Multiuser interference cancellation aided adaptation of a MMSE receiver for direct-sequence code-division multiple-access systems

We consider the application of iterative interference cancellation (IC) schemes to improve the convergence speed of the linear adaptive minimum mean-squared error (MMSE) receiver in a direct-sequence code-division multiple-access (DS-CDMA) system. Our aim is to reduce the overhead introduced during the training period. This will be achieved using an iterative interference cancellation algorithm such as the parallel interference cancellation (PIC) algorithm. The proposed system makes use of the available knowledge of the training sequences for all users (i.e., at the base station) to jointly cancel the multiple access interference (MAI) and adapts to the MMSE optimum filter taps using the combined adaptive MMSE/PIC. An examination of the adaptive MMSE/PIC receiver reveals that it is near-far resistant. Moreover, it is shown that using the least-mean-square (LMS) as a simple form of the adaptive MMSE receiver, the proposed algorithm requires only a few tens of symbols for convergence as compared to a few hundred training symbols needed for the conventional adaptive LMS receiver. Also employing a more complex, yet faster, a recursive-least squares algorithm (RLS) convergence is reached with few training symbols when PIC is used.