Abstract:
A system and method used for controlling an attenuator in a two-way communication device ( 100, 200 ) includes the steps of measuring ( 211 ) a relative signal strength indication (RSSI) at the input of the two-way communications device. The RSSI measurement is then compared ( 213, 217, 223 ) to both at least one predetermined interference threshold value (T 1 , T 2 , T 3 ) and a selective squelch state ( 219, 225 ) for determining whether to switch an in-line attenuator to the input of the two-way communications device. The invention provides a method by which an attenuator may be used only when interference is present without it being continually switched in-line to cause signal degradation.

Description:
TECHNICAL FIELD 
     This invention relates in general to two-way communications systems and more particularly to interference reduction through the use of an attenuator in a two-way radio receiver. 
     BACKGROUND 
     Two-way radios are typically designed with a robust amount of gain in their front end amplifier stages. As is well known in the art, one drawback of providing too much gain occurs when multiple communications systems are operated in close proximity to one another. The radio frequency (RF) gain stages of the radio receiver can both provide gain but can also act to enhance interfering signals. Generally, the cause of this type of problem stems from intermodulation distortion (IMD) and adjacent channel interference, which degrade radio performance in the form of poor radio reception. Intermodulation distortion interference occurs from adjacent channel interference that mixes with other RF signals to produce an unwanted RF signal on or near the receiver filter pass-band. This type of interference is becoming more common every day as radio spectrum becomes more crowded with differing types of users, RF signal power levels, and modulation schemes that all attempt to fit into a finite space. 
     In the past, one common way to help the receiver reduce this type of interference has been through the use of an attenuator device. The attenuator is a circuit the may be inserted between the antenna of the radio receiver and the RF amplifier circuitry in order to reduce the amount of RF energy reaching the receiver. As a general rule since the intermodulation product is a third order non-linear expression, every one decibel (dB) of attenuation that is switched in circuit provides a three dB reduction in the amount of intermodulation interference in the radio receiver. 
     One method currently used to control an attenuator for controlling this type of interference is through the use of a radio signal strength indication (RSSI) measurement. In this method, the signal strength is typically measured at the antenna or at some point in the RF amplifier chain in order to determine the absolute signal strength of the input radio signal. The signal strength may also be mathematically computed in a digital signal processor (DSP). If the radio signal reaches some predetermined threshold, then an attenuator may be switched into the receiving circuit in order to reduce the amount of interference that might be caused by this high signal condition. In some schemes, attenuation may even be switched in a stepped fashion where a greater amount of attenuation is used depending on the level of the RF input signal. Other schemes may use the RSSI measurement in conjunction with a carrier squelch (CSQ) detector circuit or software. However, CSQ is solely dependent on the RSSI measurement and not some “known” information. 
     The problem associated with these types of arrangements is that the receiver cannot determine if the received RF signal is an actual signal to be received or if the high RSSI level is merely interference or a combination of both. In general, when interference is present, the RSSI level is the sum of both the desired signal power and the interference power. Since both an on-channel signal and off-channel interference will provide a high RSSI level, the attenuator will continually be left in-line. This severely reduces the sensitivity of the receiver in situations where low RSSI levels need to be received. Communications are often missed since the receiver does not receive these lesser signals in view of the attenuator that is always in circuit. Still further methods have been developed and used in connection with the RSSI measurement such as measuring the incoming bit error rate (BER) against the signal strength to decide if the incoming signal is an actual signal or an interfering one. In many two way radio communications systems there is no digital bit information available to compute the BER. However, many two way radio communications systems have some form of squelch detect circuitry or software that utilizes “known” information for the purpose of unsquelching or unmuting the radio speaker. This information may be in the form of synchronization data as well as sub-audible tones such as a sinusoidal tone or a digital signal. 
     Thus, there is a need to provide a method for a radio receiver that requires the use of this “known” incoming data, for selectively switching an attenuator in connection with an RF input signal. This would work to ultimately optimize the performance of the radio receiver by enabling the user to accurately receive various signal levels of incoming information while still enabling an attenuator circuit in situations where adjacent channel interference or high on-channel signal levels are present. 
     SUMMARY OF THE INVENTION 
     Briefly, according to the invention, there is provided a method for selectively inserting an attenuation device in a two-way radio receiver based on a squelch detector circuit or software that utilizes “known” information (such as sub-audible tones or synchronization data) and radio signal strength indication (RSSI) measurements. Rather than always attenuating an receiver input signal when a relatively high on-channel or off-channel RSSI is received, the invention utilizes a plurality of interference threshold limits that are compared against an RSSI measurement in order to determine if an input signal should be attenuated. The interference threshold limits are used in connection with a receiver status condition such as whether the receiver is in a muted or un-muted state. This allows the receiver to make a determination as whether to switch the attenuator to an ON state or allow the attenuator to remain in an OFF state. Hence, the attenuator will be used at the proper time to insure the receiver maintains a high level of sensitivity at times when no off-channel interference is present. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the present invention, which are believed to be novel, are set forth particularly in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
         FIG. 1  is a block diagram illustrating the operation of the attenuator in the radio receiver according to the preferred embodiment of the invention; and 
         FIG. 2  is a flow chart diagram showing a method for determining insertion of an attenuation device in a two-way radio receiver based on squelch detect and radio signal strength indication (RSSI) according to the preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. 
     As seen in  FIG. 1 , a system describing the preferred system and method of selectively utilizing attenuation in a radio receiver  100 , includes a radio signal strength indication (RSSI) used in connection with detector  101 . The RSSI is used to determine the radio frequency (RF) signal strength of an incoming signal. Those skilled in the art will recognize that the terms radio, receiver, and transceiver are synonymous throughout this description and may be used interchangeably. The RF signal strength may include both on-channel signals as well as inter-modulation interference that is a result of off-channel interference mixing with both oscillator and amplifier stages in the radio to produce unwanted on-channel signal components that end up on the desired frequency of reception. 
     An attenuator  103  is used by the receiver  105  to reduce the amplitude of the input signal so that it may be more easily demodulated at the receiver input. This process as described herein, includes utilizing a comparator  109  to compare the RSSI measurement with a plurality of predetermined threshold levels. This comparison along with information relating to receiver status  111  is then used to control the on or off state of the attenuator  103 . The receiver status information is typically determined by a user and a digital signal processor (DSP) and indicates whether the receiver  105  is in a muted or unmuted condition. Since the radio  105  includes some type of squelch algorithm and/or software to control the muted or unmuted condition of the audio output  107  in receiver  105 , this information is then used by the comparator in order to determine when to attenuate the RF input signal. 
     Referring now to  FIG. 2 , a flow chart diagram illustrating a method  200  for determining the on or off state of an attenuation device in a two-way radio receiver based on squelch detect and radio signal strength indication (RSSI) according to the preferred embodiment of the invention includes switching  201  the radio receiver to powered “on” mode. The radio is initialized by software within the radio and controls  203  an internal attenuation device i.e. an attenuator is set to an OFF state. As will be further evident to those skilled in the art, an attenuation device is a device for reducing a radio frequency (RF) input signal magnitude level by a variable and/or fixed amount. In the preferred embodiment of the present invention, only a fixed attenuator is used however further embodiments could use a variable attenuation scheme. 
     After the attenuator is switched to an OFF state, the mode of the radio is then checked  205  to determine if it is in a transmit or receive state  207 . If the radio is in a transmit state, then the attenuator positioned in-line with the receiver chain remains in an OFF state  209  and the transceiver will again check  205  to determine its mode of operation. It will be recognized by those skilled in the art that some radio transceivers utilize an attenuator on the receiver input during transmit in order to better isolate the receiver from RF interference. In the preferred embodiment of this invention, the attenuator will remain in an OFF state during the transmit mode of the transceiver. 
     When the receiver is in a receive mode  207 , the relative signal strength indication is measured  211  at some predetermined location in the transceiver radio frequency (RF) amplifier chain. This might include measurement directly at the antenna or at any stage in the RF amplifier circuitry. In the case of a digital transceiver, the real (I) and imaginary (Q) components of the digital signal can be mathematically computed within the DSP or at the output of an analog-to-digital detector circuit. Thus, an absolute value is presented by a detector representing the RSSI received by the radio. This number is absolute and does not take into account whether the RF signal that is received is an on-channel signal or perhaps off-channel RF interference. This information is then used to determine what state the transceiver&#39;s attenuator is set in order to insure all received information is presented to a user. 
     The first comparison  213  for the transceiver is to determine if an exceptionally high amount of RF energy is being received by the receiver. For example, if the RSSI measurement is greater than a first interference threshold level T 1 , which might be set at approximately −40 dBm, then the receiver would recognize that attenuation would be required in order to protect the receiver&#39;s RF amplifier circuitry from damage. In that case, the attenuator is switched  215  to an ON state. The mode of the radio is again checked  205  to determine if the transceiver is in a transmit or receive state. In this situation the attenuator will remain in an ON state as long as the RSSI measurement remains greater than the first interference threshold level T 1 . 
     If the RSSI measurement drops below the first interference threshold level T 1 , then a second comparison is made to determine if attenuation is needed to eliminate or reduce off-channel interference that may produce intermodulation distortion (IMD). For example, the second interference threshold level T 2  might be set to −80 dBm. In the comparison  217 , if the RSSI measurement is greater than the second interference threshold level T 2 , the status of the receiver is determined  219 . This determination is based upon whether the receiver is squelched or muted  221 . As is well known in the art, muting is generally based on the selective squelch setting of the transceiver either in an analog or digital mode. In each of these modes, a host and digital signal processor (DSP) determines if the squelch criteria is met before setting an un-mute flag. Un-muting would permit audio to be ported to the radio&#39;s internal speaker. In an analog trunking mode this might take the form of a connect tone on the voice channel similar to a private line (PL) tone. Moreover, there is also low speed data from a control channel that may be used for this purpose. 
     In the digital trunking mode, either control channel data or information related to identification (ID) embedded in each data frame used for synchronization may be used by the inventor to determine a mute or un-mute status. In an analog simplex mode, a user may employ PL or digital private line (DPL) information as used with a selective squelch and known user identification. In a digital simplex mode, the receiver may utilize frame synchronization information as described for a commonly implemented digital trunking mode. With a plurality of apriori information based on mode of operation, the transceiver can determine when to un-mute and consequently when any attenuation should be switched to an ON state. 
     If the receiver is muted  221 , then the attenuator is switched  215  to an ON state and the process begins again where the mode of the transceiver is checked  205  and new RSSI measurements are calculated  211 . If the receiver remains muted i.e. the squelch or muting algorithm is not triggered however the RSSI remains greater than the second interference threshold level T 2 , the transceiver determines that interference is being received and switches the attenuator to an ON state. This occurs in order to reduce attention levels at the receiver input so that reasonable on-channel signal levels can be attained. This allows valid information to be demodulated without interference and/or distortion. If the transceiver is not muted  221 , then the transceiver determines that it must be receiving valid information that is not subject to interference. In this case, no attenuation would be needed and the receiver is again checked  205  for operating mode and new RSSI measurements are calculated  211 . 
     If the RSSI measurements are less than the second interference threshold level T 2 , the measurement is compared  223  against a third interference threshold level T 3 . For example, the third interference threshold level T 3 , might be set at −120 dBm (or the radio sensitivity level). If the RSSI is greater than the third interference threshold level T 3 , then the receive status is again checked  225 . If the receiver is muted  227 , then the received signal level is deemed lower than what the receiver can detect and the attenuator is turned to an OFF state. The process continues again through the loop where the mode is checked  205  and new RSSI values are measured  211 . If the radio is un-muted and if the RSSI measurement is less than the third interference threshold level T 3 , then the receiver determines that valid information is still being received and the process begins anew with a check of the radio mode  205 . It will be further evident to those skilled in the art that state timers may be implemented for use in connection with the attenuator on or off state threshold crossings. Generally, the timers may be similar to a debounce circuit in that the algorithm will not consider a threshold crossing until that threshold remains crossed for some predetermined period of time. The use of threshold crossings timers helps to combat problems associated with the effects of fading and random noise. 
     Thus, in view of this new process for determining when to switch the attenuator in-line and/or to an ON state, the primary advantage this invention outperforms other well known RSSI-only methods and algorithms, is that RSSI-only algorithms always use an attenuator regardless of whether IM interference is present. The present invention offers a novel method over the prior art by using the attenuator only when needed. Thus, attenuation is provided only when IM interference is a problem. Thus, the receiver&#39;s sensitivity will remain high when no interference is being received. Since the selective use of an attenuator offers a substantial gain in the receiver&#39;s IM performance figures, the invention offers a great advantage for use in communications systems used within crowded frequency spectrum where intermodulation interference is often a concern. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.