1. A receiver circuit for processing a received spread spectrum signal containing two or more signals of interest and one or more interfering signals with known spreading codes, said receiver circuit comprising: a despreading circuit including a group of correlators for each of the signals of interest, wherein each group of correlators is configured to generate a despread signal vector for a corresponding signal of interest, for a given symbol time; and an impairment covariance estimation circuit configured to calculate a covariance matrix for the given symbol time, the covariance matrix representing impairment correlations among the correlators from sources other than the signals of interest as well as from inter-symbol interference in and among the signals of interest, wherein the covariance matrix comprises diagonal blocks that each represent impairment correlations among a single one of the groups of correlators and wherein said impairment covariance estimation circuit is configured to calculate said diagonal blocks based on first terms that account in a code-specific manner for same-symbol-time interference to the desired signals from each of the interfering signals.