Timing-Free Blind Multiuser Detection for Multicarrier DS/CDMA Systems with Multiple Antennae
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Timing-Free Blind Multiuser Detection for Multicarrier DS/CDMA Systems with Multiple Antennae Stefano Buzzi DAEIMI, Universit`a degli Studi di Cassino, Via Di Biasio 43, 03043 Cassino (FR), Italy Email: [email protected]
Emanuele Grossi DAEIMI, Universit`a degli Studi di Cassino, Via Di Biasio 43, 03043 Cassino (FR), Italy Email: [email protected]
Marco Lops DAEIMI, Universit`a degli Studi di Cassino, Via Di Biasio 43, 03043 Cassino (FR), Italy Email: [email protected] Received 30 December 2002; Revised 30 July 2003 The problem of blind multiuser detection for an asynchronous multicarrier DS-CDMA system employing multiple transmit and receive antennae over a Rayleigh fading channel is considered in this paper. The solutions that we develop require prior knowledge of the spreading code of the user to be decoded only, while no further information either on the user to be decoded or on the other active users is required. Several combining rules for the observables at the output of each receive antenna are proposed and assessed, and the implications of the different options are studied in depth in terms of both detection performance and computational complexity. A closed form expression is also derived for the conditional error probability and a lower bound for the near-far resistance is provided. Results confirm that the proposed blind receivers can cope with both multiple access interference suppression and channel estimation at the price of a limited performance loss as compared to the ideal linear receivers which assume perfect channel state information. Keywords and phrases: MC CDMA, multiple antennae, MIMO systems, channel estimation, timing-free detection, near-far resistance.
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INTRODUCTION
Multicarrier code division multiple access (MC-CDMA) has been conceived as a transmission format which retains the potentials of direct sequence CDMA (DS-CDMA)— and in particular its resistance to multipath effects induced by the radio channel as the communication rate grows larger and larger [1]—while relaxing some very demanding requirements posed by its competitor. In particular, the efficacy of DS-CDMA on wireless channels is mainly due to the recombination of multiple rays so as to increase the average signal-to-noise ratio, but this inevitably poses the problem of a tight synchronization so as to avoid heavy mismatch losses in the replicas-retrieving process. MC-CDMA, instead, by partitioning the available bandwidth in many subbands, no larger than the channel coherence bandwidth, and allocating in each subband independently modulated digital signals, achieves two advan-
tages, that is, (a) the propagation channel in each subband is frequency-flat, and (b) the symbol duration for the data signals occupying the frequency subbands grows linearly with the number of subbands, thus implying that the need for fast electronics and high-performance synchronization schemes is less stringent. The combination of the MC concept with the CDMA technology has led to the birth of three main access schemes, that is, multitone CDMA [2, 3],
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