Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-06-30
2004-04-20
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Interference or noise reduction
C348S607000
Reexamination Certificate
active
06724844
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed in general to digital signal processing, and in particular to receiver based methods and devices for improving decision feedback equalizer (“DFE”) performance in a trellis coded system.
2. Background of the Invention
In many digital communication scenarios (e.g. telephone transmission, broadcast TV transmission, cable etc.) The transmitted signal arrives at the receiver through more than one path in addition to the direct path. This condition is called “multipath” and leads to intersymbol interference (“ISI”) in the digital symbol stream. This ISI is compensated for in the receiver through an equalizer which in many cases is a DFE as shown in FIG.
1
. U.S. Pat. No. 5,572,262 shows one method of combating these multipaths.
A DFE
10
(
FIG. 1
) has two filter sections, a forward filter
12
and a feedback filter
16
. The input to the forward filter
12
is the received data which includes the transmitted symbol sequence a
k
, noise n
k
and multipath h
i
. The input to the feedback filter
16
is the quantized equalizer output â
k
. The output of both the sections are summed
18
to form the final equalizer output ã
k
which is also the input to the next stage in a trellis-coded system, the trellis decoder. While a DFE performs better than a linear equalizer in severe ISI, the performance is limited by error propagation through the feedback filter
16
of the DFE
10
. Error propagation occurs in the feedback filter
16
when the quantized equalizer output â
k
is not the same as the transmitted symbol a
k
. If an error is made in determining the symbol â
k
at the output of the slicer
14
, this incorrect symbol is fed back to the input of the feedback filter
16
and propagates. In many systems which employ error correction codes like trellis codes and/or Reed-Solomon codes to obtain very low error rates at moderate SNRs, the “raw” symbol error rate (SER) at the equalizer output can be extremely high. For example, in the VSB system, at threshold in white noise the SER at the equalizer output is about 0.2. The increased error propagation due to these high SERs can cause the DFE to lose a couple of dB in performance as compared to the case of no error propagation. Additionally, the error propagation causes the error sequence at the equalizer output to be correlated, since it depends on past incorrect symbol decisions. This correlation has an adverse effect on the subsequent trellis decoder which is usually designed for a white noise sequence.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to reduce the mean-squared-error (MSE) and correlation of the DFE output error sequence. This object is achieved by placing a short adaptive predictor filter after the DFE in order to whiten the error sequence. The ISI introduced into the data symbol stream by this filter is then compensated in the trellis decoder by the use of delayed decision feedback sequence estimation (DDFSE) as described in “Delayed decision-feedback sequence estimation,” by A. Duel-Hallen and C. Heegard, IEEE Trans. Commun., vol. COM-37, no. 5, pp. 428-436, May 1989.
The invention has applications in any trellis coded system that utilizes a DFE for equalization, especially in cases where the SER after the equalizer is high and error propagation causes problems. An example is terrestrial transmission of digital TV signals.
The invention can also be used in trellis coded systems with DFEs which are subject to colored noise interference. Again, the DFE cancels much of the interference but the adaptive predictor serves to whiten the noise further. This helps improve the trellis decoder performance.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.
REFERENCES:
patent: 5052000 (1991-09-01), Wang et al.
patent: 5293402 (1994-03-01), Crespo et al.
patent: 5572262 (1996-11-01), Ghosh
patent: 5784415 (1998-07-01), Chevillat et al.
patent: 6012161 (2000-01-01), Ariyavisitakul et al.
patent: 6134265 (2000-10-01), Long
“Delayed Decision-Feedback Sequence Estimation”, By Alexandra Duel-Hallen et k\al., IEEE Transactions on Communications, vol. 37, No. 5, May 1989, pp. 428-436.
Adapative Signal Processing, by Bernard Widrow et al., pp. 99-115.
Chin Stephen
Gatham Laurie E.
Williams Lawrence
LandOfFree
Method and device for improving DFE performance in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for improving DFE performance in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for improving DFE performance in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3209029