Dual Switched Predictive DIR MLSD Receiver for Dynamic Channels
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Dual Switched Predictive DIR MLSD Receiver for Dynamic Channels Michael Boyle Department of Electronic and Electrical Engineering, University College Dublin, Belfield, Dublin 4, Ireland Email: [email protected]
Anthony D. Fagan Department of Electronic and Electrical Engineering, University College Dublin, Belfield, Dublin 4, Ireland Email: [email protected] Received 1 August 2001 and in revised form 15 March 2002 A linear prefilter can be used to reduce the required complexity of a maximum likelihood sequence detector Viterbi algorithm (MLSD-VA) by shortening the overall channel and prefilter impulse response in dynamic communication systems. The combination of channel and prefilter should have the effect of producing a desired impulse response (DIR) at the detector. Falconer and Magee (1973) showed that for a finite length DIR there are a limited number of possible DIRs that are optimal. For a DIR of length two symbols, there are only two optimal DIRs, for a length three DIR there exists a range of possibly optimal DIRs. In this paper, we present a novel receiver architecture in which we use two equalisers and two Viterbi detectors. Each equaliser has a different target DIR. A selection device chooses between the output of the two VAs. It is demonstrated that, using the two optimal length two DIRs can be preferable to both switched triple DIR system and adaptive DIR strategies. It is also demonstrated in this paper that there exists a range of environments where adaptive DIR MLSD-VA receivers fail, however the proposed dual switched DIR MLSD-VA is successful in these environments. The efficacy of the switched dual DIR MLSD-VA is also shown using doubly selective fading channels. Keywords and phrases: maximum-likelihood sequence detection, fading channels, equalisation.
1.
INTRODUCTION
The detection of a signal transmitted through a communication channel that contains intersymbol interference (ISI) and additive Gaussian noise has been widely studied for a broad range of channel models. Maximum likelihood sequence detection (MLSD) implemented using the Viterbi algorithm, proposed by Forney [1], is an optimal equalisation method to combat ISI. The computational complexity of the Viterbi algorithm (VA) grows exponentially with the length of the channel impulse response (CIR). One technique [2] that has been used to reduce the complexity of the VA is to use a prefilter to truncate the CIR. The cascade of the prefilter and the channel produces an equivalent channel impulse response (ECIR) at the input to the VA, which is close to the adaptive desired impulse response (DIR) being used by the VA. The DIR is shorter than the CIR and hence the complexity of the VA is reduced. In [3], different length DIRs were investigated using the mean square error (MSE) criterion. In [4], the optimal DIR using the effective signal-to-noise ratio (SNR) criterion was determined. In both cases, it was shown
that for the case of a length two DIR there exist just two optimal DIRs. If the power of the DIRs is√constrained√to one, √ then √ the
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