GB2111311A

GB2111311A - Adaptive antenna arrays

Info

Publication number: GB2111311A
Application number: GB08136510A
Authority: GB
Inventors: Christopher Robert Ward
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-12-03
Filing date: 1981-12-03
Publication date: 1983-06-29
Also published as: GB2111311B

Abstract

An adaptive antenna array system imposes phase mode constraints on the elements of a circular array 10 prior to adaptive processing 12 with the aid of orthogonal transformation means 11 such as a Butler matrix network. The order of phase mode constraint is selected by a clamping signal C applied to the appropriate output port of the Butler network. <IMAGE>

Description

SPECIFICATION Improvement in or relating to adaptive antenna arrays The present invention relates to antenna arrays and more particularly to adaptive antenna arrays.

The invention is concerned in the main with radio communications.

In general, it is known that the radiation or sensitivity beam pattern of an antenna array is determined by the type of elements in the array, their orientations and position in space and the amplitude and phase of the currents induced in the elements. An adaptive antenna array modifies the pattern in accordance with some control criteria. An adaptive processor can act to apply complex weightings to the amplitudes and phases of the signals from the elements of the array to provide such adaptive control.

In many radio communications systems, especially mobile systems, the optimum reception of desired signals may be adversely affected by the presence of one or more unwanted interference or jamming signals. By employing an adaptive antenna array for the receiver it is however possible to modify the associated radiation pattern of the array to create a null centred on the direction of the incoming jamming signal(s). Where the direction of a desired signal is unknown, it is desirable to exclude the desired signal from the adaptive processing and to ensure the radiation pattern provides maximal angular coverage in the azimuth plane. In the absence of any incoming signals when the receiver is in a quiescent state, the radiation pattern should be ideally omnidirectional.This can be accomplished by enabling just one element of the array and by disabling all the other remaining elements. The adaptive processor should then modify the radiation pattern in the presence of a jamming signal to create the aforesaid null centred on the source of this jamming signal without unduly degrading the sensitivity of the remaining of the pattern. If angular coverage is to be optimized in the modified beam pattern, the width and location of the or each null needs to be controlled precisely.

Circular array configurations allow null patterns to be steered uniformly in azimuth without degradation of null-width. This is a distinct from the operation of linear array operation where the null-width broadens significantly at endfire steering directions.

With regard to the foregoing, a general object of the invention is to provide an improved adaptive antenna array system.

In one aspect the present invention provides a method of processing signals in an adaptive circular antenna array system which includes the step of imposing phase mode constraints on the signals from the elements of the array prior to adaptive processing.

An adaptive antenna system utilizing a circular array configuration constituted in accordance with the invention thus employs means for imposing phase mode constraints on the signals from the elements of the array prior to adaptive processing.

As described hereinafter one convenient realization of the invention is to utilize an orthogonal transformation means. One example of a device suitable for such transformation is a Butler type matrix network. It is desirable to select the order of phase mode constraint imposed on the signals and control means can be provided, inter alia, to change the order of phase mode constraint selectively. With a Butler matrix network the control means can simply clamp the appropriate output port of the network.

In a particular application, such as mobile radio communications, the imposition of phase mode constraints can provide an omnidirectional radiation sensitivity pattern for the receiver at quiescence and can improve the angular coverage in the azimuth plane. The adaptive processor would subject the signals to complex amplitude and phase weightings after the phase mode constraints and prior to summing to modify the radiation pattern when necessary to provide a null centred on the source of a jamming interference signal.

The imposition of phase mode constraints on a circular array can be thought of as phase mode.

"excitation" where the aperture excitation consists of a constant amplitude plus an integral number of cycles of phase variation around the circumference to create a response with constant amplitude and a cycling phase change pattern.

With small arrays with a limited number of elements, it is necessary to adjust the radius of the array to excite a pure phase mode.

The invention may be understood more readily, and various other features of the invention may become apparent, from consideration of the following description.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, wherein: Figure 1 is a schamatic representation of an adaptive antenna array system utilizing a circular antenna array and constructed in accordance with the invention; Figures 2, 3 and 4 are graphical representations of the performance of the system of Figure 1 under various operating conditions; Figure 5 is an equivalent graphical representation of the performance of a linear antenna array under various operating conditions; Figures 6 to 9 depict radiation response patterns for the system of Figure 1 and the linear antenna array under various operating conditions.

Figure 1 depicts an adaptive antenna array system for a receiver used, for example in mobile radio communications subjected to jamming. A six element circular antenna array 10 has the signal outputs from the individual elements 1-6 fed to orthogonal transformation means 11. The transformation means 11 subjects the signals to phase mode constraints prior to adaptive processing. The transformation means 11 can take the form of a Butler matrix network which imposes the selected order of phase mode constraint, i.e., the zero, 1, 2, 3, etc., order by clamping the appropriate selected output port of the network.An adaptive processor 1 2 employs weighting means W2-W6 which subjects the signals from the remaining output ports to complex amplitude and phase weighting prior to their combination in a combining or summing means 13. The summing means 13 provides an output R for further processing by the receiver.

Control means 14 serves to control the adaptive process by providing the appropriate control signal S to the weighting means W2-W6 in accordance with pre-determined algorithms and selects the order of their phase mode constraint by clamping signal C applied to the appropriate output port of the network 11.

Assuming the elements 1-6 are equi-spaced around the circumference of the array 10 the arcuate spacing 'd' between adjacent elements 1-6 is dependent on the radius of the circle describing the array 10. The purity of the selected phase mode can be ensured by adjusting the radius of the circle. If the radius is increased the phase modes follow a Bessel function as represented in Figure 4 which shows relative field strengths for various radii. By selecting the fourth order phase mode as a constraint and by selecting d2 as a spacing or radius for the array 10 the second order phase response is eliminated.

Figures 2 and 3 depict the angular coverage of the array 10 at various gain thresholds. In Figure 2 the adaptive processor 12 of Figure 1 has been adjusted to minimize the response sensitivity of a single jamming source. The individual curves in both Figures 2 and 3 denoted 0, 1 and 4 correspond to zero, first and fourth order phase mode constraints at which the radius of the array 10 is also adjusted to 0.16.1. 0.3.1 and 0.81 to ensure purity of the selected phase modes. For comparison, Figure 1 5 depicts the equivalent angular coverage to Figure 2 but for an array of six linear elements equi-spaced by half a wavelength (.l/2) for curves i), ii) and iii) and differentially spaced for curve iv) where one element of the linear array denoted + is clamped.Comparison between Figures 2 and 1 5 show that the higher order phase mode constraints imposed on the circular array 10 produce improved angular coverage akin to the effect obtained by clamping an element of a linear array of increasing distance from the phase centre of the array aperture.

Figure 6 shows the adapted radiation patterns for the system of Figure 1 for various phase mode constraints and array radii where the response of independent jamming sources with directions denoted by K are minimized. The zero order phase mode i) produces the best response with clear null zones and no spurious null zones as produced by the higher order phase modes ii) and iii). However.

the peak gain is reduced to 2.96 dBi - c.f. 5.08 dBi for the fourth order mode constraint.

Figure 7 shows the comparative adapted radiation response patterns for the system of Figure 1 with a simple clamped element i), with a fourth order phase mode constraint ii) and a six element linear array with imposed clamping iii) in the presence of a single jamming source with a direction denoted by arrow K. The fourth order mode constraint ii) provides a much better null resolution of 9.5 as compared with 250 for the clamped constraint i) for the circular array 10.

Although the clamped linear array provides a narrower resolution of 3 the boresight single jamming source especially favours this configuration and the null resolution directly represents the greater slot or aperture of the linear array. The fourth order constraint response ii) also produces slightly less gain ripple than the clamped case i).

Figure 8 shows the adapted radiation response patterns for the system of Figure 1 with low order phase mode constraints and adjusted array radii in the presence of a single jamming source denoted by arrow K.

In general, the imposition of higher order phase mode constraints prior to adaptive processing for a circular array provides enhanced angular coverage but such systems may become over sensitive to phase errors and incoming signal bandwidth and operation at certain angles to the azimuth plance can create problems.

Figure 9 shows the adapted radiation response patterns for the system of Figure 1 under quiescent conditions without jamming with the array 10 inclined at various angles 9 to the azimuth plane. As the angle 0 increases from zero (i) through 100 ii) 300 iii) to 600 iv) the desired omni-direction pattern degrades. In these circumstances the imposition of lower order phase mode constraints may be desirable to compromise between the resultant effects.

Claims

1. An adaptive antenna array system comprising a circular array and adaptive processor means for processing the signals from the elements of the array to modify the radiation pattern thereof characterized in that means is provided for imposing phase mode constraints on the signals from the elements prior to the adaptive processing.

2. A system according to claim 1, wherein the means for imposing phase mode constraints is an orthogonal transformation means followed by a clamped weight adaptive process.

3. A system according to claim 2, wherein said transformation means is a Butler matrix network.

4. A system according to claim 3, wherein control means serves to select the order of phase mode constraint by clamping the appropriate output port of the network.

5. A system according to claim 1, wherein control means is provided to select the order of the imposed phase mode constraint.

6. A system according to any one of claims 1 to 5, in the adaptive processor means subjects the signals to complex amplitude and phase weightings after the imposition of the phase mode constraints and the weighted signals are combined in summing means.

7. A system according to any one of claims 1 to 6, wherein the array has six elements.

8. An adaptive antenna array system substantially as described with reference to, and as illustrated in, Figure 1 of the accompanying drawings.

9. In an adaptive circular antenna array system; a method of processing signals from the elements of the array which comprises imposing phase mode constraints on the signals prior to their adaptive processing.

1 0. A method according to claim 9, wherein the phase mode constraints are imposed by passing the signals through a Butler matrix network and by clamping a selected output port thereof.

11. A method of processing signals from the elements of an array in an adaptive antenna array system substantially as described with reference to Figure 1 of the accompanying drawings.