An Improved Overloading Scheme for Downlink CDMA
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An Improved Overloading Scheme for Downlink CDMA Frederik Vanhaverbeke Telecommunications and Information Processing (TELIN) Department, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium Email: [email protected]
Marc Moeneclaey Telecommunications and Information Processing (TELIN) Department, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium Email: [email protected] Received 24 July 2003; Revised 1 March 2004 An improved overloading scheme is presented for single-user detection in the downlink of multiple-access systems based on OCDMA/OCDMA (O/O). By displacing in time the orthogonal signatures of the two user sets that make up the overloaded system, the cross-correlation between the users of the two sets is reduced. For random O/O with square-root cosine rolloff chip pulses, the multiuser interference can be decreased by up to 50% (depending on the chip pulse bandwidth) as compared to quasiorthogonal sequences (QOS) that are presently part of the downlink standard of CDMA2000. This reduction of the multiuser interference gives rise to an increase of the achievable signal-to-interference-plus-noise ratio for a particular channel load. Keywords and phrases: CDMA, quasiorthogonal sequences, downlink transmission, oversaturated channels, power control.
1.
INTRODUCTION
In any synchronized multiple-access system based on codedivision multiple access (CDMA), the maximum number of orthogonal users equals the spreading factor N. In order to be able to cope with overloading of a synchronized CDMA system (i.e., with a number of users K = N + M : N < K ≤ 2N), several schemes have been proposed in literature. Apart from the trivial random spread system (PN) [1], one can look for signature sets that are “as orthogonal as possible.” A popular measure for the quasiorthogonality of a signature set is the total squared correlation (TSC) [2]; signature sets that minimize TSC among all possible signature sets are called Welch bound equality (WBE) sequences [3, 4]. A third approach consists of the OCDMA/OCDMA (O/O) systems [5, 6, 7, 8, 9, 10, 11, 12], where a complete set of orthogonal signature sequences are assigned to N users (“set 1 users”), while the remaining M users (“set 2 users”) are assigned another set of orthogonal sequences. The motivation behind the latter proposal is that the interference levels of the users are decreased considerably as compared to other signature sequence sets (e.g., random spreading), since each user suffers from interference caused by the users of the other set only. WBE sequences have some very interesting properties: they maximize both the sum capacity [3, 4] and achieve the network capacity [13] for synchronous systems based on CDMA. Unfortunately, two major drawbacks of WBE sequences seriously complicate their application to cellular
systems: (1) they give rise to an unscalable system,1 and (2) the chips of the sequences can be taken binary only if K is a multiple of 4 [14, 15]. As a result, in spite of their superior performance,
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