KR100421957B1 - Method for acquirement of echo impulse response of echo canceller in DMT - Google Patents

Abstract

The present invention provides a method for acquiring an echo impulse response of a DMT echocancer, comprising: a first step of obtaining a complex one tap using a reverb signal; By including the second step of obtaining the time domain echo impulse response by IFFT the complex one tap obtained in the first step, it is possible to obtain the echo impulse response of the time domain without grasping the echo frame synchronization at the central station side .

Description

Method for acquirement of echo impulse response of echo canceller in DMT}

The present invention relates to a Discrete Multitone modulation (DMT) echo canceller, and is particularly suitable for obtaining an echo impulse response in the time domain at the central office (CO) side. The present invention relates to a method for acquiring an echo impulse response of a DMT echocancer.

In general, DMT is an American National Standard Institute (ANSI) standard modulation technology used for Asynchronous Digital Subscriber Line (ADSL). Simplifying ADSL technology is a new modem technology that allows high-speed data transmission over existing telephone lines.

This ADSL technology is generally composed of three channels: high speed downstream channel, medium speed full duplex (Duplex) channel, and low speed Plain Old Telephone Service (POTS) channel. Frequency division multiplexing (Frequency Division) It is divided into multiplexing (FDM) method and echo cancellation method. The FDM method uses a medium speed channel among upstream, downstream and POTS channels as an upstream dedicated channel, and the echo cancellation method uses a medium speed channel as an upstream and downstream channel.

In addition to channel division by these frequencies, the most important factor in ADSL technology is modulation. This modulation technology is an element technology that determines the standard of ADSL technology itself and is currently divided into a Discrete Multi Tone (DMT) method and a Carrierless Amplitude and Phase (CAP) method.

The technical characteristics of the DMT and CAP schemes are as follows.

First of all, in the modulation technology, the DMT method divides the frequency band used by 4KHz into 2,560 channels and modulates each channel with QAM or QPSK using multiple carriers. The CAP method uses one carrier. It is a technique to modulate with QAM method using. Therefore, the speed difference is relatively faster than the CAP method because the modulation is performed for each channel in the DMT method. In power consumption, since the DMT method transmits for each channel, it consumes more power than the CAP method and heat is generated relatively in the equipment. The difference in the delay rate is that the DMT method is generally faster, but the delay rate is relatively larger than that of the CAP method because the speed provided by each unit channel is limited.

In addition, ADSL technology allows the modem to adjust the data rate through Negotiation depending on the condition of the line or the overall environmental factors. This is called speed adaptation. In the DMT method, this speed adaptation is performed in units of 32 Kbps, and the CAP method is made in units of 200 Kbps, so that the DMT method provides a relatively improved speed adaptation capability than the CAP method.

1 is a block diagram of a conventional DMT.

In this case, reference numeral 11 is a first IFFT unit which performs an inverse fast fourier transform (IFFT) on the input data and sends a reverb signal to 256 bins, and 12 is a digital output from the first IFFT unit 11. Digital to Analog (D / A) conversion unit for converting data into 2.208 MHz analog data, and 13 is a transmission and reception for receiving the output of the D / A conversion unit 12, and performing transmission and reception processing for the data The hybrid filter 14 is an A / D (Analog to Digital) converter which converts the analog data of the transmit / receive hybrid filter 13 into digital data of 276 KHz by performing down sampling.

Reference numeral 15 denotes an echo frame synchronizer which obtains and synchronizes an echo frame sync from digital data of the A / D converter 14, and 16 denotes an output of the echo frame synchronizer 15. Is an FFT unit that performs an FFT (Fast Fourier Transform), 17 is an adder which subtracts an output of an echo canceller (EC) from an output of the FFT unit 16, and 18 is an adder 17 Is an echo canceler performing echo canceling using the output of the first IFFT unit 11 and the output of the first IFFT unit 11, and 19 denotes a complex one tap output from the echo canceler 18. ) Is a second IFFT unit for obtaining a time domain echo impulse response m by IFFT.

2 is a flowchart illustrating a method of obtaining an echo impulse response of a conventional DMT echocancer.

As shown therein, a step ST11 of sending a reverb signal to 256bin; Acquiring an echo frame after sending the reverb signal (ST12); Transmitting a random data signal after synchronizing the echo frame (ST13); Obtaining a complex one tap after sending the random data signal (ST14); IFFT the complex one tap to obtain a time domain echo response impulse m (ST15).

Referring to the operation of the prior art configured as described above is as follows.

First, a process of obtaining m of a time domain echo impulse response in the echocancer 18 of the central station CO DMT-ADSL is as follows.

The transmission signal Tx is transmitted through the first IFFT unit 11, the D / A converter 12, and the transmission / reception hybrid filter 13, and the reception signal Rx is received through the transmission / reception hybrid filter 13. It is transmitted via the A / D converter 14, the echo frame synchronizer 15, the FFT unit 16, and the adder 17. Therefore, the signal is transmitted from the transmitting side through the transmit / receive hybrid filter 13, and the receive signal is received by the transmit / receive hybrid filter 13, and the FFT unit 16 FFTs the echoes coming into the receiving side to LMS (Least Mean Squared) The update is made, and a complex one tap is obtained from the ecocancellor 18. The second IFFT unit 19 performs IFFT again to obtain an echo impulse response m.

In the case of ADSL, since the upstream and the downstream have an asymmetric structure, the sampling rates of the transmitting side Tx and the receiving side Rx are different. In other words, assuming that there is no interpolator, the sampling rate at the transmitting side is 2.208 MHz, and the sampling rate at the receiving side is 276 KHz.

Therefore, the signal transmitted from the transmitter is downsampled to 1/8 of the receiver. For this reason, an aliasing phenomenon occurs in which a correct frequency and a wrong frequency are generated together.

The down-sampled echo is FFTed by the FFT unit 16, and the echocancer 18 obtains a frequency domain complex one tap as shown in Equation 1 below.

Where (n = 0, 1, 2, 3,, 31), D _{n, k} is a modeled echo, X _{n, k} is a complex symbol (echo source) of N bins, and W _{n, k} is N bin Complex one tap.

Therefore, if the actual echo (Echo) is E _{n, k} , the following equation (2) holds.

Where e _{n, k} is an error.

With e _{n, k} in Equation 2 _, W _{n, k} , W _{64-n, k} , W _{64 + n, k} , W _{128-n, k} , W _{128 + n, k} , W _{192-n, k} , W _{192 + n, k} and W _{256-n, k} are obtained by the Least Mean Squared (LMS) update.

In addition, in order to obtain W _{n, k} (n = 0, 1, 2, 3,, 31) in Equation 1, a sender sends a reverb signal, which is a sine wave having periodic characteristics. )

When generalized to Equation 1 again, it is expressed as Equation 3 below.

If the reverb signal continues to be sent, there are a myriad of sets of "A ₁ , A ₂ , A ₃ , ..., A ₈ " which satisfy the equation (3). However, there is only one echo impulse response.

Therefore, to obtain the correct set of "A ₁ , A ₂ , A ₃ , ..., A ₈ ", we need several sets of "X ₁ , X ₂ , X ₃ ,, X ₈ " rather than a fixed signal. Shall be.

However, since the frame synchronization of Rx is set to the remote signal, the frame synchronization is mismatched if the echo is not a reverb signal.

Therefore, in the related art, the echo frame synchronization unit 15 first obtains the echo frame synchronization, synchronizes the synchronization, and then sends a random signal, and the echo canceler 18 sends "A ₁ , A ₂ , A ₃ ,..., A ₈ ". Was obtained, and the second IFFT unit 19 was subjected to IFFT to obtain an impulse response m.

However, since this conventional technique cannot obtain an exact set of "A ₁ , A ₂ , A ₃ ,, A ₈ " as a reverb signal, an echo frame synchronizer 15 which is a block functioning to match echo frame synchronization There was a necessary weakness.

In addition, there is a disadvantage in that the periodic characteristics disappear by using a random signal rather than a reverb signal having periodic characteristics to obtain a set.

Above all, since the band characteristic used by the echo path is not linear, a problem occurs in the process of acquiring the echo frame synchronization due to the phase delay between the bins. Due to such a problem, it is difficult to obtain a precise impulse response m, and there is a problem that results in deterioration of the overall echocancer.

Accordingly, the present invention has been proposed to solve the conventional problems as described above, and an object of the present invention is to provide an echo of a DMT echocancer that can obtain an echo impulse response in a time domain without having to acquire an echo frame in the central station. It is to provide an impulse response acquisition method.

In order to achieve the above object, the method of obtaining an echo impulse response of the DMT echocancer according to an embodiment of the present invention,

Obtaining a complex one tap with a reverb signal; And a second step of obtaining a time domain echo impulse response by IFFTing the complex one tap obtained in the first step.

1 is a block diagram of a conventional DMT.

2 is a flowchart illustrating a method of obtaining an echo impulse response of a conventional DMT echocancer.

3 is a block diagram of a DMT to which the present invention is applied.

4 is a flowchart illustrating a method of obtaining an echo impulse response of a DMT echocancer according to the present invention.

Explanation of symbols on the main parts of the drawings

11: first IFFT unit 12: D / A conversion unit

13: Hybrid transmission and reception filter 14: A / D converter

15: echo frame synchronization unit 16: FFT unit

17: addition unit 18: ecocancer

19: second IFFT unit

Hereinafter, an embodiment of the present invention configured as described above, according to the technical concept of the method for obtaining an echo impulse response of the DTC echocanser will be described in detail with reference to the accompanying drawings.

3 is a block diagram of a DMT to which the present invention is applied. 4 is a flowchart illustrating a method of obtaining an echo impulse response of a DMT echocancer according to the present invention.

As shown therein, a first step (ST21 to ST25) of obtaining a complex one tap using a reverb signal; And performing a second step ST26 to IFFT the complex one tap obtained in the first step to obtain a time domain echo impulse response.

The first step ST21 to ST25 may include sending some bins of the reverb signal (ST21); Sending a complex one tap of the some bins (ST22); Storing a complex one tab of the some bins and then determining whether the last bin has been reached (ST24); If the last bin is not performed, the method further includes performing a next bin to send some bins of the reverb signal (ST21), and if the last bin is set, performing the second step (ST25).

The operation of the method for acquiring an echo impulse response of the DMT echocancer according to the present invention configured as described above will be described in detail with reference to the accompanying drawings.

First, in the case of the central station DMT-ADSL, 256 bins are used at the transmitter and 32 bins are used at the receiver. Therefore, since downsampling occurs for synchronization between the transmitting side and the receiving side, the receiving end affects eight signals corresponding to the echo components for each bin.

Therefore, the FFT section 16 FFTs the echoes from the transmitting side to the receiving side, and the complex canceler 18 obtains the complex one tap in the frequency domain. IFFT of the second IFFTQ unit 19 to obtain the echo impulse response m is the same as the conventional method.

However, in the present invention, the echo frame is taken differently from the remote frame in order to obtain an accurate set of "A ₁ , A ₂ , A ₃ ,..., A ₈ ". In addition, a random symbol is transmitted from the transmitting end to obtain an accurate set of "A ₁ , A ₂ , A ₃ , ..., A ₈ ", and the correct "A ₁ , A only with the reverb signal as the pattern data is obtained. ₂ , A ₃ , ..., A ₈ "can be obtained. In other words, the correct "A ₁ , A ₂ , A ₃ , ..., A ₈ " is obtained only by the reverb signal without the echo frame synchronization.

To do this, the transmitter sends a reverb signal divided by 32 bins when transmitting a symbol.

M thus obtained does not have a phase delay for each bin that occurs in the process of synchronizing the echo frames, so that the performance of the echocancer 18 is not degraded.

Referring to the operation of the present invention in more detail as follows.

First, aliasing occurs due to down-sampling at the receiving side, and aliasing makes it impossible to obtain a complex one tap in an accurate frequency domain. In addition, even if the second IFFT unit 19 performs the IFFT, the correct m cannot be obtained.

Therefore, conventionally, in order to obtain the correct m, the echo signal frame synchronization is performed separately from the frame synchronization of the remote signal, and then the sender sends a random data signal instead of the reverb signal and sends the correct "A ₁ , A ₂ , A ₃ ,... , A ₈ ″ was obtained, and IFFT was performed in the second IFFT unit 19 to obtain m.

However, the present invention does not use this conventional method and does not capture the frame synchronization of the echo.

Obtain the correct set of "A ₁ , A ₂ , A ₃ , ..., A ₈ " even with the reverberation signal alone, and then IFFT in the second IFFT unit 19 to obtain the echo impulse response m. do.

In Equation 3, at least eight or more to simultaneously transmit "X ₁ , X ₂ , X ₃ , ..., X ₈ " to obtain a set of "A ₁ , A ₂ , A ₃ , ..., A ₈ " Different sets of "X ₁ , X ₂ , X ₃ , ..., X ₈ " are required.

That is, a random data signal must be sent, and the prior art used this signal.

However, if A ₁ is obtained by sending "X ₁ , 0, 0, ..., 0", the A ₁ is stored, the value is saved, and "0, X ₂ , 0, ..., 0" is sent to A ₂ and the value is obtained. Save and repeat this process to save all values up to A ₈ .

And when using the method of combining "A ₁ , A ₂ , A ₃ , ..., A ₈ " obtained in each step, there is no need to send a random data signal.

Of course, the "X ₁ , 0, 0, ..., 0" to "0, 0, 0, ..., X ₈ " transmitted above transmits the reverb signal only to the corresponding bin.

In this way, when the complex one tap of all bins is obtained, “X ₁ , X ₂ , X ₃ ,…, X ₈ ” all become periodic reverb signals, and therefore, frame synchronization must be performed separately at the Rx stage. This eliminates the need for a process and also prevents the performance of the ecocancer.

The overall operation of the present invention is as follows.

Tx of the central station DMT-ADSL uses 0 to 255 bins, and Rx uses 0 to 31 bins. Since the sampling rates are different from each other, the receiver needs downsampling. The effect on bins of 0 ~ 31 is shown in Table 1 below.

Rx bin Tx bin One 1, 63, 64, 127, 128, 191, 192, 254 2 2, 62, 66, 126, 130, 190, 194, 253 3 3, 61, 67, 125, 131, 189, 195, 252 … … 30 30, 34, 94, 98, 158, 162, 222, 226 31 31, 32, 95, 96, 159, 160, 223, 224

However, in Table 1, 0 and 256 bins on the transmitting side do not bit load.

Therefore, there are 256 complex one taps to be obtained, and 1 to 31 taps are obtained by storing only 1 to 31 bins at the transmitting side, and 32 to 63 taps are stored at 32 to 63 taps at the transmitting side. Transmit and store only bins and remember them. For 64 to 95 taps, only 64 to 95 bins are sent and stored on the sending side. Also, 224 ~ 254 taps are obtained by transmitting only 224 ~ 254 bins from the transmitter and remember them.

After all the values obtained are summed, IFFT is used in the second IFFT unit 19 to obtain an accurate echo impulse response m.

As such, the present invention obtains the echo impulse response in the time domain without having to acquire the echo frame synchronization at the central station.

Although the preferred embodiment of the present invention has been described above, the present invention may use various changes, modifications, and equivalents. It is clear that the present invention can be applied in the same manner by appropriately modifying the above embodiments. Accordingly, the above description does not limit the scope of the invention as defined by the limitations of the following claims.

As described above, the method for acquiring the echo impulse response of the DMT echocancer according to the present invention has the effect of obtaining the echo impulse response in the time domain without grasping the echo frame synchronization at the central station.

In addition, the present invention eliminates the need for an echo frame synchronizing block used in the prior art, and only sequentially (1 to 31, 32 to 63, 64 to 95, 96 to 127, 128 to 159 when transmitting reverb signals). , 160 to 191, 192 to 223, and 224 to 254 are transmitted on the transmitting side, thereby obtaining an accurate impulse response.

Claims (2)

Obtaining a complex one tap with a reverb signal; And performing a second step of obtaining a time domain echo impulse response by IFFTing the complex one tap obtained in the first step. The method of claim 1, wherein the first step, Sending some bins of the reverb signal; Sending a complex one tab of the some bins; Storing a complex one tab of the some bins and then determining whether a last bin has occurred; If the last bin is not performed, performing the next bin again to send some of the reverb signals, and if the last bin, further comprising the step of performing the second step echo of the DMT echocancer How to get an impulse response.