Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2000-05-15
2003-11-18
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Equalizers
Automatic
C375S347000, C455S137000
Reexamination Certificate
active
06650702
ABSTRACT:
BACKGROUND
The present invention relates generally to equalizers, and more particularly, to a decision feedback equalizer and method that provides for blind initialization of the decision feedback equalizer using an antenna array.
The closest prior art known to the inventor relating to the present invention is an article entitled “Adaptive Decision Feedback Equalization: Can You Skip the Training Period?” by Labat et al., IEEE Transactions on Communications, Volume 46, No. 7, July 1998, pages 921-930. As is stated in this article, decision feedback equalizers (DFE's) are often used to combat distortion of communication channels. Even with severe and noisy channels, DFE's can have good steady-state performance, a small output mean-square error (MSE), at a very much lower computational cost than other techniques, such as maximum-likelihood sequence estimation. Since the channels are unknown, the DFE must be implemented in an adaptive way.
In a classical DFE, adaptation cannot be started without the transmission of a known training data sequence d(k), referred to as the training period. Then the proper transmission begins and the tracking of channel distortion is pursued by unsupervised adaptation using the detected data d(k) in place of the true data d(k), which is the decision-directed mode.
The channel can drastically change during the tracking period. Consequently, a continued adaptation requires the periodic transmission of the training sequence, which decreases the effective bit rate and is not always possible. That is why unsupervised DFE's, without any training sequence, are necessary. Adaptation starts as soon as the (unknown) data d(k) are transmitted.
Heretofore, unsupervised adaptive algorithms have been designed for transversal equalizers based on the assumption that the sequence d(k) is zero-mean independent identically distributed. Unfortunately, with a recursive equalizer such as a DFE, the phenomenon of error propagation restricts the use of unsupervised adaptation to the case of an initial “open eye,” corresponding to a mild channel. On the other hand, severe channels cannot be properly corrected with a transversal equalizer, especially when there is additive noise, and hence there is a dilemma.
The Labat et al. article discloses an unsupervised (blind) adaptive decision feedback equalizer (DFE that attempts to overcome the problems associated with prior approaches. The Labat et al. DFE is a cascade of four devices, whose main components are a purely recursive filter (R) and a transversal filter (T). The major feature of the Labat et al. DFE is its ability to deal with severe, quickly time-varying channels, unlike prior adaptive DFEs. The equalizer modifies, in a reversible way, both its structure and its adaptation according to a performance measure such as the mean-square error (MSE).
In the starting mode, R comes first and whitens its own output by means of a prediction principle, while T removes the remaining intersymbol interference (ISI) using a Godard (or Shalvi-Weinstein) algorithm. In tracking mode, the equalizer becomes a classical DFE controlled by a decision-directed least-mean-square algorithm. With the same computational complexity, the unsupervised equalizer exhibits the same convergence speed, steady-state MSE, and bit-error rate (BER) as the trained conventional DFE, but it requires no training. The disclose DFE has been implemented on a digital signal processor (DSP) and tested on underwater communications signals-its performances are really convincing.
However, the Labat et al. article discloses or suggests nothing regarding the use of inputs derived from an antenna array to blindly initialize a decision feedback equalizer. It is therefore an objective of the present invention to provide for an improved decision feedback equalizer and nethod that provides for blind initialization of the decision feedback equalizer using an antenna array.
SUMMARY OF THE INVENTION
To accomplish the above and other objectives, the present invention provides for blind initialization of a decision feedback equalizer (DFE) using an antenna array. Various apparatus and methods are disclosed that provide for blind initialization of the decision feedback equalizer.
The decision feedback equalizer comprises an antenna array including two or more antenna elements or sensors that comprise input paths. Outputs of the two or more antenna elements or sensors are coupled to respective finite impulse response (FIR) filters. Outputs of the FIR filters are coupled to an adder that combines the respective signals. The output of the adder is coupled to a decision device. The output of the decision device is coupled to a feedback FIR filter whose output is fed back to the adder and combined with signals derived from the antenna elements and FIR filters. A switch is disposed in the decision feedback path between the decision device and the feedback FIR filter. A switch and an attenuator are disposed in parallel in all but one of the input paths to the adder.
The initialization scheme for the decision feedback equalizer has three modes of operation. In initialization mode, the antenna array is blindly adapted using any of a variety of blind array algorithms (e.g., a constant modulus algorithm (CMA), or prediction error method, for exmaple). In transition mode, feedback and feed-forward structures of the decision feedback equalizer are allowed to adapt in a decision-directed mode as inputs from all but one element of the antenna array are slowly attenuated to zero. In decision feedback equalizer mode, the decision feedback equalizer operates independently of the other array elements, using a single antenna element, the decision device, decision feedback FIR filter, and adder. The other array elements may be used for other tasks.
The present invention thus provides for a means to initialize the decision directed adaptation of the decision feedback equalizer in a strong multipath environment. The present invention provides for blind reception of digitally modulated communication signals in the strong multipath environment.
An exemplary method in accordance with the present invention that blindly initializes a decision feedback equalizer is as follows. A decision feedback equalizer is provided that comprises a plurality of antenna elements, or sensors that provide input paths a plurality of feedforward finite impulse response (FIR) filters, a decision feedback FIR filter, an adder for combining signals output by the feedforward and decision feedback FIR filters, and a decision device disposed between the adder and the decision feedback FIR filter.
In implementing the present method, digitally modulated communication signals are transmitted in a strong multipath environment. The digitally modulated communication signals are received by each of the plurality of antenna elements, or sensors, of the decision feedback equalizer. The digitally modulated communication signals received by each of the antenna elements, or sensors, are processed as follows.
The decision feedback equalizer is initialized using a blind adaptation algorithm while the feedback FIR filter is not used. The decision feedback equalizer transitions to normal operation wherein the feedforward FIR filters and the decision feedback FIR filter are allowed to adapt while inputs from all but one element of the antenna array are slowly attenuated to zero, using variable attenuators in the input paths. The decision feedback equalizer then operates in the normal mode, wherein the decision feedback FIR filter operates independently of the other FIR filters receiving input from only one antenna element and feed forward FIR filter.
REFERENCES:
patent: 5727032 (1998-03-01), Jamal et al.
patent: 5844951 (1998-12-01), Proakis et al.
patent: 5901174 (1999-05-01), Richard
patent: 6115419 (2000-09-01), Meehan
patent: 6400761 (2002-06-01), Smee et al.
Bocure Tesfaldet
Lockheed Martin Corp.
Nelson Kenneth W.
LandOfFree
Blind initialization of decision feedback equalizer using an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Blind initialization of decision feedback equalizer using an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Blind initialization of decision feedback equalizer using an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3164818