Generalized Multicarrier CDMA: Unification and Linear Equalization
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Generalized Multicarrier CDMA: Unification and Linear Equalization Georgios B. Giannakis Department of Electrical and Computer Engineering, University of Minnesota, 200 Union Street SE, Minneapolis, MN 55455, USA Email: [email protected]
Paul A. Anghel Department of Electrical and Computer Engineering, University of Minnesota, 200 Union Street SE, Minneapolis, MN 55455, USA Email: [email protected]
Zhengdao Wang Department of Electrical and Computer Engineering, Iowa State University, 2215 Coover Hall, Ames, IA 50011, USA Email: [email protected] Received 11 August 2003; Revised 21 January 2004 Relying on block-symbol spreading and judicious design of user codes, this paper builds on the generalized multicarrier (GMC) quasisynchronous CDMA system that is capable of multiuser interference (MUI) elimination and intersymbol interference (ISI) suppression with guaranteed symbol recovery, regardless of the wireless frequency-selective channels. GMC-CDMA affords an alldigital unifying framework, which encompasses single-carrier and several multicarrier (MC) CDMA systems. Besides the unifying framework, it is shown that GMC-CDMA offers flexibility both in full load (maximum number of users allowed by the available bandwidth) and in reduced load settings. A novel blind channel estimation algorithm is also derived. Analytical evaluation and simulations illustrate the superior error performance and flexibility of uncoded GMC-CDMA over competing MC-CDMA alternatives especially in the presence of uplink multipath channels. Keywords and phrases: multicarrier CDMA, multipath fading channels, blind equalization.
1.
INTRODUCTION
Mitigation of frequency-selective multipath and elimination of multiuser interference (MUI) have received considerable attention as they constitute the main limiting performance factors in wireless CDMA systems. Orthogonal frequency-division multiple access (OFDMA) [1], the multiuser counterpart of orthogonal frequency-division multiplexing (OFDM) [2], is capable of complete MUI elimination in the downlink provided that a sufficiently long cyclic prefix is used at the transmitter. However, plain OFDMA cannot collect multipath diversity and therefore suffers from frequency-selective fading. Alternatives like OFDMA with frequency hopping and/or channel coding [1, pages 213– 228] have been proposed to increase frequency diversity and therefore enhance robustness to multipath, at the price of increased complexity and/or reduced bandwidth efficiency. Multicarrier (MC) CDMA systems have been developed to capitalize on both OFDMA’s resilience to MUI and directsequence (DS) CDMA’s robustness against frequency selectivity to mitigate both MUI and the intersymbol interference
(ISI) caused by time-dispersive channels [3, 4, 5, 6, 7, 8, 9]. The spread-spectrum multicarrier multiple-access schemes developed in [6, 10, 11] rely on bandwidth-expanding repetition or convolutional codes for MUI elimination and mitigation of frequency-selective uplink channels. But no existing MC- or DS-CDMA scheme guarantees (blind or
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