Abstract
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 direct-sequence 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.
Highlights
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
The techniques of low-rate spreading and coding have been combined with orthogonal code-division transmit diversity (CDTD) and iterative “turbo” processing at the receiver
In contrast to layered ST turbo-coded (TC) CDTD, where a turbo encoder is required for every transmit diversity branch available, super-orthogonal turbo transmit diversity (SOTTD) requires a single turbo encoder-decoder pair, making it attractive for CDMA wireless applications, the only requirement being that the number of constituent encoders Z be greater or equal to the transmit diversity order MT
Summary
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. It has been illustrated that the maximum theoretical CDMA capacity can only be achieved by employing very low-rate FEC codes utilizing the entire bandwidth, without further spreading by the multipleaccess sequence [14, 15, 16]. A layered ST super-orthogonal turbo transmit diversity (SOTTD) architecture for a downlink DSCDMA system, operating over a frequency-selective fading channel, is investigated This open-loop transmit diversity technique is well-suited for code-spread CDMA systems where downlink performance is improved by using a small number of transmit antennas (MT = 3) at the base station and a single antenna (MR = 1) at the mobile handset receiver. Low-rate super-orthogonal codespread CDMA is married with code-division transmit diversity (CDTD), and at the mobile receiver, ST RAKE CDTD processing is combined with iterative turbo code-spread decoding.
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Published Version
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