Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-06-29
2002-09-17
Moskowitz, Nelson (Department: 3662)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S349000, C375S350000, C455S277200, C342S162000
Reexamination Certificate
active
06452988
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an adaptive sensor array apparatus and a method of obtaining interference rejection.
2. Discussion of Prior Art
Arrays of sensors connected to associated signal processing units are well known. The sensors generate signals in response to received radiation for subsequent processing in the units to provide output signals. Each sensor signal is scaled and phase shifted by an associated weighting vector w in a processing unit to provide a corresponding conditioned signal. Conditioned signals from the sensors are summed in the unit to provide a processed output signal therefrom in a process known as beamforming. By phase shifting and amplitude scaling each of the signals in a controlled manner prior to combining them, the processing unit exhibits in its output signal a polar gain response to received radiation comprising one or more directions of enhanced gain and one or more directions of attenuation; the directions of enhanced gain are referred to as lobes or beams of the response, and the directions of attenuation as nulls thereof. By appropriate choice of the weighting vectors w, the contributions in the output signal arising from radiation from unwanted interfering sources within a field of view in which the sensors are receptive to radiation are at least partially cancellable relative to contributions arising from radiation from wanted sources therein. For this to be possible, the wanted sources must lie in different directions to the interfering sources relative to the sensors, so that response nulls are steerable towards interfering sources and lobes towards wanted sources.
In other words, the sensors and their associated processing unit exhibit a steerable polar gain response to radiation determined by the weighting vectors w. The vectors are calculable to provide enhanced gain in directions of wanted sources and reduced gain in directions of interfering sources. Values for the vectors w are calculable automatically under computer control from the sensor signals themselves to provide at least partial rejection of contributions in the output signal from interference sources, even when directions of arrival of radiation at the array are not known a priori.; this is known as adaptive beamforming, and is described in a publication “Adaptive Array Principles” by J E Hudson, published by IEE and Peter Peregrinus, London 1981.
Apparatus incorporating arrays of sensors capable of adaptive beamforming are often operated on moving platforms such as aircraft and ships. As a result, the arrays are not always stationary with respect to wanted targets and unwanted sources of jamming and interfering radiation within fields of view of the apparatus.
There are a number of algorithms presently in use for computing the weighting vectors w described above. These algorithms rely on adjusting the vectors w gradually to track more slowly changing components of the sensor signals and assume that more rapidly changing random signal components are removed by integration and are hence not tracked. However, as disclosed in a publication “A Kalman-type algorithm for adaptive radar arrays and modelling of non-stationary weights” IEE Conference Publication, 180, 1979 by J E Hudson, the assumption may be invalid for apparatus incorporating adaptive sensor arrays operating on future agile platforms which will be capable of performing more rapid trajectory changes in comparison to current platforms.
A more general solution than the Kalman-type algorithm for coping with rapid variations in the sensor signals is described by S D Hayward in a publication “Adaptive beamforming for rapidly moving arrays”, Radar 96, Beijing, China October 1996. In the solution, instantaneous weighting vectors w
k
for scaling the sensor signals are calculated from Eq. 1:
w
k
=w
o
+k&Dgr;w
Eq. 1
where
w
k
=weighting vectors for use in scaling sensor signals to obtain an adaptive directional polar gain response;
k=a sample time within a time interval T during which the vectors w
k
are updated;
w
o
=initial values of weighting vectors w
k
; and
&Dgr;w=incremental weighting vector change for rapidly tracking a scene.
In the solution, the weighting vectors w
o
and &Dgr;w are calculated from a vector z using Eq.2:
[
w
0
Δ
w
]
≡
z
Eq
.
2
The vector z is in turn computed by solving Eq. 3:
Rz=&agr;C
Eq. 3
where
C=a matrix of constraints defining mainbeam gain direction;
&agr;=a scalar constant chosen to satisfy the constraints C; and
R=a covariance matrix of augmented sensor signal data as provided by Eq. 4:
R
=
1
T
∑
k
=
1
T
[
x
k
x
∼
k
]
[
H
H
x
k
x
∼
k
]
Eq
.
4
where
X
k
=sensor signal data arriving at the sample time k;
X{tilde over ( )}
k
=augmenting sensor signal data including f(k) X
k
where f(k) is a complex data scaling function which varies with the sample time k; and
H=a Hermitian transpose.
The function f(k) is chosen to match anticipated dynamic characteristics of a platform onto which apparatus implementing the solution is mounted for operation; it is often referred to as a penalty function. Although the solution can provide improved tracking of more rapidly changing scenes, it suffers a problem of providing poor cancellation of interference when there are multiple interference sources within a field of view of the apparatus. Moreover, the solution is more computationally complex than conventional solutions for adaptive beam forming.
Alternative solutions for computing the vectors w are described by Riba et al. in a publication “Robust Beamforming for Interference Rejection in Mobile Communications”, IEEE Trans. Sig. Proc., Vol. 45, No. 1 January 1997 and in a publication by Gersham et al. in a publication “Adaptive Beamforming Algorithms with Robustness Against Jammer Motion”, IEEE Trans. Sig. Proc., Vol. 45 No. 7, July 1997. In these alternative solutions, rapidly time-varying weighting vectors are not computed; instead, nulls in polar gain response provided by vectorially multiplying and then summing the sensor signals together are broadened in a slowly varying adaptive manner to ensure that sources of interference always lie within directions of the nulls. These alternative solutions have a disadvantage that a polar gain response of an apparatus provided thereby becomes unacceptably distorted when sources of jamming are located in a direction of a mainbeam response provided by the apparatus, or where there are multiple jamming and interference sources located in directions away from the direction of the mainbeam response where a residual polar gain sidelobe response is provided by the apparatus.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an alternative adaptive sensor array apparatus providing enhanced interference rejection characteristics.
The invention provides an adaptive sensor array apparatus for generating an output signal in response to received radiation, the apparatus incorporating:
(a) multielement receiving means for generating a plurality of element signals in response to received radiation;
(b) processing means for processing the element signals to provide corresponding augmented signals in which element signals with and without such processing are grouped;
(c) adaptive computing means for adaptively computing weighting vectors from the augmented signals, and for processing the augmented signals using the weighting vectors to provide the output signal, characterised in that the processing means incorporates beamforming means for preconditioning the element signals when generating the augmented signals to enhance interference rejection characteristics of the apparatus when generating the output signal.
The invention provides the advantage of enhancing interference rejection characteristics of the apparatus by improving its performance to track sources of interference which are non-stati
Moskowitz Nelson
Qinetiq Limited
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