KR970009680B1 - Apparatus for recovering clock and method thereof in discrete multitone system - Google Patents

Abstract

An apparatus for restoring a clock signal of a discrete multi-tone system and a method for operating the apparatus are disclosed. The apparatus comprises a signal converter(200) for converting an inputted analog signal to a digital signal; a first filter(202) for reducing the length of a channel so as to reduce the signal distortion; a temporary storing unit(204) for receiving the output signal from the first filter to store it so as to provide it in parallel; a demodulation unit(206) for demodulating the output signal from the temporary storing unit(204); a phase compensating unit(208) for receiving the demodulated digital signal, compensating the phase distortion generated at the channel, and generating sub-channel tones having a low frequency band; a phase offset unit(210); a frequency offset unit(212); a second filter(214) and a frequency controller(216).

Description

Clock recovery device and its operation method in discontinuous multitone (DMT: DISCRETE MULTITONE) system

1 is a block diagram illustrating a clock restoration apparatus in a discontinuous multitone system according to the present invention.

2 is a flowchart for explaining a clock recovery method in a discrete multitone system according to the present invention.

3 is a flowchart for explaining in detail the flowchart shown in FIG.

4 is a detailed configuration diagram of the second filter unit illustrated in FIG. 1.

FIG. 5 is a graph showing the degree of convergence of the frequency offset according to α and β when α of the second filter unit shown in FIG. 4 is fixed and β is increased.

FIG. 6 is a graph showing the degree of convergence of the frequency offset according to α and β when α of the second filter part shown in FIG. 4 becomes large and β is fixed.

The present invention relates to a Discrete MultiTone (DMT) system, and more particularly, to an apparatus for recovering a clock of a transmitter from a receiver and a method of operating the apparatus in a DMT system.

In general, a DMT system is a transmission method for maximizing channel efficiency using multiple carriers. That is, in the transmission signal using the multicarrier, the center frequency of each subchannel is fi i = 1,... N (N = 256) and consists of the sum of independent carrier signals with the same bandwidth (4.3125 MHz). In particular, if the characteristics of the transmission path are not good, the high-speed data transmission over the telephone line enables the DMT system to transmit data of 6Mbps or higher, thereby providing quality service. However, there is a problem in that the clock synchronization between the transmitter and the receiver does not match when configuring such a system.

The clocks on the transmitter side and the receiver side are shifted while the transmission signal is transmitted through the transmission path. Therefore, there is a problem in that they must be synchronized.

Accordingly, an object of the present invention is to provide a clock restoring apparatus in a DMT for synchronizing the clock of a transmitter and a clock of a receiver in a DMT system.

Another object of the present invention is to provide a clock recovery method in a DMT for synchronizing the clock of a transmitter and a clock of a receiver in a DMT system.

In order to achieve the above object, the clock restoring apparatus in the DMT system of the present invention is configured to receive a digital signal by receiving an arbitrary analog signal received at the receiving end of a discrete multi-tone (DMT) device. A signal conversion means for converting the signal into a first signal; first filter means for reducing the length of a channel to reduce distortion of the signal so as to eliminate interference between units receiving the output of the signal conversion means and performing fast Fourier transform; Temporary storage means for storing the signal for receiving the digitized signal output in series from the first filter means in parallel and outputting in parallel, and the output of the temporary storage means to the modulation of the transmitting end of the DMT device Demodulation means for performing a corresponding demodulation and the demodulated digital signal to compensate for phase distortion occurring in the channel; One of the sub channel tones in the first mode, which is an initialization process of the DMT device, to synchronize the phase compensation means for generating sub channel tones of the low frequency band and the synchronization of the clock of the transmitter and the receiver. A frequency offset that removes the frequency offset of the signal by using a first tone and tracks and removes the frequency offset from a second mode, which is a normal operation of the apparatus, to a second tone, which is another one of the subchannel tones. Means, and in order to match the synchronization of the transmitting and receiving clocks, in the first and second modes, the second tone is input to calculate a phase error so that the phase compensating means compensates the phase of each subchannel. A phase for supplying the calculated error value to the phase compensating means and tracking and removing the phase offset of the signal while updating the coefficient of the phase compensating means. Controlled by the offset means and the frequency offset portion, and the internal natural frequency should be adjusted to match the sampling clock of the signal conversion means of the transmitter, and the sampling rate of the signal conversion means is adjusted to adjust the frequency offset. And second filter means for effectively tracking the frequency offset, making the frequency control means insensitive to noise, and supplying output to the frequency control means.

In order to achieve the above object, the clock recovery method of the DMT system of the present invention is a signal conversion method for receiving an analog signal and converting it to a digital signal in order to synchronize the clocks of the transmitter and the receiver of the DMT device. And a first initialization preparation step of reducing distortion of the signal at an initial stage of a first mode which is an initialization process of the device, and demodulating the signal at an initial stage of the first mode after the first initialization preparation step, and A second initialization preparatory step of compensating for a phase offset of the second channel; and a sub-channel generated after the second initialization preparatory step to remove a frequency offset of the signal in the first mode and the second mode during normal operation of the apparatus. Tone angular velocity calculation step of calculating the angular velocity of one of the frequency tones, and the amount of change in the angular velocity is greater than or equal to a threshold value; And a frequency adjusting step of controlling the signal conversion step so that the frequency offset can be adjusted if the determination step is not satisfied. A data alignment step of aligning the data of the signal to recognize the start and end of the signal, a phase offset step of removing the phase offset of the signal after the data alignment step, and the second mode after the phase offset step And tracking the frequency and phase offset of the signal by the frequency offset tracking result to the determination step, and the phase offset tracking result to the phase offset step to include an offset tracking step to remove the frequency and phase offset. It is characterized by.

FIG. 1 is a block diagram illustrating a clock restoration apparatus for synchronizing clock synchronization between a transmitting and receiving end in a DMT system according to the present invention. The signal converting unit 200, the first filter unit 202, and the temporary storage unit (FIG. 204, a demodulator 206, a phase compensator 208, an error offset unit 210, a frequency offset unit 212, a second filter unit 214, and a frequency control unit 216.

2 is an overall flowchart for explaining a clock recovery method in a DMT system according to the present invention.

3 is a flowchart for explaining in detail the flowchart shown in FIG.

The operation and clock recovery method of the clock recovery apparatus in the DMT system will be described in detail with reference to the configuration of FIG. 1 and FIGS. 2 and 3 as described above.

The analog signal received through the input terminal IN is converted into a digital signal by the signal converter 200 shown in FIG. 1 (steps 400 and 600), wherein the sampling rate of the signal converter 200 is a frequency controller. 216, and the frequency of the frequency adjusting section 216 should be adjusted to match the sampling rate of the signal converting section (typically a D / A converter) of the transmitting end of the DMT system. The first filter unit 202, which is a finite impulse response (FIR) filter and is a time domain equalizer (TEQ), receives a digitized signal and adjusts the memory length of the channel to reduce distortion of the signal. In this case, the interference between the DMT symbol blocks, which is the execution unit of the fast Fourier transform, is eliminated (step 602). The signal passing through the first filter unit 202 is stored in the signal storage unit 204 and then output in parallel to the demodulation unit 206, corresponding to the modulation using an inverse fast Fourier transform on the transmitting side of the DMT system. The demodulator 206 performs demodulation, and the phase compensator 208, which is implemented as a frequency-domain equalizer (FEQ), is adapted to compensate for the phase distortion generated in the channel by inputting the demodulated signal. As a filter, it compensates for the phase of each subchannel from the phase error value received from the phase offset unit 210 (step 604). In addition, the phase compensator 208 generates subchannel tones in a plurality of low frequency bands.

Among the plurality of sub-channel tones generated in the phase compensator 208, the 32nd tone (1/16 times the sampling frequency) is referred to as the first tone 218 and the 64th tone as the second tone 220. At this time, if there is a frequency offset between the transmit frequency and the receive frequency, the constellation point of the first tone measured after the demodulation of the signal will rotate.

Actual simulation results show that when the sampling rate is 2,208 MHz, the constellation point of the first tone 218 rotates about 11.5 ° from every block for the 2,208 MHz (0.1%) frequency offset [about 2 second tones]. 25 °]. Therefore, the frequency offset unit 214 removes the frequency offset by adjusting the frequency of the frequency control unit 216 in consideration of the rotation amount and direction. The frequency offset unit 212 shown in FIG. 1 receives the first and second tones and calculates the angular velocity of the second tone in the first mode (step 606). After step 606, the frequency offset unit 212 temporarily stores the change amount of the angular velocity in the buffer and determines whether the change amount is equal to or less than the threshold value T (step 608). If the frequency offset is not satisfied in step 608, the frequency offset unit 212 controls the frequency of the frequency control unit 216 shown in FIG. 1 so that the frequency offset unit 212 may adjust the frequency of the signal converter 200. The sampling rate is adjusted (step 610).

Steps 602 to 610 described above refer to a frequency offset step in which the frequency offset is removed and adjusted (step 402 shown in FIG. 2).

After satisfying step 608, if the result of the correlation is measured while delaying by the sample unit using the correlation in the phase compensator 208, the peak of the correlation result value is detected up to 512 to align the DMT block. (Steps 404 and 612). After the step 612, the phase offset unit 210 shown in FIG. 1 calculates an error from a second tone by a direct decision method to determine a tap coefficient of the phase compensator 208. The phase offset is removed while updating (steps 406 and 614).

The phase offset may remove the phase offset using the error offset unit 210 using a direct determination direction by comparing the value received in the second tone with a promised constellation value. If the phase error calculated in the second tone is θe, the phase error is 2 × θd in the 128th third tone among the subchannel tones of θd / 2 in the first tone subchannel with frequency 1/2 and the subchannel tones with twice the frequency. The phase error values generated for each subchannel are θi = θe × i ÷ 64, i = 1-N (N is the total number of subchannels), and θk = -θe × K ÷ 64 which compensates this value. , Multiplying each subchannel by K = 1-N, compensates for the phase error. At this time, the part of multiplying the sub-channel error by each subchannel is the phase compensator 208, and the phase offset and the frequency offset can be continuously tracked in the second mode.

After the step 614, in order to remove the frequency and phase offsets in the second mode, the frequency offset unit 212 shown in FIG. 1 tracks the frequency offset from the second tone, and the phase offset unit 210 adjusts the phase offset. Track (step 408).

In the second mode, the frequency offset unit 212 controls the frequency controller 216 by tracking the second tone to adjust the sampling rate of the signal converter 200 to remove the frequency offset.

Here, the frequency of the frequency controller 216 should be matched with the sampling clock of the signal converter of the transmitter. This is because the frequency offset is so large that the criterion of the rotation direction becomes ambiguous when the rotation range of the first tone exceeds ± 180 °. In case of adjusting the frequency of initial receiver, it should be adjusted within ± 1% of sampling frequency. In this process, when the second tone is used instead of the first tone, the response to the frequency offset is twice as large. Therefore, the first tone is used because the initial adjustment of the frequency controller 216 needs to be made more accurate.

On the other hand, the second filter unit 212 is installed so that the frequency control unit 216 does not react sensitively to noise in order to effectively perform the frequency offset, and at this time, it is necessary to select α and β as appropriate values as shown in FIG. .

5 and 6 show the convergence degree of the frequency offset according to α and β of the second filter unit 214 shown in FIG. 4 when the initial frequency offset is 1% (22.04 MHz) for the 2.208 MHz sampling rate. The graph shown.

As shown in the graphs of FIGS. 5 and 6, the larger the beta 802 when the α 800 is fixed, the larger the swing width converges, and the faster the α becomes when the β 802 is fixed. Convergence, but after converging, the wave becomes severe. Therefore, the appropriate α (800) and β (802) should be selected according to the convergence speed and the degree of wave. In the clock recovery initialization of the first mode DMT system, the frequency offset can be removed using the first tone, and in the second mode in the normal state, only the second tone is transmitted so that the standard transmits an open signal. Use two tones to stay in sync. That is, the clock synchronization in the DMT system can be stably maintained by using the first tone in the initialization and the second tone in the steady state.

Claims (4)

At a receiving end of a discrete multitone (DMT) device, signal converting means for receiving an analog signal received at the receiving end and converting the received analog signal into a digital signal; First filter means for reducing the length of the channel to reduce distortion of the signal to receive the output of the signal converting means and to eliminate interference between units performing fast Fourier transform; Temporary storage means for receiving the digitized signals output in series from the first filter means and storing the signals for outputting in parallel; Demodulation means for receiving an output of the temporary storage means and performing demodulation corresponding to modulation of a transmitting end of the DMT device; Phase compensation means for receiving the demodulated digital signal to compensate for phase distortion generated in the channel and generating sub-channel tones in a low frequency band; In order to synchronize the clock of the transmitter and the synchronization of the receiver, the frequency offset of the signal is removed using a first tone, which is one of the subchannel tones, in a first mode which is an initialization process of the DMT device. Frequency offset means for tracking and eliminating the frequency offset to a second tone, the other of said subchannel tones, in a second mode during normal operation of the apparatus; In order to match the synchronization of the transmitter and the receiver clocks, in the first and second modes, the second tone is received and the phase error is calculated so that the phase compensating means compensates the phase of each subchannel. Phase offset means for supplying an error value to the phase compensating means and tracking and removing the phase offset of the signal while updating the coefficient of the phase compensating means; A frequency control means controlled by the frequency offset unit, the internal frequency of which is to be adjusted to match the sampling clock of the signal conversion means of the transmitter, and a sampling rate of the signal conversion means for adjusting the frequency offset; ; And a second filter means for effectively tracking the frequency offset, making the frequency control means insensitive to noise, and supplying an output to the frequency control means. The clock restoring apparatus of claim 1, wherein the demodulation means and the phase compensating means align the data of the signal to recognize the start and end of the signal transmitted from the DMT device. . A signal conversion step of receiving and converting an analog signal into a digital signal in order to synchronize the transmitting and receiving clocks of the DMT device at a receiving end of the DMT device; A first initialization preparation step of reducing distortion of the signal at an initial stage of the first mode, which is an initialization process of the device, and demodulating the signal at an initial stage of the first mode after the first initialization preparation stage, and phase offset of the signal A second initialization preparatory step of compensating for the sub-channel frequency tones generated after the second initialization preparatory step to remove the frequency offset of the signal in the first mode and the second mode which is a normal operation of the apparatus. A tone angular velocity calculation step of calculating an angular velocity of one of the first tones, a determination step of determining whether the change in the angular velocity is greater than or equal to a threshold value, and a frequency offset if the determination step is not satisfied. A frequency offset step of controlling the signal conversion step; And a data alignment step of aligning data of the signal to recognize the start and end of the signal when the determination step is satisfied. A phase offset step of removing the phase offset of the signal after the data alignment step; After the phase offset step, the frequency and phase offset of the signal are tracked in the second mode so that the frequency offset tracking result proceeds to the determination step, and the phase offset tracking result proceeds to the phase offset step to adjust the frequency and phase offset. And an offset tracking step for removing the clock. 4. The method of claim 3, wherein the initializing step includes removing the frequency and phase offsets for clock recovery of the DMT device, and including the frequency offset step, the data alignment step, and the offset step to align the data of the signal. Clock recovery method in the DMT device characterized in that.