Abstract:
Radio frequency transmission systems often suffer from the problem of co-site interference, where the frequency band of a strong radio transmitter overlaps with the frequency band of a co-located and/or remote radio receiver, such that the transmitter interferes with the ability of the receiver to detect a weak signal of interest. There is a need for a device that can process both the transmitted radio signal and the received radio signal to eliminate such interference. Previous attempts to solve this problem have been unable to cancel in-band interference in excess of 20 to 40 dB stronger than the signal of interest over a broad bandwidth, with large dynamic range, and with a high degree of linearity. Disclosed is a robust system and method for cancelling broadband in-band RF interference that operates in a dynamically changing multipath environment.

Description:
CROSS-REFERENCE TO PRIOR FILED APPLICATION 
     This application claims priority to earlier filed U.S. provisional patent application No. 61/488,521 filed on May 20, 2011, which is herein incorporated by reference in its entirety. 
    
    
     UNITED STATES GOVERNMENT RIGHTS 
     This invention was made with government support under Subaward #96183NBS68 from Booz Allen Hamilton, Inc. to Princeton University (PRIME: U.S. Army, Grant #W15P7T-06-D-E401) and Subaward #S12-119176 from CACI Technology, Inc. to Princeton University (PRIME: U.S. Army—Fort Monmouth, Grant #TESS W15P7T-09-D-P013). The government has certain rights in this invention. 
    
    
     FIELD OF INVENTION 
     The present disclosure generally relates to a new system and method for broadband RF interference cancellation that will allow co-located and/or remote interferers and communication equipment to operate in harmony. 
     BACKGROUND 
     Due to exponential growth in the demand for radio frequency (RF), the radio spectrum is extremely crowded and becoming more crowded every day. Multiple wireless systems are allocated in close proximity or even in the same radio spectrum. As a result, optimum performance of one system cannot be achieved due to interference caused by another system, including narrowband interference of a wideband signal, remote wideband interference, and co-site interference. Each of these interference-related issues is challenging and critically important to efficient spectrum use. For maximum utilization of wireless equipment a system that seamlessly allows existing communication equipment to operate in harmony with interfering transmitters is required. 
     SUMMARY OF THE INVENTION 
     A system and method for broadband RF interference cancellation are disclosed. The system and method allows co-located and/or remote interferers and communication equipment to operate in harmony including operation on the same channel. The disclosed interference cancellation system (ICS) substantially reduces interference that cannot be removed by receiver RF front end filters. 
     A coherent approach uses a dual parallel electrical RF signal to optical signal converter (converter), e.g., a dual drive Mach Zehnder modulator. Cancellation is accomplished by destructive interference of the optical field, rather than by incoherent addition of intensities. The result is annihilation of the optical signal rather than adding to a quiescent DC optical level. The advantages include improvement of SNR by removing the DC pedestal, and increased linearity and dynamic range due to the use of linear phase modulation rather than nonlinear intensity modulation. The advantages also include elimination of the S 21  mismatch problem entirely. Matched filtering between the transmitted and received signal may also be done electrically. 
     An optical matched filter may be integrated into one of the arms of the Mach Zehnder modulator. This allows for the whole system to reside on a single chip. As this places a limit on the length of the delay, the optical matched filter may be used for fine tuning in conjunction with electrical matched filtering. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a system architecture block diagram showing a radio frequency (RF) interferer, radio transmitter/receiver (T/R) and an interference cancellation system (ICS); 
         FIG. 2  is a block diagram of an interference canceller and processor; 
         FIG. 3  is a detailed system diagram; and 
         FIG. 4  is a block diagram of an interference canceller and processor including an on-chip optical matched filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Disclosed is a system and method that allows co-located and/or remote interferers and communication equipment to operate in harmony.  FIG. 1  is a schematic of an example system architecture and illustrates the relationship between a radio frequency (RF) interferer  20 , interference cancellation system (ICS)  30  and radio  40 . Power sources for these devices are not shown for purposes of clarity. The provision of appropriate power for such devices, e.g., AC or DC power, from the power grid batteries or other sources, is well known to those skilled in the art. The examples disclosed herein focus on the radio  40  receive functionality. However, it should be understood that the radio  40  may be capable of both transmit and receive functionality. The ICS  30  is configured to substantially reduce jamming interference that cannot be removed by known radio receiver RF front end filters. 
     The RF interferer  20  includes an interferer antenna  22  configured to transmit an interferer signal shown graphically by arrow  23 . It should be understood that a wide variety of RF ramming devices having a variety of signal formats may be used, e.g., random noise, random pulse, stepped tones, warbler, random keyed modulated CW, tone, rotary, pulse, spark, recorded sounds, gulls, sweep-through techniques and the like. An interferer sample coupler  24  is configured to generate an interferer reference signal  26 . In this example, the interferer sample coupler  24  is coupled between the RF output  28  of the RF interferer  20  and the interferer antenna  22 . It should also be understood that a variety of sampling devices may be used without departing from the scope of the invention, including devices tapped into various locations in the RF interferer circuitry or output stages. It should be understood that RF interferer  20  may have a variety of other inputs, outputs and controls that are not shown. The provision of such features is well known to those skilled in the art. 
     The ICS  30  has an ICS antenna  32 . In this example, the ICS antenna  32  is configured to receive RF signals such as an interferer signal plus a desired signal as shown by arrow  33 . The interferer reference signal  26  is coupled to the ICS  30  interferer reference input  34 . The ICS  30  generally has an interference canceller and processor  36  as described below. The ICS  30  also has a desired signal output  37  and a transmit/receive (T/R) control input  38 . The ICS  30  is configured to remove at least a portion of the interferer signal  23  from the signal received by ICS antenna  23 . The resulting signal is output via the desired signal output  37  and is effectively the desired signal, e.g., an RF transmission of interest, with the interferer signal  23  significantly reduced in amplitude. In typical applications, the ICS may be capable of a 35-50+ dB reduction in the jamming signal  23  at the desired signal output  37 . 
     The radio  40  has an antenna input  42  coupled to the ICS  30  desired signal output  37 . The radio  40  also has a transmit/receive (T/R) control output  44 , e.g., coupled via a T/R bypass switch, coupled to the ICS  30  transmit/receive (T/R) control input  38 . It should be understood that radio  40  may have a variety of other inputs and outputs, e.g., voice and data ports, as well as a variety of controls that are not shown. The provision of such features is well known to those skilled in the art. 
     The ICS  30  generally includes an interference canceller and processor  36 . The interference canceller and processor  36  performs RF to optical conversion with a dual parallel electrical RF signal to optical signal converter (converter), e.g., a Dual Parallel Mach-Zehnder modulator (DPMZ), configured for a “Coherent Optical” cancellation approach as shown in  FIG. 2 . The advantages of the DPMZ in this configuration are improved RF amplitude and phase tracking, minimal DC offset, and reduced distortion as compared with systems using two nearly identical MZ modulators on the same chip. It should be understood that other converters may be used to implement an interference canceller based on the disclosure herein, e.g., a Dual Drive Mach-Zehnder (DDMZ). In this example, the interference canceller and processor  36  includes a laser light source (laser)  50  having an output  52  coupled to input  61  of DPMZ  60 . The output  79  of the DPMZ  60  is coupled to a photo detector  80 . The output  82  of the photo detector  80  may then be coupled to a radio antenna input, e.g., antenna input  42  of radio  40  ( FIG. 1 ). It should be understood that various filtering and/or amplitude adjustments may be implemented in the various optical links between the laser  50 , DPMZ  60 , photo detector  80  and the antenna input  42 . 
     The DPMZ  60  can be implemented using a crystal, such as lithium niobate, whose refractive index varies as a function of the strength of the local electric field. Suitable converter units may be obtained from various manufacturers including JDS UNIPHASE Corporation (www.jdsu.com) of Milpitas, Calif., COVEGA TECHNOLOGY (now THORLABS—www.thorlabs.com) and FUJITSU (www.fujitsu.com) of Tokyo Japan. The DPMZ  60  includes an input  61  and output  79 . Two optical paths are defined between the input  61  and output  79 . The DPMZ includes a splitter  62  feeding a first arm  64  and a second arm  66 . The first and second arms  64 ,  66  terminate at a combiner  63 . The first and second arms  64 ,  66  include first and second electrodes  73 ,  75  coupled to input terminals  74 ,  76  respectively. Input terminals  74 ,  76  are used to vary the electric field and therefore the refractive index of the first and second arms  64 ,  66  respectively. For matters of simplicity, ground terminals are not shown. Each arm  64 ,  66  functions as a linear phase modulator. The second arm  66  also includes a phase compensator  77  that is configured to shift the phase of the light traveling through the second arm by 180 degrees. The phase compensator  77  may be externally adjustable via the phase compensator terminal  78 , e.g., adjusted based on the laser frequency and other factors. In general, the interferer+desired signal (output of ICS antenna  32 ) is coupled to the first terminal  74 . The interferer reference signal  26  is coupled to second terminal  76 . 
     In operation, light from laser  50  enters the DPMZ input  61  and is split between arms  64  and  66 . With two identical RF signal inputs coupled to the input terminals  74 ,  76 , the DPMZ optically cancels the carrier, resulting in RF cancellation (zero light output). If a desired signal is present along with the interferer signal, the interferer signal is optically cancelled by the DPMZ and the desired signal with the interferer signal significantly reduced is output via DPMZ output  79 . The disclosed coherent optical approach generates minimal DC offset compared to non-coherent approaches, which cancel only the RF envelope but not all the light (carrier), leaving a residual DC offset at the photo detector output. The disclosed coherent optical approach converts the interferer reference signal  26  to optical using a single laser modulator, providing better linearity than the incoherent MZ modulator approaches. 
       FIG. 3  is a detailed system diagram. The system includes an RF interferer  120  having an output  121  coupled to an interferer antenna  122  via an interferer sample coupler  124 . The output of the interferer sample coupler  124  is coupled to a laser modulator  127 . A length of optical cable may be used to provide the interferer reference signal  126  to a tapped delay line  129  with a delay very close to the antenna coupling delay, to minimize dispersion for broadband cancellation in addition to RF isolation. This approach also relies on optical delay and weighting of the interferer reference signal  126 . 
     Variable optical attenuators and delays may be used for the weighting network  131  to achieve the RF phase shift and delays that are needed for RF signal cancellation. The tapped delay line/weighting network summed as shown by block  133 . The output of the summer  133  is converted back to RF using a photodiode detector  135  for minimal distortion. The resulting signal is coupled to electrode  176  of DPMZ  137 . 
     On the receive side, transceiver antenna  132  receives the interferer signal and the desired signal as shown by arrow  134 . The received signal passes through T/R bypass switch  147  and is coupled to electrode  174  of DPMZ  160 . The inputs to electrodes  174  and  176  are used by the DPMZ  160  for coherent cancellation of the interferer signal. The cancelled interferer signal residue plus desired receive signal are output via DPMZ output  179 . The DPMZ output  179  is converted back to RF using a photo diode  143  as shown. The resulting signal is coupled to the radio antenna input  142 . A sample of the cancelled output, filtered and correlated with the sample of the interferer signal, is provided by coupler  157 . This signal functions as a control signal for the tapped delay and weighting networks to minimize the jamming signal. The output of the coupler  157  is routed to a preselect filter  155 . The resulting filtered output is correlated by block  153  and is routed via an RF connector to weighting network  151 . A weighted control signal is then routed from the weighting network  141  to summer  133 . A portion of the interferer reference signal  126  is routed to a photodiode detector  149  and then a preselect filter  151 . The resulting filtered output is correlated by block  153  as discussed above. The radio also includes a T/R control output that is coupled to a control interface  161 . The control interface  161  generates outputs that are coupled to the preselect filter  155  and the T/R bypass switch  159 . 
     In general, the adaptive control loop amplitude and phase control inputs are supplied through correlation of the interferer signal sample with a sample of the summed weighted interferer and coupled interferer signals at the transceiver input. Both the interferer sample signal and the cancelled interferer plus desired receive signal are converted to RF using photodiodes and correlated using an RF correlator. Any resultant interferer signal present at the transceiver input causes a correlator output, which is then used to control both amplitude and phase of the weighting network. The loop controls both amplitude and phase for zero correlator output, indicating a completely cancelled interferer signal. Any DC offsets in the control loop reduce the cancellation depth. These DC offsets are due to RF coupling of the interferer signal into the ICS correlator input path, in addition to component DC offsets. The interferer cancellation depth is a function of the correlator dynamic range. 
     The disclosed coherent ICS provides interferer multipath cancellation for the second and third multipath coupling, since the larger multipath delays are considerably lower in amplitude due to the higher path loss. A tapped delay line with weighted taps provides the delays and phasing necessary for direct and multipath cancellation. The tapped delay line implementation can be achieved optically or using RF components. RF-only ICS techniques are limited in cancellation bandwidth due to the RF component amplitude and phase dispersion vs. frequency. 
     With the specific embodiment described, multipath cancellation would typically be performed by adaptive matched filtering in the electrical domain, prior to the Dual MZ Modulator, as shown in  FIG. 3 . Any bandwidth limitation of such an electrical filter compared to the bandwidth of the optical cancellation in the MZ Modulator may be overcome by integration on the modulator chip of an optical waveguide-based adaptive matched filter, including the appropriate weights and delays to compensate for the multipath channel characteristic. 
       FIG. 4  shows a DPMZ  260  implemented with an optical adaptive matched filter  286 . The DPMZ  260  includes an input  261  and output  279 . Two optical paths are defined between the input  261  and output  279 . The DPMZ includes a splitter  262  feeding a first arm  264  and a second arm  266 . The first and second arms  264 ,  266  terminate at a combiner  263 . The first and second arms  264 ,  266  include first and second electrodes  273 ,  275  coupled to input terminals  274 ,  276  respectively. Input terminals  274 ,  276  are used to vary the electric field and therefore the refractive index of the first and second arms  264 ,  266  respectively. For matters of simplicity, ground terminals are not shown. Each arm  264 ,  266  functions as a linear phase modulator. The second arm  266  includes a phase compensator  277  that is configured to shift the phase of the light traveling through the second arm by 180 degrees. The phase compensator  277  may be externally adjustable via the phase compensator terminal  278 , e.g., adjusted based on the laser frequency and other factors. The second arm also includes an optical adaptive matched filter  286  configured to supplement the adaptive electrical matched filtering at the front-end. The optical adaptive matched filter  286  may be externally adjustable via the optical adaptive matched filter compensator terminal  284 , e.g., adjusted based on the laser frequency and feedback from the adaptive elements, as well as other factors. In general, the interferer+desired signal, e.g., output of ICS antenna  32 , is coupled to the first terminal  274 . The interferer reference signal, e.g., as shown by reference number  26  in  FIG. 1 , is coupled to second terminal  276  as discussed above in connection with  FIG. 2 . 
     In operation, light from laser  250  enters the DPMZ input  261  and is split between arms  264  and  266 . With two identical RF signal inputs coupled to the input terminals  274 ,  276 , the DPMZ optically cancels the carrier, resulting in RF cancellation (zero light output). If a desired signal is present along with the interferer signal, the interferer signal is optically cancelled by the DPMZ and the desired signal with the interferer signal significantly reduced is output via DPMZ output  279 . 
     The converter may be configured with an optical adaptive matched filter with a series of optical weights and delays. The optical adaptive matched filter may be based on a photonic implementation of a finite impulse response (FIR) filter, which is a common and well-known filter used for signal processing. In conjunction with RF matched filtering, the adaptive optical filter may aid in the cancellation of multipath reflections. The adaptive optical matched filter, along with a front-end RF matched filter, may compensate for the aggregate effect of multipath reflections by emulating the channel response of the environment. Such multipath compensation is achieved via a series of taps and delays, both in the RF filter as well as the optical filter. The optical matched filter achieves the weighting and delaying effects via arrays of variable optical attenuators and optical delay lines. 
     The use of both an RF matched filter and an adaptive optical matched filter allows for coarse and fine-tuning (respectively) of multipath compensation. The RF/electrical matched filter at the front-end provides the ability to coarsely adjust multipath compensation through the use of traditional digital signal processing (DSP)-based filtering algorithms. A suitable electrical matched filter may be implemented with a series of weights and delays. In operation the electrical match filter sums the various taps together at the filter output. In effect, the electrical matched filter roughly approximates the channel response between the interferer and receiver, and applies this to the interferer reference signal. This modified reference signal is then fed to the optical matched filter via terminal  276 . The optical adaptive matched filter is similar to the electrical filter in that it applies a series of weights and delays to the input signal. The matched optical filter is located in the bottom arm of the DPMZ ( 266 ). Specifically, the optical filter begins at terminal  284 . A 1:n optical splitter splits the optical signal n ways. These n signals then enter an n-channel array of variable optical attenuators, where each of the n signals can be individually attenuated by some amount. Each of the attenuated signals is then delayed by some fixed amount, and the weighted+delayed signals are then “summed” by a single mode to multi-mode (SM:MM) optical coupler. The signal from both arms ( 264  and  266 ) are then combined at terminal  263 , and the total signal is then to a multi-mode photodetector where the desired signal is then converted back to the electrical domain. The purpose of using an RF matched filter in conjunction with an adaptive optical matched filter is that DSP-based filtering is able to accommodate large delay adjustments that optical components cannot provide. Essentially, the electrical matched filter provides a coarse approximation of the channel response, and then the optical adaptive matched filter provides the fine-tune adjustments to the interferer reference signal, such that the interferer reference signal matches the interferer signal nested within the (interferer+desired) signal. 
     The following papers are related to the invention and are incorporated by reference in their entirety as if fully set forth herein: John Suarez, Paul R. Prucnal, “Incoherent Method of Optical Interference Cancellation for Radio Frequency Communications”, IEEE Journal of Quantum Electronics, Vol. 45, NO. 4, pp. 402-408; John Suarez, Paul R. Prucnal, “System Level Performance and Characterization of Counter-phase Interference Cancellation”, Journal of Lightwave Technology, Vol. 28, Issue 12, pp. 1821-1831 (2010); Ward, M. J., Keefer, C. W., Andrews II, H. G., “Design and Fabrication of a Multichannel Adaptive OPTICAL Processor (MADOP)”, In-House Report, RL-TR-92-333, December 1992; H. Brahimi, P. H. Merrer, and O. Llopis, “CAD of Microwave Optical Systems for Time and Frequency Applications”, LAAS-CNRS, Toulouse University, France, 2006; and T. Akajoki, O. Pekonen, and A. Tanskanen, “Model Optical Transmitters with a Circuit Simulator”, Microwaves &amp; RF, April 2005 
     Although features and elements are described above in particular combinations, each feature or element may be used alone without the other features and elements or in various combinations with or without other features and elements.