Abstract:
A system and method for detecting the presence of a signal on a channel mixes signals received with an antenna with a local signal having a controllable frequency within a band of frequencies defined by the channel. The local signal is produced with a sequentially adjusted frequency within the channel. The mixing operation (for each local signal frequency) produces an output signal having a frequency indicative of a difference between a frequency of signals received with the antenna and a frequency of the local signal. The output signal is converted to a baseband frequency band, and is then analyzed to determine whether the signals received with the antenna occupy the channel.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system and method of detecting channel occupancy, such as in a listen-before-talk (LBT) communication protocol. 
     In many communication systems, a transceiver of a device follows an LBT protocol to ensure that a communication channel is clear before transmitting on the channel. In typical systems that employ communication channels having a relatively wide bandwidth, devices employ a transceiver that has the ability to detect a signal that is present anywhere within the channel bandwidth. The sensitivity of the transceiver is also typically quite high, to ensure that any signal that might be present on the channel is detected. As a result of the performance characteristics that are required, transceivers in typical systems can be quite expensive to implement, and can result in detection of false positives due to their high sensitivity across the entire channel bandwidth. 
     An improved system and method for detecting the presence of signals on a communication channel would be a useful advance in the state of the art. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a system and method for detecting the presence of a signal on a channel. Signals received with an antenna are mixed with a local signal having a controllable frequency within a band of frequencies defined by the channel. The local signal is produced with a sequentially adjusted frequency within the channel. The mixing operation (for each local signal frequency) produces a baseband output signal having a frequency indicative of a difference between a frequency of signals received with the antenna and a frequency of the local signal. The output signal is analyzed to determine whether the signals received with the antenna occupy the channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the functional components of a receiver for detecting the presence of a signal on a channel in accordance with the present invention. 
         FIG. 2  is a flow diagram illustrating a method of operating a receiver to detect the presence of an interfering signal on a wideband communication channel. 
         FIGS. 3A-3C ,  4 A- 4 C,  5 A- 5 C and  6 A- 6 C are graphs illustrating the frequency response of a receiver according to the present invention to various input signals. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram illustrating the functional components of receiver  10  for detecting the presence of a signal on a channel in accordance with the present invention. Antenna  12  is provided to receive signals, and is coupled to frequency mixer  14 . Antenna  12  is coupled to frequency mixer  14  through optional front end filter  16  and optional low noise amplifier (LNA)  18  in some embodiments. Local oscillator  20  is connected to frequency mixer  14 , and is controllable to provide an output at a number of frequencies. Frequency mixer  14  is connected to baseband circuitry  22 , which amplifies and filters the baseband signal output by frequency mixer  14 . Baseband filter  24 , which may be fixed or adjustable, is connected to baseband circuitry  22 , and the output of baseband filter  24  is connected to tone detector  26 . Tone detector  26  may be an analog or digital tone detector circuit, or may include an analog-to-digital converter (ADC) and a digital signal processor (DSP) for performing an algorithm such as a Goertzel algorithm or a fast Fourier transform (FFT) algorithm. All of these tone detecting techniques are known and well understood in the art. 
     In operation, receiver  10  receives signals via antenna  12 . In embodiments that include front end filter  16 , the received signal is filtered to pass only signals of interest. Furthermore, in embodiments that include LNA  18 , the signal is amplified as well. The received signal is input to frequency mixer  14 , which mixes the incoming signal with a signal produced by local oscillator  20 , producing a baseband signal having a frequency indicative of the difference between the frequency of the incoming signal and the signal produced by local oscillator  20 . The difference signal is amplified and filtered by baseband circuitry  22  (which includes low pass filter and amplifier components), and is then filtered by baseband filter  24 . This process eliminates the high frequency signals from the output of frequency mixer  14 , so that low frequency signals (indicative of an incoming signal having a frequency that nearly matches the frequency of local oscillator  20 ) are isolated. Tone detector  26  receives the filtered signal from baseband filter  24  and determines whether a signal matching the frequency of the signal produced by local oscillator  20  has been detected. In this way, receiver  10  is able to detect the presence of a signal in a particular frequency band of a communication channel. 
       FIG. 2  is a flow diagram illustrating a method of operating a receiver (such as receiver  10  shown in  FIG. 1 ) to detect the presence of an interfering signal on a wideband communication channel. The steps shown in the flow diagram of  FIG. 2  will be explained with respect to operation of the components shown in  FIG. 1  (and referred to by reference number). Local oscillator  20  is set to an initial frequency (step  30 ), and the signal produced by local oscillator  20  is mixed by frequency mixer  14  with the signal received from antenna  12  to produce a difference signal (step  32 ). A baseband frequency range is then monitored (i.e., by ADC  26 ) to determine whether the difference signal is present in the baseband frequency range (step  34 ). If a baseband difference signal is detected (step  36 ), it is concluded that channel interference is present (step  37 ). If a baseband difference signal is not detected (step  36 ), the system determines whether all of the local oscillator frequencies within the range of channel frequencies have been checked (step  38 ). For example, a system may be configured to check for interference signals at a frequency below the center frequency of the channel, at a frequency above the center frequency of the channel, and at the center frequency of the channel. Other configurations may also be used. If not all local oscillator frequencies (that is, all of the frequencies that local oscillator  20  is configured to step through) have been checked, local oscillator  20  is stepped to the next frequency to be checked (step  40 ), and the local oscillator signal is again mixed with the signal received from antenna  12  to produce a difference signal (step  32 ). If all local oscillator frequencies have been checked, and no interference has been detected, it is concluded that the channel is clear (step  41 ). 
       FIGS. 3A-3C  are graphs illustrating the response of a Goertzel tone detector (employed as element  26  shown in  FIG. 1 ) to an input signal (received on antenna  10  shown in  FIG. 1 ) at the center channel frequency having a power level of −55 dBm, with local oscillator  20  (shown in  FIG. 1 ) having a frequency that is offset from the center channel frequency by −37.5 kHz.  FIG. 3A  is a graph of the input signal,  FIG. 3B  is a graph of the frequency response of a MATLAB® generated Goertzel tone detector algorithm, and  FIG. 3C  is a graph of the frequency response of a Simulink® generated tone detector algorithm (MATLAB® and Simulink® are both commercially available mathematical modeling software packages). As shown in the graphs of  FIGS. 3B and 3C , the frequency index labeled “5” (which corresponds to detection of a signal offset from the local oscillator frequency by 37.5 kHz) has a magnitude of 1×10 7  units, indicating the presence of a signal at this frequency. 
       FIGS. 4A-4C  are graphs illustrating the response of a Goertzel tone detector (employed as element  26  shown in  FIG. 1 ) to an input signal (received on antenna  10  shown in  FIG. 1 ) at the center channel frequency having a power level of −96 dBm, with local oscillator  20  (shown in  FIG. 1 ) having a frequency that is offset from the center channel frequency by −37.5 kHz.  FIG. 4A  is a graph of the input signal,  FIG. 4B  is a graph of the frequency response of a MATLAB® generated Goertzel tone detector algorithm, and  FIG. 4C  is a graph of the frequency response of a Simulink® generated tone detector algorithm. As shown in the graphs of  FIGS. 4B and 4C , the frequency index labeled “5” (which corresponds to detection of a signal offset from the local oscillator frequency by 37.5 kHz) has a magnitude of 4×10 5  units, indicating the presence of a signal at this frequency. 
       FIGS. 5A-5C  are graphs illustrating the response of a Goertzel tone detector (employed as element  26  shown in  FIG. 1 ) to an input signal (received on antenna  10  shown in  FIG. 1 ) at the center channel frequency having a power level of −105 dBm, with local oscillator  20  (shown in  FIG. 1 ) having a frequency that is offset from the center channel frequency by −37.5 kHz. The −105 dBm level of the input signal is below the required sensitivity of the system.  FIG. 5A  is a graph of the input signal,  FIG. 5B  is a graph of the frequency response of a MATLAB® generated Goertzel tone detector algorithm, and  FIG. 5C  is a graph of the frequency response of a Simulink(&amp; generated tone detector algorithm. As shown in the graphs of  FIGS. 5B and 5C , the frequency index labeled “5” (which corresponds to detection of a signal offset from the local oscillator frequency by 37.5 kHz) has a magnitude of less than 1×10 5  units, providing only a slight indication of the presence of a signal at this frequency. 
       FIGS. 6A-6C  are graphs illustrating the response of a Goertzel tone detector (employed as element  26  shown in  FIG. 1 ) to no input signal (received on antenna  10  shown in  FIG. 1 ).  FIG. 6A  is a graph of the input signal (which is essentially just random noise),  FIG. 6B  is a graph of the frequency response of a MATLAB® generated Goertzel tone detector algorithm, and  FIG. 6C  is a graph of the frequency response of a Simulink® generated tone detector algorithm. As shown in the graphs of  FIGS. 6B and 6C , the frequency index labeled “5” (which corresponds to a frequency offset from the local oscillator frequency by 37.5 kHz) has no indication of a signal at this frequency, illustrating that the tone detector does not detect a false positive when no signal is present. 
     In application, the tone detector utilized in the receiver system of the present invention will be calibrated so that magnitudes of input signals above a threshold will be interpreted as interfering signals, while magnitudes of input signals below the threshold will be interpreted as a clear channel. 
     The stepped narrowband frequency approach of listening for interference on a wideband communication channel provides savings in both the cost and the complexity of the receiver. In prior systems, listening for interference on a wideband channel required a receiver that was highly sensitive to any signals within the communication channel bandwidth. When the channel bandwidth is wide, this is an expensive piece of equipment. The stepped approach offered by the present invention allows a receiver to be sensitive to a narrow bandwidth of signals at a time, which is much less expensive to implement. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.