Adaptive Space-Time-Spreading-Assisted Wideband CDMA Systems Communicating over Dispersive Nakagami- Fading Channels
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Adaptive Space-Time-Spreading-Assisted Wideband CDMA Systems Communicating over Dispersive Nakagami-m Fading Channels Lie-Liang Yang School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Email: [email protected]
Lajos Hanzo School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Email: [email protected] Received 23 May 2004; Revised 18 December 2004 In this contribution, the performance of wideband code-division multiple-access (W-CDMA) systems using space-timespreading- (STS-) based transmit diversity is investigated, when frequency-selective Nakagami-m fading channels, multiuser interference, and background noise are considered. The analysis and numerical results suggest that the achievable diversity order is the product of the frequency-selective diversity order and the transmit diversity order. Furthermore, both the transmit diversity and the frequency-selective diversity have the same order of importance. Since W-CDMA signals are subjected to frequencyselective fading, the number of resolvable paths at the receiver may vary over a wide range depending on the transmission environment encountered. It can be shown that, for wireless channels where the frequency selectivity is sufficiently high, transmit diversity may be not necessitated. Under this case, multiple transmission antennas can be leveraged into an increased bitrate. Therefore, an adaptive STS-based transmission scheme is then proposed for improving the throughput of W-CDMA systems. Our numerical results demonstrate that this adaptive STS-based transmission scheme is capable of significantly improving the effective throughput of W-CDMA systems. Specifically, the studied W-CDMA system’s bitrate can be increased by a factor of three at the modest cost of requiring an extra 0.4 dB or 1.2 dB transmitted power in the context of the investigated urban or suburban areas, respectively. Keywords and phrases: CDMA, space-time spreading, Nakagami-m fading, transmit diversity.
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BACKGROUND ON LINK ADAPTATION
It is widely recognised that the channel quality of wireless systems fluctuates over a wide range and hence it is irrealistic to expect that conventional nonadaptive systems might be able to provide a time-invariant grade of service. Hence in recent years various near-instantaneously adaptive-coding-and-modulation- (ACM-) assisted arrangements have been proposed [1, 2], which have found their way also into the high-speed downlink packet access (HSDPA) mode of the third-generation wireless systems [3] and in other adaptively reconfigurable multicarrier orthogonal 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.
frequency division multiplex (OFDM) systems [4] as well as into single-carrier and multi-carrier DS-CDMA schemes [5]. The family of multi-carrier systems is now widely considered to be the most potent candi
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