  In this paper, taking the view that a linear parallel interference canceller
(LPIC) can be seen as a linear matrix filter, we propose new linear matrix
filters that can result in improved bit error performance compared to other
LPICs in the literature. The motivation for the proposed filters arises from
the possibility of avoiding the generation of certain interference and noise
terms in a given stage that would have been present in a conventional LPIC
(CLPIC). In the proposed filters, we achieve such avoidance of the generation
of interference and noise terms in a given stage by simply making the diagonal
elements of a certain matrix in that stage equal to zero. Hence, the proposed
filters do not require additional complexity compared to the CLPIC, and they
can allow achieving a certain error performance using fewer LPIC stages. We
also extend the proposed matrix filter solutions to a multicarrier DS-CDMA
system, where we consider two types of receivers. In one receiver (referred to
as Type-I receiver), LPIC is performed on each subcarrier first, followed by
multicarrier combining (MCC). In the other receiver (called Type-II receiver),
MCC is performed first, followed by LPIC. We show that in both Type-I and
Type-II receivers, the proposed matrix filters outperform other matrix filters.
Also, Type-II receiver performs better than Type-I receiver because of enhanced
accuracy of the interference estimates achieved due to frequency diversity
offered by MCC.
