Space-Time Joint Interference Cancellation Using Fuzzy-Inference-Based Adaptive Filtering Techniques in Frequency-Select
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Space-Time Joint Interference Cancellation Using Fuzzy-Inference-Based Adaptive Filtering Techniques in Frequency-Selective Multipath Channels Chia-Chang Hu,1 Hsuan-Yu Lin,1 Yu-Fan Chen,2 and Jyh-Horng Wen1, 3 1 Department
of Electrical Engineering, National Chung Cheng University, Min-Hsiung, Chia-Yi 621, Taiwan of Communications Engineering, National Chung Cheng University, Min-Hsiung, Chia-Yi 621, Taiwan 3 Institute of Communication Engineering, National Chi Nan University, Puli, Nantou 545, Taiwan 2 Department
Received 7 March 2005; Revised 29 May 2005; Accepted 19 July 2005 Recommended for Publication by Helmut Bolcskei An adaptive minimum mean-square error (MMSE) array receiver based on the fuzzy-logic recursive least-squares (RLS) algorithm is developed for asynchronous DS-CDMA interference suppression in the presence of frequency-selective multipath fading. This receiver employs a fuzzy-logic control mechanism to perform the nonlinear mapping of the squared error and squared error variation, denoted by (e2 ,Δe2 ), into a forgetting factor λ. For the real-time applicability, a computationally efficient version of the proposed receiver is derived based on the least-mean-square (LMS) algorithm using the fuzzy-inference-controlled step-size μ. This receiver is capable of providing both fast convergence/tracking capability as well as small steady-state misadjustment as compared with conventional LMS- and RLS-based MMSE DS-CDMA receivers. Simulations show that the fuzzy-logic LMS and RLS algorithms outperform, respectively, other variable step-size LMS (VSS-LMS) and variable forgetting factor RLS (VFF-RLS) algorithms at least 3 dB and 1.5 dB in bit-error-rate (BER) for multipath fading channels. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
Direct-sequence code-division multiple access (DS-CDMA), a specific form of spread-spectrum transmission, has been adopted as the multiaccess technology for nonorthogonal transmission in the third-generation (3G) mobile cellular systems, such as wideband CDMA (W-CDMA) or multicarrier CDMA (MC-CDMA). This sort of the code-division multiaccess techniques requires no time or frequency coordination among the mobile stations. However, the so-called near-far problem and the multipath fading are the major impediments to maintain reliable communication links in CDMA systems. It is well known that an adaptive minimum mean-square error (MMSE) linear receiver [1] has immunity to the nearfar problem and the interference floor in performance exhibited by the conventional matched filter reception. In addition, a linear MMSE receiver can be implemented as an adaptive tapped delay line (TDL), analogous to a linear equalizer, with a relatively low complexity. However, the computation
of the MMSE solution involves the calculation of the inverse of the input autocorrelation matrix, which costs a complexity of O((MN)3 ). Here M denotes the size of the MMSE receiving array and N is the processing gain of the CDMA system so that MN indicates the number of t
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