GB2323969A

GB2323969A - Adaptive antenna arrays

Info

Publication number: GB2323969A
Application number: GB8316094A
Authority: GB
Inventors: Philip John Hargrave; John Edward Hudson; J G Searle; C R Ward
Original assignee: Northern Telecom Europe Ltd; Standard Telephone and Cables PLC; Nortel Ltd
Current assignee: STC PLC; Nortel Networks Optical Components Ltd; Nortel Ltd
Priority date: 1983-06-11
Filing date: 1983-06-11
Publication date: 1998-10-07
Anticipated expiration: 2003-06-11
Also published as: GB2323969A8; GB2323969B; GB8316094D0

Abstract

In an adaptive antenna array system the outputs of the array elements (10 1 .....10 n ) are weighted in a beam forming network (11) under the control of a gradient descent weight processor (12) before being summed to provide the array output. To provide an error signal for the processor (12) the array output is fed to a limiter and the bandpass filter (20) and to a modulus detector (40). The output of the modulus detector is filtered (41) and used to control the gain of an amplifier (42) whereby the limiter output is amplitude scaled in proportion with the modulus of the array output. The amplifier output is subtracted from the array output to form an error signal feedback to the processor.

Description

ADAPTIVE ANTENNA ARRAYS This invention relates to adaptive antenna arrays and is particularly useful for spread spectrum systems.

An adaptive antenna array uses real-time pattern control to provide nulls on interference signals in radar and communications systems. Generally the array output is introduced into a feedback loop together with an injected reference signal to derive an error signal.

This error signal is then used to control weights applied to the outputs from the array elements. The effect of the feedback loop on the antenna array is to minimise the error signal, thereby enhancing the desired signal output from the array.

Fig. 1 shows a generalize schematic diagram of a reference signal adaptive array. The array elements 101, 102 10n feed a beam forming network 11 in which the individual element outputs are subjected to weighting by a gradient descent weight processor 12. The weighted outputs are then summed to produce the array output signals. The array output signal then has subtracted from it a reference signal to provide an error signal to which the processor 12 responds to adjust the weights. The reference signal is itself derived from the array output. This is achieved by feeding the array output signal to a desired signal estimator network 13 in which the output signal is generated in accordance with preconceptions concerning the form of the desired signal.

One pre-conception that is tenable is that, given a frequency or phase modulated signal, the carrier amplitude will be constant.

According to the present invention there is provided an adaptive antenna array system for the reception of frequency or phase modulated signals in which the array element outputs are weighted to minimise an error signal being the difference between the summed array output signal and a reference signal, characterised in that the array output signal is passed through a bandpass limiter to form the reference signal.

In an embodiment of the invention there is further included means for scaling the amplitude of the limiter output in proportion with the mean modulus of the output voltage of the array output signal.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: - Fig. 2 illustrates an adaptive antenna array system using a fixed level bandpass limiter to generate a reference signal; Fig. 3 illustrates adaptive response characteristics of a system as shown in Fig. 2; Fig. 4 illustrates an adaptive antenna array system using adaptive scaling of the bandpass limiter output to generate a reference signal; and Fig. 5 illustrates typical adaptive response characteristics of a system as shown in Fig. 4.

In the embodiment shown in Fig. 2 the desired signal estimator of Fig. 1 is implanted by the bandpass limiter 20. This preserves the phase of the array output signal but removes any amplitude modulation. The resultant fixed level signal forms the reference signal which is subtracted from the array output signal to provide the error signal which is used as the feedback in a conventional adaptive, gradient descent processor 12.

The adaptive processor adjusts the weight coefficients applied to the beam forming network 11 until the error signal is minimised. This minimum will occur when the output from the antenna array contains predominantly a constant amplitude, frequency or phase modulated signal corresponding to the desired transmission. In order to have reached this condition the adaptive beamformer will have nulled unwanted signals, including jamming signals and other external interference, whilst maximising gain in the desired signal direction. A computer simulation of the typical convergence characteristics of such a system is shown in Fig. 3. This shows adaptation of the array for a weak desired signal (-20 dB) in the presence of a strong jamming signal (0 dB) for a small array of 3 elements. This example illustrates the self-acquiring nature of the system even though the initial signal/jammer power ratio is extremely poor.

Another pre-conception concerning the desired signal can be derived from the type of modulation used.

With bi-phase modulation the two phase states will be known a-priori and this knowledge can be used to aid the estimation.

However, under adverse conditions, whilst convergence will be achieved the convergence time may be unreasonably long for the purpose in hand.

For example, in the simulation shown in Fig. 3 the desired signal is not brought out from the interference until over 1000 samples have occurred.

A further pre-conception which can be included in the estimation process to improve significantly the rate of convergence concerns the level of the injected reference signal. Any estimate of the desired signal component from the output of the antenna array must be bounded by the modulus of the total signal level at the estimator input. One possibility is to scale the amplitude of the bandpass limiter output in proportion with the mean modulus of the output voltage of the array. An implementation of this scheme is shown in Fig.

4. In addition to feeding the array output to the bandpass limiter 20 the output is fed to a modulus detector 40. The detector output is then low pass filtered in filter 41 and applied as a gain control signal to amplifier 42 which adjusts the level of the estimator. Fig. 5 illustrates the performance improvement offered by this scheme for the case of a simulated reception of an FM desired signal in the presence of a strong noise jamming signal. Acquisition of the desired FM signal is extremely rapid.

The technique of adaptively controlling the amplitude of the derived reference has an advantage in terms of the dynamic range of the weighting networks.

For a fixed level reference, the demanded weight dynamic range may be high depending on the range of possible desired signal levels. In the preferred scheme described here, the necessary dynamic range of the weights will be greatly reduced; in fact, it will now be only the reference signal processor which is required to track the desired signal level.

Claims

CLAIMS:

1. An adaptive antenna array system for the reception of frequency or phase modulated signals in which the array element outputs are weighted to minimise an error signal being the difference between the summed array output signal and a reference signal, characterised in that the array output signal is passed through a bandpass limiter to form the reference signal.

2. An adaptive antenna array system according to claim 1 including means for scaling the amplitude of the limiter output in proportion with the mean modulus of the output voltage of the array output signal.

3. An adaptive antenna array system according to claim 2 wherein the scaling means comprises a modulus detector to which the array output is applied, a low pass filter to which the detector output is applied, and a controlled gain amplifier the input to which is the bandpass limiter output, the low pass filter output providing a gain control signal for the amplifier the output of which is the reference signal.

4. An adaptive antenna array system substantially as described with reference to Fig. 2 or Fig. 4 of the accompanying drawings.

5. A method of operating an adaptive antenna array system wherein an estimate of the desired signal is used to control a gradient descent processor to apply weights to the array element outputs in a beam forming network, characterised in that the array output is bandpass limited and the limited output is then amplitude scaled in proportion with the mean modulus of the array output to form the estimate of the desired signal.

Amendments to the claims have been filed as folows 1. An adaptive antenna array system for the reception of frequency or phase modulated signals in which the array element outputs are weighted to minimise an error signal being the difference between the summed array output signal and a reference signal, characterised in that the array output signal is passed through a limiting and bandpass filtering means to form the reference signal whereby the phase of the array output is preserved whilst amplitude modulation of tne output is removed.

2. An adaptive antenna array system according to claim 1 including means for scaling the amplitude of the limiter output in proportion with the mean moaulus of the output voltage of the array output signal.

3. An adaptive antenna array system according to claim 2 wherein the scaling means comprises a modulus detector to which the array output is applied, 2 low pass filter to which the detector output is applied, and a controlled gain amplifier the input to which is the bandpass limiter output, the low pass filter output providing a gain control signal for the amplifier the output of which is the reference signal.

4. An adaptive antenna array system substantially as described with reference to Fig. 2 or Fig. z of the accompanying drawings.

5. A method of operating an adaptive antenna array system wherein an estimate of the desired signal is used to control a gradient descent processor to apply weights to the array element outputs in a beam forming network, characterised in that the array output is limited and bandpass filtered and the limited and bandpass filtered output is then amplitude scaled in proportion with the mean modulus of the array output to form the estimate of the desired signal.

6. A method of operating an adaptive antenna array system in which frequency or phase modulated signals are received, comprising weighting the array element outputs to minimise an error signal, being the difference between the summed array output signal and a reference signal, and passing the array output signal through a limiting and bandpass filtering means to form the reference signal, whereby the phase of the array output is preserved whilst amplitude modulation of the output is removed.

7. A method o operating an adaptive antenna array system, sunstanziclly as hereinbefore described zith rererer-.oe to Figs. 2 to 5 of the accompanying drawings.