Abstract:
A system and method for maintaining privacy in transmission of a high bandwidth analog signal. A preferred system and method breaks an input signal into bands and then uses Walsh codes to overlay many signals on top of each other with a set of phase modulators to establish scrambling of the input signal. The scrambled signal is then transmitted. The received scrambled signal has the phase modulation removed and an inverse Walsh transform recovers the original bands. The recovered bands are added together to recover the original signal. The system and method use techniques that establish synchronization and phase coherency complete the process.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to scrambling systems and methods, and more particularly, to an analog privacy scrambling system and method that scrambles analog signals such that a casual receiver cannot recover the analog information contained in the signal.  
           [0002]    Historically, privacy for signals is established by digitizing the signals, then scrambling bits of the signal with a noise sequence to generate a transmitted signal. The modulation by the noise sequence is removed at the receiver resulting in a digital signal that is a replica of the original digitized signal. If an analog signal is desired, the signal may be passed through an digital-to-analog converter to recover the analog signal.  
           [0003]    When the bandwidth to be transmitted is high enough, this process is not possible. Conversion of the signal to digital form is difficult for high bandwidth signals. If the conversion to digital format were possible, the transmission of the digital format requires several times the bandwidth of the original analog signal. Several bits are transmitted for each sample of the input signal, resulting in bit rates for communication that are typically more than 16 times the bandwidth of the signal. For example, transmission of a 1 GHz bandwidth with a 2 giga-sample per second sample rate, each sample with 8 bits per sample, requires a transmission bit rate of 16 giga-bits per second. Scrambling of these high data rates and the transmission bandwidth required, even for sophisticated modulation techniques, are very difficult.  
           [0004]    Even when the bandwidth is not high, privacy for analog signals is difficult. If the dynamic range is high, conversion to a digital format can generate many bits of data, as discussed above. If an analog technique is to be used, the dynamic range should be preserved through the privacy processing. The privacy processing cannot introduce noise that reduces the dynamic range of the signal by an unacceptable amount.  
           [0005]    Analog scrambling schemes have included inverting the spectrum of the signal at regular time intervals. The random inversion of the signal frequencies obscures the individual signals in the bandwidth. However, the privacy afforded by this technique is limited since the possible inversion of the signal bandwidth is performed at regular intervals. Processing of the signals with a Fourier transform can identify the time of the transition from the upright signal to the inverted signal and back. In addition the transitions generate transients in the signal that interfere with the quality of the signal after it has been reconstructed.  
           [0006]    It is therefore an objective of the present invention to provide for analog privacy scrambling systems and methods that improves upon the above-discussed techniques.  
         SUMMARY OF THE INVENTION  
         [0007]    To accomplish the above and other objectives, the present invention provides for systems and methods that scramble analog signals such that a casual receiver cannot recover the analog information contained in the signal. The systems and methods obscure signals with many overlaying interfering signals and phase modulates the signals to prevent casual recovery without knowing a phase modulation signal used at the transmitter. The systems and methods hide signal components in similar bands and provides for phase modulation that cannot be recovered by a casual receiver.  
           [0008]    The present invention extends the usual phase modulation of a digital signal to provide privacy for an analog signal. The technique starts by segmenting an analog signal, then combines the segments using a Walsh transform, for example. This process obscures individual signals by burying the signal in a number of other added signals. The phase modulation introduces a random transformation of the segments that cannot be undone by a casual receiver of the signal.  
           [0009]    A desired receiver of the signal recovers the segments using segment selection that removes the phase modulation of the received segments. Applying the Walsh transform to the segments recovers the original baseband segments. Remodulating the segments to their original frequencies and adding the segments reproduces the original signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural element, and in which:  
         [0011]    [0011]FIG. 1 a  illustrates a transmitter of an exemplary analog scrambler in accordance with the principles of the present invention;  
         [0012]    [0012]FIG. 1 b  illustrates a receiver of the exemplary analog scrambler that is employed with the transmitter shown in FIG. 1;  
         [0013]    [0013]FIG. 2 illustrates a band selection process employed in the present invention;  
         [0014]    [0014]FIG. 3 illustrates Walsh coding of the signal employed in the present invention;  
         [0015]    [0015]FIG. 4 illustrates phase modulation of the signal employed in the present invention;  
         [0016]    [0016]FIG. 5 illustrates that individual band segments are modulated back to their RF positions and combined to form the original signal; and  
         [0017]    [0017]FIG. 6 is a flow diagram illustrating an exemplary scrambling method in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring to the drawing figures, FIG. 1 a illustrates a transmitter  11  of an exemplary analog scrambler  10  in accordance with the principles of the present invention, while FIG. 1 b  illustrates a receiver  20  employed with the transmitter  11  shown in FIG. 1 a . The exemplary analog scrambler  10  employs a technique based on the use of Walsh codes and continuous phase modulation.  
         [0019]    Referring to FIG. 1 a , the transmitter  11  of the exemplary analog scrambler  10  comprises a band subdivision circuit  12  that receives an analog input signal that is to be scrambled. The band subdivision circuit  12  segments the analog signal and applies the segmented signal to a coding circuit  13 , such a Walsh coding circuit  13 . The Walsh coding circuit  13  combines the segments using a Walsh transform, for example.  
         [0020]    The outputs of the Walsh coding circuit  13  are input to a phase modulation circuit  14 . The phase modulation circuit  14  introduces a random transformation of the combined segments. The phase modulated combined segments are then applied to a transmit circuit  15  for transmission as a scrambled signal.  
         [0021]    Referring to FIG. 1 b , the receiver  20  of the exemplary analog scrambler  10  comprises a band selection circuit  21  that receives the transmitted scrambled signal and segments the received scrambled signal. The scrambled signal is applied to a phase demodulation circuit  22  that has phase modulation synchronized with the phase modulation imposed at the transmitter  11  to shift the segmented signal to baseband and remove the phase modulation. The phase demodulated signal is applied to an inverse coding circuit  23 , such as an inverse Walsh coding circuit  23 . The inverse Walsh coding circuit  23  applies a Walsh transform to the phase demodulated signal to recover the original baseband segments. The original baseband segments are applies to a signal composition circuit  24  that remodulates the segments to their original frequencies and adds the segments to reproduce the original signal.  
         [0022]    In operation, the transmitter  11  breaks an input signal up into frequency segments. Preferably the number of segments is a power of two. Eight or sixteen segments are convenient numbers. The individual segments are downconverted to base band forming an in-phase and quadrature signal for each of the band segments. A Walsh function is applied across the baseband signals. The Walsh function applies a multiplication of +1 or −1 for each baseband and adds the basebands together. The number of Walsh codes is a power of two and is equal to the number of basebands that are added together.  
         [0023]    Details of the band subdivision circuit  12  and its operation to break the input signal into frequency segments is shown in FIG. 2. The band subdivision circuit  12  comprises a plurality of band selection circuits  31  that include I and Q multipliers  34 ,  36  that each receive the analog input signal at a first input. One of a plurality of oscillators  32  is coupled to a 90 degree hybrid  33  whose respective outputs are coupled to second inputs of the I and Q multipliers  34 ,  36 . Outputs of the I and Q multipliers  34 ,  36  are coupled to a filter  35 ,  37 , such as a Bessel filter  35 ,  37 .. Each individual band selection circuit  31  selects a different part of the input spectrum based on the frequency of the oscillator  32  input to the 90 degree hybrid  33  that is used for band selection.  
         [0024]    Phase modulation may additionally be employed in the band subdivision circuit  12  to vary segmentation of the input signal. To achieve this, the input signal is phase modulated before it is input to the I and Q multipliers  34 ,  36 . This may be implemented using phase modulators  38  coupled to the oscillators  32  as shown in FIG. 2.  
         [0025]    The operation of the Walsh coding circuit  13  is most easily understood when the number of basebands is two. One Walsh code is [1,1], while the second Walsh code is [1,−1]. The application of the first Walsh code indicates that two baseband signals are to be added together, while the application of the second Walsh code indicates that the two signals are to be subtracted. Transmission of the two Walsh coded signals sends signals that have the contents of the two basebands overlayed on each other. At the receiver  20 , the original signals are recovered by adding the two Walsh coded signals to form one of the basebands, and subtracting the second Walsh coded signal from the first to recover the second baseband signal. The two basebands may then be shifted to their original frequencies and added to form the original signal. By extension, when multiple Walsh codes are used, the signal can be broken into as many parts as there are Walsh codes and the Walsh transform used to combine signals for transmission, then decompose signals on reception.  
         [0026]    [0026]FIG. 3 illustrates Walsh coding of the signal. Walsh coding adds and subtracts signals to form multi-signal components. The inputs are in-phase and quadrature components from the band subdivision (selection) circuit  12 , forming a complex signal for input to the Walsh coding.  
         [0027]    The Walsh coding adds the individual parts together, producing a large set of interfering signals for any one signal that is transmitted, since the signals are overlayed on any Walsh coded segment. The receiver  20  can apply the Walsh transform across the set of signals and recover the original signals. The present invention uses phase modulation of the Walsh coded segments to achieve privacy.  
         [0028]    Each individual Walsh coded segment is phase modulated in the phase modulation circuit  14  with a random signal that can be reproduced at the receiver  20 . If this were a digital signal, privacy would be introduced by modulating bits of the communication signal with a random bit stream known to the receiver  20 . The random modulation amounts to  180  degree phase modulation of the signal. For these analog signals, the phase modulation is a random phase modulation with a bandwidth that can be controlled.  
         [0029]    An exemplary phase modulation circuit  14  for phase modulating the signal is shown in FIG. 4. As is shown in FIG. 4, the phase modulation of the individual segments randomly modifies the transmitted signals. The exemplary phase modulation circuit  14  comprises a waveform generator  41  that is coupled to a voltage controlled oscillator  42 . The output of the voltage controlled oscillator  42  is coupled to a 90 degree hybrid  43 . Respective outputs of the 90 degree hybrid  43  are input to first inputs of I and Q multipliers  44 ,  45 . Signals output from the Walsh coding circuit  13  are input to second inputs of the I and Q multipliers  44 ,  45 . Outputs of the I and Q multipliers  44 ,  45  are summed in an adder  46  to produce an RF signal.  
         [0030]    The bandwidth of the phase modulation is one of the parameters of the processing performed by the present invention. If the modulation has a bandwidth that is comparable to the bandwidth of a segment and the modulation index is high, the modulated bandwidth of the segment will be increased to nearly twice the original bandwidth. Less protection is provided when the modulation bandwidth and modulation index are lower.  
         [0031]    Comparing the process provided by the present invention with the protection of a digital bit stream, the digital bit stream is usually modulated with a phase modulation that has the same symbol rate as the information. Because of the digital nature of the modulation, the bandwidth of the digital bit stream is not increased, but the basic process is very similar to that discussed above for an analog phase modulation.  
         [0032]    The process of phase modulating the signal results in an RF signal that can be transmitted. This RF signal is combined with the other RF signals from other band segments to form the transmitted signal. The transmitter  11  must be substantially linear or be compensated to act as a linear transmitter to prevent intermodulation products between the segments from interfering.  
         [0033]    The receiver  20  for the signal is the inverse of the transmitter  11 . Band selection circuitry  21  uses the oscillator  32  that has phase modulation synchronized with the phase modulation imposed at the transmitter  11  to shift the signal to baseband and to remove the phase modulation. Only the desired receiver  20  knows the phase modulation signal and can synchronize to the phase modulation. Once the baseband signal for each of the segments has been recovered, the receiver  20  applies the inverse Walsh transform. The form of the inverse Walsh transform is exactly the same as that of the transmitter  11  as shown in FIG. 3, since the Walsh transform is its own inverse.  
         [0034]    When the individual band segments have been recovered, the signals are modulated back to their original band positions using the signal composition circuit  23  as shown in FIG. 5. As is shown in FIG. 5, individual band segments are modulated back to their RF positions and combined to form the original signal.  
         [0035]    An exemplary signal composition circuit  23  comprises a plurality of oscillators  51  that are respectively coupled to inputs of a 90 degree hybrid  52 . Respective outputs of the 90 degree hybrid  52  are input to first inputs of I and Q multipliers  53 ,  54 . Signals output from the inverse Walsh coding circuit  22  are input to second inputs of the I and Q multipliers  53 ,  54 . Outputs of the I and Q multipliers  53 ,  54  are summed in an adder  55  to produce the original RF analog signal.  
         [0036]    The process implemented by the present invention requires synchronization between the transmitter  11  and receiver  20 . The phase modulation must be synchronized at the receiver  20  with the transmitted phase modulation. Early-late gate processing that is commonly used with digital signals may be used in this application as well. Inserting a low-level carrier in the Walsh coded signal before phase modulation can aid in this process. The phase of the Walsh coded segments must be recovered accurately in order that the inverse Walsh transform can recover the original segments accurately.  
         [0037]    The individual bands are phase synchronized such that the signals that are near the cross-over between band segments are added coherently in the combination of segments in the receiver  20 . This requires phase and frequency locking the reconstructed segments to each other. This process can be aided by inserting a carrier near to the cross-over frequencies between the segments before the signal is broken into segments. The carrier can be made to be coherent between the segments being combined using phase locked loop techniques in the reconstruction processing.  
         [0038]    Referring now to FIG. 6, it is a flow diagram illustrating an exemplary analog signal scrambling method  60  in accordance with the principles of the present invention. The exemplary scrambling method  60  comprises the following steps.  
         [0039]    An analog signal is segmented  61  into a plurality of segments. Optionally, the input signal is phase modulated  61   a . The plurality of segments are combined  62  or coded  62 , such as by using a Walsh transform, for example. The combining or coding process obscures individual signals by burying the signal in a number of other added signals. The combined or coded signals are phase modulated  63  to introduce a random transformation of the segments that cannot be undone by a casual receiver of the signal. The phase modulated signals are then transmitted  64 .  
         [0040]    The transmitted phase modulated signals are received  65 . The received signals are segmented  66  using segment selection. The segmented signals are phase demodulated  67  to remove the phase modulation of the segmented signals. The phase demodulated signals are recovered  68 , such as by using inverse Walsh coding, to recover the original baseband segments. The original baseband segments are remodulated  69  to their original frequencies and added  70  to reproduce the original analog signal.  
         [0041]    Thus, a technique that extends the usual phase modulation of a digital signal to provide privacy for an analog signal has been disclosed. The technique starts by segmenting an analog signal, then combining the segments using a Walsh transform. This process obscures individual signals by burying the signal in a number of other added signals. The phase modulation introduces a random transformation of the segments that cannot be undone by a casual receiver of the signal.  
         [0042]    The desired receiver of the signal will recover the segments with a segment selection that removes the phase modulation of the received segments. Applying the Walsh transform to the segments will recover the original baseband segments. Remodulating the segments to their original frequencies and adding the segments will reproduce the original signal.  
         [0043]    Thus analog privacy scrambling systems and methods have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.