A method and arrangement of increasing impairment co-variance matrix Ru estimation accuracy in downlink in a user equipment in a communication network system. De-spread is performed on HS-DSCH symbols to form a matrix X of de-spread symbols. The matrix X and channel estimates hc from CPiCH and modulation scheme information are used to form a matrix S of recovered symbols in hard value. The matrix X and the channel estimates hc, the modulation scheme information and the matrix S output are used to increase the estimation accuracy of the matrix S. The previous step is repeated until the output symbols are the same as the input symbols or the number of iterations reaches a pre-defined maximum value. The matrix X and the matrix S with increased estimation accuracy are used to form an impairment co-variance matrix Ru estimate.