It is disclosed a first device 80; 90 and a second device 100; 110 adapted to communicate wirelessly with each other, and methods performed therein. A guard interval length is determined 54 based on a degree of beamforming to be applied in communication of a first transmission block from the first device to the second device. A guard interval having the determined guard interval length will be applied 56 between the first transmission block communicated to the second device and a second transmission block subsequent to the first transmission block. Transmission efficiency may be increased and overhead may be decreased.