Transmitter and receiver for an othogonally multiplexed QAM signal of a sampling rate N times that of PAM signals, comprising an N/2-point offset fourier transform processor

A converter for digitally carrying out conversion between a plurality of baseband real PAM digital signal sequences, 2L in number, having a slow sampling rate of 1/T hertzes and synchronized with one another and an orthogonally multiplexed real QAM digital signal sequence of a fast sampling rate of N/T hertzes, where N represents an even integer that is not less than 2L, said converter being operable as a predetermined one of a transmitter for converting said real PAM digital signal sequences to said real QAM digital signal sequence and a receiver for converting said real QAM digital signal sequence to said real PAM digital signal sequences and comprising:

• first means for digitally carrying out conversion between said real PAM digital signal sequences and a plurality of first-type complex digital signal sequences of twice said slow sampling rate, equal in number to a predetermined one of N and N/2 and to N/2 when said converter is operable as said transmitter and said receiver, respectively;
• N/2-point offset discrete Fourier transform processing means for subjecting prescribed ones of said first-type complex digital signal sequences and a plurality of second-type complex digital signal sequences of twice said slow sampling rate, equal in number to a prescribed one of N and N/2, to N/2-point offset discrete Fourier transform processing at a period of T/2 second to produce said second-type and said first-type complex digital signal sequences when said converter is operable as said transmitter and said receiver, respectively;
• filter means comprising a plurality of digital filter units, equal in number to said prescribed one of N and N/2, said digital filter units being operable at twice said slow sampling rate and having a prescribed effective bandwidth and linear phase slopes stepwise different from one another among a bank of the digital filter units, N/2 in number, for carrying out band limitation on preselected ones of said second-type complex digital signal sequences and a plurality of third-type complex digital signal sequences of twice said slow sampling rate, respectively, said third-type complex digital signal sequences being equal in number to N/2 and to said predetermined one of N and N/2 when said converter is operable as said transmitter and said receiver, respectively, said filter means thereby producing said third-type and said second-type complex digital signal sequences when said converter is operable as said transmitter and said receiver, respectively;
• and
• second means for digitally carrying out conversion between said third-type complex digital signal sequences and said real QAM digital signal sequence.