BACKGROUND OF THE INVENTION (a) Field of the Invention The present invention relates to a method for determining timing advance in a zipper DMT (discrete multi-tone) system.

(b) Description of the Related Art Demands of multimedia service supply through networks has proliferated, and xDSL (digital subscriber line) methods for providing data transmission rates of from several hundred kbps to several tens of Mbps using conventional copper cables without amplifiers or repeaters have been developed so as to satisfy these demands.

The xDSL methods have evolved into the HDSL (high-data-rate DSL), the SDSL (single-line HDSL), the ADSL (asymmetric DSL), the UADSL (universal ADSL), and the VDSL (very-high-bit-rate DSL) which has been developed to send data at a high data rate in a short distance of from 300 to 1,500m.

Modulation and demodulation methods used for the xDSL include the CAP (carrierless AM/PM) method and the QAM (quadrature amplitude modulation) method both for the SCM (single carrier modulation) method, and the DMT (discrete multi-tone) method for the MCM (multi-carrier modulation) method.

The DMT method divides a total transmission band into a plurality of narrowband subchannels, and transmits them, to thereby increase the transmission period of each subchannel by the number of subchannels, and to compensate for channel distortion through a simple single tap equalizer. Also, by adding a cyclic prefix as a guard interval to a DMT symbol, the method maintains orthogonality between the subchannels and removes inter-symbol interference, thereby providing a simple equalizer configuration at a receiver part. Further, the method can realize high-speed modulation and demodulation processes using the IFFT (inverse fast Fourier transform) and the FFT (fast Fourier transform).

The biggest performance-lowering factors of the DMT system include near end crosstalk signals and echo signals. The near end crosstalk signals are generated when two different transmitters concurrently send data, that is, when they concurrently transmit the data of the same frequency band in the identical binder group. The echo signals are noise that affects signals received when the transmitter at the same part sends data.

The zipper DMT method has been developed to prevent the near end crosstalk signals and the echo signals, and a cyclic suffix is added to the end of

the signal in addition to the cyclic prefix, and then it is used. To minimize the interference of the near end crosstalk signals and the echo signals, the orthogonality between each frequency is to be maintained. Therefore, to guarantee the orthogonality between frequencies, the zipper DMT method adds a cyclic suffix to the source signal so that only one symbol of the near end crosstalk signals and the echo signals may be included in the symbol interval of the received signal of each network's end.

Further, with reference to a U2 interface in the zipper DMT method, the U2 interface of a remote site Tx/Rx (transmitter and receiver) unit transmits signals prior to the start point of the symbol interval of the received signal by the degree of timing advance so that an ONU (optical network unit) Tx/Rx unit and the remote site Tx/Rx unit may concurrently transmit the signals.

Conventionally, the sample number of the cyclic suffixes is set to be greater than the sample number of the timing advances to remove the influences of the near end crosstalk and echo.

In the prior art, the timing advance is determined from a channel's impulse response or transfer function. Accordingly, an estimation process for the channel's impulse response or transfer function is needed so as to determine the timing advance in the like manner of the above-noted description.

SUMMARY OF THE INVENTION It is an advantage of the present invention to concurrently transmit and

receive signals by the ONU Tx/Rx unit and the remote site Tx/Rx unit with reference to the U2 interface.

It is another advantage of the present invention to remove the near end crosstalk signals and echo signals using a cyclic suffix with a minimum length.

It is still another advantage of the present invention to measure a group delay of a channel and determine the timing advance.

The present invention achieves the above-noted advantages by measuring the group delay and determining the timing advance.

In one aspect of the present invention, A zipper DMT system comprises: an ONU Tx/Rx unit including: a first Tx buffer for temporarily storing a sample to be transmitted; a first Tx sample counter for showing a first Tx sample counter value for indicating a position, within a symbol, of an output sample of the first Tx buffer; a first Rx buffer for temporarily storing a received sample ; and a first Rx sample counter for showing a first Rx sample counter value for indicating a position, within a symbol, of an input sample of the first Rx buffer, the ONU Tx/Rx unit for extracting a sample from the first Tx buffer according to the first Tx sample counter value, transmitting the sample, extracting a sample from the first Rx buffer according to the first Rx sample counter value, and receiving the sample ; and a remote site Tx/Rx unit including : a second Tx buffer for temporarily storing a sample to be transmitted; a second Tx sample counter for showing a second Tx sample counter value for indicating a position, within a symbol, of an output sample of the second Tx buffer; a second Rx buffer for temporarily storing a received sample; and a second Rx

sample counter for showing a second Rx sample counter value for indicating a position, within a symbol, of an input sample of the second Rx buffer, the remote site Tx/Rx unit for extracting a sample from the second Tx buffer according to the second Tx sample counter value, extracting a sample from the second Rx buffer according to the second Rx sample counter value, and receiving the sample, wherein the remote site Tx/Rx unit modifies the second Tx sample counter value, and transmits a signal in advance of time when a sample interval of the received signal ends in a U2 interface of the remote site Tx/Rx unit by a first timing advance, and when receiving a second timing advance from the ONU Tx/Rx unit, the remote site Tx/Rx unit transmits a signal in advance of time when the sample interval of the received signal ends in the U2 interface of the remote site Tx/Rx unit by the second timing advance, and wherein the ONU Tx/Rx unit calculates the number of delayed samples in the Tx signal interval and the Rx signal interval of the U2 interface of the ONU Tx/Rx unit, and determines the second timing advance in consideration of the first timing advance and the delayed samples.

The ONU Tx/Rx unit and the remote site Tx/Rx unit updates a cyclic prefix with (the cyclic prefix + the first timing advance-the second timing advance), and a cyclic suffix with (the cyclic suffix-the first timing advance + the second timing advance).

In another aspect of the present invention, a method for determining a timing advance in a zipper DMT system for data exchange by an ONU Tx/Rx unit and a remote site Tx/Rx unit, comprises: (a) the remote site Tx/Rx unit

receiving a training start signal from the ONU Tx/Rx unit, transmitting a response signal in advance of a time when a sample interval of the received sample ends in the U2 interface of the remote site Tx/Rx unit by the first timing advance; (b) the ONU Tx/Rx unit receiving the response signal, and calculating the number of delayed samples in a Tx signal interval and an Rx signal interval of the ONU Tx/Rx unit; (c) the ONU Tx/Rx unit determining the second timing advance in consideration of the number of delayed samples and the first timing advance; (d) transmitting the second timing advance to the remote site Tx/Rx unit; and (e) the remote site Tx/Rx unit transmitting a response signal in advance of time when the sample interval of the received signal ends in the U2 interface of the remote site Tx/Rx unit by the second timing advance.

The (e) comprises updating a cyclic prefix with (the cyclic prefix + the first timing advance-the second timing advance), and a cyclic suffix with (the cyclic suffix-the first timing advance + the second timing advance).

The (b) comprises determining the number of the delayed samples by subtracting a group delay from a transmitter side of the ONU Tx/Rx unit to the U2 interface of the ONU Tx/Rx unit and a group delay from a receiver side of the ONU Tx/Rx unit to the U2 interface of the ONU Tx/Rx unit from a Tx sample count value for showing a position, within a symbol, of a transmission sample. <BR> <BR> <BR> <BR> <BR> <BR> <P> The (c) comprises determining the second timing advance through<BR> (TA0+#)modNT<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> TA= where TA is the second timing advance, TA0 is the first timing advance, # is the number of delayed samples, NT is a sample length,

and mod is a modulo operation.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: FIG. 1 shows a block diagram of a zipper DMT Tx/Rx (transmitting and receiving) system according to a preferred embodiment of the present invention; FIG. 2 shows a diagram for illustrating a sample number of a symbol used by the zipper DMT Tx/Rx system according to a preferred embodiment of the present invention; and FIGs. 3 (a) through 3 (c) show a timing diagram of a method for measuring time advances according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor (s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and

description are to be regarded as illustrative in nature, and not restrictive.

A zipper DMT system for determining timing advance, and a method for determining timing advance in the system will now be described with reference to drawings.

Detailed descriptions of the O-SIGNATURE, the R-MSG1, the R-ACK, the O-UPDATE, and the R-IDLE messages are provided on the recommendation of T1 E1. 4 VDSL (i. e. , VDSL metallic interface, part 3: Technical specification of a multi-carrier modulation transceiver).

Referring to FIGs. 1 and 2, the zipper DMT Tx/Rx system according to a preferred embodiment of the present invention will be described.

FIG. 1 shows a block diagram of a zipper DMT Tx/Rx (transmitting and receiving) system according to a preferred embodiment of the present invention, and FIG. 2 shows a sample number of a symbol used by the zipper DMT Tx/Rx system according to a preferred embodiment of the present invention.

As shown in FIG. 1, the zipper Tx/Rx system comprises an ONU Tx/Rx unit 100, and a remote site Tx/Rx unit 200. The Tx/Rx units 100 and 200 are respectively connected to U2 interfaces 400 and 500 through a channel 300 such as a UTP (unshielded twisted pair) cable.

The Tx/Rx units 100 and 200 convert digital signals into physical layer signals of the U2 interfaces 400 and 500, or converts physical layer signals into digital signals. The Tx/Rx units 100 and 200 respectively comprise a transmitter 110 and 210, a transmission buffer 120 and 220, a transmission sample counter 130 and 230, a receiver 140 and 240, a receiving buffer 150 and 250, a

receiving sample counter 160 and 260, and an AFE (analog front end) 170 and 270.

The transmitter 110 and 210 includes a Tx front-end controller 111 and 211. The Tx front-end controller 111 and 211 temporarily stores data IFFT- performed by the transmitter 110 and 210 in the transmission buffer 120 and 220, and extracts data from the transmission buffer 120 and 220 according to counting by the transmission sample counter 130 and 230 to send the extracted data to the AFE 170 and 270, and also controls count values of the transmission sample counter 130 and 230.

The receiver 140 and 240 includes an Rx front-end controller 141 and 241. The Rx front-end controller 141 and 241 temporarily stores data provided by the AFE 170 and 270 in a receiving buffer 150 and 250, extracts data from the receiving buffer 150 and 250 according to counting by the receiving sample counter 160 and 260, and transmits the extracted data to an FFT unit (not illustrated) of the receiver 140 and 240, and also controls count values of the receiving sample counter 160 and 260.

The AFE 170 and 270 converts digital signals provided by the transmitter 110 and 210 into physical layer signals (i. e. , analog signals), and respectively transmits them to the remote site Tx/Rx unit 200 and the ONU Tx/Rx unit 100 through the U2 interface 400 and 500, and also converts the physical layer signals received from the remote site Tx/Rx unit 200 and the ONU Tx/Rx unit 100 through the U2 interface 400 and 500 into digital signals to transmit them to the receiver 140 and 240.

As shown in FIG. 2, when the number of symbol tones is set to be Nsc, the symbol of the zipper DMT has Lcp cyclic prefix samples, 2NSc samples, and Lcs cyclic suffix samples. That is, the sample number NT of a single symbol is given as Equation 1.

Equation 1 NT=LCP+2NSC+LCS Tx sample count values OTxSampleCnt and RTxSampleCnt that are count values of the transmission sample counters 130 and 230 represent positions, within the symbol, of output samples of the transmission buffers 120 and 220, ranging from 0 to NT-1. Tx sample counter values OTxSampleCntAtU2 and RTxSampleCntAtU2 from the U2 interfaces indicate positions, within the symbol, of transmission samples of the U2 interfaces 400 and 500. They are expressed in Equation 2 in consideration of group delays AOTX and ARTX from outputs of the transmission buffers 120 and 220 to the U2 interfaces 400 and 500 in the transmission sample count values OTxSampleCnt and RTxSampleCnt.

Equation 2 OTxSampleCntAtU2 = OTxSampleCnt - #OTX RTxSainpleCntAtU2 = RTxSampleCnt - #RTX In a like manner, the Rx sample count values ORxSampleCnt and RRxSampleCnt that are count values of the receiving sample counters 160 and 260 represent positions, within a symbol, of input samples of the receiving buffers 150 and 250, ranging from 0 to NT-1. Rx sample counter values

ORxSampleCntAtU2 and RRxSampleCntAtU2 of the U2 interfaces 400 and 500 indicate positions, within a symbol, of receiving samples of the U2 interfaces 400 and 500. They are in consideration of group delays SORr and Brrr from inputs of the receiving buffer 120 and 220 to the U2 interfaces 400 and 500 in the Rx sample count values ORxSampleCnt and RRxSampleCnt as expressed in Equation 3.

Equation 3 ORxSampleCntAtU2 = ORxSampleCnt - #ORX RRxSmapleCntAtU2 = RRxSampleCat-ARr In general, it is required to perform an initialization step so as to transmit data in the above-noted zipper DMT system. The initialization step includes an activation (or handshake) step, a training step, and a channel analysis and exchange step. The activation (or handshake) step is a step for checking whether the transmitter and the receiver are ready to transmit signals.

The training step is a step for synchronizing symbols and training an equalizer.

The channel analysis and exchange step is a step for measuring an SNR (signal to noise ratio) for each subchannel, generating a suitable bit table that has bit loading information, and allowing various parameters to be exchanged between the transmitter and the receiver. In the training step, the timing advance is determined.

Referring to FIGs. 3 (a) through 3 (c), a method for determining the timing advance according to the preferred embodiment of the present invention will be described.

FIGs. 3 (a) through 3 (c) show a timing diagram of a method for measuring time advances according to a preferred embodiment of the present invention.

After performance of the initialization in the training step, the ONU Tx/Rx unit 100 repeatedly transmits the 0-SIGNATURE message to the remote site Tx/Rx unit 200 until receiving an R-MSG1 from the remote site Tx/Rx unit 200 in step S301. The O-SIGNATURE message that has fields including a band, an RFI band, and a PSD is used for setting default values needed for data transmission. The remote site Tx/Rx unit 200 finishes symbol synchronization and starts equalizer convergence in step S302.

The remote site Tx/Rx unit 200 receives an O-SIGNATURE message when finishing the equalizer convergence, and the Tx front-end controller 211 sets the Tx sample count RTxSampleCnt of the remote site Tx/Rx unit 200 when starting to receive the O-SIGNATURE, as NT-TA0+#RTX+#RRX in step S303. In this instance, the sample count RTxSampleCntAtU2 in a U2-R interface becomes NT-TA0 +ARRr in consideration of channel delay.

Next, the remote site Tx/Rx unit 200 repeatedly sends to the ONU Tx/Rx unit 100 the R-MSG1 which is a confirmation message that the remote site Tx/Rx has received the O-SIGNATURE message, and it starts sending the same when the Tx sample count RTxSampleCnt becomes 0 in step S304.

When the Tx sample count RTxSampleCnt reaches ARTY the sample count at the U2-R interface becomes 0, that is, the U2-R interface starts transmitting the

R-MSG1. As a result, the U2-R interface transmits a Tx message in advance prior to an Rx message by TAo When the ONU Tx/Rx unit 100 finishes symbol synchronization, the equalizer starts to be converged, and a difference A of sample number between Tx signals and Rx signals in a U2-O interface is set as OTxSampleCnt-AoRr-ooTr in step S305. The ONU Tx/Rx unit 100 receives an R-MSG1 message when finishing the equalizer convergence in step S306, and transmits an O-UPDATE message before the sample interval of the R-MSG1 is finished in step S307. In this instance, the ONU Tx/Rx unit 100 sends the O- UPDATE message when it receives the R-MSG1 and the Tx sample count OTxSampleCnt becomes 0. The O-UPDATE message includes a field for recording a correction value of the timing advance, and the ONU Tx/Rx unit 100 records a corrected timing advance in the field and transmits the field. The corrected timing advance is given as Equation 4.

Equation 4 TA = (TAO +A) mod NT 2 where TA is a corrected timing advance, TAo is a previous timing advance, NT is a sample length, and mod is a modulo operation.

The remote site Tx/Rx unit 200 receives an O-UPDATE message, and stores TA included in the O-UPDATE message in step S308, and when the Tx sample counter value RTxSampleCnt becomes 0, the remote site Tx/Rx unit 200 transmits to the ONU Tx/Rx unit 100 an R-ACK message which is a

confirmation message that the remote site Tx/Rx unit 200 has received the O- UPDATE message in step S309, and repeatedly transmits an R-IDLE message which is a meaningless message after transmitting the R-ACK message in step S310.

When the Tx sample count OTxSampleCnt is NT- (TA-A) after receiving the R-IDLE message, the controller of the ONU Tx/Rx unit 100 sets the Tx sample count OTxSampleCnt as 0, and sets a new cyclic prefix and a cyclic suffix as LCP + TAo-TA and LS- (TAo-TA) to transmit a new message in step S311. In this instance, when the Tx sample count RTxSampleCnt is 0, the controller of the remote site Tx/Rx unit 200 sets the new cyclic prefix and the cyclic suffix as LCP +TAO-TA and LCS-(TAO-TA) to transmit a new message.

The ONU Tx/Rx unit 100 calculates the number of delayed samples and a new timing advance in a like manner of the above process, and repeats the above step when the new timing advance is different from the previous timing advance.

Therefore, the timing advance can be determined by measuring the group delay of a channel, and the length of the cyclic suffix can be reduced by the difference between the existing timing advance and the new timing advance.

Also, by setting the timing advance according to the above-noted method, the ONU Tx/Rx unit and the remote site Tx/Rx unit can concurrently transmit and receive signals with reference to the U2 interface.

While this invention has been described in connection with what is

presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.