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This invention relates to systems for receiving wave energy signals and
particularly to systems which adaptively suppress interference in the
received signal.
Prior art adaptive canceling systems have most often been used in
connection with radar systems. A radar system usually uses a highly
directional antenna which has a main beam and many sidelobes. A signal
within the received frequency band of the radar may cause radar jamming by
having sufficient amplitude to saturate the radar receiver when that
signal is incident on the antenna sidelobes as well as the main beam.
Prior art systems have been developed for cancelling this type of
interference in a radar system. These prior art systems usually use an
omni-directional auxiliary antenna to receive the interfering signal in an
auxiliary channel. The interfering signal in the auxiliary channel is
combined with the signal received in the main radar channel to adaptively
cancel the interfering signal when the interfering signal is present on
the radar sidelobes.
This system limits the adverse effects of the interfering signal to only
those portions of the radar scanning time when the main beam is pointed at
the interfering signal source. This facilitates location of the
interfering signal source and prevents that source from interfering with
the radar detection of other targets.
The prior art radar adaptive sidelobe canceller usually discriminates
between an interfering signal and a desired radar return signal on the
basis of signal format. For example, a radar usually transmits short
pulses of R.F. energy to facilitate range finding. An interfering signal
to be effective is most often a more continuous signal and therefore the
adaptive canceling circuit may be designed to respond only to signals
having a duration substantially greater than the duration of the desired
radar pulse. In other cases the radar signal may be a phase coded or
frequency chirped signal which may be distinguished from the undesired
interfering signal according to the known characteristics of the desired
radar signal.
While useful in systems wherein the signal format of the desired signal is
easily distinguished from the signal format of the interfering signal,
such as radar systems and communications systems using predetermined
codes, the above described prior art adaptive sidelobe canceling systems
are not usable in systems where there is no distinguishing feature between
the desired signal and the interfering signal. For example, in a system
where the desired signal is an amplitude modulated signal and the
interfering signal is also an amplitude modulated signal in the same
frequency band, discrimination on the basis of signal format is not
possible. In some of these systems it is possible to distinguish between a
desired signal and an interfering signal on the basis of a priori
knowledge of the location of the source of the desired signal. Very often
sufficient discrimination between the desired signal and the interfering
signal may be accomplished using only a directional antenna. In some
instances, however, the interfering signal may be of such a greater
intensity than the desired signal that the interfering signal received on
an antenna sidelobe has greater signal strength in the receiver than the
desired signal received on the antenna main beam. This is possible, for
example, in an instance where a shore based communications terminal which
is seeking to receive a desired signal from ships far at sea is located in
the vicinity of another transmitting terminal and receives interference
from that transmitting terminal.
R. J. Masak in his co-pending application, Ser. No. 496,494 filed Aug. 12,
1974, entitled, "Interference Rejection System For Multi-Beam Antenna"
which is assigned to the same assignee as the present application, has
described a system for suppressing interference on the basis of direction
of signal origin. That system utilizes a multiple beam antenna, having
multiple output ports and provides for adaptive circuitry responsive to
the output of each antenna port except the port corresponding to the
direction of the desired signal.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a system for
receiving desired wave energy signals and suppressing interfering signals.
It is a further object of the present invention to provide such a system
wherein discrimination between the desired signal and the interfering
signal is on the basis of direction of arrival of the respective signals.
It is a still further object of the present invention to provide such a
system using an antenna which is responsive to signals originating in
multiple regions of space and has multiple output ports each primarily
responsive to signals originating in one of the regions of space.
It is a still further object of the present invention to provide such a
system using only a single circuit responsive to the combined outputs of
the antenna ports.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a system for
receiving desired wave energy signals from a selected angular region of
space within the area covered by the system and for suppressing
interfering signals from all other angular regions within the coverage
area. The system includes antenna means, having a first plurality of
antenna elements and a second plurality of output ports for receiving wave
energy signals from said coverage area and for developing at each of said
output ports a signal primarily representative of the wave energy signals
received from a corresponding predetermined angular region of said
coverage area. The system further includes first means for combining
supplied signals in accordance with a predetermined function to develop an
output signal. It includes second means for combining supplied signals in
accordance with a predetermined function to develop a reference signal. It
also includes means for coupling signals, including desired and
interfering signal components, from a selected output port of said antenna
means to an input of said first combining means and for coupling a portion
of the signals from at least the remainder of said antenna ports to inputs
of said second combining means. Also included are means for developing a
correction signal. It includes means for mixing the output signal from
said first combining means and the reference signal from said second
combining means thereby to develop a control signal. Further included are
means for mixing said control signal and said reference signal thereby to
develop a correction signal. Finally, there is included means for coupling
said correction signal to an input of said first combining means thereby
causing suppression of said interfering signal components in said output
signal.
For a better understanding of the present invention together with other and
further objects thereof reference is had to the following description
taken in conjunction with the accompanying drawings and its scope will be
pointed out in the appended claims.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic diagram of a system constructed in accordance with
the present invention.
FIG. 2 shows an alternate antenna unit for use in the antenna system of
FIG. 1.
Description and Operation of the FIG. 1 System
FIG. 1 shows a system constructed in accordance with the present invention.
Shown in FIG. 1 is antenna unit 10 which includes a plurality of antenna
elements 12 and an antenna beam forming network 14 connected to the
antenna unit 12. Beam forming network 14 may be a well-known Butler Matrix
or any other type suitable for providing antenna element phasing. Antenna
unit 10 is capable of generating antenna beams 16a, 16b, 16c and 16d in
response to wave energy signals supplied to the ports 18a, 18b, 18c and
18d of antenna network 14. Because of the reciprocal nature of antenna
unit 10, signals originating in angular region 20a will be primarily
received by antenna beam 16a and primarily supplied to antenna port 18a.
Likewise, signals from angular regions of space 20b, 20c and 20d will be
primarily supplied to corresponding antenna ports 18b, 18c and 18d,
respectively.
Each of the antenna ports 18 is therefore primarily responsive to signals
received from a corresponding region of space, but those skilled in the
art will also recognize that signals from directions outside of the
corresponding angular region of space will also appear in each of the
antenna ports because of the imperfections of antenna beam shaping,
principally beam fall off and antenna sidelobes. In FIG. 1 antenna beam
16d is shown, for example, to include a main beam primarily responsive to
signals originating in angular region of space 20d, and also a sidelobe
which is responsive to a lesser degree to signals originating in region of
space 20a. Hence antenna port 18d is primarily responsive to signals
originating in angular region 20d, and also responsive, to a lesser
extent, to signals originating in angular region 20a. It may also be seen
that antenna port 18d will be responsive to signals originating in
portions of spacial region 20c because of the antenna beam fall off in
this angular region.
In the FIG. 1 antenna system there is further provided directional couplers
22a, 22b, 22c and 22d each connected to a corresponding output port 18.
The output port of each of the couplers 22 is connected to a corresponding
one of the poles of single pole multi-throw switch 25. The coupled ports
of couplers 22 are connected to combining unit 24 by single pole switches
23a, 23b, 23c, and 23d. Multi-throw switch 25 may be used to connect any
of the output ports 18 to a receiver by means of combiner 30. There is
included between switch 25 and combiner 30 in the embodiment of FIG. 1 a
local oscillator 44 and mixer 46 to affect an IF conversion of the signal
from the selected antenna port 18.
Combiner 24 forms a reference signal in response to the signals supplied
from the coupled ports of directional couplers 22. In some cases it may be
desirable to exclude from the reference signal generated in combiner 24
the coupled output of the antenna port 18 corresponding to the direction
of the desired signal. To achieve this the appropriate switch 23 may be
opened while the remaining switches 23 are in the closed position. It is
entirely possible, however, to practice the present invention without
having any of the switches 23 opened and therefore have the reference
signal generated in combiner 24 representative of the combination of the
signals in all of the antenna ports 18.
The output of combiner 24 is supplied to mixer 42, and also to mixer 36 by
limiter 34. Also supplied to mixer 36 is a portion of the output signal
from combiner 30. The combination of the reference signal from combiner 24
and the output signal from combiner 30 in mixer 36 causes the generation
of a control signal which is supplied to mixer 42 by integrator 38 and
amplifier 40. Since the output of combiner 30 has undergone a frequency
conversion in mixer 46, the control signal will have the same frequency as
local oscillator 44.
The operation of the FIG. 1 antenna system is very similar to that
described in the above-referenced co-pending application. If it is desired
to receive a signal which originates in region 20a, switch 25 is
appropriately set to couple the output of antenna port 18a to combiner 30
by means of mixer 46. In this case switch 23a may be opened while switches
23b, 23c, and 23d are in the closed position. Combiner 24 is therefore
supplied with a portion of the output signal of antenna ports 18b, 18c and
18d which are combined to form a reference signal. The relative amplitude
of the coupling from antenna ports 18b, 18c and 18d to combiner 24 is
considerably lower than the coupling from antenna port 18a to combiner 30
by reason of the coupling ratio selected for directional couplers 22.
Typically the coupling ratio selected for directional couplers 22 would be
approximately equal to the sidelobe level expected from antenna beams 16.
When there is only a desired signal incident from region of space 20a, that
signal is supplied from antenna port 18a to combiner 30 by switch 25. If
switch 23a is in the open position, only signals from antenna ports 18b,
18c and 18d are supplied to combiner 24 to form the reference signal.
Since the desired signal from region 20a is primarily supplied to antenna
port 18a, only a small amount of the incident desired signal,
corresponding to the antenna beam fall-off or sidelobe level will be
supplied to the remaining antenna ports 18b, 18c and 18d. The magnitude of
the signals at these antenna ports if further reduced by the coupling
ratio of couplers 22b, 22c and 22d prior to supplying the signals to
combiner 24 to form the reference signal. The reference signal supplied to
mixers 36 and 42 will therefore be representative of the desired signal,
but at a considerably lower signal level than the desired signal supplied
directly to combiner 30. A control signal will be developed at mixer 36
having a correspondingly low signal level and only a small correction
signal will be generated in mixer 42 because of the low signal level of
both the reference signal and the control signal.
The system will also operate in a similar manner if switch 23a is closed,
thereby allowing the desired signal from antenna port 18a to be supplied
to combiner 24 at a signal level which is reduced by only the coupling
ratio of coupler 22a. This condition corresponds to there being no
switches 23 present in the system. In this case the reference signal and
control signals will have greater amplitude than in the case where switch
23a is opened, but because the signals have been reduced by the coupling
ratio of coupler 22a, the correction signal generated will have
insufficient amplitude to cancel the desired signal supplied to combiner
30 by switch 25.
When in addition to the desired signal from region 20a, there is incident
on the antenna system an interfering signal from another region of space
20b, 20c or 20d, the interfering signal as well as the desired signal will
appear at antenna port 18a. The interfering signal may reach port 18a by
reason of a sidelobe of antenna beam 16a or by reason of the antenna beam
fall-off in region 20b. In this case, although antenna port 18a is
primarily responsive to signals originating in region of space 20a, the
interfering signal may have such greater signal level incident on the
antenna system that it appears at antenna port 18a with greater magnitude
than the desired signal. The interfering signal and desired signal are
both supplied to combiner 30 by switch 25.
When the interfering signal is incident from a region of space
corresponding to another antenna port, for example region 20b,
corresponding to antenna port 18b, the interfering signal is supplied to
port 18b with greater amplitude than the interfering signal supplied to
port 18a. In that case it is also supplied by coupler 22b to combiner 24
and appears in the reference signal formed in combiner 24. The coupling
ratio of coupler 22b is selected so that the reference signal formed by
combiner 24 has approximately the same amplitude of interfering signal as
is supplied to mixer 46 by switch 25. Thus if the response of antenna port
18a to signals incident from regions 20b, 20c and 20d is 10 dB below the
response of ports 18b, 18c and 18d to those signals as combined in
combiner 24, a 10 dB coupling ratio would be appropriate.
The reference signal formed in combiner 24, which is primarily
representative of the interfering signal is supplied to mixer 36 by
limiter 34. Limiter 34 is used to achieve greater system dynamic range as
is fully explained in the co-pending application of R. J. Masak, Ser. No.
489,623, filed July 18, 1974, entitled "Spectrum Notcher", which is
assigned to the same assignee as the present application. Also supplied to
mixer 36 is a portion of the output signal from combiner 30, which
includes both the desired signal and the interfering signal following
frequency conversion in mixer 46. Since the reference signal supplied to
mixer 36 is primarily representative of the interfering signal, there will
be developed a control signal representative of the amplitude and phase of
the interfering signal present in the output signal supplied to mixer 36.
The control signal is supplied to mixer 42 by integrator 38 and amplifier
40. Integrator 38 is typically a very narrow band filter which determines
the response time of the circuitry as is fully explained in the above
referenced co-pending application entitled Spectrum Notcher. The control
signal supplied to mixer 42 has the same frequency as local oscillator 44
since that is the amount that the output signal supplied to mixer 36 has
been shifted in frequency with respect to the reference signal. At mixer
42 the control signal causes a frequency conversion and modulation of the
reference signal to develop a correction signal suitable for causing
cancellation of the interfering signal at combiner 30. Those skilled in
the art will recognize that appropriate adjustment of the phase of either
the reference signal or the output signal supplied to mixer 36 may be
required to assure that the correction signal has a phase appropriate to
cancel the interfering signal in output combiner 30.
While it is convenient to implement the invention utilizing IF conversions
as has been described, those skilled in the art will recognize that such
conversions are not necessary. R. J. Masak's above-referenced co-pending
application entitled Spectrum Notcher has described such alternate methods
for developing similar correction signals without IF conversions.
It will also be recognized by those skilled in the art that other types of
antenna units, other than the one illustrated in FIG. 1, may be used in
practicing the invention. FIG. 2 illustrates an antenna unit 50 comprising
individual radiating means 52a, 52b, 52c and 52d, each connected to a
corresponding antenna port 18. Each of the radiating means 52 is capable
of radiating a directional antenna beam 54, and consequently of receiving
signals primarily from a corresponding region of space.
While there have been described what are believed to be the preferred
embodiments of this invention, it will be obvious to those skilled in the
art that various changes and modifications may be made therein without
departing from the invention and it is, therefore, aimed to cover all such
changes and modifications as fall within the true spirit and scope of the
invention.
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