A method and arrangement for noise variance and SIR estimation in a UTRAN Node B or User Equipment estimates the SIR (SIR(1) . . . SIR(K)) at the output of a detector by using an estimate ({circumflex over (σ)}2) of the detector input noise variance to provide an estimate ({circumflex over (σ)}2) of the detector output noise variance. The detector input noise variance is derived from a midamble portion in the received signal. By deriving the transfer function of the detector an estimate of the detector output noise variance is estimated. The estimated output noise variance then allows an improved estimate of the SIR (SIR(1) . . . SIR(K)) at the detected output.