The ITU Radiocommunication Assembly considering a)that an increasing demand is noted for high-speed data transmission over HF radio circuits and further increase in such demand may be expected; b)that recent radiocommunication technologies and operational techniques are leading to systems having greatly improved bandwidth efficiency, i.e. a larger capacity in bits per second per unit bandwidth;c)that it is desirable that the effects of the random variations and disturbances in the propagation medium be the ultimate factors governing the performance obtainable with such systems.