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| publisher = IET Communications
}}</ref> has been investigated for the uplink that exploits the differences between users' fading channel signatures to increase the user capacity well beyond the spreading length in the MAI-limited environment. The authors show that it is possible to achieve this increase at a low complexity and high [[bit error rate]] performance in flat fading channels, which is a major research challenge for overloaded CDMA systems. In this approach, instead of using one sequence per user as in conventional CDMA, the authors group a small number of users to share the same spreading sequence and enable group spreading and despreading operations. The new collaborative multi-user receiver consists of two stages: group multi-user detection (MUD) stage to suppress the MAI between the groups and a low-complexity maximum-likelihood detection stage to recover jointly the co-spread users' data using minimal Euclidean-distance measure and users' channel-gain coefficients.
Further to note that research in the area is going on and in 2004, Prof. Li ping has introduced the new concept of enhanced CDMA version known as INTERLEAVE DIVISION MULTIPLE ACCESS (IDMA) scheme. It uses the orthogonal interleaved as the only means of user separation in place of signature sequence used in CDMA system. Many researchers has introduced various orthogonal interleaving mechanisms including tree based interleaved suggested my Prof. Manoj Kumar Shukla of Harcourt Butler Technical University.
==See also==
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