Abstract:
The invention creates a method for transmitting an analog data stream in which secondary minima are prevented during an equalization of the analog data stream at the receiving end when approximating the channel transfer function of a transmission channel, wherein the analog data stream is received via the transmission channel, the received analog data stream is converted into a digital data stream in an analog/digital converter, the digital data stream is decimated in a decimation device in order to obtain a decimated digital data stream, a control signal is derived from either the digital data stream or the decimated digital data stream in accordance with a predeterminable adjustment of a switching device, the derived control signal is supplied to a coefficient determining device and wherein equalization coefficients are provided, together with an information item provided by the reference signal, for the equalizer for equalization of the decimated digital data stream in the time domain in the coefficient determining device via the channel transfer function of the transmission channel.

Description:
TECHNICAL FIELD 
     The present invention relates to a method for transmitting an analog data stream and, in particular, relates to a method for transmitting an analog data stream and a circuit arrangement in which secondary minima are prevented during an equalization of the analog data stream at the receiving end. 
     BACKGROUND ART 
     Usually, a multitone method (DMT—discrete multitone) is used for asymmetric data stream transmission via normal telephone lines, normal telephone lines usually being constructed as asymmetric digital subscriber lines (ADSL). 
     An essential advantage of ADSL transmission techniques consists in being able to use conventional cable networks for a transmission, twisted copper conductors normally being used. 
     High-speed digital subscriber lines of the prior art are described, for example, in the publication “High-speed digital subscriber lines, IEEE Journal Sel. Ar. In Comm., Vol. 9, No. 6, August 1991”. 
     Among the transmission methods with a high data rate, which are based on digital subscriber lines (DSL), a number of VDSL (Very High Data Rate DSL) arrangements are known and, for example, methods such as carrierless amplitude/phase (CAP), discrete wavelet multitone (DWMT), single line code (SLC) and discrete multitone (DMT) can be used for these. In the DMT method, the transmit signal is provided from multiple sinusoidal or cosinusoidal signals, where both the amplitude and the phase can be modulated of each individual sinusoidal or cosinusoidal signal. The multiple modulated signals thus obtained are provided as quadrature-amplitude modulated (QAM) signals. 
       FIG. 4  shows a conventional data stream receiver for receiving an analog data stream  101  which contains multitone signals. The multitone signals are provided by a data stream transmitter and transmitted via a transmission channel as will be described in greater detail below. After the analog data stream  101  has been received in a preprocessing device  301 , a preprocessed digital data stream  302  is provided for further processing. 
     The preprocessing device  301  contains in conventional manner an analog/digital converter  104  by means of which the analog data stream  101  is converted into a digital data stream  103 . The digital data stream  103  is then converted in conventional manner into a filtered data stream by means of a first filtering device  401 , the first filtering device  401  providing a decimation of the incoming digital data stream  103 . 
     The data thus decimated and filtered by the first filtering device  401  are provided to a second filtering device  402  in which a time domain equalization is carried out. The second filtering device  402  is constructed as an adaptive transversal filter which operates at a symbol sampling rate Fs which is, for example, 276 kHz with ADSL at a switching center. The signal equalized by the second filtering device  402  is supplied as a preprocessed digital data stream  302  to a transformation device  110  in which, for example, a fast Fourier transformation (FFT) is carried out. 
     The transformation signals  111   a – 111   n  formed as a complex number which is defined, for example, in accordance with amount and phase, are then supplied to a correction device  112  in which a correction of a transfer characteristic of the transmission channel is provided. The corrected transformation signals  113   a – 113   n  are also supplied to a determining device  116  in which pairs of amount signals  114  and phase signals  115  are determined in accordance with the multitone signals in the analog data stream  101 . The pairs of amount signals  114  and phase signals  115  are supplied to a decoding device  117  in which the pairs of amount signals and phase signals are decoded in a decoder data stream  118 . The decoded data stream  118  is then output via a data output device  119 . 
     The frequencies of the multitone signal contained in the analog data stream  101  to be transmitted are usually equidistantly distributed and become calculable in accordance with the following formula: 
                 f   l     =         i   ·     1   T       ⁢           ⁢   i     =   1       ,   2   ,     …   ⁢           ⁢     N   /   2             
where T is a period and N is a number of samples of a DMT symbol.
 
     For example, conventional DMT methods use 256 tones which can be modulated in amount and phase in each case as sinusoidal tones. The fundamental frequency is 4.3 kHz and the frequency spacing between successive tones is also 4.3 kHz. Thus, a frequency spectrum from 4.3 kHz (fundamental frequency) to (4.3 kHz+256×4.3 kHz)=1.1 MHz is transmitted. Each DMT symbol is thus represented by a sinusoidal tone which can be modulated in amount and phase, a maximum of 15 bits per symbol usually being represented as complex number. During the transmission of a multitone signal of this type, the problem occurs, however, that transient effects are produced by the transmission channel which, for example, can be constructed as a twisted copper dual wire, which effects have decayed after, for example, M samples. 
     In the transmitter device, the last M samples of a DMT symbol are appended to a block start after an inverse fast Fourier transformation (IFFT), where the following relation applies: M&lt;N. Due to this cyclic extension (cyclic prefix), a periodic signal can be simulated for the data stream receiver when the transient effect caused by the transmission channel has decayed after M samples and mutual interference between different DMT symbols, i.e. inter-symbol interference (ISI), can be avoided. 
     As a result, an equalization effort in an equalizer arranged in the data stream receiver can be considerably reduced in conventional methods since after demodulation of the received analog data stream  101  in the data stream receiver, only a simple correction with the inverse frequency response of the transmission channel must be performed in the correction device  112 . 
     In methods according to the prior art, identification of a transmission channel is provided by a transfer function which is given by the following equation:
 
 H ( z )= B ( z )/ A ( z ).
 
     An equalizer is conventionally adjusted in such a manner that a cascading of the channel transfer function of the transmission channel and of the transfer function of the equalizer provides a resultant transfer function Hr as follows:
 
 Hr=B ( z ).
 
     It can be clearly seen that a length of a remaining impulse response is thus determined by the order of the numerator polynomial B(z). 
     In the known methods described above, the equalizer operates at a sampling rate Fs which is, for example, Fs=276 kHz. The order of the numerator polynomial B(z) is thus defined by the length of the cyclic prefix predetermined by the respective transmission standard, for example M=4. 
     An approximation of the channel transfer function, for example by means of a rational transfer function H(z) by using mathematical optimization methods, for example the method of least error squares, disadvantageously achieves a global optimum only if the order of the numerator polynomial B(z) and the order of the denominator polynomial A(z) can be selected to be sufficiently large. 
     In the method according to the prior art it is also disadvantageous that the order of the polynomials is restricted by the length of the cyclic prefix so that secondary minima occur when the channel transfer function of the transmission channel is approximated. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide a method for transmitting an analog data stream in which secondary minima are prevented during an equalization of the analog data stream at the receiving end when a channel transfer function of the transmission channel is approximated. 
     According to the invention, this object is achieved by the method specified in claim  1  and by a circuit arrangement for transmitting an analog data stream having the features of claim  9 . 
     Further embodiments of the invention are obtained from the subclaims. 
     An essential concept of the invention consists in deriving [lacuna] by deriving a control signal which is provided either from a digitized analog data stream or a decimated digital data stream via a switching device, and supplying this control signal to a coefficient determining device which, together with a reference signal  124  supplied via a reference signal input device  123 , determines coefficients for the equalizer  105  in such a manner that secondary minima are prevented when the channel transfer function of the transmission channel is approximated. 
     It is thus an advantage of the present invention that the length in time of a cyclic prefix does not change and, in particular, the condition M&lt;&lt;N can be met. It is also advantageous that a variable adjustment of coefficients is provided for the equalizer by oversampling the analog data stream. 
     The method according to the invention for transmitting an analog data stream in which secondary minima are prevented during an equalization of the analog data stream at the receiving end, essentially exhibits the following steps:
     a) receiving the analog data stream via a transmission channel in a data stream receiver;   b) converting the received analog data stream into a digital data stream by sampling the analog data stream with a sampling rate in an analog/digital converter;   c) decimating the digital data stream in a decimation device in order to provide a decimated digital data stream;   d) deriving a control signal  128  from either the digital data stream  103  or the decimated digital data stream  106  according to a predeterminable adjustment of a switching device;   e) supplying the derived control signal to a coefficient determining device;   f) supplying a reference signal, input via a reference signal input device, to the coefficient determining device, the reference signal representing a measure of the channel transfer function of the transmission channel;   g) determining at least one coefficient of an equalizer by means of the coefficient determining device in dependence on the reference signal supplied and in dependence on the control signal supplied, in such a manner that secondary minima are prevented when the channel transfer function of the transmission channel is approximated;   h) equalizing the decimated digital data stream in the time domain in the equalizer to which at least one determined coefficient has been applied, in order to provide an equalized decimated digital data stream;   i) transforming the decimated equalized digital data stream from the time domain into the frequency domain by means of a transformation device, in order to provide transformation signals which are defined in amount and phase;   j) correcting the transformation signals in a correction device in order to provide corrected transformation signals;   k) determining at least one amount signal and at least one phase signal from at least one corrected transformation signal in a determining device;   l) decoding the at least one amount signal determined in the determining device and the at least one phase signal in a decoding device in order to provide a decoded data stream; and   m) outputting the decoded data stream via a data output device.   

     The subclaims contain advantageous developments and improvements of the respective subject matter of the invention. 
     According to a preferred development of the present invention, a suitable control signal  128  is selected by means of the switching device  127 , in which process a received signal oversampled with the sampling rate Fa can be optionally used for an identification which requires that the coefficients determined in the coefficient determining device must be transformed to a lower sampling rate. 
     According to a further preferred development of the present invention, the analog data stream is oversampled at a sampling rate in such a manner that an oversampled mode is provided for the coefficient determining device and the equalizer. 
     According to yet another preferred development of the present invention, the decimated digital data stream is sampled with a predeterminable rate in the equalizer. 
     According to yet another preferred development of the present invention, the reference signal is provided for determining a channel transfer function of the transmission channel. 
     According to yet another preferred development of the present invention, a fast Fourier transformation (FFT) is used for transforming the decimated equalized digital data stream from the time domain into the frequency domain. 
     According to yet another preferred development of the present invention, the transformation signals are weighted with an inverse channel transfer function of the transmission channel for correcting the transformation signals in a correction device in order to provide corrected transformation signals. 
     According to yet another preferred development of the present invention, a transformation signal pair formed of an amount signal and a phase signal is in each case provided during a determination of the at least one amount signal and of the at least one phase signal from at least one corrected transformation signal in the determining device. 
     The circuit arrangement according to the invention for transmitting an analog data stream in which secondary minima do not occur during an equalization of the analog data stream at the receiving end also exhibits the following:
     a) a transmission channel for transmitting the analog data stream from a data stream transmitter to a data stream receiver;   b) an analog/digital converter, arranged in the data stream receiver, for converting the received analog data stream into a digital data stream by sampling the analog data stream at a sampling rate;   c) a decimation device for decimating the digital data stream in order to provide a decimated digital data stream;   d) a coefficient determining device for determining coefficients for the equalizer in dependence on a control signal supplied;   e) a reference signal input device for inputting a reference signal into the coefficient determining device;   f) an equalizer for equalizing the decimated digital data stream according to the coefficients adjusted by the coefficient adjusting device in order to provide an equalized decimated digital data stream;   g) a transformation device for transforming the decimated equalized digital data stream from the time domain into the frequency domain in order to provide transformation signals which are defined by amount and phase;   h) a correction device for correcting the transformation signals in order to provide corrected transformation signals;   i) a determining device for determining at least one amount signal and at least one phase signal from at least one corrected transformation signal;   j) a decoding device for decoding the at least one amount signal determined in the determining device and of the at least one phase signal in order to provide a decoded data stream; and   k) a data output device for outputting the decoded data stream.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are explained in greater detail in the description following and are shown in the drawings, in which: 
         FIG. 1  shows a data stream receiver with a coefficient determining device for receiving an analog data stream according to an exemplary embodiment of the present invention; 
         FIG. 2   a  shows a block diagram of a DMT transmission link with data stream transmitter, transmission channel and data stream receiver; 
         FIG. 2   b  shows a diagrammatic structure of a DMT symbol with cyclic prefix; 
         FIG. 3  shows the transmission arrangement illustrated in  FIG. 2   a  for transmitting an analog data stream in more detailed representation; and 
         FIG. 4  shows a conventional data stream receiver. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the figures, identical reference symbols designate identical or functionally equal components or steps. 
     The circuit arrangement according to an exemplary embodiment of the present invention, as shown in  FIG. 1 , exhibits a coefficient determining device  125  which provides equalization coefficients  126  to an equalizer  105 . 
     In this arrangement, an analog data stream  101  is supplied to an analog/digital converter  104  which oversamples the analog data stream  101  at a sampling rate  108 . The analog data stream  101  thus sampled is supplied to a decimation device  107  as a digital data stream  103 . At the same time, information of the digital data stream  103  is supplied to a switching device  127 . 
     In the decimation device  107 , the digital data stream  103  is decimated so that a decimated digital data stream  106  is obtained. Information from the decimated digital data stream  106  is supplied to the switching device  127 . 
     The decimated digital data stream  106  is supplied to the equalizer which exhibits equalization coefficients  126  determined in the coefficient determining device  125 . An equalized decimated digital data stream  109  is supplied to a transformation device  110  as output signal from the equalizer  105 . 
     The transformation device  110  provides a transformation of the decimated equalized digital data stream  109  into transformation signals  111   a – 111   n , where n represents the maximum number, 256 in the present example, of the cosinusoidal or sinusoidal signals defined in amount and phase. It should be pointed out that the transformation device  110  performs a digital transformation of a signal which is digitally present in the time domain into a signal which is digitally present in the frequency domain. 
     The transformation signals  111   a – 111   n  correspond, for example, to complex numbers for each of the multitones, evaluation being provided in amount and phase or, respectively, as a real component and imaginary component. Furthermore, the complex numbers can be provided as amplitudes of cosinusoidal (real component) and sinusoidal oscillations (imaginary component) to be sent out within a block, the frequencies being provided equidistantly distributed in accordance with the equation specified above, the data to be transmitted being combined in blocks. 
     It should be pointed out that more or fewer than 256 different tones can be transmitted as cosinusoidal or sinusoidal signals which are defined and can be modulated in amount and phase, resulting in a correspondingly different number of transformation signals  111   a – 111   n . The first transformation signal is here designated as  111   a  and the last transformation signal as  111   n . The transformation device  110  preferably carries out a fast Fourier transformation (FFT) in order to provide a fast transformation from the time domain into the frequency domain. 
     In a correction device  112 , the transformation signals  111   a – 111   n  are weighted with a known correction function which is input to the correction device  112 . This correction function input into the correction device  112  is preferably but not exclusively an inverse of the channel transfer function of the transmission channel. This makes it possible to compensate for influences of the transmission channel with respect to frequency response, phase etc., so that corrected transformation signals  113   a – 113   n  are obtained at the output of the correction device  112 . The corrected transformation signals  113   a – 113   n  are then supplied to a determining device  116  in which at least one amount signal  114  and at least one phase signal  115  or, respectively, a real component and an imaginary component, of a corrected transformation signal is determined. 
     The amount signals  114  and phase signals  115 , determined in the determining device, are then decoded by supplying the amount signals  114  and the phase signals  115  to a decoding device  117 . 
     In the decoding device  117 , decoding according to a coding of the data stream performed in the data stream transmitter  210  (described below) is provided. The decoding device  117  thus outputs a decoded data stream  118  which is finally supplied to a data output device  119  and can be output from there and processed further. 
     The switching device  127  chooses between an information item of the digital data stream  103  and an information item of the decimated digital data stream  106  as a control signal  128  and supplies the resultant choice to the coefficient determining device  125  as the control signal  128 . The coefficient determining device  125  is also supplied with a reference signal  124  input into the reference signal input device  123 . This reference signal is a measure of a channel transfer function of the transmission channel so that the information is included in a determination of the equalization coefficient  126 . According to the invention, the analog data stream  101  oversampled at the sampling rate  108  is processed in such a manner that the number of degrees of freedom during a determination of the equalization coefficients is increased with a predetermined length of a cyclic prefix. 
     The correction device  112  provided for supplying corrected transformation signals  113   a – 113   n  can be constructed as a frequency domain equalizer. The equalizer  105  also exhibits a variably adjustable filter order. 
       FIG. 2   a  shows a basic block diagram of an arrangement for transmitting an analog data stream according to the DMT method, the data stream transmitter  210 , the transmission channel  102  and the data stream receiver  211  being illustrated. 
     Data stream transmitter  210  and data stream receiver  211  consist of separately identifiable blocks which will be briefly described in the text which follows. A data input device  201  is used for inputting data to be transmitted, the input data being forwarded to a coding device  202 . In the coding device  202 , the data stream is decoded in accordance with a conventional method and supplied to a retransformation device  203 . 
     The retransformation device  203  provides a transformation of data present in the frequency domain into data present in the time domain. The retransformation device  203  can be provided, for example, by a device in which an inverse fast Fourier transformationation (IFFT) is performed. 
     It should be pointed out that the transformation from the frequency domain into the time domain performed in the retransformation device  203  represents a transformation which is inverse to the transformation performed by the transformation device  110  shown in  FIG. 1 . 
     Finally, the digital data stream output by the retransformation device  203  is converted into an analog data stream by means of a digital-analog converter  204 . The analog data stream, which is now present in the time domain, is supplied to a transmission channel  102  which provides the data transmission described above, and for the transmission, there can be band-pass filtering, high-pass filtering and/or low-pass filtering and an application of noise to the analog data stream  101 . The analog data stream  101  is furthermore supplied to the analog-digital converter  104  arranged in the data stream receiver  211 , which converts the received analog data stream  101  into a digital data stream  103 , the converted digital data stream  103  being supplied to the transformation device  110 . 
     After a transformation, which is the inverse to that in the retransformation device  203 , from the frequency domain into the time domain, the transformed data stream, after passing through a correction device (not shown) and a determining device (not shown), is decoded in the decoding device  117 . The decoded data stream is finally output via the data output device  119 . 
       FIG. 2   b  shows an arrangement of a discrete multitone symbol, the analog data stream to be transmitted being provided as a sequence of multitone symbols. Before the data transformed in the transformation device  203  are forwarded to the digital-analog converter  204 , the last M samples of a multitone symbol are again appended to the start of the block which defines a cyclic prefix and where the following applies:
 M&lt;N 
     This makes it possible to simulate a periodic signal for a data stream receiver if the transient effect caused by the transmission channel has decayed after M samples, i.e. there is no inter-symbol interference (ISI). 
     As shown in  FIG. 2   b , the original multitone symbol has a length of N samples, for example N=64 whereas, for example, the last four values are placed at the start of the symbol  205  as a cyclic prefix  212 , where:
 
M=4.
 
     The total length of a multitone symbol  208 , together with the end of DMT symbol values  213  appended to the start of the symbol  205 , is then M+N from the start of prefix  207  to the end of DMT symbol  206 . 
     It should be pointed out that the number of end of DMT symbol values  213  cyclically appended to the start of symbol  205  must be kept as small as possible, i.e. M&lt;&lt;N in order to obtain the least possible reduction in transmission capacity and quality. 
     In another example, a multitone symbol  208  consists of 256 complex numbers which means that 512 time samples (real and imaginary component) must be transmitted as a periodic signal. In this example, if a total of 32 end of DMT symbol values  213  are copied to the start of the symbol as cyclic prefix  212 , a total length of the time sample to be transmitted is calculated to be 544; which results in a sampling period T A  of 544×10 −6 /2.208 s or 0.25 ms, at a maximum tone frequency of a DMT signal of 2.208 MHz, the symbol transmission frequency being calculated from f DMT =1/T A ≈4 kHz. 
       FIG. 3  shows a method for transmitting an analog data stream and a circuit arrangement in a more detailed representation. 
     The data stream supplied to the data input device  201  is combined into blocks and a certain number of bits to be transmitted is allocated to a complex number depending on scaling. In the coding device  202 , finally, coding takes place in accordance with the selected scaling, the coded data stream finally being supplied to the retransformation device  203 . 
     A multitone signal  303  provided by the retransformation device  203  finally forms a digital transmitter data stream which has been transformed from the frequency domain into the time domain. The multitone signal  303  formed as a digital data stream is finally converted into an analog data stream in the digital-analog converter  204  and supplied to a line driver device  304 . 
     The line driver device  304  amplifies or drives, respectively, the analog data stream  101  to be transmitted into a transmission channel  102 , the channel transfer function of which is known in principle or can be measured. In the transmission channel, noise is also superimposed on the analog data stream which is shown by a superposition device  121  in  FIG. 3 . The superposition device  121  is supplied with the analog data stream transmitted from the transmission channel and with a noise signal  122  so that, finally, an analog data stream  101  is obtained on which noise is superimposed. 
     The analog data stream  101  is supplied to a preprocessing device  301 . A preprocessed digital data stream  302  output by the preprocessing device  301  is finally supplied to the circuit unit of the data stream receiver which have already been described with reference to  FIG. 1 . The description of the components of the data stream receiver  211 , shown in  FIG. 3 , are thus left out here in order to prevent an overlapping description. 
     However, it should be pointed out that a decimation of the equalized digital data stream  106  can be suppressed and it must then be possible to apply a correspondingly higher rate to the transformation device  110  which results in the advantage that a further improvement in the quality of transmission is thus provided. 
     With respect to the conventional data stream receiver shown in  FIG. 4 , reference is made to the introduction to the description. 
     Although the present invention has been described above by means of preferred exemplary embodiments, it is not restricted to these but can be modified in any variety of ways. 
     LIST OF REFERENCE DESIGNATIONS 
     In the figures, identical reference symbols designate identical or functionally equal components or steps. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 101 
                 Analog data stream 
               
               
                   
                 102 
                 Transmission channel 
               
               
                   
                 103 
                 Digital data stream 
               
               
                   
                 104 
                 Analog/digital converter 
               
               
                   
                 105 
                 Equalizer 
               
               
                   
                 106 
                 Decimated digital data stream 
               
               
                   
                 107 
                 Decimation device 
               
               
                   
                 108 
                 Sampling rate 
               
               
                   
                 109 
                 Equalized decimated digital data stream 
               
               
                   
                 110 
                 Transformation device 
               
               
                   
                 111a– 
                 Transformation signals 
               
               
                   
                 111n 
               
               
                   
                 112 
                 Correction device 
               
               
                   
                 113a– 
                 Corrected transformation signals 
               
               
                   
                 113n 
               
               
                   
                 114 
                 Amount signal 
               
               
                   
                 115 
                 Phase signal 
               
               
                   
                 116 
                 Determining device 
               
               
                   
                 117 
                 Decoding device 
               
               
                   
                 118 
                 Decoded data stream 
               
               
                   
                 119 
                 Data output device 
               
               
                   
                 120 
                 Symbol rate 
               
               
                   
                 121 
                 Superposition device 
               
               
                   
                 122 
                 Noise signal 
               
               
                   
                 123 
                 Reference signal input device 
               
               
                   
                 124 
                 Reference signal 
               
               
                   
                 125 
                 Coefficient determining device 
               
               
                   
                 126 
                 Equalization coefficients 
               
               
                   
                 127 
                 Switching device 
               
               
                   
                 128 
                 Control signal 
               
               
                   
                 201 
                 Data input device 
               
               
                   
                 202 
                 Coding device 
               
               
                   
                 203 
                 Retransformation device 
               
               
                   
                 204 
                 Digital/analog converter 
               
               
                   
                 205 
                 Start of DMT symbol 
               
               
                   
                 206 
                 End of DMT symbol 
               
               
                   
                 207 
                 Start of prefix 
               
               
                   
                 208 
                 Discrete multitone (DMT) symbol 
               
               
                   
                 210 
                 Data stream transmitter 
               
               
                   
                 211 
                 Data stream receiver 
               
               
                   
                 212 
                 Cyclic prefix 
               
               
                   
                 213 
                 End-of-DMT symbol values 
               
               
                   
                 301 
                 Preprocessing device 
               
               
                   
                 302 
                 Preprocessed digital data stream 
               
               
                   
                 303 
                 Multitone signal 
               
               
                   
                 304 
                 Line driver device 
               
               
                   
                 401 
                 First filtering device 
               
               
                   
                 402 
                 Second filtering device