An Efficient Code-Timing Estimator for DS-CDMA Systems over Resolvable Multipath Channels
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An Efficient Code-Timing Estimator for DS-CDMA Systems over Resolvable Multipath Channels Jianhua Liu College of Engineering, Embry-Riddle Aeronautical University, 600 S. Clyde Morris Boulevard, Daytona Beach, FL 32114, USA Email: [email protected]
Jian Li Department of Electrical and Computer Engineering, University of Florida, P.O. Box 116130, Gainesville, FL 32611-6130, USA Email: [email protected] Received 31 July 2003; Revised 26 March 2004 We consider the problem of training-based code-timing estimation for the asynchronous direct-sequence code-division multipleaccess (DS-CDMA) system. We propose a modified large-sample maximum-likelihood (MLSML) estimator that can be used for the code-timing estimation for the DS-CDMA systems over the resolvable multipath channels in closed form. Simulation results show that MLSML can be used to provide a high correct acquisition probability and a high estimation accuracy. Simulation results also show that MLSML can have very good near-far resistant capability due to employing a data model similar to that for adaptive array processing where strong interferences can be suppressed. Keywords and phrases: DS-CDMA, synchronization, code timing, multipath channels.
1.
INTRODUCTION
Direct-sequence code-division multiple access (DS-CDMA) is one of the most promising multiple-access technologies for the next-generation wireless communication services. For time-dispersive fading channels, DS-CDMA can outperform other multiple-access schemes, such as FDMA (frequencydivision multiple access) and TDMA (time-division multiple access), due to its capability of exploiting the RAKE combination [1] to combat the time-dispersive fading problem effectively. The structure of the RAKE receiver is determined by the type of the time-dispersive multipath channel over which the DS-CDMA system works. There are mainly two kinds of time-dispersive multipath channels for the DS-CDMA systems. The first one is the (nearly) continuous channel, which can be represented by a finite impulse response (FIR) filter channel model; the second one is the discrete channel where the separations of the resolvable bunches of multipaths (referred to as resolvable paths in the sequel) are larger than the chip duration of the spreading code. The latter can be represented by a resolvable time-dispersive channel model. While the first model is quite effective in describing channels in urban areas, the second model is suitable for describing channels in rural or mountain areas or in the case of soft handoff [2], where a few resolvable paths exist.
In this paper, we focus on the channels represented by the second model and we refer to them as the resolvable multipath channels. For the resolvable multipath channels considered herein, the RAKE receiver assumes the knowledge of the channel parameters including the code timing, signal power, as well as the carrier phase for each resolvable path of each user. These parameters are typically unknown in practice and hence need to be estimated. In this paper, we consider the problem o
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