One frequency repeater for a digital microwave radio system with cancellation of transmitter-to-receiver interference

• US 4,701,935 A
• Filed: 01/09/1986
• Issued: 10/20/1987
• Est. Priority Date: 01/09/1985
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Namiki, Junji
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/817,380
• Time in Patent Office: 649 Days
• Field of Search: 455/7, 455/15, 455/16, 455/19, 455/20, 455/24, 455/25, 455/60, 375/3, 375/4, 178/70 R, 178/70 S, 178/71 R, 342/361, 370/20
• US Class Current: 375/214
• CPC Class Codes: H04B 7/15585 by interference cancellation

Abstract

A digital microwave radio repeater using the same carrier frequency for the transmitter and the receiver carriers for improving the frequency utilization efficiency. The transmitter-to-receiver interference signal is cancelled by producing an estimated-interference signal of the interference signal from the digital baseband signal by the use of a transversal filter and by subtracting the estimated-interference signal from the received signal. The estimated-interference signal is also produced by frequency shifting a digital modulated signal in the transmitter. The transmitter carrier frequency is locked to the receiver carrier frequency by the use of a phase-locking technique, so that the interference signal is reduced.

Claims

1. A one frequency repeater for a digital radio wave transmission system wherein a carrier is modulated by a digital baseband signal and is transmitted as a digital radio wave signal, said repeater comprising:

a first digital signal source for producing a first digital baseband signal;

transmitter circuit means coupled to said first digital signal source for producing a first digital radio wave signal modulated by said first digital baseband signal, said first digital radio wave having a first carrier frequency;

transmitting antenna means for radiating said first radio frequency signal supplied from said transmitter circuit means;

receiving antenna means for receiving a second digital radio wave signal modulated by a second digital baseband signal, said second digital radio wave signal having a second carrier frequency substantially equal to said first carrier frequency, said receiving antenna means receiving said first digital radio wave signal as a transmitter-to-receiver interference signal due to coupling between said first transmitting antenna means and said first receiving antenna means;

receiver circuit means for extracting said second digital baseband signal by demodulating said second radio wave signal received at said receiving antenna, said demodulated second digital baseband signal being accompanied by a baseband interference signal derived from said transmitter-to-receiver interference signal;

means coupled to said first digital signal source for producing an estimated-interference signal corresponding to said baseband interference signal from said first digital baseband signal, said estimated-interference signal producing means comprising transversal filter means comprising a delay line with a predetermined number of tap gains; and

interference cancelling means coupled to said receiver circuit means and said estimated-interference signal producing means for cancelling said baseband interference signal from an output signal of said receiver circuit means using said estimated-interference signal to thereby produce said demodulated second digital baseband signal without said baseband interference signal;

decision circuit means for detecting said second digital baseband signal from an output of said interference cancelling means and regenerating said second digital baseband signal; and

tap gain control means for controlling said tap gains, said tap gain control means comprising first subtracting means for subtracting said regenerated second digital baseband signal from said output signal of said interference cancelling means and providing a first error signal, first multiplier means for multiplying said first digital baseband signal by said first error signal and providing a multiplied signal, and low-pass filter means for smoothing said multiplied signal, said smoothed multiplied signal being supplied to said transversal filter means to control said tap gains so that said first error signal is zero.

2. A one frequency repeater as claimed in claim 1, wherein said transmitting antenna means and said receiving antenna means are disposed side by side and directed to a common direction, said transmitter-to-receiver interference signal being caused by side-to-side coupling between said transmitting and said receiving antenna means.

3. A one frequency repeater as claimed in claim 1, wherein said transmitting antenna means and said antenna receiving means are disposed on opposite sides of said repeater and directed to opposite direction, said regenerated second digital baseband signal being supplied to said transmitter circuit means as said first digital baseband signal, said transmitter-to-receiver interference signal being caused by front-to-back coupling between said transmitting and said receiving antenna means.

4. A one frequency repeater as claimed in claim 1, wherein said transmitting antenna means and said receiving antenna means are disposed side by side and directed to a common direction, said transmitter circuit means including variable frequency oscillator means for fine adjusting said first carrier frequency, said variable frequency oscillator means being controlled by an output signal of said first multiplier means so that said first carrier frequency is made equal to said second carrier frequency.

5. A one frequency repeater as claimed in claim 1, which comprises said transmitter circuit means having first variable frequency oscillator means for providing a first signal, said first signal being used to produce said first digital radio wave signal from said first digital baseband signal, said receiver circuit means having second variable frequency oscillator means for providing a second signal, said second signal being used to extract said second digital baseband signal by demodulating said received second digital radio wave signal, means for detecting from an output signal of said receiver circuit means a phase difference of said second signal from said received second digital radio wave signal, said detected phase difference being applied to said second variable frequency oscillator means for controlling said second signal, and phase comparator means for phase-comparing said second signal and said first signal and providing a second error signal, and said second error signal being applied to said first variable frequency oscillator means so that said first signal is phase-locked to said second signal.

6. A one frequency repeater as claimed in claim 1, which comprises said transmitter circuit means having first variable intermediate frequency oscillator means for providing a first intermediate frequency signal, modulating means for modulating said first intermediate frequency signal by said first digital baseband signal, and frequency-up converter means for converting said first intermediate frequency of said modulated signal to said first carrier frequency and producing said first digital radio wave signal, said receiver circuit means having frequency-down converter means for converting a second digital radio wave signal received at said receiving antenna means to a down-converted signal having a second intermediate frequency, second variable intermediate frequency oscillator means for providing a second intermediate frequency signal, and demodulating means for demodulating said down-converted signal by said second intermediate frequency signal and providing a demodulated signal, means for detecting from said demodulated signal a phase difference between said down-converted signal and said second intermediate signal, said second variable intermediate frequency oscillator means being controlled by said phase difference, and phase comparator means for phase-comparing said second intermediate signal and said first intermediate signal and providing a second error signal, said second error signal being applied to said first variable intermediate oscillator means so that said first intermediate frequency signal is phase-locked to said second intermediate signal.

7. A repeater as claimed in claim 6, which comprises fixed frequency oscillator means for providing a fixed radio frequency signal having said second carrier frequency, said frequency-up converter means comprising multiplier means for multiplying said modulated signal by said fixed radio frequency signal, and said frequency-down converter means comprising multiplier means for multiplying said second digital radio wave signal received at said receiving antenna means by said fixed radio frequency signal.

8. A one-frequency repeater for a digital radio wave transmission system wherein a carrier is modulated by a digital baseband signal and is transmitted as a digital radio wave signal, said repeater comprising:

a first digital signal source for producing a first digital baseband signal;

transmitter circuit means coupled to said first digital signal source for producing a first digital radio wave signal modulated by said first digital baseband signal, said first digital radio wave having a first carrier frequency;

transmitting antenna means for radiating said first radio frequency signal supplied from said transmitter circuit means;

receiving antenna means for receiving a second digital radio wave signal modulated by a second digital baseband signal, said second digital radio wave signal having a second carrier frequency substantially equal to said first carrier frequency, said receiving antenna means receiving said first digital radio wave signal as a transmitter-to-receiver interference signal due to coupling between said first transmitting antenna means and said first receiving antenna means;

receiver circuit means for extracting said second digital baseband signal by demodulating said second radio wave signal received at said receiving antenna, said demodulated second digital baseband signal being accompanied by a baseband interference signal derived from said transmitter-to-receiver interference signal;

means coupled to said first digital signal source for producing an estimated-interference signal corresponding to said baseband interference signal from said first digital baseband signal, said estimated-interference signal producing means comprising;

first local oscillator means for generating a first frequency signal;

first phase shifter means for phase-shifting by π

/2 said first frequency signal;

first multiplier means for multiplying said first digital baseband signal by said first frequency signal;

second multiplier means for multiplying said first digital baseband signal by said phase-shifted first frequency signal;

second local oscillator means for generating a second frequency signal, said first and second frequency signals having a frequency difference corresponding to a frequency difference between said first and second carrier frequencies;

second phase shifter means for phase-shifting by π

/2 said second frequency signal;

third multiplier means for multiplying an output from said first multiplier means by said second frequency signal;

fourth multiplier means for multiplying an output from said second multiplier means by said phase-shifted second frequency signal;

adder means for adding an output signal of said third multiplier means and an output signal of said fourth multiplier means;

coefficient generating means for generating a coefficient representing an amount of said transmitter-to-receiver interference; and

fifth multiplier means for multiplying an output signal of said adder means by said coefficient, said fifth multiplier means providing said estimated-interference signal; and

interference cancelling means coupled to said receiver circuit means and said estimated-interference signal producing means for cancelling said baseband interference signal from an output signal of said receiver circuit means using said estimated-interference signal to thereby produce said demodulated second digital baseband signal without said baseband interference signal.

9. A one frequency repeater as claimed in claim 8, which further comprises delay means connected to said fifth multiplier means and for delaying said estimated-interference signal by a time period equal to a propagating time of said first digital radio wave signal from said transmitting antenna to said receiving antenna means.

10. A one frequency repeater as claimed in claim 8 which further comprises decision circuit means for deciding said second digital baseband signal from an output signal of said interference cancelling means and regenerating said second digital baseband signal, first subtracting means for subtracting said regenerated second digital baseband signal from said output signal of said interference cancelling means and providing an error signal, sixth multiplier means for multiplying said error signal by said estimated-interference signal and providing a frequency control signal, one of said first and second local oscillator means being variable oscillator means, said variable oscillator means being controlled by said frequency control signal.

11. A one-frequency repeater as claimed in claim 8, wherein said transmitter circuit means includes first variable frequency oscillator means for providing a first signal, said first signal being used to produce said first digital radio wave signal from said first digital baseband signal, said receiver circuit means including second variable frequency oscillator means for providing a second signal, said second signal being used to extract said second digital baseband signal by demodulating said received second digital radio wave signal, means for detecting from an output signal of said receiver circuit means a phase difference between said second signal and said received second digital radio wave signal, said detected phase difference being applied to said second variable frequency oscillator means to control the generation of said second signal, and phase comparator means for phase-comparing said second signal and said first signal and for providing an error signal, said error signal being applied to said first variable frequency oscillator means so that said first signal is phase-locked to said second signal.

12. A one-frequency repeater as claimed in claim 8, wherein said transmitter circuit means includes first variable intermediate frequency oscillator means for providing a first intermediate frequency signal, modulating means for modulating said first intermediate frequency signal by said first digital baseband signal, and frequency up converter means for converting said first intermediate frequency of said modulated signal to said first carrier frequency and for producing said first digital radio wave signal, said receiver circuit means including frequency down converter means for converting a second digital radio wave signal received at said receiving antenna means to a down-converted signal having a second intermediate frequency, second variable intermediate frequency oscillator means for providing a second intermediate frequency signal, and demodulating means for demodulating said down-converted signal by said second intermediate frequency signal and for providing a demodulated signal, means for detecting from said demodulated signal a phase difference between said down-converted signal and said second intermediate frequency signal, said second variable intermediate frequency oscillator means being controlled by said phase difference, and phase comparator means for phase-comparing said second intermediate frequency signal and said first intermediate frequency signal and providing an error signal, said error signal being applied to said first variable intermediate frequency oscillator means so that said first intermediate frequency signal is phase-locked to said second intermediate frequency signal.

13. A repeater as claimed in claim 8, further comprising fixed frequency oscillator means for providing a fixed radio frequency signal having said second carrier frequency, said frequency-up converter means comprising multiplier means for multiplying said modulated signal by said fixed radio frequency signal, and said frequency-down converter means comprising multiplier means for multiplying said second digital radio wave signal received at said receiving antenna means by said fixed radio frequency signal.

14. A one-frequency two-way repeater for a digital transmission system wherein first and second digital microwave signals are received at first and second receiving antenna means and regenerated as third and fourth digital microwave signals, respectively, said third and fourth digital microwave signals being radiated from first and second transmitting antenna means, respectively, said first and second digital microwave signals carrying first and second digital baseband signals, respectively, said first, second, third and fourth digital microwave signals having a first, a second, a third and a fourth carrier frequency, respectively, said first, second, third and fourth carrier frequencies being substantially equal to one another, said repeater comprising:

said first receiving antenna means and said second transmitting antenna means being disposed side by side and directed in a common first direction, said first receiving antenna means and said second antenna means having a side-to-side antenna coupling whereby said first receiving antenna means receives said fourth digital microwave signal as a first transmitter-to-receiver interference signal;

said first transmitting antenna means and said second receiving antenna means being disposed side by said and directed in a common second direction opposite to said first direction, said first transmitting antenna means and said second receiving antenna means having a side-to-side antenna coupling whereby said second receiving antenna means receives said third digital microwave signal as a second transmitter-to-receiver interference signal;

first receiver circuit means connected to said first receiving antenna means for extracting said first digital baseband signal by demodulating said first digital microwave signal received at said first receiving antenna means, said demodulated first digital baseband signal being accompanied by a first baseband interference signal derived from said first transmitter-to-receiver interference signal;

first decision circuit means for detecting said demodulated first digital baseband signal and regenerating said first digital baseband signal as a third digital baseband signal;

first transmitter circuit means for producing said third digital microwave signal modulated by said third digital baseband signal and for supplying said third digital microwave signal to said first transmitting antenna means;

second receiver circuit means connected to said second receiving antenna means and for extracting said second digital baseband signal by demodulating said second digital microwave signal received at said second receiving antenna means, said demodulated second digital baseband signal being accompanied by a second baseband interference signal derived from said second transmitter-to-receiver interference signal;

second decision circuit means for detecting said demodulated second digital baseband signal and regenerating said second digital baseband signal as a fourth digital baseband signal;

second transmitter circuit means for producing said fourth digital microwave signal modulated by said fourth digital baseband signal and for supplying said fourth digital microwave signal to said second transmitting antenna means;

first estimated-interference signal producing means coupled to said second decision circuit means for producing a first estimated-interference signal corresponding to said first baseband interference signal from said fourth digital baseband signal, said first estimated-interference signal producing means comprising transversal filter means comprising a delay line having a predetermined number of tap gains;

first interference cancelling means coupled to said first estimated-interference signal producing means for cancelling said first baseband interference signal from an output signal of said first receiver circuit means using said first estimated-interference signal, to thereby obtain said demodulated first digital baseband signal without said first baseband interference signal;

second estimated-interference signal producing means coupled to said first decision circuit means for producing a second estimated-interference signal corresponding to said second baseband interference signal from said third digital baseband signal; and

second interference cancelling means coupled to said second estimated-interference signal producing means for cancelling said second baseband interference signal from an output signal of said second receiver circuit means using said second estimated-interference signal, to thereby obtain said demodulated second digital baseband signal without said second baseband interference signal.

15. A repeater as claimed in claim 14, wherein said second estimated-interference signal producing means comprises a transversal filter means comprising a delay line with a predetermined number of tap gains.

16. A repeater as claimed in claim 14, which further comprises said first receiving antenna means and said first transmitting antenna means having a front-to-back antenna coupling so that said third digital microwave signal is received at said first receiving antenna means as a third transmitter-to-receiver interference signal, said first receiver circuit means providing an output signal with a third baseband interference signal derived from said third transmitter-to-receiver interference signal, third estimated-interference signal producing means for producing a third estimated-interference signal corresponding to said third baseband interference signal from said third digital baseband signal, said third estimated-interference signal being applied to said baseband signal, said first interference cancelling means so that said third baseband interference signal is cancelled from said output signal of said first receiver circuit means.

17. A repeater as claimed in claim 16, which further comprises said second receiving antenna means and said second transmitting antenna means having a front-to-back antenna coupling so that said fourth digital microwave signal is received at said second receiving antenna means as a fourth transmitter-to-receiver interference signal, said second receiver circuit means providing an output signal with a fourth baseband interference signal derived from said fourth transmitter-to-receiver interference signal, fourth estimated-interference signal producing means for producing a fourth estimated-interference signal corresponding to said fourth baseband interference signal from said fourth digital baseband signal, said fourth estimated-interference signal being applied to said second interference cancelling means so that said fourth baseband interference signal is cancelled from said output signal of said second receiver circuit means.

18. A repeater as claimed in claim 17, wherein said third estimated-interference signal producing means is first transversal filter means comprising a delay line with a predetermined number of tap gains, and said fourth estimated-interference signal producing means is second transversal filter means comprising a delay line with a predetermined number of tap gains.

19. A repeater as claimed in claim 16, wherein said third estimated-interference signal producing means comprises:

first local oscillator means for oscillating a first frequency signal;

first phase shifter means for phase-shifting by π

/2 said first frequency signal;

first multiplier means for multiplying said third digital baseband signal by said first frequency signal;

second multiplier means for multiplying said third digital baseband signal by said phase-shifted first frequency signal;

second local oscillator means for oscillating a second frequency signal, said first and second frequency signals having a frequency difference corresponding to a frequency difference between said first and third carrier frequencies;

second phase shifter means for phase-shifting by π

/2 said second frequency signal;

third multiplier means for multiplying an output from said first multiplier means by said second frequency signal;

fourth multiplier means for multiplying an output from said second multiplier means by said phase-shifted second frequency signal;

adder means for adding an output signal of said third multiplier means and an output signal of said fourth multiplier means;

coefficient generating means for generating a coefficient representing an amount of said third transmitter-to-receiver interference;

fifth multiplier means for multiplying an output signal of said adder means by said coefficient, said fifth multiplier means providing said third estimated-interference signal; and

delay means being connected to said fifth multiplier means and for delaying said third estimated-interference signal by a time equal to a propagation time of said third digital radio wave signal from said first transmitting antenna means to said first receiving antenna means.

20. A repeater as claimed in claim 17, wherein said fourth estimated-interference signal producing means comprises;

first local oscillator means for oscillating a first frequency signal;

first phase shifter means for phase-shifting by π

/2 said first frequency signal;

first multiplier means for multiplying said fourth digital baseband signal by said first frequency signal;

second multiplier means for multiplying said fourth digital baseband signal by said phase-shifted first frequency signal;

second local oscillator means for oscillating a second frequency signal, said first and second frequency signals having a frequency difference equal to a frequency difference between said second and fourth carrier frequencies;

second phase shifter means for phase-shifting by π

/2 said second frequency signal;

third multiplier means for multiplying an output from said first multiplier means by said second frequency signal;

fourth multiplier means for multiplying an output from said second multiplier means by said phase-shifted second frequency signal;

adder means for adding an output signal of said third multiplier means and an output signal of said fourth multiplier means;

coefficient generating means for generating a coefficient representing an amount of said fourth transmitter-to-receiver interference;

fifth multiplier means for multiplying an output signal of said adder means by said coefficient, said fifth multiplier means providing said fourth estimated-interference signal; and

delay means connected to said fifth multiplier means and for delaying said fourth estimated-interference signal by a time equal to a propagation time of said fourth digital microwave signal from said second transmitting antenna means to said second receiving antenna means.

21. A one-frequency two-way repeater for a digital transmission system wherein first and second digital microwave signals are received at first and second receiving antenna means and regenerated as third and fourth digital microwave signals, respectively, said third and fourth digital microwave signals being radiated from first and second transmitting antenna means, respectively, said first and second digital microwave signals carrying a first and a second digital baseband signal, respectively, said first, second, third and fourth digital microwave signals having first, second, third and fourth carrier frequencies, respectively, said first, second, third and fourth carrier frequencies beinbg substantially equal to one another said repeater comprising:

said first receiving antenna means and said second transmitting antenna means being disposed side by side and directed in a common first direction, said first receiving antenna means and said second antenna means having a side-to-side antenna coupling whereby said first receiving antenna means receives said fourth digital microwave signal as a first transmitter-to-receiver interference signal;

said first transmitting antenna means and said second receiving antenna means being disposed side by side and directed in a common second direction opposite to said first direction, said first transmitting antenna means and said second receiving antenna means having a side-to-side antenna coupling whereby said second receiving antenna means receives said third digital microwave signal as a second transmitter-to-receiver interference signal;

first receiver circuit means connected to said first receiving antenna means for extracting said first digital baseband signal by demodulating said first digital microwave signal received at said first receiving antenna means, said demodulated first digital baseband signal being accompanied by a first baseband interference signal derived from said transmitter-to-receiver interference signal;

first decision circuit means for detecting said demodulated first digital baseband signal and regenerating said first digital baseband signal as a third digital baseband signal;

first transmitter circuit means for producing said third digital microwave signal modulated by said third digital baseband signal and for supplying said third digital microwave signal to said first transmitting antenna means;

second receiver circuit means connected to said second receiving antenna means for extracting said second digital baseband signal by demodulating said second digital microwave signal received by said second receiving antenna means, said demodulated second digital baseband signal being accompanied by a second baseband interference signal derived from said second transmitter-to-receiver interference signal;

second decision circuit means for detecting said demodulated second digital baseband signal and regenerating said second digital baseband signal as a fourth digital baseband signal;

second transmitter circuit means for producing said fourth digital microwave signal modulated by said fourth digital baseband signal and for supplying said fourth digital microwave signal to said second transmitting antenna means;

first estimated-interference signal producing means coupled to said second decision circuit means for producing a first estimated-interference signal corresponding to said first baseband interference signal from said fourth digital baseband signal, said first estimated-interference signal producing means comprising;

first local oscillator means for generating a first frequency signal;

first phase shifter means for phase-shifting by π

/2 said first frequency signal;

first multiplier means for multiplying said fourth digital baseband signal by said first frequency signal;

second multiplier means for multiplying said fourth digital baseband signal by said phase-shifted first frequency signal;

second local oscillator means for generating a second frequency signal, said first and second frequency signals having a frequency difference corresponding to a frequency difference between said first and fourth carrier frequencies;

second phase shifter means for phase-shifting by π

/2 said second frequency signal;

third multiplier means for multiplying an output from said first multiplier means by said second frequency signal;

fourth multiplier means for multiplying an output from said second multiplier means by said phase-shifted second frequency signal;

first adder means for adding an output signal of said third multiplier means and an output signal of said fourth multiplier means;

first coefficient generating means for generating a first coefficient representing the amount of said first transmitter-to-receiver interference; and

fifth multiplier means for multiplying an output signal of said adder means by said first coefficient, said fifth multiplier means providing said first estimated-interference signal;

first interference cancelling means coupled to said first estimated-interference signal producing means for cancelling said first baseband interference signal from an output signal of said first receiver circuit means using said first estimated-interference signal, to thereby obtain said demodulated first digital baseband signal without said first baseband interference signal;

second estimated-interference signal producing means coupled to said first decision circuit means for producing a second estimated-interference signal, corresponding to said second baseband interference signal, from said third digital baseband signal; and

second interference cancelling means coupled to said second estimated-interference signal producing means for cancelling said second baseband interference signal from an output signal of said second receiver circuit means using said second estimated-interference signal, to thereby obtain said demodulated second digital baseband signal without said second baseband interference signal.

22. A repeater as claimed in claim 21, wherein said second estimated-interference signal producing means comprises;

third local oscillator means for generating a third frequency signal;

third phase shifter means for phase-shifting by π

/2 said third frequency signal;

sixth multiplier means for multiplying said third digital baseband signal by said third frequency signal;

seventh multiplier means for multiplying said third digital baseband signal by said phase-shifted third frequency signal;

fourth local oscillator means for generating a fourth frequency signal, said third and fourth frequency signals having a frequency difference equal to a frequency difference between said second and third carrier frequencies;

fourth phase shifter means for phase-shifting by π

2 said fourth frequency signal;

eighth multiplier means for multiplying an output from said sixth multiplier means by said fourth frequency signal;

ninth multiplier means for multiplying an output from said seventh multiplier means by said phase-shifted fourth frequency signal;

second adder means for adding an output signal of said eighth multiplier means and an output signal of said ninth multiplier means;

second coefficient generating means for generating a second coefficient representing the amount of said second transmitter-to-receiver interference; and

tenth multiplier means for multiplying an output signal of said second adder means by said second coefficient, said tenth multiplier means providing said second estimated-interference signal.

23. A one-frequency repeater for a digital radio wave transmission system wherein a carrier is modulated by a digital baseband signal and is transmitted as a digital radio wave signal, said repeater comprising:

first transmitter circuit means comprising first modulating means for modulating a first intermediate frequency signal by a first digital baseband signal and providing a first modulated signal, and first frequency up converter means for converting a frequency of said first modulated signal to a first carrier frequency and providing a first digital radio wave signal;

first transmitting antenna means for radiating said first digital radio wave signal;

first receiving antenna means for receiving a second digital radio wave signal modulated by a second digital baseband signal, said second digital radio wave signal having a second carrier frequency substantially equal to said first carrier frequency, said first receiving antenna means receiving said first digital radio wave signal as a first transmitter-to-receiver interference signal due to coupling between said first transmitting antenna means and said first receiving antenna means;

first receiver circuit means comprising frequency down converter means for converting a frequency of a radio wave signal received at said first receiving antenna means to a second intermediate frequency slightly different from said first intermediate frequency, said frequency down converter means providing a first digital intermediate frequency signal, said first digital intermediate frequency signal having a first signal component derived from said second digital radio wave signal and a first interference signal derived from said first transmitter-to-receiver interference signal;

first frequency shifter means for frequency-shifting said first modulated signal from said first modulating means by a frequency difference of said first and second intermediate frequencies and providing a first frequency-shifted signal, said first frequency shifter means comprising;

first local oscillator means for generating a first local signal having a frequency equal to one of said first and second intermediate frequencies;

first phase shifter means for phase-shifting by π

/2 said first local signal;

second multiplier means for multiplying said first modulated signal by said first local signal;

third multiplier means for multiplying said first modulated signal by said phase-shifted first local signal;

first low-pass filter means connected to an output of said second multiplier means;

second low-pass filter means connected to an output of said third multiplier means;

second local oscillator means for generating a second local signal having a frequency equal to the other of said first and second intermediate frequencies;

second phase shifter means for phase-shifting by π

/2 said second local signal;

fourth multiplier means for multiplying an output from said first low-pass filter means by said second local signal;

fifth multiplier means for multiplying an output from said second low-pass filter means by said phase-shifted second local signal; and

first adder means for adding an output signal of said fourth multiplier means and an output signal of said fifth multiplier means, said first adder means providing said first frequency-shifted signal;

first coefficient generating means for generating a first coefficient representing an amount of said first transmitter-to-receiver interference;

sixth multiplier means coupled to said first frequency shifter means and said first coefficient generating means for multiplying said first frequency-shifted signal by said first coefficient and providing a first interference-removing signal; and

first interference cancelling means coupled to said frequency down converter means and said sixth multiplier means for cancelling said first interference signal from said first digital intermediate signal using said first interference-removing signal.

24. A one frequency repeater as claimed in claim 23, wherein said first transmitting antenna means and said first receiving antenna means are disposed side by side and directed to a common direction, said first transmitter-to-receiver interference signal being caused by side-to-side coupling between said first transmitting and said first receiving antenna means.

25. A one frequency repeater as claimed in claim 23, wherein said first transmitting antenna means and said first receiving antenna means are disposed on opposite sides of said repeater and directed to opposite direction, said first transmitter-to-receiver interference signal being caused by front-to-back coupling between said first transmitting and said first receiving antenna means, said first receiver circuit means further comprising first demodulating means for demodulating an output of said first interference cancelling means and extracting said second digital baseband signal, first decision circuit means for deciding said second digital baseband signal from an output of said first demodulating means and regenerating said second digital baseband signal, said regenerated second digital baseband signal being supplied to said first transmitter circuit means as said first digital baseband signal.

26. A repeater as claimed in claim 25, which further comprises:

second transmitter circuit means comprising second modulating means for modulating a third intermediate frequency signal by a third digital baseband signal and providing a second modulated signal, said second and third intermediate frequency signals having a frequency difference and second frequency-up converter means for converting a frequency of said second modulated signal to a third carrier frequency substantially equal to said second carrier frequency and providing a third digital radio wave signal;

second transmitting antenna means for radiating said third digital radio wave signal, said second transmitting antenna means and said first receiving antenna means being disposed side by side and directed in a common direction, said radiated third digital radio wave signal being received at said first receiving antenna means as a second transmitter-to-receiver interference signal due to a side-by-side antenna coupling between said second transmitting antenna means and said first receiving antenna means;

said frequency-down converter means providing said first digital intermediate frequency signal, said first digital intermediate frequency said further having a second interference signal derived from said second transmitter-to-receiver interference signal;

second frequency shifter means for frequency shifting said second modulated signal from said second modulating means by the frequency difference of said second and third intermediate frequencies and providing a second frequency-shifted signal;

second coefficient generating means for generating a second coefficient representing an amount of said second transmitter-to-receiver interference;

seventh multiplier means for multiplying said second frequency-shifted signal by said second coefficient and providing a second interference-removing signal, said second interference-removing signal being supplied to said first interference cancelling means so that said second interference signal is cancelled from said first digital intermediate signal.

27. A one-frequency repeater as claimed in claim 26, wherein said second frequency shifter means comprises:

third local oscillator means for generating a third local signal having a frequency equal to one of said second and third intermediate frequencies;

third phase shifter means for phase-shifting by π

/2 said third local signal;

eighth multiplier means for multiplying said second modulated signal by said third local signal;

ninth multiplier means for multiplying said second modulated signal by said phase-shifted third local signal;

third low-pass filter means connected to an output of said eighth multiplier means, fourth low-pass filter means connected to an output of said ninth multiplier means;

fourth local oscillator means for generating a fourth local signal having a frequency equal to the other of said second and third intermediate frequencies;

fourth phase shifter means for phase-shifting by π

/2 said fourth local signal;

tenth multiplier means for multiplying an output from said third low-pass filter means by said fourth local signal;

eleventh multiplier means for multiplying an output from said fourth low-pass filter means by said phase-shifted fourth local signal; and

second adder means for adding an output signal of said tenth multiplier means and an output signal of said eleventh multiplier means, said second adder means providing said second frequency-shifted signal.

28. A frequency repeater as claimed in claim 25, which further comprises delay means being connected to said sixth multiplier means and for delaying said first interference-removing signal by a time equal to a propagation time of said first digital radio wave signal from said first transmitting antenna means and said first receiving antenna means.

29. A one frequency repeater as claimed in claim 23, which comprises said first receiver circuit means further comprising first demodulating means for demodulating an output of said first interference cancelling means and extracting said second digital baseband signal, first decision circuit means for deciding said second digital baseband signal from an output signal of said first demodulating means and regenerating said second digital baseband signal, first subtracting means for subtracting said regenerated second digital baseband signal from said output signal of said first demodulating means and providing an error signal, seventh multiplier means for multiplying said error signal by said first interference-removing signal and providing a frequency control signal, one of said first and second local oscillator means being variable oscillator means, said variable oscillator means being controlled by said frequency control signal.

30. A one frequency repeater as claimed in claim 23, which comprises said first transmitter circuit means having first variable intermediate frequency oscillator means for providing said first intermediate frequency signal, said receiver circuit means having second variable intermediate frequency oscillator means for providing a second intermediate frequency signal, and demodulating means for demodulating said first digital intermediate frequency signal by said second intermediate frequency signal and providing a demodulated signal, means for detecting from said demodulated signal a phase difference between said first digital intermediate frequency signal and said second intermediate signal, said second variable intermediate frequency oscillator means being controlled by said phase difference, and phase comparator means for phase-comparing said second intermediate signal and said first intermediate signal and providing an error signal, said error signal being applied to said first variable intermediate oscillator means so that said first intermediate frequency signal is phase-locked to said second intermediate signal.

31. A repeater as claimed in claim 30, which comprises fixed frequency oscillator means for providing a fixed radio frequency signal having said second carrier frequency, said first frequency-up converter means comprising multiplier means for multiplying said first modulated signal by said fixed radio frequency signal, and said frequency-down converter means comprising multiplier means for multiplying said second digital radio wave signal received at said receiving antenna means by said fixed radio frequency signal.

32. A one-frequency repeater for a cochannel dual polarization transmission system wherein two independent digital baseband signals are carried by orthogonally related paired waves of a single carrier frequency signal, said repeater comprising:

first and second signal sources for generating first and second digital baseband signals, respectively;

first transmitter circuit means coupled to said first and second signal sources for producing a first pair of horizontally and vertically polarized waves of a first carrier frequency which carry said first and said second digital baseband signals;

first transmitting antenna means for radiating said first pair of horizontally and vertically polarized waves;

first receiving antenna means for receiving a second pair of horizontally and vertically polarized waves of a second carrier frequency which carry a third and a fourth digital baseband signal;

said second carrier frequency being substantially equal to said first carrier frequency means, said first pair of horizontally and vertically polarized waves being received at said first receiving antenna means as a first transmitter-to-receiver interference signal due to a coupling between said first transmitting antenna means and said second receiving antenna means;

receiver circuit means comprising first and second demodulating means for independently modulating said second horizontally polarized wave signal and said second vertically polarized wave signal and extracting said third digital baseband signal and said fourth baseband signal, respectively, said extracted third digital baseband signal being accompanied by a first interference signal and a second interference signal caused by said first transmitter-to-receiver interference signal, said extracted fourth digital baseband signal being accompanied by a third interference signal and a fourth interference signal caused by said first transmitter-to-receiver interference signal;

first estimated-interference signal producing means coupled to said first signal source for producing a first estimated-interference signal corresponding to said first interference signal from said first digital baseband signal;

second estimated-inteference signal producing means coupled to said second signal source for producing a second estimated-interference signal corresponding to said second interference signal from second digital baseband signal;

first interference cancelling means coupled to said first and second estimated-interference signal producing means for cancelling said first and second interference signal from an output of said first modulating means using said first and second estimated-interference signals;

third estimated-interference signal producing means coupled to said first signal source for producing a third estimated-interference signal corresponding to said third interference signal from said first digital baseband signal;

fourth estimated-interference signal producing means coupled to said second signal source for producing a fourth estimated-interference signal corresponding to said fourth interference signal from said second baseband signal; and

second interference cancelling means coupled to said third and fourth estimated-interference signal producing means for cancelling said third and fourth interference signals from an output of said second demodulating means using said third and fourth estimated-interference signals.

33. A one frequency repeater as claimed in claim 32, wherein each of said first to fourth estimated-interference signal producing means is a transversal filter means comprising a delay line with a predetermined number of tap gains.

34. A one frequency repeater as claimed in claim 32, wherein said first transmitting antenna means and said first receiving antenna means are disposed side by side and directed to a common direction, said first transmitter-to-receiver interference signal being caused by a side-to-side coupling between said first transmitting and said first receiving antenna means.

35. A one frequency repeater as claimed in claim 32, which further comprises first decision circuit means for deciding said third digital baseband signal from an output of said first interference cancelling means and regenerating said third digital baseband signal, second decision circuit means for deciding said fourth digital baseband signal from an output of said second interference cancelling means and regenerating said fourth digital baseband signal, said regenerated third and fourth digital baseband signals being supplied to said first transmitter circuit means as said first and second digital baseband signals, said transmitting antenna means and said receiving antenna means being disposed on opposite sides of said repeater and directed to opposite directions, said first transmitter-to-receiver interference signal being caused by front-to-back coupling between said first tranmitting and said first receiving antenna means.

36. A one frequency repeater as claimed in claim 35, which further comprises:

second transmitter circuit means for producing a third pair of horizontally and vertically polarized means of a third carrier frequency substantially equal to said second carrier frequency;

second transmitting antenna means for radiating said third pair of said horizontally and vertically polarized waves, said second transmitting antenna means and said first receiving antenna means being disposed side by side and directed in a common direction, said radiated third pair of polarized waves being received at said first receiving antenna means as a second transmitter-to-receiver interference signal due to a side-to-side antenna coupling between said second transmitting antenna means and said first receiving antenna means;

said first and second demodulating means demodulating said second transmitter-to-receiver interference signal whereby said extracted third digital baseband signal being further accompanied with a fifth interference signal and a sixth interference signal while said extracted fourth digital baseband signal being further accompanied with a seventh interference signal and an eighth interference signal;

fifth estimated-interference signal producing means for producing a fifth estimated-interference signal corresponding to said fifth interference signal from said fifth digital baseband signal, said fifth estimated-interference signal being supplied to said first interference cancelling means so that said fifth interference signal is cancelled;

sixth estimated-interference signal producing means for producing sixth estimated-interference signal corresponding to said sixth interference signal from said sixth digital baseband signal, said sixth estimated-interference signal being supplied to said first interference cancelling means so that said sixth interference signal is cancelled;

seventh estimated-interference signal producing means for producing a seventh estimated-interference signal corresponding to said seventh interference signal from said fifth digital baseband signal, said seventh estimated-interference signal being supplied to said second interference cancelling means so that said seventh interference signal is cancelled; and

eighth estimated-interference signal producing means for producing eithth estimated-interference signal corresponding to said eighth interference signal from said sixth digital baseband signal, said eighth estimated-interference signal being supplied to said second interference cancelling means so that said eighth interference signal is cancelled.

37. A one frequency repeater as claimed in claim 36, wherein each one of said fifth to eighth estimated-interference signal producing means is a transversal filter means comprising a delay line with a predetermined number of tap gains.