Clutter rejection in a passive radar receiver of OFDM signals

• US 6,924,763 B2
• Filed: 01/18/2002
• Issued: 08/02/2005
• Est. Priority Date: 02/07/2001
• Status: Expired due to Fees

• Current Assignee: ONERA
• Original Assignee: ONERA
• Inventors: Poullin, Dominique
• Primary Examiner(s): Gregory, Bernarr E.
• Application Number:
  US10/470,713
• Publication Number:
  US 20040066331A1
• Time in Patent Office: 1,292 Days
• Field of Search: 375130-153, 342/27, 342/28, 342/59, 342 89-103, 342/134, 342/135, 342159-175, 342192-197
• US Class Current: 342/159
• CPC Class Codes: G01S 13/003 Bistatic radar systems; Multistatic radar systems
• CPC Class Codes: G01S 13/524 based upon the phase or frequency shift resulting from movement of objects, with reference to the transmitted signals, e.g. coherent MTi
• CPC Class Codes: G01S 7/414 Discriminating targets with respect to background clutter

Abstract

The invention concerns a passive radar receiver for a received orthogonal frequency division multiplex-type signal consisting of symbol frames each emitted on coded orthogonal carriers. After formatting the received signals into digital symbols (S₁ S₁), a filtering circuit (2) eliminates by subtraction or using a covariance matrix, in the symbol signal at least unwanted signals with null Doppler effect so as to apply a filtered signal (X′) including essentially signals backscattered by mobile targets to a Doppler-distance correlator (4).

Claims

1. A radar receiver adapted to process a radio signal received via a propagation channel and comprising COFDM frames of symbols, each symbol being coded on a plurality of emitted orthogonal carriers, said radar comprising shaping means for converting said received signal into a digital symbol signal, Doppler-distance correlation means for discriminating mobile targets, and filtering means for, eliminating in said symbol signal at least unwanted zero Doppler effect signals in order to apply a filtered signal including essentially signals backscattered by targets to said correlation means.

2. The radar receiver according to claim 1, wherein said filtering means comprises means for producing spectral lines of said symbol signal corresponding to said orthogonal carriers, means for detecting said unwanted zero Doppler effect signals respectively in said spectral lines by estimating coefficients of a transfer function of said propagation channel for thereby deriving spectral lines of said unwanted zero Doppler effect signals deduced from the estimated coefficients of said transfer function, means for subtracting said spectral lines of said unwanted zero Doppler effect signals from said spectral lines of said symbol signal, and means for synthesizing spectral lines produced by said subtraction means into said filtered signal.

3. The radar receiver according to claim 2, wherein said spectral lines of said unwanted zero Doppler effect signals are estimated for each symbol and averaged over each frame in said detecting means before being subtracted from said spectral lines of said symbol signal.

4. A radar receiver according to claim 2, comprising means for estimating an emitted signal replica as a function of said spectral lines of said unwanted zero Doppler effect signals, said estimated replica being correlated to said filtered signal in said correlation means.

5. A radar receiver according to claim 1, wherein said shaping means comprises a plurality of receivers for shaping a plurality of signals received by said receivers in the form of digital symbol signals, and said filtering means comprises means for producing groups of spectral lines of said symbol signals respectively corresponding to said orthogonal carriers, means for estimating covariance matrices each depending on products of spectral lines of said symbol signals two by two in a group relating to a respective carrier, means for deriving inverse matrices of said covariance matrices, means for filtering said groups of spectral lines respectively relating to said carriers by multiplying said groups of spectral lines by said respective inverse matrices to produce filtered groups of spectral lines, and means for synthesizing said filtered groups of spectral lines into said form of filtered symbol signals which includes essentially signals backscattered by targets and which are applied to said correlation means.

6. The radar receiver according to claim 5, wherein said products of spectral lines of said symbol signals on which said covariance matrices are dependent depend on spectral lines of symbols and are averaged over each frame.

7. A radar receiver according to claim 5, comprising means for estimating an emitted signal replica as a function of said spectral lines of one of said symbols signals by estimating coefficients of a transfer function of said propagation channel, said estimated replica being correlated to said filtered symbol signals in said correlation means.

8. A radar receiver adapted to process a radio signal received via a propagation channel and comprising COFDM frames of symbols, each symbol being coded on a plurality of emitted orthogonal carriers, said radar comprising a converter for converting said received signal into a digital symbol signal, a Doppler-distance correlator for discriminating mobile targets, and a filter for eliminating in said symbol signal at least unwanted zero Doppler effect signals, the filter being connected to apply a filtered signal including signals backscattered by targets to said correlator.

9. The radar receiver according to claim 8, wherein said filter is arranged for:

(a) producing spectral lines of said symbol signal corresponding to said orthogonal carriers,

(b) detecting said unwanted zero Doppler effect signals respectively in said spectral lines by estimating coefficients of a transfer function of said propagation channel for thereby deriving spectral lines of said unwanted zero Doppler effect signals deduced from the estimated coefficients of said transfer function,

(c) subtracting said spectral lines of said unwanted zero Doppler effect signals from said spectral lines of said symbol signal, and

(d) synthesizing spectral lines produced by said subtracting into said filtered signal.

10. The radar receiver according to claim 9, wherein said filter is arranged for causing spectral lines of said unwanted zero Doppler effect signals to be (a) estimated for each symbol and (b) averaged over each frame during detecting of said unwanted zero Doppler effect signal and before being subtracted from said spectral lines of said symbol signal.

11. The radar receiver according to claim 9, wherein said filter is arranged for (a) estimating an emitted signal replica as a function of said spectral lines of said unwanted zero Doppler effect signals, and (b) correlating said estimated replica to said filtered signal.

12. The radar receiver according to claim 8, wherein said converter comprises a plurality of receivers for shaping a plurality of signals received by said receivers in the form of digital symbol signals, and said filter is arranged for (a) producing groups of spectral lines of said symbol signals respectively corresponding to said orthogonal carriers, (b) estimating covariance matrices each depending on products of spectral lines of said symbol signals two by two in a group relating to a respective carrier, (c) deriving inverse matrices of said covariance matrices, (d) filtering said groups of spectral lines respectively relating to said carriers by multiplying said groups of spectral lines by said respective inverse matrices to produce filtered groups of spectral lines, (e) synthesizing said filtered groups of spectral lines into said form of filtered symbol signals which includes essentially signals backscattered by targets, and (f) correlating the synthesized, filtered groups of spectral lines.

13. The radar receiver according to claim 12, wherein said filter is arranged for causing products of spectral lines of said symbol signals on which said covariance matrices are dependent to depend on spectral lines of symbols and to be averaged over each frame.

14. The radar receiver according to claim 12, wherein the filter is arranged for estimating an emitted signal replica as a function of said spectral lines of one of said symbol signals by estimating coefficients of a transfer function of said propagation channel, said estimated replica being correlated to said filtered symbol signals in said correlator.

15. A method of processing a radio signal received via a propagation channel and comprising COFDM frames of symbols, each symbol being coded on a plurality of emitted orthogonal carriers, said method comprising converting said received signal into a digital symbol signal, discriminating mobile targets by using Doppler-distance correlation, and eliminating from said symbol signal at least unwanted zero Doppler effect signals and correlating the signal having the unwanted Doppler effect signals eliminated from it so signals backscattered by targets are correlated.

16. The method according to claim 15, wherein the unwanted Doppler effect signals are eliminated from the symbol by:

(a) producing spectral lines of said symbol signal corresponding to said orthogonal carriers,

(b) detecting said unwanted zero Doppler effect signals respectively in said spectral lines by estimating coefficients of a transfer function of said propagation channel thereby deriving spectral lines of said unwanted zero Doppler effect signals deduced from the estimated coefficients of said transfer function,

(c) subtracting said spectral lines of said unwanted zero Doppler effect signals from said spectral lines of said symbol signal, and

(d) synthesizing spectral lines produced by said subtracting into said filtered signal.

17. The method according to claim 16, further including estimating said spectral lines of said unwanted zero Doppler effect signals for each symbol arid averaging said estimates of the spectral lines of said unwanted zero effect Doppler signals before they are subtracted from said spectral lines of said symbol signal.

18. The method according to claim 16, further comprising estimating an emitted signal replica as a function of said spectral lines of said unwanted zero Doppler effect signals, and correlating said estimated replica to said symbol signal having the unwanted zero Doppler effect signals estimated from it.

19. The method according to claim 15, wherein said converting (a) is performed by a plurality of receivers and (b) shapes a plurality of signals received by said receivers in the form of digital symbol signals, and said eliminating step includes (a) producing groups of spectral lines of said symbol signals respectively corresponding to said orthogonal carriers, (b) estimating covariance matrices each depending on products of spectral lines of said symbol signals two by two in a group relating to a respective, carrier, (c) deriving inverse matrices of said covariance matrices, (d) producing filtered groups of spectral lines by filtering said groups of spectral lines respectively relating to said carriers by multiplying said groups of spectral lines by said respective inverse matrices, and (e) synthesizing said filtered groups of spectral lines into said form of filtered symbol signals which includes essentially signals backacattered by targets and which are correlated.

20. The method according to claim 19, wherein said products of spectral lines of said symbol signals on which said covariance matrices are dependent depend on spectral lines of symbols and are averaged over each frame.

21. The method according to claim 19, further including estimating an emitted signal replica as a function of said spectral lines of one of said symbol signals by estimating coefficients of a transfer function of said propagation channel, and correlating said estimated replica to said filtered symbol signals.