The present invention relates to a device and method for optimal estimation of the distortion caused by the transmission medium by means of the sequential emission of pairs of quadrature complementary sequences, estimating the frequency spectrum of a transmission channel allows obtaining information of considerable interest to analyze its characteristics or correcting the distortion effects introduced in a communication system. This invention describes a new method for extracting said characteristics generating a transmission preamble using complementary sequences, which allows upon reception identifying the distortion caused in each of the I/Q phases of a quadrature modulation system on the transmitted symbols, canceling the phase distortion effects caused by the medium and reducing the equalization of the channel to a single equalizing baseband filter identical for both I2/Q2 phases. Since the distorted frequencies are received in the receiver, they also allow extracting the temporal and/or frequency characteristics of the distortion caused by the medium they traverse with a precision depending on the length of the sequences used in coding.