There is a need for HF data transmission systems to provide reliable service in an efficient manner with multi-tone frequency shift keying (FSK) modems, as described in this Recommendation, or multi-tone phase shift keying (PSK) modems, as described in Recommendation ITU-R F.763. To compensate for the unfavourable nature of the selective fading phenomenon of the transmission medium, in-band or other frequency diversity techniques are widely utilized.