Abstract:
A radio carrier signal is detected ( 20 ) after being converted to digital form and is filtered ( 22 ). A noise estimator ( 24 ) generates a noise estimate signal by estimating the noise in the detected signal due to atmospheric conditions and the noise due to the gain in noise figure properties of the circuitry. The output of the noise estimator is used to calculate a threshold signal ( 50 ) and another detection operation ( 52 ) determines whether the power of a signal derived from the detected signal exceeds the threshold. If the threshold is exceeded, the detected signal is passed through a signal conditioner, such as a switch ( 54 ) to an output path ( 56 ).

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to radio reception, and more particularly relates to the squelching of a signal received by a radio. 
     Squelching techniques have been known in the past. Typically, such techniques are based on an envelope detected signal which has already been subjected to automatic gain control. That is, the squelching is based on a normalized envelope detected signal. In addition, frequency division of signal and noise is used in most commercial radios. 
     New regulations confining aviation radio channels to an 8.33 KHz channel separation have decreased the performance capability of prior known squelch techniques. The 8.33 KHz channel separation makes the frequency separation of signal plus noise power and noise power for use in squelch operation unreliable due to carrier frequency uncertainty and the closeness of adjacent channels. As a result, a reliable method of detecting signal presence prior to baseband AGC and of deriving a measure of signal plus noise power divided by signal power is desired. Such method must not overload the processor executing baseband processing. The present invention addresses these problems and provides a solution. 
     BRIEF SUMMARY OF THE INVENTION 
     The preferred embodiment is useful in a radio receiver adapted to receive a radio frequency carrier signal. In such an environment, signal squelching is provided by generating a baseband carrier signal in response to the radio frequency carrier signal. A detected signal is generated in response to the baseband carrier signal and a noise estimate signal is generated by estimating at least the thermal noise in the detected signal. A threshold signal defining a predetermined signal level is generated in response to the noise estimate signal and one or more predetermined signal parameters. A first signal is generated in the event that a signal derived from the detected signal has a first predetermined relationship with respect to the predetermined signal level, and a second signal is generated in the event that the signal derived from the detected signal has a second predetermined relationship with respect to the predetermined signal level. The detected signal is passed in response to the first signal, and the detected signal is squelched in response to the second signal. 
     By using the foregoing techniques, a squelch algorithm of acceptable computational complexity for a CPU processor can be provided. An estimate of signal quality can be made even with an 8.33 KHz channel spacing. The estimate does not require extensive filtering to separate signal power plus noise and noise power alone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram and functional block diagram of a preferred form of algorithm for squelch processing made in accordance with the present invention. 
     FIG. 2 is a flow diagram illustrating in more detail a portion of the functional block diagram shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a preferred form of radio receiver  10  made in accordance with the present invention includes a conventional antenna  12  which receives a radio frequency carrier signal that is modulated by amplitude modulation and is transmitted in the VHF range of about 118-137 MHz. The radio frequency carrier signal is processed by carrier processing circuitry  14  that generates a baseband carrier signal in response to the radio frequency carrier signal. 
     A preferred form of carrier processing circuitry is described in the patent application Ser. No. 09/677,317 entitled Radio Receiver Automatic Gain Control Techniques, filed on the same date as this application, filed in the name of the same inventor, assigned to the same assignee, and incorporated by reference into this application. The carrier processing circuitry  14  provides AGC control for the radio frequency (RF) carrier signal, as well as an intermediate frequency (IF) carrier signal generated by circuitry  14 . The carrier processing circuitry  14  converts the IF carrier to digital form and then converts the IF carrier to a baseband digital carrier signal. Circuitry  14  also resamples the digital baseband carrier to 42 kilosamples per second and generates orthogonal I and Q signals representing the baseband carrier. The conversion of the IF carrier to the baseband digital carrier and resampling is preferably accomplished by a digital signal processor  16  which performs the additional functions illustrated by blocks  20 - 52  in the remainder of FIG.  1 . 
     The algorithm performed by processor  16  executes in real time floating point arithmetic. The algorithm performs an envelope detection operation  20  on the envelope of the baseband carrier signal which has been partially normalized by an IF and RF automatic gain control, but not by any baseband automatic gain control. The detected signal has a residual dynamic range of 67 decibels from which is derived a reliable estimate of signal with noise power and of noise power alone. Narrow band Mode  0  channels have an assigned 8.33 KHz frequency separation, making it difficult and unreliable to try to filter outside the band for a noise estimate. The detected signal is filtered by a decimate by 2 frequency impulse response (FIR) filter operation  22  with a six decibel bandwidth of 2.7 KHz and 60 decibel bandwidth of 3.5 KHz. 
     Basically, the algorithm passes the detected and filtered audio signal when the average power out of FIR  22  exceeds a threshold based on the best estimate of the noise floor from the receiver gain distribution, the desired detection probability, and the measured noise whenever a signal is absent. At initialization, the noise floor is calculated from the known gain distribution. When the channel is turned on, noise is measured when signal is absent, and the noise floor is updated in a Kalman filter. On a given channel, the noise estimate generally improves with time. 
     At the start of the receive mode, the threshold is initialized to 6 decibels (adjustable) above an estimate of the noise floor. The estimate changes based on the known noise figure of the receiver, the desired probability of detection and false triggering, the known gain distribution which changes as the RF and IF attenuators change (driven by the AGC loops shown in the above-referenced application), on the minimum signal strength (sensitivity) desired, and the desired minimum signal to noise ratio (SNR). operation  26  and an averaging operation  28  which generates a signal PO corresponding to the averaged power out of the FIR filter operation  22 . A Kalman filter operation  32  estimates the noise floor in-band value based on the PO value which is transmitted through a gate  30  when the PO value is below a threshold. Filter operation  32  also is based on the gain of the radio frequency stages (Grf) and the gain of the intermediate frequency stages (Gif), as well as a noise figure calculated for the various circuits in processing circuitry  14 . The Grf and Gif values change depending on the gains dictated by the AGC operation in circuitry  14 . 
     The noise floor is transmitted along a signal path  44  and is used by a squelch threshold compute operation  50  which is based on a desired minimum signal to noise ratio (SNR) and a minimum signal strength or sensitivity (SENS). The SNR and SENS values are constants in the algorithm. The threshold calculated in operation  50  is transmitted to a threshold detection operation  52  which generates a first signal along a path  53  to gate  30  in the event that the PO value is less than the threshold and generates another signal along a path  55  in the event that the PO value is greater than the threshold (i.e., when the averaged power of the detected signal is greater than the threshold). In response to the signal in path  55 , a signal conditioner  54 , such as a switch, passes the detected signal to an output path  56 . 
     As a result of the foregoing operation, gate  30  passes the PO signal to a Kalman filter operation  32  when it consists primarily of noise power alone. Additional details about Kalman filter operation  32  are shown in FIG.  2 . In response to the signal on path  53 , the average of the received noise power (i.e., the average of signal PO when signal PO consists at least primarily of noise) is calculated in an operation  36 . In addition, the noise floor (NkTFB) and the gain distribution from the RF and IF frequency stages of circuitry  14  (Grf and Gif) are calculated in operation  34 . NkTFB means: NkRT=Thermal noise (−204 dbw for T=290° K normally), F=receiver noise figure and B=receive bandwidth. 
     The values resulting from operations  34  and  36  are subtracted in a subtraction operation  38  and the results are latched in a latching operation  40  when a signal is present on path  53  indicating that the PO value is less than the threshold determined in operation  50 . The signal resulting from latching operation  40  and noise floor computation operation  34  are added in an adding operation  42  to result in the smoothed noise floor value transmitted on path  44  to the squelch threshold computing operation  50 . 
     Referring to FIG. 2, noise power alone is updated by operations  34  and  36  whenever PO is less than the threshold and is taken over several samples before the detection occurs to avoid signal contamination. The combined signal power with noise power (PS) is computed when the threshold calculated by operation  50  is exceeded, making the signal to noise estimates orthogonal in time. The algorithm assumes that the channel noise statistics are quasi-stationary between noise and signal power estimates (i.e., are slowly varying on the same channel). 
     Those skilled in the art will recognize that the preferred embodiments may be altered and modified without departing from the true spirit and scope of the invention as defined in the appended claims.