23 research outputs found

    Blind decorrelating rake receivers for long-code WCDMA

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    Burst-by-burst adaptive multiuser detection cdma: a framework for existing and future wireless standards

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    Enhanced Wireless Access Technologies and Experiments for W-CDMA Communications

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    This article reviews enhanced wireless access technologies and experimental evaluations of the wideband DS-CDMA physical layer employing intercell asynchronous operation with a three-step fast cell search method, pilot symbol-assisted coherent links, signal-to-interference plus background noise power ratio-based fast transmit power control, site diversity (soft/softer handover), and transmit diversity in the forward link. The article also presents link-capacity-enhancing techniques such as using an interference canceller and adaptive antenna array diversity receiver/transmitter, and experimental results in a real multipath fading channel. The laboratory and field experiments exemplify superior techniques of the W-CDMA physical layer and the potential of the IC and AAAD transceiver to decrease the mobile transmit power in the reverse link and multipath interference from high-rate users with large transmit power in the forward link

    Chip and Signature Interleaving in DS CDMA Systems

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    Siirretty Doriast

    Interference characterization and suppression for multiuser direct-sequence spread-spectrum system

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 175-184).In this thesis we investigate efficient modulation and detection techniques for the uplink (i.e. transmission from mobile to base station) of a DS-CDMA network. Specifically, the thesis contains three parts. In the first part, we focus on the mobile transmitter. In particular, we evaluate and compare the spectral efficiency of two promising variable rate DS-CDMA transmission techniques, multicode (MCD) and variable-spreading-gain (VSG), under the presence of multiple-access (user-to-user) interferences (MAI) and multipath interferences. The uniqueness of our study is that in bit-error-rate evaluation, instead of approximating the interference as Gaussian noise (which has been done in most of the previous studies), we incorporate both power and distribution of interferences into consideration. We show where the Gaussian assumption may give misleading answers and how our results in these cases are different from those obtained in the past. In part two and three of the thesis, we focus on the base station receiver. Specifically, we present effective joint detection techniques that have good performance-complexity tradeoff. Part two of the thesis introduces a class of novel multistage parallel interference cancellation algorithms based on stage-by-stage minimum mean-squared error (MMSE) optimization. We show that this scheme is capable of achieving significantly better performance than other algorithms with similar complexity. Part three of the thesis presents a low-complexity dual-mode multiuser detector that dynamically switches its detection mode between the matched-filter receiver and the decorrelator. We show that this detector is capable of achieving the performance of a decorrelator but with significant savings in processing power and complexity.by Mingxi Fan.Ph.D

    Performance Comparison Between Variable and Fixed Signature Codes in DS-CDMA Systems

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    In a CDMA or SCMA system, users can use a fixed signature for the whole transmission interval. But there is the possibility for two users to have highly correlated signatures. Hence, high interference exists in the system, and it will degrade the performance. On the other hand, if the users use variable signatures for spreading each modulation symbol, then the interference between the users will not be fixed for all the transmissions.In this way, we are avoiding clustering error symbols resulted from the high interference. As we show in this thesis, this prediction about better performance of variable spreading is not always true. We have discussed several scenarios and shown the performance for both the fixed and variable signature codes

    A Redundant Residue Number System Coded Burst-by-Burst Adaptive Joint-Detection Based CDMA Speech Transceiver

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    Blind iterative multiuser detection for error coded CDMA systems.

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    Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.Mobile communications have developed since the radio communications that were in use 50 years ago. With the advent of GSM, mobile communications was brought to the average citizen. More recently, COMA technology has provided the user with higher data rates and more reliable service, and it is apparent that it is the future of wireless communication. With the introduction of 3G technology in South Africa, it is becoming clear that it is the solution to the country's wireless communication requirements. The 3G and next-generation technologies could provide reliable communications to areas where it has proven difficult to operate and maintain communications effectively, such as rural locations. It is therefore important that the se technologies continue to be researched in order to enhance their capabilities to provide a solution to the wireless needs of the local and global community. Whilst COMA is proving to be a reliable communications technology, it is still susceptible to the effects of the near-far problem and multiple-access interference. A number of multiuser detectors have been proposed in literature that attempt to mitigate the effects of multiple-access interference. A notable detector is the blind MOE detector, which requires only the desired user 's spreading sequence , and it exhibits performance approximating that of other linear multiuser detectors. Another promising class of multiuser detector operate using an iterative principle and have a joint multiuser detection and error-correcting coding scheme. The aim of this research is to develop a blind iterative detector with FEC coding as a potential solution to the need for a detector that can mitigate the effects of interfering users operating on the channel. The proposed detector has the benefits of both the blind and iterative schemes: it only requires the knowledge of the desired user ' s signature, and it has integrated error-correcting abilities. The simulation results presented in this dissertation show that the proposed detector exhibits superior performance over the blind MOE detector for various channel conditions. An overview of spread-spectrum technologies is presented, and the operation of OS-COMA is described in more detail. A history and overview of existing COMA standards is also given . The need for multiuser detection is explained, and a description and comparison of various detection methods that have appeared in literature is given. An introduction to error coding is given , with convolutional code s, the turbo coding concept and method s of iterative detection are described in more detail and compared, as iterat ive decoding is fundamental to the operation of an iterative COMA detector. An overview of iterative multiuser detection is given , and selected iterative methods are described in more detail. A blind iterative detector is proposed and analysed. Simulation results for the propo sed detector, and a comparison to the blind MOE detector is presented, showing performance characteristics and the effects of various channel parameters on performance. From these results it can be seen that the proposed detector exhibits a superior performance compared to that of the blind MOE detector for various channel conditions. The dissertation is concluded, and possible future directions of research are given
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