Variable-bandwidth frequency division multiplex communication system

• US 6,014,366 A
• Filed: 04/15/1997
• Issued: 01/11/2000
• Est. Priority Date: 04/15/1996
• Status: Expired due to Fees

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Ichiyoshi, Osamu
• Primary Examiner(s): HSU, ALPUS
• Application Number:
  US08/839,753
• Time in Patent Office: 1,001 Days
• Field of Search: 370/206, 370/210, 370/480, 370/481, 370/484, 370/488, 370/497, 375/260, 375/261, 375/295, 375/298, 375/316, 375/324, 375/328, 375/350, 708/300, 708/400, 708/403, 708/404, 708/405
• US Class Current: 370/210
• CPC Class Codes: H04J 4/005 Transmultiplexing

Abstract

A variable-bandwidth frequency-division multiplex communication system which needs no interpolation circuits, but uses switch element of simple structure. The system includes a signal combining circuit which has N A/D converters associated respectively with independent information signals to be transmitted, a complex local oscillator for generating complex signals each having a frequency kΔf (1≦k≦N-1), Δf being the channel frequency interval, N complex multipliers for multiplying the outputs of the A/D converters by the respective complex signals, and producing N output signals, a circuit for effecting an inverse Fourier transform of N points, an N×N switch circuit connected between the outputs of the complex multipliers and input terminals of the inverse Fourier transform circuit, N digital subfilters connected respectively to the outputs of the inverse Fourier transform circuit, N delay units connected respectively to outputs of the digital subfilters, and an adder for adding output signals from the delay units and outputting a sum signal.

Claims

1. A variable-bandwidth frequency-division multiplex communication system having a transmission device including a signal combining circuit and a reception device including a signal branching circuit, for establishing channels having a frequency band wider than a channel frequency interval Δ

f,

said signal combining circuit comprising;

a sampling timing generator for generating a sampling frequency f_s which is a multiple by a natural number of said channel frequency interval Δ

f;

a plurality of A/D converters associated respectively with N independent information signals to be transmitted, N being a natural number, for sampling the corresponding information signals at said sampling frequency f_s and converting the information signals to digital signals;

a complex local oscillator for generating complex signals each having a frequency which is k times said channel frequency interval Δ

f, k being an integer ranging from 0 to N-1;

N complex multipliers for multiplying the digital signals from said A/D converters by the respective complex signals from said complex local oscillator, and producing N output signals;

an inverse Fourier transform circuit for effecting a complex inverse Fourier transform of N points;

an N-input/N-output switch circuit connected between output terminals of said complex multipliers and input terminals of said inverse Fourier transform circuit;

a plurality of digital subfilters connected respectively to N output terminals of said inverse Fourier transform circuit, for filtering output signals from said inverse Fourier transform circuit;

a plurality of delay units connected respectively to output terminals of said digital subfilters; and

an adder for adding output signals from said delay units and outputting a sum signal;

wherein a k-th delay unit (1≦

k≦

N) applies a time lag expressed by (k-1)/f_s to an output signal from a corresponding one of said digital subfilters.

2. The variable-bandwidth frequency-division multiplex communication system according to claim 1, wherein said sampling frequency f_s is represented by NΔ

f, and said transmission device further comprises a high-speed D/A converter for converting a complex sample sequence outputted from said signal combining circuit to a complex continuous signal, and a quadrature amplitude modulation circuit for quadrature-amplitude-modulating a carrier with an output signal from said high-speed D/A converter.

3. The variable-bandwidth frequency-division multiplex communication system according to claim 1, wherein said switch circuit comprises N 3-input/1-output switch elements each represented as a k-th switch element, and wherein said k-th switch element has:

a first input terminal connected to a (k-1)th input terminal of said switch circuit;

a second input terminal connected to a k-th input terminal of said switch circuit;

a third input terminal connected to a (k+1)th input terminal of said switch circuit; and

an output terminal connected to a k-th output terminal of said switch circuit.

4. The variable-bandwidth frequency-division multiplex communication system according to claim 1, wherein said complex local oscillator comprises an N-output direct digital synthesizer.

5. The variable-bandwidth frequency-division multiplex communication system according to claim 1, wherein said digital subfilters serve as a single digital filter as a whole, and have a frequency characteristic curve which becomes completely flat in frequency regions [0, Δ

f] when folded and superposed at a frequency of Δ

f/2.

6. The variable-bandwidth frequency-division multiplex communication system according to claim 2, wherein said switch circuit comprises N 3-input/1-output switch elements each represented as a k-th switch element, and wherein said k-th switch element has:

a first input terminal connected to a (k-1)th input terminal of said switch circuit;

a second input terminal connected to a k-th input terminal of said switch circuit;

a third input terminal connected to a (k+1)th input terminal of said switch circuit; and

an output terminal connected to a k-th output terminal of said switch circuit.

7. A variable-bandwidth frequency-division multiplex communication system having a transmission device including a signal combining circuit and a reception device including a signal branching circuit, for establishing channels having a frequency band wider than a channel frequency interval Δ

f,

said signal branching circuit comprising;

a sampling timing generator for generating a sampling frequency f_s which is a multiple by a natural number of said channel frequency interval Δ

f;

an N-output shift register for shifting a complex digital numerical sequence of a received digital signal in synchronism with said sampling frequency f_s, N being a natural number;

a sampling circuit for sampling output signals from respective stages of said shift register in synchronism with said sampling frequency f_s ;

a plurality of digital subfilters for filtering respective N signals outputted parallel from said sampling circuit;

a Fourier transform circuit for effecting a complex Fourier transform of N points on respective output signals from said digital subfilters;

an N-input/N-output switch circuit connected to output terminals of said Fourier transform circuit;

a complex local oscillator for generating complex signals each having a frequency which is k times said channel frequency interval Δ

f, k being an integer ranging from 0 to N-1;

N complex multipliers for multiplying output signals from said switch circuit by the respective complex signals from said complex local oscillator, and producing N output signals; and

a plurality of D/A converters connected respectively to output terminals of said complex multipliers.

8. The variable-bandwidth frequency-division multiplex communication system according to claim 7, wherein said sampling frequency f_s is represented by NΔ

f, and said reception device further comprises a quadrature amplitude demodulation circuit for quadrature-amplitude-demodulating a received intermediate-frequency signal, and an A/D converter for sampling an output signal from said quadrature amplitude demodulation circuit at said sampling frequency f_s and converting the sampled signal to a complex digital signal, which is applied to said shift register.

9. The variable-bandwidth frequency-division multiplex communication system according to claim 7, wherein a received signal which is converted to a baseband signal and converted to a digital signal is applied to said signal branching circuit, and said D/A converters produce frequency-divided multiplex output signals.

10. A variable-bandwidth frequency-division multiplex communication system according to claim 7, wherein said switch circuit comprises N 1-input/3-output switch elements, N first adders each represented by a k-th first adder, and N second adders each represented by a k-th second adder, and wherein:

said k-th second adder adds an output signal from a first output terminal of a (k+1)th switch element and an output signal from a second output terminal of a k-th switch element, and outputs a sum signal to a k-th output terminal of said switch circuit; and

said k-th first adder adds an output signal from a third output terminal of a (k-1)th switch element and a signal from a k-th input terminal of said switch circuit, and inputs a sum signal to an input terminal of said k-th switch element.

11. The variable-bandwidth frequency-division multiplex communication system according to claim 7, wherein said complex local oscillator comprises an N-output direct digital synthesizer.

12. The variable-bandwidth frequency-division multiplex communication system according to claim 7, wherein said digital subfilters serve as a single digital filter as a whole, and have a frequency characteristic curve which becomes completely flat in frequency regions [0, Δ

f] when folded and superposed at a frequency of Δ

f/2.

13. The variable-bandwidth frequency-division multiplex communication system according to claim 8, wherein said switch circuit comprises N 1-input/3-output switch elements, N first adders each represented by a k-th first adder, and N second adders each represented by a k-th second adder, and wherein:

said k-th second adder adds an output signal from a first output terminal of a (k+1)th switch element and an output signal from a second output terminal of a k-th switch element, and outputs a sum signal to a k-th output terminal of said switch circuit; and

said k-th first adder adds an output signal from a third output terminal of a (k-1)th switch element and a signal from a k-th input terminal of said switch circuit, and inputs a sum signal to an input terminal of said k-th switch element.

14. A variable-bandwidth frequency-division multiplex communication system for achieving a multiplex access using a relay station having antennas corresponding to respective geographic regions, to provide a complete connection between said regions, said relay station having:

as many sets of a reception device and a transmission device as the number of said antennas, a sampling timing generator for generating a sampling frequency f_s which is a multiple by a natural number of said channel frequency interval Δ

f, a complex local oscillator for generating complex signals each having a frequency which is k times said channel frequency interval Δ

f, k being an integer ranging from 0 to N-1 and N being a natural number, a baseband switch matrix, signal branching circuits connected between the reception devices and said baseband switch matrix, and signal combining circuits connected between the transmission devices and said baseband switch matrix;

each of said signal combining circuits comprising;

a first complex multiplying circuit having complex multipliers associated respectively with N independent information signals outputted from said baseband switch matrix, for multiplying the information signals by the respective complex signals from said complex local oscillator and producing N output signals;

an inverse Fourier transform circuit for effecting a complex inverse Fourier transform of N points;

an N-input/N-output first switch circuit connected between output terminals of said first complex multiplying circuit and input terminals of said inverse Fourier transform circuit;

a plurality of first digital subfilters connected respectively to N output terminals of said inverse Fourier transform circuit, for filtering output signals from said inverse Fourier transform circuit;

a plurality of delay units connected respectively to output terminals of said digital subfilters; and

an adder for adding output signals from said delay units and outputting a sum signal to said transmission device;

wherein a k-th delay unit (1≦

k ≦

N) applies a time lag expressed by (k-1)/f_s to an output signal from a corresponding one of said first digital subfilters;

each of said signal branching circuits comprising;

an N-output shift register for shifting a complex digital numerical sequence from said reception device in synchronism with said sampling frequency f_s ;

a sampling circuit for sampling output signals from respective stages of said shift register in synchronism with said sampling frequency f_s ;

a plurality of second digital subfilters for filtering respective N signals outputted parallel from said sampling circuit;

a Fourier transform circuit for effecting a complex Fourier transform of N points on respective output signals from said second digital subfilters;

an N-input/N-output second switch circuit connected to output terminals of said Fourier transform circuit; and

a second complex multiplying circuit having N complex multipliers for multiplying output signals from said second switch circuit by the respective complex signals, producing N output signals, and supplying the N output signals as frequency-divided multiplex signals to said baseband switch matrix.