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. Over-sampling 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, where P is the processing gain at the slowest rate. A u_k times faster user k behaves as if it were a linear combination of u_k users at the slowest rate. The problem boils down to classical multiuser detection with time-varying interference cancelling filters for faster users. The processing is time-invariant, however, when performed at rate P. In that case, u_k symbols for the kth user get estimated simultaneously through u _k parallel receivers. We discuss the blind estimation of the channels, required for determining the linear multirate receivers, via a constrained minimum output energy approach