (k) is added to the OFDM signal s (k) transmitted by the matching filter 205 to the transmission channel 204. Is received and matched, the Fourier transformer 206 is configured to Fourier transform the output signal y (k) of the matched filter 205 to output the signal Y [k].

Looking at the characteristics of each signal in this configuration is as follows.

The OFDM symbol b (k) input from the inverse Fourier transformer 202 to the transmission filter 203 is expressed as shown in Equation (1) below.

----- Formula (1)

Where n is the number of OFDM samples and n = 0,1,2, ..., (N-1) has a value.

The signal B (k) input to the inverse Fourier transformer 202 is data carried on the k-th subcarrier as a QAM mapped complex symbol.

The data b (k), which has undergone the inverse Fourier transform (IDFT), is transmitted to the transmission channel 204 through the transmission filter 203 having the characteristic g (k) and is transmitted to the NTSC interference signal i (t). ) Is added.

Thus, the signal received at the OFDM receiver ( r _m (n) Is represented by the following equation (2).

r _m (n) = b (n) * g (n) + i (n) ----- Equation (2)

Here, "*" represents convolution.

The signal received above ( r _m (n) ) Becomes a signal y (n) as shown in equation (3) below via a matching filter 205 having a characteristic f (n).

----- Equation (3)

Therefore, the signal Y (k) finally Fourier transformed (FFT) by the Fourier transformer 206 is represented by the following equation (4).

----- Formula (4)

here, b (n) * g (n) = s (n) F (k) is a value already known as the frequency response characteristic of the matched filter 205.

The Fourier transformed signal S [k] is a de-mapping signal of the QAM signal B (k) and has an equal probability, so that the average value is '0'.

At this time, the interference signal I (k) by NTSC is a frequency domain signal of the interference signal i (t), which results in the addition of the signal in the frequency domain when viewed in the OFDM signal.

That is, in the above formula (4) S (k) ⋅F (k) Is a Fourier transformed signal of the portion to which the interference signal after passing through the matched filter 205 is not added. I (k) ⋅F (k) Is a Fourier transformed signal of the NTSC interference signal.

Therefore, Equation (4) does not become '0' in the added interference signal portion.

In this case, in order to minimize the interference of the NTSC signal, an estimated value corresponding to the interference signal I (k) ( ) Is inserted into Equation (4) and is expressed as Equation (5) below.

----- Formula (5)

here, Is a signal corrected by estimating the amount of interference in the received signal, and S '(k) is an interference component of the NTSC signal input to the OFDM receiver.

Therefore, Y (k) ≈ The error caused by interference is minimized.

That is, the predicted interference signal ( ) Can be attenuated to perform the same operation as in Equation (5).

Thus, the predicted interfering signal (a) for attenuating the NTSC signal I (k) with interference in channel 204 In order to estimate), an interference signal predictor 210 is added as shown in FIG. 9, and the operation of the interference signal predictor 210 will be described as follows.

First, the interference signal predictor 210 stores the signal Y (k) output from the Fourier transformer 206 by one frame and then averages the time value ( )

Since the received signal Y (k) is a signal to which the interference signal I (k) is added, in the 2K mode without a guard interval, 1 OFDM symbol is composed of 1705 data, and 68 sets of such 1 OFDM symbols are obtained. It is referred to as one frame, and the value during one frame, that is, 68 OFDM symbols is shown in FIG.

If it is assumed that the QAM mapped signal has the same probability as the input signal, if the OFDM signal is 2K mode during channel transmission, the OFDM signal of one frame is received and stored, and then the average is calculated on the time axis. .

----- Formula (6)

For example, if the interval for obtaining the average value is 68 symbols (= 1 frame), and the guard interval is 1/4 in the 2K mode, the interval of the entire symbol is 1705. × 0.1 μ 0.17 mS for S × 68 symbols ≒ 11.6 μ The delay time for removing the interference signal by S is not so large, so there is no big problem in practicality.

On the other hand, the characteristic of the OFDM signal is a signal having an equal probability, so the average value is '0', but a large value appears in the portion where the interference signal i (t) exists, that is, in the image, color, and voice carrier portion of the NTSC. Since the interference signal i (t) is additional noise, the interference signal i (t) does not become '0' at the portion where interference occurs.

Therefore, when the added null is extracted from the averaged value during the 1-frame, only the interference signal appears purely, and thus the magnitude and shape of the interference signal are predicted, so that the overall prediction signal ( ) Can be obtained.

In addition, the interference signal also has a Gaussian distribution around each carrier due to the characteristics of NTSC. When the time average is obtained, the values of the image that change with time become nearly '0', so Only the distribution characteristic remains.

Therefore, after extracting the shape and size of the interference from the small number of transmitted '0' data, it is possible to minimize the influence of the interference by correcting the newly input OFDM symbol with this value.

That is, since the position where interference occurs in the received OFDM signal is a portion having strong energy in the NTSC signal, the position can be known in the OFDM symbol, and the center frequency at which the NTSC interference signal appears can be known. The amount of calculation can be reduced by storing the amount and taking the average.

By applying this method, we can save the entire frame only for the first 1 frame, get the average of the time base, and reduce the calculation amount according to the size of the interfering signal for the subsequent signal. can do.

For example, the sample of the m frame l-th symbol of the received OFDM signal ( Y _l , _m (N) If interference occurs only in the video carrier portion of the NTSC signal during one frame of), the time axis average can be obtained as shown in Equation (7).

----- Formula (7)

Here, p is a center frequency at which interference occurs in OFDM subcarriers.

N is the number of subcarriers of interference appearing in the OFDM by NTSC signal, depending on the size of the interference signal. The larger N is, the greater the amount of interference, and the number of nulls also increases.

However, since the interference signal does not change rapidly with time, the size and shape in the frequency domain of the signal due to the interference signal remain almost constant.

Therefore, in order to predict the amount of interference, initially, the shape and size of the interference are obtained from a signal of one frame, and if the number of nulls is known, only one period (= 1 symbol) is added to the OFDM symbols received thereafter. It can be calculated in the form of a sliding window that discards the oldest value.

----- Formula (8)

here, N _g Is a frame that reflects the symbol added after the first frame.

The reason for making the prediction signal is to reduce the number of null signals to 'N' or less.

As a way to do this, α Assuming that there are 'null signals, the receiver is approximately' α Since as many errors will occur as shown in FIG. α The average of OFDM signals with 'n' null signals is obtained on the time axis for one frame, and then the null signal is extracted to predict the magnitude and shape of the overall interference signal.

If, 'N- α If 'N' interferences occur around the position (p) of the video carrier of the NTSC signal in the OFDM signal inserted with 'n null' N- α After predicting the degree of interference from the average value of one frame signal of 'n null signals, N' is obtained, and the ranges k1 and k2 to be corrected in the received signal can be obtained.

----- Formula (9)

This is a prediction of the interval (N ') of the interference signal in the frequency domain with respect to the distribution of interference appearing at the OFDM receiver for each carrier signal of the image, color, and voice of the NTSC signal.

Here, the position p of the image carrier is the center of the interference.

In the same manner, the positions of the color subcarriers and the voice carriers can be obtained, and the estimated interference signals of the respective carriers can be obtained from the center positions of the interferences.

The value obtained by the above expression (9) is an average value of the interference signals during one frame.

Therefore, in order to correct the interference signal with the predicted signal, the prediction range setting unit 213 predicts a section to be corrected from the average value of the interference signal for one frame, and then inputs an NTSC signal to the OFDM receiver as shown in Equation (10) below. The interference component of can be obtained.

----- Formula (10)

here, Is the average value of one frame of the interference signal.

Therefore, the interference component obtained in Equation (10) can be substituted into Equation (5) to eliminate the interference.

That is, the interference signal predictor 210 determines the average value of the interference signals during one frame ( ), The adder 220 subtracts from the output signal Y (k) of the Fourier transformer 206 to remove the interference.

As above, in an environment with NTSC interference that requires N null signals (N- α The overall data rate can be improved by transmitting errors by correcting errors by transmitting) null signals.

If the insertion position and the number of nulls are set in advance, a fast predictive signal can be generated by performing an average operation only on a null signal, so that a fast-changing moving object can have a fast response characteristic.

As described in detail above, the present invention obtains a statistical value of the interference signal by obtaining a time-base average value for a predetermined period of the received OFDM signal, and then predicts the interference signal based on this to correct the received signal. Even reducing it to about 1/2 has the effect of eliminating interference signals with similar performance.

In addition, the present invention can be applied to any analog system, such as NTSC or PAL as long as the size of the null data is determined at the transmitting side by observing the received signal to identify the statistical characteristics of the interference signal. There is.

Claims (7)

A method of removing interference by receiving a signal in which '0' data is inserted and transmitted to remove interference of an analog broadcast in an OFDM system, the method comprising: storing a received signal for a predetermined time and obtaining a time axis average; A second step of predicting a degree of interference from the average value, a third step of obtaining a correction range according to the degree of interference obtained above, a fourth step of predicting an interference signal using the obtained correction range, Performing a fifth step of reducing the number of nulls by correcting the received signal by using the predicted interference signal and a sixth step of removing the interference signal from the corrected received signal. How to remove channel interference 2. The method of claim 1, further comprising presetting the number of nulls. The method of claim 1, wherein the first step calculates a time axis average value of a received signal for a predetermined time period and performs time axis averaging only on a null signal. The method of claim 1 or 3, wherein the amount of the received signal for the average of the time axis is set to one frame. The method of claim 1, wherein the second step extracts a null signal from an average value of the received signal during one frame to predict the magnitude and shape of the overall interference signal. The method of claim 1, wherein the third step estimates a correction range only for a carrier portion of an analog broadcast signal. An OFDM system for removing interference by receiving and encoding a signal containing null data to remove interference of analog broadcasting, comprising: a matching filter for receiving and matching OFDM samples, and a Fourier output signal of the matching filter. A Fourier transformer for converting, an interference signal predictor for storing the output signal of the Fourier transformer for a predetermined time, obtaining a mean of the time axis, and predicting the magnitude and shape of the interference signal according to the degree of interference to generate a predicted interference signal; And an adder for subtracting an output signal of a Fourier transformer to an output signal of the interference signal predictor and outputting an original OFDM transmission signal.