MODEM INCLUDING AN EQUALIZER FOR AMPLITUDE JITTER

Abstract: A modem (100) includes an equalizer for equalizing a received signal (10) for amplitude jitter distortion. The received signal includes modulated symbols based on a predetermined constellation of points, each point representing a symbol. The modem first extracts a modulated symbol to provide an extracted symbol, R(k), which includes amplitude jitter distortion. The modem then identifies a signal point Pn(k) of the predetermined constellation corresponding to the extracted symbol. Further, the modem determines an error signal (76) based on the amplitude jitter distortion, and then uses the error signal to compensate subsequent extracted symbols for the amplitude jitter distortion.

Description: MODEM INCLUDING AN EQUALIZER FOR AMPLITUDE JITTER 10 Incorporation by Reference of another U. S. Patent The applicant hereby incorporates by reference U. S. Patent 4,777,640, Michael D. Turner et al., "Frequency Adaptive Phase Jitter Canceler", issued October 11 , 1988, verbatim and with the same effect as though the same patent were fully and completely set forth herein. 15 Field of the Invention This application relates to communication signal receiving apparatus including, but not limited to, apparatus for compensating the received communication signal for at least amplitude jitter in order to 20 retrieve modulated digital data therefrom. Background of the Invention Presently, data communication equipment ("DCE"), such as modems, for example, are being used to transport digital data between 25 data terminal equipment ("DTE") such as personal computers, workstations and the like, over channels such as, for instance, telephone lines. In some instances, the communication signal is modulated to include digital data in the form of a train of symbols based on a predetermined transmit constellation of signal points. Each signal point 30 of the transmit constellation represents a digital code or value. Each „ symbol is modulated substantially at the signal point of the transmit , ; constellation corresponding to the value desired to be represented thereby in the train, and thus transported over the channel. At the receiving DCE, the symbols are extracted from the communication signal and a signal point is identified from a receive constellation of signal points corresponding to each extracted symbol. The signal points are used to retrieve the digital data from the modulated communication signal. In communicating the communication signals over the telephone network, a source of distortion thereof is known as jitter distortion and is caused primarily by the coupling of sinusoidal distortion from 60 Hz power lines and/or the 20 Hz ringing signal. An equalizer function is commonly used in the receiving DCE to remove linear distortion from the received communication signals, but because of low update gains, an equalizer is unable to remove the jitter distortion components. Jitter distortion is comprised of phase and amplitude components. It has been recognized that the phase jitter component of the jitter distortion has the major effect on modem performance when using conventional receive constellations for standard data communication formatting between modems. Accordingly, contemporary modem receivers include one or more phase jitter cancelling circuits for reducing the sinusoidal components of phase jitter in the communications channel. An example of such a phase jitter canceling circuit may be found in the Michael D. Turner et al. U. S. Patent 4,777,640, which patent has been incorporated by reference hereinabove. Because of the spread in signal points of the conventional receive constellations, the amplitude jitter component of the jitter distortion has little effect on the modem's performance. However, higher data bit rate communication is contemplated in the near future. Since the bandwidth of the communication channel is substantially fixed, increasing data bit rates is accomplished by increasing the number of signal points of a receive constellation, thereby compressing the distances therebetween. Consequently, high speed modems incorporating these compressed receive constellations are likely to be more susceptible to amplitude jitter. For example, in a contemplated 19.2 K bits per second high speed modem receiver as little as 2% amplitude jitter can affect the modem's performance. Under these conditions, amplitude jitter affects the signal to distortion performance of the high speed modem in the same way that phase jitter affects performance of a lower speed modem. Accordingly, it is desirable for the contemplated higher speed modems to include not only phase jitter cancellation to compliment the equalizer function thereof, but also to include an amplitude jitter cancelling function to further improve performance. Summary of the Invention A modem is disclosed for receiving a signal, the signal including modulated symbols based on a predetermined constellation of points, each point representing a symbol. The modem includes: means for extracting modulated symbols to provide an extracted symbol, R(k), the extracted symbol including amplitude jitter distortion, identifying means for identifying a signal point P n (k) of the predetermined constellation corresponding to the extracted symbol, based on a predetermined criteria; determining means for determining an error signal representative of the amplitude jitter distortion based on the extracted symbol and the signal point; and compensating means responsive to the error signal for compensating at least one subsequent extracted symbol for the amplitude jitter distortion prior to the identifying means identifying the signal point corresponding thereto. Brief Description of the Drawings FIG. 1 is a block diagram schematic of a first embodiment of a modem including an equalizer for amplitude jitter, in accordance with the invention. FIG. 2 is a graph depicting a signal point constellation. FIG. 3 is a block diagram schematic of a first amplitude jitter tracker. FIG. 4 is a block diagram schematic depicting further detail for the amplitude jitter tracker of FIG. 3. FIG. 5 is a block diagram schematic depicting a second amplitude jitter tracker. Det ailed Description FIG. 1 is a block diagram schematic of a modem 100 including a receiver that is being used, by way of example, as a receiving portion of a DCE. Referring to FIG. 1 , the received signal 10 that has been transported by a channel which, for example, may be a telephone line, includes jitter distortion. Generally the signal 10 will include modulated symbols based on a predetermined constellation. As is known, the constellation includes a plurality of points, each point representing a symbol. Each symbol, in turn, represents a value to be transported. The incoming signal 10 is first filtered and amplified by a conventional low pass filter 12 which may include an automatic gain control circuit. Next, the filtered signal undergoes analog-to-digital ("A/D") conversion by a converter 14 at a controlled sampling rate governed by a sampling signal 16. The sampled digital data of the A/D converter 14 is demodulated by a conventional demodulator 18 and the demodulated output 20 is equalized by an equalizer function 22. In the present embodiment, the equalizer may be of the classical ieast-means- squared type, for example. The output of the equalizer 22 is an equalized receive signal 24 which governs a conventional timing loop 26 to provide the sampling signal 16. in the present embodiment, the sampling rate is adjusted to approximately four times the symbol or baud rate of the incoming train of symbols forming the received signal 10. The combination of elements 14, 18, 22 and 26 provide the function of extracting the modulated symbols of the train from the received signal 10 and providing them, as extracted symbols, to a decision algorithm 198 via the signal line 28. In general, the kth extracted symbol may be referred to as R(k). In the decision algorithm 198, a signal point P n (k) of the predetermined receive constellation corresponding to an extracted symbol R(k) is identified based on a predetermined criteria. In the present embodiment, the criteria used for signal point identification is based on the closest signal point of the receive constellation to the extracted symbol. Once the signal point P n (k) is identified, it may be used to retrieve the value corresponding thereto. FIG. 2 is a graph which illustrates the signal point identification process. Since each extracted symbol includes an inphase I and quadrature Q component, the abscissa (or "X") axis and ordinate (or Υ") axis of the graph represent the respective I and Q components. While the graph of FIG. 2 exhibits only quadrant 1 of the receive constellation including signal points P1-P4, it is understood that the principles described also apply to the other three quadrants of the receive constellation as well. Accordingly, an extracted symbol may be represented by a vector such as that shown by vector R and each signal point may be represented by vectors as well. In the present embodiment, error vectors E may be measured from each of the signal points of the received constellation to the extracted symbol vector R under consideration and the signal point having the smallest magnitude error vector is considered the identified signal point. In FIG. 2, signal point P1 has the smallest error vector to the extracted symbol vector R and thus is considered the closest. Once the decision algorithm 198 identifies the signal point P n (k) corresponding to the extracted symbol R(k), the signal point P n (k) is applied to a decoder 30 over signal path 32. In the decoder 30, the digital data corresponding to the identified signal point P n (k) is retrieved. The data is then output over path 34 as received bits. In addition, the decision algorithm 198 applies the associated error signal E(k), that is based on the extracted symbol R(k) and the signal point P n (k), to the equalizer 22 over signal paths 36 and 66. The equalizer 22 includes compensating means responsive to the error signal for compensating the subsequent extracted symbol, R(k+1 ). Since the equalizer 22 is situated before the decision algorithm 198 in the signal path, the subsequent compensation will occur prior to the decision algorithm identifying the signal point P n (k+1 ) corresponding to the subsequent extracted symbol. Thus, the equalizer 22 utilizes the error signal E(k) for adjustment thereof in order to reduce the magnitude of the subsequent error for the subsequent extracted symbol, R(k+1 ). As discussed above, the equalizer 22 is generally unable to remove jitter distortion from the received signal. For purposes of explanation, the extracted symbol R, as depicted in the graph of FIG. 2, is considered to include only jitter type distortion. Accordingly, the error vector E then is comprised of both an amplitude jitter distortion component and a phase jitter distortion component. In the graph, the amplitude jitter distortion component is represented by E a and is in line with the vector of the identified signal point P1. In addition, the phase jitter distortion component is represented by E p and is perpendicular to the identified signal point vector. To compensate for the phase jitter distortion component, the error signal E(k) of path 36 is applied to a phase error calculator 40 which separates out a phase jitter distortion distortion error signal 46 therefrom that is applied to a phase jitter tracker 42 and also to a summer 44. The phase jitter tracker 42 is responsive to the phase jitter distortion error signal 46 to provide a phase jitter distortion angle signal 48 that is applied to a summer 50 and also to a delay function 52 which delays the phase jitter distortion angle term approximately 1 baud or symbol time interval before subtracting it from the phase jitter distortion term 46 in the summer 44. The output of the summer 44 governs a carrier loop function 54 to provide a carrier angle term 56 that is summed with the phase jitter distortion angle term 48 in the summer 50 to produce a total phase angle compensation term 58 that is represented by the symbol q. A phase compensating signal eft is formed from the compensating signal 58 in the block 60 and used to compensate the subsequent extracted symbol R(k+1) of path 28 utilizing a multiplier 62 disposed in the signal path 28. In addition, another compensating signal e~\ ~ is generated in a block 64 from the compensating signal 58 and is used to compensate the error signal of path 36 to effect a phase compensated error signal 66 for adjusting the equalizer 22. In the present embodiment, the compensating signal of block 64 multiplies the error signal using a multiplier 68 disposed in the error signal path 36. For more details of the foregoing described phase jitter distortion compensation portion of the modem, refer to the aforementioned U. S. Patent 4,777,640 which has been incorporated by reference he re in above. Further in regard to the present embodiment, the error signal E(k) of path 36 is applied to an amplitude error calculator 70 for deriving a normalized amplitude jitter distortion error signal 72. In the present embodiment, the calculator 70 projects the error vector E(k) of the corresponding extracted signal R(k) onto the vector of the identified signal point P n (k) to derive an unnormaiized error signal E a (k). In addition, the error signal E a (k) is normalized by dividing it by the absolute value of the identified signal point vector P n (k) to derive the normalized error signal E n (k) 72. In the present embodiment, the unnormaiized error signal E a (k) is derived in the calculator 70 based on the following equation: t = ,_,-_ E X Pχ + EyPy + 1/2 (E X Py - E V Pχ) Ea(k) = |Pn(k)| where: Ex = the inphase component of E(k); E y = the quadrature component of E(k); P x = the inphase component of P n (k); and, P y = the quadrature component of P n (k). Moreover, the normalized error signal E n (k) is derived based on the following equation: E a (k) E n (k) = |Pn(k)| In connection with the compensation of the amplitude jitter distortion, an amplitude jitter tracker 74 is responsive to the normalized error signal E n (k) 72 to provide a compensating signal over path 76. FIG. 3 is a block diagram schematic of an amplitude jitter tracker 74 suitable for use in the embodiment of FIG. 1. Since the amplitude jitter distortion includes at least one sinusoidal component having an amplitude and a frequency, the amplitude jitter tracker 74 includes a second-order phase locked loop 94 and a phase accumulator 98 responsive to the normalized error signal E n (k) for providing a compensating signal frequency value 84 and a circuit 82 responsive to the error signal for providing a compensating signal amplitude value c-i at 86. The amplitude value 86 and the frequency value 84 are applied to a multiplier 88, which provides the composite compensating signal 76, represented by the Greek symbol a. The function of the amplitude jitter tracker 74 in the present embodiment, is to match the compensating signal 76 to the amplitude jitter distortion of the extracted symbols such that compensation thereof converges the corresponding error signals to a steady state value, preferably substantially zero. More specifically, for each extracted symbol R(k), the normaiizer block 90 normalizes the corresponding error signal E n (k) 72 by dividing it with the derived amplitude term ci to remove substantially amplitude variations thereof. The output of the normaiizer 90 is then applied to one input of a multiplier 92. The output of the multiplier 92 drives a second- order phase lock loop 94 that is used to track and update an incremental phase 96 of the frequency term in the proper direction. The resulting incremental phase tracking signal 96 is applied to a phase accumulator 98 which generates a phase term f j for the subsequent extracted symbol R(k+1). The phase term f j is delayed by approximately one-symbol time interval by the delay block 100 and applied to block 102 wherein the sign of cos f j is derived and applied to another input of the multiplier 92 for multiplying the normalized error signal E n (k) 72 to insure that the phase lock loop 94 is updating the incremental phase 96 in the proper direction for each extracted symbol. In addition, the compensating signal frequency value 84 is computed by establishing the sine of the phase term f j in block 104. In operation, the compensating signal frequency value 84 is matched to the frequency of the sinusoidal amplitude jitter distortion of subsequent extractθd symbols such that compensation of the subsequent extracted symbols with the frequency value 84 converges the corresponding error signals to substantially zero values. Moreover, the circuit 82 of the amplitude jitter tracker 74 operates to correlate the normalized error signal E n (k) 72 with the frequency value 84 via a one-symbol delay through block 106, thus substantially removing any sinusoidal variations from the amplitude value 86. The circuit 82 operates to match the amplitude value 86 to the amplitude of the sinusoidal component of the amplitude jitter distortion such that compensation of subsequent extracted symbols with the amplitude value 86 converges the corresponding error signals to substantially zero values. FIG. 4 depicts the second-order phase lock loop 94, phase accumulator 98 and the circuit 82 in further detail. Referring to FIG. 4, the output of the multiplier 92 is operated on by a gain term Wj 2* ! "2 in block 110. Further, the output of block 110 is accumulated in an accumulator formed by the summer 112 and a one-symbol time interval delay 114. The accumulated result is applied to one input of a summer 116. The output of multiplier 92 is also operated on by a gain term awjT in block 118, where a is a constant, and the result is applied to another input of the summer 116. The result of the summer 116 becomes the estimated incremental phase 96 for the subsequent extracted symbol and is applied to the accumulator 98 that is formed by a summer 120 and a one- symbol delay 122. The output of the accumulator 98 provides the phase term f j for the subsequent extracted symbol. Moreover, the circuit 82 includes an adaptive update filter. More particularly, the error signal E n (k) 72 is multiplied by the frequency value 84, delayed by a one-symbol time interval 106, in a multiplier 126. The output of the multiplier 126 is operated on by a gain K in block 128 and the resultant signals for each of the extracted symbols are accumulated by an accumulator formed by a summer 130 and a one-symbol time interval delay 132. The accumulation results in the amplitude value 86, which is substantially free of sinusoidal variations, and has an average that is correlated with the amplitude of the error signal E n (k) 72. FIG. 5 depicts an alternate embodiment 500 of an amplitude jitter tracker 74' suitable for use in the modem of FIG. 1. This alternate embodiment includes a linear predictor function that is responsive to the normalized error signal E n (k) 72 to provide the compensating signal a 76 for use in compensating subsequent extracted symbols. The error signal E n (k) 72 is compensated by a signal formed as a function of the compensating signal a 76 in a multiplier 140 to provide an intermediate error signal EAW. In the present embodiment, a block 142 forms the signal for compensating the error signal as 1-a. Still further, the compensating signal a 76 is subtracted from the intermediate error signal EA(k) in a summer 144 to provide a second intermediate error signal b(k). A delay line 146 having a plurality m of tapped delay stages is operative to conduct the second intermediate error signal b(k) through the plurality of delay stages thereof. The contents of each stage i being operated on by a corresponding stage gain q to provide a corresponding plurality of tapped output signals which are summed in a summer 148 to produce an estimate 149 of the compensating signal a 76 for the subsequent extracted symbol. Each of the stage gains q is updated as a function of the intermediate error signal EA(k) as shown in the block 150. A one- symbol delay 152 operates on the estimate 149 to derive the compensating signal a 76. Referring again to FIG. 1 , a signal is formed as a function of the compensating signal of path 76 in the block 160 for use in compensating the subsequent extracted symbol over path 28 to provide a compensated extracted symbol for use in the decision algorithm 198 to identify the signal points corresponding thereto. In the present embodiment, the extracted symbol of path 28 is compensated by the signal (1/1 -a) formed by block 160 by multiplying the extracted symbol thereby utilizing a multiplier 162 disposed in the path 28. Similarly, the error signal over path 66 for use in adjusting the equalizer 22 is compensated by a signal (1-a) formed in block 164 as a function of the compensating signal a 76 to provide a compensated error signal over path 66. In the present embodiment, the signal formed by block 164 multiplies the error signal of path 66 by a multiplier 166 disposed in the path 66. Moreover, the equalized extracted symbols of path 28 are compensated for both amplitude jitter distortion and phase jitter distortion utilizing the respectively corresponding amplitude jitter tracker 74 and phase jitter tracker 42 in connection therewith prior to identifying the signal points thereof through use of the decision algorithm 198. Once the extracted symbols are compensated for the jitter distortion, the resultant compensated symbols are closer to their intended signal points so that identification of the proper signal point will be achieved, thereby improving performance of the modem receiver. Returning again to FIG. 1 , those skilled in the art will appreciate that the filter 12, the A/D converter 14, the demodulator 18, the timing loop 26, the decoder 30, and the decision algorithm 198 depicted therein perform typical modem functions, whereas the remaining elements collectively interact to perform a general equalizing function. As explained hereinabove, jitter distortion includes both a phase component and an amplitude component. A major shortcoming of prior modem equalizers is that they have not addressed the problem of amplitude jitter distortion. At higher bit rates such as, for instance, 19.2 KBits per second, the amplitude jitter distortion is crucial to the satisfactory performance of the modem. For example, an amount of amplitude jitter distortion as low as 5% will cause relatively high levels of received bit errors, probably at least 3%. Thus, the bit error rate ("BER") = 0.03. In modem communication applications, such a high bit error rate is normally unacceptable, and it would effectively mean that the modem could not be used in the desired application. The present modem including an equalizer for amplitude jitter, in accordance with the present invention, combats this problem, however, by effectively compensating for at least ninety-nine percent (99%) of the unwanted amplitude jitter distortion. Stated otherwise, the amount of uncompensated amplitude jitter distortion remaining in the extracted symbols is reduced by a ratio of approximately 100 to 1. Moreover, such a modem, in accordance with the present invention, provides an even greater improvement in performance when measured in terms of bit error rate, inasmuch as the corresponding bit errors in the received data stream will be reduced by an even more substantial ratio-an improvement, in fact, of at least a ratio of 10,000 to 1. Thus, the end user may be expected to enjoy a bit error rate of approximately 0.0003%. Thus, the BER = 0.000003. The over-all benefit to the user, of course, is obtaining a modem product that will work in a high-speed application. This is because, as a direct result of a modem in accordance with the present invention, the amplitude jitter distortion and corresponding bit rate are improved to a degree that modem speeds of 19.2 KBrts per second, and perhaps greater, may reliably be achieved, with satisfactory levels of received bit errors. While various embodiments of a modem including an equalizer for amplitude jitter, according to the present invention, have been described hereinabove, the scope of the invention is defined by the following claims.

Claims: What is claimed is: Claims: 1. A modem (100) for receiving a signal, the signal including modulated symbols based on a predetermined constellation of points, each point representing a symbol, the modem including: means (14, 18, 22, 26) for extracting modulated symbols to provide an extracted symbol, R(k), the extracted symbol including amplitude jitter distortion, identifying means (198) for identifying a signal point Pn(k) of the predetermined constellation corresponding to the extracted symbol, based on a predetermined criteria; determining means (70, 74) for determining an error signal (72) representative of the amplitude jitter distortion based on the extracted symbol and the signal point; and compensating means (22) responsive to the error signal for compensating at least one subsequent extracted symbol for the amplitude jitter distortion prior to the identifying means identifying the signal point corresponding thereto. 2. The modem of claim 1 wherein R(k) and Pn(k) are complex signals including inphase and quadrature components and the error signal, E(k), is based on the difference between R(k) and Pn(k), and wherein the determining means includes projecting means for projecting E(k) onto Pn(k) to derive an unnormaiized error signal. 3. The modem of claim 2 wherein the determining means includes means for normalizing the unnormaiized error signal to the absolute value of Pn(k) to form a normalized error signal for use by the compensating means. 4. The modem of claim 2 wherein the determining means includes means for deriving the unnormaiized error signal, E a (k), based on the following equation: E x Pχ + EyPy + 1 /2 (E x Py - E v Pχ) |Pn( )| where: Ex = the inphase component of E(k); Ey = the quadrature component of E(k); P x = the inphase component of Pn(k); Py = the quadrature component of Pn(k). 5. The modem of claim 4 wherein the determining means further includes means for deriving a normalized error signal, E n (k), based on the following equation: Ea(k) |Pn(k)| 6. The modem of claim 1 wherein the compensating means includes an amplitude jitter tracker means (74) responsive to the error signal for providing a compensating signal (76) for compensating the at least one subsequent extracted symbol. 7. The modem of claim 6 wherein the amplitude jitter tracker means includes means for matching the compensating signal to the amplitude jitter distortion of the extracted symbols such that compensation thereof converges the corresponding error signals to a steady state value. 8. The modem of claim 6 wherein the amplitude jitter distortion includes at least one sinusoidal component having a sinusoidal component amplitude and a sinusoidal component frequency, and wherein the amplitude jitter tracker means includes: first providing means (94, 98) responsive to the error signal for providing a compensating signal frequency value; second providing means (82) responsive to the error signal for providing a compensating signal amplitude value; and combining means (88) for combining the compensating signal frequency value and compensating signal amplitude value to form a composite compensating signal. 9. The modem of claim 1 wherein the compensating means includes linear predictor (74') means responsive to the error signal (72) for providing a compensating signal (76) for compensating the at least one subsequent extracted symbol. 10. The modem of claim 9 wherein the linear predictor means includes: means for compensating the error signal (72) with a signal based on the compensating signal (76) to provide an intermediate error signal; means for combining the intermediate error signal with a delayed compensating signal to provide a second intermediate error signal; delay line means having a plurality of tapped delay stages (146), the delay line means arranged to conduct the second intermediate error signal through the plurality of tapped delay stages thereof, the contents of each stage being operated on by a corresponding stage gain to provide a corresponding plurality of tapped output signals; means for updating the plurality of stage gains based on the intermediate error signal; and means for combining the plurality of tapped output signals to provide the compensating signal.

Inventors: BLACKWELL STEVEN RAY, RUSHING MICKEY C, BLACKWELL, STEVEN, RAY, RUSHING, MICKEY, C.
Applicants: UNIVERSAL DATA SYSTEM INC, UNIVERSAL DATA SYSTEM INC.
Application year: 1993
Main classification: H03H7
IPC codes: H04B3, H04B7, H04L27