Method and device for echo cancellation

• US 20070041440A1
• Filed: 07/24/2006
• Published: 02/22/2007
• Est. Priority Date: 07/25/2005
• Status: Abandoned Application

• Current Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Original Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Inventors: Schoenbeck, Dietmar, Kaufmann, Friedrich
• Application Number:
  US11/491,734
• Publication Number:
  US 20070041440A1
• Time in Patent Office: Days
• US Class Current: 375/235
• CPC Class Codes: H04B 7/155 Ground-based stations H04B7/204 takes precedence
• CPC Class Codes: H04H 20/02 Arrangements for relaying broadcast information
• CPC Class Codes: H04H 20/67 Common-wave systems, i.e. using separate transmitters operating on substantially the same frequency

Abstract

The invention relates to a repeater and a method for re-transmitting single frequency signals, in particular of the type used in single frequency signal repeaters for use in single frequency networks (SFNs) like digital video/audio broadcasting (DVB/DAB) networks. The estimation of echo feedback is derived from the measured power density spectrum with an inclusion of a variable phase shifter (57) in the signal path. Preferably, the phase shifter (57) is controlled by control means (56), which also set the adaptive filter (54) for feedback compensation, such that a signal coupled from the transmitter antenna back to the receiver antenna can substantially be cancelled out. The method according to the invention exhibits quick and stable regulation behavior and the re-transmitted signal does not suffer from degradation due to added interference. The quality of the signal at the transmitter antenna is improved even if dynamically changing feedback paths from the transmitter to the receiver antenna exist. The transmitter output power can be increased without increasing the risk of system instabilities, such as oscillations.

Claims

1. Method for re-transmitting single frequency signals, particularly of the type used in single frequency signal repeaters for use in single frequency networks (SFNs) like digital video/audio broadcasting (DVB/DAB) networks, the method comprising the following steps:

receiving a first single frequency signal by means of at least one first antenna,

converting said first single frequency signal to another frequency, preferably by down-mixing it to an intermediate frequency (IF) signal,

input filtering said first single frequency or said IF signal, preferably by a band-pass filter, to produce a filter input signal,

amplifying said filtered input signal to produce an amplified input signal, preferably such that the amplification gain is controlled automatically to result in a substantially constant amplitude of said amplified input signal,

quantizing said amplified input signal, preferably by analogue-to-digital conversion to produce a quantized input signal,

demodulating said amplified input signal or said quantized input signal to produce a demodulated input signal,

equalizing said filtered, amplified, quantized and/or demodulated input signal to provide an equalized signal to provide an equalized signal, wherein said equalization at least reduces a coupling signal between said first and second antenna by generating a cancellation signal on the basis of said equalized signal, the cancellation signal being fed back to said equalized signal,

converting said equalized signal into a second single frequency signal, preferably of substantially the same frequency as said first single frequency signal,

filtering said second single frequency signal to produce a filtered second single frequency signal,

amplifying said filtered second single frequency signal to produce an amplified second single frequency signal,

transmitting said amplified second single frequency signal by means of at least one second antenna, and

said equalizing comprises controlled phase shifting of said equalized signal by at least one predetermined phase angle Ψ

.

2. Method according to claim 1, wherein the equalized signal is at least partially analyzed and wherein the analysis results are used for the generation of the cancellation signal and/or the analysis results are used to control the phase shifting of the equalized signal.

3. Method according to claim 1, wherein said cancellation signal is generated based on said equalized signal using an adaptive filter with a transfer function F(z).

4. Method according to claim 1, wherein said equalization step comprises capturing said filtered, amplified, quantized and/or demodulated input signal continuously or at predetermined points in time to produce a captured signal.

5. Method according to claim 4, wherein said equalization step comprises analyzing the captured signal and controlling said phase shifting using the result of the analysis of said captured signal and/or controlling the generation of said cancellation signal using the result of the analysis of said captured signal.

6. Method according to claim 1, wherein said equalization step comprises delaying said equalized signal by at least one, preferably by a set of predetermined and/or fixed time intervals to form the basis of said cancellation signal.

7. Method according to claim 6, wherein said equalization step comprises controlling said delay by said analyzed captured signal.

8. Method according to claim 1, wherein said equalization step comprises measuring the spectral power densities |M|² of said equalized signal for at least two, preferably three values of Ψ

, and most preferably for

$\mathrm{\psi <br><br>}\in <br><br>\left\{0,\frac{2<br><br>\mathrm{\pi <br><br>}}{3},\frac{4<br><br>\mathrm{\pi <br><br>}}{3}\right\}$

resulting in the corresponding spectral power densities |M_A|², |M_B|² and |M_C|² of said equalized signal.

9. Method according to claim 8, wherein said cancellation signal is generated by adaptively filtering said equalized signal shifted in phase by Ψ

(phase shifted equalized signal) using said transfer function F(z).

10. Method according to claim 8, wherein said first single frequency signal or said IF signal has a spectral function X(z), said signal coupling between said first and second antenna has a transfer function A(z), said input filtering has a transfer function H(z), said phase shifting can be expressed by a transfer function e^jΨ

, said processing signal has a spectral function M(z) and said processing signal shifted in phase has a spectral function Y(z), wherein said spectral function M(z) can be expressed as

M=X·

H+Y·

A·

H−

Y·

F, with Y=M·

e<sup>jΨ

.</sup>

such that

$M=X·<br><br>H+M·<br><br>{\mathrm{e<br><br>}}^{\mathrm{j\psi <br><br>}}·<br><br>\left(A·<br><br>H-F\right)$ $\mathrm{or}$ $M=X·<br><br>\frac{H}{1-{\mathrm{e<br><br>}}^{\mathrm{j\psi <br><br>}}·<br><br>\left(A·<br><br>H-F\right)}=X·<br><br>\frac{H}{1-{\mathrm{e<br><br>}}^{\mathrm{j\psi <br><br>}}<br><br>\mathrm{\Delta <br><br>}},$

and wherein

Δ

=H·

A−

F=R·

e^jΨ

.

11. Method according to claim 10, wherein said equalization step comprises using a help function T, where

$T=\frac{\frac{1}{{}^{}}}{}$

M A 

2 + 1 

M B 

2 ·

ⅇ

- j ⁢

2 ⁢

π

3 + 1 

M C 

2 ·

ⅇ

- j ⁢

4 ⁢

π

3 1 

M A 

2 + 1 

M B 

2 + 1 

M C 

2

based on said measured spectral power densities |M_A|², |M_B|² and |M_C|² of said equalized signal for determining and minimizing H·

A to yield

$M=X·<br><br>\frac{H}{1-{\mathrm{e<br><br>}}^{\mathrm{j\psi <br><br>}}<br><br>\mathrm{\Delta <br><br>}}<br><br>\stackrel{\mathrm{\Delta <br><br>}\to <br><br>)}{⟶<br><br>}<br><br>X·<br><br>H$

by iteratively adapting said filter transfer function F(z), wherein an accordingly modified impulse response f_i+1=f_i+δ

of said filtering step results from the addition of δ

, i.e., the inverse Fourier transform of Δ

, δ

=iFFT(Δ

), to the former impulse response f, of said filtering step.

12. Method according to claim 3 wherein during an initialization step a training signal sequence and preferably a pseudo-random sequence is used as an input signal to obtain an initial value f₁ of the impulse response f_i of said adaptive filtering step.

13. Method according to claim 12, wherein said adaptive filtering step uses a difference signal between said captured processing signal and said training signal sequence, which is delayed according to the delay of said captured processing signal with respect to said input signal, as a criterion to optimize the filter coefficients of said adaptive filtering step by minimizing the spectral power density of said difference signal.

14. Method according to claim 3, wherein said adaptive filtering step comprises an optimization algorithm selected from a group comprising a Least Mean Squares (LMS) algorithm, a steepest descent algorithm, a differential steepest descent algorithm, a gradient algorithm, a stochastic gradient algorithm and a Recursive Least Squares (RLS) algorithm.

21. A method for re-transmitting single frequency signals according to claim 1 in a re-transmitter for single frequency networks (SFNs), preferably digital video/audio broadcasting (DVB/DAB) networks and most preferably DVB-T networks.

15. Re-transmitter (Repeater) for single frequency signals, in particular of the type used in single frequency signal repeaters for use in single frequency networks (SFNs) like digital video/audio broadcasting (DVB/DAB) networks comprising:

at least one first antenna which receives a first single frequency signal,

a converter which converts said first single frequency signal to another frequency, preferably by down-mixing to an IF signal,

an input filter, preferably a band-pass filter, which filters said first single frequency or said IF signal and having a filtered input signal as an output,

an input amplifier, which amplifies said filtered input signal to produce an amplified input signal, preferably such that the amplification gain is controlled automatically to result in a substantially constant amplitude of said amplified input signal,

a quantizer, preferably an analogue-to-digital converter, which quantizes said amplified input signal or said filtered input signal to produce a quantized input signal,

a demodulator which demodulates said amplified input signal, said filtered input signal or said quantized input signal to result in a demodulated input signal,

an equalizer which equalizes said filtered, amplified, quantized and/or demodulated input signal, wherein said equalizer (50) is equipped to at least reduce a coupling signal between said first and second antenna,

a generator that generates a cancellation signal based on said equalized signal and feed-back means to feed back said cancellation signal to said equalized signal,

a converter which converts said equalized signal into a second single frequency signal, preferably of substantially the same frequency as the first single frequency signal,

an output filter which filters said second single frequency signal to result in a filtered second single frequency signal,

an output amplifier which amplifies said filtered second single frequency signal to result in an amplified second single frequency signal,

at least one second antenna which transmits said amplified second single frequency signal, and

said equalizer further comprises a variable phase shifter which shifts the phase of said equalized signal by an at least one predetermined phase angle Ψ

.

16. Re-transmitter according to claim 15, wherein said equalizer includes adaptive filter means having a transfer function F(z).

17. Re-transmitter according to claim 15, including capturing means for capturing said filtered, amplified, quantized and/or demodulated input signal continuously or at predetermined points in time to produce a captured signal.

18. Re-transmitter according to claim 15, including analyzer means which at least partially analyze said equalized signal and/or said captured signal.

19. Re-transmitter according to claim 15, including delayer means for delaying said analyzed captured signal.

20. Re-transmitter according to claim 15, wherein the equalizer comprises digital signal processing.