Efficient Implementation of Multiuser Detectors for Asynchronous CDMA

We consider a linear multistage detector with universal (large system) weighting for large asynchronous code-division multiple-access (CDMA). A convenient choice of the basis of the projection subspace allows a joint projection of all users’ signals and avoids truncation effects due to finite time windowing. Thanks to the joint projection the complexity per bit of the proposed detector scales linearly with the number of users. Under the assumption that the system is symbol asynchronous but chip synchronous the detector performance is analyzed analytically and shown to be identical to synchronous CDMA in the large system limit thanks to the absence of truncation effects. The proposed detector is compared to the finite-window linear minimum meansquare error detector (LMMSE). While in the synchronous case the LMMSE detector always outperforms weighted multistage detectors, we show that, with a sufficiently large delay, the proposed multistage detector can outperform the LMMSE detector in asynchronous CDMA due to constraints on the observation window.