This invention relates to a method and apparatus for channel estimation. The guard electrode is maintained at a slightly lower potential than the pad and the measure electrode is at an intermediate potential thereto. By means of two inflection points, there is a concavely designed suction side contour segment in a segment from 35% to 50% for the suction side contour, and a convexly implemented pressure side contour segment for the pressure side contour. The base is connected to the bonnet by folding the ridges over the peripheral rim of the bonnet. By pulling the lever-actuator away from the cage assembly, the fiber optic module is withdrawn from the cage assembly. The radio receiver receives signals transmitted via a propagation channel from a transmitter, and determines that the propagation channel is an AWGN channel when a filtered version of a minimum value of a metric is lower than a threshold value.