Asynchronous CDMA Systems with Random Spreading-Part II: Design Criteria

Totally asynchronous code-division multiple-access (CDMA) systems are addressed. In Part I, the fundamental limits of asynchronous CDMA systems are analyzed in terms of spectral efficiency and SINR at the output of the optimum linear detector. The focus of Part II is the design of low-complexity implementations of linear multiuser detectors in systems with many users that admit a multistage representation, e.g. reduced rank multistage Wiener filters, polynomial expansion detectors, weighted linear parallel interference cancellers. The effects of excess bandwidth, chip-pulse shaping, and time delay distribution on CDMA with suboptimum linear receiver structures are investigated. Recursive expressions for universal weight design are given. The performance in terms of SINR is derived in the large-system limit and the performance improvement over synchronous systems is quantified. The considerations distinguish between two ways of forming discrete-time statistics: chip-matched filtering and oversampling. Index Terms Asynchronous code-division multiple-access (CDMA), channel capacity, effective interference, minimum mean-square error (MMSE) detector, multistage detector, multiuser detection, random matrix theory, random spreading sequences. This work was presented in part at the IEEE Information Theory Workshop (ITW 2006), Punta de l’Este, Uruguay, March 2006 and at the IEEE Wireless Communications and Networking Conference, Hong Kong, March 2007. It partly appears in Laura Cottatellucci, “Low Complexity Multistage Detectors for Randomly Spread CDMA Systems”, Ph.D. thesis, Vienna University of Technology, March 2006. This work was supported in part by the French ANR ”Masses de Donnes” project SESAME and by the Research Council of Norway under grant 171133/V30. Laura Cottatellucci is with Eurecom, Sophia Antipolis, France (e-mail: [email protected]). She was with Institute of Telecommunications Research, University of South Australia, Adelaide, SA, Australia. Ralf Müller is with Norwegian University of Science and Technology, Trondheim, Norway, (e-mail: [email protected]). Mérouane Debbah was with Eurecom, Sophia Antipolis, France. He is currently with SUPELEC, 91192 Gif-sur-Yvette, France (e-mail: [email protected]).