Abstract:
The invention relates to a device for correcting a receiver signal which is associated with an emission signal transmitted in a distorted transmission system. The emission signal comprises periods which can be determined by analyzing the received signal wherein determined properties are exhibited which are suitable for adjusting the correction. According to one embodiment, the device comprises a component for adjusting the correction based upon an analysis of the received signal and a component for monitoring and enabling the adjusting component when the received signal associated with the transmission signal exhibits certain characteristics.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a device for equalizing a received signal and in particular to a device for equalizing a received signal of a transmission system. 
   DESCRIPTION OF PRIOR ART 
   In the case of digital transmission methods, adaptive equalization is an effective measure to compensate in particular for time-dependent distortions on the transmission link, such as due to ageing, temperature fluctuations or changing connection configurations. In view of the complexities available in the case of modern CMOS technologies for digital signal processing, such as for example the number of available logical gates and the associated signal processing possibilities, it is also attractive for cost reasons to reduce the requirements with regard to the tolerances of analog components/modules and to compensate for the resulting system imperfections that are caused by process fluctuations, but also by time-variant external influences, such as for example temperature fluctuations, by means of an adaptive equalizer. Time-variant phenomena, such as for instance the changing of the transmission function of analog components on account of a temperature drift, in this case occur on a time scale which is several orders of magnitude longer than the symbol period. This is therefore a quasi-static problem, i.e. the optimum equalization, such as for example the optimum coefficient setting for a filter, of an equalizer is virtually time-independent. Only the spectral properties of the transmission spectrum are time-variant under some circumstances. 
   The setting of an adaptive equalizer can be carried out with the aid of the transmission of a known data sequence (data-aided), such as for example a preamble. In the case of many transmission methods, however, in particular in the case of continuous transmission by contrast with burst mode, or for example block-based methods, such as multi-carrier transmission, the transmission of such a known data sequence is not envisaged. An adaptive equalizer may therefore alternatively be constructed in such a way that the information for the setting of the adaptive equalizer is derived from the received and possibly falsified data alone. This presupposes that the transmitted data sequence has particular properties. 
   One particular property of such a data sequence is, for example, that the input spectrum must cover the entire frequency range of the equalizer. This property is not ensured for example in the case of decimating equalizers, which reduce the data rate, for example by subsampling of the signal, from the input to the output. The frequency range of the equalizer in this case extends to f sampling /2, where f sampling  is for example an integral multiple of f symbol . The signal energy is essentially restricted to a range &lt;f symbol /2. The lacking signal energy above f symbol /2 may cause a problem which is known by the term “tap wandering” or coefficient wandering. f symbol  here is the symbol frequency of transmitted symbols, which are represented for example by voltage levels in a transmitted signal. 
   A further particular property of the above-mentioned data sequence is, for example, that the successive data must not be correlated, i.e. the input spectrum must be “white” and must ideally have a flat frequency profile with a low-pass characteristic. This property can be ensured if a pseudo-random sequence is generated from the data before the transmission by means of a scrambler. 
   There are, however, a whole series of applications in which nothing can be stated about the properties of the transmission spectrum over time. The reason for this is that the coding schemes used are chosen merely with a view to ensuring special properties of the system, such as for example facilitating clock recovery or the absence of a direct component in the transmission spectrum. In particular, a scrambler is not usually provided. 
   One example of such an application is digital data transmission within the PDH (=Plesiochronous Digital Hierarchy) according to the T x  standard or DS x  standard in the USA (x=1:1.544 Mbit/s data transmission rate; x=3:44.736 Mbit/s data transmission rate) or the Ex standard in Europe (x=1:2.048 Mbit/s data transmission rate; x=3:34.368 Mbit/s data transmission rate), for which components are currently required in large volume (large numbers of items). A further example of such an application is the digital transmission between a physical layer (PHY; PHY=Physical Layer) of a 10 gigabit/s transmission via glass fibre and the associated intermediate access layers (MAC; MAC=Medium Access Layer) via the so-called “Ten Gigabit Attachment Unit Interface (XAUI)”, which are specified within the IEEE Standard P802.3ae. The aim is to permit the greatest possible physical separation of these layers. This application will be of enormous significance in the near future. 
   Furthermore, in the case of many systems, the transmission spectrum deviates more or less significantly from the ideal of a “flat” baseband spectrum with a low-pass character. When a line coding as in the aforementioned Tx/Ex transmission is used, in the case of which the transmission spectrum is not intended to have any direct component, there occurs for instance a passband characteristic, in the case of which the spectrum in the pass band is also not flat. 
   Individual effects, such as the frequency-dependent attenuation in the case of transmission via two-wire copper lines, which is determined by the length of the line, or the distortions which are induced by tolerances in the cut-off frequency of an analog anti-aliasing filter with a known transmission function, can be compensated by means of an equalizer which is fixed but can be set, in the form of a programmable filter. 
   The setting of such an equalizer on a chip can take place on the one hand externally, such as for example by the selection of coefficients from sets of coefficients which are stored on the same chip, or by external programming. In this case, for example, a setting at which the lowest bit error rate is achieved may be chosen. However, also possible on the other hand is a setting by a control unit on the chip, which can select between the coefficients which are stored on the chip by means of a suitable criterion. 
   In both cases, the stored coefficients can approximate the transmission function that is inverse to the distortion and be determined for instance by minimizing a suitable cost function, such as for example a criterion of a least mean square error (LMS; LMS=Least Mean Square). In the case of many applications, however, there are practical limits to such a procedure, since only a limited number of sets of coefficients can be stored. Such a procedure may be useful whenever it is intended to compensate essentially for a dominant distortion, which moreover is determined by few parameters. Time-variant phenomena, such as for example temperature fluctuations, ageing etc., cannot generally be taken into account in this way. 
   A problem in the prior art is therefore that the known settable equalizers can only be used to a limited extent to compensate for distortions of, for example, a transmission link and analog components. 
   A further problem in the prior art is that virtually no time-variant distortions, caused for example by temperature fluctuations, ageing etc., can be compensated with the known settable equalizers. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is consequently to provide a transmission system which comprises a device for equalizing the received signal permitting flexible equalization of time-variant distortions that are caused in particular by the transmission system. In this case, the device for equalizing a received signal must be designed in such a way that equalization is also achieved when nothing can be stated a priori about the time with respect to the spectrum of the received signal. 
   The idea on which the present invention is based is that automatic adaptation methods which have proved successful in the case of continuous transmission of data with a white spectrum are combined with a time-limited, that is block-by-block, evaluation of the data stream, in a way similar to in the case of data-aided methods, in which the transmitted data are known. 
   By contrast with such methods, which for example use a known frame structure and a preamble transmitted along with it, in the case of the proposed method the blocks of the received signal that can be used for the adaptation are determined automatically. Such a method with a corresponding extra effort for the evaluation of the received signal and the corresponding activation of the adaptation is attractive in view of the technological advancement (or the increasing available complexity on a chip). 
   One advantage of the present invention is that, with the aid of the present invention, the advantages presented above of adaptive equalization can also be used in the case of transmission methods in which nothing can be stated about the properties of the transmission spectrum over time. The invention can also be used in those cases in which there is no transmitted signal at all in a temporally unforeseeable way. 
   A further advantage of the present invention is that it can be extended in a simple manner to be used in cases in which the convergence behavior of the adaptive equalization is poor. 
   According to a preferred development of the invention, the same also has a first means for taking decisions and for supplying a first estimate signal, which estimates the transmitted symbols of the transmitted signal and supplies the estimated symbols with the first estimate signal. 
   According to a further preferred development of the device, the same also has a first means for supplying a first error signal, which determines the first error signal from the difference between the first estimate signal, which is supplied by the first means for taking decisions, and the first equalized received signal, which is supplied by the first means for equalizing. 
   According to a further preferred development of the invention, the means for setting the settable equalization determines the first settable equalization of the first means for equalizing the received signal from the received signal and the first error signal. 
   According to a further preferred development of the device, the means for setting the settable equalization derives the first settable equalization of the first means for equalizing the received signal from the correlation of the received signal and the first error signal. 
   According to a further preferred development of the device, the means for monitoring the received signal switches the first error signal through to the means for setting the settable equalization and activates the means for setting the settable equalization when the transmitted signal assigned to the received signal has the particular properties. 
   According to a further preferred development of the device, the same also has a second means for equalizing the received signal and for supplying a second equalized received signal, the second means for equalizing the received signal having a second settable equalization. 
   According to a further preferred development of the device, the same also has a second means for taking decisions and for supplying a second estimate signal, which estimates the transmitted symbols of the transmitted signal and supplies the estimated symbols in the second estimate signal. 
   According to a further preferred development of the device, the same also has a second means for supplying a second error signal, which determines the second error signal from the difference between the second estimate signal, which is supplied by the second means for taking decisions, and the second equalized received signal, which is supplied by the second means for equalizing the received signal. 
   According to a further preferred development of the device, the means for setting the settable equalization determines the second settable equalization of the second means for equalizing the received signal from the received signal and the second error signal. 
   According to a further preferred development of the device, the means for setting the settable equalization derives the second settable equalization of the second means for equalizing the received signal from the correlation of the received signal and the second error signal. 
   According to a further preferred development of the device, the means for monitoring the received signal switches the second error signal through to the means for setting the settable equalization and activates the means for setting the settable equalization when the transmitted signal assigned to the received signal has the particular properties. 
   According to a further preferred development of the device, the same also has a means for comparing, which compares a first quality level, which is derived from the first error signal, and a second quality level, which is derived from the second error signal, and, if the second quality level is greater than the first quality level, instigates that the set equalization of the second means for equalizing the received signal is supplied by the means for setting the settable equalization to the first means for equalizing the received signal. 
   According to a further preferred development of the device, upstream of the means for comparing there are arranged a first and a second means for ascertaining a quality level, which derive the first and second quality levels from the first error signal and the second error signal and supply them to the means for comparing. 
   According to a further preferred development of the device, the first and second quality levels are derived from the mean square error of the first error signal and the second error signal. 
   According to a further preferred development of the device, the means for monitoring the received signal monitors optionally either the first equalized received signal, which is supplied by the first means for equalizing the received signal, or the first estimate signal, which is supplied by the first means for taking decisions. 
   According to a further preferred development of the device, the means for monitoring the received signal monitors optionally either the second equalized received signal, which is supplied by the second means for equalizing the received signal, or the second estimate signal, which is supplied by the second means for taking decisions. 
   According to a further preferred development of the device, the same also has a first shaping filter, which supplies a shaped received signal to the second means for equalizing the received signal and the means for setting the settable equalization, and a second shaping filter, which has a filter function corresponding to the first shaping filter and supplies a shaped second estimate signal to the means for supplying a second error signal. 
   According to a further preferred development of the device, the same also has a first shaping filter, which supplies a shaped received signal to the means for setting the settable equalization, and a second shaping filter, which has a filter function identical to the first shaping filter and supplies a shaped second error signal to the means for monitoring the received signal. 
   According to a further preferred development of the device, the first and/or second means for equalizing the received signal have a programmable filter, the equalization of which can be set by means of filter coefficients by the means for setting the settable equalization. 
   According to a further preferred development of the device, the first and/or second means for equalizing the received signal also respectively have a first memory for storing a first set of filter coefficients. 
   According to a further preferred development of the device, the first set of filter coefficients has filter coefficients which are used in the initialization of a transmission of transmitted signals. 
   According to a further preferred development of the device, the first and/or second means for equalizing the received signal also have a second memory for storing a second set of filter coefficients. 
   According to a further preferred development of the device, the second set of filter coefficients has filter coefficients which are supplied by the means for setting the settable equalization. 
   According to a further preferred development of the device, the means for monitoring the received signal has a filter bank, in order to determine the energy distribution in the received signal. 
   According to a further preferred development of the device, the filter bank has bandpass filters. 
   According to a further preferred development of the device, the means for monitoring the received signal carries out a Fourier transformation, in order to determine the energy distribution in the received signal. 
   According to a further preferred development of the device, the transmitted signal has as a particular property of the particular properties of the transmitted signal a flat baseband spectrum with a low-pass characteristic. 
   According to a further preferred development of the device, deviations of the transmitted signal from a flat baseband spectrum with a low-pass characteristic are taken into account in the means for monitoring the received signal in the form of a corresponding reference spectrum. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a general representation of the present invention; 
       FIG. 2  shows a first exemplary embodiment according to the present invention; 
       FIG. 3  shows a second exemplary embodiment according to the present invention; 
       FIG. 4  shows a third exemplary embodiment according to the present invention; and 
       FIG. 5  shows a fourth exemplary embodiment according to the present invention. 
   

   DETAILED DESCRIPTION 
   In the figures, the same reference numerals or reference numerals which differ only in the first digit designate component parts that are the same or functionally the same. 
     FIG. 1  shows a general representation of the present invention. A transmission system  100 , in which the present invention is used, comprises a transmission channel  102 , which is arranged between a transmitter  104  and a receiver, which comprises the components  112 ,  114  and  106 . When a digital transmitter  104  is used, the transmission path of the transmission system  100  has, following the transmitter, a digital/analog converter  108 , which converts the digital transmitted signal of the transmitter  104  into an analog transmitted signal, followed by further analog components  110 , by means of which the analog transmitted signal is fed into the transmission channel  102 . The reception path also has, equivalent to this, analog reception components  112 , for example anti-aliasing filters, and an analog/digital converter  114  for converting the analog received signal into a digital received signal. The reception path may also comprise further digital components  106 , such as means for raising or reducing the sampling rate. 
     FIG. 1  also generally shows a device according to the invention for equalizing a received signal, which equalizes the received signal of the transmitter after the digital components  106 . The received signal is assigned to the transmitted signal that is transmitted in the distorting transmission system  100 . The transmitted signal has time segments in which the same has particular properties, such as for example a white frequency spectrum, in the case of which successive data are not correlated with one another, and/or a flat frequency profile with a low-pass characteristic. 
   The device for equalizing the received signal has a means  116  for equalizing the received signal and for supplying an equalized received signal, the means  116  for equalizing the received signal having a settable equalization. The device for equalizing the received signal also has a means  118  for setting the settable equalization (automatic adaptation) of the means  116  for equalizing the received signal in accordance with the received signal and a means  120  for monitoring the received signal and for activating the means  118  for setting the settable equalization when the transmitted signal assigned to the received signal has particular properties, such as for example a white frequency spectrum, that are suitable for setting an equalization of the distortion of the received signal caused by the transmission system. 
   The means  116  for equalizing the received signal and for supplying a first equalized received signal preferably has a programmable filter  122 , the equalization of which can be set by means of filter coefficients by the means  118 . The means  116  for equalizing also has a first memory  124  for storing a first set of filter coefficients and a second memory  126  for storing a second set of filter coefficients. The first set of filter coefficients preferably has filter coefficients that are used in the case of the initialization of a transmission of transmitted signals or in the case of a cold start, and may also contain various coefficients by means of which the means  116  for equalizing can be set differently during operation. The second set of filter coefficients is preferably supplied by the means  118  for setting the settable equalization and preferably contains the momentary equalizer setting with regard to a warm start. 
   For the function of the device according to the present invention, it is assumed that there are time segments during which the transmitted signal has the properties required for an automatic adaptation in the means  118  for setting. By means of the means  120  for monitoring the received signal or by means of a monitor, the statistical properties of the transmitted signal can be monitored and the time segments in which the transmitted signal or the assigned received signal is suitable for the adaptation can be determined. During such time segments, an automatic coefficient adaptation can then be activated by the means  118  for setting. In this case, different methods, known from the literature, can be used for the adaptation. If no suitable received signal is available for the automatic adaptation, the adaptation is deactivated by the means  118  for setting the settable equalization. In certain time segments, the coefficients ascertained by means of adaptation by the means  118  for setting can be taken over into the programmable filter  122 . It is expedient to store the newly taken-over coefficients in the second memory  126 . These coefficients are then available once again for example for a possible warm start. The taking over of the coefficients from the means  118  for setting into the second memory  126  of the means  116  for equalizing may take place at regular time intervals, controlled by the means  118  for setting, but also by the means  120  for monitoring the received signal by means of switches  140 . It is advantageous, however, to check by means of a suitable cost function whether the coefficient setting produces any improvement of the reception properties at all. In this way, the taking over of erroneous or unfavourable coefficient settings, which cannot be ruled out in view of the problems described with the transmission spectrum, can be avoided. 
     FIG. 2  shows a first exemplary embodiment of a device for equalizing a received signal, which is assigned to a transmitted signal that is transmitted in a distorting transmission system  200 , the transmitted signal having time segments in which the same has particular properties, such as for example a white frequency spectrum, that are suitable for analyzing the distortion of the received signal caused by the transmission system  200  and for setting the equalization of the received signal. As already shown in  FIG. 1 , the transmission system  200  preferably has a transmission channel  202 , which is arranged between a digital transmitter  204  and a receiver, which comprises the components  212 ,  214  and  206 . Arranged downstream of the digital transmitter  204  are preferably a digital/analog converter  208  followed by further analog components  210 . The reception path has analog components  212 , for example anti-aliasing filters, an analog/digital converter  214  and further digital components  206 , such as means for raising or reducing the sampling rate. The device for equalizing the received signal receives the received signal from the digital components  206 . 
   The device for equalizing a received signal has a means  216  for equalizing the received signal and for supplying a first equalized received signal, which has a settable equalization. The device for equalizing a received signal also has a means  218  for setting the settable equalization of the means  216  for equalizing the received signal in accordance with the received signal and a means  220  for monitoring the received signal and for activating the means  218  for setting when the transmitted signal assigned to the received signal has the particular properties. 
   In a way similar to in  FIG. 1 , the means  216  for equalizing the received signal preferably has a programmable filter, the equalization of which can be set by means of filter coefficients by the means  218  for setting. The programmable filter may in this case have a first memory for storing a first set of filter coefficients, which is preferably used in the initialization (cold start) of a transmission of transmitted signals, and a second memory for storing a second set of filter coefficients, which is supplied by the means  218  for setting and is obtained from an adaptation or optimization. 
   The means  220  for monitoring the received signal preferably has a filter bank, in order to determine the energy distribution in the received signal. The filter bank allows evaluation of whether the estimated transmission spectrum has adequate energy in all frequency ranges, the nominal transmission spectrum serving as a reference. In many cases, a filter bank with a small number of bandpass filters is adequate for this. As an alternative to such a modular and regular arrangement with the known advantages with regard to implementation on a small surface area and with little loss of performance, the means  220  for monitoring the received signal may alternatively or additionally carry out a Fourier transformation, such as for example a discrete Fourier transformation or an FFT (=Fast Fourier Transform), in order to determine the energy distribution in the received signal. 
   The device for equalizing a received signal also has a means  223  for taking decisions and for supplying an estimate signal, which estimates the transmitted symbols of the transmitted signal and supplies the estimated signals with the estimate signal, and a means  225  for supplying an error signal ê, which determines the error signal ê from the difference between the estimate signal, which is supplied by the means  223  for taking decisions, and the equalized received signal, which is supplied by the means  216  for equalizing. 
   As shown in  FIG. 2 , the means  218  determines the settable equalization of the means  216  for equalizing the received signal automatically from the received signal and the first error signal ê. The settable equalization of the means  216  for equalizing the received signal is in this case preferably derived from the correlation of the received signal and the error signal ê. The means  220  for monitoring the received signal switches the error signal through to the means  218  for setting the settable equalization for updating the setting of the means  216  and activates the means  218  for setting when the transmitted signal assigned to the received signal has the particular properties, such as for example a white frequency spectrum or adequate signal energy in all frequency ranges. The means  220  for monitoring the received signal monitors optionally either the equalized received signal, which is supplied by the means  216  for equalizing the received signal, or the estimate signal, which is supplied by the means  223  in the form of a decision with respect to the symbol alphabet used. 
     FIG. 2  presents these two possible options, which can be selected by means of the switch  227 . The first option, as mentioned, comprises the selection of the received signal equalized by the means  216  for equalizing before the means  223  for taking decisions, which is used for example when the means  216  for equalizing is set close to its optimum, and the errors are in practice not correlated with the transmitted signal, and consequently meaningful monitoring is possible. The second option comprises the selection of the estimate signal after the means  223  for taking decisions, which is advantageous when an error-free estimate of the transmitted symbols takes place by the means  223  for taking decisions and therefore no error contribution enters the means  220  for monitoring the received signal. In view of the in some circumstances strong distortions in the transmission channel  202 , the equalized received signal downstream of the means  223  for taking decisions comes into consideration primarily for monitoring of the transmission spectrum, on the basis of which the transmitted symbols can be estimated. 
   The coefficient adaptation in  FIG. 2  is shown for the so-called MMSE algorithm, in the case of which the updating of the individual coefficients is derived from the correlation of the received signal or the input signal of the means  216  for equalizing with the correspondingly delayed error signal ê after the means  223  for taking decisions. This algorithm is preferably also used in the following exemplary embodiments. 
   Since it cannot be ensured how long the time segments available for the adaptation are and in what time interval they follow one another, the adaptation rate of the method is not known from the outset. Moreover, in spite of the monitoring of the data, it is not possible to rule out the chance that the means  216  for equalizing has in the meantime assumed a setting that is unfavourable, for example with respect to the mean square error (MSE) after the means  223  for taking decisions and only very slowly converges in the direction of the optimum equalization. Losses of performance on account of such “inadvertent states” can be obviated if an additional second means for equalizing a received signal or an additional adaptive equalizer is provided parallel to the reception path with the first means for equalizing the received signal, as described below. 
     FIG. 3  shows a second exemplary embodiment according to the present invention, and the elements shown in  FIG. 3  that differ from the elements in  FIG. 2  only by the first digit of the reference numeral represent component parts that are the same or functionally the same and are not described again below. This similarly applies to the further figures that follow. 
   With reference to  FIG. 3 , the device for equalizing has in addition to a first means  316  for equalizing the received signal and for supplying a first equalized received signal an adaptive equalizer or second means  328  for equalizing the received signal and for supplying a second equalized received signal, the second means  328  for equalizing the received signal having a second settable equalization, which differs from the first settable equalization of the first means  316  for equalizing. The device for equalizing a received signal also has in addition to the first means  323  for taking decisions and for supplying a first estimate signal a second means  330  for taking decisions and for supplying a second estimate signal, which estimates the transmitted symbols of the transmitted signal and supplies the estimated symbols in a second estimate signal. 
   In comparison with the first exemplary embodiment of  FIG. 2 , the device for equalizing a received signal also has in addition to the first means  325  for supplying a first error signal ê a second means  332  for supplying a second error signal ê′, which determines the second error signal ê′ from the difference between the first estimate signal, which is supplied by the second means  330  for taking decisions, and the second equalized received signal, which is supplied by the second means  328  for equalizing the received signal. 
   In the case of this second exemplary embodiment, the means  318  for setting the settable equalization of the received signal determines the second settable equalization of the second means  328  for equalizing the received signal from the received signal and the second error signal ê′, which is supplied by the second means  332  for supplying a second error signal ê′. The means  318  for setting or automatically updating the setting of the second means  328  for equalizing preferably derives the settable equalization from the correlation of the received signal and the second error signal ê′. 
   The means  320  for monitoring the received signal switches the second error signal ê′ through to the means  318  for setting and activates the means  318  for setting the settable equalization when the transmitted signal assigned to the received signal has particular properties, such as for example a white frequency spectrum. As in the case of the first exemplary embodiment, the means  320  for monitoring the received signal optionally supplies by means of a switch  327  either the first equalized received signal, which is supplied by the first means  316  for equalizing the received signal, or the first estimate signal, which is supplied by the first means  323  for taking decisions. 
   By contrast with the first exemplary embodiment, the device for equalizing a received signal according to the second exemplary embodiment also has a means  334  for comparing, which compares a first quality level, which is derived from the first error signal ê, and a second quality level, which is derived from the second error signal ê′, and, if the second quality level is greater than the first quality level, instigates that the set equalization of the second means  328  for equalizing the received signal is supplied by the means  318  to the first means  316  for equalizing the received signal by means of a switch  340 . Preferably arranged upstream of the means  334  for comparing the first and second quality levels, for ascertaining the first and second quality levels, are a first means  336  for ascertaining a first quality level and a second means  338  for ascertaining a second quality level, which for example derive the first and second quality levels from the mean square errors of the first error signal ê and the second error signal ê′, respectively, and supply them to the means  334  for comparing. 
   If the second means  328  for equalizing a received signal has a programmable coefficient filter, the error ê′, which is ascertained after the second means  330  for taking decisions, which follows the second means  328  for equalizing a received signal, is to be used for the coefficient adaptation of the second means  328  for equalizing the received signal. 
   As already mentioned, the transmission spectrum may deviate from the ideal of a flat baseband signal with a low-pass characteristic. As already mentioned above, this deviation is to be taken into account in the means  220 ,  320  for monitoring in  FIGS. 2 and 3  in the form of a reference spectrum. 
   As in the case of the exemplary embodiments described below, an improvement of the convergence behavior is generally achieved, however, by using a shaping filter, which compensates as far as possible for the difference from the ideal spectrum. In this way, a spectrum that is as flat as possible can be achieved for example in the transmission band. A new reference spectrum is obtained for the signal processing downstream of a shaping filter. There are various different ways of extending the structures according to the present invention by adding a shaping filter. However, it must always be ensured that only signals which relate to the same reference spectrum are combined, such as for example the correlation of a signal before a means for equalizing the error signal after a means for taking decisions. Therefore, it will generally be required to insert a number of shaping filters at different points of the signal processing. 
   The introduction of a shaping filter is expedient in particular in the case of signal processing parallel to the direct reception path, such as for example of the second means for equalizing a received signal or the adaptive equalizer shown in  FIG. 3 , since the received spectrum must not be “shaped” in comparison with the transmission spectrum. However, insertion of a shaping filter and a downstream shaping filter which has a filter function that is inverse to the filter function of the first shaping filter into the direct reception path is not advisable in practice. 
     FIG. 4  shows a third exemplary embodiment of a device according to the present invention. By contrast with the second exemplary embodiment, the means  420  for monitoring the received signal monitors optionally either the second equalized received signal, which is supplied by the second means  428  for equalizing the received signal, or the second estimate signal, which is supplied by the second means  430  for taking decisions, that is signals after the adaptive equalizer. 
   By contrast with the second exemplary embodiment of  FIG. 3 , the device for equalizing a received signal also comprises a first shaping filter  442 , which supplies a shaped received signal to the second means  428  for equalizing the received signal and to the means  418  for setting or automatically updating the equalizer setting, and a second shaping filter  444 , which has a filter function corresponding to the first shaping filter  442  and supplies a shaped second estimate signal to the means  432  for supplying a second error signal ê″. The second shaping filter  444 , which is arranged downstream of the second means  430  for taking decisions, ensures that the calculation of the second error ê″ takes place after the second means  430  for taking decisions with respect to the same reference to which the shaped received signal for the means  414  for setting the settable equalization also relates. The error ê″ ascertained in this way, or the second error signal ê″ ascertained in this way, is not identical to the ascertained error ê′ or the second error signal ê′ of  FIG. 3 . The error signal ê′ is obtained from ê″ by means of a shaping filter with a filter function which is inverse to the filter function of the second shaping filter  444 . The error ê″ can be used, however, in first approximation for the MMSE calculation. 
     FIG. 5  shows a fourth exemplary embodiment according to the present invention. By contrast with the third exemplary embodiment of  FIG. 4 , provided in the case of this exemplary embodiment are a first shaping filter  546 , which supplies a shaped received signal to the means  518  for setting the settable equalization, and a second shaping filter  548 , which has a filter function identical to the first shaping filter  546  and supplies a shaped second error signal ê′ to the means  520  for monitoring the received signal. 
   While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and systems of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.