1. A method for estimating parameters of a received radio signal that includes a sequence of transmitted symbols, in a receiver having a plurality of processing-delay elements, the method comprising: computing a quality metric of a first type, from samples of the received radio signal, for each of a first group of channelization codes for which an associated modulation scheme and an associated transmit diversity scheme, if any, are known, but for which the associated transmitted symbol values, if any, are unknown; determining that at least one of the channelization codes of the first group has a quality metric higher than a first threshold value and that at least one of the channelization codes of the first group has a quality metric lower than a second threshold value; computing a covariance matrix for each of the channelization codes of the first group having a quality metric higher than the first threshold value, using per-delay channel estimates calculated from per-delay de-spread symbol values at two or more processing delays and quasi-pilot symbol values obtained from combined per-delay de-spread symbol values; computing a correlation matrix for each of the channelization codes of the first group having a quality metric lower than the second threshold value, wherein each of the correlation matrices is computed directly from per-delay de-spread symbol values at two or more processing delays; and calculating an impairment covariance matrix for the received radio signal from the computed covariance matrices and correlation matrices.