[0001] This invention relates to beam steering, and in particular it relates to apparatus
for steering the beam of electromagnetic radiation associated with an array of antenna
elements.
[0002] It is of course well known that the combined output from an array of antenna elements
generally has a lobed structure with the output greater in some directions than others.
It is useful to be able to control the exact nature of this structure; either to rotate
the beam for directionality or to maximise the reception or transmission in some directions
and minimise it from other directions where unwanted signals may occur or in which
direction signals are not to be transmitted. Several methods are available to achieve
such beam steering although all are heavily processing intensive. One such method
involves knowing in advance the precise gains and phase shifts that must be applied
to the signals associated with each element within the array to achieve any particular
beam configuration and direction. When a chosen configuration is desired, signals
with the appropriate amplitudes and phases are then automatically fed to all the elements
within the array to achieve that desired radiation pattern. As will be appreciated,
many different combinations are possible even with a relatively small array and hence
the apparatus requires a large and costly processing and/or memory capability. There
is also a degree of inflexibility associated with such a system.
[0003] Perturbation methods also exist in which a desired result is achieved basically by
a trial and error process by sequentially altering the gain and phase of the signals
from each array element until the desired beam configuration is achieved. Although
this method would seem to take a long time to achieve any desired beam direction,
in fact it is not significantly slower than in the previously described method since
the speed of response depends mainly upon the processing speed, i.e. the data rate
of the processing computer. Inevitably, a feed-back loop is required in such an arrangement
although all systems up to now have only required the use of one feedback loop.
[0004] The present invention represents an improved apparatus for achieving such a perturbation
method which is more flexible in terms of achieving any particular desired result.
[0005] According to the present invention there is provided apparatus for steering the beam
from an array of antenna elements, comprising: a first and a second feedback loop
including respective means for obtaining a first signal representative of a first
chosen parameter of the total output of the array and a second signal representative
of a second chosen parameter of the total output of the array; and processing means
for increasing or decreasing the output of each element in turn by discrete steps,
the first and second signals being used as inputs to the processing means which is
adapted to alter the outputs of each element in a way that maximises a chosen relationship
between the first and second signals.
[0006] Preferably, the first signal is representative of the total output, including any
noise or unwanted information, from the entire antenna array, and the second signal
is representative of the useful or wanted information which has been extracted from
the outputs of the array.
[0007] The chosen relationship may be the ratio between the wanted and unwanted signals.
[0008] The apparatus may further include means for varying the step size used for increasing
or decreasing the output of each element in accordance with the past history of the
process; such that if, for instance, the ratio between the wanted and unwanted signals
is poor then large steps can be used to more quickly reach an optimum but coarsely
defined relationship and if the ratio of the wanted to unwanted signals is good, i.e.
in most instances is high, then smaller steps can be used to finely adjust and maximise
the relationship.
[0009] In a preferred embodiment the steps may range from 1/8 to 1/256 of the total available
control output of each element.
[0010] The apparatus may be adapted to operate on one particular element in turn until the
chosen relationship is maximised for that element and then proceed to adjust a second
element, until all elements have been adjusted and the relationship is maximised.
Various elements can be used as reference elements if desired, in a manner which will
become clear.
[0011] The apparatus may include means for applying weightings to the wanted and unwanted
signals to reflect the relative importance of each of these at any stage. These weightings
may adaptively change as the relative importance varies throughout the use of a system.
This means that although at some point during the processing it might be desirable
to maximise the signal to noise ratio, at other times it may be desirable merely to
minimise the unwanted signal without worrying too much about the signal to noise ratio,
i.e. without being concerned about useful information being obtained. The criteria
affecting these choices can of course vary throughout the use of a system and hence
the control perameters may be adaptively altered. The control parameters may, for
example, be phase or amplitude values.
[0012] The use of the apparatus with a given antenna array may be schematically described
as follows: firstly a start condition is set in which for example the output of one
element may be set at its maximum and the output of all other elements set at their
minimum. The output signals from the array are then fed via the two feedback loops
to the processing means which adjust the output of the elements in firstly coarse
and then in fine steps to attempt to find an acceptable solution to any given problem.
If no acceptable solution can be found, perhaps because the start condition was wrong,
then the start condition can be changed, perhaps by setting a different element as
the 'reference' or fully on element and setting all other elements to zero. The feedback
process is then repeated until an acceptable solution is found. Note that the reference
element need not be held at maximum during the adjustment process. Generally, an acceptable
value for the ratio of wanted to total signal will be 25dB although of course this
will vary depending upon the circumstances. Weightings can be applied to the wanted
and total signals, as described above, to reflect the relative importance of each
of these.
[0013] An embodiment of the invention will now be described by way of example only with
reference to the accompanying drawings in which:
Figure 1 shows schematically a control arrangement according to the present invention,
and
Figure 2 shows some initial start conditions for the antenna array.
[0014] Referring to Figure 1 there is shown an antenna array 1 which is shown, by way of
example only, as having five regularly spaced elements 2. The spacing and number of
these elements may of course vary depending upon the particular application. Outputs
are taken from each of the antenna elements 2 to a first processing unit 3. This analyses
the output signals and produces two respective outputs, the first of which is representative
of the total output of the array, and hence includes unwanted signals and noise as
well as the useful data. This signal is output to a unit which may be called the total
signal analyser 4. A second signal from unit 3 which is representative only of the
useful or wanted data is fed to a wanted data analyser 5. Outputs from the total and
wanted analysers, 4 and 5 respectively are fed to a second or main processing unit
6, which will generally be a computer. This computer 6 is used to control the signals
applied to the antenna elements over control lines 7.
[0015] The use of the apparatus will now be described.
[0016] Firstly, an initial start condition is set on the array. This may be for instance
the condition shown in Figure 2A where the output of the central element is set at
its maximum value 1 and the outputs of all other elements are set at 0. Initial weightings
are also given at this stage to the wanted and total signals, where the terms wanted
and total are as hereinbefore described. These ratings will depend upon whether it
is merely the ratio between the wanted and total signals that is to be maximised for
the particular application or whether some other application is desired, such as to
reduce totally the output from, or sensitivity of, the array in a given direction
to suppress the effect of deliberate signal jamming from that direction for instance,
in which case in that direction the total or unwanted signal is to be minimised and
the level of the actual wanted signal is unimportant. The weightings given to the
two signals will reflect this and thus may perhaps be termed 'relative importance
values'.
[0017] Having set the desired condition in processing computer 6 and decided what the final
beam is to look like, the total output signal from the array is analysed in unit 3
which functions, in known manner, to produce a signal representative of the total
signal to unit 4 and of the total wanted or desirable signal to unit 5. These units
then analyse the data received and output may be taken from them to further processing
units (not shown) if required. Outputs from units 4 and 5 are used as feedback loops
into the computer 6 which applies the previously determined weightings to the respective
outputs and then compares them. If a desired ratio is not adequate then the unit serves
to change the gain and phase of the signal applied to one of the array elements step
wise, initially with steps of the largest step size, which in this embodiment is 1/8
of the total output of that element. Some form of attenuator network may be used to
alter gain and phase, such an PIN diode attenuators. After an element, which may be
for example element 8 in Figure 2A has been adjusted once by one discrete step then
naturally the total array output will change. This is analysed by units 3, 4 and 5
and the resulting ratio is measured in computer 6 which determines whether or not
the ratio has been improved by altering the output of the one particular array element.
By measuring the change in ratio after each stepwise alteration in an array element
control line 7, it is seen that the total output of the array will gradually be shifted
towards that which gives the desired result or beam configuration. After the output
of one element has been adjusted through its entire usable range then this element
will be held at its value which maximises the ratio and a second element will begin
to be varied. Alternatively, after the first element has been adjusted by one step
a second element may be adjusted by one step and so on. In either case after all the
elements have been varied to maximise the ratio it is seen that the desired result
is likely to be achieved.
[0018] The elements are initially adjusted in coarse steps in order that the system may
rapidly be brought to a state as close as possible to the desired one. As it approaches
the desired beam configuration then smaller steps may be used to finely tune the system.
The exact size of these steps and the time when the steps are reduced will be determined
by the processing computer 6 dependent upon the past history of the system. Factors
such as the number of previous attempts to reach a desired state, the particular element
being varied, and whether or not the ratio of the wanted to unwanted ratio is low
or high (i.e. poor or good) will all be used to determine the exact step size at any
instant. The smallest step size envisaged in the present embodiment is 1/256 of the
total output of any element, although of course this will vary depending upon the
particular application.
[0019] There may however be circumstances in which given the initial start conditions set
up in the array, the ratio of the unwanted to wanted signals will never be adequately
maximised for any chosen desired application. A typical criterion for this is that
the wanted to unwanted signal ratio should be better than 25dB. If this is not achieved
after stepwise varying of the array elements then a second start condition may be
selected, for instance that in Figure 2b where it is seen that a different element
to that of Figure 2a is set at its maximum output and all other elements are set at
their minimum output as the start condition. This start condition could automatically
be selected if a chosen number of attempts have been made to maximise the wanted to
unwanted ratio without success. The method described above is then repeated from this
new start condition. Any other desired start condition may of course be applied if
the second one is not successful.
1. Apparatus for steering the beam from an array of antenna elements (1), characterised
by; a first and a second feedback loop including respective means (4,5) for obtaining,
a first signal representative of a first chosen parameter of the total output of the
array and a second signal representative of a second chosen parameter of the total
output of the array; and processing means (6) for increasing or decreasing the output
of each element in turn by discrete steps, the first and second signals being used
as inputs to the processing means which is adapted to alter the outputs of each element
in a way that maximises a chosen relationship between the first and second signals.
2. Apparatus as claimed in claim 1 wherein the first signal is representative of the
total output from the entire antenna array and the second signal is representative
of the useful or wanted information which has been extracted from the outputs of the
array.
3. Apparatus as claimed in claim 1 or claim 2 wherein the chosen relationship in the
ratio between the second and first signals.
4. Apparatus as claimed in any of the preceding claims, including means for varying
the step size in accordance with the past history of the process.
5. Apparatus as claimed in claim 4 wherein the step size is variable between 1/8 to
1/256 of the total available control output of each element.
6. Apparatus as claimed in any of the preceding claims further including means for
applying weightings to the first and second signals to reflect the relative importance
of the signals.
7. Apparatus as claimed in claim 6 wherein the weighting means are arranged to adaptively
alter the weightings to reflect any variation of the relative importance of the signals
with time.
8. An antenna system, including beam steering apparatus as claimed in any one of the
preceding claims.