Radio communications system comprising a transmitter and a receiver; wherein the transmitter is configured to: • generate or receive digital symbols having a given symbol rate associated with a corresponding symbol period; • generate, every S digital symbols generated/received, a respective multi-mode digital signal, which has a predefined time length shorter than S times the symbol period, which is sampled with a predefined sampling rate higher than the symbol rate, and which carries said S digital symbols by means of a plurality of orthogonal harmonic modes comprising - a main mode which is a real harmonic mode and carries P of said S digital symbols, and - one or more secondary modes carrying the other S-P digital symbols, each secondary mode being a complex harmonic mode time-shifted by half the symbol period with respect to the main mode; and • transmit a radio frequency signal carrying a sequence of the generated multi-mode digital signals; wherein S is an integer higher than three and P is an integer lower than S; and wherein the receiver is configured to: • receive the radio frequency signal transmitted by the transmitter; • process the received radio frequency signal so as to obtain a corresponding incoming digital signal; and • extract, from successive, non-overlapped portions of the incoming digital signal sampled with the predefined sampling rate, the S digital symbols respectively carried by each incoming digital signal portion by means of the orthogonal harmonic modes; wherein each of the successive, non-overlapped portions of the incoming digital signal has the predefined time length. • generate or receive digital symbols having a given symbol rate associated with a corresponding symbol period; • generate, every S digital symbols generated/received, a respective multi-mode digital signal, which has a predefined time length shorter than S times the symbol period, which is sampled with a predefined sampling rate higher than the symbol rate, and which carries said S digital symbols by means of a plurality of orthogonal harmonic modes comprising - a main mode which is a real harmonic mode and carries P of said S digital symbols, and - one or more secondary modes carrying the other S-P digital symbols, each secondary mode being a complex harmonic mode time-shifted by half the symbol period with respect to the main mode; and - a main mode which is a real harmonic mode and carries P of said S digital symbols, and - one or more secondary modes carrying the other S-P digital symbols, each secondary mode being a complex harmonic mode time-shifted by half the symbol period with respect to the main mode; and • transmit a radio frequency signal carrying a sequence of the generated multi-mode digital signals; • receive the radio frequency signal transmitted by the transmitter; • process the received radio frequency signal so as to obtain a corresponding incoming digital signal; and • extract, from successive, non-overlapped portions of the incoming digital signal sampled with the predefined sampling rate, the S digital symbols respectively carried by each incoming digital signal portion by means of the orthogonal harmonic modes; wherein each of the successive, non-overlapped portions of the incoming digital signal has the predefined time length.