Super-Orthogonal Space-Time Turbo Transmit Diversity for CDMA
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uper-Orthogonal Space-Time Turbo Transmit Diversity for CDMA ยจ J. van Wyk Daniel RapidMobile (Pty)Ltd, Persequor Park, Pretoria 0020, South Africa Email: [email protected]
Louis P. Linde Department of Electrical, Electronic and Computer Engineering, University of Pretoria, Pretoria 0002, South Africa Email: [email protected]
Pieter G. W. van Rooyen Broadcom, 15435 Innovation Drive, San Diego, CA 92128, USA Email: [email protected] Received 8 September 2003; Revised 30 July 2004 Studies have shown that transmit and receive diversity employing a combination of multiple transmit-receive antennas (given ideal channel state information (CSI) and independent fading between antenna pairs) will potentially yield maximum achievable system capacity. In this paper, the concept of a layered super-orthogonal turbo transmit diversity (SOTTD) for downlink directsequence code-division multiple-access (CDMA) systems is explored. This open-loop transmit diversity technique improves the downlink performance by using a small number of antenna elements at the base station and a single antenna at the handset. In the proposed technique, low-rate super-orthogonal code-spread CDMA is married with code-division transmit diversity (CDTD). At the mobile receiver, space-time (ST) RAKE CDTD processing is combined with iterative turbo code-spread decoding to yield large ST gains. The performance of the SOTTD system is compared with single- and multiantenna turbo-coded (TC) CDTD systems evaluated over a frequency-selective Rayleigh fading channel. The evaluation is done both by means of analysis and computer simulations. The performance results illustrate the superior performance of SOTTD compared to TC CDTD systems over practically the complete useful capacity range of CDMA. It is shown that the performance degradation characteristic of TC CDTD at low system loads (due to the inherent TC error floor) is alleviated by the SOTTD system. Keywords and phrases: transmitter diversity, space-time coding, code-division transmit diversity, layered super-orthogonal turbo transmit diversity, low-rate spreading and coding, CDMA wireless communications.
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INTRODUCTION
Space-time (ST) processing techniques, such as receive diversity and antenna beamforming, can significantly improve the downlink and uplink capacity of cellular direct-sequence (DS) code-division multiple-access (CDMA) systems. Recent studies have explored the limits of multiple-antenna systems performance in frequency-selective multipath fading environments from an information-theoretic point of view [1, 2]. It has been shown that, with perfect receiver channel state information (CSI) and independent fading between pairs of transmit-receive antennas, maximum system This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
capacity may potentially be achieved. When multiple receive antennas are not available, multiple transmit
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