A set of channelization codes to be monitored is divided into two groups. The first group includes those codes for which an associated symbol modulation and transmit-diversity scheme is known. In the second group are those codes that are characterized by an unknown symbol modulation or unknown transmit-diversity scheme. The quality of the transmission of each code is then evaluated, using a metric. The metric in turn is used to determine whether the code should be used in estimating the covariance matrix by correlating the RAKE data corresponding to the code (i.e., by computing a correlation matrix for the code) or by first subtracting the channel estimates from the channel samples before correlation (i.e., by computing a covariance matrix for the code). An impairment covariance matrix is computed from the covariance matrices and correlation matrices so computed.