Spectral efficiency is a key design issue for all wireless communication systems. Orthogonal Frequency Division Multiplexing (OFDM) is a very well-known technique for efficient data transmission over many carriers overlapped in frequency. Recently, several papers have appeared which describe spectrally efficient variations of multi-carrier systems where the condition of orthogonality is dropped. Proposed techniques suffer from two weaknesses: the complexity of generating the signal is increased and the signal detection is computationally demanding. Known methods suffer either unusably high complexity or high error rates because of the inter-carrier interference.