Blind Multiuser Detection in Multirate CDMA Based on Cyclic LMS Adaptation

The problem of blind adaptive multiuser detection in multirate CDMA systems is considered. Indeed, since symbol detection in multirate CDMA systems requires periodically time-varying processing of the observables, classical LMS and RLS adaptive algorithms, which assume that the solution to be tracked is time-invariant or slowly time-varying, are not suited for blind adaptive multiuser detection in a multirate system. While a cyclic RLS algorithm has recently appeared in the literature, this paper focuses on the development of LMS-based cyclic filtering algorithms. In particular, cyclic versions of the standard LMS algorithm, of the LMS algorithm with iterate averaging and of the LMS algorithm with adaptive step-size are derived. Interestingly, the last two algorithms are shown to exhibit a convergence speed close to that of the cyclic RLS procedure, but with an order of magnitude lower computational complexity. An adaptive procedure for the automatic selection of the algorithm periodicity is also presented, which is based on a minimum mean-output-energy criterion, and that obviates the need for knowledge of the transmitted data-rates from the interfering signals. Moreover, the case of known multipath fading channels is also examined. In particular, it is shown that the proposed cyclic LMS algorithms can be used to achieve RLS-like performance also in the presence of multipath distortion. Extensive computer simulation results, along with some analytical convergence results, confirm that the proposed algorithms are effective and achieve very satisfactory performance.