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MMSE interference suppression for timing acquisition and demodulation in direct-sequence CD.pdf

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MMSE interference suppression for timing acquisition and demodulation in direct-sequence CD

MMSE Interference Suppression for Timing Acquisition andDemodulation in Direct-Sequence CDMA SystemsUpamanyu Madhow1AbstractIt has been recently shown that Minimum Mean Squared Error (MMSE) demodulators arean eective means of interference suppression in CDMA systems. The MMSE demodulatorcan be implemented adaptively using an initial training sequence, followed by decision-directedadaptation. This requires that the symbol-level timing of the desired user be known prior totraining. In this paper, we remove this requirement by providing a method for timing acquisitionin which the output of the acquisition process is a near-far resistant demodulator which auto-matically accounts for the delays and amplitudes of both the desired signal and the interferencewithout explicitly estimating these parameters. The only requirements are a training sequencefor the desired user and a nite uncertainty regarding the symbol timing. Even if the absolutetiming uncertainty is arbitrarily large, the latter condition can be realized by using a periodictraining sequence.Keywords: Direct-sequence, CDMA, interference suppression, training, timing acquisition, near-farproblem, MMSE, equalization1 IntroductionThe MMSE criterion leads to demodulators for direct-sequence CDMA which do not suer from thenear-far problem or from the interference oor in performance exhibited by conventional matched lter demodulation. Application of the linear MMSE criterion to centralized multiuser detectionwas suggested in [21]. More recently, it was recognized that, since the received signal in linearlymodulated CDMA systems with short spreading waveforms (i.e., with spreading waveforms whoseperiod equals the symbol interval) is cyclostationary, the linear MMSE criterion is amenable to1The author is with the ECE Department and the Coordinated Science Laboratory, University of Illinois, 1308W. Main, Urbana, IL 61801, USA (Phone: 217 333-0943, Fax: 217 244-1642, Email: madhow@). This workwas supported by the Oce