A new class of M-ary communication systems is introduced in which information is transmitted using long-duration waveforms that are synthesised from complementary sequences. The signal waveforms, which are detected with a correlation receiver, are such that information can be transmitted using a baud period that is much shorter than the duration of the waveforms, without any intersymbol interference being introduced. The class of systems described have considerable potential and may well be of particular value in overcoming the effects of impulsive noise, or more general burst-type noise.