Mmse-zf Receiver and Blind Adaptation for Multirate Cdma

We consider multirate users in a DS-CDMA system operating in an asynchronous fashion in a multipath environment. Oversampling w.r.t. the chip rate is applied to the cyclostationary received signal and multi-sensor reception is considered, leading to a linear multichannel model. Channels for different users are considered to be FIR and of possibly different lengths, depending upon their processing gains. We consider an individualized linear MMSE-ZF or projection receiver for a given user, exploiting its spreading sequence and timing information. Due to the periodically varying nature of the symbol spreading sequences for fast users in a multirate setup, the symbol rate cyclostationarity no longer holds for all users. However, the multiuser signal is still cyclostationary with the period P , whereP is the processing gain at the slowest rate. A uk times faster user k behaves as if it were a linear combination of uk users at the slowest rate. The problem boils down to classical multiuser detection with timevarying interference cancelling filters for faster users. The processing is time-invariant, however, when performed at rate P . In that case, uk symbols for the kth user get estimated simultaneously through uk parallel receivers. We discuss the blind estimation of the channels, required for determining the linear multirate receivers, via a constrained minimum output energy approach.