xDSL transceiver

An xDSL transceiver comprising a transmission unit for transmitting a DMT-modulated signal through a subscriber line as a transmission path and a receiving unit for receiving the DMT-modulated signal from the subscriber line. The xDSL transceiver further comprises an echo signal suppression unit for suppressing the echo signal from the transmission unit to the receiving unit by matching the phase between the frame of the transmission signal and the frame of the receiving signal.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an xDSL (Digital Subscriber Line) transceiver using a discrete multitone modulation scheme utilizing the existing subscriber line as a high-speed data communication line, or in particular to an xDSL transceiver for realizing the suppression of an echo signal from a transmission unit to a receiving unit.

(2) Description of the Related Art

(i) Description of ADSL Technique

xDSL is known as a technique providing a digital subscriber transmission system utilizing an existing subscriber line as a high-speed communication line. xDSL is a transmission scheme utilizing a telephone line and one of the modulation-demodulation methods. It is roughly classified into two types according to whether the up transmission speed from a remote terminal (hereinafter referred to as a RT side) to a central office (hereinafter referred to as a CO side) is symmetric or asymmetric with the down transmission speed from the CO side to the RT side.

As one of the asymmetric xDSLs, ADSL (Asymmetric DSL) is known. ADSL may be a G.dmt type with the down transmission speed of about 6M bits/sec and a G.lite type with the down transmission speed of about 1.5 M bits/sec, both of which employ the DMT (Discrete Multitone) modulation scheme.

(ii) Description of DMT Modulation Scheme

FIG. 1is a block diagram showing a configuration of a conventional ADSL transceiver. In the drawing, only the transmission unit on the CO side and the receiving unit on the RT side are shown, and the receiving unit on the CO side and the transmission unit on the RT side are not shown. The receiving unit on the CO side has substantially the same configuration as the receiving unit on the RT side shown in the figure, and the transmission unit on the RT side has substantially the same configuration as the transmission unit on the CO side shown in the figure.

The DMT modulation scheme will be explained with reference to the ADSL transceiver shown in FIG.1and taking G.lite as an example. This explanation refers to the modulation-demodulation in down direction from the CO to the RT. Nevertheless, the modulation-demodulation in up direction from the RT to the CO is carried out in a similar manner.

InFIG. 1, the transmission unit on the CO side includes a serial-to-parallel buffer (S-P buffer)10for converting serial transmission data to parallel transmission data, an encoder20, a 256-point inverse fast Fourier transformer (hereinafter referred to as IFFT)30, a cyclic prefix adder40, a parallel-to-serial buffer (P-S buffer)41, a D/A converter50and a transmission bit map60.

The receiving unit on the RT side includes an A/D converter80for converting an analog signal from the subscriber line70to a digital signal, a time domain equalizer (TEQ)90, a cyclic prefix remover100, a serial-to-parallel buffer (S-P buffer)101, a 256-point fast Fourier transformer110, a frequency domain equalizer (FEQ)120, a decoder130and a parallel-to-serial buffer (P-S buffer)140.

Now, the operation will be explained. First, on the CO side, the transmission data is input to the ADSL transceiver and stored in one symbol time (4 kHz) in the serial-to-parallel buffer10. The stored data is segmented into sections each having a number of transmission bits per carrier predetermined by the transmission bit map60and output to the encoder20. In the encoder20, each input bit string is converted into signal points for quadrature amplitude modulation and output to an IFFT30. The IFFT30performs the quadrature amplitude modulation for each signal point by inverse fast Fourier transform and outputs the resulting signal to the parallel-to-serial buffer41. In the process, a cyclic prefix adder40adds 240 to 255 samples of the output of the IFFT30to the head of the DMT symbol as a cyclic prefix. The output of the P-S buffer41is sent to the D/A converter50where it is converted into an analog signal at a sampling frequency of 1.104 MHz and, through a metallic line70, is transmitted to the subscriber side.

In the receiving unit in the ADSL transceiver on the RT side, the analog signal is converted into a digital signal at 1.104 MHz by the A/D converter80, and output to the time domain equalizer (TEQ)90. In the TEQ90, the signal is processed in such a manner that the inter-symbol interference (ISI) may be contained within the 16-sample cyclic prefix, and then stored in an amount corresponding to one DMT symbol in the S-P buffer101. At the same time, the cyclic prefix is removed by the cyclic prefix remover100. The outputs of the S-P buffer101are input to the FFT110. In the FFT110, a fast Fourier transform is performed to generate (demodulate) signal points. After that, the demodulated signal points are applied to the FEQ120where the effect on the amplitude and phase caused by the passage through the metallic line70is compensated for each carrier of a different frequency, and is decoded by the decoder130according to the receiving bit map150holding the same values as those in the transmission bit map60. The decoded data is stored in the P-S buffer140and constitutes the receiving data as a bit string.

(iii) Explanation of the Echo Signal

FIG. 2is a block diagram for explaining the echo signal in the conventional ADSL transceiver. In the drawing, unit #1is one ADSL transceiver and unit #2is the other ADSL transceiver. The transceivers are connected to each other by a subscriber line240. Unit #1includes a transmission unit210having the component elements designated by reference numerals10to60inFIG. 1, a receiving unit230having the component elements designated by reference numerals80to150inFIG. 1, and a hybrid circuit220for sending the transmission signal to the subscriber line240and delivering the receiving signal from the subscriber line240to the receiving unit. Unit #2has the same configuration as unit #1.

The signal output from the transmission unit210of unit #1is output to the subscriber line240through the hybrid circuit220. The hybrid circuit220is so designed that the transmission signal thereof does not echo into the receiving unit230. In an ideal hybrid circuit having a perfect impedance matching with the subscriber line, an echo of the transmission signal to the receiving unit230does not occur. Actually, however, the line characteristic varies from one subscriber line to another depending on the length, diameter, the condition of the bridge tap and the temperature, etc. of the subscriber line240. Therefore, an impedance mismatch is caused in the hybrid circuit220so that the transmission signal from the transmission unit210echoes into the receiving unit230in the same system. This echo component constitutes a noise for the receiving unit230and is a cause of deterioration of the data transmission characteristics.

FIG. 3is a diagram showing a spectrum of the transmission signal and the receiving signal in the conventional ADSL transceiver on the RT side. The spectrum of the transmission signal and the receiving signal of the ADSL transceiver on the CO side is similar to that of FIG.3and is not shown.

Conventionally, an effort has been made to remove the echo making up an echo signal from the transmission unit to the receiving unit using a sub-band filter. This conventional method will be explained with reference to FIG.3.

InFIG. 3, a thin solid line indicates the receiving signal (down signal) from the CO side, and a two-dot chain indicates the transmission signal (up signal) from the RT side to the CO side. The receiving signal has a main component (in-band component) on the comparatively high-frequency band side and an out-of-band component on the comparatively low-frequency band side. The transmission signal, on the other hand, has a main component on the comparatively low-frequency band side, and an out-of-band component on the comparatively high-frequency band side. The receiving signal (down signal) is attenuated while passing through the subscriber line, and therefore in the drawing, is lower in gain than the transmission signal. As shown inFIG. 3, conventionally, in order to secure a sufficient S/N of the receiving signal against the out-of-band component of the transmission signal, a transmission low-pass filter (transmission LPF) for suppressing the out-of-band component of the receiving signal and passing only the main component of the transmission signal is arranged on the output side of the transmission unit210, while a receiving high-pass filter (HPF) for removing the main component of the transmission signal and passing only the main component of the receiving signal is arranged on the input side of the receiving unit230, thereby suppressing the echo signal from the transmission unit to the receiving unit.

For the S/N ratio of the receiving signal to be sufficiently large against the out-of-band component of the transmission signal, however, a transmission LPF and a receiving HPF having a characteristic with a sharp rise and a sharp fall are required, resulting in a large number of stages in the respective filters.

As explained above, in the conventional method using the transmission LPF and the receiving HPF, the order of the filter for suppressing the echo component must be increased and, therefore, the hardware including a filter, if any, is also increased in size. In the case where the filter is digitally configured with a digital signal processor (DSP) or the like, on the other hand, the processing amount is so increased as to require a DSP high in processing capacity. In either case, therefore, the system cost is increased. It is for this reason that inexpensive means for suppressing the echo from the transmission unit to the receiving unit is desired.

SUMMARY OF THE INVENTION

In order to solve the problem described above, the object of the present invention is to provide an xDSL transceiver capable of suppressing the echo signal by adjusting the frame boundaries of the transmission signal and the receiving signal.

To attain the above object, according to a first aspect of the invention, there is provided an xDSL transceiver comprising an echo signal suppression unit for suppressing the echo signal from the transmission unit to the receiving unit by matching the phase of the frame of the transmission signal and that of the receiving signal with each other.

According to a second aspect of the invention, there is provided an xDSL transceiver of the first aspect, in which the echo signal suppression unit is included in the transmission unit and has a phase compensator for making the frame boundaries of the carriers of the echo signals from the transmission unit to the receiving unit to be the same for all the frequencies.

According to a third aspect of the invention, there is provided an xDSL transceiver of the second aspect, in which the receiving unit includes phase compensation degree determining unit for determining the degree of phase compensation of the phase compensator in the transmission unit by analyzing the receiving signal.

According to a fourth aspect of the invention, there is provided an xDSL transceiver of the first aspect, in which the echo signal suppression unit is included in the receiving unit and has a phase compensator for making the same frame boundary of the carrier of the echo signal from the transmission unit to the receiving unit for all the frequencies.

According to a fifth aspect of the invention, there is provided an xDSL transceiver of the fourth aspect, in which the phase compensator is configured with a cascade connection of a delay equalizer for correcting the group delay distortion of the echo signal from the transmission unit and a time domain equalizer for reducing the inter-block interference in the output of the delay equalizer.

According to a sixth aspect of the invention, there is provided an xDSL transceiver of the fourth aspect, in which the phase compensator is configured with a single equalizer of an FIR Filter type having the same function as a cascade connection of a delay equalizer for correcting the group delay distortion of the echo signal from the transmission unit and a time domain equalizer for reducing the inter-block interference in the output of the delay equalizer.

According to a seventh aspect of the invention, there is provided an xDSL transceiver of the first aspect, in which the transmission unit includes a portion of the echo signal suppression unit and the receiving unit includes the other portion of the echo signal suppression unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will be explained in detail below with reference to the drawings. In all the drawings, the same reference numerals designate the same or similar component parts, respectively.

(i) Phase Matching of Receiving Frames

FIG. 4is a diagram for explaining a discontinuous point in the transmission signal at the time of demodulating the receiving signal using the conventional ADSL transceiver. As shown inFIG. 4, for the demodulation, an FFT is performed in the frame boundaries of the receiving signal to extract signal points of each carrier. Thus, the spectrum of the receiving signal component contains substantially no out-of-band component. However, in view of the fact that the frame boundary of the transmission frame which has been reflected as an echo is not always coincident with the FFT section for demodulation of the receiving signal, a discontinuous point is often contained in the FFT section. As a result, a transmission out-of-band component is generated and constitutes noise in the received signal.

FIG. 5is a graph showing the state in which the transmission out-of-band component of the transmission spectrum enters the band of the receiving spectrum to constitute noise in the received signal as described above.

(ii) Phase Control of Transmission Frames

FIG. 6is a block diagram showing a portion of the configuration of the ADSL transceiver according to a first embodiment of the invention. As shown inFIG. 6, the ADSL transceiver according to this embodiment comprises the same component parts as the ADSL transceiver shown inFIG. 1, i.e. the receiving unit including the A/D converter80, the S-P buffer100and the FFT110, and the transmission unit including the IFFT30, the cyclic prefix adder40, the P-S buffer41and the D/A converter50. In addition, the ADSL transceiver according to this embodiment comprises, between the receiving unit and the transmission unit, an echo signal suppression unit650configured with a receiving frame phase detector610, a FFT boundary phase controller620, an echo frame phase detector630and a transmission frame phase controller640.

Now, the operation will be explained. A fixed pattern signal for training is generated between the ADSL transceivers by the IFFT30at the time of initial training. The data is passed through the cyclic prefix adder40without doing anything at the time of initial training, enters the P-S buffer41and is converted into an analog signal at the sampling rate by the D/A converter (DAC)50. This transmission signal is used for training the equalizer in the receiving unit of the ADSL transceiver on the opposite party. At the same time, the receiving unit of the ADSL transceiver of the own party is rendered to perform the following operation thereby to match the frame phases for transmission and receiving.

Specifically, the training pattern transmitted leaks into the receiving side as an echo through the hybrid circuit220(FIG.2). This echo is converted into a digital signal by the A/D converter80and held in the buffer100. The echo, when reaching an amount corresponding to one frame, is subjected to fast Fourier transformation (hereinafter referred to as FFT) by the FFT110. An echo frame phase detector630compares the FFT result signal with the transmitted pattern, and calculates the transmission delay from the phase difference between the carriers.

Next, the ADSL transceiver at the opposite party transmits a training signal, which is output from the FFT110through the A/D converter (ADC)80and the S-P buffer100. Also, in view of the fact that this training signal is continuously transmitted in a predetermined transmission pattern, the difference between the frame boundary of the receiving signal and the present FFT section can be detected from the output of the FFT110by the receiving frame phase detector610. In order to assure coincidence between the frame boundary of the receiving signal and the FFT section, the FFT boundary phase controller620changes the timing of input to the FFT110from the S-P buffer100by the resolution of one sample from the A/D converter80. In the transmission frame phase controller640, the amount of change of the transmission timing for matching the phase between the transmission frame and the receiving frame is determined from the value determined previously by the echo frame phase detector630and the amount of change of the FFT timing on receiving side, and the timing of sending out the transmission frame is changed by controlling the P-S buffer41.

These steps of the process makes it possible to match the phases of the transmission frame and the receiving frame.

FIG. 7is a flowchart for explaining the operation of the apparatus shown inFIG. 6in more detail.

InFIG. 7, the first step S1is for a transmission pattern generating unit600to generate a transmission pattern X(n) for training, followed by step S2for generating and transmitting the transmission signal x(t) by modulating the transmission pattern X(n) in the IFFT30. This transmission signal x(t) leaks to the receiving unit from the hybrid circuit220(FIG. 2) and is converted into a digital signal by the A/D converter80as x′ (t). This digital signal x′ (t) is subjected to fast Fourier transformation by the FFT110thereby to determine X′ (n) in step S3. Then, in step S4, the product of X′ (n) and X*(n) which is a complex conjugate of the training transmission pattern X(n) described above is subjected to the inverse fast Fourier transform by the IFFT30thereby to calculate the phase (A) of the echo frame.

Then, the signal y(t) obtained by modulating the training pattern Y(n) in the IFFT at the opposite party is transmitted. This signal is transmitted to the subscriber line and enters the receiving unit of the ADSL transceiver of the own party as a receiving signal y′ (t). In step S5, the receiving unit determines Y′ (n) by fast Fourier transformation of the signal y′ (t) by the FFT110. Then, in step S6, the product of the complex conjugate Y′ (n) of the known Y(n) and Y′ (n) is subjected to inverse fast Fourier transformation in the IFFT30thereby to calculate the phase (B) of the receiving frame. Then, the output phase of the S-P buffer100of the receiving unit is shifted by B samples in order to match the FFT boundary of the receiving frame in step S7. As the last step, the output phase of the P-S buffer41of the transmission unit is sample shifted by (B-A) samples in step S8, thereby completing the phase matching of the echo frame.

FIG. 8is a diagram showing the state in which the phase is matched between the transmission frame and the receiving frame. As shown inFIG. 8, no discontinuous point is contained in the FFT section, and therefore substantially no out-of-band component of the transmission signal is generated in the band of the receiving signal.

FIG. 9is a graph showing the effect of matching the phases between the transmission frame and the receiving frame as described above. As seen from the drawing, the configuration shown inFIG. 6makes it possible to retrieve only the receiving signal by almost completely separating the echo signal, from the transmission unit to the receiving unit, in the demodulation by the FFT.

(iii) Adjustment of Group Delay Distortion of Echo

FIGS. 10A and 10Bare waveform diagrams for explaining the group delay distortion removed by the ADSL transceiver according to a second embodiment of the invention. The echo path usually has a group delay distortion, and therefore arrives at the receiving unit at a different time depending on the DMT carrier frequency. As shown inFIG. 10A, the group delay characteristic of the echo path is the one in which the larger delay is the lower the frequency is. Therefore, according to this embodiment, a transmission signal having a lower frequency (a larger delay) is transmitted before others in such a manner as to secure the coincidence of the delay of each frequency component of the receiving signal resulting from the echo of the transmission signal. Specifically, the transmission signal is transmitted with the inverse characteristic of the group delay distortion. In this way, as shown inFIG. 10B, the frame boundaries of the DMT carriers come to coincide in the receiving unit.

(iv) Application of Group Delay Corrector to Transmission Unit

FIG. 11is a block diagram showing a portion of the configuration of the ADSL transceiver according to a third embodiment of the invention. In this embodiment, group delay distortion correction means according to the foregoing embodiment is built in the transmission unit. Specifically, an IIR filter110is interposed between the cyclic prefix adder40and the D/A converter (DAC)50in the transmission unit. Also, the receiving unit includes an energy detector112for estimating the attenuation characteristic of the subscriber line, a coefficient selector113for selecting a coefficient of the IIR filter110and a filter coefficient table114having N filter coefficients of the group delay IIR filter110in accordance with the characteristic of the subscriber line.

Now, the operation will be explained. First, the known initialization signal transmitted from the opposite party is converted into a digital signal by the A/D converter80(FIG.1), and then applied to the energy detector112. The energy detector112calculates the root mean square of each sample value digitized as a mean energy. This root square mean is compared with the root square mean of the known initialization signal, and the frequency attenuation characteristic of the subscriber line is estimated from the attenuation amount. The coefficient selector113selects the optimum filter coefficient corresponding to the attenuation characteristic of the subscriber line estimated by the energy detector112from the filter coefficient table114, and sets the selected filter coefficient in the transmission IIR filter110of the transmission unit. This alleviates the adverse effect that the group delay distortion of the echo from the transmission unit has on the receiving signal.

FIG. 12is a graph showing the frequency characteristic against the group delay time of the echo described above, andFIG. 13is a graph showing the frequency characteristic of the IIR filter coefficient having a characteristic inverse to the group delay characteristic of the echo shown in FIG.12. The characteristic of the IIR filter110for the group delay correction coefficient of the transmission unit described above will be explained with reference toFIGS. 12 and 13. As shown inFIG. 12, the group delay characteristic of the echo is varied depending on the characteristics such as the distance of the subscriber line. In the IIR filter110, in order to correct the group delay distortion, the filter coefficients having a characteristic inverse to the group delay characteristic for each main type of the subscriber line are set in a table in advance, and the optimum filter coefficient is selected from the attenuation amount of the subscriber line determined by the energy detector112shown in FIG.11and set in the IIR filter110of the transmission unit.

(v) Application of Group Delay Corrector to Receiving Unit

FIG. 14is a block diagram showing a portion of the configuration of the ADSL transceiver according to a fourth embodiment of the invention. According to this embodiment, the group delay distortion correction means according to the second embodiment described above is built in the receiving unit. Specifically, the group delay distortion correction unit140in the receiving unit is configured with a cascade connection between a delay equalizer (delay EQL)141for correcting the echo path group delay distortion and a time domain equalizer (TEQ)142for alleviating the inter-block interference.

Now, a method of determining the characteristic of the delay equalizer141, i.e. a method of determining the coefficient of the delay equalizer141will be explained.

The frequency domain signal obtained by FFT of the echo signal of the transmission signal is multiplied by the complex conjugate of the transmitted signal, i.e. the complex conjugate of the IFFT input signal on the transmission side thereby to calculate the delay of each frequency component. Then, the inverse characteristic of the calculation result, i.e. the complex conjugate of the calculation result is used as a coefficient in the frequency domain of the delay equalizer141. As a result, the delay equalizer141comes to assume a characteristic for correcting the group delay of the echo signal from the transmission signal. In the last step, a time domain coefficient is determined by IFFT of this signal and used as an actual coefficient of the delay equalizer141.

Now, the operation will be explained. First, the known initialization signal transmitted from the opposite party is converted into a digital signal by the A/D converter80(FIG. 1) through the hybrid circuit220, and then applied to the delay equalizer141. The delay equalizer141develops a delay of a characteristic inverse to the line characteristic, and therefore the adverse effect that the group delay distortion in the echo path from the transmission unit has on the receiving signal is alleviated.

According to the fifth embodiment of the invention, the TEQ142and the delay equalizer141can be integrated into a single equalizer143of FIR type by making the delay equalizer141of a FIR-type filter, thereby making it possible to reduce the processing capacity. The technique for integrating the TEQ142and the delay equalizer141into a single unit can be implemented by use of either a method in which the coefficients are integrated into a single coefficient in time domain or a method in which the coefficients are integrated into a single coefficient in frequency domain. Each method will be explained in (vi) and (vii) below.

(vi) Method of Integrating Coefficients Into One in Time Domain

FIG. 15is a diagram showing a method of integrating the filter coefficients into a single coefficient in time domain. As shown inFIG. 15, a new coefficient can be obtained as a single coefficient of the FIR filter by cyclic convolution of the coefficient a optimized for the receiving signal of the TEQ142configured with a FIR-type filter and the coefficient b of the delay equalizer141optimized for the group delay of the transmission echo.

(vii) Method of Integrating Coefficients Into One in Frequency Domain

FIG. 16is a diagram showing a method of integrating the filter coefficients into a single coefficient in-frequency domain. As shown inFIG. 16, the frequency characteristic optimized for the receiving signal of the TEQ142is assumed to be T(ω), and the frequency characteristic optimized for the transmission echo of the delay equalizer141for group delay of the transmission echo, i.e. the result of FFT of the equalizer coefficient is assumed to be E(ω). Assuming that these two coefficients are integrated into a single F(ω), the relation holds that F(ω)=T(ω)×E(ω). Then, the filter coefficient f(t) can be obtained by IFFT of F(ω).

(viii) Hybrid Type

FIG. 17is a block diagram showing a configuration of a portion of the ADSL transceiver according to a sixth embodiment of the invention. This embodiment employs a hybrid type in which the group delay corrector described above is provided in both the transmission unit and the receiving unit.

As shown inFIG. 17, the transmission unit includes an IIR filter110like the ADSL transceiver shown inFIG. 11, and the receiving unit includes an energy detector112, a coefficient selector113and a filter coefficient table114. At the same time, like the ADSL transceiver shown inFIG. 14, the delay equalizer141and the TEQ142are connected in cascade in the receiving unit.

In operation, the attenuation amount of the receiving signal is determined by the energy detector112, the line characteristic is estimated from the attenuation amount, and the coefficient selector113selects the filter coefficient most suitable for the estimated line characteristic from the filter coefficient table114and sets it in the transmission IIR filter110.

Also, by employing the hybrid type for correcting the group delay distortion in the transmission and receiving units using the delay equalizer141for correcting the echo path group delay distortion in the receiving unit, it becomes possible to reduce the size of the IIR filter110in the transmission unit and the size of the delay equalizer141in the receiving unit.

In the embodiments mentioned above, the ADSL transceiver was referred to for explanation. Nevertheless, it is apparent to those skilled in the art that the present invention is applicable also to the xDSL transceivers other than ADSL transceiver.

As described above, this invention has the following effects.

By matching the phase of the transmission frame with that of the receiving frame, the effect of the noise component, which is influenced by the echo from the transmission unit to the receiving unit, in the receiving signal, can be suppressed. Also, the quality of echo removal can be improved by correcting the group delay distortion of the echo path. Also, the amount of data to be processed can be reduced by combining with the TEQ filter.