HK1114722A - Adaptive equalizer tap stepsize - Google Patents

Description

Adaptive equalizer tap step size

Cross Reference to Related Applications

This application claims benefit of U.S. provisional application No. 60/700630 filed on 19/7/2005.

Technical Field

The present invention relates generally to communication systems, and more particularly to adaptive filters, such as are used to form filter elements such as equalizers (equalizers).

Background

Many digital data communication systems employ adaptive equalization to compensate for the effects of changing channel conditions and interference on the signal transmission channel. The ability of the equalizer to adaptively acquire and track (track) the time-varying channel is a function of how much gain is applied to the tap update process. Greater gain results in the ability to handle changing channel conditions faster, but up to only one point. Once this point is exceeded, the gain causes excessive jitter in the taps, which degrades the fidelity (fidelity) of the equalizer output.

One way to control this self-induced tap noise under high gain control is to implement a bias on the tap that drives it to zero when the only other driving force on the tap is substantially random. The disadvantages of this method are: as the gain continues to increase, the value of the offset relative to zero must also increase, i.e., become stronger. This causes the offset value to significantly limit the amount of gain that can be applied.

Disclosure of Invention

The inventors have observed that: a high gain can be applied to the equalizer-independent of any offset value (if present) -and still prevent excessive noise generation. Thus, the ability of the equalizer to quickly adapt to changing conditions is further improved. Specifically, and in accordance with the principles of the present invention, an apparatus comprises: an adaptive filter having groups of taps, each group including at least one tap having an associated tap value; and a controller for selecting a scaling factor (scaling factor) for at least one group of taps as a function of the tap value for that group, and adjusting the error value as a function of the selected scaling factor; wherein the adaptive filter adapts (adapt) the tap values of the at least one group of taps as a function of the adjusted error value. Thus, high gain can be applied to only those taps of the filter that are adaptively found to have a significant effect on the filter response, thereby obtaining the benefit of high gain on the taps that require high gain while preventing excessive noise generation.

In accordance with an embodiment of the present invention, a receiver includes an equalizer having groups of taps, each group including at least one tap having an associated tap value; and wherein the equalizer adjusts tap values in each group, wherein tap values of at least one group are adjusted as a function of a step size (stepsize), the value of the step size being selected as a function of the tap values of the group.

Drawings

Preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings.

FIG. 1 illustrates a prior art decision (decision) feedback equalizer;

FIG. 2 shows an illustrative block diagram of a receiver in accordance with the principles of the invention;

FIG. 3 shows an illustrative decision feedback equalizer in accordance with the principles of the invention;

fig. 4 further illustrates the inventive concept in the context of the decision feedback equalizer of fig. 3;

FIG. 5 is an illustrative flow chart illustrating a method in accordance with the principles of the invention;

FIG. 6 shows illustrative thresholds for the flowchart of FIG. 5; and

fig. 7 shows another illustrative embodiment in accordance with the principles of the invention.

Detailed Description

Other than the inventive concept, the elements shown in the figures are well known and will not be described in detail. Also, familiarity with television broadcasting and receivers is assumed and is not described in detail herein. For example, in addition to the inventive concept, familiarity with existing and proposed recommended TV standards such as NTSC (national television systems Committee), PAL (phase alternating line), SECAM (sequential and storage color television System), and ATSC (advanced television systems Committee) (ATSC) is assumed. Also, in addition to the inventive concept, transmission concepts such as eight-level (level) vestigial sideband (8-VSB), Quadrature Amplitude Modulation (QAM), receiver components such as Radio Frequency (RF) front-ends or receiver components such as low noise blocks, tuners, demodulators are assumed. Similarly, formatting and encoding methods for generating transport bitstreams, such as the Moving Picture Experts Group (MPEG) -2 systems Standard (ISO/IEC13818-1), are well known and not described herein.

Turning now to fig. 1, a prior art Decision Feedback Equalizer (DFE)100 is shown. DFE100 includes a feed-forward (FF) filter 115, an adder 120, a slicer 125, a Feedback (FB) filter 130, and an error calculator 135. Both FF filter 115 and FB filter 130 are adaptive filters known in the art, each filter comprising a plurality of taps (also referred to in the art as coefficients) (not shown), each tap having a tap value (or coefficient value). To promote hardware efficiency, the taps of each filter are typically arranged in groups that share expensive resources such as large multipliers. Operationally, unequalized data enters FF filter 115 via signal 114, which provides FF output signal 116 to adder 120. Adder 120 sums FF output signal 116 with FB output signal 131 from FB filter 130 to provide equalized output signal 121. The equalized output signal 121 is provided to other parts of the receiver (not shown) and to a slicer 125. Equalized output signal 121 represents a sequence of signal points, each having in-phase (I) and quadrature (Q) values in a constellation space. DFE100 is a feedback device and the feedback path includes slicer 125 and FB filter 130. Slicer 125 is a decision device well known in the art and makes "hard decisions" on the possibly transmitted symbols from the equalized output signal. Specifically, for each signal point of equalized output signal 121, slicer 125 compares the signal point to a symbol constellation (not shown) in the constellation space and selects the symbol of the symbol constellation that is closest to the value of the signal point. Slicer 125 thus provides the sequence of symbols to FB filter 130 via signal 126. (hence the term decision feedback equalizer.) FB filter 130 filters the sequence of symbols and provides FB output signal 131 to adder 120 (as described above).

As described above, both FF filter 115 and FB filter 130 are adaptive filters, i.e., the tap values are adjusted over time so that the overall filter response can adapt to changing channel conditions. The adjustment of the tap values of FF filter 115 and FB filter 130 is performed as a function of the amount of equalized data error (or simply "error") determined by error calculator 135. The error calculator 135 determines the error in any of a number of ways, the most common being a constant modulus (module) algorithm (CMA), a Decision-Directed (Decision-Directed) method, or by training (training). Training and CMA methods only require an equalized output signal (also referred to herein as a "soft equalizer output signal") to derive the error, while decision directed methods use both the soft equalizer output signal and hard decisions from the slicer to derive the error. Thus, fig. 1 shows error calculator 135 receiving both signals 121 and 126. The error is scaled differently for each filter due to the inherent gain difference in FF filter 115 and FB filter 130. This is represented in fig. 1 by the use of separate adjustment signals 136 and 137 for FF filter 115 and FB filter 135, respectively.

As previously described, the ability of the equalizer to adaptively acquire and track the time-varying channel is a function of how much gain is applied to the tap update process. Unfortunately, large gain values may require the use of offset values in the tap update process to limit the amount of self-induced tap noise. In addition, this method of using the offset value to control the self-induced tap noise limits how much gain can be applied to the tap update process. However, the inventors observed that: a high gain can be applied to the equalizer-independent of any offset value (if present) -and still prevent excessive noise generation. Thus, the ability of the equalizer to quickly adapt to changing conditions is further improved. Specifically, and in accordance with the principles of the present invention, an apparatus comprises: an adaptive filter having groups of taps, each group including at least one tap having an associated tap (coefficient) value; and a controller for selecting a scaling factor for at least one tap group as a function of the tap values of that group, and adjusting the error value as a function of the selected scaling factor; wherein the adaptive filter adapts the tap values of the at least one group of taps as a function of the adjusted error value. Thus, high gain can be applied to only those taps of the filter that are adaptively found to have a significant effect on the filter response, thereby obtaining the benefit of high gain on the taps that require high gain while preventing excessive noise generation.

A high-level block diagram of an illustrative television set 10 in accordance with the principles of the invention is shown in fig. 2. A Television (TV) set 10 includes a receiver 15 and a display 20. Illustratively, receiver 15 is an ATSC-compatible receiver. It should be noted that: receiver 15 may also be NTSC (national television systems committee) -compatible, i.e., have an NTSC mode of operation and an ATSC mode of operation such that television set 10 is capable of displaying video content from an NTSC broadcast or an ATSC broadcast. For simplicity in describing the inventive concept, only the ATSC mode of operation is described herein. Receiver 15 receives (e.g., via an antenna (not shown)) broadcast signal 11 for processing to recover (receiver) therefrom, e.g., an HDTV (high definition TV) video signal for application to display 20 for viewing video content thereon.

Referring now to fig. 3, an illustrative embodiment of a Decision Feedback Equalizer (DFE)200 of receiver 15 in accordance with the principles of the invention is shown. DFE200 includes a feed-forward (FF) filter 215, an adder 220, a slicer 225, a Feedback (FB) filter 230, an error calculator 235, an error sealer 250, and an error sealer 255. Both FF filter 215 and FB filter 230 are adaptive filters, each filter comprising a plurality of taps (coefficients) (not shown), each tap having a tap value (or coefficient value). In addition to the inventive concept, DFE200 also operates in a similar manner as described above for DFE 100. Specifically, the unequalized data enters FF filter 215 via signal 214, which provides FF output signal 216 to adder 220. Adder 220 sums FF output signal 216 with FB output signal 231 from FB filter 230 to provide equalized output signal 221. The equalized output signal 221 is provided to other parts of the receiver (not shown) and to a slicer 225. Equalized output signal 221 represents a sequence of signal points, each having in-phase (I) and quadrature (Q) values in a constellation space. Slicer 225 makes "hard decisions" on the possibly transmitted symbols from the equalized output signal and provides a sequence of symbols 226 to FB filter 230. FB filter 230 filters the sequence of symbols and provides FB output signal 231 to adder 220.

As before, the error calculator 235 determines the amount of equalized data error (error). As noted above, any of a variety of techniques may be used, most commonly a Constant Modulus Algorithm (CMA), a decision-directed approach, or by training. Training and CMA methods only require an equalized output signal (also referred to herein as a "soft equalizer output signal") to derive the error, while decision directed methods use both the soft equalizer output signal and hard decisions from the slicer to derive the error. Thus, fig. 2 shows error calculator 235 receiving both signals 221 and 226, although only one of them may be required. The actual method for determining the equalized data error is not relevant to the inventive concept. As described above, since there may be an inherent gain difference in FF filter 215 and FB filter 230, the error is scaled differently for each filter. This is represented in fig. 2 by the use of separate adjustment signals 236 and 237 for FF filter 215 and FB filter 235, respectively. It should be noted, however, that the inventive concept is not so limited and one adjustment signal may be provided to both filters.

In accordance with the principles of the present invention, an adaptive filter is coupled to at least one error sealer (also referred to herein as a controller). The error sealer may be part of the adaptive filter or external to the adaptive filter. In the context of the example illustrated by DFE200, there are two scalers 250 and 255, although the invention is not so limited. For example, there may be one error sealer that processes tap values from more than one adaptive filter (e.g., FF filter 215 and FB filter 230). For this example, error scalers 250 and 255 are similar in operation except for the tap values they process. Thus, the error scaler 250 is used to further illustrate the principles of the present invention.

Turning now to fig. 4, the relevant portions of DFE200 are shown. FB filter 230 includes a plurality of taps T (305). The plurality of taps 305 are divided into K groups, each group including N taps, i.e., T ═ ((K) (N)), where K > 0 and N > 0. This is illustrated in fig. 4 by tap groups 305-1 through 305-K. The tap set is further illustrated in FIG. 4 by tap set 305-j, which includes N taps as represented by taps 306-j-1 through 306-j-N, where 0 < j ≦ K. It should be noted that: although this example shows each tap group having the same number of taps N, the invention is not so limited and the number of taps in each tap group may vary. As shown in fig. 4, the tap values for each tap group are coupled to a selector 255. For example, signal 232-1 conveys N tap values for tap group 305-1; the signal 232-j conveys the N tap values of the tap group 305-j (as represented by signals 231-j-1 through 232-j-N); and signal 232-K conveys the N tap values of tap group 305-K.

In accordance with the principles of the present invention, each group of taps within the adaptive filter receives an error term (error term) to be used in their tap update process, which error term has been specifically (specularly) scaled for that group as a function of tap size (magnitude). One illustrative way of doing this is shown in fig. 4. Selector 255 includes a plurality of selection elements, where each selection element selects an error term or sealer that further adjusts the error from calculator 235. This further adjusted error is then provided to FB filter 230 for its tap update processing. This is illustrated by selection element 310 of selector 255. Selection element 310 processes the N tap values of tap group 305-j and provides an error term to multiplier 315 via signal 311. Multiplier 315 multiplies the error from calculator 235 by an error term (which is conveyed via signal 237) to provide the further adjusted error described above back to FB filter 230 via signal 316 (part of signal 256 of fig. 3). Thus, and in accordance with the principles of the invention, the amount of error to be used in the tap update process of FB filter 230 has been scaled specifically for each tap group of FB filter 230. It should be noted that: the method by which the selector 255 checks the taps of the tap group may vary. For example, the selector 255 may check the taps in parallel (as shown in FIG. 4), or the selector 255 may check the taps serially, e.g., serially scan out the tap values for processing by the selector 255. If in serial fashion, it is assumed that the group boundaries are predetermined and selector 255 knows where they are within the serial stream of generated tap values. However, it should be noted that: the group boundaries are also programmable in the context of the present invention.

Turning now to FIG. 5, an illustrative flow chart for use in a selection element (e.g., selection element 310 of FIG. 4) is shown. In step 505, the selection element receives the N tap values for a particular tap group. In step 510, the selection element 510 selects a sealer or scaling factor (also referred to herein as a step size) as a function of the received N tap values for the tap group. A diagram of the selection function is shown in fig. 6. It should be noted that: the inventive concept is not so limited and other selection functions may be used. The selection process shown in fig. 6 selects the scaling factor as a function of the largest tap size in the tap group. Axis 301 illustrates the values of the increased tap size. Selection element 510 determines the largest tap size of the tap set 305-j of fig. 4 and selects the appropriate scaling factor. In particular, if the determined maximum tap size is smaller than "threshold 1", the scaling factor K is selected₀(ii) a Selecting a scaling factor K if the determined maximum tap size is less than "threshold 2" but greater than or equal to "threshold 1₁And so on. The selected scaling factor is then used to adjust the error (e.g., multiplier 315 of fig. 4), at step 515. Finally, in step 520, the adjusted error is provided to an adaptive filter for use therein in a tap update process. It should be noted that: the threshold may be adjustable or programmable. In addition, if there is only one tap in the set, the scale factor selection is based only on the size of the one tap.

As described above and in accordance with the principles of the invention, a receiver includes an equalizer having groups of taps, each group including at least one tap having an associated tap value; and wherein the equalizer adjusts tap values in each group, wherein at least one group of tap values is adjusted as a function of a step size, the value of the step size being selected as a function of the group of tap values.

Another illustrative embodiment of the present inventive concept is shown in fig. 7. In this illustrative embodiment, an Integrated Circuit (IC)605 for use in a receiver (not shown) includes a DFE620 and at least one register 610 coupled to a bus 651. Illustratively, IC605 is an integrated analog/digital television decoder. However, only those portions of IC605 relevant to the inventive concept are shown. For example, analog-to-digital converters, other filters, decoders, etc. are not shown for simplicity. Bus 651 provides communication to and from other components of the receiver as represented by processor 650. Register 610 represents one or more registers of IC605, where each register comprises one or more bits as represented by bit 609. The registers or portions of registers of IC605 may be read-only, write-only, or read/write. In accordance with the principles of the invention, DFE620 includes the above-described coefficient adjustment or mode of operation, and at least one bit (e.g., bit 609) of register 610 is a programmable bit that can be set, for example, by processor 650 to enable or disable this tap value adjustment mode of operation. In the context of fig. 7, IC605 receives IF signal 601 for processing via an input pin or lead (lead) of IC 605. The correlated signal 602 is applied to DFE620 for filtering. The tap values of DFE620 are further adjusted as described above (e.g., see fig. 4, 5, and 6). DFE620 provides a signal 621 that represents a filtered signal (e.g., signal 221, above). Although not shown in fig. 7, signal 621 may be provided to circuitry external to IC605 and/or may be accessed via register 610. DFE620 is coupled to register 610 via internal bus 611, internal bus 611 representing other signal paths and/or components of IC605 for connecting DFE620 to register 610. IC605 provides one or more recovered signals (e.g., a composite video signal) as represented by signal 606. It should be noted that: other variations of IC605 are possible in accordance with the principles of the invention, e.g., IC605 may simply perform the tap adjustment described above at all times without requiring external control of the tap adjustment mode of operation, e.g., via bits 610.

The present invention can be realized in hardware, software, or a combination of hardware and software. Aspects of the present invention also may be implemented as a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which-when loaded in a computer system-is able to carry out these methods. Computer program or application in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduced in different material forms.

In view of the above, the foregoing merely illustrates the principles of the invention and it will thus be appreciated that: those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly described herein, embody the principles of the invention and are within its spirit and scope. For example, although illustrated in the context of discrete functional elements, these functional elements may be implemented on one or more Integrated Circuits (ICs). Similarly, although shown as discrete elements, any or all of the elements may be implemented in a stored-program-controlled processor (e.g., a digital signal processor) executing associated software, e.g., corresponding to one or more of the steps shown, e.g., in fig. 5. Additionally, although shown as elements included within television 10, the elements therein may be distributed in different units in any combination thereof. For example, receiver 15 of fig. 2 may be part of a device, or a box such as a set-top box physically separate from the device, or a box containing display 20, etc. Moreover, it should be noted that: although described in the context of terrestrial broadcast, the principles of the present invention are applicable to any type of communication system in which filtering is desired, such as, but not limited to, satellite, cable, wireless, and the like. It should therefore be understood that: various modifications may be made to the illustrative embodiments and other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims (19)

1. A receiver, comprising:

an adaptive filter having groups of taps, each group including at least one tap having an associated tap value; and

a controller for selecting a scaling factor for at least one tap group as a function of the tap value for that group and adjusting an error value as a function of the selected scaling factor;

wherein the adaptive filter adapts the tap values of the at least one group of taps as a function of the adjusted error value.

2. The receiver of claim 1, wherein the adaptive filter is part of an equalizer.

3. The receiver of claim 1, wherein the controller multiplies the error value by a selected scaling factor to provide an adjusted error value.

4. The receiver of claim 1, wherein the controller determines a maximum tap value for the at least one group of taps and selects the scaling factor as a function of the determined maximum tap value.

5. The receiver of claim 4, wherein the controller selects the scaling factor by comparing the determined maximum tap value to a plurality of thresholds, each threshold being associated with a particular scaling factor.

6. A receiver, comprising:

an equalizer having groups of taps, each group including at least one tap having an associated tap value; and is

Wherein the equalizer adjusts tap values in each group, wherein the tap values of at least one group are adjusted as a function of a step size, the value of the step size being selected as a function of the tap values of the group.

7. The receiver of claim 6, further comprising:

a selector for providing the selected step size, wherein the selector determines a maximum tap value of the at least one group and selects the step size as a function of the determined maximum tap value.

8. The receiver of claim 7, wherein the selector is part of an equalizer.

9. The receiver of claim 7 wherein the selector multiplies the error value by the selected step size to provide an adjusted error value, the adjusted error value being used by the equalizer to adjust the tap values of the at least one group.

10. The receiver of claim 7, wherein the selector selects the step size by comparing the determined maximum tap value to a plurality of thresholds, each threshold being associated with a particular step size.

11. A method for use in a receiver, the method comprising:

adaptively filtering a signal with an adaptive filter having a plurality of taps, wherein the plurality of taps includes a plurality of tap groups, each tap group having at least one tap;

determining an error value as a function of the filtered signal;

adjusting the error value as a function of tap values of at least one of the tap groups to provide an adjusted error value; and

the taps of the at least one tap group are adapted as a function of the adjusted error value.

12. The method of claim 11, wherein the adjusting step comprises:

selecting a scaling factor as a function of tap values of the at least one tap group; and

multiplying the error value by the selected scaling factor to provide an adjusted error value.

13. The method of claim 12, wherein the selecting step comprises:

determining a maximum tap value for the tap group; and

the scaling factor is selected as a function of the determined maximum tap value.

14. The method of claim 13, wherein the step of selecting a scaling factor comprises:

the determined maximum tap value is compared to a plurality of thresholds, each threshold being associated with a particular scaling factor.

15. A method for use in a receiver, the method comprising:

equalizing the signal with an equalizer to provide an equalized signal, the equalizer having a plurality of tap groups, each tap group having at least one tap;

determining an error value as a function of the equalized signal;

adjusting the error value to provide an adjusted error value; and

the tap value of at least one of the plurality of tap groups is adapted as a function of the adjusted error value.

16. The method of claim 15, wherein the adjusting step comprises:

selecting a step size as a function of tap values of the at least one tap group; and

the error value is adjusted as a function of the selected step size to provide an adjusted error signal.

17. The method of claim 16, wherein the adjusting the error value comprises:

the error value is multiplied by the selected step size to provide an adjusted error signal.

18. The method of claim 16, wherein the selecting step comprises:

determining a maximum tap value of the tap values of the at least one tap group; and

the step size is selected as a function of the determined maximum tap value.

19. The method of claim 18, wherein the step of selecting the step size comprises:

the determined maximum tap value is compared to a plurality of thresholds, each threshold being associated with a particular step size.