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

• US 4,300,229 A
• Filed: 02/15/1980
• Issued: 11/10/1981
• Est. Priority Date: 02/21/1979
• Status: Expired due to Term

• Current Assignee: Nippon Electric Company Limited (NEC Corporation)
• Original Assignee: Nippon Electric Company Limited (NEC Corporation)
• Inventors: Hirosaki, Botaro
• Primary Examiner(s): Olms, Douglas W.
• Application Number:
  US06/121,943
• Time in Patent Office: 634 Days
• Field of Search: 370/20, 370/70, 370/19, 370/23
• US Class Current: 370/206
• CPC Class Codes: H04J 4/005 Transmultiplexing
• CPC Class Codes: H04L 27/2634 Inverse fast Fourier transform [IFFT] or inverse discrete Fourier transform [IDFT] modulators in combination with other circuits for modulation
• CPC Class Codes: H04L 27/26414 Filtering per subband or per resource block, e.g. universal filtered multicarrier [UFMC] or generalized frequency division multiplexing [GFDM]
• CPC Class Codes: H04L 27/26524 Fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators in combination with other circuits for demodulation
• CPC Class Codes: H04L 27/2654 Filtering per subcarrier, e.g. filterbank multicarrier [FBMC]

Abstract

In a transmitter for digitally converting PAM signals, 2L in number, of a common slow sampling rate of 1/T hertzes to an orthogonally multiplexed QAM signal of a fast sampling rate of N/T hertzes, N/2-point input data into which the PAM signals are pre-processed with addition thereto of dummy signals, only (N/2-L) in number, are supplied to at least one N/2-point ODFT (offset discrete Fourier transform) processor (61), a pair of which (61, 62) must have been an N-point ODFT processor accompanied by switches controlled at a T/2-second period. After caused to pass through respective filter units of a bank (71), N/2-point output data are time-division multiplexed into the QAM signal. The filter units may either be complex band-pass or real low-pass filter units. A receiver comprises similar parts and carries out entirely reversed operation.

Claims

1. 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.

2. A converter as claimed in claim 1 and operable as said transmitter, wherein:

said first means comprises pre-processing means for digitally converting said real PAM digital signal sequences to pre-processing means output complex digital signal sequences of twice said slow sampling rate, equal in number to said prescribed one of N and N/2, said pre-processing means output complex digital signal sequences serving as said first-type complex digital signal sequences, respectively;

said offset discrete Fourier transform processing means subjecting said pre-processing means output complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce at least one set of N/2-point offset discrete Fourier transform processed complex digital signal sequences that serve as said second-type complex digital signal sequences, respectively;

the digital filter units of said filter means carrying out band limitation on said offset discrete Fourier transform processed complex digital signal sequences to produce filter unit output complex digital signal sequences, respectively, said filter means comprising filter output means responsive to said filter unit output complex digital signal sequences for producing filter means output complex digital signal sequences of twice said slow sampling rate, N/2 in number, said filter means output complex digital signal sequences serving as said third-type complex digital signal sequences, respectively;

said second means digitally converting said filter means output complex digital signal sequences to said real QAM digital signal sequence.

3. A converter as claimed in claim 2, said prescribed one of N and N/2 being N, wherein:

said pre-processing means comprises a pre-processing circuit for digitally converting said real PAM digital signal sequences to pre-processed complex digital signal sequences of a first kind of twice said slow sampling rate, N/2 in number, and also to pre-processed complex digital signal sequences of a second kind of twice said slow sampling rate, again N/2 in number, said first-kind and said second-kind pre-processed complex digital signal sequences serving in combination as said pre-processing means output complex digital signal sequences;

said offset discrete Fourier transform processing means comprising a first N/2-point offset discrete Fourier transform processor for subjecting said first-kind pre-processed complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a first kind, N/2 in number, and a second N/2-point offset discrete Fourier transform processor for subjecting said second-kind pre-processed complex digital signal sequences also to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a second kind, again N/2 in number, said first-kind and said second-kind offset discrete Fourier transformed complex digital signal sequences serving in combination as two sets of said N/2-point offset discrete Fourier transform processed complex digital signal sequences;

the digital filter units of said filter means comprising a first bank of complex band-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/T hertzes and said phase slopes for carrying out band limitation on said first-kind offset discrete Fourier transformed complex digital signal sequences to produce filtered complex digital signal sequences of a first kind, respectively, and a second bank of complex band-pass digital filter units, again N/2 in number, operable also at twice said slow sampling rate and having said 1/T-hertz effective bandwidth and said phase slopes for carrying out band limitation on said second-kind offset discrete Fourier transformed complex digital signal sequences to produce filtered complex digital signal sequences of a second kind, respectively, said filter output means being responsive to said first-kind and said second-kind filtered complex digital signal sequences for producing real parts of said first-kind filtered complex digital signal sequences and imaginary parts of said second-kind filtered complex digital signal sequences as real and imaginary parts of said filter means output complex digital signal sequences, respectively;

said second means comprising a post-processing circuit for digitally converting said real parts of first-kind filtered complex digital signal sequences and said imaginary parts of second-kind filtered complex digital signal sequences to said real QAM digital signal sequence.

4. A converter as claimed in claim 3, wherein said post-processing circuit comprises:

a post-processor for subjecting said real parts of first-kind filtered complex digital signal sequences and said imaginary parts of second-kind filtered complex digital signal sequences to a predetermined frequency shift to produce post-processed complex digital signal sequences, N/2 in number; and

a time-division multiplexing circuit for time-division multiplexing real parts of said post-processed complex digital signal sequences into said real QAM digital signal sequence.

5. A converter as claimed in claims 3 or 4, wherein said pre-processing circuit comprises:

a pre-processor for digitally converting said real PAM digital signal sequences to pre-processor output complex digital signal sequences of a first kind of twice said slow sampling rate, L in number, and also to pre-processor output complex digital signal sequences of a second kind of twice said slow sampling rate, again L in number; and

means for digitally producing complex dummy signals, (N/2-L) in number;

said first-kind pre-processor output complex digital signal sequences and said complex dummy signals serving in combination as said first-kind pre-processed complex digital signal sequences, said second-kind pre-processor output complex digital signal sequences and said complex dummy signals serving in combination as said second-kind pre-processed complex digital signal sequences.

6. A converter as claimed in claim 2, said prescribed one of N and N/2 being N/2, wherein:

said pre-processing means comprises a pre-processor for digitally converting said real PAM digital signal sequences to pre-processed complex digital signal sequences of twice said slow sampling rate, L in number, and means for digitally producing complex dummy signals, (N/2-L) in number, said pre-processed complex digital signal sequences and said complex dummy signals serving in combination as said pre-processing means output complex digital signal sequences;

said offset discrete Fourier transform processing means comprising an N/2-point offset discrete Fourier transform processor for subjecting said pre-processing means output complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce N/2-point offset discrete Fourier transformed complex digital signal sequences as the respective offset discrete Fourier transform processed complex digital signal sequences of a single set;

the digital filter units of said filter means comprising a single bank of complex band-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/T hertzes and said phase slopes for carrying out band limitation on said offset discrete Fourier transformed complex digital signal sequences to produce filtered complex digital signal sequences, respectively, said filter output means being responsive to said filtered complex digital signal sequences for producing real parts of said filtered complex digital signal sequences as real parts of said filter means output complex digital signal sequences, respectively;

said second means comprising a time-division multiplexing circuit for time-division multiplexing said real parts of filtered complex digital signal sequences into said real QAM digital signal sequence.

7. A converter as claimed in claim 2, said prescribed one of N and N/2 being N, wherein:

said pre-processing means comprises a pre-processing circuit for digitally converting said real PAM digital signal sequences to pre-processed complex digital signal sequences of a first kind of twice said slow sampling rate, N/2 in number, and also to pre-processed complex digital signal sequences of a second kind of twice said slow sampling rate, again N/2 in number, said first-kind and said second-kind pre-processed complex digital signal sequences serving in combination as said pre-processing means output complex digital signal sequences;

said offset discrete Fourier transform processing means comprising a first N/2-point offset discrete Fourier transform processor for subjecting said first-kind pre-processed complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a first kind, N/2 in number, and a second N/2-point offset discrete Fourier transform processor for subjecting said second-kind pre-processed complex digital signal sequences also to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a second kind, again N/2 in number, said first-kind and said second-kind offset discrete Fourier transformed complex digital signal sequences serving in combination as two sets of said offset discrete Fourier transform processed complex digital signal sequences;

the digital filter units of said filter means comprising a first bank of real low-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/(2T) hertzes and said phase slopes for carrying out band limitation on said first-kind offset discrete Fourier transformed complex digital signal sequences to produce filtered complex digital signal sequences of a first kind, respectively, and a second bank of real low-pass digital filter units, again N/2 in number, operable also at twice said slow sampling rate and having said 1/(2T)-hertz effective bandwidth and said phase slopes for carrying out band limitation on said second-kind offset discrete Fourier transformed complex digital signal sequences to produce filtered complex digital signal sequences of a second kind, respectively;

said filter output means comprising a first offset circuit for subjecting said first-kind filtered complex digital signal sequences to a prescribed frequency offset to produce real parts of filtered and frequency offset complex digital signal sequences of a first kind, respectively, and a second offset circuit for subjecting said second-kind filtered complex digital signal sequences to said prescribed frequency offset to produce imaginary parts of filtered and frequency offset complex digital signal sequences of a second kind, respectively;

said real parts of first-kind filtered and frequency offset complex digital signal sequences and said imaginary parts of second-kind filtered and frequency offset complex digital signal sequences serving as real and imaginary parts of said filter means output complex digital signal sequences, respectively;

said second means comprising a post-processing circuit for digitally converting said real parts of first-kind filtered and frequency offset complex digital signal sequences and said imaginary parts of second-kind filtered and frequency offset complex digital signal sequences to said real QAM digital signal sequence.

8. A converter as claimed in claim 7, wherein said post-processing circuit comprises:

a post-processor for subjecting said real parts of first-kind filtered and frequency offset complex digital signal sequences and said imaginary parts of filtered and frequency offset complex digital signal sequences to a predetermined frequency shift to produce post-processed complex digital signal sequences, N/2 in number; and

a time-division multiplexing circuit for time-division multiplexing real parts of said post-processed complex digital signal sequences into said real QAM digital signal sequence.

9. A converter as claimed in claims 7 or 8, wherein said pre-processing circuit comprises:

a pre-processor for digitally converting said real PAM digital signal sequences to pre-processor output complex digital signal sequences of a first kind of twice said slow sampling rate, L in number, and also to pre-processor output complex digital signal sequences of a second kind of twice said slow sampling rate, again L in number; and

means for digitally producing complex dummy signals, (N/2-L) in number;

said first-kind pre-processor output complex digital signal sequences and said complex dummy signals serving in combination as said first-kind pre-processed complex digital signal sequences, said second-kind pre-processor output complex digital signal sequences and said complex dummy signals serving in combination as said second-kind pre-processed complex digital signal sequences.

10. A converter as claimed in claim 2, said prescribed one of N and N/2 being N/2, wherein:

said pre-processing means comprises a pre-processor for digitally converting said real PAM digital signal sequences to pre-processed complex digital signal sequences of twice said slow sampling rate, L in number, and means for digitally producing complex dummy signals, (N/2-L) in number, said pre-processed complex digital signal sequences and said complex dummy signals serving in combination as said pre-processing means output complex digital signal sequences;

said offset discrete Fourier transform processing means comprising an N/2-point offset discrete Fourier transform processor for subjecting said pre-processing means output complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce N/2-point offset discrete Fourier transformed complex digital signal sequences as the respective offset discrete Fourier transform processed complex digital signal sequences of a single set;

the digital filter units of said filter means comprising a single bank of real low-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/(2T) hertzes and said phase slopes for carrying out band limitation on said offset discrete Fourier transformed complex digital signal sequences to produce filtered complex digital signal sequences, respectively;

said filter output means comprising an offset circuit for subjecting said filtered complex digital signal sequences to a prescribed frequency offset to produce real parts of filtered and frequency offset complex digital signal sequences as said filter means output complex digital signal sequences, respectively,

said second means comprising a time-division multiplexing circuit for time-division multiplexing said real parts of filtered and frequency offset complex digital signal sequences into said real QAM digital signal sequence.

11. A converter as claimed in claim 1 and operable as said receiver, wherein:

said second means comprises pre-processing means for digitally converting said real QAM digital signal sequence to pre-processing means output complex digital signal sequences of twice said slow sampling rate, equal in number to said prescribed one of N and N/2, said pre-processing means output complex digital signal sequences serving as said third-type complex digital signal sequences, respectively;

the digital filter units of said filter means carrying out band limitation on said pre-processing means output complex digital signal sequences to produce filter means output complex digital signal sequences, respectively, said filter means thereby producing said filter means output complex digital signal sequences as said second-type complex digital signal sequences, respectively;

said offset discrete Fourier transform processing means comprising offset discrete Fourier transform processor means for subjecting said filter means output complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce at least one set of N/2-point offset discrete Fourier transform processed complex digital signal sequences and processor output means responsive to said offset discrete Fourier transform processed complex digital signal sequences for producing a single set of offset discrete Fourier transform processed complex digital signal sequences, N/2 in number, that serve as said first-type complex digital signal sequences, respectively;

said first means digitally converting said single-set offset discrete Fourier transform processed complex digital signal sequences to said real PM digital signal sequences.

12. A converter as claimed in claim 11, said prescribed one of N and N/2 being N, wherein:

said pre-processing means comprises a demultiplexing circuit for demultiplexing said real QAM digital signal sequence into a plurality of demultiplexed real digital signal sequences of twice said slow sampling rate, N/2 in number, and a pre-processing circuit for digitally converting said demultiplexed real digital signal sequences to pre-processed complex digital signal sequences of a first kind of twice said slow sampling rate, N/2 in number, and also to pre-processed complex digital signal sequences of a second kind of twice said slow sampling rate, again N/2 in number, said first-kind and said second-kind pre-processed complex digital signal sequences serving in combination as said pre-processing means output complex digital signal sequences;

the digital filter units of said filter means comprising a first bank of complex band-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/T hertzes and said phase slopes for carrying out band limitation on said first-kind pre-processed complex digital signal sequences to produce filtered complex digital signal sequences of a first kind, respectively, and a second bank of complex band-pass digital filter units, again N/2 in number, operable also at twice said slow sampling rate and having said 1/T-hertz effective bandwidth and said phase slopes for carrying out band limitation on said second-kind pre-processed complex digital signal sequences to produce filtered complex digital signal sequences of a second kind, respectively, said first-kind and said second-kind filtered complex digital signal sequences serving in combination as said filter means output complex digital signal sequences;

said offset discrete Fourier transform processor means comprising a first N/2-point offset discrete Fourier transform processor for subjecting said first-kind filtered complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a first kind, N/2 in number, and a second N/2-point offset discrete Fourier transform processor for subjecting said second-kind filtered complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a second kind, again N/2 in number;

said second-kind offset discrete Fourier transformed complex digital signal sequences corresponding to said first-kind offset discrete Fourier transformed complex digital signal sequences, respectively;

said processor output means being responsive to said first-kind and said second-kind offset discrete Fourier transformed complex digital signal sequences for producing, as said single-set offset discrete Fourier transform processed complex digital signal sequences, predetermined ones, substantially equal in number to a half of N/2, of said first-kind offset discrete Fourier transformed complex digital signal sequences and those of said second-kind offset discrete Fourier transformed complex digital signal sequences which correspond to the first-kind offset discrete Fourier transformed complex digital signal sequences except said predetermined first-kind offset discrete Fourier transformed complex digital signal sequences, respectively;

said first means comprising a post-processing circuit for digitally converting the last-mentioned single-set offset discrete Fourier transform processed complex digital signal sequences to baseband complex PAM digital signal sequences, L in number, and also to complex dummy signals, (N/2-L) in number, and an output circuit for converting said baseband complex PAM digital signal sequences to said baseband real PAM digital signal sequences.

13. A converter as claimed in claim 12, wherein said pre-processing circuit comprises means for digitally converting said demultiplexed real digital signal sequences to said first-kind pre-processed complex digital signal sequences and means for digitally converting the last-mentioned first-kind pre-processed complex digital signal sequences to conjugate complex digital signal sequences thereof, respectively, said conjugate complex digital signal sequences serving as said second-kind pre-processed complex digital signal sequences, respectively.

14. A converter as claimed in claim 11, said prescribed one of N and N/2 being N/2, wherein:

said pre-processing means comprises a demultiplexing circuit for demultiplexing said real QAM digital signal sequence into a plurality of demultiplexed real digital signal sequences of twice said slow sampling rate, N/2 in number, said demultiplexed real digital signal sequences serving as said pre-processing means output complex digital signal sequences, respectively;

the digital filter units of said filter means comprising a single bank of complex band-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/T hertzes and said phase slopes for carrying out band limitation on said demultiplexed real digital signal sequences to produce filtered complex digital signal sequences, respectively, said filtered complex digital signal sequences serving as said filter means output complex digital signal sequences, respectively;

said offset discrete Fourier transform processor means comprising an N/2-point offset discrete Fourier transform processor for subjecting said filtered complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce a signal set of N/2-point offset discrete Fourier transformed complex digital signal sequences, said processor output means producing said single-set offset discrete Fourier transformed complex digital signal sequences as said single-set offset discrete Fourier transform processed complex digital signal sequences, respectively;

said first means comprising a post-processing circuit for digitally converting said single-set discrete Fourier transformed complex digital signal sequences to baseband complex PAM digital signal sequences, L in number, and also to complex dummy signals, (N/2-L) in number, and an output circuit for converting said baseband complex PAM digital signal sequences to said baseband real PAM digital signal sequences.

15. A converter as claimed in claim 11, said prescribed one of N and N/2 being N, wherein:

said pre-processing means comprises a demultiplexing circuit for demultiplexing said real QAM digital signal sequence into a plurality of demultiplexed real digital signal sequences of twice said slow sampling rate, N/2 in number, and a pre-processing circuit for digitally converting said demultiplexed real digital signal sequences to pre-processed complex digital signal sequences of a first kind of twice said slow sampling rate, N/2 in number, and also to pre-processed complex digital signal sequences of a second kind of twice said slow sampling rate, again N/2 in number, said first-kind and said second-kind pre-processed complex digital signal sequences serving in combination as said pre-processing means output complex digital signal sequences;

the digital filter units of said filter means comprising a first bank of real low-pass digital filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/(2T) hertzes and said phase slopes for carrying out band limitation on said first-kind pre-processed complex digital signal sequences to produce filtered complex digital signal sequences of a first kind, respectively, and a second bank of real low-pass digital filter units, again N/2 in number, operable also at twice said slow sampling rate and having said 1/(2T)-hertz effective bandwidth and said phase slopes for carrying out band limitation on said second-kind pre-processed complex digital signal sequences to produce filtered complex digital signal sequences of a second kind, respectively, said first-kind and said second-kind filtered complex digital signal sequences serving in combination as said filter means output complex digital signal sequences;

said offset discrete Fourier transform processor means comprising a first N/2-point offset discrete Fourier transform processor for subjecting said first-kind filtered complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a first kind, N/2 in number, and a second N/2-point offset discrete Fourier transform processor for subjecting said second-kind filtered complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce offset discrete Fourier transformed complex digital signal sequences of a second kind, again N/2 in number;

said second-kind offset discrete Fourier transformed complex digital signal sequences corresponding to said first-kind offset discrete Fourier transformed complex digital signal sequences, respectively;

said processor output means being responsive to said first-kind and said second-kind offset discrete Fourier transformed complex digital signal sequences for producing, as said single-set offset discrete Fourier transform processed complex digital signal sequences, predetermined ones, substantially equal in number to a half of N/2, of said first-kind offset discrete Fourier transformed complex digital signal sequences and those of said second-kind offset discrete Fourier transformed complex digital signal sequences which correspond to the first-kind offset discrete Fourier transformed complex digital signal sequences except said predetermined first-kind offset discrete Fourier transformed complex digital signal sequences, respectively;

said first means comprising a post-processing circuit for digitally converting said single-set offset discrete Fourier transformed complex digital signal sequences to baseband complex PAM digital signal sequences, L in number, and also to complex dummy signals, (N/2-L) in number, and an output circuit for converting said baseband complex PAM digital signal sequences to said baseband real PAM digital signal sequences.

16. A converter as claimed in claim 15, wherein said pro-processing circuit comprises means for digitally converting said demultiplexed real digital signal sequences to said first-kind pre-processed complex digital signal sequences and also to conjugate complex digital signal sequences of said first-kind pre-processed complex digital signal sequences and means for giving a common frequency shift of 1/T hertzes to said conjugate complex digital signal sequences to produce said second-kind pre-processed complex digital signal sequences.

17. A converter as claimed in claim 11, said prescribed one of N and N/2 being N/2, wherein:

said pre-processing means comprises a demultiplexing circuit for demultiplexing said real QAM digital signal sequence into a plurality of demultiplexed real digital signal sequences of twice said slow sampling rate, N/2 in number, and a pre-processor for giving a frequency offset of 1/T hertzes to said demultiplexed real digital signal sequences to produce pre-processed complex digital signal sequences of twice said slow sampling rate, N/2 in number, that serve as said pre-processing means output complex digital signal sequences, respectively;

the digital filter units of said filter means comprising a single bank of real low-pass filter units, N/2 in number, operable at twice said slow sampling rate and having an effective bandwidth of 1/(2T) hertzes and said phase slopes for carrying out band limitation on said pre-processed complex digital signal sequences to produce filtered complex digital signal sequences serving as said filter means output complex digital signal sequences, respectively;

said offset discrete Fourier transform processor means comprising an N/2-point offset discrete Fourier transform processor for subjecting said filtered complex digital signal sequences to said N/2-point offset discrete Fourier transform processing to produce a signal set of N/2-point offset discrete Fourier transformed complex digital signal sequences, said processor output means producing said N/2-point offset discrete Fourier transformed complex digital signal sequences as said single-set offset discrete Fourier transform processed complex digital signal sequences, respectively;

said first means comprising a post-processing circuit for digitally converting said single-set offset discrete Fourier transformed complex digital signal sequences to baseband complex PAM digital signal sequences, L in number, and also to complex dummy signals, (N/2-L) in number, and an output circuit for converting said baseband complex PAM digital signal sequences to said baseband real PAM digital signal sequences.