Abstract:
A weaker signal receiving system inclusive of stronger nearby-sourced interference signal cancellation capability. Stronger interference signal cancellation is accomplished by actively canceling or subtracting from the received signal an intermediate signal in which the weaker signal has been attenuated but the stronger signal remains. Attenuation of the weaker signal in this intermediate signal is accomplished in a feedback loop arrangement by an amplitude-responsive signal processing element embodied from for example a ferrite material such as yttrium iron garnet disposed in a physical wave propagating and wave amplitude sensitive film. The cancellation or subtracting is accomplished using received signals and without need for a direct output sample of the stronger signal at its source. Military aircraft use of the disclosed system in the microwave and other spectral regions with transmission mode as opposed to reflection mode signal amplitude discrimination by the ferrite device is included.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of radio frequency receivers and transmitters operating without disruptive signal interference in close physical proximity while located for example in an aircraft or other vehicle. 
     The conventional present day technique for providing continuous wave signal interference mitigation between a co-located or common platform radio frequency transmitter and receiver, in an aircraft setting for example, involves installing signal couplers in the transmission path between the transmitter and the transmitting antenna. Such couplers obtain a copy of a receiver-interfering strong transmitter signal for use in a signal subtraction or cancellation arrangement. This technique is represented generally in the FIG. 8 drawing herein. The copied strong transmitter signal is then provided with the proper phase and amplitude characteristics and coupled into the signal path between the receiving antenna and the receiver to achieve cancellation of the offending signal. Using this technique a significant amount of host platform, or e.g. host aircraft, characterization must be performed to install the signal suppression architectures. Common platforms requiring suppression of multiple offending transmitter signals in multiple receiver signal lines, will benefit from a better approach to interference mitigation. 
     SUMMARY OF THE INVENTION 
     The present invention provides active interference signal cancellation protection for a low level receiver operating near a transmitter antenna and provides this protection with a nonlinear ferrite element and without need to access a sample of the transmitter signal at its source. 
     It is an object of the present invention therefore to protect a radio receiver apparatus operating in the vicinity of a radio transmitter from transmitter-sourced interference signals. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference without requiring direct access to a transmitter signal. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference through use of receiver antenna-collected signals. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference by way of an adaptive signal cancellation-based signal processing algorithm. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference by way of introducing a frequency selective amplitude-sensitive nonlinear ferrite element into the generation of a transmitter interference-cancellation signal. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference by way of introducing an amplitude and frequency responsive nonlinear ferrite element such as yttrium iron garnet into the generation of a transmitter interference-cancellation signal. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference by way of introducing an amplitude and frequency responsive yttrium iron garnet ferrite element disposed in one of film form and spherical form into the generation of a transmitter interference-cancellation signal. 
     It is another object of the invention to provide radio receiver protection from adjacent transmitter interference in a manner providing low received signal loss. 
     It is another object of the invention to provide a system of radio receiver protection from adjacent transmitter interference usable in a co-located platform environment such as in a military aircraft. 
     It is another object of the invention to provide radio receiver protection from adjacent continuous wave, pulsed or frequency hopped transmitter interference. 
     It is another object of the invention to provide adaptive cancellation-based radio receiver protection from adjacent continuous wave, pulsed or frequency hopped transmitter interference. 
     It is another object of the invention to provide a radio receiver protection from adjacent transmitter interference arrangement usable over a frequency range such as 50 megahertz to 18 gigahertz. 
     It is another object of the invention to provide active mode low signal loss radio receiver protection from adjacent transmitter interference. 
     It is another object of the invention to provide an active interference cancellation algorithm for use with continuous wave transmitter interference signals. 
     Additional objects and features of the invention will be understood from the following description and claims and the accompanying drawings. 
     These and other objects of the invention are achieved by the method of receiving a weaker desired radio frequency signal in the presence of a nearby transmitter-sourced adjacent frequency strong interference signal, said method comprising the steps of: 
     generating a first electrical signal having both said weaker desired radio frequency signal and said nearby transmitter-sourced adjacent frequency strong interference signal as components thereof; 
     forming, in a first electrical signal-connected first path apparatus, a second electrical signal in which said weaker desired radio frequency signal component is attenuated with respect to said nearby transmitter-sourced adjacent frequency strong interference signal component; 
     subtracting, in a first electrical signal-connected second path apparatus, said second electrical signal from said first electrical signal to form a third electrical signal of suppressed nearby transmitter-sourced adjacent frequency strong interference signal content and substantially unaltered weaker desired radio frequency signal content; 
     applying said third electrical signal of suppressed nearby transmitter-sourced adjacent frequency strong interference signal content and substantially unaltered weaker desired radio frequency signal content to an input port of a radio receiver apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical military aircraft together with several of the transmitting and receiving antenna locations often used in such aircraft. 
     FIG. 2 shows an active cancellation strong signal interference suppression arrangement according to the present invention. 
     FIG. 3 shows a manually operable strong signal interference suppression laboratory simulation apparatus disposed according to the present invention. 
     FIG. 4 shows a power transfer function relationship for a nonlinear material such as yttrium iron garnet usable in the present invention. 
     FIG. 5 shows an exemplary received signal relationship in a non amplitude responsive first signal path of the present invention. 
     FIG. 6 shows a processed signal relationship in an amplitude responsive second signal path of the present invention. 
     FIG. 7 shows a canceled signal relationship from a combination of FIG.  5  and FIG. 6 signals according to the present invention. 
     FIG. 8 shows a prior art strong signal interference suppression arrangement. 
     FIG. 9 shows typical small signal passband characteristics for a yttrium iron garnet signal enhancer device. 
     FIG. 10 shows typical above threshhold large signal passband characteristics for a yttrium iron garnet signal enhancer device. 
    
    
     DETAILED DESCRIPTION 
     The purpose of the present invention is to provide a low complexity reduced cost large signal interference suppression capability for receivers co-located with or in near proximity to transmitters on, for example, a common platform. Active cancellation interference suppression using frequency selective ferrite devices, as disclosed, herein allows the suppression to be accomplished without disruption of a transmitter signal path thus making interference mitigation a platform independent procedure. 
     In order to better appreciate the invention it is believed helpful to consider the military aircraft shown in the FIG. 1 drawing herein and the transmitter to receiver interference possibilities often found in such an aircraft. In the FIG. 1 drawing, a RC 135 reconnaissance military aircraft, as currently employed by the U.S. Air Force, is represented at  100  together with indications of multiple radio frequency energy transmitting and receiving antennas used in such aircraft. One engine of the FIG. 1 aircraft is indicated at  150  for reference. In the FIG. 1 drawing, for example, are represented multiple ultra high frequency and very high frequency (UHF and VHF) transmitter and receiver antennas as are used for communication and other mission related purposes on such aircraft. Such usage typically involves transmitter power levels of 100 watts and frequencies in the 20 to 500 Megahertz range. 
     Without delving into great and probably unneeded detail, the multiple antennas shown on the FIG. 1 aircraft  100  include the four aircraft underside-mounted UHF mucels antennas indicated at  102 , the seven aircraft underside-mounted P band antennas  104 , the two aircraft underside-mounted AT 741  system antennas  106 , the single aircraft underside-mounted AT 256  system antenna  108  and the three aircraft underside-mounted mucels system VHF low band antennas  110 . Additionally appearing on the FIG. 1 aircraft  100  are the three aircraft underside-mounted AT 1076  system antennas  112  (two antennas being located in the rightmost  112  position of FIG.  1 ), the three aircraft underside-mounted mucels system VHF high band antennas  114 , the mucles system test antenna  116 , the four aircraft underside-mounted muscles system UHF antennas  118 , the aircraft underside-mounted mucels all band antenna  120  and the aircraft side mounted G 369  system antenna  122 . 
     Additionally appearing on the FIG. 1 aircraft  100  are the rudder top mounted JTIDS antenna  124 , the rudder heel mounted patch antenna  126 , the aircraft fuselage top-mounted CHF 9  (IDM) antenna  125 , the short-wave long wire antenna  128 , the aircraft fuselage top-mounted SATCOM UHF antennas  130 ,  132  and  138 , the UHF 1 /VHF 1  antenna  136 , the aircraft fuselage top right-mounted and top left mounted UHF 3  and UHF 5  antennas  142  and  140  and the AIMS IFF AT 741  antenna  144 . Other antennas mounted on the aircraft  100  include the ultra high frequency/very high frequency transmitter antenna  146  and a plurality of radar and other antennas received in the aircraft nose cone radome  148 . Some of the signals communicating by way of the FIG. 1 antennas may be of a frequency hop, pulsed, spread spectrum, or other non-simple signal nature. 
     With the FIG. 1 illustrated numbers of transmitting and receiving antennas in frequent simultaneous use on certain military aircraft and indeed with some of the same antennas and systems being present in airline and corporate aircraft and in other civilian aircraft it is not difficult to recognized that interference between concurrently operated radio frequency transmitter and receiver equipment is to be expected. Such concurrent operation and real time data communication are a necessity for the mission of a FIG. 1 type of aircraft. Transmitter to receiver interference can therefore result from such real world occurrences as lower level parasitic oscillations and harmonic frequency signals being present in a transmitter signal, from the Fourier components of a pulse modulated signal, from signal leakage events along coaxial and other transmission lines, from equipment grounding difficulties and from other mechanisms. The limited availability of antenna shielding by aircraft parts, the limited physical separation of antennas achievable in the confines of an aircraft and the omni directional field strength patterns attending many antennas increase the tendency toward interference in the FIG. 1 aircraft. The use of wideband signal frequencies, adjacent frequencies or harmonically related frequencies also increases the tendency toward interference in the FIG. 1 aircraft and increases the magnitude of interfering signals. Such interference is not limited to aircraft mounted equipment and is often encountered in ships, ground vehicles, spacecraft (i.e., in “fluid supported” vehicles) and in small platform fixed-site situations as often occur at a military base, at airports and in other locations. The instant invention addresses an active arrangement for elimination of these interferences. 
     As recited above, the present day conventional technique for providing interference mitigation between common platform radio frequency transmitter and receiver apparatus involves installing a signal coupler in the energy path between a transmitter and the associated transmitting antenna as is illustrated in the FIG. 8 prior art drawing herein. Such couplers, as represented at  804  in FIG. 8, obtain a copy of the receiver-offending transmitter signal from some point along a transmission line  802  between transmitter  800  and antenna  806 . The copied transmitter signal is split into two parts and one of these parts is provided to the block  820  signal conditioner through transmission line  834 . The other part is provided via transmission line  832  to block  828 , the comparator. Simultaneously an error correction signal from transmission line  816  and coupler  817  is provided for signal comparison in block  828 . The comparator data is presented to block  830 , the signal controller, for conversion from its I and Q data form to phase and amplitude information. This information is then sent to block  820  to maintain the desired phase and amplitude in the offending interference signal provided by the transmission line  834  and conditioned in the block  820  conditioner. The copy of the offending signal (with proper amplitude and phase characteristics) is then injected into the receive path of transmission line  812  by the coupler  814  to prevent the interfering signal received at antenna  810  from entering the receiver  818 . 
     In contrast with the FIG.  8 -represented conventional signal cancellation technique, the present invention achieves signal suppression or cancellation of unwanted transmitter signals of magnitude above some predetermined threshold without requiring historical knowledge of the offending interfering signals. The invention also obviates the need to signal sample the offending transmitter to obtain a copy of the unwanted signal. The present invention suppression utilizes the non-linear characteristics of a device such as a ferrite (e.g. a Yttrium Iron Garnet ferrite) and disposes this ferrite device in a feedback loop location where its characteristic (of converting both the interfering large signal and the desired small signal to heat energy) can be advantageously used. 
     The present invention as is illustrated in the preferred embodiment of FIG. 2 couples a sample of both the offending (large) signal and desired (small) signal from the signal path  222  between the receiving antenna  230  and the low noise input amplifier of receiver  232  and feeds these signals into a signal separator device, a nonlinear ferrite device signal processor (e.g. a Yttrium Iron Garnet element) as represented at  202 . The utilized characteristic of such a ferrite signal separator device  202  is that it provides for a signal magnitude threshold effect. An incoming signal above this threshold will pass through the nonlinear ferrite device separator with some attenuation. A signal below this threshold will nearly all be dissipated as heat. The offending passed-through signal is then amplified to the proper amplitude and phase adjusted 180 degrees. It is then coupled back into the receiver path to achieve subtractive cancellation of the undesired large signal. The below-threshold signal of interest passes through to the receiver front end with minimal attenuation. 
     FIG. 2 therefore shows a preferred arrangement of the present invention. The FIG. 2 apparatus represents a two path active cancellation signal processor usable between an antenna and a receiver in a transmitter interference environment such as that shown in FIG. 1 of the drawings. In FIG. 2 signals from the receiving antenna  230  communicate along the path  210  to the receiver  232  and signals derived from this antenna to receiver path in the coupler  200  communicate along the multiple component inclusive path  212 . The signals appearing on path  210  are represented as the large and interfering transmitter signal  216  and the small and usually distally sourced desired receiver signal  218  in FIG. 2. A sample of each of these signals from coupler  200  is applied to a signal separator device  202  in which a ferrite element such as a film-disposed layer of yttrium iron garnet material is used to selectively propagate the combined signals according to a magnetostatic surface wave phenomenon. 
     The transfer function of the YIG device in block  202  of FIG. 2 is nonlinear in nature and results in a signal amplitude-determined separation of large and small components of the composite signal from coupler  200 . FIG. 4 in the drawings illustrates the nature of this amplitude responsive nonlinearity in a film-disposed ferrite material such as yttrium iron garnet. In FIG. 4 the varying power level range of an input signal is represented by the curve  402  which covers the input power range of −32 dBm to +13 dBm—as is measured along the horizontal scale  408 . The output power level provided by the surface wave coupling of the ferrite device is represented by the curve at  400  in FIG.  4  and is measured on the scale  406 . The curve  400  also extends from extreme left to right in the FIG. 4 drawing. The curves  400  and  402  appear coincident to the left of the point  408 . The FIG. 4 output power curve  400  also covers the input power range from −32 dBm to +13 dBm and results in output power levels of −70 dBm to 0 dBm. 
     In the lower power level portion of the FIG. 4 curve  400  at the point  404 , for example, there is little difference between the curves  400  and  402  and the curves coincide to show a nearly constant difference of −40 dBm between ferrite input and output power levels. Commencing at the threshold point  408  in FIG. 4, at an input power level of about −12 dBm and an output power level of −52 dBm, the increasing power level represented by the rightmost portion of the curve  402  input signal results in a nonlinear increasing response from the yttrium iron garnet material and significantly lower losses between input and output power levels ensues. At the FIG. 4 rightmost +13 dBm input power level point, for example, the power loss between input and output powers has decreased from −40 dBm to only about −13 dBm (i.e., an input power of +13 dBm on curve  402  provides a curve  400  output power of 0 dBm). 
     The FIG. 2 yttrium iron garnet element  202  in responding according to the FIG. 4 curves in the presence of a composite signal having one component signal below the threshold point  408  and one component above the threshold point  408  essentially absorbs or converts to heat energy the smaller signal while passing the larger signal with significantly lower losses (losses which are easily compensated by amplification as included at  204  in FIG.  2 ). The resulting signal includes only the larger of the two signals at  216  and  218 , i.e., the transmitter interference signal. 
     The yttrium iron garnet element of block  202  in FIG. 2 includes the customary magnetic bias flux provisions for a yttrium iron garnet element even though representations of such provisions are omitted from the FIG. 2 drawing. Additional details concerning such magnetic bias flux and the relationship between magnetic bias flux and the frequency response of a yttrium iron garnet element are disclosed in the U.S. Pat. No. 5,615,473 of M. Dydyk et al., a patent which is hereby incorporated by reference herein. Usable nonlinear characteristics are contemplated for the yttrium iron garnet element of the present invention over a frequency range of at least 50 megahertz to 18 gigahertz. Improved doping materials and techniques for the yttrium iron garnet material of block  202  are seen as a key to the achievement of wider range of frequencies for the yttrium iron garnet nonlinear characteristics. 
     The error feedback signal on path  214  in FIG. 2 is compared for magnitude and phase differences with the offending interference signal from transmission line  224  in the signal comparator  207 . The error feedback signal on path  214  is derived in the directional coupler or coupler  234  from the difference between the injected cancellation signal of path  228  and the received large signal  216  entering the antenna  230  and appearing on the antenna to receiver path  210 . The comparator data is presented to block  204 , the signal controller, for conversion from its I and Q data form to phase and amplitude information. In the I and Q data form, I data is the in-phase comparator output and Q the quadrature comparator output. The direction of signal coupling for the directional coupler  234  of FIG. 2 is indicated by the coupler-adjacent arrow shown at  235 , such arrows repeat for other drawing-illustrated couplers. The resistor shown at  237  in FIG. 2 represents a characteristic impedance termination or load for the directional coupler  234  and is also repeated for other couplers shown in the drawings. 
     The phase and amplitude information from the block  204  signal controller is sent to the signal conditioner of block  206  to maintain the desired phase and amplitude of the processed offending signal or cancellation signal output on transmission line  228 , i.e., to maintain the desired phase and amplitude of the cancellation signal processed from the offending interference signal received on transmission line  226 . The processed offending signal or cancellation signal on transmission line  228  is injected into the receiver path  210  via coupler  208  to prevent the undesired interfering signal received via antenna  230  from entering the receiver  232 . 
     By way of the active interference cancellation two path signal processing shown in FIG. 2 including the amplitude selective “filter” provided by the yttrium iron garnet ferrite device of block  202 , an interfering large transmitter signal impressed on the receiver antenna has been removed and precluded from saturating or otherwise blocking the input stage(s) of a receiver  232  while attempting to receive an adjacent frequency low level signal  218  arriving from a distant source. Notably this active interference cancellation is achieved in the FIG. 2 present invention apparatus through use of signals collected from the receiver antenna and without need for a sample of the offending transmitter signal obtained directly from the interfering transmitter or its output energy transmission line. The FIG. 2 active interference cancellation has also been achieved without need for the significant amount of host platform characterization needed with the conventional signal cancellation practiced prior to the present invention. 
     Since host platform characterization as used with conventional signal cancellation arrangements involves measuring distance separation between transmitters and receivers and determining actual energy levels attributable to offending transmitters in a receiver&#39;s input path along with accomplishing these activities for each channel where signal suppression is required, it may be appreciated that eliminating the need for such tedious characterization is a significant advantage of the present invention. The present invention allows a given receiver to maintain it&#39;s sensitivity in the presence of a co-located transmitter operating simultaneously. 
     FIGS. 9 and 10 of the drawings show the frequency versus attenuation passband characteristics provided by a yttrium iron garnet film signal enhancer of the type usable at  202  in the FIG. 2 drawing. In the presence of an input signal of amplitude below a −10 dBm threshold, the yttrium iron garnet device has a frequency independent, substantially flat passband characteristic as is shown in the FIG. 9 drawing. In the presence of an input signal sufficiently large to exceed a −10 dBm device threshold however the yttrium iron garnet film device provides the frequency selective, diminished attenuation characteristic represented at  1001  in FIG.  10 . The diminished attenuation characteristic  1001  is found to be of about five megahertz in frequency width, of frequency location determined by the input signal frequency and also somewhat dependent on the yttrium iron garnet material&#39;s composition and construction. In applying the yttrium iron garnet film device represented in FIG. 10 to an apparatus of the FIG. 2 type it is, of course, preferable for the desired signals to be of a frequency located in the regions  1003  and  1005  i.e., located below or above the frequency of the diminished attenuation characteristic  1001 . The −40 dB attenuation indicated at  900  in FIG.  9  and at  1000  in FIG. 10 is the typical attenuation encountered by a below-threshold signal applied to the yttrium iron garnet device. 
     FIG. 3 in the drawings shows in block diagram form a manually adjustable laboratory apparatus useful for achieving a controlled environment better understanding of the invention and its performance and capabilities. In the FIG. 3 drawing the large interfering transmitter signal and small desired signal are represented at  300  and  302  respectively and a 3 dB coupling circuit for these signals represented at  304 . The two signal paths discussed in connection with the FIG. 2 drawing are represented at  318  and  320  in the FIG. 3 drawing and include the first path yttrium iron garnet signal separator “amplitude filter” of block  308  and the second path attenuator and phase shifter  310  and  311 ; these two paths are provided by a 10 dB coupler circuit represented by block  306 . Notably in the FIG. 3 apparatus the direct path  320  signal is obtained by way of signal coupling in the 10 dB coupler  306  while the “indirect path”  318  signal is provided by an attenuated straight-through signal in the coupler  306 . A second 3 dB coupler at  312  provides subtractive recombination of the FIG. 3 first and second path signals into an output signal appearing on path  313 . A spectrum analyzer  316  and associated attenuator or amplifier devices as shown at  314  may be used to appraise the effectiveness of the FIG. 3 apparatus in removing large transmitter interference signals without significant effect on the processed signals. 
     FIG. 5, FIG.  6  and FIG. 7 in the drawings show graphic representations of the performance of a signal cancellation apparatus according to the present invention as this performance is demonstrated by a simulation apparatus of the FIG. 3 manually-adjusted type. In these FIG.  5 , FIG.  6  and FIG. 7 drawings each horizontal division represents a frequency change of 5.0 megahertz or 0.005 gigahertz with the center frequency being 3.000 gigahertz. Each vertical divisions in these drawings represents an amplitude change of 10.0 dB with the top most horizontal line representing a 0.0 dBm reference level. In the FIG. 5 drawing which represents signal along the direct channel path  320  in FIG. 3, and therefore is also representative of the composite input signal received from the output of the 3 dB coupler  304 , a strong interference signal is represented by the center-most 3.000 gigahertz waveform at  500  and the desired lower level receiver signal is represented by the 20 dB lesser amplitude 3.01 gigahertz waveform at  502 . 
     The FIG. 5 waveforms are applied to the yttrium iron garnet “amplitude filter”  308  in FIG. 3 where the desired lower level receiver signal represented at  502  in FIG. 5 is attenuated with respect to the strong interference signal at  500  by the above-described characteristics of the yttrium iron garnet film device. Phase adjustment of one FIG.  5  and FIG. 6 signal to enable their direct subtraction in the 3 dB coupler  312  is accomplished in the direct channel path  320  in FIG. 3; amplitude adjustment preparing for this subtraction is performed in the block  310 . 
     The FIG. 6 yttrium iron garnet film-processed signal is subtracted from the FIG. 5 direct channel signal in the 3 dB coupler  312  in FIG. 3 to provide the output waveform represented in the FIG. 7 drawing. As illustrated in the FIG. 7 drawing the interfering waveform signal at  500  in FIG. 5 has been substantially removed by the FIG. 3 cancellation signal processing. The remainder portion of the interfering waveform signal at  500  in fact is shown in FIG. 7 to have an amplitude of only some 3 dB following the FIG. 3 signal processing. The waveforms of FIG. 7 as well as FIGS. 5 and 6 are obtained by way of the spectrum analyzer instrumentation indicated at  316  in FIG.  3 . Signal level attenuation needed for adjusting signal magnitudes from levels compatible with the threshold of yttrium iron garnet device  308  to levels within range of the spectrum analyzer instrument  316  is provided in block  314 . 
     The disclosed invention therefore is believed to provide a continuous wave transmitter interference signal cancellation arrangement especially useful in a small platform situation such as is encountered in an aircraft or other vehicle. The signal cancellation is accomplished in a two path signal processing in which signal separation is performed by attenuation of the desired small signal using a nonlinear ferrite device characteristic and a feedback arrangement. 
     While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.