Abstract:
Embodiments of a method and system for determining the presence or absence of a digital communication signal are disclosed. One embodiment of the inventive method comprises the steps of collecting a plurality of signal samples at a selected frequency for a known period of time, determining a metric dependent upon at least one measured characteristic associated with the collected signal samples, and indicating signal status as present when said determined metric is greater than a known value. In another embodiment, the aforementioned method may indicate the signal status as absent when the determined metric is less than the known value. As should be appreciated, embodiments of the inventive method may be used to determine the known value by determining the metric performance for a plurality of samples for a plurality of signal conditions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Many communication systems or networks, both wired, e.g., Ethernet, and wireless, e.g., HF, VHF, UHF, radio, utilize a form of Carrier Sense Multiple Access (“CSMA”) to determine whether a frequency or radio channel is being used by another station or stations before using the frequency or channel. However, this technique does not work well in the noise- and interference-rich environment of radio communications. In some systems, a known sequence, i.e., a training sequence, is interspersed in modem waveforms to assist in signal detection and channel tracking.  
           [0002]    However, signal detection of standard HF modem waveforms is further hampered by the use of heavily filtered Phase Shift Keyed (“PSK”) waveforms (e.g., M-PSK). These waveforms are extremely noise-like in nature and difficult to distinguish from background noise and interference. Without the presence of the training sequence, the ability to determine the presence or absence of these waveforms is extremely difficult to achieve.  
           [0003]    Hence, there is a need in the industry for a system that is able to determine the presence or absence of a PSK signal without the need of a training sequence. Embodiments of the present invention are related to the field of digital communications and more specifically determining whether a communication signal is present or absent to meet the above-identified needs. More specifically, methods and systems for determining the presence or absence of a digital communication signal are disclosed. One method comprises the steps of collecting a plurality of signal samples at a selected frequency for a known period of time, determining a metric dependent upon at least one measured characteristic associated with the collected signal samples, and indicating signal status as present when the determined metric is greater than a known value. In another aspect, the method may indicate the signal status as absent when the determined metric is less than said known value. Other embodiments of the present inventive method may further be used to determine the known value by determining a metric for a plurality of samples for a plurality of signal conditions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 illustrates a block diagram of a detection circuit in accordance with the principles of the invention;  
         [0005]    [0005]FIG. 2 illustrates a flow chart of an exemplary process for determining a first signal characteristic in accordance with the principles of the invention;  
         [0006]    [0006]FIG. 3 illustrates a flow chart of an exemplary process for determining a second signal characteristic in accordance with the principles of the invention;  
         [0007]    [0007]FIG. 4 illustrates a chart of measured performance in accordance with the principles of the present invention; and  
         [0008]    [0008]FIG. 5 illustrates a system for executing the processing of the present invention. 
     
    
       [0009]    It is to be understood that these drawings are solely for purposes of illustrating the concepts of the invention and are not intended as a definition of the limits of the invention. The embodiments shown in FIGS. 1 through 5 and described in the accompanying detailed description are to be used as illustrative embodiments and should not be construed as the only manner of practicing the invention. Also, the same reference numerals, possibly supplemented with reference characters where appropriate, have been used to identify similar elements.  
       DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 1 illustrates a block diagram of an exemplary PSK receiving system  100  in accordance with the principles of the invention. In this exemplary system, a digital signal  110  is conventionally applied to multipliers  120 ,  125 , which demodulate digital signal  110  into an in-phase and quad-phase component,  126 ,  127 , respectively. The demodulated components are optionally applied to excision  130  to remove any narrow-band signal, e.g., tone jammer, that may be superimposed on signal  110 . The demodulated signals are then applied to vector magnitude determinator  140 . Determination of signal vector magnitude values as a complex value, i.e., magnitude and phase, is well known in the art. For example, real magnitude may be determined as the square root of the sum of the squares of the in-phase  126  and quad-phase  127  components of the signal.  
         [0011]    The vector magnitudes are then applied to processor  145  to determine characteristics of the received signal over a known period of time. In a preferred aspect of the invention, the determined characteristics are the mean and variance of the real magnitude of the complex value associated with the collected sample. Processor  145  is further operable to determine a metric value associated with the determined characteristics. The determined metric is then applied to comparator  147  concurrently with a known threshold value  148 . An indication  150  of signal presence occurs when the determined metric value is greater than the known threshold. Otherwise, an indication of no signal detected is provided.  
         [0012]    [0012]FIG. 2 illustrates a flow chart  200  of an exemplary process for determining a preferred first characteristic value as a real magnitude of the complex magnitude, i.e., amplitude, associated with the signals received during a known time period. In the preferred embodiment, shown in FIG. 2, a mean value of the complex vector amplitudes associated with the collected signal samples may be determined as:  
               A   _     =         ∑     n   =   1     N                     A   n       N             [   1   ]                               
 
         [0013]    In this preferred embodiment, a first sample is obtained at block  210 . The amplitude associated with the sample is added to an accumulated sum of amplitudes at block  215 . A determination is made at block  220  whether more samples are available. If the answer is in the affirmative, then a next sample is obtained at block  225  and processing continues at block  215 . However, if the answer is in the negative, then a mean value is determined in accordance with equation 1 at block  230 .  
         [0014]    [0014]FIG. 3 illustrates a flow chart  300  of an exemplary process for determining a preferred variance value of the amplitudes associated with the signals received during a known time period. In the preferred embodiment shown in FIG. 3, a variance value of the complex vector amplitudes associated with the collected signal samples may be determined as:  
                 V   =           ∑     n   =   1     N                     A   n   2       N     -         (         ∑     n   =   1     N                     A   n       N     )     2                   for                 N                 1           [   2   ]                               
 
         [0015]    or more specifically,  
             V   =         ∑     n   =   1     N                       (       A   n     -     A   _       )     2       N             [   3   ]                               
 
         [0016]    In this preferred embodiment, a first sample is obtained at block  310 . The amplitude associated with the sample is squared and added to an accumulated sum of vector magnitudes at block  315 . A determination is made at block  320  whether more samples are needed. If the answer is in the affirmative, then a next sample is obtained at block  325  and processing continues at block  315 .  
         [0017]    However, if the answer is in the negative, then an average value of the squares of the vector magnitudes or amplitude values is determined at block  330 . At block  335 , a mean value of the collected amplitudes is determined, similar to the process shown in FIG. 2. At block  340 , a variance value, in accordance with equation 2, is then determined.  
         [0018]    As would be understood, the received data samples are collected over a known period of time. In one aspect of the invention, the period is preferably 450 milliseconds. In a second aspect, the period is 900 milliseconds. However, it would be appreciated that any time period that provides for sufficient data sample collection may be selected without altering the scope of the invention.  
         [0019]    A performance metric may be determined at block  350  in relation to the first and second measured characteristic, in the preferred embodiment mean and variance, as:  
             m   =         A   _     2     V             [   4   ]                               
 
         [0020]    where m is the metric value;  
         [0021]    {overscore (A)} is the mean value; and  
         [0022]    V is the variance.  
         [0023]    In this case, the ratio shown in equation 3 provides an indication of dispersion of the received demodulated samples in IQ vector space. A low dispersion value indicates the received signal resembles a PSK signal received with an unknown sub-carrier frequency offset.  
         [0024]    At block  360 , a determination is made whether the determined metric is greater than a known threshold value. If the answer is negative, then processing is ended. However, if the answer is in the affirmative, then the signal status is deemed present at block  370 .  
         [0025]    Although the present invention has been described in accordance with a preferred embodiment of the invention, it should be appreciated that other similar metrics based on statistical characteristics of the received signal sample, such as geometric mean, harmonic mean, weighted mean, mode, median, quartile, percentiles, mean deviation, standard deviation, root mean square, etc., may also be used without altering the scope of the invention.  
         [0026]    [0026]FIG. 4 illustrates the performance metric shown in equation 3 using a plurality of samples associated with different signal conditions. Histogram  410  is representative of a plurality of metric values determined by equation 1 for a no signal and an on-air noise only condition. Histogram  420  is representative of a plurality of metric values determined by equation 1 for a white-Gaussian noise only signal. Histogram  430  is representative of a plurality of metric values determined by equation 1 for a signal having a signal-to-noise ratio of 2 dB. Histogram  440  is representative of a plurality of metric values determined by equation 1 for a signal having a signal-to-noise ratio of 10 dB. As should be appreciated, no signal or noise only signals have a large dispersion and a corresponding small metric value. On the other hand, relatively high SNR signals have a small dispersion and a corresponding large metric value.  
         [0027]    [0027]FIG. 4 further illustrates a metric related threshold value that may be used in comparator  147  of FIG. 1 to determine the presence of a signal. In this case, a value of four (4) units may be selected as a threshold value in comparator  147  that may be used to determine the presence of signal. Hence, data collected having a metric value less than the selected threshold are classified “not-present,” while data collected having a metric value greater that the selected threshold are classified as “present”.  
         [0028]    [0028]FIG. 5 illustrates an exemplary embodiment of a system  500  containing processor  145  that may be used for implementing the principles of the present invention. System  500  may represent a desktop, laptop or palmtop computer, a personal digital assistant (PDA), as well as portions or combinations of these and other devices, and may be embedded in a radio product, cellular phone or other wireless communication device or product. System  500  may contain one or more input/output devices  502 , processors  503  and memories  504 , that are operable to execute the processing illustrated in FIGS. 2 and 3.  
         [0029]    Processor  145  may access vector magnitude determinator  501  that contains information items regarding vector magnitudes associated with the collected samples. Information contained in vector magnitude determinator  501  may be stored in permanent or semi-permanent media, such as RAM, ROM, hard disk drive, optical disk drive or other image storage devices or may also be obtained dynamically and in real-time.  
         [0030]    Input/output devices  502 , processors  503  and memories  504  may communicate over a communication medium  506 . Communication medium  506  may represent, for example, an internal communication bus or network, one or more internal connections of a circuit, circuit card or other apparatus, as well as portions and combinations of these and other communication media. Input data from vector magnitude determinator  501  may be received by I/O device  502  and, in accordance with one or more software programs operable to execute the exemplary processing shown in FIGS. 2 and 3, may be stored in memories  504  and executed by processors  503 . The output of the processing, i.e., a metric value, may then be applied to comparator  147  for further processing.  
         [0031]    Processors  503  may be any means, such as general purpose or special purpose computing system, or may be a hardware configuration, such as a laptop computer, desktop computer, handheld computer, dedicated logic circuit, integrated circuit, Programmable Array Logic (PAL), Application Specific Integrated Circuit (ASIC), etc., that provides a known output in response to known inputs.  
         [0032]    In one embodiment, coding and decoding employing the principles of the present invention is implemented by computer readable code executed by processor  503 . The code may be stored in the memory  504  or read/downloaded from a memory medium such as a CD-ROM or floppy disk (not shown) in communication with processor  503  or I/O device  502 . In other embodiments, hardware circuitry may be used in place of, or in combination with, software instructions to implement the invention. For example, the elements illustrated herein may also be implemented as discrete hardware elements.  
         [0033]    As would be understood, the term processor may represent one or more processing units or computing units in communication with one or more memory units and other devices, e.g., peripherals, connected electrically, electronically, or wirelessly to and communicating with the at least one processing unit. Furthermore, the devices may be electrically, electronically or wirelessly connected to the one or more processing units via internal busses, e.g., ISA bus, microchannel bus, PCI bus, PCMCIA bus, wireless, etc., or one or more internal connections of a circuit, circuit card or other device, as well as portions and combinations of these and other communication media or an external network, e.g., the Internet and Intranet.  
         [0034]    While there has been shown, described, and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the apparatus described, in the form and details of the devices disclosed, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. For example, although the present invention has been described with regard to declaring the presence of a digital signal at a known frequency, it would be within the knowledge of one skilled in the art to provide to indication when presence is detected and an indication when a signal is not detected. Accordingly, while the word “presence” is used to describe the present invention, the determination of the “absence” of a signal is also contemplated to be within the scope of the invention. It should further be appreciated that the present invention is operable when a large frequency offset is present. Hence, the tracking of a frequency offset, which is preformed in conventional modem technology, is not needed for the operation of the present invention.  
         [0035]    It is expressly intended that all combinations of those elements that perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated.