Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1999-12-23
2001-08-14
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C375S341000, C714S795000
Reexamination Certificate
active
06275525
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to any digital wireless mobile system which employs a training sequence. More specifically, the present invention relates to a demodulation technique for decoding a received signal under channel impairment conditions.
In digital wireless mobile system technology demodulating a received signal under channel impairment conditions may require the use of specialized digital signal processing techniques. Demodulation in this case refers to the extraction of a message signal from a modulated carrier after it has propagated through a transmission (radio) channel.
In wireless systems, the radio channel may impose severe signal impairments. For example, the mobility of a wireless unit causes channel variations known as Doppler effect. Moreover, the reception of a signal via multiple propagation paths also results in delay spread, which introduces inter-symbol interference within the received signal and may add destructively.
In addition, due to the movement of the mobile unit, the characteristics of the delay spread and its resultant inter-symbol interference can change rapidly with time. Therefore, to optimize reception, propagation signal impairments must be eliminated, or else utilized in ways which can improve the demodulation of the signal data.
To demodulate an impaired signal in wireless technology, the receiver must first estimate the channel via a training sequence, which is a sequence of symbols either prefixed, postfixed, or positioned in the middle of the frame to a transmitted sequence of symbols, and is known by the receiver. Assuming that the model for the impairments, in the form of multi-tap channel coefficients, is known by the receiver, and that the input-output characteristics of the channel may be regarded as constant during at least one symbol interval, the MLSE (Maximum likelihood Sequence Estimation) equalizer process can be used to demodulate the received signal. This detection technique uses an estimated model for the channel characteristics to update the branch metrics used in a Viterbi decoding process. Since the channel characteristics are a function of the movement of the mobile station, the channel may vary every signaling interval. Therefore, for reliable demodulation performance under severe channel impairment conditions, frequent updates of the channel model are required.
In prior art approaches to the demodulation of a signal with a delay spread impairment, a decision feedback equalizer is often used. Also, the General MLSE approach and adaptive MLSE approaches are used as well. However, most commonly used prior art channel estimation techniques may not provide sufficient accuracy, or they may suffer from excessive lag, due to inherent decision delays. As such, overall receiver performance can be significantly degraded.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art by providing an adaptive-predictive estimation scheme which provides a reliable demodulation process for a received signal with severe channel impairments.
BRIEF SUMMARY OF THE INVENTION
The present invention is, in one embodiment, a method for adaptively equalizing an output of a digital communication channel, including steps of: generating an initial channel estimation, determining an initial set of metric states of a trellis, based on known symbols of a received input signal comprising a sequence of symbols; updating the set of metric states according to the initial channel estimation and the received signals updating a set of paths for the metric states using a delayed tentative decision of a symbol of the received signal, to determine a set of survivor paths; and updating the channel estimation, based on the updated metrics, delayed tentative decision, and the received signal.
This process enables reliable demodulation performance under conditions of severe channel impairments.
Illustrative embodiments of the present invention are more fully described below in conjunction with the following drawings.
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Yow-Jong Liu, “Performance of Adaptive Equalization and Solf-Decision Decoding Techniques over TDMA Digital Radio Channels” IEE, 1992.
Bahai Ahmad Reza
Sanwal Kumud Kumar
Armstrong Teasdale LLP
National Semiconductor Corporation
Pham Chi
Phu Phuong
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