Abstract:
A system for short range communications includes a transmitter capable of transmitting a colored noise-like preamble. A receiver receives the colored noise-like preamble and the receiver includes an antenna with an antenna pattern. A direction of the antenna is controllable by the receiver. A signal processor is connected to and responsive to the receiver. The signal processor detects and estimates the strength of the colored noise-like preamble.

Description:
BACKGROUND OF THE INVENTION 
   This invention generally relates to a spread spectrum communication system and, more particularly, to narrowband interference mitigation for a spread spectrum communication system using electronic processing of low complexity. 
   Industrial spread spectrum communications have benefited greatly from the rules of the U.S. Federal Communications Commission (FCC) Part 15 relating to unlicensed spread spectrum communications. The advent and universal acceptance of Industrial, Scientific, and Medical (ISM) communications has benefited many organizations faced with a requirement to transport data over short distances. Worldwide response has been extremely positive for the ISM communications of data over short distances. For example, the ISM band from 2400–2483.5 MHz is almost universally available. 
   Spread spectrum communications are often asymmetric in cost and complexity. For example, spread spectrum signals can be generated using circuitry of relatively low complexity. However, detection and successful demodulation of such signals is typically a complex and expensive task. The cost/complexity asymmetry is especially true in an interference environment. Since the communications are unlicensed and quite often used to support host missions on mobile platforms, it is prudent to plan for the contingency that a relatively strong narrowband interfering signal (“interferer”) must be removed or excised in order to gain a sufficient signal-to-interferer-plus-noise ratio for the requisite data transportation quality. Therefore, a desire exists for systems that can be used to enable short-range spread spectrum communications using circuits and signal processing techniques of low complexity. 
   BRIEF SUMMARY OF THE INVENTION 
   In one exemplary embodiment, a communications system is provided comprising a transmission unit and a receiving unit. In one embodiment, the transmission unit comprises a noise source for generating a noise signal. A signal generator is connected to the noise generator and generates a colored noise-like preamble from at least the noise signal. A modulator is connected to the signal generator and modulates the colored noise-like preamble. A switching device has at least a first input, a second input and an output. The first input is connected to the modulator. An ISM spread spectrum modulator is connected to the second input of the switching device and provides an ISM transmission signal. A transmitter is connected to the output of the switching device. When the switching device is in a first position, the colored noise-like preamble is provided as a transmitter output signal. When the switching device is in a second position, the ISM transmission signal is provided as the transmitter output signal. The colored noise-like preamble is transmitted by the transmitter before the ISM transmission signal. In one embodiment, the receiving unit comprises an antenna that receives the transmitter output signal transmitted by the transmission unit. The antenna adjusts an antenna pattern for improving reception of the transmitter output signal by the transmission unit. A signal processor is connected to the antenna. The antenna produces an antenna output signal that includes the antenna pattern and the transmitter output signal. The signal processor evaluates the antenna output signal and determines at least the presence of the colored noise-like preamble in the antenna output signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram view of one exemplary embodiment of a transmitter for ISM transmissions; 
       FIG. 2  is a block diagram view one exemplary embodiment for generating sampled signal values of a colored noise-like preamble; 
       FIG. 3  is a block diagram view of another exemplary embodiment for generating sampled signal values of a colored noise-like preamble; 
       FIG. 4  is a flow diagram of the one exemplary method of operation forming the n-samples of a colored noise-like preamble using an interleaving accumulator; 
       FIG. 5  is a block diagram view of one exemplary embodiment of an ISM receiver with augmented capability of searching for the transmitted spectrally colored noise-like preamble and mitigating an off-angle narrowband interferer: 
       FIG. 6  is a block diagram of another exemplary embodiment of an ISM receiver having a two antenna system with a signal processing module; and 
       FIG. 7  is a block diagram view of one exemplary embodiment of a signal processing submodule within the signal processing module. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In one embodiment, a communication system includes an ISM transmission unit  100  that creates a preamble to a spread spectrum transmission and a ISM receiver unit  500 ,  600  that recognizes the preamble received from the ISM transmission unit  100 . Such that the preamble complies with various governmental regulations pertaining to spread spectrum communications, the preamble also spreads energy over a wide frequency band. 
   As shown in  FIG. 1 , one representative embodiment of an ISM transmission unit  100  creates an ISM transmission that is preceded by a spectrally colored noise-like preamble. In one embodiment, colored noise comprises a signal where the power spectral density of the noise is not substantially flat, “white”, over a significant bandwidth of the preamble. An output signal of a noise source  110  is input to a signal generator  120  that generates sampled signal values of a colored noise-like preamble. It should be appreciated that, in one embodiment, that the noise source  110  comprises a broadband noise source. The sampled signal values output from the signal generator  120  are input to a modulator  130  that modulates the sampled signal values. In one embodiment, the modulator  130  converts the sampled signal values into a form suitable for transmission, such as, for example, heterodyning or shifting the sampled signal values to a higher frequency for transmission. An output signal from the modulator  130  is connected to a switch  150 . As shown in  FIG. 1 , in one embodiment, the switch  150  comprises a two-position switch having a first input  152  and a second input  154 . In the embodiment shown in  FIG. 1 , the first input  152  is connected to the modulator  130  and an ISM spread spectrum modulator  160  is connected to the second input  154  of the switch  150 . In one embodiment, the switch  150  is activated by a timer (not shown) that allows the output signal from the modulator  130  to be passed to the antenna  180  for a predetermined amount of time before the ISM spread spectrum modulator  160  is connected to the antenna  180 . When the switch  150  is in position A, the output signal from the modulator  130  via the first input  152  is passed to the transmitter  170  via the output  156 . When in position A, the transmitter  170  drives an antenna  180  with an output signal from the modulator  130 . When the switch  150  is in position B, the output signal of the ISM spread spectrum modulator  160  is supplied from the second input  154  to the transmitter  170  via the output  156  of the switch  150 . When in position B, the transmitter  170  drives the antenna  180  with the output signal from the ISM spread spectrum modulator  160 . In operation, the ISM transmission unit  100  operates by starting with switch  150  in position A whereby the spectrally colored noise-like preamble is provided to the signal transmitter  170  and transmitted via the antenna  180 . After the spectrally colored noise-like preamble is transmitted, the conventional ISM transmission is commenced after the switch  150  is placed in position B. It should be appreciated that the switch  150  can be electronically programmed using, such as for, example, software or a timing device, to switch from position A to position B. 
   As shown in  FIG. 2 , one exemplary embodiment of the signal generator  120  that generates sampled signal values of a colored noise-like preamble comprises a low pass filter  121  that colors the noise input from the noise source  110 . In one embodiment, the noise input is colored such that the power spectral density of the noise is not substantially flat, “white”, over the significant bandwidth of the signal. The output signal from the low pass filter  121  is input to a sampler  122  that produces a signal composed of periodic samples of the output signal of the colored noise from the low pass filter  121 . The output signal of the sampler  122  is input to the modulator  130 . In the embodiment shown in  FIG. 2 , the broadband noise signal from the noise source  110  is severely narrowed in spectral content using the low pass filter  121  in order to induce a significant inter-sample correlation. 
   In another embodiment, as shown in  FIG. 3 , the colored noise-like preamble comprises a bandwidth closer to the original noise-like signal provided by the noise source  110 . It should be appreciated that, in one embodiment, the noise source  110  comprises a broadband noise source. In  FIG. 3 , the output signal from the noise source  110  is input to a sampler  122  that produces a signal composed of periodic samples from the output signal of the noise source  110 . The output signal from the sampler  122  is provided to an interleaver accumulator  124  that produces a signal composed of interleaved sequences of samples of colored noise. The output signal of interleaver accumulator  124  is input to the modulator  130 . 
   If it is assumed that the white noise process is a zero mean, unit variance, memoryless Gaussian process with samples {g(n)}, the interleaver accumulator  124  forms a colored noise output signal {s(n)} by forming:
 
 s ( i ) =αs ( i−d )+(1−α) g ( i ), 0&lt;α&lt;1
 
The signal power of the output signal {s(n)} is
 
             σ   s   2     =       1   -   α       1   +   α             
and the autocorrelation coefficient of lag k, α(k), is
 
             ρ   ⁡     (   k   )       =     {             α          d   /   k            ,     d   ❘   k                 0   ,   otherwise                   
The normalized power spectral density of {s(n)}, Φ s /σ s   2 , is
 
                     Φ   s     /     σ   s   2       =       ⁢       ∑     k   =     -   ∞       ∞     ⁢       ρ   ⁡     (   k   )       ⁢     ⅇ       -   j     ⁢           ⁢   2   ⁢           ⁢   π   ⁢           ⁢   f   ⁢           ⁢   k                       =       ⁢       -   1     +       ∑     k   =   0     ∞     ⁢       ρ   ⁡     (   k   )       ⁢     ⅇ       -   j     ⁢           ⁢   2   ⁢           ⁢   π   ⁢           ⁢   f   ⁢           ⁢   k                         =       ⁢       -   1     +     4   ⁢       ∑     m   =   0     ∞     ⁢       α   m     ⁢     cos   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢   f   ⁢           ⁢   d   ⁢           ⁢   m     )                           =       ⁢       -   1     +       1   -     α   ⁢           ⁢     cos   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢   fd     )             1   +     α   2     -     2   ⁢   α   ⁢           ⁢     cos   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢   fd     )                           
illustrating the coloring of the noise signal spectrum. It should be appreciated that the ISM transmission unit  100  can be adjusted to the requirements of any applicable governmental regulations such that the preamble signal is sufficiently spread for ISM usage. In one embodiment, this adjustment can be made by selecting the appropriate values for d and α. It should be appreciated, in one embodiment, that the symbol d represents the span or memory of the interleaver accumulator  124  as described in equation hereinabove, and the symbol α represents the degree to which the noise is colored as described in the same equation hereinabove. In one representative embodiment, for example, setting d=25 and α=0.95 will divide the signaling energy into 12 lobes over the baseband frequency spectrum.
 
   In one embodiment as shown in  FIG. 4 , the interleaver accumulator  124  produces n-samples of the colored noise-like preamble. The interleaver accumulator  124  starts the process (step  410 ). The noise source  110  is sampled “d” times and the samples are stored in s( 1 ), s( 2 ), . . . s(d) (step  420 ). It should be appreciated that, in one embodiment, the noise source  110  comprises a broadband Gaussian noise source. An index “i” is set to the value d+1 (step  430 ). In step  440 , the i-th sample “s(i)” of the colored noise-like preamble is formed by the interleaver accumulator  124  by computing:
 
 s ( i )=α s ( i−d )+(1−α) g ( i ).
 
The index “i” is incremented by unity (step  450 ). A test is made to determine if the index “i” exceeds a predetermined value “n”. In one embodiment, the value “n” comprises a default value of the number of samples and can be selected such that the preamble transmission time (the number of samples “n” divided by the symbol transmission rate) is about 100 milliseconds (ms). If index “i” does exceed the predetermined value “n”, the process finishes (step  470 ). If the index “i” does not exceed the predetermined value “n”, the next i-th “s(i+1)” sample is formed (step  440 ) and the process continues.
 
   When not receiving an ISM transmission, an ISM receiver unit  500 ,  600  constantly scans for the presence of the colored noise-like preamble that is transmitted by the ISM transmission unit  100 . In one embodiment, the scanning can comprise moving a directed spatial null around and testing for the presence of the colored noise-like preamble by computing a lag d autocorrelation of a output signal from the ISM receiver unit  500 . In one embodiment, the complexity and costs are kept down by using an arc-sine law to process the output signal and determine the presence of the colored noise-like preamble. 
   In one embodiment as shown in  FIG. 5 , an ISM receiver unit  500  is provided that receives transmissions from the ISM transmitting unit  100 . In one embodiment, an antenna  510  continually rotates a spatial null. It should be appreciated, in one embodiment, that the spatial null is part of an antenna pattern where the antenna gain is substantially zero or equal to zero. In addition, it should be appreciated that the antenna  510  adjusts an antenna pattern to improve reception of the transmission received from the ISM transmitting unit  100 . Also, the improvement of the reception of the transmission received from the ISM transmitting unit includes receiving the transmission that has a stronger signal than a previously received transmission. In one embodiment, the adjustment of the antenna pattern  510  can comprise rotating a spatial null. The antenna  510  is connected to a signal processor  515 . The signal from the antenna  510  is provided to a downconverter  520  of the signal processor  515 . The output signal from the downconverter  520  is provided to an ISM receiver  540  of the signal processor  515 . In one embodiment, the ISM receiver  540  is a conventional ISM receiver. In another embodiment, the output signal from the downconverter  520  is provided to a capacitor  530  that blocks any DC level in the signal. It should be appreciated that, in even another embodiment, the capacitor  530  is not provided in the ISM receiver unit  500 , and as such, DC levels are not blocked from the signal. The signal provided by the capacitor  530  is then input to a sampler  550  of the signal processor  515 . The output signal from the sampler  550  is provided to a one-bit quantizer  560  of the signal processor  515 . The one bit output signals from the one-bit quantizer  560  are input to an arc-sine law processor  570  of the signal processor  515 , and the arc-sine law processor  570  searches for the presence of the colored noise-like preamble. If the arc-sine law processor  570  does not detect the colored noise-like preamble then the arc-sine law processor  570  instructs the antenna  510  to continue to rotate the spatial null. If the arc-sine law processor  570  detects the colored noise-like preamble, the arc-sine law processor  570  instructs the antenna  510  to adjust the angular placement of the spatial null in order to maximize the reception of the colored noise-like preamble. In so doing, the ISM transmission following the spectrally colored noise-like preamble will be received under an optimal signal-to-noise condition. 
   In  FIG. 6 , another embodiment of the ISM receiving unit  600  receives transmissions from the transmitter  100 . In this embodiment, the colored noise-like preamble of the transmitter  100  is highly correlated to temporal lag d. The ISM receiving unit  600  includes antennas  610  and  620  that have antenna patterns exhibiting a single mainlobe. In one embodiment, the antennas  610 ,  620  are spatially separated on the order of a wavelength or more, and the mainlobes of the antennas  610 ,  620  are oriented in slightly different directions. The output signals of the antennas  610 ,  620  are processed in signal processor  515  that forms a sum signal and a difference signal from the output signals of the antennas  610 ,  620 . The sum signal corresponds to a beam pattern in the far field and is designated the sum beam  710  ( FIG. 7 ). The difference signal corresponds to a beam pattern in the far field and is designated the difference beam  730  ( FIG. 7 ). The output signal from the signal processor  515  is provided to the ISM receiver  540 . 
   Typically, the angle of arrival of a signal can be estimated from comparison of the sum beam  710  and difference beam  730 . This procedure is known as monopulse angle estimation. The standard angle estimation procedure can be modified to estimate the angle-of-arrival of a colored noise signal. In  FIG. 7 , the signal processing performed by the signal processor  515  that follows the formation of the sum beam  710  and difference beam  730  is provided. From the antennas  610  and  620 , the sum beam  710  is input to unit  718  that converts the sum beam  710  to a complex baseband. In one embodiment, the unit  718  is not provided and the sum beam  710  is provided directly to a delay unit  712  via the antennas  610  and  620 . The output of the unit  718  is input to a delay unit  712 . The output signal of the delay unit  712  is provided to a complex conjugator  714 . In one embodiment, the complex conjugator  714  is used to negate the imaginary portion of the output signal from the delay unit  712 . The output signal of the complex conjugator  714  is multiplied by the output signal from unit  718  (complex baseband converted sum beam  710 ) in a multiplier  716 . The output signal of the multiplier  716  is provided to an integrator  720 . The output signal of the integrator  720  is denoted ρ s  and is input to the monopulse processing module  750 . From the antennas  610  and  620 , the difference beam  730  is input to unit  738  that converts the difference beam  730  to a complex base band. In one embodiment, the unit  738  is not provided and the difference beam  730  is provided directly to a delay unit  732  via antenna  610  and  620 . The output from the unit  738  is input to a delay unit  732 . The output signal of the delay unit  732  is input to a complex conjugator  734 . In one embodiment, the complex conjugator  734  is used to negate the imaginary portion of the output signal from the delay unit  732 . The output signal from the complex conjugator  734  is multiplied by the complex baseband version of the sum beam  710  in a multiplier  736 . The output signal from the multiplier  736  is provided to an integrator  740 . The output signal from the integrator  740  is denoted ρ sd  and is input to the monopulse processing module  750 . The monopulse processing module  750  calculates the angle of arrival, β, from the ρ s  signal and the ρ sd  signal corresponding to the lag or delay d specified in the delay units  712  and  732 . The monopulse processing module  750  determines the angle of arrival, β, by calculating the ratio of ρ sd  to ρ s . This ratio is an estimate of the ratio of the response of the difference beam  730  at the angle-of-arrival of the signal to the response of the sum beam  710  at the angle-of-arrival of the signal. This ratio can be converted to the angle-of-arrival estimate by a look-up table this is obtained from the specific sum beam  710  and difference beam  730  patterns that are produced by antennas  610  and  620  as combined by signal processor  515 . Once β is determined, if the lag or delay d corresponds to the colored noise-like preamble, the monopulse processing unit  750  causes a high gain lobe of the antennas  610  and  620  to be steered in the direction of β. If the lag or delay d corresponds to the narrowband jamming process and/or narrow bandwidth interfering signal, the monopulse processing unit  750  causes an antenna pattern, such as, for example, a spatial null to be steered to the antennas  610 ,  620  in the direction of β. In one embodiment, since the interfering signals have a narrow bandwidth, the correlation function is periodic. Therefore, if the lag or delay d is a multiple of the period of the narrow bandwidth interfering signal, the angle-of-arrival of the interfering signal can be determined. Further, in one embodiment, the steering of the antennas  610  and  620  is provided by forming a weighted difference of first antenna output a 1 (t) and second antenna output a 2 (t) using the following equation: Difference = K 1  · a 1 (t) − K 2  · a 2 (t). In one embodiment, a null will exist at angle θ in the difference beam  730  when the ratio of K 1 /K 2  is equal to the ratio of b 2 (θ)/b 1 (θ) where b 1  and b 2  are the beam patterns of the antennas  610  and  620  at the temporal frequency of the interfering signal. When the interfering signal has a narrow bandwidth, the difference beam  730  could be used to receive a desired signal when nulling out the narrow bandwidth interfering signal. To steer antennas  610  and  620  having an antenna pattern with high gain lobes, such as, for example, wideband, a time delay is used to compensate for the delay difference between the two elements. In one embodiment, one element would be delayed with respect to the other element so that the signals coming from the desired direction are added in-phase regardless of the frequency of the signals. It should be appreciated that this technique is a standard beamforming technique to one skilled in the art.