Digital Adaptive Interference Canceller

The present invention relates generally to the field of devices for reducing interference to a desired signal caused by interfering signal sources. More particularly, the present invention relates to radio frequency apparatus and to those devices which are used to cancel unwanted, interfering signals that may appear in a transmission link between an antenna and a receiver.

There are a number of techniques which have been developed to reduce the effects of interference which may appear on a transmission line which also carries a desired signal. For instance, if the desired signal and the interference are separated in frequency, filters may be used which pass the desired signal and suppress the interference. However, if a filter is used, frequency components of the desired signal which fall within the rejection band will also be suppressed. Filters can introduce significant attenuation even for the desired signal. A filter can also introduce significant absolute and differential group delay times. It is known, for instance, that the absolute group delay time through a band reject filter is approximately the inverse of the pass bandwidth of the filter. For example, the absolute delay time through a 5 MHz bandwidth reject filter will be about 200 nanoseconds.

A further alternative to cancelling interfering signals is the technique of blanking the input to the receiver during the time the interference is present. With this technique, however, no signal can be received during the blanking period. Therefore, this technique is only effective if the blanking does not eliminate a significant fraction of the desired signal. Therefore blanking cannot be used if the interfering signal is continuous or has a high duty cycle. Furthermore blanking requires either a hard-wired connection to the interfering signal or some other technique e.g., predictive analysis, to derive timing information for blanking activation.

Another alternative is that of combining blanking with the use of band reject filters. In this technique, the filter is switched in only during the time the interference is present. However, utilization of this technique still results in significant time-varying group delay propagation times through the filter during the time period in which the filter is switched in and out and also still introduces time-varying differential group delay time.

A still further alternative solution to the problem of cancelling interference is the use of a signal canceller. A prior art signal canceller system is illustrated by way of example in FIG. 1. As is illustrated in FIG. 1, an interfering signal is generated by interfering transmitter 12 and is propagated along transmission line 14 to antenna A.sub.i which radiates the interfering signal. Antenna A.sub.s receives a desired signal and this desired signal is propagated along transmission line 16 to signal receiver 18. Because of coupling from A.sub.i to A.sub.s the signal receiver receives an admixture of both the desired and interfering signals. The interfering signal level S.sub.i, at the output of A.sub.s, is:

where

S.sub.i =amplitude of interfering signal at input to A.sub.i ;

G=the coupling coefficient from DC1 to DC2 via the A.sub.i -A.sub.s antenna coupling path; G is a complex coefficient specifying both the amplitude and phase of the coupling coefficient;

t.sub.i =group delay time from DC1 to DC2 via the A.sub.i -A.sub.s antenna coupling path.

In order to cancel the interfering signal, S.sub.i, a sample of the interfering signal taken at directional coupler DC1 is injected through directional coupler DC2 into the receiver transmission line 16 at a signal level S.sub.j. The level of cancelling signal S.sub.j is:

where

s.sub.i =amplitude of interfering signal at input to A.sub.i ;

g=the coupling coefficient from DC1 to DC2 via the interference injection coupling path; g is a complex coefficient specifying both the amplitude and phase of the coupling coefficient;

tj=group delay time from DC1 to DC2 via the interference injection coupling path.

The resultant sum will be:

where

t'=t-ti;

td=t.sub.j -t.sub.i, the difference in group delay propagation time between the antenna coupling path and the injection path.

As can be seen from equations (1), (2), and (3) above, for perfect cancellation g=-G and td=0. In practice there will be errors, possibly time-varying errors which will result in a residual interfering signal which can be sampled at coupler DC3. The residual signal must be measured and refinements made in the amplitude, phase and group delay propagation time of the injected signal to further reduce the amplitude of the residual signal. Thus the canceller utilizes a null-seeking feed-back loop. Analog implementations of the feed-back loop to control the amplitude and phase of the injected signal have been utilized for a number of canceller applications. However, hard-wired analog control of the feed-back loop is unsatisfactory or difficult to implement, and is inflexible in any case, for many system configurations, particularly those in which there is low-duty cycle frequency-hopping and/or wide-band interference, variations in coupling G or group delay t.sub.d e.g., mechanical or electrical rotation of A.sub.i, and/or A.sub.s, and a wide variety of other complex system configurations and modes.

Accordingly, the present invention overcomes the shortcomings of the analog canceller illustrated in prior art FIG. 1 by providing digital, computer control of the minimization of the residual signal appearing on the transmission link between the signal receiver and the antenna used for receiving the desired signal. Because the present invention is digitally implemented, the processing of pulsed interference elimination is facilitated. Since in many cases the interfering emitter can also provide a pre-trigger pulse, such pre-trigger pulse, suitably delayed, can serve as a sampling pulse for analog-to-digital conversion precisely centered on the peak of the residual signal.

In accordance with the present invention the computer used in the canceller controls the time delay introduced into the signal injected into the desired signal transmission link between the antenna and the signal receiver and also controls the amplitude and phase modulation of the injected signal in accordance with a minimization algorithm. The computer is furnished with information from an amplitude/phase detector/correlator that receives a sample of the residual signal propagating in the desired signal transmission link and compares that signal sample against the injected signal. By using a matrix-inversion minimization algorithm the difference between the residual signal and the injected signal is determined and the delay time, the amplitude and the phase of the injected signal are corrected to minimize the residual signal to as nearly close to a zero value as is possible.

One of the advantages of the digital canceller of the present invention is that ancilliary external digital data can be integrated into the control of the canceller. For example, in one suitable application for the present invention, the interfering signal is electronically rotated from beam position to beam position. Since the amplitude and phase of the interfering signal varies in a predictable, periodic fashion as the beam rotates, the computer controlled digital adaptive interference canceller of the present invention can use the digital beam number as a memory pointer to retrieve the vector modulator control coefficients appropriate for each beam position.

Even if the interfering signal is radiated from a mechanically rotated antenna, the computer of the present invention makes it possible to measure its amplitude, phase and group delay as a periodic function of time. If the antenna is rotated at a constant rate, the vector modulator and RF delay line control words generated by the computer can be periodically clocked out of canceller-dedicated memory to achieve cancellation. Interference from emitter antennas which are not rotated at a constant rate can be cancelled on the basis of prior measurements if the antenna bearing angle can be read into the canceller. Finally, interference from a periodically rotating antennas with no bearing angle read out can be cancelled through pulse-to-pulse injection and residual measurement. In this case the effectiveness of the cancellation depends upon the extent to which the phase and amplitude varies from pulse to pulse as the relative antenna positions change.

The digital adaptive interference canceller of the present invention is particularly suitable for use in applications where the interfering emitter emits a frequency-hopping signal even on a pulse-to-pulse time scale. For applications of the present invention for utilization with such frequency hopping emitters, a "pre-hop" digital control word specifying frequency can be outputted from the frequency hopping emitter to the adaptive interference canceller of the present invention. Again, the appropriate vector modulator control coefficients can be computed from measurements and stored.

The present invention has the further advantage of having the ability to be time shared if there are a number of interfering emitters provided only that the emitters are pulsed. In such a configuration there would be required only one directional coupler sampler such as DC1 in FIG. 1 for each emitter and a digital data word to the digital adaptive interference canceller of the present invention indicating the identification of the current interfering emitter. Again, in this application appropriate vector modulator and RF delay line control coefficients can be clocked from the computer memory. The emitter identification word would also switch the injection input to the interference canceller of the present invention to the current interfering emitter. Thus, the expensive resources of the interference canceller could be time shared among all interfering emitters with only very occasional conflicts between emitters for control of the interference canceller.

Accordingly, it is the primary object of the present invention to disclose a signal interference canceller that is digital and computer controlled.

It is a further object of the present invention to disclose a novel digital signal interference canceller.

It is another object of the present invention to disclose a signal interference canceller that is programmable.

Another object of the present invention is the disclosure of a signal interference canceller that is suitable for use to cancel signals emanated from frequency hopping interfering emitters.

A still further object of the present invention is to disclose an interference canceller that may be time shared if there are a number of interfering emitters.

These and other objects of the invention will become more readily apparent from the ensuing specification when taken together with the drawings.

FIG. 1 is a schematic block diagram of a prior art signal interference canceller.

FIG. 2 is a schematic block diagram of the digital adaptive interference canceller of the present invention.

Referring now to FIG. 2 the preferred embodiment of the present invention will be described. FIG. 2 is a block diagram showing the digital adaptive interference canceller 22 of the present invention. The interference canceller 22 is illustrated in FIG. 2 by way of example for cancelling out an interfering signal generated by interfering transmitter 24 and radiated by interfering radiating antenna 26. In the system illustrated in FIG. 2 a desired signal is to be received by antenna 28 and propagated along transmission link 30 to signal transmitter/receiver 32. As can readily be appreciated the interfering antenna 26 may be in such close proximity to antenna 28 that the signal radiated from interfering antenna 26 may be received by desired signal antenna 28 and thereby appear at the input to signal receiver 32. This interfering signal is cancelled out by the components of the present invention described below. Directional coupler 34 is operably coupled to transmission link 25 between the interfering transmitter 24 and the interfering radiating antenna 26. The output of directional coupler 34 at, for example, 960-1215 MHz, is input to mixer 36; the other input is from tunable local oscillator 38, at, for example, a frequency of 800-1055 MHz.

Thus, the down-converted output of mixer 36, at a frequency of, for example, 160 MHz, is provided as one input to computer controlled RF delay line 40. The other input to computer controlled RF delay line 40 is derived from control computer 42 of the present invention. The output of RF delay line 40 is split by power splitter 41 into equal outputs onto transmission lines 44 and 46, respectively. Vector modulator 48 receives the signal propagated along transmission line 44 and phase and amplitude modulates the signal under the control of computer 42.

The delayed, phase and amplitude modulated signal output of vector modulator 48 is furnished to mixer 50 which receives its other input from local oscillator 38 and up-converts that signal. The up-converted signal, in two bands, for example, one at 960-1215 MHz and the other at 640-895 MHz, is amplified by amplifier 52; the lower frequency signal output from mixer 50 is filtered out by band reject filter 54. In order to protect the signal injection system modules 52 and 54 for system configurations in which a transmitter shares line 30, computer controlled switch 56 may be utilized between band reject filter 54 and directional coupler 58. The computer controlled switch 56 may be selectively opened and closed under control of the computer 42 or by other means (not shown) according to the application of the invention.

Directional coupler 58 is utilized to inject the cancellation signal onto transmission line 30 in order to cancel out the interfering signal received by antenna 28 from interfering signal antenna 26.

Directional coupler 60 is operably coupled to transmission line 30 for deriving a sample of the residual signal resulting from the sum of the interfering signal and the injected signal as will be described further below. This residual signal sample is down-converted by means of mixer 62 which receives its other input from local oscillator 38. The down-converted residual signal sample is provided as an input to amplitude/phase detector/correlator 64. The amplitude/phase detector correlator 64 receives its other input on transmission line 46 from the signal splitter 41 and comprises the down-converted time delayed interfering signal derived from RF delay line 40 via down-converter 36 and directional coupler 34. The amplitude/phase detector correlator 64 is a well known device and may comprise, for example, RHG Company model number DPD16020. Generally, amplitude/phase detector 64 compares the down-converted residual signal derived from directional coupler 60 and mixer 62 with the interfering signal derived from transmission line 46 as previously described. Any other signals such as spurious signals appearing on transmission link 30, or such as the desired signal appearing on transmission link 30, which are sampled by directional coupler 60 and which thereby appear at the output of mixer 62 and consequently at the input 66 of amplitude/phase detector correlator 64, are rejected by the correlator function of amplitude/phase detector 64. Accordingly, such signals are not compared with the interfering signal propagated on transmission line 46. The in-phase and quadrature-phase outputs of amplitude/phase detector 64 are furnished to analog-to-digital converter 68 which converts these analog signal components to digital signals which may, for example, be 12-bit signals for input to computer 42.

Computer 42 receives the output of analog-to-digital converter 68 and executes a residual minimization algorithm which adjusts the amplitude and phase of the injected signal via vector modulator 48 and which also may adjust the time delay of the injected signal via RF delay line 40. A listing attached as Appendix I hereto is provided by way of example and constitutes a source code listing written in HP Basic, including the above referred to minimization algorithm and suitable for use in programming computer 42 in implementing the residual minimization algorithm.

Referring again to FIG. 2 the interference canceller of the present invention operates as follows. A sample of the interfering signal generated by interfering transmitter 24, which will hereafter be referred to as the injection signal, S.sub.i, defined in Equation (1) above, is taken via directional coupler 34 from transmission line 25 that couples interfering transmitter 24 with radiating antenna 26. It is assumed for purposes of this invention that the injection signal derived by directional coupler 34 is a time-shifted replica of the interfering signal that will be received by antenna 28 from antenna 26 through the free space path. The directional coupler 34 contributes only a few tenths of a dB attenuation to the interfering signal path between interfering transmitter 24 and interfering radiating antenna 26. The interfering signal sample which may, for instance, be in the frequency range of 960-1215 MHz, is then down-converted to an intermediate frequency by means of mixer 36. For this application, local oscillator 38 may be tuned, for instance, to 800-1055 MHz. The output of mixer 36 then comprises sum and difference frequencies resulting from the mixing function of mixer 36. The sum frequency output of mixer 36 is filtered out by RF delay line 40 and the difference frequency which may, for example, be in the frequency range of 160 MHz is passed through the computer controlled RF delay line 40 which fine tunes the delay through the transmission line, electronic component cancellation path between directional coupler 34 and injection directional coupler 58 to be the same as the delay in the coupling path from directional coupler 34 via antenna 26 via free space to the antenna 28 and then through transmission line 30 to directional coupler 58.

Power splitter 41 divides the signal output of RF delay line 40 into equal amplitude components onto transmission lines 44 and 46, respectively. The transmission line 44 component of the time-delayed, down-converted injection signal is furnished to the input of computer controlled vector modulator 48 which adjusts the amplitude of the injected signal S.sub.j injected via directional coupler 58 into transmission line 30 to be the same as the amplitude of the interfering signal S.sub.i. Further, the vector modulator 48, under control of computer 41, adjusts the phase of the injected signal S.sub.j to differ by 180.degree. from that of S.sub.i. The resultant signal from the output of vector modulator 48 is then up-converted back to its original frequency and amplified by amplifier 52. The difference component output of mixer 50 is filtered out by bandpass filter 54 and the sum component output at, for example, 960-1215 MHz is passed through filter 54 through switch 56 to directional coupler 58 which injects the signal S.sub.j into transmission line 30 for cancellation of the interfering signal S.sub.i. Filter 54 also eliminates spurious components introduced in the up-conversion process that appear at the output of mixer 50. Switch 56 is used to isolate and protect the output of amplifier 52 from any transmitter sharing line 30.

The residual signal sum S.sub.i +S.sub.j is sampled by directional coupler 60 and this signal sum is down-converted by means of

mixer 62. Amplitude/phase detector/correlator 64 has two inputs. The first input is the time-delayed down-converted sample portion of interfering signal S.sub.i appearing on transmission line 46 at the output of power splitter 42. The other input to amplitude/phase detector correlator 64 is the down-converted residual signal sum S.sub.i +S.sub.j. Amplitude/phase detector correlator 64 suppresses signals other than S.sub.i and S.sub.i +S.sub.j.

On the assumption that a pre-trigger pulse is available from interfering transmitter 24, as could be the case where interfering transmitter 24 and receive antenna 28 are located on the same ship, the analog-to-digital sampling time of analog-to-digital converter 68 relative to the pre-trigger pulse is precisely set in accordance with the present invention using the digital delay generator 70 which receives both the pre-trigger pulse and digital instruction from computer 42. Thus, digital delay generator 70 is under control of computer 42 to establish the A/D sampling time of A/D converter 68. The delay-time algorithm for residual interference sampling is implemented in the computer listing of Appendix I. This delay time algorithm is executed to precisely evaluate the signal levels measured over a range of delay times and to determine the optimum delay, i.e. the maximum sampled signal level. Once the A/D sampling time, or the digital delay generator 70 time delay, is determined, the computer 42 enters the residual minimization algorithm which adjusts the amplitude and phase of the signal input to vector modulator 48. Computer 42 then adjusts the delay through computer controlled RF delay line 40 to further minimize the residual signal and again adjusts phase and amplitude via vector modulator 48 to minimize the residual. Unless antennas 26 and 28 are moving with respect to each other no further adjustments are required in the RF delay so that all additional refinements and minimization can be accomplished through phase and amplitude adjustments via vector modulator 48.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. ##SPC1##