Computationally Efficient Blind Code Synchronization for Asynchronous DS-CDMA Systems with Adaptive Antenna Arrays
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Computationally Efficient Blind Code Synchronization for Asynchronous DS-CDMA Systems with Adaptive Antenna Arrays Chia-Chang Hu Department of Electrical Engineering, National Chung Cheng University, Min-Hsiung, Chia-Yi 621, Taiwan Email: [email protected] Received 28 July 2003; Revised 18 February 2004 A novel space-time adaptive near-far robust code-synchronization array detector for asynchronous DS-CDMA systems is developed in this paper. There are the same basic requirements that are needed by the conventional matched filter of an asynchronous DS-CDMA system. For the real-time applicability, a computationally efficient architecture of the proposed detector is developed that is based on the concept of the multistage Wiener filter (MWF) of Goldstein and Reed. This multistage technique results in a self-synchronizing detection criterion that requires no inversion or eigendecomposition of a covariance matrix. As a consequence, this detector achieves a complexity that is only a linear function of the size of antenna array (J), the rank of the MWF (M), the system processing gain (N), and the number of samples in a chip interval (S), that is, O(JMNS). The complexity of the equivalent detector based on the minimum mean-squared error (MMSE) or the subspace-based eigenstructure analysis is a function of O((JNS)3 ). Moreover, this multistage scheme provides a rapid adaptive convergence under limited observation-data support. Simulations are conducted to evaluate the performance and convergence behavior of the proposed detector with the size of the J-element antenna array, the amount of the L-sample support, and the rank of the M-stage MWF. The performance advantage of the proposed detector over other DS-CDMA detectors is investigated as well. Keywords and phrases: code-timing acquisition, rank reduction, smart antennas, adaptive interference suppression, generalized likelihood ratio test.
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INTRODUCTION
Spread-spectrum communication systems have been used successfully in military applications for several decades. Recently, direct-sequence (DS) code-division multiple access (CDMA), a specific form of spread-spectrum transmission, has become an important component in third-generation (3G) mobile communication systems, such as wideband CDMA (W-CDMA) or multicarrier CDMA (MC-CDMA) for 3G cellular radio systems, because of its many advantages compared with the conventional frequency- and/or time-division multiple-access (FDMA/TDMA) systems. In a DS-CDMA communication system, all users are allowed to transmit information simultaneously and independently over a common channel using preassigned spreading waveforms or signature sequences that uniquely identify the users. In [1], Verdu´ demonstrates that a DS-CDMA receiver is not fundamentally multiple-access interference (MAI) limited and can be near-far resistant. The proposed optimal multiuser detector for DS-CDMA signals comprises a bank of matched filters followed by a maximum-likelihood sequence detector whose decision algorithm is the Viterbi algorithm. ´ deUnfortunately, th
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