Crosspole interference canceling receiver for signals with unrelated baud rates

• US 5,838,740 A
• Filed: 04/17/1996
• Issued: 11/17/1998
• Est. Priority Date: 04/17/1996
• Status: Expired due to Term

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Kallman, Kurt Albert, Turcotte, Randy Lee
• Primary Examiner(s): TSE, YOUNG TOI
• Application Number:
  US08/633,636
• Time in Patent Office: 944 Days
• Field of Search: 375/234, 375/235, 375/346, 375/347, 375/349, 375/350, 342/361, 342/362, 455/295, 455/296, 455/307
• US Class Current: 375/346
• CPC Class Codes: H04B 1/126 having multiple inputs, e.g. auxiliary antenna for receiving interfering signal aerials in general H01Q
• CPC Class Codes: H04B 7/005 Control of transmission; Equalising

Abstract

Crosspolarized signals of unrelated baud rates transmitted through a communication channel (10) become depolarized due to channel distortions such as rain and antenna imperfections. The resulting interference is canceled in a Modified Adaptive Crosspole Interference Canceler (MAXPIC) receiver (50, 70) by adjustment the timing in the crosspolarization path to compensate for the differential delay. Near equivalent bit error rate (BER) performance is achieved for systems utilizing crosspolarized signals with independent baud rates. In one embodiment, the receiver uses a fractionally spaced finite impulse response (FIR) filter (78) that operates at an integer multiple of the direct channel signal baud rate. In another embodiment, a variable delay (54) is used to time-align the received crosspolarized channel signal with the crosspolarization interference contained in the received direct signal.

Claims

1. A receiver for canceling crosspolarization interference caused by transmission of crosspolarized signals of unrelated baud rates through a dual polarization frequency reuse channel, said receiver comprising:

a direct channel portion for receiving a direct channel signal of a first baud rate through said dual polarization frequency reuse channel, said direct channel signal having a direct channel component and a cross channel interference component, said direct channel portion providing an equalized output signal;

a crosspolarization interference canceling portion receiving a cross channel signal of a second baud rate through said dual polarization frequency reuse channel, said crosspolarization interference canceling portion providing a combined output signal substantially without crosspolarization interference; and

a data decision and error estimating portion receiving said combined output signal and providing data decisions based on said combined output signal, said data decision and error estimating portion providing an error estimate based on said combined output signal and a threshold value,

said crosspolarization interference canceling portion including a variable delay element for delaying said cross channel signal based a delay estimate signal,

said crosspolarization interference canceling portion including a cross channel demodulator for demodulating said cross channel signal;

said crosspolarization interference canceling portion including a cross channel adaptive filter providing a cancellation signal based on adaptive filter coefficients and a delayed and demodulated version of said cross channel signal provided through the variable delay element and the cross channel demodulator, said adaptive filter coefficients based on said error estimate and the delayed cross channel signal;

said crosspolarization interference canceling portion including a delay estimator providing said delay estimate signal to said variable delay element, said delay estimate signal based on said error estimate and said cancellation signal,

said crosspolarization interference canceling portion including a summer for combining the cancellation signal with said equalized output signal whereby said cross channel interference component is canceled from said direct channel signal.

2. A receiver as claimed in claim 1 wherein said data decision and error estimating portion provides feedback signals to said direct channel portion.

3. A receiver as claimed in claim 2 wherein said cross channel interference signal represents an inverse function of said crosspolarization interference.

4. A receiver as claimed in claim 3 wherein said cross channel adaptive filter includes a finite impulse response filter (FIR), said FIR filter substantially matching channel characteristics of said dual polarization frequency reuse channel.

5. A receiver as claimed in claim 4 wherein said FIR filter comprises a fractionally spaced adaptive filter having a tapped delay line with taps spaced apart by a tap spacing that is a function of said first baud rate.

6. A receiver as claimed in claim 5 wherein said data decision and error estimating portion provides said error estimate by comparing said combined output signal with data decision thresholds, said data decision thresholds being based on a modulation format of said dual polarization frequency reuse channel.

7. A receiver for canceling crosspolarization interference caused by transmission of crosspolarized signals through a dual polarization frequency reuse channel, said receiver comprising:

a direct channel receiver portion receiving a direct channel signal of a first baud rate through said dual polarization frequency reuse channel to provide complex direct channel equalized signals, said direct channel signal including crosspolarization interference;

a data decision and error estimating portion; and

a crosspolarization interference canceling portion, said crosspolarization interference canceling portion comprising;

a cross channel baseband demodulator receiving through said dual polarization frequency reuse channel, a crosspolarized channel signal of a second baud rate, said cross channel baseband demodulator providing complex cross channel baseband signals;

a cross channel adaptive filter receiving said complex cross channel baseband signals and providing complex interference canceling signals based on cross channel filter coefficients;

a cross channel coefficient estimator for calculating said cross channel filter coefficients for said cross channel adaptive filter based on said complex cross channel baseband signals and a complex error signal;

a summer adding said complex interference canceling signals to said complex direct channel equalized signals to provide output direct channel signals substantially without crosspolarization interference; and

a variable delay element delaying said complex interference canceling signals by an amount based on said complex interference canceling signals and said complex error signal, said complex error signal based on said output direct channel signals and threshold values, said complex error signal provided by said data decision and error estimating portion.

8. A receiver as claimed in claim 7 wherein said complex interference canceling signals represent an inverse function of said crosspolarization interference.

9. A receiver as claimed in claim 8 wherein said variable delay element includes means for delaying said crosspolarized channel signal by a time delay to time-align said crosspolarization interference of said complex direct channel equalized signals with said complex interference canceling signals and wherein said crosspolarization interference canceling portion further comprises a delay estimator for determining said time delay based on said complex error signal and said complex cross channel baseband signals.

10. A receiver as claimed in claim 9 wherein said cross channel adaptive filter comprises a finite impulse response filter (FIR), said FIR filter matching channel characteristics of said dual polarization frequency reuse channel.

11. A receiver as claimed in claim 10 wherein said FIR filter comprises a fractionally spaced adaptive filter having a tapped delay line with taps spaced apart by a tap spacing that is a function of said first baud rate.

12. A receiver as claimed in claim 11 wherein said data decision and error estimating portion provides said complex error signal by comparing said output direct channel signals with data decision thresholds, said data decision thresholds being based on a modulation format of said dual polarization frequency reuse channel.

13. A receiver as claimed in claim 12 wherein said direct channel receiver portion comprises:

a direct channel baseband demodulator for receiving said direct channel signal, said direct channel baseband demodulator providing complex direct channel baseband signals;

a direct channel adaptive equalizer for receiving said complex direct channel baseband signals from said direct channel baseband demodulator and providing said complex direct channel equalized signals based on direct channel equalizer coefficients; and

a direct channel coefficient estimator for calculating said direct channel equalizer coefficients for said direct channel adaptive equalizer based on said complex error signal,

said direct channel adaptive equalizer implementing a transfer function that is inverse to frequency response characteristics of a direct channel portion of said dual polarization frequency reuse channel.

14. A receiver as claimed in claim 13 wherein said direct channel coefficient estimator calculates said direct channel equalizer coefficients by correlating said complex direct channel baseband signals with said complex error signal.

15. A receiver as claimed in claim 14 wherein said direct channel coefficient estimator calculates said direct channel equalizer coefficients by correlating said complex direct channel baseband signals with said complex error signal, wherein said direct channel coefficient estimator uses a zero-forcing algorithm to correlate said complex direct channel baseband signals with said complex error signal, said zero-forcing algorithm using said complex direct channel equalized signals.

16. A receiver as claimed in claim 14 wherein said direct channel coefficient estimator calculates said direct channel equalizer coefficients by correlating said complex direct channel baseband signals with said complex error signal, wherein said direct channel coefficient estimator uses a least-mean-square algorithm to correlate said complex direct channel baseband signals with said complex error signal, said least-mean-square algorithm using said complex direct channel baseband signals.

17. A receiver as claimed in claim 14 wherein said direct channel signal and said crosspolarized channel signal are orthogonally polarized and of different baud rate intervals.

18. A method of canceling crosspolarization interference from a direct channel signal of a first baud rate received through a dual polarization frequency reuse channel, said direct channel signal having a direct channel component and a cross channel interference component, said method comprising the steps of:

receiving a cross channel signal of a second baud rate through said dual polarization frequency reuse channel;

filtering a demodulated version of said cross channel signal to produce a cancellation signal, the filtering

time-aligning said cross channel signal with said cross channel interference component by delaying said cross channel signal an amount based on said cancellation signal and an output error signal; and

combining said direct channel signal with said cancellation signal to provide an output signal substantially without crosspolarization interference.

19. A method as claimed in claim 18 wherein the time-aligning step further comprises providing a time delay with a filter to time-align said cross channel signal with said direct channel signal, said filter matching channel characteristics of said dual polarization frequency reuse channel.

20. A method as claimed in claim 19 wherein the time-aligning step, said filter includes a finite impulse response (FIR) filter that comprises a fractionally spaced adaptive filter having a tapped delay line with taps spaced apart by a tap spacing that is a function of a baud rate interval of said direct channel signal.