Dual decision feedback equalizer with selective attenuation...

Details Dual decision feedback equalizer with selective attenuation... Dual decision feedback equalizer with selective attenuation...

1998-05-29

2001-10-23

Chin, Stephen (Department: 2734)

Pulse or digital communications

Equalizers

Automatic

C375S229000, C375S232000, C375S340000

Reexamination Certificate

active

06307884

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of data communication channels, and more particularly, but not by way of limitation, to a read channel of a disc drive having an improved dual decision feedback equalizer (DDFE) that employs feedback attenuation during an erasure window to improve channel readback performance.
BACKGROUND OF THE INVENTION
Data communication channels are used to transmit and receive information in an efficient and reliable manner, Such channels are utilized in a variety of applications from wireless communication networks, such as mobile cellular and satellite communication systems, to computer data storage devices, such as hard disc drives.
In a receiving portion of a data communication channel, detection circuitry is necessary to detect and decode the information being transmitted, One basic type of detection circuitry is known as a detection feedback equalizer (DFE). As will be recognized by those skilled in the art, a typical DFE comprises a slicer which applies a selected threshold to a prefiltered input signal to generate a corresponding decision nominally indicative of the data value of the input signal (such as +1 or −1). A feedback filter having a response excited by past decisions provides an output which is subtracted from the input signal in order to cancel trailing intersymbol interference (ISI), with the resulting sum characterized as a decision variable. An error term is also generated in relation to the presence of noise in the channel as a difference between the decision variable and the decision.
In the absence of noise, the decision variable will generally be equal in magnitude to the input signal, the decision will be correct and the error term will be zero. However, as the amount of noise in the channel increases, the error term will grow; when the noise is sufficiently large, the decision variable will change polarity and an erroneous decision will be made. This erroneous decisions can propagate for some time due to the feedback provided by the feedback filter. For examples of communication channels utilizing various DFE based architectures, see U.S. Pat. No. 4,985,902 issued Jan. 15, 1991 to Gurcan, U.S. Pat. No. 5,027,369 issued Jun. 25, 1991 to Kuenast, and U.S. Pat. No. 5,430,661 issued Jul. 4, 1995 to Fisher et al.
To provide improved error rate performance over a single DFE, a dual decision feedback equalizer (DDFE) architecture was recently discussed by Bergmans et al. in a paper entitled “Dual Decision Feedback Equalizer,” Philips Research Laboratories, Prof. Holstlaan 4, 5656 Eindhoven. The Netherlands, Nov. 27, 1996. In this paper, Bergmans et al, propose the use of two DFEs which are operated in parallel so that each independently makes decisions on an input sequence. The DFEs are nominally identical, except that the first DFE utilizes a slicer threshold of &agr; and the second DFE utilizes a complementary slicer threshold of −&agr; (the interval −&agr; to &agr; defining an “erasure zone”).
When noise is small, the decisions of both DFEs will be correct and identical. However, when one or both of the decision variables fall within the erasure zone, the first DFE is caused to output a decision +1, the second DFE is caused to output a decision −1, and an erasure flag is set. At this point the slicer thresholds are both temporarily set to zero. The respective error terms are then accumulated for a selected amount of time during an erasure period and the DFE with less error energy is selected as the correct sequence of decisions for the period. This is based on the reasoning that the erroneous decision sequence will likely contain a larger error energy than the error energy associated with the correct decision sequence. At the conclusion of the erasure period, the slicer thresholds are reset and the DFEs resume independent operation.
A DDFE provides improved error rate performance over a DFE, approaching the higher rates of performance achievable by more complex types of detectors, such as the well-known Viterbi type. Improvements in the error rate performance of a DDFE are nonetheless desirable, especially for high data rate transfer applications such as disc drives, and it is to this end that the present invention is directed.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for improving error rate performance of a communication channel, such as a read channel of a disc drive.
As exemplified in a preferred embodiment, the communication channel comprises a dual decision feedback equalizer with selective attenuation (DDFE/SA). The DDFE/SA comprises a first decision feedback equalizer operably coupled to receive a series of input signals z
k
and comprising a first slicer which utilizes a selected threshold −&agr; to generate a series of decisions â ;
1
k
nominally indicative of corresponding peak data values of the series of input signals z
k
. The first decision feedback equalizer further comprises a first feedback filter, operably coupled to receive the series of decisions â ;
1
k
from the first slicer, which provides a filtered output (a*p)
1
k
based on previous decisions, and a first summing junction, operably coupled to receive and sum the filtered output (a*p)
1
k
from the first feedback filter and the series of input signals z
k
to generate a series of decision variables ã
1
k
which are input to the first slicer.
The DDFE/SA further comprises a second decision feedback equalizer operably coupled in parallel with the first decision feedback equalizer to receive the series of input signals z
k
and comprising a second slicer which utilizes a selected threshold &agr; to generate a series of decisions â ;
2
k
nominally indicative of corresponding peak data values of the series of input signals z
k
, a second feedback filter, operably coupled to receive the series of decisions â ;
2
k
from the second slicer, which provides a filtered output (a*p)
2
k
based on previous decisions, and a second summing junction, operably coupled to receive and sum the filtered output (a*p)
2
k
from the second feedback filter and the series of input signals z
k
to generate a series of decision variables ã
2
k
which are input to the second slicer.
The DDFE/SA further comprises first and second attenuators, operably coupled to the first and second feedback filters, respectively, which apply the selective attenuation to the first and second feedback filters during an erasure window when a mismatch is detected between the series of decisions â ;
1
k
and â ;
2
k
(i,e., when the series of decisions are unequal). During normal operation when no mismatch is detected, the first and second attenuators apply an attenuation gain of one (i,e,, no attenuation is applied during normal operation).
Further, the DDFE/SA is provided with first and second multi-stage buffers, operably coupled to receive the series of decisions â ;
1
k
and â ;
2
k
, respectively, first and second squaring blocks, operably coupled to receive and square the series of error terms e
1
k
and e
2
k
, respectively, a third summing junction, operably coupled to receive and sum the squared series of error terms e
1
k
and e
2
k
, respectively, and an accumulator, operably coupled to receive a summed output of the third summing junction to generate an accumulated error energy. A switch, operably coupled to the first and second multi-stage buffers and the accumulator, selects a correct series of decisions from the series of decisions â ;
1
k
and â ;
2
k
stored in the first and second multi-stage buffers, respectively, in relation to a polarity of the accumulated error energy at such time that the mismatch between the series of decisions â ;
1
k
and â ;
2
k
is resolved; that is, in relation to the series having the lower error energy.
Generally, the DFE producing the correct decision sequence will likely continue to make correct decisions, whereas the DFE producing the erroneous decision sequence will likely make additionally incorrect decisions. Thus, by attenuating the feed

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