Patent Publication Number: US-9887799-B2

Title: Methods and apparatus for data communication using bandwidth modulation

Description:
BACKGROUND 
     As is known in the art, while jamming signals of an adversary, friendly nearby systems are also prevented from using the same bandwidths that are being jammed. Since many communications systems use the same methods for communicating, it may not be possible to allow one to operate while disabling the other. Conventional techniques of communicating while jamming involve separating the signals in either the time or frequency domain. Either the communications are sent during times the jamming signal is turned off, or a small band is left unjammed for the communications system to use. However, this allows the adversary to use the same technique to communicate if they can determine the right time/frequency. 
     SUMMARY 
     The present invention provides method and apparatus for adding communication data onto a jamming signal by varying pulse parameters. A jammer can be modeled to look like pure noise in order to make it difficult to filter out. In embodiments of the invention, the jamming signal still appears as noise to an adversary, while still conveying useful data to an ally. In general, jamming effectiveness remains unaffected. It is understood that embodiments of the invention are applicable to continuous waveforms, such as noise, and non-continuous waveforms, e.g., coherent waveforms, and polymorphic waveforms by modulating the bandwidth (BW), center frequency (CF), and pulse width (PW) as appropriate to embed communications. 
     Embodiments of the invention enable communication data to be modulated onto a jamming waveform, thereby removing the need to separate one signal from the other at the receiver. Digital bits are represented by small changes, or jitters, in the jamming signal spot coverage (bandwidth) and/or pulse-width. These jitters are large enough to be detected, but small enough to allow the jamming to maintain effectiveness against the adversary. In embodiments, jitter is controlled such that a threat system does not have sufficient sensitivity to observe the jitter communications, while a friendly system maintains the capability to detect the jitter communications. Main-lobe jam energy is applied to the target, while side-lobe energy is used to communicate with friendly aircraft via the jitters placed within the waveform. In embodiments, this could work while the friendly aircraft is in the main-lobe. 
     In one aspect of the invention, a method of data jitter to modulate data onto a signal comprises: modulating a data jitter value above a threshold corresponding to a sum of inherent spot jitter and inherent transmission error to generate an output signal; and transmitting the output signal with a transmitter. 
     The method can further include one or more of the following features: modulating a center frequency to generate the output signal, modulating a center frequency to provide the output signal having first and second channels, modulating different data jitter values on each side of the signal to generate the output signal, encrypting the output signal, determining signal characteristics between the transmitter and a receiver including the inherent spot jitter, directing a signal lobe at the receiver, the data jitter has a corresponding number of states, the signal comprises a continuous waveform signal, adjusting the threshold to achieve a given signal-to-noise ratio, determining a number of states for transmitting the output signal, receiving the transmitted signal and demodulating the data jitter value, communicating with a receiver to determine the threshold, dynamically adjusting the threshold, and/or modulating a pulse width jitter value onto the signal in a buffer area adjacent to a pulse edge of the signal having inherent jitter to generate the output signal. 
     In another aspect of the invention, a system comprises: a module configured to modulate a data jitter value onto a signal above a threshold corresponding to a sum of inherent spot jitter and inherent transmission error for generating an output signal for transmission by a transmitter. 
     The system can further include one or more of the following features: modulating a center frequency to generate the output signal, modulating a center frequency to provide the output signal having first and second channels, modulating different data jitter values on each side of the signal to generate the output signal, encrypting the output signal, determining signal characteristics between the transmitter and a receiver including the inherent spot jitter, directing a signal lobe at the receiver, the data jitter has a corresponding number of states, the signal comprises a continuous waveform signal, adjusting the threshold to achieve a given signal-to-noise ratio, determining a number of states for transmitting the output signal, receiving the transmitted signal and demodulating the data jitter value, communicating with a receiver to determine the threshold, dynamically adjusting the threshold, and/or modulating a pulse width jitter value onto the signal in a buffer area adjacent to a pulse edge of the signal having inherent jitter to generate the output signal. 
     In a further aspect of the invention, a system comprises: a module configured to demodulate a data jitter value modulated onto a signal above a threshold corresponding to a sum of inherent spot jitter and inherent transmission error. The system can be further configured to demodulate a modulated center frequency providing first and second channels. 
     In another aspect of the invention, a method of pulse width jitter to modulate data onto a signal, comprises: modulating a pulse width jitter value onto the signal in a buffer area adjacent to a pulse edge of the signal having inherent jitter to generate an output signal; and transmitting the output signal with a transmitter. 
     The method can further include one or more of the following features: modulating a center frequency to generate the output signal, modulating a center frequency to provide the output signal having first and second channels, modulating different data jitter values on each side of the signal to generate the output signal, encrypting the output signal, determining signal characteristics between the transmitter and a receiver, directing a signal lobe at the receiver, the signal comprises a non-continuous waveform signal, adjusting the buffer area to achieve a given signal-to-noise ratio, receiving the transmitted signal and demodulating the pulse width jitter value, communicating with a receiver to determine the buffer area, dynamically adjusting the buffer area, and/or modulating a data jitter value above a threshold corresponding to a sum of inherent spot jitter and inherent transmission error to generate the output signal. 
     In another aspect of the invention, a system comprises: a module configured to modulate a pulse width jitter value onto a signal in a buffer area adjacent to a pulse edge of the signal having inherent jitter to generate an output signal. 
     The system can further include one or more of the following features: modulating a center frequency to generate the output signal, modulating a center frequency to provide the output signal having first and second channels, modulating different data jitter values on each side of the signal to generate the output signal, encrypting the output signal, determining signal characteristics between the transmitter and a receiver, directing a signal lobe at the receiver, the signal comprises a non-continuous waveform signal, adjusting the buffer area to achieve a given signal-to-noise ratio, receiving the transmitted signal and demodulating the pulse width jitter value, communicating with a receiver to determine the buffer area, dynamically adjusting the buffer area, and/or modulating a data jitter value above a threshold corresponding to a sum of inherent spot jitter and inherent transmission error to generate the output signal. 
     In a further aspect of the invention, a system comprises: a module configured to demodulate a pulse width jitter value modulated onto a signal in a buffer area adjacent to a pulse edge of the signal having inherent jitter to generate an output signal. The system can further include the module being further configured to demodulate a modulated center frequency providing first and second channels, and/or the module being further configured to modulate a data jitter value onto the signal above a threshold corresponding to a sum of inherent spot jitter and inherent transmission error for generating an output signal for transmission by a transmitter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
         FIG. 1  is a schematic representation of a bandwidth communication system; 
         FIG. 2  is a schematic representation of data jitter communication; 
         FIG. 3  is a schematic representation of spot jitter communication; 
         FIG. 4  is a flow diagram showing bandwidth communication; 
         FIGS. 5A and 5B  show eight states on both sides of a modulated waveform and  FIGS. 5C and 5D  shows the modulated waveforms of  FIGS. 5A and 5B  with a modulated center frequency; 
         FIGS. 6A , B are schematic representations of illustrative data communication with bandwidth modulation; 
         FIGS. 7A-C  are a graphical representation of bandwidth modulation signal; 
         FIG. 8  is a schematic representation of a wireless communication having bandwidth modulation; and 
         FIG. 9  is a schematic representation of an illustrative computer than can perform at least a portion of the processing described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an illustrative system  100  for transmitting signals having bandwidth modulation in accordance with illustrative embodiments of the invention. A controller  102  controls overall operation of the system. A signal processor module  104  processes information to be transmitted after an encoder module  106  performs bandwidth modulation, as described more fully below. A beamformer  108  receives the information to be transmitted and steers one or more beams in a desired direction. A series of transmit/receive (TR) modules  110   a -N are coupled to an antenna  112  for transmitting the information into free space. In illustrative embodiments, the antenna is provided as an active electronically scanned array (AESA) antenna and the TR modules  110  include both amplitude and phase adjustment circuits (e.g. amplifiers and or phase shifting elements) as is generally known. Amplitude and phase setting in either or both of beamformer  108  and TR modules  110  are selected to provide an AESA antenna having a desired radiation pattern and beam pointing direction for one or more beams. 
     Received signals are provided to a receiver  150  that downconverts the frequency of the received signals for processing by a receive signal processor  152 . For example, the receive signal processor  152  may process the downconverted receive signals to extract the position, velocity, direction of motion, and/or type of target. 
       FIG. 2  shows an illustrative implementation of what is referred to herein as data jittering  200  in a CW (continuous waveform) application in which a signal is periodically sampled in a jamming environment. Data jitter  202  is communicated as ones and zeroes over an inherent spot jitter. There is some level of error inherent in detecting signals. Jitter can be random or deterministic. In embodiments, data jitter value  202  is added slightly above a level  204  corresponding to a sum  204  of the inherent spot jitter level and the system inherent transmission error. The data jitter  202  is controlled so as to provide a one or a zero for enabling data communication in the presence of jamming signals or generally during waveform transmission since this could be more than just a ‘jamming signal.’ 
       FIG. 3  shows an illustrative implementation of what is referred to herein as pulse width jitter  300 . In general, pulse width jittering is suitable for non-CW modulation techniques. It is understood that during the pulse, BW jitter can also be performed. The PW jitter allows one to jitter the time of transmission for a given time, while the BW jitter can occur at a significantly faster rate during that shorter pulse. It should be noted that the data rate is significantly higher by combining jitter techniques on a non-CW waveform. Pulses contain an inherent jitter  302  at the location of the beginning and ending of a given pulse. A relatively small buffer  304  is added to a pulse edge to indicate a one or a zero. The buffer  304  is adjacent to the earliest (or latest) time the signal pulse (with inherent jitter) can occur. 
     In general, a main lobe or side lobe of sufficient energy from the transmitter is pointed towards receiver. A ‘norm’ is established by the receiver of what the normalized signal “chip” is with no communications modulation. This can be normalized “spot width” in the case of spot jitter or normalized “pulse width” in the case of pulse width jitter. A priori, an “error range” is established between transmitter and receiver systems of expected pulse width jitter or spot width jitter. In other embodiments, an error range is dynamically adjusted. Once a normalization period has been established, any change in received signal above the error range is interpreted as data communication. Changes include the received spot width (SJ) or pulse width (PWJ) or both simultaneously (SJ+PWJ). 
     In an embodiment, the number of bits per “chip” can be determined as follows:
 
Num bits =log 2 ( M )=log 2  jitter_range/jitter_resolution)
 
where M is the number of states, jitter_range is the data jitter that can be applied by the transmitter beyond the “error range,” and jitter resolution is the resolving ability of the receiver, which is lower bounded by the actual jitter in the system. It is understood that while  FIGS. 2 and 3  show a single 0 or 1 state on each bit, there can be multiple steps, for example as shown in  FIGS. 5A-D .
 
     The bit error rate can arise from frequency agile threats and the application of CW techniques to that threat. However, the spot jittering can be re-synced with minimal lost bits. It is understood that Forward Error Correction (FEC) or any suitable technique can be used to determine these lost bits. If however, a significant number of bits have been lost and FEC cannot properly disambiguate the signal, then the signal can be repeated with larger jitter values. By dynamically adjusting the jitter values to converge to a sufficient jitter Signal-to-Noise Ratio (SNR), the system can ensure the jitter communications link is established between systems. Finally, the jitter value would have an upper bound defined limiting the degradation to the primary mission. 
     The loss in PSD (power spectral density) is relatively minimal since the jitter magnitude can be a small multiple of the minimal transmit and receive tone width of the system. 
     In general, PW Jitter (PWJ) is suited to non-CW techniques. PWJ applies a slight buffer to the pulse widths to create a 0 or 1. PWJ works as intended while simultaneously transmitting data to “in the know” receivers. Bit Error Rate (BER) can derive agile PWs; however, it can quickly be re-synched. Existing processing techniques can determine these lost bits. If multiple non-CW techniques are interleaved, this communication technique can be superimposed between those techniques. 
     A new chip for pulse width jitter (PWJ) is resolved each sampling clock cycle and a new chip is resolved on spot width jitter every two sampling clock cycles to meet Nyquist criteria, for example. With pulse width (PW), and pulse repetition frequency (PRF), and where M states can be established, the communications rate with PWJ is then:
 
Bit rate   _   sj =PRF·log 2   M  
 
     For spot jitter (SJ), with sampling rate Fs, and duty factor (Duty) and M states, the communications rate with SJ is then 
     
       
         
           
             
               Bit 
               
                 rate 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 _ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 pwj 
               
             
             = 
             
               
                 2 
                 · 
                 
                   ( 
                   
                     Fs 
                     2 
                   
                   ) 
                 
                 · 
                 
                   log 
                   2 
                 
               
               ⁢ 
               
                 M 
                 · 
                 Duty 
               
             
           
         
       
     
     It is understood that the “2” in the above equation is present because when performing spot jitter it is assumed there is jittering of both the bandwidth and the center frequency of the waveform. There is still overlap of the desired transmit bandwidth but this allows M different states on each side of the waveform (see, e.g.,  FIG. 5D ). These methods can be used for either a single channel, a double redundant single channel, and a two-channel scenario (see, e.g.,  FIGS. 5C ,D) to double communication effectiveness. 
     Spot jitter and pulse width jitter can be combined for both continuous and non-continuous waveforms such that:
 
Bit rate =PWJ rate +SJ_rate
 
       FIG. 4  shows a process to provide bandwidth modulation in accordance with illustrative embodiments of the invention. In embodiments, a receiver has the capability to transmit different types of signals. While processing is described for the various signals, it is understood that in embodiments, only steps for the particular signals may be performed. 
     In the illustrated embodiment, in  402  a desired primary signal is determined, which can include radar signals, electronic attack (EA), e.g., jamming, etc. In step  404 , a desired communication signal is determined. In step  406 , a desired communication signal pointing direction is determined. In step  408 , desired primary signal pointing directed is determined. 
     In step  410 , the system determines a normalized pulse width error on using previously transmitted data. In step  412 , the system determines a normalized bandwidth error on using transmitted data. Steps  410  and  412  can also be used to determine when environmental errors prevented a communication link. Steps  410  and  412  render the system able to adjust the jitter parameters beyond the typical error magnitudes to guarantee the target receiver can disambiguate jitter magnitudes. In step  414 , the system determines weights to steer a waveform main and side lobes. 
     Based on the signal type transmitted and pulse width and/or bandwidth error and steering, the system determines the number of states required to transmit the signal in step  416 . In step  418 , the system determines the magnitude of states for communication signals for a given target range and receiver capabilities. Step  418  ensures that the waveform has sufficient jitter magnitude (in time and frequency) so that the target system can resolve the transmitted signal from environmental and system interference. Step  418  also determines the number of states required to facilitate the desired communication data. In step  420 , the system determines bandwidth, center frequency, and pulse width variations for the desired signal to also contain both sufficient communication jitter characteristics and the original signal characteristics. In step  422 , the system synthesizes the signal for transmission including  422   a  synthesizing calibration pulses and  422   b  synthesizing pulse for transmission based upon signal variations  420 . The synthesized signals from step  422  are then fed back to step  416  to facilitate adaptive adjustments of the pulse jitter. In step  424 , the pulses are transmitted. In optional step  426 , the system can wait for acknowledgement of the transmitted pulse from the receiver to further drive the jitter magnitudes to gain sufficient jitter SNR at the target receiver to close-link and/or prevent detection. 
     On the receiver side, in step  450 , the transmitted pulses are received. In step  452 , the receiver identifies a number of states required for signal transmission and in step  454  identifies a magnitude of each state for communication signals. In step  456 , the receiver processes the received communication signal using calibration values. An optional acknowledgment signal can be sent to the transmitter in step  458  to provide adaptive tuning of the jitter parameters by the transmitting system. After signal processing, the communication is signal is provided in step  460 . 
       FIGS. 5A-D  show a sequence of bandwidth modulation for an illustrative sequence for states 1-M, where M=8 for eight states on both sides of a modulated waveform.  FIGS. 5C and 5D  show the modulated waveforms of  FIGS. 5A and 5B  with a modulated center frequency.  FIGS. 5A and 5B  show a double redundant, single-channel jitter communication during a pulse and  FIGS. 5C and 5D  shows a dual channel (channel 1 is on the left-hand side and channel 2 is on the right hand side), where the Y axis is time and the X axis is frequency. The illustrative modulation embodiments cover the same desired bandwidth, but each embeds a different example of pulse jitter communications within the shown pulse. 
     For the illustrative modulation of  FIGS. 5C and 5D , first and second channels of communication are provided (one on the left-hand side and another on the right-hand side). First and second channels are enabled by having different jitter values on each side of the transmit signal so as to double communication efficiency. 
     In embodiments, encrypted information can be transmitted via bandwidth modulation. Because the transmitted signal is a series of bit values, any encryption process can be adopted in pre-processing (on transmit) and post-processing (on receive). In  FIG. 5 , encryption could be already embedded on transmit; while a decryption device manages the encryption on receive. 
       FIG. 6  shows an illustrative pulse jitter test sequence of “Test Test” transmitted using first and second frequencies F1, F2. At the beginning of the sequence and at the beginning of the new frequency, there are a series standardization pulses. Pulse width jitter uses forty-five pulses and non-CW PW/frequency transmission uses thirteen modulation pulses, as shown in  FIGS. 6A, 6B . Pulse 1 and pulse 2 are used for standardization. F1P3 includes “T” and half of “e” and F1P4 includes the second half of “e” and “s.” F1P5 includes half of “t” and standardization for the new frequency. As can be seen, for the first frequency, each pulse contains 12 bits. For the second frequency, each pulse contains 8 bits. 
     After standardization, F2P3 includes the second half of “t” and half of “ ” and F2P4 contains half of “ ” and half of “T.” The remaining 8-bit F2 pulses contain “est.” 
     While the illustrative embodiment is shown as 1 bit/chip, it will be appreciated that throughput increases with multiple chips. In embodiments, the frequency jitter rate is about equal to twice the receiver (or the transmitter clock frequency, whichever is lower) processor clock frequency and the PW data jitter rate is limited to twice the PRF. 
     It will be appreciated that data jitter as described herein will appear to receivers not ‘in the know’ as conventional transmit error. If the friendly systems have a more precise clock than unfriendly systems, it may not be possible for the unfriendly systems to detect the data jitter at all. It will be further appreciated that range can work in the favor of friendly systems since the greater the distance the receiver is from the transmitter, the lower the signal-to-noise ratio, and thus, the more difficult to extract the encoded information. In addition, coherent signal processing eliminates the data jitter. 
       FIGS. 7A-C  shows communication information disguised as (an exaggerated for example purposes) jitter on the exterior of an original spot, e.g., outside a 3 dB BW.  FIG. 7A  shows a FFT of one pulse with data jitter of ONE,  FIG. 7B  shows a FFT of one pulse with data jitter of ZERO.  FIG. 7C  shows the pulses resulting from the data jitter of  FIG. 7A . FFT and FIR filtering is used by a receiver to detect the inserted data jitter. 
       FIG. 8  shows an illustrative wireless communication system  800  providing data jitter to a variety of wireless devices  802 . Messages  804  are encoded with data jitter using bandwidth modulation  806 , as described above. A cell tower  808 , for example, transmits the information to the wireless devices  802 , which decode the data jitter using known pulse information for bandwidth modulation  810  to provide the received message  812 . 
     Referring again to  FIGS. 5A-D , since the signals being shown are not just jamming (but could also be communications, radar, etc.) the same processing is applicable. For this scenario, the communication system uses the entire jittered bandwidth (giving a slightly higher communication rate), while the bits placed on the magnitude of the jitter add additional channels to that communication signal (either pulse width jitter modulation or frequency/BW jitter modulation), further increasing the communication rate by the amount shown in the Bit_rate equation a few pages back. 
     The result would be approximately:
 
Bit_Rate=bits/s/hz (of the communication tower)*BW (desired BW+the jitter bandwidths)+PWJ_rate+SJ_rate
 
     In embodiments, bandwidth modulated data can be transmitted such that only receivers within a certain range can detect the bandwidth modulation It is understood that detection of the bandwidth may depend upon device characteristics, such as receiver clock accuracy. For example, for first and second devices at the same range, only the first device, which has a more accurate clock than the second device, can detect the bandwidth modulated signal. In embodiments, an entity may charge additional costs or fees for devices that can receive, or be enabled to receive, the bandwidth modulation data in addition to data modulated in the conventional way. In other embodiments, additional security may be provided in the bandwidth modulated data. Additionally, in other embodiments the additional channels, provided by the jitter technique, may communicate metadata about the primary signal such as transmit time, conditions for data collection, transmitter location, etc. between capable systems. 
       FIG. 9  shows an exemplary computer  900  that can perform at least part of the processing described herein. The computer  900  includes a processor  902 , a volatile memory  904 , a non-volatile memory  906  (e.g., hard disk), an output device  907  and a graphical user interface (GUI)  908  (e.g., a mouse, a keyboard, a display, for example). The non-volatile memory  906  stores computer instructions  912 , an operating system  916  and data  918 . In one example, the computer instructions  912  are executed by the processor  902  out of volatile memory  904 . In one embodiment, an article  920  comprises non-transitory computer-readable instructions. 
     Processing may be implemented in hardware, software, or a combination of the two. Processing may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a storage medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform processing and to generate output information. 
     The system can perform processing, at least in part, via a computer program product, (e.g., in a machine-readable storage device), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a storage medium or device (e.g., CD-ROM, hard disk, or magnetic diskette) that is readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device is read by the computer. 
     Processing may also be implemented as a machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate. 
     Processing may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field programmable gate array) and/or an ASIC (application-specific integrated circuit)). 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety. 
     Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. Other embodiments not specifically described herein are also within the scope of the following claims.