Abstract:
Systems and methods are disclosed for removing interference in a time-harmonic received composite signal, comprising, sampling an in-band interfering signal from a stationary transmitter at a stationary receiver; sampling a composite signal having the in-band interfering signal and an in-band desired signal from a stationary transmitter at the receiver, deriving at least one of a phase and time difference from the composite signal and sampled in-band interfering signal, applying the derived at least one of phase and time difference to the sampled in-band interfering signal to estimate a synchronized current in-band interfering signal, and generating the un-interfered, in-band desired signal by subtracting the synchronized current in-band interfering signal from a current composite signal.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention is assigned to the United States Government. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone 619-553-2778; email: T2@spawar.navy.mil. Reference Navy Case No. 100,421. 
    
    
     FIELD 
     This disclosure relates generally to radio frequency reception/transmission antenna systems capable of selectively discriminating between spatially displaced locations of wanted and unwanted signal reception/transmissions. 
     BACKGROUND 
     When two signals are on different frequencies, they can be separated by simple filtering methods. But when the two signals are on the same frequency, they will combine into one (composite) signal having the same frequency. Filtering out one signal will inadvertently filter out the other signal. So separating the two signals using filters is not a viable option. The only known solution is to add to the composite signal a negative of the signal that is desired to be removed. In essence, this is called cancellation via subtraction. U.S. Pat. Nos. 3,696,429; 4,093,923; 4,135,159; 5,966,684; and 4,870,424 describe classic approaches to filtering “unwanted” signals. However, these examples are only effective when the amplitude and particularly the phase (timing) of the interferer is known. In some situations, these requirements cannot be met. 
     Accordingly, there has been a long-standing need for new systems and methods for co-site, in-band signal extraction and/or recovery, as further described in detail below. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview, and is not intended to identify key/critical elements or to delineate the scope of the claimed subject matter. Its purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
     In one aspect of the disclosed embodiments, a method of removing interference in a time-harmonic received composite signal is provided, comprising: sampling an in-band interfering signal from a stationary transmitter at a stationary receiver; sampling a composite signal having the in-band interfering signal and an in-band desired signal from another stationary transmitter at the receiver; deriving at least one of a phase and time difference from the composite signal and sampled in-band interfering signal; applying the derived at least one of phase and time difference to the sampled in-band interfering signal to estimate a synchronized current in-band interfering signal; and generating the un-interfered, in-band desired signal by subtracting the synchronized current in-band interfering signal from a current composite signal. 
     In one aspect of the disclosed embodiments, a time-harmonic in-band signal interference removing system is provided, comprising: an in-band interfering signal from a stationary transmitter; an in-band desired signal from another stationary transmitter; a stationary receiver receiving the in-band interfering signal and in-band desired signal; an in-band interfering signal removal module, wherein the removal module derives at least one of a phase and time difference from a composite signal formed from a sampled in-band interfering signal and sampled in-band desired signal, applies the derived at least one of phase and time difference to the sampled in-band interfering signal to estimate a synchronized current in-band interfering signal, and generates an un-interfered in-band desired signal by subtracting the synchronized current in-band interfering signal from a current composite signal. 
     In another aspect of the disclosed embodiments, a system of removing interference in a time-harmonic received composite signal is provided, comprising: means for sampling an in-band interfering signal from a stationary transmitter at a stationary receiver; means for sampling a composite signal having the in-band interfering signal and an in-band desired signal from another stationary transmitter at the receiver; means for deriving at least one of a phase and time difference from the composite signal and sampled in-band interfering signal; means for applying the derived at least one of phase and time difference to the sampled in-band interfering signal to estimate a synchronized current in-band interfering signal; and means for generating the un-interfered, in-band desired signal by subtracting the synchronized current in-band interfering signal from a current composite signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a multi-transmitter/receiver co-site arrangement. 
         FIG. 2  is a flow chart diagram summarizing an exemplary approach for extracting the desired signal from the composite signal. 
         FIG. 3  is a plot showing results of an experimental model operating at f=350 MHz. 
         FIG. 4  is a followup plot comparing results of the experimental model of  FIG. 3 . 
         FIG. 5  is another plot showing results of an experimental model operating at a frequency of f=150 MHz. 
         FIG. 6  is followup plot comparing results of the experimental model of  FIG. 5 . 
         FIG. 7  is a plot of measured VSWRs for experimental model antennas spanning different frequencies. 
     
    
    
     DETAILED DESCRIPTION 
     The following description details a wireless implementation to extract a desired signal from co-site in-band interference. It should be appreciated that the desired signal can be weaker in strength than the interfering signal. The interfering signal can be a periodic signal, either pulsed or continuous wave (CW). Aspects of the disclosed embodiments are useful in constrained environments where the interferer(s) are located within a known distance from the receiving antenna and are in a steady state operating mode. As one example, the distance between transmitters on a ship or mobile platform is well known, being fixed on the platform. The “fixed” transmitter for an interfering signal provides the property that its recorded version has the same Fourier properties for the time under consideration. With this consideration, aspects of the described embodiments may be implemented on various mobile platforms without departing from the spirit and scope herein. 
       FIG. 1  is an illustration of a multi-transmitter/receiver co-site arrangement suitable for application of the exemplary methods and systems described herein. The desired signal  1  emanating from transmitter/antenna TX 1    2  is combined with an interfering signal  5  emanating from interfering transmitter/antenna TX 2    6 , to result in the composite signal  9  which is received by receiver/antenna RX  10 . It is understood that in this non-limiting exemplary scenario the distances between the desired signal transmitter/antenna TX 1    2  and interfering signal transmitter/antenna TX 2    6  from the receiver/antenna RX  10  are known, shown in  FIG. 1  by distances D 1  and D 2 , respectively. 
       FIG. 2  is a flow chart diagram summarizing a non-limiting exemplary approach for extracting the desired signal from the composite signal. The exemplary approach starts  20  with sampling  21  only the co-site in-band interferer signal at the receiver. This sampled data is stored for later use. Next the composite signal at the receiver is sampled  23  and also stored for later use. Note, between steps  21  and  23 , no time relationship is assumed between the capture events. Also no time relationship is assumed between the interfering signal and the desired signal. From the above, the exemplary approach then obtains  25  the salient features (time relationships) from the composite signal by modifying/aligning or synching the recorded interference signal  21  up with the composite signal. One of several approaches to accomplish this is to estimate the phase difference between the current composite signal (on-the-fly, so to speak) and the recorded interference signal  21 . This can be achieved by recognizing that the phase difference (phase_diff) between the two signals is approximately equal to difference of the current composite signal&#39;s Fourier-transform phase and that of the recorded interference signal. That is, phase_diff˜phase(FFT (composite))−phase(FFT(recorded interference)). Other approaches within the scope of one of ordinary skill in the art may be used. 
     Next, an estimation  27  of the synchronization between the interference signal and desired signal is performed. This can be obtained by recognizing that the above-obtained phase_diff can be converted to a time value according to: time latency (t_lat)=phase_diff/(2*π). This time latency (t_lat) is applied to the recorded interference signal to result in a modified interference=recorded interference (t−t_lat), where t represents time. The modified interference represents a very good approximation of the actual interfering signal, accounting for time delay. The exemplary approach then subtracts  29  the interference signal with the correct time relationship (i.e., modified interference) from the composite signal to extract the desired signal. The exemplary process then stops  30 . 
     To demonstrate the efficacy of the above exemplary approach, an experimental model was built using three (3) antennas configured with signal generators coupled to two antennas (transmit antennas) and an oscilloscope receiver coupled to the third antenna (receive antenna) in the arrangement depicted in  FIG. 1 . Both the desired signal generator and interfering signal generator were configured to beam continuous waves at the oscilloscope receiver. The interfering signal&#39;s power level was set at least 10 times larger than that of the desired signal&#39;s power. To simulate in-band interference, both generators&#39; frequencies were set at f˜350 MHz. All antennas used in the experimental setup were ¼ wave monopole antennas of length L˜8.75″ and also terminated with 50Ω. Of course, other types of antennas as well as frequencies may be used according to design preference. 
     For associating the above signal generators/oscilloscope receiver and antennas in the context of the framework of  FIG. 1 , the corresponding elements will be used in the following explanation. That is, the interfering signal generator and antenna combination will correspond to interfering transmitter/antenna TX 2    6  of  FIG. 1  and the desired signal generator and antenna combination will correspond to desired signal transmitter/antenna TX 1    2  of  FIG. 1  Similarly, the oscilloscope receiver and receive antenna will correspond to receiver/antenna RX  10  of  FIG. 1 . 
     Applying now the steps show in  FIG. 2 , first, in order to establish a reference, the interfering signal  5  is received from the receiver/antenna RX  10  while the interfering transmitter/antenna TX 2    6  is transmitting with the desired signal transmitter/antenna TX 1    2  not transmitting. This establishes a baseline representation of the interfering signal  5  at the receiver/antenna RX  10 . This is performed understanding that the interfering transmitter/antenna TX 2    6  is displaced from the receiver/antenna RX  10  by an arbitrarily fixed distance D i =6.5″. 
     Next, the composite signal  9  from the receiver/antenna RX  10  is captured (or sampled) while the interfering transmitter/antenna TX 2    6  and the desired signal transmitter/antenna TX 1    2  are both transmitting. This is accomplished with an understanding that the respective transmitter-to-receiver/antenna distance is arbitrarily fixed at D i =6.5″. Non-similar distances may be used, according to design. By applying steps  25 - 29  of  FIG. 2 , the desired signal  1  is obtained by subtracting the modified interference signal from the composite signal  9 . It should be noted that the exemplary approach described herein does not rely on a known capture time for either the composite signal or interference signal. The Applicants believe this to be one of several aspects of the disclosed subject matter that is unique to the art. That is, timing of the capture events is not necessary to arrive at the desired results. 
       FIGS. 3-4  are plots showing the results of an experimental model operating at 350 MHz using measurement equipment that is standard to the art. The ordinate&#39;s scale represents a normalized relative value and the abscissa&#39;s scale is a data index which corresponds to a time scale. Referring to  FIG. 3 , Line  31  represents the reference interference signal having a power of approximately 10 dBm. Line  33  represents the raw or actual composite signal from the combination of the interference signal  31  and desired signal  37  (as shown in  FIG. 4 ). Line  35  is a plot of the estimated or expected composite signal that is obtained using the method/approaches described above. 
     Referring to  FIG. 4 , a comparison of the extracted or estimated desired signal to the actual signal is shown. Specifically, pre-recorded or original desired signal line  37  is recognized as having a lower power lever (0.1 dBm) and is shifted from the reference/interference signal (line  31  of  FIG. 3 ). The extracted or estimated desired signal is line  39  which compares very favorably with the actual desired signal  37 . Again it is noted that only the interference signal (line  31 ) and composite signal (line  33 ) of  FIG. 3  are known. All the other signal lines (with the exception of the actual desired signal—line  37 , used for comparison purposes) are derived. By using the exemplary procedures described above, it has been shown that by utilizing the interference signal (line  31 ) and composite signal (line  35 ) only, an extracted desired signal (line  39 ) can be generated which is seen to compare favorably to the actual desired signal (line  37 ). 
       FIGS. 5-6  are plots showing the results of the experimental model operating at 150 MHz using measurement equipment that is standard to the art. The ordinate&#39;s scale represents a normalized relative value and the abscissa&#39;s scale is a data index which corresponds to a time scale. Referring to  FIG. 5 , line  51  represents the reference interference signal, line  53  represents the raw or actual composite signal from the combination of the interference signal  51  and desired signal  57  (as shown in  FIG. 6 ). Line  55  is a plot of the estimated or expected composite signal that is obtained using the method/approaches described above. 
     Referring to  FIG. 6 , a comparison of the extracted or estimated desired signal against the actual signal is shown. Specifically, pre-recorded or original desired signal line  57  is recognized as having a lower power lever (0.1 dBm) and is shifted from the reference/interference signal (line  51  of  FIG. 5 ). The extracted or estimated desired signal is line  59  which compares very favorably with the actual desired signal  57 . Again it is noted that only the interference signal (line  51 ) and composite signal (line  53 ) of  FIG. 5  are known. All the other signal lines (with the exception of the actual desired signal—line  57 , used for comparison purposes) are derived. By using the exemplary procedures described above, it has been shown that by utilizing the interference signal (line  51 ) and composite signal (line  55 ) only, an extracted desired signal (line  59 ) can be generated which is seen to compare favorably to the actual desired signal (line  57 ). 
     It should be noted that it is evident that the composite signal (line  53 ) is due to antenna mismatch loss at the operating frequency of f=150 MHz being much higher than that at f=350 MHz. This mismatch loss is apparent when examining the antenna VSWRs. 
       FIG. 7  is a plot of measured VSWRs for the antennas described above at different frequencies. The VSWR response for interfering antenna is shown as line  72 , for receiving antenna is line  74 , and for desired signal antenna as line  76 . Examining this plot shows that at f=150 MHz, a VSWR of ˜65:1 corresponds to a 12.24 dB mismatch loss; whereas at f=350 MHz, a VSWR of ˜2.5:1 corresponds to a mismatch loss of 0.88 dB. Accordingly, the dimensions of the experimental model are shown to be sensitive to lower frequencies and is better suited for f&gt;280 MHz or higher. While the experimental model may have limitations, it is expressly understood that the exemplary principles articulated herein are not frequency limited and may be applied to frequencies below or beyond those that have been demonstrated here. 
     The above exemplary embodiments show that co-site interference can be easily extracted from a receiver using an elegant approach that does not require timing information; and can be applied to any form of time-harmonic signal that is measurable, whether in the form of time radio waves or non-radio waves. For example, sound waves or other time-harmonic functions can be applied without departing from the spirit and scope herein. In view of the described approaches, it is possible to tolerate or accommodate interfering signals without degrading reception of the intended or desired signal. Concomitantly, co-site radiators can be designed with less stringent radiation patterns (for example, lobes) due to the ability to “remove” their interference from the received composite signal. 
     In view of this, it should be appreciated that the above exemplary embodiments may be applied to ships, airplanes, buildings, vehicles and so forth, that utilize antennas that experience co-site or near co-site interference. Additionally, software running on a processor may be devised to implement the various methods and/or steps elucidated above. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     The various illustrative logical blocks, modules, and processes described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. In the alternative, the processor and the storage medium may reside as discrete components. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principal and scope of the invention as expressed in the appended claims.