CDMA downlink equalization with imperfect channel estimation

Abstract

In CDMA systems the downlink user signals are orthogonal, and intra-cell interference arises through multipath. Interference cancellation may be achieved by a low complexity equalizer-receiver without knowledge of the in-cell users' activity states, codes, powers, etc. As only knowledge of the channel is exploited, the scheme may be highly vulnerable to channel estimation errors. We observe that a "mismatch" phenomenon occurs when the zeros of the estimated channel transfer function do not include the zeros of the noise spectral density. We show that this mismatch may be corrected by projecting the noisy channel estimate back into the subspace spanned by the delays of the chip auto-correlation function.