METHOD AND APPARATUS FOR IMPLEMENTING A SELF COMPENSATING BAND STOP FILTER ON A DIGITAL DATA COMMUNICATIONS LINK

Abstract: The present patent publication discloses a method and apparatus for realizing a self-compensating bandstop filter on a digital communications channel. According to the method, the signal is subjected in the transmitter to bandstop filtering (H(z<-1>)) and precoding (K(z<-1>)) using the Tomlinson principle so that a common computing engine is used for both the bandstop filtering and the precoding operations.

Description: Method and apparatus for implementing a self compensating band stop filter on a digital data communications link. The invention relates to a method according to the preamble of claim 1 for realizing a self-compensating bandstop filter on a digital communications path. The invention also relates to an apparatus suitable for realizing a self-compensating bandstop filter on a digital communications path. On modem connections serving wideband communications, the signal spectrum extends to frequencies that can be as high as from 10 MHz to 30 MHz. At such high frequencies, a portion of the signal energy propagating over a copperwire connection emanates as RF interference to the environment. A portion of the energy emanated to the environment falls on frequency bands allocated to radio amateurs. This portion of the signal spectrum must be attenuated to prevent a modem connection from disturbing radio amateur activities. In practice, RFI attenuation can be accomplished either by allocating frequency bands by means of bandstop filters so that no signal is transmitted on the radio amateur bands or by filtering the transmitter signal of the modem connection so that the signal spectrum components falling on the radio amateur bands are attenuated to a sufficiently low power level (e.g., to meet the ETSI standard specifying an emanated power level of less than 80 dBm/Hz at frequency bands allocated to radio amateurs). The bandstop filters used herein may be realized using analog or digital techniques or both. The benefits of the filtering technique over the band allocation solution are: easy switch-on and switch-off of the filters and more relaxed allocation of transmission bands without being so much restricted by the established allocation of radio amateur bands. Disadvantages of the filtering approach include a more complicated structure of channel equalizers and the fact that the relative reduction in the data transmission capacity is higher than the relative reduction of bandwidth due to the filtration. In the prior art, two different basic techniques are available for equalizing the signal distortion caused by a bandstop filter placed in a modem transmitter. The distortion caused by the filter is equalized either in conjunction with compensation of the overall signal distortion of the transmission channel (including its bandstop filters) by means of separate equalizers that may be of the fixed or adaptive type and that further may be located in the transmitter (TX) or the receiver (RX) or partially in both. Another technique is to compensate for the distortion caused by the filters by means of equalizers specifically developed for equalization of filter distortion, whereby an essential complication in the design of equalizers intended for correcting other types of channel distortion is avoided. A shortcoming of the prior-art techniques is that the implementation of equipment may become very complicated, because the equalization of distortion caused by bandstop filters either require separate equalizers tailored for the filters or the distortion compensation capability of equalizers used for correction of channel distortion must be improved. The complexity of such implementations bears a plurality of shortcomings, whereby many problems are encountered, e.g., in regard to processing capacity and the amount of memory for storing program code in a processor-based system and, respectively, in an ASIC implementation, in regard to power consumption and the number of logic gates. It is an object of the present invention to provide an entirely novel type of method and apparatus capable of overcoming the drawbacks of the above-described prior-art techniques. The goal of the invention is achieved by way of realizing the digital bandstop filters of the transmitter (TX) so that the filter itself also performs as a precoder of the Tomlinson type, whereby an essential increase in the complexity of the channel equalizers can be avoided. The construction of equalizers becomes more complex only therein that the decision-feedback equalizer (DFE) must be designed capable of extended handling symbols. As the outgoing signal leaving the output of a self- compensating bandstop filter comprises extended symbols, both the linear equalizer (FFE) and the decision-feedback equalizer (DFE) can operate using the same tap coefficient values as those used in a situation not involving the above-mentioned type of bandstop filter. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 3. The invention offers significant benefits. Inasmuch the same computing engine performs both the filtering and precoding functions, the circuit design or processing program can be implemented in a manner simpler than that possible by means of prior-art techniques, whereby such benefits as lower power consumption are gained over conventional techniques or, alternatively, it is possible to use a simpler and cheaper signal processor. Lowered power consumption brings about such additional benefits as improved reliability and higher packaging density of the modem equipment. In the following, the invention is described with reference to exemplary embodiments elucidated in the appended drawings in which FIG. 1 shows a block diagram of a realization according to the invention for constructing a self-compensating bandstop filter, whereby the diagram makes it clear that the computation of both the bandstop filtration H(z _1 ) and precoding K(z _1 ) can be based on the same result of F(z _1 ); FIG. 2 shows a block diagram of the embodiment of FIG. 1 transformed into a realizable form, wherein the output signal level adjustment coefficient g can be left unnoted without compromising the general validity of the implementation; FIG. 3 shows the embodiment of FIG. 2 modified by a topology utilizing the fact that subtraction and adding are functions canceling each other; FIG. 4 shows a system according to the invention in a more generalized form; FIG. 5 shows a block diagram of a module in a conventional bandstop filter comprised of successive modules, the module realizing one pole and one zero in the transfer function of the bandstop filter; FIG. 6 shows a block diagram of a module in a conventional precoder comprised of successive modules, the module eliminating distortion by compensating for one pole and one zero in the transfer function of the bandstop filter causing the distortion; FIG. 7 shows a block diagram of a prior-art realization having a number modulo CN*2m summed at the input of the precoder so that the output value of the precoder is limited; and FIG. 8 shows a block diagram of one module in a conventional precoder comprised of successive modules but now modified so that it is possible to move the summing of the number modulo CN*2m illustrated in FIG. 7 to take place at the output of the last block N z . A self-compensating bandstop filter according to the invention can be realized so that the system shown in FIGS. 7 and 8 is adapted into block "Precoder" of FIG. 4. The mathematical analysis presented later in the text proves that the thus according to the invention realized system implements the functions of both a precoder and a bandstop filter. Abbreviations used in the text: ASIC Application-Specific Integrated Circuit CAP Carrierless Amplitude and Phase Modulation DFE Decision-Feedback Equalizer FFE Feed-Forward Equalizer FIR Finite-Impulse Response IIR Infinite-Impulse Response PAM Pulse- Amplitude Modulation QAM Quadrature- Amplitude Modulation RX Receiver TX Transmitter TML Tomlinson-Harashima precoder H(z _1 ) Bandstop filter transfer function, wherein z "1 is the intersymbol delay K(z _1 ) Precoder transfer function, wherein z "1 is the intersymbol delay. Accordingly, the invention relates to a method and apparatus capable of realizing a digital bandstop filter suitable for use in a transmitter so that no separate equalizer is needed and, simultaneously, no essential increase is caused in the complexity of the equalizers required for compensating for distortion caused by other elements of the channel, such as cables. A limitation to the use of the bandstop filter according to the invention is that the sampling rate of the filter must be equal to the symbol trans- mission rate and that the filter input signal must also be the equal to the symbol sequence to be transmitted, because the filter-precoder engine also carries out the precoding and filtration functions. The theoretical basis of the method will be evident to the reader from the discussion given below. The transfer function of a digital bandstop filter using the symbol transmission rate as its sampling rate can be presented in the z-plane as: wherein Zzj (complex number) is a zero i in the z-plane, z pl (complex number) is a pole i in the z-plane and g (real number) is a coefficient that gives the filter a desired gain. The discussion below is valid for both an FIR as well as an HR filter. For an FIR filter, Q(z _1 ) = 1, which means that z p ; = 0 for all values of i. Function H(z _1 ) can be written as: where cj is a complex number and M is the larger of N z -1 and N p -1. The latter form of the equation results from the fact that constant term is 1 in both of the polynomials P(z _I ) and Q(z _1 ). The latter form is denoted as: H(z- I ) = g - (l + z- 1 F(z- 1 )). (3) A bandstop filter having this kind of transfer function can be compensated by means of precoding that has a transfer function of the form: K(z- l ) = (4) l + z- l F(z ~l ) Herein, it is necessary to add to the precoder input a number modulo CN*2m so that the real and imaginary parts of the output signal do not exceed limits -m, +m. Number m is +1 (as the largest value in the symbol constellation) +1 and CN is a complex number, whose real and imaginary parts are integral numbers. It can be seen that the computation of both H(z _1 ) and K(z _1 ) can be carried out using the same result of F(z _1 ) as is evident from FIG. 1. The system shown in FIG. 1 can be transformed as shown in FIG. 2 into a realizable form (wherein coefficient g may be left unnoted without compromising the general validity of the implementation), wherein the system architecture is straightforward if the bandstop filter H(z _1 ) is such that it can be realized relatively easily in an expanded form 1 + z "1 F(z "1 ). If the bandstop filter is a multistage HR filter, it is possible that at least the computation of F(z _1 ) according to Eq. (2) becomes complicated to implement on silicon. In the following is described such a preferred embodiment of the invention that realizes the computing engine only for the precoder but not for the bandstop filter. The output signal of the system comprises extended symbols whose transmission spectrum includes the desired band stops. Extended symbols are input signal symbols to which are added modulo numbers defined in the precoder. The system shown in FIG. 2 can be transformed into the form shown in FIG. 3 (inasmuch subtraction and addition are mutually canceling functions): As can be seen from FIG. 3, it is sufficient to compute only the precoder functions and to generate the required modulo numbers that are then summed with the system input signal. The system can be presented in a more generalized form shown in FIG. 4. From FIG. 4 is evident that the implementation architecture of the precoding function can be selected without constraints. Hence, the system is not difficult to implement even if the realization of the computation of F(z _1 ) might be tricky. The discussion below concerns the implementation of a system according to the invention when the bandstop filter is a multistage IIR filter. As an example, we can assume an 1TR filter having an equal number of nodes and zeros, and the zeros coincide on the same frequencies (z pl = rjZ z i and N p = N z , where r; is a real number and 0 < ri < 1). This type of filter is suited for implementing a plurality of different bandstop filters in a sufficiently flexible manner. The filter may be realized using an architecture based on a succession of modules, one of which is shown in FIG. 5. The transfer function of the precoder can be realized using modules similar to those described above but now having the roles of each zero and pole interchanged as shown in FIG. 6. A number modulo CN*2m must be added to the precoder input so that the output signal of the precoder is limited as shown in FIG. 7. While the addition of number modulo CN*2m illustrated in FIG. 7 may be transferred to take place at the output of the last block N z , this requires that the addition of the number modulo CN*2m must also be performed in each subblock, e.g., in fashion shown in FIG. 8. As an entity, a self-compensating bandstop filter is such as shown in FIG. 4, wherein each precoder block is comprised of a succession of subblocks of the type illustrated in FIG. 8. The number modulo CN is selected such that the real and imaginary parts of the output signal of the last subblock do not exceed the limits -m, +m where m is the largest value of the symbol constellation +1.

Claims: What is claimed is: 1. A method for realizing a self-compensating bandstop filter on a digital communications channel, the method comprising performing in the transmitter the steps of - bandstop filtering the signal by a digital filter whose transfer function can be written as H(z -1 ) = g-(l+z '1 F(z "1 )), - precoding by means of a fixed precoder realized according to the Tomlinson principle, whereby the transfer function of the precoder can be written as K(z _1 ) = 1/(1+ z _1 F(z _1 )), complemented with a modulo number operation, whereby - the sampling rate of the filtration (H(z -1 )) and precoding (K(z -1 )) operations must be equal to the symbol transmission rate and the input signal equal to the transmitted symbol sequence, characterized in that a common computing engine is used for realizing the signal processing function (F(z -1 )) for both the bandstop filter (H(z -1 )) and the precoder (K z "1 )). 2. A method according to claim 1, characterized in that the output signal, whose transmission spectrum includes the desired band stops, is generated by way of adding to the symbol sequence serving as the input signal the modulo numbers which are determined in the precoder according the Tomlinson principle. 3. An apparatus for realizing a self-compensating bandstop filter on a digital communications channel, the apparatus comprising in the transmitter - means for bandstop filtering the signal by a digital filter whose transfer function can be written as H(z _1 ) = g-(l+z "1 F(z "1 )), - means for precoding the signal by means of a fixed precoder realized according to the Tomlinson principle, whereby the transfer function of the precoder can be written as K(z _1 ) = 1/(1+ z "1 F(z "1 )), complemented with a modulo number operation, andp i] - the sampling rate of the filtration (H(z -1 )) and precoding (K(z -1 )) operations must be equal to the symbol transmission rate and the input signal must be equal to the transmitted symbol sequence, characterized in that a common computing engine is used for realizing the signal processing function (F(z -1 )) for both the bandstop filter (H(z -1 )) and the precoder (K z- 1 )). 4. An apparatus according to claim 3, characterized in that the apparatus includes means for generating an output signal, whose transmission spectrum includes the desired band stops, by way of adding to the symbol sequence serving as the input signal the modulo numbers which are determined in the precoder according the Tomlinson principle.

Inventors: VAEAENAENEN JANNE, VAEAENAENEN, JANNE
Applicants: TELLABS OY, VAEAENAENEN JANNE, VAEAENAENEN, JANNE
Application year: 2001
Main classification: H04L27
IPC codes: H04L25