The present invention relates to a method and a circuitry for estimating a spreading factor and/or data transmission rate in a communication system that employs variable data transmission rates. A received transmission signal is analysed in order to estimate noise variances for those spreading factors that are possible for a data frame in the signal. The estimate is based on information that is determined by assuming that the smallest possible spreading factor was used for the transmission of the signal. The estimates are then scaled by predefined constants, each of the constants depending on the respective possible spreading factor. The smallest one of the scaled estimates is determined, whereafter the most probable spreading factor of the received data frame is determined based on the smallest variance estimate.