the present invention utilizes transmission of cyclic signals and a non - iterative method for the estimation of finite impulse response ( fir ) filter coefficients for use in an echo canceller which is part of an asymmetric digital subscriber line ( adsl ) modem . in accordance with an embodiment of the present invention , the response of a given echo cancellation filter is calculated in the frequency domain based on frequency domain measurements of the echo cancellation ( ec ) path and the echo path . the echo path is the path going from an ifft output to a digital - to - analog ( d / a ) converter and , then , through leakage to an analog - to - digital ( a / d ) converter and a fft , as described in detail below with reference to the drawings . the calculated frequency response is consequently converted to the time domain of the fir filter . fig4 and 5 schematically illustrate schemes for calculating ec coefficients in accordance with two preferred embodiments of the present invention . the scheme shown in fig4 is suitable for use in conjunction with an adsl atu - c modem . the scheme shown in fig5 is suitable for use in conjunction with an adsl atu - r modem . in the embodiment of fig4 , a signal generated by a constellation encoder 16 is transformed by an ifft 18 , e . g . an ifft 512 , and transmitted via a parallel to serial ( p / s ) converter 20 , at a rate of 2 . 208 msps , to a digital to analog ( d / a ) converter 22 , producing an analog transmitted signal 23 , as is known in the art . for some applications of the invention , the combination of constellation encoder 16 , ifft 18 and p / s converter 20 may be replaced by a look - up table which stores at least one cycle of the signal to be transmitted . when such a look - up table is used , the output signal may include a cyclic repetition of the look - up table . the signal from p / s converter 20 is decimated by a factor of 4 , as indicated at block 30 , and provided as input x ( n ) to an echo cancellation signal generator 10 , which generates an echo cancellation signal , y ( n ), received by a substracter 14 . at the receive path , an analog received signal 25 is converted by an analog to digital ( a / d ) converter 24 into a 552 ksps digital signal . substracter 14 subtracts echo cancellation signal y ( n ) from the digital signal , yielding an echo - canceled signal , z ( n ). the echo - canceled signal z ( n ) is then converted by a serial to parallel ( s / p ) converter 26 and transformed by a fast fourier transform ( fft ) 28 , e . g ., a fft 128 , yielding an integrated signal , z ( k ). as indicated by path 102 , the integrated signal z ( k ) is also input to an echo - cancellation coefficient estimator 100 which calculates echo - canceling coefficients , c i , to be used by echo - cancellation signal generator 10 , as described in detail below . coefficient estimator 100 also provides a control signal 104 to constellation encoder 16 , as described below . the control signal controls the timing of the transmitted signals , i . e ., which signal would be transmitted at which time . the format of the control signal may vary and generally depend on specific implementations of the constellation encoder ( ce ). the control signal assists in the calculation of the ec coefficients by ensuring that channel response measurements are performed on designated transmitted signals . i some preferred embodiments it is sufficient to synchronize the cancellation coefficient calculator with the constellation encoder and , based on this synchronization , the coefficient calculator may determine the times at which the constellation encoder transmits appropriate reverb signals . in the embodiment of fig5 , a signal generated by a constellation encoder 116 is transformed by an ifft 118 , e . g . an ifft 128 , and transmitted via a parallel to serial ( p / s ) converter 120 , at a rate of 552 ksps , to a digital to analog ( d / a ) converter 122 , producing an analog transmitted signal 123 , as is known in the art . the signal from p / s converter 120 is also supplied as input to echo cancellation signal generator 10 which generates echo cancellation signal y ( n ). the echo cancellation signal is interpolated by a factor of 4 , as indicated at block 132 , and then received by substracter 14 . at the receive path , an analog received signal 125 is converted by an analog to digital ( a / d ) converter 124 into a 2 . 208 msps digital signal . substracter 14 subtracts the interpolated echo cancellation signal from the digital signal , yielding echo - canceled signal z ( n ). the echo - canceled signal z ( n ) is converted by a serial to parallel ( s / p ) converter 126 and then transformed by a fast fourier transform ( fft ) 128 , e . g ., a fft 512 , yielding integrated signal z ( k ). as indicated by path 202 , the integrated signal z ( k ) is supplied as input to an echo - cancellation coefficient estimator 200 , analogous to estimator 100 of fig4 , which calculates the echo - canceling coefficients , c i , to be used by echo - cancellation signal generator 10 , as described in detail below . coefficient estimator 200 also provides a control signal 204 to constellation encoder 116 , as described below . as explained above , with reference to fig4 , the control signal controls the timing of the transmitted signals , thereby to assists in calculating ec coefficients by ensuring that channel response measurements are performed on designated transmitted signals . in an embodiment of the present invention , the echo - cancellation coefficient estimators , 100 or 200 , perform the following functions : 1 . initializing the fft and ifft serial to parallel interfaces such that no cyclic prefix e . g ., at a fft / ifft rate of 4 . 3125 khz , is added or removed . this ensures that the channel response measurements are performed on a signal without a cyclic prefix . the addition of a cyclic prefix changes the symbol rate , e . g ., from 4 . 3125 khz to 4 . 0588 khz ; however the system described herein is based on an assumption that the bin spacing of the fft is 4 . 3125 khz , i . e ., does not have a cyclic prefix . 2 . commanding constellation encoder , 16 or 116 , respectively , to transmit a reverb signal without a cyclic prefix , to ensure that the channel response measurements are performed on a signal without a cyclic prefix , as explained above . the reverb signal may be as defined in ansi ti . 413 - 1998 — network and customer installation interfaces — asymmetric digital subscriber line ( adsl ) metallic interface , incorporated herein by reference in its entirety . 3 . disabling the echo - cancellation path . this function may be performed by setting all the filter coefficients , c i , to zero . 4 . integrating the output of the fft for l symbols to obtain an echo response measurement , echo ( k ). the purpose of this integration is to reduce the effect of noise on the signal measurements ( by “ averaging out ” noise ) and , thus , to increase the signal - to - noise ( s / n ) ratio of the measurements . it is appreciated that a larger number of symbols , l , results in a higher s / n ration ; however integration over a larger number of symbols also results in a longer execution time . the allowed execution time may also be constrained by the signal intervals in the adsl start - up sequence . for example , values of l = 16 , l = 32 and l = 64 have been determined to provide a reasonable tradeoff between the s / n ration and the execution time in certain applications . 5 . enabling the echo - cancellation ( ec ) path with the filter coefficients , c i , set to values which yield a unity gain filter . 6 . integrating the output , z ( k ), of fft 28 or 128 , respectively , over l symbols , to obtain an echo + ec response measurement . 7 . calculating an ec response , ec ( k ), by determining the difference between the echo + ec response measurement and the echo response measurement ( echo ( k )). alternatively , by disabling the transmission path , the ec response ( ec ( k )) can be measured directly . 8 . based on the echo + ec response and the echo response ( echo ( k )) measurements , calculating the appropriate ec filter coefficients to be used by the echo - canceling signal generator 10 , as described in detail below . 9 . setting the ec filter to operate with the ec coefficients calculated in the previous step . all the above listed steps ( 1 – 9 ) may be performed by coefficients estimator 10 . as explained above , in some implementations of the invention , the coefficient estimator may be synchronized with the signal generated by the constellation encoder ( 16 or 116 ) instead of separately controlling the constellation encoder ( ce ) and the p / s converter ( 20 or 120 ). for example , in the adsl startup stage , the ce and p / s converter may be configured by a controller to transmit a sequence of signals as defined by the standard for the adsl startup sequence . in such a scheme , coefficients estimator 10 may also be synchronized to the standard startup sequence and , thus , the measurements may be timed with the appropriate signals . in a preferred embodiment of the present invention , the ec filter coefficients are calculated , using suitable hardware and / or software in coefficient estimators , 100 or 200 , as described in detail below . the following notations are used throughout the description of the calculation process : echo ( k )— the complex value of an echo path response measurement . ec ( k )— the complex value of an ec path response measurement . c ( k )— the complex value of a desired echo cancellation response . { overscore ( c )}— the desired echo cancellation coefficients . ĉ ( m )— the selected echo cancellation coefficients . w ( m )— a windowing function , which may be used to “ smooth the edges ” of the filter response . it is noted that setting w ( m )= 1 for all m &# 39 ; s yields the desired smoothing in most cases . sharp drops in the signal at the response end of ec filter ( i . e ., in the time domain ) may lead to a residual echo noise with a wide - band content . this spread of the noise over a relatively wide frequency band may degrade modem performance , because the modem operates more efficiently with lower frequency noise ( i . e ., frequencies below the operation band of the modem ). this problem may be mitigated by “ smoothing ” the time domain response of the fir near the edges of the transmitted frequency band , e . g ., by applying to the fir coefficients a windowing function with smooth transitions to zero near the edges . k — the fft bin index which represents a discrete equivalent of the fft frequency . for a fft of size m and sampling rate s the actual frequency may be derived from k using the formula f = k * s / m . for example , if the sampling rate is 2 . 208e6 hz , the fft size is 128 , and k = 10 , then f = 2 . 208e6 * 10 / 128 = 43 . 125 khz . ℑ − 1 { }— an inverse fourier transform ( e . g ., ifft ) l — a filter length , defined as the number of taps in the echo - cancellation fir filter . using the above notations , the calculation in accordance with a preferred embodiment of the invention includes the following steps : the desired echo cancellation response is calculated using the following formula : c ⁡ ( k ) = echo ⁡ ( k ) ec ⁡ ( k ) ⁢ ⁢ k = - 63 ⁢ ⁢ … ⁢ ⁢ 64 ( 2 ) then , the appropriate ec filter coefficients may be calculated based on the echo - cancellation response , using an inverse fourier transform ( e . g ., an ifft ) as follows : as explained above , the channel response integration is performed in the frequency domain , i . e ., after the fft . however , the ec filter operates in the time domain . therefore , an inverse fft function is performed to transform the desired response from the time domain to the frequency domain . the actual filter coefficients are then calculated using the following formula : ĉ ( m )= w ( m )· c ( m ) m = 0 . . . ( l − 1 ) ( 4 ) wherein w ( m ) is a temporal window designed to reduce the high frequency components of the filter coefficients , c ( m ). it should be appreciated that generating a reverb signal is not fundamental to the ec filter coefficient calculation scheme of the present invention . the signal which is used may be generated either by the constellation encoder ( 16 or 116 ) and ifft ( 18 or 118 ) of fig4 and 5 , respectively , or by a cyclic lookup table or by any other suitable methods known in the art . further , it should be appreciated that the use of a specific type of reverb signal , as mentioned above with reference to the prior art , is not critical for the purposes of the present invention . in fact , any cyclic signal having non - zero components for all the frequencies utilized by the ec filter coefficient calculation process may be used instead of the above - mentioned reverb signal . integration of fft output , z ( k ), may be performed either directly on the fft output or using any linear ( i . e ., per bin ) manipulation of the fft output . for example , the output of a frequency equalizer ( not shown ) may be used as the basis for integrating z ( k ). in an alternative embodiment , the input received by the fft ( 28 or 128 ) may be integrated in the time domain , and then the fft transformation may be applied to the accumulated ( integrated ) vector to produce vectors echo ( k ) and ec ( k ). in calculating filter coefficients , c i , it is generally preferred to select from the measured response only bins corresponding to frequencies lower than or equal to the filter operation rate nyquist frequency . in some embodiments of the invention , all the bins in this range are selected ( e . g ., − 63 . . . 64 ). alternatively , any evenly spaced subset of this range may be selected , provided that such subset includes the dc bin and spans substantially the entire frequency range up to the nyquist frequency . to fully represent the desired response , it is desired to include bins that span the entire frequency range , i . e ., a range of zero to the nyquist frequency in the sampled domain . the bin spacing defines the length of the desired response in the time domain . the larger the bin spacing , the shorter the response time . for a given ec fir length l ′ ( e . g . 32 taps ), it may be a unnecessary to perform an ifft that yields a desired response longer that l ′, because a desired response of such length would be truncated to length l ′. for example , with an ifft 128 , a bin spacing of 1 in the frequency domain yields a desired response length of 128 in the time domain . if the filter response length is only 32 , a bin spacing of 4 may be used and an ifft32 may be applied instead of an ifft128 . it should be appreciated that the present invention may be implemented by means of computer software or dedicated hardware or any combination of hardware and software as may be required for specific applications of the invention . for example , in a preferred embodiment of the invention , step 8 and 9 of the algorithm performed by echo - cancellation coefficient estimators , 100 or 200 , as described above , may be executed using the following computer software code ( written in c ++): // symbolsize - the symbol size to use with the channel response // the vector as the following struceture [ nb na b0 b1 . . . outputshift ]. // when calling the function care should be taken to allocate memory // window - if this pointer points ( not equal o null ) to a window the coeffecint // are multiplied by a window to smoothen the time domain response // addtoresponse - if true the response is added to the the current coefs . // if false ( default ) a totaly new coef . are calculated and loaded max value & lt ; 2 { circumflex over ( )} 15 outputshift , // the output shift applied firorder = eccoefs [ 0 ]; // extract the fir order from the input vector i f ( m i n v a l i d m e a s u r e & gt ; { // if response measurements are invalid set desired response value to zero // fit the desired response into the ifft data bits with 3db margin p e a k v a l u e = — — m a x ( p e a k v a l u e , // transfer the channel response to time domain using the ifft transform // calculate the extra shift so that largest coef will full in the // integer coefs . flttointfactor = 1 & lt ;& lt ; ( mulshift + outputshift + 16 bit to 20 if ( window ) // if a window was supplied multiply the coefs . by window e c c o e f s [ i + 2 ] − = e c c o e f s [ i + 2 ] = − it will be appreciated by persons skilled in the art that the present invention is not limited by the preferred embodiments described above with reference to the accompanying drawings . rather , the scope of the present invention is defined by the following claims :