CA2178664A1 - Signal conditioner with symbol addressed lookup table based transversal filters - Google Patents
Signal conditioner with symbol addressed lookup table based transversal filtersInfo
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- CA2178664A1 CA2178664A1 CA 2178664 CA2178664A CA2178664A1 CA 2178664 A1 CA2178664 A1 CA 2178664A1 CA 2178664 CA2178664 CA 2178664 CA 2178664 A CA2178664 A CA 2178664A CA 2178664 A1 CA2178664 A1 CA 2178664A1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
- H03H17/06—Non-recursive filters
- H03H17/0607—Non-recursive filters comprising a ROM addressed by the input data signals
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
- H03H17/0219—Compensation of undesirable effects, e.g. quantisation noise, overflow
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
- H03H17/0248—Filters characterised by a particular frequency response or filtering method
- H03H17/0264—Filter sets with mutual related characteristics
- H03H17/027—Complementary filters; Phase complementary filters
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- Engineering & Computer Science (AREA)
- Computer Hardware Design (AREA)
- Mathematical Physics (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
Abstract
A signal conditioner comprising several embodiments of transversal (FIR) filters that utilize symbol addressed multiplier lookup tables. In particular, five embodiments in accordance with the principles of the present invention are described.
The present invention generates a predistorted waveform within the transversal filter that provides for compensation for the effects of nonlinear amplifiers and intersymbol interference. The present invention may be employed to digitally generate an arbitrary baseband waveform that represents a limited alphabet of symbols. The present invention dramatically simplifies the implementation of FIR filters where the application of the FIR filter is to provide a transversal filter function for waveforms that represent symbols. The degree of simplification is related to the number of bits that define the size of the symbol alphabet as compared to the number of bits required to adequately represent a digital sample of the waveform. Typically 2, 3, or 4 bits represent the alphabet of communication symbols, whereas waveforms require as a minimum 8 bitsof quantization. A second aspect of the present invention comprises combining the function of generating arbitrary waveforms with the symbol addressed lookup tables that are part of the FIR filter. The waveform is precompensated to minimize the effect of intersymbol interference and is prewarped to compensate for transmitter nonlinearities.
The present invention generates a predistorted waveform within the transversal filter that provides for compensation for the effects of nonlinear amplifiers and intersymbol interference. The present invention may be employed to digitally generate an arbitrary baseband waveform that represents a limited alphabet of symbols. The present invention dramatically simplifies the implementation of FIR filters where the application of the FIR filter is to provide a transversal filter function for waveforms that represent symbols. The degree of simplification is related to the number of bits that define the size of the symbol alphabet as compared to the number of bits required to adequately represent a digital sample of the waveform. Typically 2, 3, or 4 bits represent the alphabet of communication symbols, whereas waveforms require as a minimum 8 bitsof quantization. A second aspect of the present invention comprises combining the function of generating arbitrary waveforms with the symbol addressed lookup tables that are part of the FIR filter. The waveform is precompensated to minimize the effect of intersymbol interference and is prewarped to compensate for transmitter nonlinearities.
Description
SIGNAL CONDITIONER WITH SYMBOL ADDRESSED
LOOKUP TABLE BASED TRANSVERSAL FILTERS
BACKGROUND
The present invention generally relates to signal conditioners, and more particu-larly, to transversal (FIR) filters that use symbol addressed lookup tables.
Telecor""""-ication power ll;~ ers, such as traveling wave tube amplifiers.
exhibit nonlinear behavior in their amplitude and phase transfer function. These non-5 linear characteristics result in distorted transmit waveforms that degrade the perfor-mance of telecommunication channels. When applying digital mod~ ting waveforms to a ~ r ~mplifi~.r channel, additional linear distortions are experienced due to intersymbol in~ relence (ISI). ISI is generated as a result of non-ideal phase and frequency chald~elislics of the transmitter chain. Some compensation for these 10 distortions can be achieved by predistorting the signal prior to its application to a power amplifier. Predistortion is conceptually performed in two steps. The first step is to predistort the digital modulation in amplitude and phase so that the resultant signal at the output of the power amplifier is undistorted. The second step is to pre-equalize the mo(lnl:-ting waveform so that the effect of ISI is minimi7erl In the absence of nonlin-15 earities, the act of pre equalization in the tr~ncmitt~r is equivalent to performing equal-ization of the tr~n.cmitt~l waveform in the receiver.
Prior art techniques for generating arbitrary digital waveforms that represent asymbol include the use of lookup tables. The symbol is used to address a lookup table that is programmed to output a representative digital waveform. Generally this wave-20 form has two quadrature components. Compensation for nonlinear distortion isaccomplished by precomputing the requisite corrections and progr~mming the lookup tables with the predistorted w~v~fc lllls. Prior art techniques that compensate for ISI
make use of a Ll~u1s~;lsal filter ar~,hitt-ctme to distort the w~v~follll such that intersym-bol i~ Çele~lce at the output of the Ll~ ' iS ~ ;7.PA A finite impulse response (~'1~) filter with taps spaced by a delay of one symbol ~rOlllls such a function. Tne 5 ~ filter forms the weighted sum of the waveforms which represent a finite number of past symbols and future symbols. The w~i~htin~ factors are prede~e~ cl such thatwhen this sum is added to the waveform represçntin~ the current symbol, the factors which cause s~lbseql~P-nt ISI are effectively c~nrçllP,d In a digital implel l l~ t;on of such a ~ filter pre-eq~u~li7ert the w~v~follll is commonly multiplied at each tap by 10 means of a mllltirli~r circuit and a register CO~ g the w~ htin~ factor. An alterna-tive impl.-mlo,nt~ion is by application of digital lookup tables that are addressed by the digital word represçnting the waveform at each tap. The 100KUP tables are pro~ lled to output a weighted represçnt~tion of the input address. Quadrature waveforrns require a FIR filter for each of the two quadrature components. An additional pair of ~'lK
filters may be required to collll~e~c~tto for any "cross" effects.
Conventional techniques have not provided for a transversal (~) filter that utilizes symbol addressed lookup tables. U.S. Patent No. 5,113,414 issued to Kararn et al. provides an ex,~ nt description of the prior art for predistorting waveforrns in digital tr~ncmicc,ion system systems. However, the Karam et al. technique for CG",l ei-c~tion is quite different and more complicated than the technique of the present invention. Karam makes use of a Ll~lsl---L flter (and does not in~lic~tç if it is digital or analog) that applies an oversarnpled waveform to a lookup table which in tum generates a correction that is applied to a multiplier.
It is therefore an objective of the present invention to provide for a signal con-ditioner that implements a transversal filter and uses symbol addressed lookup tables. It is also an objective of the present invention to provide for a transmit conditioner that co,.... ...l.el-c~tPs for the effects of nonlinear amplifiers and intersymbol interference.
SUMMARY OF THE INVENTION
The present invention is a signal conditioner that comprises several embodi-ments of transversal (~) filters that utilize symbol addressed multiplier lookup tables.
In particular, five embodiments in accordance with the principles of the present inven-tion are described. The present invention generates a predistorted waveform within the transversal filter. The present invention also provides for an extension of this idea, with no additional complexity, that compensates for the effects of nonlinear amplifiers and intersymbol interference. The present invention may be employed to digitallygenerate any arbitrary b:lceb~nd waveforrn that represents a limited alphabet of symbols. The ~avèfollll is precnm~.nc~t~.~l to 111;~illl;7~ the effect of hlLelaylllbol intelrèleilce (ISI) and ~lcw~ed to comr~n.c~ts for tl;~ nonlin~riti~s.
A first embodiment colll~lises a LldllsvGl~.al filter having a symbol addressed multiplier lookup table. In the first embo-iim.ont the L.~.vcl~.al filter processes input 5 symbols to generate b~ceb~n(i output wavèfolllls comrricing qll~nti7~d signal wave-forms that are lc~lcsellLdLivè of the input symbols. The ~alls~.al filter comprises an input for receiving binary codes lcplcsç~l;ng input symbols, and a plurality of sêrially coupled one symbol delay devices for sequentially delaying the input symbols. A
plurality of symbol addressed lookup tables are respectively coupled to the input and 10 one symbol delay devices for receiving the input symbols and delayed versions thereof and for providing respective output signals that cc,llesl)ond to scalêd ~mplitn(le values of the input symbols. The output signals comprise a predistorted waveform that Co,..penc~les for the effects of hltcl~ylllbol h~te.rèlcilce and ~l;.n.~ r~ nonlin~q~riti~c. A
plurality of s~lmming devices are individually coupled to pre~1~t~rmin~ocl pairs of the 15 plurality of symbol addressed lookup tables that process the respective output signals to geneldte the b~ceb~n-l output wavcrolllls that are 1~; lesellLaLivè of the input symbols.
A second embodiment provides for combined through and crosss multiplier lookup tables in a single lookup table used for procescing respective in-phase and quadrature signals. In the second embo~lim-ont, the transversal filter includes both in-20 phase and quadrature equalizers. In the second embodiment, the transversal filtercomprises an input for receiving binary codes representing input symbols, and a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols. First and second symbol addressed lookup tables are respectively coupled to the input and to the plurality of serially coupled one symbol delay devices 25 fomcspe~;Lively receiving the input symbols and delayed versions thereof and for providing respective in-phase and quadrature output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol inLelrèlcnce and tr~n~mitt~r nonlinearities.
A plurality of sets of ~u"""il-g devices are respectively coupled to pre~et~rrnined pairs 30 of the plurality of symbol addressed lookup tables that process the respective in-phase and quadrature output signals to generate in-phase and quadrature baseband output waveforrns that are representative of the input symbols.
A third embodiment provides for common symbol delay elements for delaying both in-phase and quadrature signals prior to the multiplier addressed lookup tables. In 35 the third embodiment, the transversal filter comprises an input for receiving binary codes representing input symbols, a plurality of common one symbol delay devices for delaying both the in-phase and quadrature signals that comprise the input symbols. A
plurality of symbol addressed lookup table are applo~fiately coupled to the input and to outputs of the respective one symbol delay devices for receiving the input symbols and delayed versions thereof and for providing a plurality of output signals that correspond to scaled ~mp1ih1de values of the input symbols. The output signals comprise a predis-5 torted waveform that co,llpGnsates for the effects of intersymbol intGlfGlcllce and . non1in~riti~s A plurality of sets of ~,,,.,.~.;I~g devices are coupled to the lookup tables for ~.. ing the lG~e~;LivG in-phase and quadrature output signal to provide in-phase and quadrature waveform output signals that comprises baseband output waveforms that are representative of the input symbols.
A fourth embodiment provides for a transversal filter that colllbhles the çlem~o-ntc of each of the previous three filters into one a single large symbol addressed ml1ltir1ier lookup table. This large symbol addressed multiplier lookup table has a size that is substantially the same as the combined sizes of the individual lookup tables used in the prior embo-lim~ntc. In the fourth embodiment, the ~ vGl~al filter comprises an input 15 for receiving binary codes representin~ input symbols, and the single symbol addressed lookup table coupled to the input for receiving the input symbols and for providing a plurality of output signals that co.lG~ond to scaled amplitude values of the input symbols. The output signals comprise a predistorted waveform that compenc~t~c for the effects of intersymbol interference and tr:lncmitter non1inP~riti~oS A first one symbol 20 delay device is coupled to the delay device for delaying a first output signal therefrom.
A first s11mming device is coupled to the lookup table and to the first one symbol delay device for s11mming the first output signal and a second output signal from the lookup table to provide a summPc~ output signal. A second one symbol delay device for delaying the summ~d output signal from the first ~-...."-i,-g device. A second snmming 25 device is coupled to the lookup table and to the second one symbol delay device for s11mming the second output signal and a third output signal from the lookup table to provide a second s11mm~cl output signal that comprises baseband output waveforms that are representative of the input symbols.
The above-s11mm:lri7~ four transversal filter embodiments provide for signifi-30 cant simplifications in ~IK filter applications such as in pre-equ~1i7ing intersymbol interference in tr~n.cmi~ter waveforrns.
A fifth embodiment incorporates prewarping of the input symbols without any change in hardware complexity. In the fifth embodiment, the transversal filter comprises an input for receiving binary codes representing input symbols, and a 35 symbol addressed lookup table based prewarper and pre-equ~1i7tor coupled to the input for receiving the input symbols and for providing output signals that correspond to prewarped and pre-equalized scaled amplitude values of the input symbols. The output signals co~ .. ;ce a predistorted and plcvv~ed wavcfollll that co~"pc~ tes for the effects of illlcl~ylllbol illlclçclcnce and l~ lll;llr~ nonlin~ itiPS
The present invention rlr~rn~tic~lly ~imrlifies the implelll~ tion of FIR filters where the aprli~til~n of the FIR filter is to provide a ll~vcl~al filter function for 5 waveforms that lc~ sellt symbols. The degree of simplification is related to the llulllbcr of bits that define the size of the symbol alphabet as colll~a~cd to the llulllbcr of bits required to adequately rc~l~ selll a digital sample of the waveform. Typically 2,3, or 4 bits lcplcsclll the alphabet of c~ " " - -- -~ tion symbols, whereas the waveforms require as a ",;.~;,..,.,.. 8 bits of qu~nti7~tion. One aspect of the present invention is to use symbol addressed lookup tables rather than digital sample addressed lookup tables in imrl~ ,t;"g ~. filter functions. A second aspect of the present invention comprises combing of the function of gen~ lg ~ubi~ waveforms with the symbol addressed lookup tables that are part of the FIR filter function.
The present invention thus provides for a much simper and efficient implemen-tation of an ;~ waveform gellel~tor and FIR filter for applications where the input waveform is defined by a symbol alphabet that may be specified with fewer bits then an equivalent q~l~n~i7,o~ waveform.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken inconjunction with the accompanying drawings, wherein like reference numerals design~te like structural elements, and in which:
Fig. l a illustrates a conventional three tap filter using multipliers to scale the delayed input waveforms;
Fig. Ib illustrates a conventional three tap filter using multiplier lookup tables to scale the delayed input waveforms;
Fig. 2 illustrates a first embodiment of a three tap filter using symbol addressed multiplier lookup tables in accordance with the principles of the present invention;
Fig. 3 illustrates a conventional equalizer for quadrature waveforms;
Fig. 4 illustrates a second embodiment of an equalizer for quadrature waveforms in accordance with the principles of the present invention;
Fig. S illustrates a third embodiment of a three tap filter using symbol addressed multiplier lookup tables in accordance with the principles of the present invention;
Fig. 6 illustrates a fourth embodiment of a three tap filter using a common symbol addressed multiplier lookup table in accordance with the principles of the present invention:
Fig. 7 illustrates a conventional technique for pre-warping and pre-equ~li7ing adigital waveform; and Fig. 8 illu~Llates a first embodiment of a transversal filter for pre-warping and pre-equalizing a digital waveform in accordance with the principles of the present invention.
DETAILED DESCRIPTION
By way of introduction, the present invention comprice,c a finite impulse response (FDR) filter for use in digital telecollllll.l.,ication tr~ncmiccion systems. By itself, the present FIR filter results in simplified filter design and better ~lrollllance than conventional FIR filters. Pre-equalization of the transmit w~v~follll is one possi-ble application of the FIR filter. The present invention may also provide a nonlinear plCW~i~lg function without adding to the complexity of the filter design. A single processing function in accordance with the present invention pelrolllls waveformpredistortion so that the tr~ncmitted waveform co~ ellsates for the nonlinear effects of a power :lmplifi~r and so that the illL~l~ylllbol inL~lrerence caused by a non-ideal tr~ncmitt~r chain is reduced.
Referring to Fig. l a it illustrates a conventional three tap FIR filter 10 using multipliers 12 for scaling the input waveform and its delayed replicas. The conven-tional FIR filter 10 is comprised of a tapped delay line 11, or discrete delay elements 1 la, l lb, means 14 for scaling the signal at each tap of the tapped delay line 11 by multiplication with a scaling coefficient, and a plurality of summing devices 13 to add the result of the scaled tap signals. The signals are lc~lcsel,l~d by digital waveforms such as baseband signals. Sampling is usually performed at the symbol rate and the delay elements 1 la, 1 lb are usually digital registers that are clocked at the symbol rate.
Scaling coefficients (C) may reside in digital registers, or if they are not to be modified.
may be hardwired into the multiplier structure.
A well known ~1tPrn~tive to using multipliers 15 is to use digital lookup tables12. Fig. lb illustrates a conventional three tap filter 10a using multiplier lookup tables 12a, 12b, 12c to scale the delayed input waveforms. The operation of the filter 10b is similar to that of filter 10a, except that the scaled tap signals are derived from the multiplier lookup tables 12 prior to sl-mming Each digital lookup table 12 may be comprised of a prograrnrnable memory device (such as a RAM or PROM) or a fixed memory (ROM). The signal that is to be scaled is used as an input address to thelookup table 12. Each address position the lookup table 12 contains an applopliately scaled value of the input signal. Such a lookup table scheme is effective if the precision of the input signal is limited, since the size of the lookup table 12 increases exponen-tially with the number of signal qll~nti7~tinn levels. For example, with an 8 bit input and N bit output, the table is of size 256xN bits. On the other hand, for a 16 bit input and N bit output the table is of si_e 65,536xN bits.
One aspect of the present invention is to recognize that when filtPrine digital 5 modlll~tine waveforms, it is not nPcPs~ry to represent the input wavefo~ in a FIR
filter as a digital ~mplit~ P, of the desired waveform. It is suffi~ient to represent the input waveform as a symbol value. A FIR filter 20 embodying this concept is shown in Fig. 2. More specific~lly~ and by way of ex~rnrkP, Fig. 2 illustrates a three tap filter 20 using symbol addressed mllltirliPr lookup tables 12 in accol~lce with the principles of 10 the present invention. Inputs to the filter 20 are binary codes representin~ symbols, and outputs are qu~nti7Pd signal wavefolllls. The selection of a symbol code is of no conceql~enre It is also to be understood that the illustration of the present invention using a three tap filter 20 is by way of ç~mple only. Any number of taps may be utilized in the filter 20 of the present invention.
For n-ary cien~lline, the llull~ber of bits that define a symbol are of size n. For example, in QPSK ci~n~line, only two bits represent a symbol. At each filter tap, the lookup table 12 has size nxN. At each address position, the lookup table 12 contains the ap~lopliately scaled ~mplihl~l~p value of the input symbol. The primary benefit of this scheme is to dr~m~tit~lly reduce the size of the lookup tables 12. A secondary benefit is increased ~ l", Irlil' precision since the lookup table 12 gent;l~les an exact (within the qu~nti7:ltion limits of the number of output bits) scaled amplitude value of the symbol rather then the scaled value of the qll~nti7Pd symbol. Another benefit is that the delay ekPmPntc 1 la, 1 lb are of size n bits (for n-ary cign~lling) rather then the number of bits required to adequately represent the signal waveform amplitude (usually at least 8 bits).
In a practical application for n-ary orthogonal waveforms, the mo~ ting signal is complex in the sense that it has in-phase and quadrature components. Consider a FIR filter implemPnt~tion of a conventional pre-equalizer 30. Fig. 3 illustrates a conventional pre-equalizer 30 for quadrature waveforms that comprises a conventional transversal filter. The pre-equalizer 30 includes four FIR filters 31a, 31b, 32a, 32b comprising a pair of pre-equalizers 30a that generate in-phase signals and a pair of pre-equalizers 30b that generate quadrature signals. In particular, to generate the pre-equalized in-phase waveform the classic scheme is to pre-equalize the in-phase wave-form with "through" coefficients, pre-equalize the quadrature waveform with "cross"
coefficients and to add the results. Except for a change in sign of the cross coefficients an analogous scheme is used to ~,ent;l~t~ the pre-equalized quadrature waveform.
The previously discussed lookup table implemrnt~tion may also be applied to the just-described pre-equalizer arc~itect~lre but with another dramatic ~implifir~tion as will be ~esrrihe-l with reference to Fig. 4. Fig. 4 illustrates a simplified pre-equalizer 40 for in-phase and quadrature waveforms in accordance with the l" ;.~ lPs of the 5 present invention. Since input signals (symbols) are no longer represented by a digital representation of a symbol waveform, the through and cross equalizers 31a, 31b, 32a, 32b of Fig. 3 may be integrated into two lookup tables and thus a first equivalent equalizer 41a is provided for ~e,1eldti,lg the pre-equalized in-phase waveforms, and a second equ~li7~r 41b provides for gen.,ld~ g the pre-equalized quadrature waveforrns.
Each of the equalizers 41a, 41b compricing the pre-equalizer 40 may be constructed in the manner described with reference to Fig. 2 above, for example. The lookup tables in the Ica~e~;~ive equalizers 41a, 41b of Fig. 4 comprise signals that are not the same. The in-phase colllponc;lll of the product of the complex waveform and the complex scaling coeffirient is stored in the first equalizer 41a. The quadrature co",~onent of the product of the complex wavefollll and the complex scaling coefficient is stored in the second equalizer 41b.
Fig. S illu~l-dtes this archit~ctllre in more detail and also in~lir~tes anothersimplification provided by the present invention. The delay elements 11 for the two pre-equ~li7ers 41a, 41b contain the same signal (i.e. symbol) and therefore they may be combined in the manner shown in Fig. 5 to produce the illustrated transversal filter 50.
The transversal filter 50 is comprised of the tapped delay line 11, or discrete delay elements 1 la, 1 lb, a plurality of lookup tables 12 that are adapted to output scaled in-phase and quadrature output signals, and two sets of ~ g devices 13a, 13b to addthe result of the respective scaled in-phase and quadrature output signals.
Another variation of the present invention which may be advantageous for some implementations is shown in Fig. 6, and comprises a pre-equalizer 60. This architec-ture represents an alternative to the equalizer 20 of Fig. 2, or each of the equalizers 41 a, 41b shown in Fig. 4. The ~ g devices 13 (adders) are separated by one symbol delay elements 11. A single lookup table 12 is employed in the pre-equalizer 60 of Fig.
6. However, fr~gmrnt~tion into several tables may be n--cesc~ry due to the largenumber of bits. One advantage of the architecture for the pre-equalizer 60 is that even with fr:lgmrnt~tion, a common address decoder may be utilized to generate addresses in the lookup table 12. This has advantages in high speed applications.
If one of the pre-equalizers 20, 40, 50, 60 is to be used in conjunction with a nonlinear distortion compen~ting modulator, for example, then another dramatic improvement may be realized. As previously in~lir:lt~, the nonlinear amplitude and phase behavior of a power amplifier may be negated by warping the signal prior to applying it to the power ~mplifi~r. Fig. 7 illustrates a conventional means for ~.cw~-ing and pre-eql~li7in~ 70 co,-,l" ;cil~ a symbol ~lCWal~r 12 (lookup table 12) and a serially couplPd pre-eq~ i7lor 40. The warping may be implem~nt~d using the lookup table 12 and then the warped digital w~vefi~ l value is pre-eqll~li7~-d in the pre-equal-S izer 40 to produce a ~cw~ ed and pre-equalized w~vcfo --- output from the filter 70.
However, and in accoidance with the present invention, if the mo~ tin~ signal is in digital form, then this warping may be implem~nt.ocl in the lookup table 12 using a symbol value as an input thereto and a warped digital w~vefo~l value as an output thereof. More speçifi~lly, and with reference to Fig. 8, it illustrates a lookup-table-10 based plcwalllcl pre-equalizer 80 that provides for pre-warping and pre-eq~l~li7in~ of a digital w~vcfc,.." (input symbol) in acco,~,ce with the ~ iplfs of the present inven-tion. The p.cw~g operation conventionally pc-ro----ed in the pre-equalizer 40 isintegrated into the lookup table 12 with no additional addition in complexity. That is, the stored values in the lookup table 12 are the products of filter coefficients and the 15 l~r~w~ed symbol amplitudes. The ~ cl pre-equalizer 80 provides a universal means for pre-warping the input symbol in addition to pre-eqll~li7in~ the input symbol.
This scheme not only elimin~t~s a lookup table 12, but also reduces the ~ccoçi~tetl qll~nti7~tion errors. Furthermore, the equalizer of Fig. 8 is programmable. The scaling coefficiçntc are prograrnmable, and the waveforms are also indepen(ltontly program-20 mable. Such pro~,l;~llll,ill~ is generally understood in the art and will not be furtherdescribed herein.
The previous applications of the present invention were related to tr~ncmitt~r functions. Some receiver designs that utilize decision fee~b:l~k techniques may also benefit from the present invention. For instance, there is a well known technique 25 known as a decision feedback equalizer. A conventional decision feedb~ck equalizer has two components similar to the pre-equalizer architecture shown in Fig. 7. The first component equalizes "future" symbols in a classic FIR filter architecture. The second component utilizes the waveform gen~la~d by detected past symbols as an input to a ~1~ filter. The present invention may be applied to simplify the second component 30 which results in the embodiment of a decision feedback equalizer similar to the architec-ture shown in Fig. 8.
Thus there has been described new and improved signal conditioners employing transversal (FIR) filters that use symbol addressed lookup tables. It is to be understood that the above-described embodiments are merely illustrative of some of the many35 specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.
LOOKUP TABLE BASED TRANSVERSAL FILTERS
BACKGROUND
The present invention generally relates to signal conditioners, and more particu-larly, to transversal (FIR) filters that use symbol addressed lookup tables.
Telecor""""-ication power ll;~ ers, such as traveling wave tube amplifiers.
exhibit nonlinear behavior in their amplitude and phase transfer function. These non-5 linear characteristics result in distorted transmit waveforms that degrade the perfor-mance of telecommunication channels. When applying digital mod~ ting waveforms to a ~ r ~mplifi~.r channel, additional linear distortions are experienced due to intersymbol in~ relence (ISI). ISI is generated as a result of non-ideal phase and frequency chald~elislics of the transmitter chain. Some compensation for these 10 distortions can be achieved by predistorting the signal prior to its application to a power amplifier. Predistortion is conceptually performed in two steps. The first step is to predistort the digital modulation in amplitude and phase so that the resultant signal at the output of the power amplifier is undistorted. The second step is to pre-equalize the mo(lnl:-ting waveform so that the effect of ISI is minimi7erl In the absence of nonlin-15 earities, the act of pre equalization in the tr~ncmitt~r is equivalent to performing equal-ization of the tr~n.cmitt~l waveform in the receiver.
Prior art techniques for generating arbitrary digital waveforms that represent asymbol include the use of lookup tables. The symbol is used to address a lookup table that is programmed to output a representative digital waveform. Generally this wave-20 form has two quadrature components. Compensation for nonlinear distortion isaccomplished by precomputing the requisite corrections and progr~mming the lookup tables with the predistorted w~v~fc lllls. Prior art techniques that compensate for ISI
make use of a Ll~u1s~;lsal filter ar~,hitt-ctme to distort the w~v~follll such that intersym-bol i~ Çele~lce at the output of the Ll~ ' iS ~ ;7.PA A finite impulse response (~'1~) filter with taps spaced by a delay of one symbol ~rOlllls such a function. Tne 5 ~ filter forms the weighted sum of the waveforms which represent a finite number of past symbols and future symbols. The w~i~htin~ factors are prede~e~ cl such thatwhen this sum is added to the waveform represçntin~ the current symbol, the factors which cause s~lbseql~P-nt ISI are effectively c~nrçllP,d In a digital implel l l~ t;on of such a ~ filter pre-eq~u~li7ert the w~v~follll is commonly multiplied at each tap by 10 means of a mllltirli~r circuit and a register CO~ g the w~ htin~ factor. An alterna-tive impl.-mlo,nt~ion is by application of digital lookup tables that are addressed by the digital word represçnting the waveform at each tap. The 100KUP tables are pro~ lled to output a weighted represçnt~tion of the input address. Quadrature waveforrns require a FIR filter for each of the two quadrature components. An additional pair of ~'lK
filters may be required to collll~e~c~tto for any "cross" effects.
Conventional techniques have not provided for a transversal (~) filter that utilizes symbol addressed lookup tables. U.S. Patent No. 5,113,414 issued to Kararn et al. provides an ex,~ nt description of the prior art for predistorting waveforrns in digital tr~ncmicc,ion system systems. However, the Karam et al. technique for CG",l ei-c~tion is quite different and more complicated than the technique of the present invention. Karam makes use of a Ll~lsl---L flter (and does not in~lic~tç if it is digital or analog) that applies an oversarnpled waveform to a lookup table which in tum generates a correction that is applied to a multiplier.
It is therefore an objective of the present invention to provide for a signal con-ditioner that implements a transversal filter and uses symbol addressed lookup tables. It is also an objective of the present invention to provide for a transmit conditioner that co,.... ...l.el-c~tPs for the effects of nonlinear amplifiers and intersymbol interference.
SUMMARY OF THE INVENTION
The present invention is a signal conditioner that comprises several embodi-ments of transversal (~) filters that utilize symbol addressed multiplier lookup tables.
In particular, five embodiments in accordance with the principles of the present inven-tion are described. The present invention generates a predistorted waveform within the transversal filter. The present invention also provides for an extension of this idea, with no additional complexity, that compensates for the effects of nonlinear amplifiers and intersymbol interference. The present invention may be employed to digitallygenerate any arbitrary b:lceb~nd waveforrn that represents a limited alphabet of symbols. The ~avèfollll is precnm~.nc~t~.~l to 111;~illl;7~ the effect of hlLelaylllbol intelrèleilce (ISI) and ~lcw~ed to comr~n.c~ts for tl;~ nonlin~riti~s.
A first embodiment colll~lises a LldllsvGl~.al filter having a symbol addressed multiplier lookup table. In the first embo-iim.ont the L.~.vcl~.al filter processes input 5 symbols to generate b~ceb~n(i output wavèfolllls comrricing qll~nti7~d signal wave-forms that are lc~lcsellLdLivè of the input symbols. The ~alls~.al filter comprises an input for receiving binary codes lcplcsç~l;ng input symbols, and a plurality of sêrially coupled one symbol delay devices for sequentially delaying the input symbols. A
plurality of symbol addressed lookup tables are respectively coupled to the input and 10 one symbol delay devices for receiving the input symbols and delayed versions thereof and for providing respective output signals that cc,llesl)ond to scalêd ~mplitn(le values of the input symbols. The output signals comprise a predistorted waveform that Co,..penc~les for the effects of hltcl~ylllbol h~te.rèlcilce and ~l;.n.~ r~ nonlin~q~riti~c. A
plurality of s~lmming devices are individually coupled to pre~1~t~rmin~ocl pairs of the 15 plurality of symbol addressed lookup tables that process the respective output signals to geneldte the b~ceb~n-l output wavcrolllls that are 1~; lesellLaLivè of the input symbols.
A second embodiment provides for combined through and crosss multiplier lookup tables in a single lookup table used for procescing respective in-phase and quadrature signals. In the second embo~lim-ont, the transversal filter includes both in-20 phase and quadrature equalizers. In the second embodiment, the transversal filtercomprises an input for receiving binary codes representing input symbols, and a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols. First and second symbol addressed lookup tables are respectively coupled to the input and to the plurality of serially coupled one symbol delay devices 25 fomcspe~;Lively receiving the input symbols and delayed versions thereof and for providing respective in-phase and quadrature output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol inLelrèlcnce and tr~n~mitt~r nonlinearities.
A plurality of sets of ~u"""il-g devices are respectively coupled to pre~et~rrnined pairs 30 of the plurality of symbol addressed lookup tables that process the respective in-phase and quadrature output signals to generate in-phase and quadrature baseband output waveforrns that are representative of the input symbols.
A third embodiment provides for common symbol delay elements for delaying both in-phase and quadrature signals prior to the multiplier addressed lookup tables. In 35 the third embodiment, the transversal filter comprises an input for receiving binary codes representing input symbols, a plurality of common one symbol delay devices for delaying both the in-phase and quadrature signals that comprise the input symbols. A
plurality of symbol addressed lookup table are applo~fiately coupled to the input and to outputs of the respective one symbol delay devices for receiving the input symbols and delayed versions thereof and for providing a plurality of output signals that correspond to scaled ~mp1ih1de values of the input symbols. The output signals comprise a predis-5 torted waveform that co,llpGnsates for the effects of intersymbol intGlfGlcllce and . non1in~riti~s A plurality of sets of ~,,,.,.~.;I~g devices are coupled to the lookup tables for ~.. ing the lG~e~;LivG in-phase and quadrature output signal to provide in-phase and quadrature waveform output signals that comprises baseband output waveforms that are representative of the input symbols.
A fourth embodiment provides for a transversal filter that colllbhles the çlem~o-ntc of each of the previous three filters into one a single large symbol addressed ml1ltir1ier lookup table. This large symbol addressed multiplier lookup table has a size that is substantially the same as the combined sizes of the individual lookup tables used in the prior embo-lim~ntc. In the fourth embodiment, the ~ vGl~al filter comprises an input 15 for receiving binary codes representin~ input symbols, and the single symbol addressed lookup table coupled to the input for receiving the input symbols and for providing a plurality of output signals that co.lG~ond to scaled amplitude values of the input symbols. The output signals comprise a predistorted waveform that compenc~t~c for the effects of intersymbol interference and tr:lncmitter non1inP~riti~oS A first one symbol 20 delay device is coupled to the delay device for delaying a first output signal therefrom.
A first s11mming device is coupled to the lookup table and to the first one symbol delay device for s11mming the first output signal and a second output signal from the lookup table to provide a summPc~ output signal. A second one symbol delay device for delaying the summ~d output signal from the first ~-...."-i,-g device. A second snmming 25 device is coupled to the lookup table and to the second one symbol delay device for s11mming the second output signal and a third output signal from the lookup table to provide a second s11mm~cl output signal that comprises baseband output waveforms that are representative of the input symbols.
The above-s11mm:lri7~ four transversal filter embodiments provide for signifi-30 cant simplifications in ~IK filter applications such as in pre-equ~1i7ing intersymbol interference in tr~n.cmi~ter waveforrns.
A fifth embodiment incorporates prewarping of the input symbols without any change in hardware complexity. In the fifth embodiment, the transversal filter comprises an input for receiving binary codes representing input symbols, and a 35 symbol addressed lookup table based prewarper and pre-equ~1i7tor coupled to the input for receiving the input symbols and for providing output signals that correspond to prewarped and pre-equalized scaled amplitude values of the input symbols. The output signals co~ .. ;ce a predistorted and plcvv~ed wavcfollll that co~"pc~ tes for the effects of illlcl~ylllbol illlclçclcnce and l~ lll;llr~ nonlin~ itiPS
The present invention rlr~rn~tic~lly ~imrlifies the implelll~ tion of FIR filters where the aprli~til~n of the FIR filter is to provide a ll~vcl~al filter function for 5 waveforms that lc~ sellt symbols. The degree of simplification is related to the llulllbcr of bits that define the size of the symbol alphabet as colll~a~cd to the llulllbcr of bits required to adequately rc~l~ selll a digital sample of the waveform. Typically 2,3, or 4 bits lcplcsclll the alphabet of c~ " " - -- -~ tion symbols, whereas the waveforms require as a ",;.~;,..,.,.. 8 bits of qu~nti7~tion. One aspect of the present invention is to use symbol addressed lookup tables rather than digital sample addressed lookup tables in imrl~ ,t;"g ~. filter functions. A second aspect of the present invention comprises combing of the function of gen~ lg ~ubi~ waveforms with the symbol addressed lookup tables that are part of the FIR filter function.
The present invention thus provides for a much simper and efficient implemen-tation of an ;~ waveform gellel~tor and FIR filter for applications where the input waveform is defined by a symbol alphabet that may be specified with fewer bits then an equivalent q~l~n~i7,o~ waveform.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken inconjunction with the accompanying drawings, wherein like reference numerals design~te like structural elements, and in which:
Fig. l a illustrates a conventional three tap filter using multipliers to scale the delayed input waveforms;
Fig. Ib illustrates a conventional three tap filter using multiplier lookup tables to scale the delayed input waveforms;
Fig. 2 illustrates a first embodiment of a three tap filter using symbol addressed multiplier lookup tables in accordance with the principles of the present invention;
Fig. 3 illustrates a conventional equalizer for quadrature waveforms;
Fig. 4 illustrates a second embodiment of an equalizer for quadrature waveforms in accordance with the principles of the present invention;
Fig. S illustrates a third embodiment of a three tap filter using symbol addressed multiplier lookup tables in accordance with the principles of the present invention;
Fig. 6 illustrates a fourth embodiment of a three tap filter using a common symbol addressed multiplier lookup table in accordance with the principles of the present invention:
Fig. 7 illustrates a conventional technique for pre-warping and pre-equ~li7ing adigital waveform; and Fig. 8 illu~Llates a first embodiment of a transversal filter for pre-warping and pre-equalizing a digital waveform in accordance with the principles of the present invention.
DETAILED DESCRIPTION
By way of introduction, the present invention comprice,c a finite impulse response (FDR) filter for use in digital telecollllll.l.,ication tr~ncmiccion systems. By itself, the present FIR filter results in simplified filter design and better ~lrollllance than conventional FIR filters. Pre-equalization of the transmit w~v~follll is one possi-ble application of the FIR filter. The present invention may also provide a nonlinear plCW~i~lg function without adding to the complexity of the filter design. A single processing function in accordance with the present invention pelrolllls waveformpredistortion so that the tr~ncmitted waveform co~ ellsates for the nonlinear effects of a power :lmplifi~r and so that the illL~l~ylllbol inL~lrerence caused by a non-ideal tr~ncmitt~r chain is reduced.
Referring to Fig. l a it illustrates a conventional three tap FIR filter 10 using multipliers 12 for scaling the input waveform and its delayed replicas. The conven-tional FIR filter 10 is comprised of a tapped delay line 11, or discrete delay elements 1 la, l lb, means 14 for scaling the signal at each tap of the tapped delay line 11 by multiplication with a scaling coefficient, and a plurality of summing devices 13 to add the result of the scaled tap signals. The signals are lc~lcsel,l~d by digital waveforms such as baseband signals. Sampling is usually performed at the symbol rate and the delay elements 1 la, 1 lb are usually digital registers that are clocked at the symbol rate.
Scaling coefficients (C) may reside in digital registers, or if they are not to be modified.
may be hardwired into the multiplier structure.
A well known ~1tPrn~tive to using multipliers 15 is to use digital lookup tables12. Fig. lb illustrates a conventional three tap filter 10a using multiplier lookup tables 12a, 12b, 12c to scale the delayed input waveforms. The operation of the filter 10b is similar to that of filter 10a, except that the scaled tap signals are derived from the multiplier lookup tables 12 prior to sl-mming Each digital lookup table 12 may be comprised of a prograrnrnable memory device (such as a RAM or PROM) or a fixed memory (ROM). The signal that is to be scaled is used as an input address to thelookup table 12. Each address position the lookup table 12 contains an applopliately scaled value of the input signal. Such a lookup table scheme is effective if the precision of the input signal is limited, since the size of the lookup table 12 increases exponen-tially with the number of signal qll~nti7~tinn levels. For example, with an 8 bit input and N bit output, the table is of size 256xN bits. On the other hand, for a 16 bit input and N bit output the table is of si_e 65,536xN bits.
One aspect of the present invention is to recognize that when filtPrine digital 5 modlll~tine waveforms, it is not nPcPs~ry to represent the input wavefo~ in a FIR
filter as a digital ~mplit~ P, of the desired waveform. It is suffi~ient to represent the input waveform as a symbol value. A FIR filter 20 embodying this concept is shown in Fig. 2. More specific~lly~ and by way of ex~rnrkP, Fig. 2 illustrates a three tap filter 20 using symbol addressed mllltirliPr lookup tables 12 in accol~lce with the principles of 10 the present invention. Inputs to the filter 20 are binary codes representin~ symbols, and outputs are qu~nti7Pd signal wavefolllls. The selection of a symbol code is of no conceql~enre It is also to be understood that the illustration of the present invention using a three tap filter 20 is by way of ç~mple only. Any number of taps may be utilized in the filter 20 of the present invention.
For n-ary cien~lline, the llull~ber of bits that define a symbol are of size n. For example, in QPSK ci~n~line, only two bits represent a symbol. At each filter tap, the lookup table 12 has size nxN. At each address position, the lookup table 12 contains the ap~lopliately scaled ~mplihl~l~p value of the input symbol. The primary benefit of this scheme is to dr~m~tit~lly reduce the size of the lookup tables 12. A secondary benefit is increased ~ l", Irlil' precision since the lookup table 12 gent;l~les an exact (within the qu~nti7:ltion limits of the number of output bits) scaled amplitude value of the symbol rather then the scaled value of the qll~nti7Pd symbol. Another benefit is that the delay ekPmPntc 1 la, 1 lb are of size n bits (for n-ary cign~lling) rather then the number of bits required to adequately represent the signal waveform amplitude (usually at least 8 bits).
In a practical application for n-ary orthogonal waveforms, the mo~ ting signal is complex in the sense that it has in-phase and quadrature components. Consider a FIR filter implemPnt~tion of a conventional pre-equalizer 30. Fig. 3 illustrates a conventional pre-equalizer 30 for quadrature waveforms that comprises a conventional transversal filter. The pre-equalizer 30 includes four FIR filters 31a, 31b, 32a, 32b comprising a pair of pre-equalizers 30a that generate in-phase signals and a pair of pre-equalizers 30b that generate quadrature signals. In particular, to generate the pre-equalized in-phase waveform the classic scheme is to pre-equalize the in-phase wave-form with "through" coefficients, pre-equalize the quadrature waveform with "cross"
coefficients and to add the results. Except for a change in sign of the cross coefficients an analogous scheme is used to ~,ent;l~t~ the pre-equalized quadrature waveform.
The previously discussed lookup table implemrnt~tion may also be applied to the just-described pre-equalizer arc~itect~lre but with another dramatic ~implifir~tion as will be ~esrrihe-l with reference to Fig. 4. Fig. 4 illustrates a simplified pre-equalizer 40 for in-phase and quadrature waveforms in accordance with the l" ;.~ lPs of the 5 present invention. Since input signals (symbols) are no longer represented by a digital representation of a symbol waveform, the through and cross equalizers 31a, 31b, 32a, 32b of Fig. 3 may be integrated into two lookup tables and thus a first equivalent equalizer 41a is provided for ~e,1eldti,lg the pre-equalized in-phase waveforms, and a second equ~li7~r 41b provides for gen.,ld~ g the pre-equalized quadrature waveforrns.
Each of the equalizers 41a, 41b compricing the pre-equalizer 40 may be constructed in the manner described with reference to Fig. 2 above, for example. The lookup tables in the Ica~e~;~ive equalizers 41a, 41b of Fig. 4 comprise signals that are not the same. The in-phase colllponc;lll of the product of the complex waveform and the complex scaling coeffirient is stored in the first equalizer 41a. The quadrature co",~onent of the product of the complex wavefollll and the complex scaling coefficient is stored in the second equalizer 41b.
Fig. S illu~l-dtes this archit~ctllre in more detail and also in~lir~tes anothersimplification provided by the present invention. The delay elements 11 for the two pre-equ~li7ers 41a, 41b contain the same signal (i.e. symbol) and therefore they may be combined in the manner shown in Fig. 5 to produce the illustrated transversal filter 50.
The transversal filter 50 is comprised of the tapped delay line 11, or discrete delay elements 1 la, 1 lb, a plurality of lookup tables 12 that are adapted to output scaled in-phase and quadrature output signals, and two sets of ~ g devices 13a, 13b to addthe result of the respective scaled in-phase and quadrature output signals.
Another variation of the present invention which may be advantageous for some implementations is shown in Fig. 6, and comprises a pre-equalizer 60. This architec-ture represents an alternative to the equalizer 20 of Fig. 2, or each of the equalizers 41 a, 41b shown in Fig. 4. The ~ g devices 13 (adders) are separated by one symbol delay elements 11. A single lookup table 12 is employed in the pre-equalizer 60 of Fig.
6. However, fr~gmrnt~tion into several tables may be n--cesc~ry due to the largenumber of bits. One advantage of the architecture for the pre-equalizer 60 is that even with fr:lgmrnt~tion, a common address decoder may be utilized to generate addresses in the lookup table 12. This has advantages in high speed applications.
If one of the pre-equalizers 20, 40, 50, 60 is to be used in conjunction with a nonlinear distortion compen~ting modulator, for example, then another dramatic improvement may be realized. As previously in~lir:lt~, the nonlinear amplitude and phase behavior of a power amplifier may be negated by warping the signal prior to applying it to the power ~mplifi~r. Fig. 7 illustrates a conventional means for ~.cw~-ing and pre-eql~li7in~ 70 co,-,l" ;cil~ a symbol ~lCWal~r 12 (lookup table 12) and a serially couplPd pre-eq~ i7lor 40. The warping may be implem~nt~d using the lookup table 12 and then the warped digital w~vefi~ l value is pre-eqll~li7~-d in the pre-equal-S izer 40 to produce a ~cw~ ed and pre-equalized w~vcfo --- output from the filter 70.
However, and in accoidance with the present invention, if the mo~ tin~ signal is in digital form, then this warping may be implem~nt.ocl in the lookup table 12 using a symbol value as an input thereto and a warped digital w~vefo~l value as an output thereof. More speçifi~lly, and with reference to Fig. 8, it illustrates a lookup-table-10 based plcwalllcl pre-equalizer 80 that provides for pre-warping and pre-eq~l~li7in~ of a digital w~vcfc,.." (input symbol) in acco,~,ce with the ~ iplfs of the present inven-tion. The p.cw~g operation conventionally pc-ro----ed in the pre-equalizer 40 isintegrated into the lookup table 12 with no additional addition in complexity. That is, the stored values in the lookup table 12 are the products of filter coefficients and the 15 l~r~w~ed symbol amplitudes. The ~ cl pre-equalizer 80 provides a universal means for pre-warping the input symbol in addition to pre-eqll~li7in~ the input symbol.
This scheme not only elimin~t~s a lookup table 12, but also reduces the ~ccoçi~tetl qll~nti7~tion errors. Furthermore, the equalizer of Fig. 8 is programmable. The scaling coefficiçntc are prograrnmable, and the waveforms are also indepen(ltontly program-20 mable. Such pro~,l;~llll,ill~ is generally understood in the art and will not be furtherdescribed herein.
The previous applications of the present invention were related to tr~ncmitt~r functions. Some receiver designs that utilize decision fee~b:l~k techniques may also benefit from the present invention. For instance, there is a well known technique 25 known as a decision feedback equalizer. A conventional decision feedb~ck equalizer has two components similar to the pre-equalizer architecture shown in Fig. 7. The first component equalizes "future" symbols in a classic FIR filter architecture. The second component utilizes the waveform gen~la~d by detected past symbols as an input to a ~1~ filter. The present invention may be applied to simplify the second component 30 which results in the embodiment of a decision feedback equalizer similar to the architec-ture shown in Fig. 8.
Thus there has been described new and improved signal conditioners employing transversal (FIR) filters that use symbol addressed lookup tables. It is to be understood that the above-described embodiments are merely illustrative of some of the many35 specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.
Claims (5)
1. A transversal filter for processing input symbols to generate baseband outputwaveforms comprising quantized signal waveforms that are representative of the input symbols, said filter comprising:
an input for receiving binary codes representing input symbols;
a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols;
a plurality of symbol addressed lookup tables respectively coupled to the input and to the plurality of serially coupled one symbol delay devices for respectively receiving the input symbols and delayed versions thereof and for providing respective output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveforrn that compensates for the effects of intersymbol inteference and transmitter nonlinearities; and a plurality of summing devices individually coupled to predetermined pairs of the plurality of symbol addressed lookup tables that process the respective output signals to generate the baseband output waveforms that are representative of the input symbols.
an input for receiving binary codes representing input symbols;
a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols;
a plurality of symbol addressed lookup tables respectively coupled to the input and to the plurality of serially coupled one symbol delay devices for respectively receiving the input symbols and delayed versions thereof and for providing respective output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveforrn that compensates for the effects of intersymbol inteference and transmitter nonlinearities; and a plurality of summing devices individually coupled to predetermined pairs of the plurality of symbol addressed lookup tables that process the respective output signals to generate the baseband output waveforms that are representative of the input symbols.
2. A signal conditioner for use in a transmitter for processing input symbols togenerate baseband pre-equalized and predistorted output waveforms comprising quantized in-phase and quadrature signal waveforms that are representative of the input symbols, said signal conditioner comprising:
an input for receiving binary codes representing said input symbols;
an in-phase equalizer that comprises:
a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols;
a plurality of symbol addressed lookup tables respectively coupled to the input and to outputs of the plurality of serially coupled one symbol delay devices for respectively receiving the input symbols and delayed versions thereof and for providing respective in-phase output signals that correspond to pre-equalized, scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities;
and a first plurality of summing devices comprising a first summing device coupled to first and second ones of the plurality of symbol addressed lookup tables and a second summing device coupled to an output of the first summing device and a third one of the plurality of symbol addressed lookup tables that process the respective in-phase output signals to generate the baseband in-phase output waveforms that are representative of the input symbols; and a quadrature equalizer that comprises:
a second plurality of summing devices comprising a third summing device coupled to the first and second ones of the plurality of symbol addressed lookup tables and a fourth summing device coupled to an output of the third summing device and the third one of the plurality of symbol addressed lookup tables that process the respective quadrature output signals to generate the baseband quadrature output waveforms that are representative of the input symbols;
and wherein the plurality of symbol addressed lookup tables provide respective quadrature output signals that correspond to pre-equalized, scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities.
an input for receiving binary codes representing said input symbols;
an in-phase equalizer that comprises:
a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols;
a plurality of symbol addressed lookup tables respectively coupled to the input and to outputs of the plurality of serially coupled one symbol delay devices for respectively receiving the input symbols and delayed versions thereof and for providing respective in-phase output signals that correspond to pre-equalized, scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities;
and a first plurality of summing devices comprising a first summing device coupled to first and second ones of the plurality of symbol addressed lookup tables and a second summing device coupled to an output of the first summing device and a third one of the plurality of symbol addressed lookup tables that process the respective in-phase output signals to generate the baseband in-phase output waveforms that are representative of the input symbols; and a quadrature equalizer that comprises:
a second plurality of summing devices comprising a third summing device coupled to the first and second ones of the plurality of symbol addressed lookup tables and a fourth summing device coupled to an output of the third summing device and the third one of the plurality of symbol addressed lookup tables that process the respective quadrature output signals to generate the baseband quadrature output waveforms that are representative of the input symbols;
and wherein the plurality of symbol addressed lookup tables provide respective quadrature output signals that correspond to pre-equalized, scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities.
3. A transversal filter for processing input symbols to generate baseband outputwaveforms comprising quantized signal waveforms that are representative of the input symbols, said filter comprising:
an input for receiving binary codes representing input symbols;
a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols;
a plurality of symbol addressed lookup tables respectively coupled to the input and to the plurality of serially coupled one symbol delay devices for respectively receiving the input symbols and delayed versions thereof and for providing respective in-phase and quadrature output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for theeffects of intersymbol interference and transmitter nonlinearities; and a plurality of sets of summing devices respectively coupled to predetermined ones of the plurality of symbol addressed lookup tables that process the respective in-phase and quadrature output signals to generate in-phase and quadrature baseband output waveforms that are representative of the input symbols.
an input for receiving binary codes representing input symbols;
a plurality of serially coupled one symbol delay devices for sequentially delaying the input symbols;
a plurality of symbol addressed lookup tables respectively coupled to the input and to the plurality of serially coupled one symbol delay devices for respectively receiving the input symbols and delayed versions thereof and for providing respective in-phase and quadrature output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for theeffects of intersymbol interference and transmitter nonlinearities; and a plurality of sets of summing devices respectively coupled to predetermined ones of the plurality of symbol addressed lookup tables that process the respective in-phase and quadrature output signals to generate in-phase and quadrature baseband output waveforms that are representative of the input symbols.
4. A transversal filter for processing input symbols to generate baseband outputwaveforms comprising quantized signal waveforms that are representative of the input symbols, said filter comprising:
an input for receiving binary codes representing input symbols;
a symbol addressed lookup table coupled to the input for receiving the input symbols and for providing a plurality of output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities;
a first one symbol delay device for delaying a first output signal from the lookup table;
a first summing device coupled to the lookup table and to the first one symbol delay device for summing the first output signal and a second output signal from the lookup table to provide a summed output signal;
a second one symbol delay device for delaying the summed output signal from the first summing device; and a second summing device coupled to the lookup table and to the second one symbol delay device for summing the second output signal and a third output signal from the lookup table to provide a second summed output signal that comprises baseband output waveforms that are representative of the input symbols.
an input for receiving binary codes representing input symbols;
a symbol addressed lookup table coupled to the input for receiving the input symbols and for providing a plurality of output signals that correspond to scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities;
a first one symbol delay device for delaying a first output signal from the lookup table;
a first summing device coupled to the lookup table and to the first one symbol delay device for summing the first output signal and a second output signal from the lookup table to provide a summed output signal;
a second one symbol delay device for delaying the summed output signal from the first summing device; and a second summing device coupled to the lookup table and to the second one symbol delay device for summing the second output signal and a third output signal from the lookup table to provide a second summed output signal that comprises baseband output waveforms that are representative of the input symbols.
5. A transversal filter for processing input symbols to generate baseband outputwaveforms comprising quantized signal waveforms that are representative of the input symbols, said filter comprising:
an input for receiving binary codes representing input symbols; and a symbol addressed lookup table coupled to the input for receiving the input symbols and for providing a plurality of output signals that correspond to prewarped and pre-equalized scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities.
an input for receiving binary codes representing input symbols; and a symbol addressed lookup table coupled to the input for receiving the input symbols and for providing a plurality of output signals that correspond to prewarped and pre-equalized scaled amplitude values of the input symbols and that comprise a predistorted waveform that compensates for the effects of intersymbol interference and transmitter nonlinearities.
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| CA 2178664 CA2178664A1 (en) | 1996-06-10 | 1996-06-10 | Signal conditioner with symbol addressed lookup table based transversal filters |
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| CA 2178664 CA2178664A1 (en) | 1996-06-10 | 1996-06-10 | Signal conditioner with symbol addressed lookup table based transversal filters |
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|---|---|
| CA (1) | CA2178664A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112640306A (en) * | 2018-09-27 | 2021-04-09 | 根特大学 | Cascadable filter architecture |
-
1996
- 1996-06-10 CA CA 2178664 patent/CA2178664A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112640306A (en) * | 2018-09-27 | 2021-04-09 | 根特大学 | Cascadable filter architecture |
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