Abstract:
A system and method for mitigating co-channel interference is disclosed. A radar system detects targets from received signals at an antenna array. The received signals include direct signals and target signals transmitted from remote transmitters. An antenna array receives the signals. A signal processing system is coupled to the antenna array to perform processing operations on the received signals. The processing system includes a primary cancellation component and a secondary cancellation component. A primary illuminator signal is cancelled from the received signals by the primary cancellation component. An adaptive beam former obtains a secondary illuminator signal from the received signals. A reference regenerator regenerates the secondary illuminator signal. An adaptive cancellation filter removes noise from the secondary illuminator signal. The secondary cancellation component mitigates co-channel interference by canceling the secondary illuminator signal from the received signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 60/288,448 entitled “System and Method for Co-Channel Interference Mitigation for PCL Applications,” filed May 4, 2001, which is hereby incorporated by reference. This application also claims benefit of U.S. Provisional Patent Application No. 60/288,451 entitled “System and Method for Wideband Pre-Detection Signal Processing for PCL Applications,” filed May 4, 2001, which is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a passive coherent location (“PCL”) radar system and method, and more particularly, to a system and method for mitigating co-channel interference of received signals for PCL radar applications.  
           [0004]    2. Discussion of the Related Art  
           [0005]    PCL radar systems may be represented by a multistatic radar system. A multistatic radar system has many receivers that are separated from one or more transmitters. The radiated signal from a transmitter arrives at a receiver via two separate paths. One path may be a direct path from the transmitter to the receiver, and the other path may be a target path that includes an indirect path from the transmitter to a target to the receiver. Measurements may include a total path length, or transit time, of the target path signal, the angle of arrival of the target path signal, and the frequency of the direct and target path signals. A difference in frequency may be detected if the target is in motion according to a doppler effect.  
           [0006]    Knowledge of the transmitted signal is desirable at the receiver if information is to be extracted from the target path signal. The transmitted frequency is desired to determine the doppler frequency shift. A time or phase reference also is desired if the total scattered path length is to be determined. The frequency reference may be obtained from the direct signal. The time reference also may be obtained from the direct signal provided the distance between the transmitter and the receiver is known.  
           [0007]    Multistatic radar may be capable of determining the presence of a target within the coverage of the radar, the location of the target position, and a velocity component, or doppler, relative to the radar. The process of locating the target position may include a measurement of a distance and the angle of arrival. The measurement of distance relative to the receiving site may desire both the angle of arrival at the receiving site and the distance between transmitter and receiver. If the direct signal is available, it may be used as a reference signal to extract the doppler frequency shift.  
           [0008]    In PCL radar systems, transmitters may be known as illuminators. Illuminators may be wideband sources of opportunities that include commercial frequency modulated (“FM”) broadcast transmitters and/or repeaters, commercial high-definition television (“HDTV”) broadcast transmitters and/or repeaters, and the like. Techniques for wideband signal pre-detection processing and co-channel interference mitigation exist. Known approaches include an array of antennas used to receive the source of opportunity to be exploited, such as the primary illuminator, and any other co-channel signals present in the environment.  
           [0009]    Co-channel signals may include multipath images of the illuminator signal, delay and Doppler-shifted reflections of the illuminator from targets in the region under survelliance, and other distant broadcast sources at the same operating frequency as the primary illuminator. Targets may include aircraft, space launch vehicles, and the like. The intent of the co-channel mitigation techniques is to eliminate the undesirable sources from the received antenna outputs, such as the strong direct path and multipath components of the exploited illuminator, while leaving the reflected signals from targets of interest unattenuated. Thus, is desirable to improve co-channel mitigation techniques to better identify and track targets, and to determine target location, range and velocity.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, embodiments of the present invention are directed to a PCL application and method for signal processing within the PCL application.  
           [0011]    Thus, the present invention is directed to a system and method for mitigating co-channel interference. According to an embodiment, a method for mitigating co-channel interference in co-channel signals in a bistatic radar is disclosed. The method includes identifying a primary illuminator signal from a primary illuminator. The primary illuminator signal comprises a frequency modulated carrier at a given frequency. The method also includes regenerating the primary illuminator signal. The method also includes canceling the primary illuminator signal from the co-channel interference signals. The method also includes identifying a secondary illuminator signal from a secondary illuminator. The secondary illuminator signal comprises a frequency modulated carrier at the given frequency. The method also includes regenerating the secondary illuminator signal from the co-channel signals.  
           [0012]    According to another embodiment, a method for mitigating co-channel interference is disclosed. The method includes canceling a primary illuminator reference signal. The method also includes canceling a secondary illuminator reference signal.  
           [0013]    According to another embodiment, a method for mitigating co-channel interference in a bistatic radar receiving co-channel signals comprising target signals reflected by targets and direct signals transmitted by remote transmitters is disclosed. The method includes receiving the co-channel signals at an antenna coupled to the bistatic radar. The method also includes performing adaptive beam forming to obtain a primary illuminator reference signal. The primary illuminator reference signal is from the direct signals and comprises a frequency modulated carrier at a given frequency. The method also includes regenerating the primary illuminator reference signal from the co-channel interference signals. The method also includes performing adaptive beamforming to obtain a secondary illuminator reference signal. The secondary illuminator reference signal is from the direct signals and comprises a frequency modulated carrier at the given frequency. The method also includes regenerating the secondary illuminator reference signal from the co-channel signal from the co-channel signals. The method also includes canceling the secondary illuminator reference signal from the co-channel interference signals.  
           [0014]    According to another embodiment, a system for mitigating co-channel interference is disclosed. The system includes an antenna array to receive signals. The system also includes a primary cancellation component to cancel a primary illuminator reference signal from the received signals. The system also includes a secondary cancellation component to cancel a secondary illuminator reference signal from the received signals.  
           [0015]    According to another embodiment, a method for detecting targets by a bistatic radar system using transmitted signals and reflected signals from the targets is disclosed. The method also includes converting the received signals into co-channel signals. The method also includes adaptive beamforming a secondary illuminator signal from the co-channel signals. The method also includes regenerating the secondary illuminator signal. The method also includes canceling the secondary illuminator signal from the co-channel signals and mitigating co-channel interference.  
           [0016]    According to another embodiment, a method for mitigating interference in a bistatic radar that receives direct path signals and target path signals transmitted as commercial broadcast signals from remote transmitters is disclosed. The target path signals are reflected off targets such that the target path signals have a doppler shift with reference to the direct path signals. The method includes identifying a secondary illuminator signal within the direct path signals. The method also includes cancelling the secondary illuminator signal from the received signals.  
           [0017]    According to another embodiment, a bistatic radar system that detects targets from received signals at an antenna, the received signals include direct signals and target signals transmitted from remote transmitters. The bistatic radar system includes an adaptive beamformer to obtain a secondary illuminator signal from the received signals. The bistatic radar system also includes a reference regenerator to regenerate the secondary illuminator signal. The bistatic radar system also includes an adaptive cancellation filter to mitigate co-channel interference by canceling the secondary illuminator signal.  
           [0018]    Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or maybe learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitutes a part of this specification, illustrate embodiments of the present invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0020]    [0020]FIG. 1 illustrates a radar detection system including a PCL system, a target, and transmitters in accordance with an embodiment of the present invention;  
         [0021]    [0021]FIG. 2 illustrates a block diagram of a PCL system in accordance with an embodiment of the present invention;  
         [0022]    [0022]FIG. 3 depicts a signal processing for mitigating co-channel interference system in accordance with the present invention; and  
         [0023]    [0023]FIG. 4 depicts a flowchart of a system in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0025]    [0025]FIG. 1 depicts radar detection system  10  including a PCL system, one or more targets of interest, and a plurality of transmitters in accordance with an embodiment of the present invention. PCL system  100  represents a family of multi-static wide area moving target surveillance sensors. PCL system  100  exploits continuous wave (“CW”) electromagnetic energy, often from sources of opportunity that may be operating for other purposes. Sources of opportunity may include television broadcast stations and FM radio stations. Preferably, PCL system  100  may receive transmissions from a plurality of uncontrolled transmitters such as sources of opportunity,  111 ,  112 , and  114 . More preferably transmitters  110 ,  112 , and  114  may be wideband sources of opportunity that include commercial FM broadcast transmitters and/or repeaters and commercial HDTV TV broadcast transmitters and/or repeaters. Transmitters  110 ,  112 , and  114 , however, are limited to these sources of opportunity any may include any device, system or means to transmit uncontrolled signals.  
         [0026]    Transmitters  110 ,  112 , and  114  may transmit wideband electromagnetic energy transmissions in all directions. Some of these transmissions are reflected by one or more targets of interest  150  and received by PCL system  100 . For example, reflected transmission  130  may be reflected by target  150  and received by PCL system  100 . Further, with regard to transmitter  114 , reference transmission  140  is received directly by PCL system  100 . PCL system  100  may compare reference transmission  140  and reflected transmission  130  to determine positional information about one or more targets of interest  110 , as discussed above. Positional information may include any information relating to a position of target  110 , including location, velocity, and acceleration, and derived according to processes known to one skilled in the art.  
         [0027]    [0027]FIG. 2 depicts a block diagram of a PCL system in accordance with an embodiment of the present invention. PCL system  100  may include antenna subsystem  200 , (“ADC”) analog to digital converter subsystem  220 , processing subsystem  240 , and output device  260 . Antenna subsystem  200  receives electromagnetic energy transmissions, including reflected transmission  130  and reference transmission  140 , with at least one antenna. ADC subsystem  220  receives the output of antenna subsystem  200  and outputs digital samples of the signal at its input by sampling the signal at a sampling rate and forming a digital waveform using the magnitude for the analog signal at each sampling interval. Processing subsystem  240  receives the output of assembly subsystem  220  and processes the signal accordingly. Output device  260  receives the processing result and displays the output of processing subsystem  230 .  
         [0028]    [0028]FIG. 3 depicts a block diagram of a signal processing system  300  for mitigating co-channel interference in accordance with an embodiment of the present invention. System  300  is configured to mitigate, or reduc, co-channel interference when exploiting wideband sources of opportunity. System  300  is used within a passive coherent location system that detects targets and processes information for the targets, such as time delay, doppler, angle of arrival, and the like. Wideband sources of opportunity may include commercial FM broadcast transmissions or commercial HDTV TV broadcast transmissions. Thus, FM and HDTV TV broadcast transmitters and/or repeaters may be the illuminators for the PCL system that incorporates system  300 .  
         [0029]    Referring to FIG. 3, signal processing system  300  receives signals  340  and  342  from transmitters, or illuminators,  310  and  312 , respectively. As noted above, signals  340  and  342  preferably may be FM or HDTV signals. Signals  340  and  342  are received by PCL system  100 , as depicted in FIGS. 1 and 2. Signal processing system  300  is part of PCL system  100 . More particularly, signal processing system  300  may be incorporated into processing sub-system  240 . Alternatively, signal processing system  300  may be incorporated into ADC subsystem  220 .  
         [0030]    Signal processing system  300  includes co-channel mitigation processing system  302 . Co-channel mitigation processing system  302  seeks to eliminate the undesirable sources from the received antenna outputs while leaving the reflected signals from targets of interest unattenuated. Co-channel mitigation processing system  302  may be used in conjunction with pre-detection processing within signal processing system  300 . Specifically, co-channel mitigation processing system  302  may be used with techniques for wideband pre-detection processing. In these techniques, an array of antennas  320  may be used to receive signals from the source of opportunity to be exploited, such as a primary illuminator and any other co-channel signals present. For example, transmitter  310  may be a primary illuminator that transmits signal  340 .  
         [0031]    Reflected target signals  314  may be received by antenna array  320 . Reflected target signals  314  may be signals correlating to transmitted signals  340  and  342  that have been reflected by a target. Reflected target signals  314  may be compared to transmitted signals to determine target parameters, such as velocity, location, and the like. Target antenna array  320  also may receive any other co-channel signals within the environment as signals  314 . Target antenna array  320  then feeds co-channel signals  316  to co-channel mitigation processing system  302 . Co-channel signals  316  may reflect the co-channel signals received and to be exploited for PCL operations. These co-channel signals may include multipath images of the illuminator signal, delay, and Doppler shifted reflections of the illuminator from targets in the region under surveillance, such as aircraft or space launch vehicles. The co-channel signals also may include other distant broadcast sources at the same operating frequency as the primary illuminator. Thus, co-channel signals  316  may be a composite of transmitted signals  340  and  342  and reflected signals  314 , and may be used as an estimate of these signals. More particularly, the co-channel signals may be a good estimate of the transmitted, or direct, signals because the strong direct path signals dominates the reflected target path signals.  
         [0032]    For example, transmitter  310  may be the primary illuminator that operates at a specified frequency and transmitter  312  may be a secondary illuminator operating at the same frequency. Thus, transmitted, or direct, signals  340  and  342  may have the same frequency.  
         [0033]    Co-channel mitigation processing system  302  seeks to eliminate the undesirable sources from the received antenna outputs of target antenna array  320  while leaving the reflected signals  314  that are from targets of interest. Reflected signals  314  should be unattenuated. Undesirable sources may include the strong direct-path and multipath components of the exploited illuminator. Co-channel mitigation processing system  302  may include primary cancellation component  304  and secondary cancellation component  306 .  
         [0034]    Primary cancellation component  304  receives co-channel signals  316 . Primary cancellation component  304  includes primary illuminator adaptive beamformer  322 , reference regenerator  324 , and adaptive cancellation filter  326 . Adaptive beamformer  322  may accept co-channel signals  316  and combine them to form signals that have selectivity along specific lines of azimuth and elevation. Adaptive beamformer  322  may enhance desired signals, such as transmitted signals  340  and  342  and reflected signals  314 , while suppressing noise and interference received by target antenna array  320 . Further, adaptive beamformer  322  may be applied whenever multiple signal sources are present that may be subdivided into target, direct path and noise. Adaptive beamformer  322  may use known methods and applications to amplify and obtain a target or direct path signal while attenuating noise or undesired signals. Preferably, adaptive beamformer  322  is concerned with transmitted signals  340  and  342 , which are the direct path signals from transmitters  310  and  312 .  
         [0035]    Reference regenerator  324  receives the co-channel signals from adaptive beamformer  322  to regenerate direct path signals. Transmitters  310  and  312  may be uncontrolled transmitters in that the users of signal processing system  300  do not have control over transmitters  310  and  312 . Reference regenerator  324  identifies those direct path signals from uncontrolled transmitters and reconstructs the signals from processing with the received target signals. A constant amplitude signal estimate having approximately the frequency and phase of the direct path signals may be generated. This signal may be a frequency modulated carrier operating at a given frequency. In reference regenerator  324 , the primary direct path signal is regenerated. For example, transmitted signal  340  may be regenerated by reference regenerator  324 . Thus, transmitted signal  340  is obtained the various sources of co-channel interference have been reduced by a significant amount.  
         [0036]    The co-channel signals with the regenerated primary direct path signal is received by adaptive cancellation filter  326 . Cancellation filter  326  serves to clean up the regenerated primary direct path signal prior to cancellation by removing excess energy, such as noise. Cancellation filter  326  removes stray energy collected by target antenna array  320  that is outside of the frequency band of the direct path signal. After filtering, primary cancellation component  304  then cancels the clean, regenerated primary direct path signal from the co-channel signals received by target antenna array  320 . Primary cancellation component  304  may reduce the primary illuminator direct path and multipath components, but may still leave significant residual interference from distant co-channel illumination sources in the co-channel signals  316 .  
         [0037]    After primary cancellation, the remaining co-channel signals are received by secondary cancellation component  306  at secondary illuminator adaptive beamformer  330 . Secondary cancellation component  306  also includes reference regenerator  332  and adaptive cancellation filter  334 . Adaptive beamformer  330  correlates to adaptive beamformer  320 , but they are not necessarily the same. Adaptive beamformers  320  and  330  may have different configurations. Adaptive beamformer  330  serves to perform beamforming on a secondary direct path signal. According to the disclosed embodiments, regulatory constraints on the geographic and spectral distribution of sources of opportunities, or illuminators, may lead to situations where the secondary source of residual co-channel is a distant illumination source operating at the same frequency as the primary illuminator. For example, two transmitters may operate at the same frequency in the FM band, such as two radio stations, and transmit signals oprating at the same given frequency. According to the above disclosed examples, transmitter  312  may be a secondary illuminator transmitting at the same frequency as transmitter  310 . Thus, transmitted signal  342  may be a secondary direct path signal operating at the same frequency as transmitted signal  340 . Secondary cancellation component  306  cancels the secondary direct path signal much like primary cancellation component  304  cancels the primary direct path signal.  
         [0038]    Adaptive beamformer  330  may accept the co-channel signals  316  and combine them to form signals that have selectivity along specific lines of azimuth and elevation. Adaptive beamformer  330  may enhanced desired signals, such as transmitted signal  342  and reflected signals  314 , while suppressing noise and interference received by target antenna array  320 . Further, adaptive beamformer  330  may be applied whenever multiple signal sources are present that may be subdivided into target, direct path and noise. Adaptive beamformer  330  may use known methods and applications to amplify and obtain a target or direct path signal while attenuating noise or undesired signals, and may be known to one skilled in the art. Preferably, adaptive beamformer  330  is concerned with transmitted signal  342  that is the direct path signal from transmitter  312 .  
         [0039]    Reference regenerator  332  receives the co-channel signals from adaptive beamformer  330  to regenerate a secondary direct path signal, if applicable. Reference regenerator  332  identifies those direct path signals from uncontrolled secondary transmitters and reconstructs the signals from processing with the received target signals. A constant amplitude signal estimate having approximately the frequency and phase of the secondary direct path signals may be generated. The signal may be a frequency modulated by operating at the same given frequency as the primary direct path signal. In reference regenerator  332 , the secondary direct path signal is regenerated. For example, transmitted signal  342  may be regenerated by reference regenerator  332 . Thus, transmitted signal  342  is obtained where the various sources of co-channel interference have been reduced by a significant amount.  
         [0040]    The co-channel signals with the regenerated secondary direct path signal are received by adaptive cancellation filter  334 . Cancellation filter  334  serves to clean up the regenerated secondary direct path signal prior to cancellation. Cancellation filter  334  removes stray energy collected by target antenna array  320  that is outside of the frequency band of the secondary direct path signal. After filtering, secondary cancellation component  304  then cancels the clean, regenerated secondary direct path signal from the co-channel signals received by target antenna array  320 . By processing the primary and secondary direct path signals, the primary and secondary illuminator components are reduced without distortion of the desired signals, such as target signals. Thus, co-channel mitigation processing system  302  outputs mitigated co-channel signal  330 . Mitigated signal  330  preferably includes target signals without primary and secondary direct path signals. Mitigated signal  330  may be processed by subsequent pre-detection processing components in a more efficient manner without having to account for strong direct path signals. Further, reflected and received target signals may be more accurate in determining target parameters because co-channel interference has been reduced.  
         [0041]    [0041]FIG. 4 depicts a flowchart for mitigating co-channel interference in accordance with an embodiment of the present invention. Step  402  executes by receiving co-channel, or multipath, signals at a target antenna array. These signals may include direct path signals and reflected target signals, as well as noise and interference signals. Step  404  executes by performing primary adaptive beamforming on the co-channel signals. The primary direct path signal is enhanced for further processing operations. Step  406  executes by regenerating the primary direct path signal. Further, this step may include filtering the primary direct path signal to remove excess energy. Step  408  executes by canceling the primary direct path signal from the co-channel signals.  
         [0042]    Step  410  executes by performing secondary adaptive beamforming on the remaining co-channel signals. The secondary direct path signal is enhanced for further processing operations. Step  412  executes by regenerating the secondary direct path signal. Further, this step may include filtering the secondary direct path signal to remove excess energy. Step  414  executes by determining the frequency of the secondary direct path signal. Preferably, the secondary direct path signal has the same frequency as the primary direct path signal. Step  416  executes by canceling the secondary direct path signal from the co-channel signals. Step  418  executes by outputting the mitigated co-channel signals for further signal processing.  
         [0043]    It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided that they come within the scope of any claims and their equivalents.