Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2001-01-05
2002-04-02
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C375S233000, C333S02800T
Reexamination Certificate
active
06366613
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to digital communications systems and more particularly relates to a highly stable, fast tracking equalizer suitable for use in combating time varying channels within a digital communications system.
BACKGROUND OF THE INVENTION
Channel Reflections and Digital Television Signals
In the coming years digital television broadcasting will take on more and more of a dominant role in television broadcasting. Public broadcasting of digital television signals has already begun the United States. Some time in the early 2000s, it is forecasted that the broadcasting of most analog television signals will cease and will be replaced by TV signals that are digital in nature, whether over terrestrial links, i.e., over the air, cable or satellite.
A problem associated with the transmission of terrestrial digital TV signals is signal reflections. Signal reflections can be caused by many factors including stationary objects such as buildings and moving objects such as airplanes.
Weak reflections that are generated relatively close to the receiver do not pose as nearly as much of a problem as strong reflections that are generated relatively far away from the receiver. The latter type of reflections being very problematic to the reception of digital signals such as digital TV signals.
Channel reflections are another common form of linear distortion ISI which constitute a common impairment in digital communications channels. They can, however, be negated by employing an equalizer in the receiver.
In many cases the amplitude level and the time delay of the reflections are time varying. These amplitude level and the time delay variations are associated typically with wireless transmission, where they may result from movement of objects which reflect the transmitted signals. In order to compensate for this type of noise, the receiver must track the channel variations and adapt the parameters of the equalizer accordingly.
Decision Feedback Equalizers
A problem frequently encountered in full duplex digital data communication systems that employ a limited bandwidth channel is the presence of linear distortion introduced into the signal propagation path. The linear distortion commonly manifests itself as intersymbol interference (ISI) in the received symbol sequence. In order to reduce the effects of this distortion, it is common practice in the signal processing art to process the received signals by some form of linear and non-linear filter mechanism, such as a decision feedback equalizer (DFE).
The samples are then fed to a feed forward linear filter section. The feed forward linear filter comprises a delay line, i.e., z
−1
, and each stage of which stores a respective symbol sample. The contents of the respective stages of the delay line are multiplied by respective weighting coefficients W
i
and then summed in an adder, to yield a combined output. This output can be applied to a downstream decision feedback section, from which output data decisions are derived.
The output of the adder is adjusted by subtracting the output of the decision feedback section from the output of the adder. The effect of subtracting the output of the decision feedback section from the linear filtered section is to remove intersymbol interference due to previously detected symbols.
Note that data decision estimates are derived on a symbol by symbol basis by means of a symbol decision mechanism, such as a symbol slicer. The symbol slicer functions to slice the signal at equally spaced levels between reference levels for the received symbols. These output data decisions are then fed back to a linear delay line to remove intersymbol interference from future symbols. The contents of the respective z
−1
stages of the delay line are multiplied by respective weighting coefficients and then summed in an adder to produce a combined output to be subtracted from the output of feed forward section.
A residual error signal for adjusting the weighting coefficients of the linear section and the decision feedback section of the filter may be obtained by differentially combining data decision estimates at the output with the output of the summation block. In the ideal conventional DFE equalizer architecture the weighting coefficients W
i
for the feed forward filter section are assumed to be one-sided. i.e., anticausal, and the last, or most delayed, tap Z
−1
of the delay line is typically the largest and is commonly referred to as the main tap, reference tap or the cursor tap. The current decision on the value of a received symbol is customarily considered to have its dominant energy contribution derived through this tap.
The weighting taps of the feedback section take on values equal to samples of the postcursor or ‘tail’ of the received symbol which follows as the symbol energy decays.
Since the classical DFE structure assumes that the number of taps or stages is infinite, practical realization requires truncating the lengths of the respective feed forward and feed back delay lines at some practical number of taps per filter. In order to prevent significant degradation of the signal, the number of taps selected for the feedback stage must be sufficient to span all significant samples of the signal at the point of ISI cancellation. The number of taps of the upstream stage is not as readily apparent.
Although this number is related to the precursors, it is not necessarily equal to the significant energy span of the precursors. One method to establish the length of the filter is to either compute the coefficients or simulate the filter with a large number of coefficients and determine how many are significant. This approach, however, is heavily channel dependent since, in practice, the signal processing circuit designer does not have the freedom to implement a ‘whitened’ matched filter in the analog domain prior to sampling, which would be different for every line shape and noise spectrum. Ultimately, some prescribed fixed shaped is employed, or a simple anti aliasing filter may be used upstream of the sampling point.
In order to train the adaptive equalizer, data values or symbols corresponding to the transmitted data are used. Training is normally carried out using a predetermined training sequence. Alternatively, if the data decisions are sufficiently reliable prior to convergence, these data decisions can be used for training. When a training sequence is employed it is common practice to derive a rough approximation of the amount of delay and allow the taps to grow until the largest tap is identified. Then the amount of delay is adjusted so as to place the cursor tap at the desired location that is the last stage of the feed forward delay line.
Prior art solutions, such as that described above, are limited in their ability to track fast time variations in channels having reflections with large delays. To compensate for the linear distortion in such channels, an equalizer with a large number of parameters is needed, making fast tracking of channel variations difficult (and sometimes even not feasible). For example, a terrestrial digital television (DTV) signal may have reflections of up to 20 micro-seconds. If a linear equalizer or a decision feedback equalizer (DFE) is used to combat such reflections, then at least 200 taps will be required. These reflections may originate from a moving airplane, in which case they can vary significantly within a period of 10000 symbols, and thus be very difficult to track when using a prior art equalizer architecture.
SUMMARY OF THE INVENTION
This present invention is an adaptive equalizer structure and an equalization method that permits fast tracking of time varying reflections without sacrificing the stability of the equalizer. The ability to track fast variations in due in part to the sectioning of the equalizer into small filtering sections. The equalizer identifies sections of the equalizer that need to be adjusted rapidly due to channel variations and, consequently, the adaptation rate of the parameters of these
Segal Mordechai
Shalvi Ofir
Sommer Naftali
Brady III Wade James
Ghebretinsae Temesghen
Holmbo Dwight N.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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