Interference canceller

• US 4,992,798 A
• Filed: 07/17/1990
• Issued: 02/12/1991
• Est. Priority Date: 11/10/1987
• Status: Expired due to Fees

• Current Assignee: Fujitsu Limited
• Original Assignee: Fujitsu Limited
• Inventors: Takizawa, Toshiyuki, Nozue, Yoshihiro
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US07/554,984
• Time in Patent Office: 210 Days
• Field of Search: 342/362, 455/295, 455/296, 455/305
• US Class Current: 342/362
• CPC Class Codes: H04B 7/002 Reducing depolarization effects

Abstract

In a digital radio transmission system for transmitting independent dual data by using cross polarization, an interference canceller detects cross polarization interference either directly or indirectly. An interference compensating signal is generated and applied to the main signal to cancel the interference element. In a case where no interference element is present, deterioration of cross polarization discrimination from an interference cancelling circuit is prevented by first determining whether an interference element actually exists and applying the interference compensating signal only when it does exist. The interference compensating signal can be attenuated based on the value of interference element so as to prevent undo deterioration of the cross polarization discrimination when the interference element is minimal.

Claims

1. An interference canceller for cancelling interference in a first signal in a communication system that simultaneously processes more than one signal, said interference canceller comprising:

equalizer means for equalizing the first signal;

compensating signal generating means for generating a compensating signal from a second signal;

first timing recovery means, operatively connected to said equalizer means, for detecting the interference in the equalized first signal by detecting and extracting a beat element of amplitude based on a frequency difference element of a pair of data rates of the first and second signals; and

first control means, operatively connected to said first timing recovery means and said compensating signal generating means, for controlling application of the compensating signal to the first equalized signal based on the detected interference.

2. An interference canceller according to claim 1, wherein the interference canceller is in a receiver, and said interference canceller further comprises:

first demodulating means, operatively connected to said equalizer means, for demodulating the first signal before the first signal is equalized by said equalizing means; and

second demodulating means, operatively connected to said compensating signal generating means, for demodulating the second signal before the second signal is used to generate the compensating signal by said compensating signal generating means.

3. An interference canceller according to claim 2, wherein the first and second signals are vertical and horizontal polarizations transmitted over a single channel.

4. An interference canceller according to claim 1, wherein said first timing recovering means detects whether a value of the interference in the equalized first signal is greater than a predetermined value, and said first control means applies the compensating signal to the equalized first signal only if the value of the interference is greater than the predetermined value.

5. An interference canceller according to claim 1, wherein said first timing recovery means determines a value of the interference, and said first control means comprises attenuating means for attenuating the compensating signal based on the determined value of the interference.

6. An interference canceller according to claim 4, wherein said first control means comprises switching means for switching off the application of the compensating signal to the equalized first signal when the value of the interference is below the predetermined value.

7. An interference canceller according to claim 1, further comprising:

adding means, operatively connected to said equalizer means and said compensating signal generator, for adding the equalized first signal and the compensating signal;

second timing recovery means, operatively connected to said adding means, for recovering a clock signal from an output of said adding means;

decision means, operatively connected to said adding means and said second timing recovery means, for sampling and outputting the output of said adding means based on the clock signal; and

second control means, operatively connected to said decision means, said equalizing means and said compensating signal generating means, for receiving the output of said decision means and the compensating signal from said compensating signal generating means and controlling said equalizer means and said compensating signal generating means based on the received signals.

8. An interference canceller according to claim 1, wherein said compensating signal generating means comprises a transversal equalizer.

9. An interference canceller according to claim 1, wherein said first timing recovery means comprises:

first rectifying means, operatively connected to said equalizer means, for receiving and rectifying an output of said equalizer means;

limiter means, operatively connected to said limiter means, for limiting the rectified signal;

phase comparison means, operatively connected to said limiter means, for outputting a signal including a beat element of the limited rectified signal;

filter means, operatively connected to said phase comparison means, for extracting the beat element from the output signal;

second rectifying means, operatively connected to said filter means, for detecting the beat element upon extraction by said filter means; and

decision means, operatively connected to said second rectifying means, for obtaining a voltage proportional to the amplitude of the beat element from the extracted beat element.

10. An interference canceller according to claim 9, wherein said first control means comprises an attenuator for attenuating the compensating signal based on the voltage obtained by said decision means.

11. An interference canceller according to claim 9, wherein said phase comparison means includes a phase comparator, a loop filter, and a voltage-controlled oscillator connected in series.

12. An interference canceller according to claim 7, wherein said second timing recovery means includes:

rectifier means, operatively connected to said adder means, for receiving and rectifying an output of said adder means;

limiting means, operatively connected to said rectifier means, for limiting the rectified signal; and

phase comparison means, operatively connected to said limiting means, for recovering the clock signal from the output of said adder means, said phase comparison means comprising a phase comparator, a loop filter, and a voltage-controlled oscillator connected in series.

13. An interference canceller according to claim 12, wherein said first timing recovery means detects the interference element by detecting and extracting a beat element from the equalized first signal, and said first control means controls application of the compensating signal to the equalized first signal based on the detected and extracted beat element.

14. An interference canceller according to claim 13, wherein said first timing recovery means comprises:

first rectifying means, operatively connected to said equalizer means, for receiving and rectifying an output of said equalizer means;

limiter means, operatively connected to said first rectifying means, for limiting the rectified signal;

phase comparison means, operatively connected to said limiter means, for outputting a signal including the beat element of the limited signal;

filter means, operatively connected to said phase comparison means, for extracting the beat element from the output signal;

second rectifying means, operatively connected to said filter means, for detecting the beat element upon extraction by said filter means; and

decision means, operatively connected to said second rectifying means, for obtaining a voltage proportional to the amplitude of the beat element from the extracted beat element.

15. An interference canceller according to claim 14, wherein said phase comparison means of said first timing recovery means includes a phase comparator, a loop filter, and a voltage-controlled oscillator connected in series.

16. An interference canceller according to claim 15, wherein a single voltage-controlled oscillator is used as said voltage-controlled oscillator of said first timing recovery means and said voltage controlled-oscillator of said second timing recovery means.

17. A compensating signal generator for generating a compensating signal, to compensate for interference in a first signal transmitted with and interfered by a second signal in a communication system, from first and second channels of the second signal, comprising:

a predetermined number of first channel delay units connected in series on the first channel;

a predetermined number of second channel delay units connected in series on the second channel;

a predetermined number of first channel tap constant multipliers, the predetermined number being one more than the predetermined number of said first channel delay units, connected in branches from the first channel before and after each of said first channel delay units;

a predetermined number of second channel tap constant multipliers, the predetermined number being one more than the predetermined number of said second channel delay units, connected in branches from the second channel before and after each of said second channel delay units;

a plurality of adders, operatively connected to said first and second channel tap constant multipliers, for adding outputs from predetermined ones of said first and second channel tap constant multipliers;

a plurality of variable attenuators, respectively connected to said adders, for receiving and attenuating outputs from said respective adders; and

adder means, operatively connected to said variable attenuators, for receiving and adding outputs from said variable attenuators and outputting the compensating signal.

18. A compensating signal generator according to claim 17, further comprising:

first control means, operatively connected to said first and second channel tap constant multipliers, for generating respective tap constants for said first and second channel tap constant multipliers depending on the amplitude and shape of distortion of the first signal; and

second control means, operatively connected to said first control means, for determining respective control signals for each of said variable attenuators based on the tap constants.

19. A compensating signal generator according to claim 18, wherein said second control means determines the respective control signals for each of said variable attenuators by determining which of the tap constants, which correspond to said first and second channel tap constant multipliers the outputs of which are to be added in said adder corresponding to said attenuator to be controlled, has the largest absolute value, and applying the determined value to said corresponding attenuator.

20. A method for cancelling interference between first and second signals in a communication system, comprising the steps of:

(a) equalizing the first signal;

(b) generating a compensating signal from the second signal which is transmitted with the first signal;

(c) detecting an interference element in the equalized first signal by detecting and extracting a beat element of amplitude based on a frequency difference element of a pair of data rates of the first and second signals; and

(d) applying the compensating signal to the first equalized signal based on the detected interference element to cancel interference in the first signal.

21. A method for cancelling interference according to claim 20, wherein the first and second signals are vertical and horizontal polarizations transmitted over a single channel.

22. A method for cancelling interference according to claim 20, wherein said step (c) further comprises the substeps of:

(i) determining a value of the interference element; and

(ii) attenuating the compensating signal based on the determined value of the interference element, and wherein said step (d) further comprises the substep of;

(i) applying the attenuated compensating signal to the first equalized signal.

23. A method for cancelling interference according to claim 20, wherein said step (c) comprises the substeps of:

(i) determining a value of the interference element; and

(ii) switching off a switch so that the compensating signal will not be applied to the first equalized signal when the value of the interference element is less than a predetermined value.