Patent Publication Number: US-11646702-B2

Title: Methods and apparatuses for reflection measurements

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/559,157, filed Sep. 3, 2019, which claims benefit of U.S. Patent Application Ser. No. 62/755,951, filed Nov. 5, 2018; the entire contents of the aforementioned patent application are incorporated herein by reference as if set forth in its entirety. 
    
    
     BACKGROUND 
     Transmitters are used in communications systems such as base stations and distributed antenna systems. Transmitters typically employ power amplifiers to boost the power of a transmitted signal. 
     In the event of an impedance mismatch between an output of the power amplifier and its load (e.g. subsequent components such as a diplexer, duplexer, or antenna), reflected energy may damage the power amplifier. Impedance match can be characterized by a reflection coefficient or a voltage standing wave ratio (VSWR). 
     To protect the power amplifier, it is desirable to monitor reflected power, the reflection coefficient, and/or the VSWR. Output reflected power is the power level of the output reflected signal a 2 . Coupled reverse power means the power level of the coupled reverse signal b 4 . If the reflected power is too large, the power amplifier and other components, e.g. duplexers, can be protected by attenuating the output power of the power amplifier. Further, little or no power may be coupled to the subsequent components. 
     To protect the power amplifier and other components, it is desirable to monitor reflected power, the reflection coefficient, and/or the VSWR. If the reflected power is too large, the power amplifier can be protected by attenuating the output power of the power amplifier. 
     Reflected power may be monitored with a directional coupler. A port of the directional coupler extracts a portion of energy reflected by the load. 
     However, the accuracy of the monitored reflected power is limited by the finite directivity of a directional coupler. Directivity is a figure of merit of a coupler that defines how well a coupled port discriminates between signals propagating in opposite directions. For example, a reverse coupled port is intended to measure energy reflected into an output port, of the directional coupler by a load coupled to that output port. The coupler&#39;s finite directivity arises because a portion of the energy emitted by the power amplifier into the input port of the directional coupler is also undesirably coupled to the reverse coupled port. Thus, the power measured at the reverse coupled port does not express solely the energy reflected by the load but also a portion of the power emitted by the power amplifier. The power measured at the reverse coupled port is therefore an inaccurate measurement of the energy reflected into the output port. This inaccuracy can detrimentally affect a system&#39;s ability to transmit enough energy and to protect the power amplifier and other components from high levels of reflected energy and/or to vary the power output of the power amplifier to ensure that the power amplifier provides linear amplification. 
     SUMMARY 
     A method is provided. The method comprises: measuring amplitude and phase of a coupled forward signal at a forward coupled port of a bidirectional coupler; measuring an amplitude and a phase of a coupled reverse signal at a reverse coupled port of the bidirectional coupler; and determining an amplitude and a phase of an output reflected signal at the output port as a function of the following: the amplitude and the phase of the coupled forward signal coupled into the forward coupled port; the amplitude and the phase of the coupled reverse signal coupled into the reverse coupled port; an electrical transmission parameter from an input port of the bidirectional coupler to the forward coupled port; an electrical transmission parameter from the input port to the reverse coupled port; and an electrical transmission parameter from an output port of the bidirectional coupler to the reverse coupled port. 
    
    
     
       DRAWINGS 
       Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail using the accompanying drawings, in which: 
         FIG.  1 A  illustrates a block diagram of one embodiment of a measurement system with enhanced reflection measurement; 
         FIG.  1 B  illustrates a block diagram of one embodiment of a processing system; 
         FIG.  1 C  illustrates a block diagram of one embodiment of a power amplifier system with enhanced reflection measurement; 
         FIG.  2 A  illustrates a block diagram of one embodiment of a distributed antenna system in which the power amplifier with enhanced reflection measurement described herein is implemented; 
         FIG.  2 B  illustrates a block diagram of one embodiment of a remote antenna unit in which the power amplifier with enhanced reflection measurement described herein is implemented; 
         FIG.  3    illustrates a block diagram of one embodiment of a single-node repeater in which the power amplifier with enhanced reflection measurement described herein is implemented; and 
         FIG.  4    illustrates a flow diagram of one embodiment of a method of enhanced reflection measurement. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized, and that structural, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual steps may be performed. The following detailed description is, therefore, not to be taken in a limiting sense. 
     The embodiments described below use electrical parameters, e.g. S-parameters, characterizing a four port directional coupler, to more accurately determine power reflected by a load. The electrical parameters may be S-parameters or any other electrical parameters. The electrical parameters may be determined by measurement, e.g. with a vector network analyzer, or may be provided by the manufacturer of the four port directional coupler. Optionally, the four port directional coupler may be formed by two back to back three port directional couplers. The four port directional coupler is also referred to herein as a bidirectional coupler. 
     Some of the embodiments described below enable a power amplifier to be better protected and/or a power output of the power amplifier to be more accurately controlled based upon more accurately determined reflected power, reflection coefficient, and/or VSWR. Furthermore, because the measurements are more accurate, transmitter malfunctions can be more quickly identified and remedied. Transmitters may be used in many different types of systems including communications systems such as broadcast transmitters for radio and television, cellular base stations, distributed antenna systems, and off-air repeaters. 
       FIG.  1 A  illustrates a block diagram of one embodiment of a measurement system with enhanced reflection measurement  100  (also referred to herein as reflectance measurement system  100 ). The reflectance measurement system  100  comprises a bidirectional coupler  106 , at least one analog to digital converter circuit (ADC(s))  108 , and a processing system (or processing system circuitry)  110 . 
     The bidirectional coupler  106  has an input port  106   a , an output port  106   b , a forward coupled port  106   c , and a reverse coupled port  106   d . There is an input signal a 1  provided to the input port  106   a  from a signal source and an input reflected signal b 1  reflected from the input port  106   a  of the bidirectional coupler  106 . At the output port  106   b , there is an output reflected signal a 2  reflected from the load to the output port  106   b  of the bidirectional coupler  106  and an output signal b 2  provided to the load from the output port  106   b  of the bidirectional coupler  106 . At the forward coupled port  106   c , there is coupled forward signal b 3  coupled from the input signal a 1  and a first reflected signal a 3  reflected from, e.g. a first input of the ADC(s)  108 , to the forward coupled port  106   c . At the reverse coupled port  106   d , there is a coupled reverse signal b 4  coupled from the output reflected signal a 2  and a second reflected signal a 4  reflected from, e.g. a second input of the ADC(s)  108 , to the reverse coupled port  106   d . Optionally, the ADC(s)  108  may comprise two analog to digital converter circuits (ADCs) each having a unique input, where an input of a first ADC is coupled to the forward coupled port  106   c  and where an input of the second ADC is coupled to the reverse coupled port  106   d . Alternatively, a single ADC may be used with multiplexers at the input and optionally at the output of the single ADC; the multiplexer(s) couple the forward coupled port  106   c  and the reverse coupled port  106   d  to the input of the ADC, and the optional second multiplexer couples the output of the ADC to different inputs of the processing system  110 . However, the processing system  110  may only have a single input coupled to the output of the ADC. 
     The bidirectional coupler  106  is configured to receive input signal a 1  from a signal source such as a power amplifier, transmitter, or any other type of signal source. The bidirectional coupler  106  is configured to provide output signal b 2  to a load such as an antenna, duplexer, diplexer, or any other type of load. 
     The bidirectional coupler  106  couples a portion of the input signal a 1  incident at its input port  106   a  (“forward signal”) to a forward coupled port  106   c . The amplitude and phase of the coupled forward signal b 3  provided at the forward coupled port  106   c  can be determined using a first forward coupling factor between the input port  106   a  and the forward coupled port  106   c . The bidirectional coupler  106  couples a phase shifted portion of the output reflected signal a 2  incident upon the output port  106   b  (“reverse signal” or “reflected signal”), e.g. reflected from the load, to the reverse coupled port  106   d . The amplitude and phase of the coupled reverse signal b 4  provided at the reverse coupled port  106   d  is determined by a first reverse coupling factor between the output port and the reverse coupled port. 
     Excluding the subsequently described undesired signal, the amplitude of the coupled forward signal b 3  is proportional to the amplitude level of the forward signal. Excluding the subsequently described undesired signal, the signal at the reverse coupled port shall be referred to as the coupled reverse signal b 4 , and the amplitude of the coupled reverse signal b 4  is proportional to the amplitude level of the reverse signal. 
     As described above a phase shifted portion of the input signal a 1  can be coupled to the reverse coupled port  106   d , and can be undesirably included in the coupled reverse signal b 4 . The phase shifted portion of the input signal a 1  coupled to the reverse coupled port  106   d  can be determined using a second forward coupling factor between the input port  106   a  and the reverse coupled port  106   d.    
     Similarly, a phase shifted portion of the output reflected signal a 2  can be coupled to the forward coupled port  106   c , and can be undesirably be included in the coupled forward signal b 3 . The phase shifted portion of the output reflected signal a 2  coupled to the forward coupled port  106   c  can be determined using a second reverse coupling factor between the output port  106   b  and the forward coupled port  106   c.    
     Typically, the portion of the output reflected signal a 2  coupled to the forward coupled port  106   c  is less than the portion of the input signal a 1  coupled to the forward coupled port  106   c . Therefore, the effect of the undesired coupling of the output reflected signal a 2  is not significant, and therefore is not addressed in some embodiments. Further, each coupling factor is dependent upon coupler design. 
     The forward coupled port  106   c  and the reverse coupled port  106   d  of the bidirectional coupler  106  are coupled to at least one input of the ADC(s)  108 . Thus, at least one input of the ADC(s)  108  is configured to receive the coupled reverse signal b 4  and the coupled forward signal b 3 . 
     The ADC(s)  108  digitize the coupled forward signal b 3  and the coupled reverse signal b 4  generating respectively a digitized forward signal  107  and a digitized reverse signal  109 . Assuming that the forward coupled port  106   c  and reverse coupled port  106   d  and the at least one input of the analog to digital converter circuitry  108  are impedance matched to the input(s) of the ADC(s)  108 , the analog to digital converter circuitry  108  generates a digitized reverse signal  109  representing the amplitude and phase of the coupled reverse signal b 4 , and generates a digitized forward signal  107  representing the amplitude and the phase of the coupled forward signal b 3 . 
     The processing system  110  is configured to receive the digitized forward signal  107  and the digitized reverse signal  109  from the ADC(s)  108 . Each of the signals generated by the processing system  110  described herein may be analog or digital signals, and voltage or current signals. 
     At least one output of the analog to digital converter circuitry  108  is coupled to at least one input of the processing system  110 . Thus, the at least one input of the processing system  110  is configured to receive the digitized forward signal  107  and the digitized reverse signal  109 . The processing system  110  is configured to generate an output signal  111 . 
       FIG.  1 B  illustrates a block diagram of one embodiment of the processing system  110 . The processing system  110  comprises processing circuitry  110 A coupled to memory circuitry  110 B. The processing system  110  may be implemented with analog and/or digital circuitry. For example, the processing circuitry  110 A may be implemented with electronic analog circuitry, including circuitry used to implement electronic analog computers. 
     The memory circuitry  110 B comprises a coupler parameters database  110 B- 1  and a modelling system  110 B- 2 . The coupler parameters database  110 B- 1  stores electrical parameters, e.g. S-parameters, characterizing the bidirectional coupler. The coupler parameters database  110 B- 1  may be a conventional database, storage registers, a storage file, or any other means by which to store the electrical parameters. 
     The processing system  110  is configured to generate an output signal  111 , e.g. a digital or analog signal, related to a value of reflected power (or the coupled reflected power), the reflection coefficient, and/or the VSWR. Optionally, the output signal  111  may be related to the input power or the forward coupled power. For example, the output signal  111  may be linearly or non-linearly proportional to the reflected power (or the coupled reflected power), the reflection coefficient, the VSWR, and/or the input power (or the coupled forward power). 
     The modelling system  110 B- 2  comprises a system for more accurately modelling a reflection coefficient of the load and/or voltage standing wave ratio (VSWR) of the load. The modelling system utilizes the following models. 
     Knowing the electrical parameters of the bidirectional coupler, the amplitude and phase of the output reflected signal a 2  can be more accurately determined by subtracting the contribution of the input signal a 1  coupled to the fourth port  106   d  from the coupled reverse signal b 4 : 
                     a   2     =           b   4     -     (       a   1     *     S     4   ⁢   1         )         S     4   ⁢   2         =         b   4     -     (         b   3       S     3   ⁢   1         *     S     4   ⁢   1         )         S     4   ⁢   2                   (     Equation   ⁢         1     )               
where S 31 , S 41 , and S 42  are transmission S-parameters respectively from the input port  106   a  to the forward coupled port  106   c , from input port  106   a  to the reverse coupled port  106   d , and from the output port  106   b  to the reverse coupled port  106   d . Further, the forward coupled port  106   c  and reverse coupled port  106   d  are deemed well impedance matched, as discussed elsewhere herein, so that first reflected signal a 3  and second reflected a 4  are substantially zero.
 
     Assuming that the level of the amplitude of the output reflected signal a 2  coupled to forward coupled port  106   c  is much less than the level of the amplitude of the input signal a 1  coupled to the forward coupled port  106   c , the output signal b 2  is: 
                     b   2     =         a   1     *     S     2   ⁢   1         =         b   3       S     3   ⁢   1         *     S     2   ⁢   1                   (     Equation   ⁢         2     )               
where S 21  is a transmission S-parameter respectively from input port  106   a  to the output port  106   b . Further, the forward coupled port  106   c  and the reverse coupled port  106   d  are deemed well impedance matched so that first reflected signal a 3  and second reflected signal a 4  are zero.
 
     The reflection coefficient Γ is: 
                   Γ   =         a   2       b   2       =         (         b   4     -     (       a   1     *     S     4   ⁢   1         )         S     4   ⁢   2         )       (         b   3       S     3   ⁢   1         *     S     2   ⁢   1         )       =       (       b   4     -     (         b   3       S     3   ⁢   1         *     S     4   ⁢   1         )       )         S     4   ⁢   2       *     (         b   3       S     3   ⁢   1         *     S     2   ⁢   1         )                     (     Equation   ⁢         3     )               
The reflection coefficient formula can be further refined to remove the reflected signal a 2  coupled to the forward coupled port  106   c . As noted elsewhere herein, practically this may not be necessary as the level of the amplitude of the output reflected signal a 2  coupled to forward coupled port  106   c  is much less than the level of the amplitude of the input signal a 1  coupled to the forward coupled port  106   c.  
 
     The voltage standing wave ratio is: 
     
       
         
           
             
               
                 
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       FIG.  1 C  illustrates a block diagram of one embodiment of a power amplifier system (PAS)  120  with enhanced reflection measurement. For pedagogical reasons, the power amplifier system  120  with enhanced reflection measurement may also be referred to herein as a transmitter front end with enhanced reflection measurements (TX FE); however, the power amplifier system  120  with enhanced reflection measurement may be used in different (i.e. non-communication) applications such as in a microwave oven. The power amplifier system  120  with enhanced reflection measurement may be configured to use the enhanced reflection measurements to provide the protection function in the event the coupled reverse power or the reflection coefficient are excessive. 
     The PAS  120  may be configured to implement automatic level control (ALC) based on monitoring the coupled forward signal b 3 . The ALC can be configured so as to diminish distortion in the power amplifier by adjusting a power level of a signal at the input of the power amplifier so that it remains below a threshold level. If the power level equals or exceeds the threshold level, the power level would drive the power amplifier into saturation, and generate non-linear amplification including distortion products. Thus, the power amplifier would no longer operate as a linear power amplifier. 
     The power amplifier system  120  with enhanced reflection measurement is configured to have a first input coupled to the output of a driver, e.g. a transmitter or any other type of driver or signal source, and an output coupled to the load. If the load is an antenna, or another device, a diplexer, duplexer, or transmit/receive switch may be coupled between the output port of a bidirectional coupler  106 , and the antenna. 
     In one embodiment, the power amplifier system  120  with enhanced reflection measurement comprises a power control system  102 , a power amplifier (PA)  104 , a bidirectional coupler  106 , analog to digital converter circuitry  108 , and power control processing circuitry  110 . The power amplifier  104  has an input coupled to an output of the power control system  102 , and an output coupled to an input port  106   a  of the bidirectional coupler  106 . The power control system  102  is configured to vary the power level of the signal input into the power amplifier  104 . The power control system  102  may be a variable attenuator and/or a variable gain amplifier. The input of the power control system  102  is coupled to the output of the transmitter. 
     Optionally, a variable attenuator, e.g. having high and low attenuation states, is inserted between the forward coupling port and the corresponding input of the analog to digital converter circuitry  108  and/or the reverse coupling port and the corresponding input of the analog to digital converter circuitry  108 . The variable attenuator(s) each have an input coupled to the processing system  110 . The processing system  110  sends a signal, as required, to vary, e.g. dither, the attenuation level of the attenuator(s) so as to extend the dynamic range of the analog to digital converter circuitry  108 . 
     The processing system  110  has an output configured to provide a control signal  112  generated by the power control processing circuitry  110  to prevent the amplifier  104  from:
         a. being damaged by a level of reflected power at the output of the power amplifier  104  over a short period of time; and/or   b. being damaged by due to prolonged exposure to a high level of reflected power that is less than the level that would damage the power amplifier during the short period of time.
 
Optionally, the processing system  110  varies the control signal  112  to prevent the power amplifier  104  from saturating. A second input of the power control system  102  is coupled to the output of the processing system  110 . Thus, the power control system  102  is configured to receive the control signal  112 . The power control system  102  uses the control signal  112  to adjust its attenuation and/or gain, and thus control the power level of the signal provided to the input of the power amplifier  104 .
       

     The processing system  110  is configured to adjust the gain and/or attenuation of the power control system  102  to quickly reduce power levels of signals provided at the input of the power amplifier  104  upon the reverse power becoming too high, e.g. upon a level of the reverse signal level exceeding a first threshold level. Typically, the attenuation is increased, or the gain is reduced by a large amount, e.g. respectively to a maximum or minimum level. This prevents excessive level of reflected power from damaging the power amplifier  104  over a short time period, e.g. 1-10 microseconds. Such control is relatively fast and may be abrupt. 
     The power control processing circuitry  110  is also configured to adjust the gain and/or attenuation of the power control system  102  to prevent the power amplifier  104  from:
         (a) being damaged due to prolonged exposure, e.g. greater than 500 milliseconds, to a high level of reflected power (but not high enough to engage the aforementioned fast control) indicated by a high voltage standing wave ratio (at the output port  106   d  of the bidirectional coupler  106 ) that is equal to or greater than a second threshold value, e.g. greater than 2:1; or   (b) saturating indicated by a high forward voltage level exceeding a third threshold level.
 
Such control is relatively slow. With respect to at least saturation prevention, the gain and/or attenuation changes are typically incremental and not be abrupt. With respect to the high standing wave ratio mentioned above, the adjustment to prevent damage due to the high VSWR is based upon a linear or non-linear function of VSWR.
       

     For example, if the VSWR increases from below 2:1 to 2.5:1 and the second threshold level is a VSWR of 2:1, then the attenuation is increased (or the gain is decreased) by a fixed amount, e.g. about 7 decibels (for example from ten percent to fifty percent). For example, if the VSWR increases from below 2.25:1 to 2.5:1 and the second threshold level is a VSWR of 2.25:1, then the attenuation is increased (or the gain is decreased) by a fixed amount, e.g. about 4.8 decibels (for example from ten percent to thirty percent). Note, that as the second threshold level increases, the attenuation level drops for a given VSWR. Optionally, the second threshold level will be less than the first threshold level. 
     Optionally, the second threshold level may be a linear and/or non-linear function of one or more parameters, such as temperature. For example, if the reflected power level is below a reflected power threshold level due to the forward power level being below a forward power threshold level, then the reflected power will not damage, even over an extended period of time, the power amplifier  104 . Whether the reflected power level is below the reflected power threshold level may be ascertained by determining if the forward voltage level is below a forward voltage threshold level and/or the reverse voltage level is below a reverse voltage threshold level. Thus, for example, the second threshold level may be a function of the forward voltage level and/or the reverse voltage level. Typically, the second threshold level will be increased if the forward voltage level is below a forward voltage threshold level and/or the reverse voltage level is below a reverse voltage threshold level. The second threshold level may be increased sufficiently high to effectively disable the control loop for protecting the power amplifier  104  against prolonged exposure to a high level of reflected power, but only while the forward voltage level is below a forward voltage threshold level and/or the reverse voltage level is below a reverse voltage threshold level. 
     Optionally, to prevent saturation of the power amplifier  104 , the adjustment of the power control processing system is based upon a linear or non-linear function of the forward voltage. The forward voltage is proportional to the level of the forward power, and thus is related to the power level at the input of the power amplifier  104 . The power control system  102  is adjusted when the forward voltage is equal to or exceeds a third threshold level. The attenuation or gain of the power control system  102  is respectfully increased or diminished until the forward voltage is less than or is equal to the third threshold level. Optionally, the third threshold level corresponds to an output power level of the power amplifier  104  at or below the output power level at which the power amplifier  104  saturates. Also, the first threshold level, the second threshold level, and/or the third threshold level may be either stored in and/or generated by the power control processing circuitry  110 . Further, the first threshold level, the second threshold level, and/or the third threshold level may be determined from testing the corresponding power amplifier  104 , and/or performing mathematical analysis. 
     The power amplifier system  120  with enhanced reflection measurement described above can be implemented in various types of systems, e.g. communications systems. For example, the power amplifier system  120  with enhanced reflection measurement described above can be implemented in various types of repeater systems. Repeater systems can be implemented in various ways. 
     For example, a repeater system can be implemented as a distributed antenna system (DAS).  FIG.  2 A  illustrates a block diagram of one embodiment of a distributed antenna system  200 A in which the power amplifier system with enhanced reflection measurement described herein is implemented. 
     The DAS  200 A comprises one or more master units  202  that are communicatively coupled to one or more remote antenna units (RAUs)  204  via one or more waveguides  206 , e.g. optical fibers or cables. Each remote antenna unit  204  can be communicatively coupled directly to one or more of the master units  202  or indirectly via one or more other remote antenna units  204  and/or via one or more expansion (or other intermediary) units  208 . Each RAU  204  is configured to be coupled to one or more antennas  215 . However, in an alternative embodiment, a RAU may include the one or more antennas. 
     The DAS  200 A is coupled to one or more base stations  203  and is configured to improve the wireless coverage provided by the base stations  203 . The capacity of each base station  203  can be dedicated to the DAS or can be shared among the DAS and a base station antenna system that is co-located with the base station and/or one or more other repeater systems. 
     In the embodiment shown in  FIG.  2 A , the capacity of one or more base stations  203  are dedicated to the DAS  200 A and are co-located with the DAS  200 A. The base stations  203  are coupled to the DAS  200 A. It is to be understood however that other embodiments can be implemented in other ways. For example, the capacity of one or more base stations  203  can be shared with the DAS  200 A and a base station antenna system co-located with the base stations  203  (for example, using a donor antenna). The base stations  203  can include one or more base stations that are used to provide commercial cellular wireless service and/or one or more base stations that are used to provide public and/or private safety wireless services (for example, wireless communications used by emergency services organizations (such as police, fire and emergency medical services) to prevent or respond to incidents that harm or endanger persons or property). 
     The base stations  203  can be coupled to the master units  202  using a network of attenuators, combiners, splitters, amplifiers, filters, cross-connects, etc., (sometimes referred to collectively as a “point-of-interface” or “POI”). This network can be included in the master units  202  and/or can be separate from the master units  202 . This is done so that, in the downlink, the desired set of RF channels output by the base stations  203  can be extracted, combined, and routed to the appropriate master units  202 , and so that, in the upstream, the desired set of carriers output by the master units  202  can be extracted, combined, and routed to the appropriate interface of each base station  203 . It is to be understood, however, that this is one example and that other embodiments can be implemented in other ways. 
     In general, each master unit  202  comprises downlink (D/L) DAS circuitry  210  that is configured to receive one or more downlink signals from one or more base stations  203 . Each base station downlink signal includes one or more radio frequency channels used for communicating in the downlink direction with user equipment  214  over the relevant wireless air interface. Typically, each base station downlink signal is received as an analog radio frequency signal, though in some embodiments one or more of the base station signals are received in a digital form (for example, in a digital baseband form complying with the Common Public Radio Interface (“CPR”) protocol, Open Radio Equipment Interface (“ORP”) protocol, the Open Base Station Standard Initiative (“OBSAI”) protocol, or other protocol). The downlink DAS circuitry  210  in each master unit  202  is also configured to generate one or more downlink transport signals derived from one or more base station downlink signals and to transmit one or more downlink transport signals to one or more of the remote antenna units  204 . 
     Each RAU  204  is configured to receive the downlink transport signals transmitted to it from one or more master units  202  and to use the received downlink transport signals to generate one or more downlink radio frequency signals that are radiated from one or more antennas associated with that remote antenna unit  204  for reception by user equipment  214 . In this way, the DAS  200 A increases the coverage area for the downlink capacity provided by the base station(s)  203 . 
     Also, each RAU  404  is configured to receive one or more uplink radio frequency signals transmitted from the user equipment  414 . These signals are analog radio frequency signals. 
     Each RAU  404  is also configured to generate one or more uplink transport signals derived from the one or more remote uplink radio frequency signals and to transmit one or more uplink transport signals to one or more of the master units  402 . 
       FIG.  2 B  illustrates a block diagram of one embodiment of a remote antenna unit  200 B in which the power amplifier with enhanced reflection measurement described herein is implemented. Each remote antenna unit  204  comprises downlink DAS circuitry  212  that is configured to receive the downlink transport signals transmitted to it from one or more master units  202  and to use the received downlink transport signals to generate one or more downlink radio frequency signals that are radiated from one or more antennas  215  associated with that remote antenna unit  204  for reception by user equipment  214 . In this way, the DAS  200 A increases the coverage area for the downlink capacity provided by the base stations  203 . The downlink DAS circuitry  212  of each RAU  204  includes at least one transmitter front end having a power amplifier with enhanced reflection measurement  219  which, for example, power amplifies the downlink radio frequency signals. 
     Also, each remote antenna unit  204  comprises uplink (U/L) DAS circuitry  217  that is configured to receive one or more uplink radio frequency signals transmitted from the user equipment  214 . These signals are analog radio frequency signals. 
     The uplink DAS circuitry  217  in each remote antenna unit  204  is also configured to generate one or more uplink transport signals derived from the one or more remote uplink radio frequency signals and to transmit one or more uplink transport signals to one or more of the master units  202 . The uplink DAS circuitry  217  of each RAU  204  may include at least one receiver front end which e.g. amplifies received remote uplink radio frequency signals. 
     Returning to  FIG.  2 A , each master unit  202  comprises uplink (U/L) DAS circuitry  216  that is configured to receive the respective uplink transport signals transmitted to it from one or more remote antenna units  204  and to use the received uplink transport signals to generate one or more base station uplink radio frequency signals that are provided to the one or more base stations  203  associated with that master unit  202 . Typically, this involves, among other things, combining or summing uplink signals received from multiple remote antenna units  204  in order to produce the base station signal provided to each base station  203 . In this way, the DAS  200 A increases the coverage area for the uplink capacity provided by the base stations  203 . 
     Each expansion unit  208  comprises downlink (D/L) DAS circuitry  218  that is configured to receive the downlink transport signals transmitted to it from the master unit  202  (or other expansion unit  208 ) and transmits the downlink transport signals to one or more remote antenna units  204  or other downstream expansion units  208 . Each expansion unit  208  also comprises uplink DAS circuitry  220  that is configured to receive the respective uplink transport signals transmitted to it from one or more remote antenna units  204  or other downstream expansion units  208 , combine or sum the received uplink transport signals, and transmit the combined uplink transport signals upstream to the master unit  202  or other expansion unit  208 . In other embodiments, one or more remote antenna units  204  are coupled to one or more master units  202  via one or more other remote antenna units  204  (for example, where the remote antenna units  204  are coupled together in a daisy chain or ring topology). 
     The downlink DAS circuitry (D/L DAS circuitry)  210 ,  212 , and  218  and uplink DAS circuitry (U/L DAS circuitry)  216 ,  217 , and  220  in each master unit  202 , remote antenna unit  204 , and expansion unit  208 , respectively, can comprise one or more appropriate connectors, attenuators, combiners, splitters, amplifiers, filters, diplexers, duplexers, transmit/receive switches, analog-to-digital converters, digital-to-analog converters, electrical-to-optical converters, optical-to-electrical converters, mixers, field-programmable gate arrays (FPGAs), microprocessors, transceivers, framers, etc., to implement the features described above. Also, the downlink DAS circuitry  210 ,  212 , and  218  and uplink DAS circuitry  216 ,  217 , and  220  may share common circuitry and/or components. 
     The DAS  200 A can use digital transport, analog transport, or combinations of digital and analog transport for generating and communicating the transport signals between the master units  202 , the remote antenna units  204 , and any expansion units  208 . Each master unit  202 , remote antenna unit  204 , and expansion unit  208  in the DAS  200 A also comprises a respective controller (CNTRL or controller circuitry)  221 . The controller  221  is implemented using one or more programmable processors that execute software that is configured to implement the various control functions. The controller  221  (more specifically, the various control functions implemented by the controller  221 ) (or portions thereof) can be implemented in other ways (for example, in a field programmable gate array (FPGA), application specific integrated circuit (ASIC), etc.). Components of the power amplifier system with enhanced reflection measurement  120 , e.g. the processing system  110  or a portion thereof, may be incorporated in, e.g. the controller  221  of a remote antenna unit  204  or in another controller  221  or state machine incorporated into the distributed antenna system  200 A. Optionally, the processing system  110  may be part of one or more controllers  221 . 
     In embodiments of the invention described herein, certain components, e.g. processing system, ADC(s), power detection circuitry, and components thereof, may be illustrated as being incorporated in a specific section of a communications system, e.g. a RAU of a DAS. However, such components may be in other sections of the corresponding communications system, e.g. in a master unit, expansion unit, and/or a base station. 
     The at least one transmitter front end having a power amplifier with enhanced reflection measurement  219  includes at least one of the power amplifier systems with enhanced reflection measurement  120  described above. Further, a combination of one or more diplexers, duplexers, transmit/receive switches duplexers and/or other combiner systems can be used to couple the downlink (D/L) DAS circuitry  212  (e.g. including the at least one transmitter front end  219 ) and the uplink DAS circuitry  217  to one or more antennas  215 . The power amplifier system with enhanced reflection measurement  120  may be incorporated, e.g. in the controller  221  of a remote antenna unit  204  or in another controller  221  otherwise incorporated into the distributed antenna system  200 A. 
     Repeater systems can be implemented in other ways. For example, a repeater system can be implemented as a single-node repeater.  FIG.  3    illustrates a block diagram of one embodiment of a single-node repeater  300  in which the power amplifier with enhanced reflection measurement described herein is implemented. 
     The single-node repeater  300  is configured to facilitate wireless communications between one or more base stations  303  and user equipment  314  (e.g. a mobile phone, tablet, or computer). Such wireless communication can be through uplink repeater circuitry  320  to the base station(s)  303  and a downlink repeater circuitry  312  to the user equipment  314 . 
     The single-node repeater  300  comprises the downlink repeater circuitry  312  that is configured to receive one or more downlink signals from the one or more base stations  303 . These signals are also referred to here as “base station downlink signals.” Each base station downlink signal includes one or more radio frequency channels used for communicating in the downlink direction with user equipment (UE)  314  over the relevant wireless air interface. Typically, each base station downlink signal is received as an analog radio frequency signal. 
     The downlink repeater circuitry  312  in the single-node repeater  300  is also configured to generate one or more downlink radio frequency signals that are radiated from one or more antennas  315  associated with the single-node repeater  300  for reception by user equipment  314 . These downlink radio frequency signals are analog radio frequency signals and are also referred to here as “repeated downlink radio frequency signals.” Each repeated downlink radio frequency signal includes one or more of the downlink radio frequency channels used for communicating with user equipment  314  over the wireless air interface. In this exemplary embodiment, the single-node repeater  300  is an active repeater system in which the downlink repeater circuitry  312  comprises one or more amplifiers (or other gain elements) that are used to control and adjust the gain of the repeated downlink radio frequency signals radiated from the one or more antennas  315 . The downlink repeater circuitry  312  includes at least one transmitter front end having a power amplifier with enhanced reflection measurement  319  which, for example, power amplifies the repeated downlink radio frequency signals. 
     Also, the single-node repeater  300  comprises uplink repeater circuitry  320  that is configured to receive one or more uplink radio frequency signals transmitted from the user equipment  314 . These signals are analog radio frequency signals and are also referred to here as “UE uplink radio frequency signals.” Each UE uplink radio frequency signal includes one or more radio frequency channels used for communicating in the uplink direction with user equipment  314  over the relevant wireless air interface. 
     The uplink repeater circuitry  320  in the single-node repeater  300  is also configured to generate one or more uplink radio frequency signals that are provided to the one or more base stations  303 . These signals are also referred to here as “repeated uplink signals.” Each repeated uplink signal includes one or more of the uplink radio frequency channels used for communicating with user equipment  314  over the wireless air interface. In this exemplary embodiment, the single-node repeater  300  is an active repeater system in which the uplink repeater circuitry  320  comprises one or more amplifiers (or other gain elements) that are used to control and adjust the gain of the repeated uplink radio frequency signals provided to the one or more base stations  303 . Typically, each repeated uplink signal is provided to the one or more base stations  303  as an analog radio frequency signal. The uplink repeater circuitry  320  may include at least one receiver front end which e.g. amplifies received uplink radio frequency signals. 
     The downlink repeater circuitry  312  and uplink repeater circuitry  320  can comprise one or more appropriate connectors, attenuators, combiners, splitters, amplifiers, filters, diplexers, duplexers, transmit/receive switches, analog-to-digital converters, digital-to-analog converters, electrical-to-optical converters, optical-to-electrical converters, mixers, field-programmable gate arrays (FPGAs), microprocessors, transceivers, framers, etc., to implement the features described above. Also, the downlink repeater circuitry  312  and uplink repeater circuitry  320  may share common circuitry and/or components. 
     The at least one transmitter front end  319  includes at least one of the transmitter front end having a power amplifier with enhanced reflection measurement  120  described above. Further, a combination of two or more diplexers, duplexers, transmit/receive switches duplexers and/or other combiner systems can be used to couple the downlink DAS circuitry  312  (e.g. including the at least one transmitter front end  319 ) and the uplink DAS circuitry  320  (e.g. including the at least one transmitter front end  319 ) to one or more antennas  315 . The single-node repeater system  300  also comprises a controller (CNTRL)  321 . The controller  321  is implemented using one or more programmable processors that execute software that is configured to implement the various control functions. The controller  321  (more specifically, the various control functions implemented by the controller  321 ) (or portions thereof) can be implemented in other ways (for example, in a field programmable gate array (FPGA), application specific integrated circuit (ASIC), etc.). The components of the power amplifier system with enhanced reflection measurement  120 , e.g. the processing system  110  or a portion thereof, may be incorporated, e.g. in the controller  321  of the single-node repeater system  300 . 
       FIG.  4    illustrates a flow diagram of one embodiment of a method of enhanced reflection measurement  400 . To the extent that the embodiment of method  400  shown in  FIG.  4    is described herein as being implemented in the systems described with respect to  FIGS.  1 - 3   , it is to be understood that other embodiments can be implemented in other ways. The blocks of the flow diagrams have been arranged in a generally sequential manner for ease of explanation; however, it is to be understood that this arrangement is merely exemplary, and it should be recognized that the processing associated with the methods (and the blocks shown in the Figures) can occur in a different order (for example, where at least some of the processing associated with the blocks is performed in parallel and/or in an event-driven manner). 
     In block  440 , measure an amplitude and phase of coupled forward signal b 3  at a forward coupled port of a bidirectional coupler. In block  442 , measure an amplitude and phase of coupled reverse signal b 4  at a reverse coupled port of the bidirectional coupler. Optionally, such amplitude and phase information is digitized, and the subsequent processing is performed in the digital domain; alternatively, such information is not digitized, and the processing is performed in the analog domain. 
     In block  444 , determine the output reflected signal a 2  at the output port  106   b  using electrical parameters of the bidirectional coupler  106  such as an electrical transmission parameter from an input port  106   a  to the forward coupled port  106   c , an electrical transmission parameter from the input port  106   a  to the reverse coupled port  106   d , and an electrical transmission parameter from an output port  106   b  to the reverse coupled port  106   d . For example for electrical parameters that are S-parameters, the reflected signal is: 
     
       
         
           
             
               a 
               2 
             
             = 
             
               
                 
                   
                     b 
                     4 
                   
                   - 
                   
                     ( 
                     
                       
                         a 
                         1 
                       
                       * 
                       
                         S 
                         
                           4 
                           ⁢ 
                           1 
                         
                       
                     
                     ) 
                   
                 
                 
                   S 
                   
                     4 
                     ⁢ 
                     2 
                   
                 
               
               . 
             
           
         
       
     
     Optionally, in block  446 , determine if at least one of the output reflected power exceeds a reflected power threshold (T 1 ) and the coupled reverse power exceeds a coupled reverse power threshold (T 2 ). If at least one of the reflected power exceeds the reflected power threshold and the coupled reverse power exceeds the coupled reverse power threshold, then, in block  448 , perform at least one of: reducing the power level of the signal at the input port of the bidirectional coupler until at least one of the reflected power is less than or equal to the reflected power threshold and the coupled reverse power is less than or equal to the coupled reverse power threshold, and providing an alarm indicating that at least one of the reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold. The alarm may be a message or signal sent to a network operator and/or an indicator, such as a flashing light, e.g. an LED, in a corresponding PAS to identify a faulty part to maintenance personnel. The alarms described herein may notify communication system operators of problems arising from high reflected powers, reflection coefficients 
     Optionally, in block  448 , reduce the power level, e.g. by fifty percent or more, by seventy five percent or more, or by ninety nine percent or more, for example by adjusting the power control system  102 . Optionally, reduce the power level of the signal comprises reduce a power level of an output signal of a signal source that is a power amplifier having an output coupled to the input port. Optionally after block  448 , stop, or proceed to blocks  450 ,  456 , or  440 . If at least one of the reflected power does not exceed the reflected power threshold and the coupled reverse power does not exceed the coupled reverse power threshold, then stop, or proceed to blocks  450 ,  456 , or  440 . 
     Optionally, in block  450 , determine at least one of a reflection coefficient (Γ) at the output port and a voltage standing wave ratio (VSWR) at the output port, where the reflection coefficient and voltage standing wave ratio are determined for a load coupled to the output port, and where: 
     the reflection coefficient is a function of the output reflected signal a 2 , e.g. 
               Γ   =       a   2       (         b   3       S     3   ⁢   1         *     S     2   ⁢   1         )         ;                 VSWR   =       1   +       ❘   &#34;\[LeftBracketingBar]&#34;     r     ❘   &#34;\[RightBracketingBar]&#34;           1   -       ❘   &#34;\[LeftBracketingBar]&#34;     r     ❘   &#34;\[RightBracketingBar]&#34;             ;         
and
 
where S 21  is a transmission S-parameter from the input port to the output port. Optionally, the reflection coefficient and VSWR are calculated as set forth above, e.g. using Equation 3. However, optionally, the equation of reflection coefficient (Equation 3) can be modified to subtract the output reflected signal a 2  coupled to the forward coupled port  106   c . As discussed above, this is not typically required for the reasons described above.
 
     Optionally, in block  452 , determine if at least one of the reflection coefficient exceeds a reflection coefficient threshold (T 3 ), and the VSWR exceeds a VSWR threshold (T 4 ). If at least one of the reflection coefficient exceeds the reflection coefficient threshold and the VSWR exceeds the VSWR threshold, then in block  454  provide an alarm indicating a reflection coefficient that exceeds the reflection coefficient threshold, or a VSWR that exceeds the VSWR threshold. The alarm may be a message or signal sent to a network operator and/or an indicator, such as a flashing light, e.g. an LED, in a corresponding PAS to identify a faulty part to maintenance personnel. 
     Optionally after block  454 , stop, or proceed to blocks  456  or  440 . If at least one of the reflection coefficient does not exceed the reflection coefficient threshold and the VSWR does not exceed the VSWR threshold, then stop, or proceed to blocks  456  or  440 . 
     Optionally, in block  456 , determine if at least one of an input power at the input port is equal to or exceeds an input power threshold level (T 5 ), and the coupled forward power is equal to or exceeds a coupled forward power threshold (T 6 ). If at least one of the input power is equal to or exceeds the input power threshold level and the coupled forward power is equal to or exceeds the coupled forward power, then in block  458  perform at least one of: reduce a power level of the input power until at least one of the input power at the input port less than an input power threshold level and the coupled forward power is less than the coupled forward power threshold, and provide an alarm indicating the automatic limiting control function has been enabled. Controlling the input power may ensure that the signal source, e.g. a power amplifier, continues to provide linear power amplification and thus maintains a higher power added efficiency of the signal source. 
     Optionally, reducing the power level of the signal comprises reducing a power level of an output signal of a signal source that is a power amplifier having an output coupled to the input port. Optionally, after block  456  or block  458 , subsequently proceed to block  440  or stop. If the power output from the power amplifier does not equal or exceed the third threshold, then optionally proceed to block  440  or stop. 
     The processor circuitry described herein may include one or more microprocessors, microcontrollers, digital signal processing (DSP) elements, application-specific integrated circuits (ASICs), complex programmable logic devices, and/or field programmable gate arrays (FPGAs). In this exemplary embodiment, processor circuitry includes or functions with software programs, firmware, or other computer readable instructions for carrying out various process tasks, calculations, and control functions, used in the methods described herein. These instructions are typically tangibly embodied on any storage media (or computer readable medium) used for storage of computer readable instructions or data structures. 
     The memory circuitry described herein can be implemented with any available storage media (or computer readable media) that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device. Suitable computer readable medium may include storage or memory media such as semiconductor, magnetic, and/or optical media. For example, computer readable media may include conventional hard disks, Compact Disk-Read Only Memory (CD-ROM), DVDs, volatile or non-volatile media such as Random Access Memory (RAM) (including, but not limited to, Dynamic Random Access Memory (DRAM)), Read Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), and/or flash memory. Combinations of the above are also included within the scope of computer readable media. 
     Exemplary Embodiments 
     Example 1 includes a method, comprising: measuring amplitude and phase of a coupled forward signal at a forward coupled port of a bidirectional coupler; measuring an amplitude and a phase of a coupled reverse signal at a reverse coupled port of the bidirectional coupler; and determining an amplitude and a phase of an output reflected signal at the output port as a function of the following: the amplitude and the phase of the coupled forward signal coupled into the forward coupled port; the amplitude and the phase of the coupled reverse signal coupled into the reverse coupled port; an electrical transmission parameter from an input port of the bidirectional coupler to the forward coupled port; an electrical transmission parameter from the input port to the reverse coupled port; and an electrical transmission parameter from an output port of the bidirectional coupler to the reverse coupled port. 
     Example 2 includes the method of Example 1, wherein determining the output reflected signal at the output port is determined with S-parameters according to: 
                   b   4     -     (         b   3       S   31       *     S     4   ⁢   1         )         S     4   ⁢   2         ;         
where b 3  is the coupled forward signal coupled into the forward coupled port; where b 4  is the coupled reverse signal coupled into the reverse coupled port; where S 31  is the transmission S-parameter from the input port of the bidirectional coupler to the forward coupled port; where S 41  is the transmission S-parameter from the input port to the reverse coupled port; and where S 42  is the transmission S-parameter from the output port of the bidirectional coupler to the reverse coupled port.
 
     Example 3 includes the method of any of Examples 1-2, further comprising: determining if at least one of the output reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold; and if at least one of the output reflected power exceeds the reflected power threshold and the coupled reverse power exceeds the coupled reverse power threshold, then performing at least one of: reducing the power level of the signal at the input port of the bidirectional coupler until at least one of the output reflected power is less than or equal to the reflected power threshold and the coupled reverse power is less than or equal to the coupled reverse power threshold, and providing an alarm indicating that at least one of the output reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold. 
     Example 4 includes the method of Example 3, wherein reducing the power level of the signal comprises reducing a power level of an output signal of a signal source that is a power amplifier having an output coupled to the input port. 
     Example 5 includes the method of any of Examples 1-4, further comprising: determining at least one of a reflection coefficient (Γ) at the output port and a voltage standing wave ratio (VSWR) at the output port, where the reflection coefficient and voltage standing wave ratio are determined for a load coupled to the output port, and where: the reflection coefficient is a function of the output reflected signal; and 
     
       
         
           
             VSWR 
             = 
             
               
                 
                   1 
                   + 
                   
                     
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               . 
             
           
         
       
     
     Example 6 includes the method of Example 5, further comprising: determining if at least one of a reflection coefficient exceeds a reflection coefficient threshold and a VSWR exceeds a VSWR threshold; if at least one of the reflection coefficient exceeds a reflection coefficient threshold and the VSWR exceeds a VSWR threshold, then providing an alarm indicating that at least one of: the reflection coefficient exceeds the reflection coefficient threshold and the VSWR exceeds the VSWR threshold. 
     Example 7 includes the method of any of Examples 1-6, wherein further comprising: determining if at least one of an input power at the input port is equal to or exceeds an input power threshold level, and the coupled forward power is equal to or exceeds a coupled forward power threshold; and if at least one of the input power is equal to or exceeds the input power threshold level and the coupled forward power is equal to or exceeds the coupled forward power, performing at least one of: reducing a power level of the input power until at least one of the input power at the input port less than an input power threshold level and the coupled forward power is less than the coupled forward power threshold, and providing an alarm indicating the automatic limiting control function has been enabled. 
     Example 8 includes the method of Example 7, wherein reducing the power level of the signal comprises reducing a power level of an output signal of a signal source that is a power amplifier having an output coupled to the input port. 
     Example 9 includes a system, comprising: a bidirectional coupler having an input port, an output port, a forward coupled port, and a reverse coupled port, where the input port is configured to receive input signal from a signal source coupled to the input port, and the output port is configured to provide output signal to a load coupled to the output port; at least one analog to digital converter circuit having at least one input coupled to the forward coupled port and the reverse coupled port, and at least one output; a processing system, comprising processing circuitry coupled to memory circuitry, having at least one input coupled to the at least one output of the at least one analog to digital converter circuit, and an output configured to generate an output signal related to a level of at least one of a coupled reverse signal and an output reflected signal; and wherein the power control processing circuitry is configured to: measure an amplitude and a phase of a coupled forward signal at a forward coupled port of a bidirectional coupler; measure an amplitude and a phase of a coupled reverse signal at a reverse coupled port of the bidirectional coupler; and determine an amplitude and a phase of an output reflected signal at the output port as a function of the following: the amplitude and the phase of the coupled forward signal coupled into the forward coupled port; the amplitude and the phase of the coupled reverse signal coupled into the reverse coupled port; an electrical transmission parameter from an input port of the bidirectional coupler to the forward coupled port; an electrical transmission parameter from the input port to the reverse coupled port; and an electrical transmission parameter from an output port of the bidirectional coupler to the reverse coupled port. 
     Example 10 includes the system of Example 9, where the power control processing circuitry is configured to determine the output reflected signal at the output port is determined with S-parameters according to: 
                   b   4     -     (         b   3       S   31       *     S     4   ⁢   1         )         S     4   ⁢   2         ;         
where b 3  is the coupled forward signal coupled into the forward coupled port; where b 4  is the coupled reverse signal coupled into the reverse coupled port; where S 31  is the transmission S-parameter from the input port of the bidirectional coupler to the forward coupled port; where S 41  is the transmission S-parameter from the input port to the reverse coupled port; and where S 42  is the transmission S-parameter from the output port of the bidirectional coupler to the reverse coupled port.
 
     Example 11 includes the system of any of Examples 9-10, wherein the processing system is further configured to: determine if at least one of the output reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold; and if at least one of the output reflected power exceeds the reflected power threshold and the coupled reverse power exceeds the coupled reverse power threshold, perform at least one of: reduce the power level of the signal at the input port of the bidirectional coupler until at least one of the output reflected power is less than or equal to the reflected power threshold and the coupled reverse power is less than or equal to the coupled reverse power threshold, and provide an alarm indicating that at least one of the output reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold. 
     Example 12 includes the system of Example 11, further comprising: a power control system coupled to the processing system; wherein the signal source is a power amplifier coupled to the power control system and the input port; and wherein reducing the power level comprises reducing an output power level of the power amplifier. 
     Example 13 includes the system of any of Examples 9-12, wherein the processing system is further configured to: determine at least one of a reflection coefficient (Γ) at the output port and a voltage standing wave ratio (VSWR) at the output port, where the reflection coefficient and voltage standing wave ratio are determined for a load coupled to the output port, and where: 
     the reflection coefficient is a function of the output reflected signal; and 
     
       
         
           
             VSWR 
             = 
             
               
                 
                   1 
                   + 
                   
                     
                       ❘ 
                       &#34;\[LeftBracketingBar]&#34; 
                     
                     r 
                     
                       ❘ 
                       &#34;\[RightBracketingBar]&#34; 
                     
                   
                 
                 
                   1 
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                     r 
                     
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               . 
             
           
         
       
     
     Example 14 includes the system of Example 13, wherein the processing system is further configured to: determine if at least one of a reflection coefficient exceeds a reflection coefficient threshold and a VSWR exceeds a VSWR threshold; if at least one of the reflection coefficient exceeds a reflection coefficient threshold and the VSWR exceeds a VSWR threshold, then provide an alarm indicating that at least one of: the reflection coefficient exceeds the reflection coefficient threshold and the VSWR exceeds the VSWR threshold. 
     Example 15 includes the system of any of Examples 9-14, wherein the processing system is further configured to: determine if at least one of an input power at the input port is equal to or exceeds an input power threshold level, and the coupled forward power is equal to or exceeds a coupled forward power threshold; and if at least one of the input power is equal to or exceeds the input power threshold level and the coupled forward power is equal to or exceeds the coupled forward power, perform at least one of: reduce a power level of the input power until at least one of the input power at the input port less than an input power threshold level and the coupled forward power is less than the coupled forward power threshold, and provide an alarm indicating the automatic limiting control function has been enabled. 
     Example 16 includes the system of Example 15, further comprising: a power control system; wherein the signal source is a power control system coupled to the processing system; wherein the power amplifier coupled to the power control system and the input port; and wherein reducing the power level comprises reducing an output power level of the power amplifier. 
     Example 17 includes the system of any of Examples 9-16, wherein the processing system is further configured to: determine if the input power is equal to or exceeds an input power threshold level; if the input power is equal to or exceeds the input power threshold level, perform at least one of reduce a power level of the input power until the input power is less than or equal to the input power threshold power level and provide an alarm indicating the automatic limiting control function has been enabled. 
     Example 18 includes the system of Example 17, further comprising: a power control system; wherein the signal source is a power control system coupled to the processing system; wherein the power amplifier coupled to the power control system and the input port; and wherein reducing the power level comprises reducing an output power level of the power amplifier. 
     Example 19 includes the system of any of Examples 9-18, wherein system comprises one of a remote antenna unit of a distributed antenna system and a single-node repeater. 
     Example 20 includes a program product comprising a non-transitory processor readable medium on which program instructions are embodied, wherein the program instructions are configured, when executed by at least one programmable processor, to cause the at least one programmable processor to: measure an amplitude and a phase of a coupled forward signal at a forward coupled port of a bidirectional coupler; measure an amplitude and a phase of a coupled reverse signal at a reverse coupled port of the bidirectional coupler; and determine an amplitude and a phase of an output reflected signal at the output port as a function of the following: the amplitude and the phase of the coupled forward signal coupled into the forward coupled port; the amplitude and the phase of the coupled reverse signal coupled into the reverse coupled port; an electrical transmission parameter from an input port of the bidirectional coupler to the forward coupled port; an electrical transmission parameter from the input port to the reverse coupled port; and an electrical transmission parameter from an output port of the bidirectional coupler to the reverse coupled port. 
     Example 21 includes the program product of Example 20, wherein the program instructions are configured, when executed by at least one programmable processor, to further cause the at least one programmable processor to determine the output reflected signal at the output port is determined with S-parameters according to: 
                   b   4     -     (         b   3       S   31       *     S     4   ⁢   1         )         S     4   ⁢   2         ;         
where b 3  is the coupled forward signal coupled into the forward coupled port; where b 4  is the coupled reverse signal coupled into the reverse coupled port; where S 31  is the transmission S-parameter from the input port of the bidirectional coupler to the forward coupled port; where S 41  is the transmission S-parameter from the input port to the reverse coupled port; and where S 42  is the transmission S-parameter from the output port of the bidirectional coupler to the reverse coupled port.
 
     Example 22 includes the program product of any of Examples 20-21, wherein the program instructions are configured, when executed by at least one programmable processor, to further cause the at least one programmable processor to: determine if at least one of the output reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold; and if at least one of the output reflected power exceeds the reflected power threshold and the coupled reverse power exceeds the coupled reverse power threshold, perform at least one of: reducing the power level of the signal at the input port of the bidirectional coupler until at least one of the output reflected power is less than or equal to the reflected power threshold and the coupled reverse power is less than or equal to the coupled reverse power threshold, and providing an alarm indicating that at least one of the output reflected power exceeds a reflected power threshold and the coupled reverse power exceeds a coupled reverse power threshold. 
     Example 23 includes the program product of Example 22, wherein reducing the power level of the signal comprises reducing a power level of an output signal of a signal source that is a power amplifier having an output coupled to the input port. 
     Example 24 includes the program product of any of Examples 20-23, wherein the program instructions are configured, when executed by at least one programmable processor, to further cause the at least one programmable processor to: determine at least one of a reflection coefficient (Γ) at the output port and a voltage standing wave ratio (VSWR) at the output port, where the reflection coefficient and voltage standing wave ratio are determined for a load coupled to the output port, and where: the reflection coefficient is a function of the output reflected signal; and 
     
       
         
           
             VSWR 
             = 
             
               
                 
                   1 
                   + 
                   
                     
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                     r 
                     
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     Example 25 includes the program product of Example 24, wherein the program instructions are configured, when executed by at least one programmable processor, to further cause the at least one programmable processor to: determine if at least one of a reflection coefficient exceeds a reflection coefficient threshold and a VSWR exceeds a VSWR threshold; if at least one of the reflection coefficient exceeds a reflection coefficient threshold and the VSWR exceeds a VSWR threshold, then provide an alarm indicating that at least one of: the reflection coefficient exceeds the reflection coefficient threshold and the VSWR exceeds the VSWR threshold. 
     Example 26 includes the program product of any of Examples 20-25, wherein the program instructions are configured, when executed by at least one programmable processor, to further cause the at least one programmable processor to: determine if at least one of an input power at the input port is equal to or exceeds an input power threshold level, and the coupled forward power is equal to or exceeds a coupled forward power threshold; and if at least one of the input power is equal to or exceeds the input power threshold level and the coupled forward power is equal to or exceeds the coupled forward power, perform at least one of: reducing a power level of the input power until at least one of the input power at the input port less than an input power threshold level and the coupled forward power is less than the coupled forward power threshold, and providing an alarm indicating the automatic limiting control function has been enabled. 
     Example 27 includes the program product of Example 26, wherein reducing the power level of the signal comprises reducing a power level of an output signal of a signal source that is a power amplifier having an output coupled to the input port. 
     The terms “about” or “substantially” indicate that the value or parameter specified may be somewhat altered, as long as the alteration does not result in nonconformance of the process or structure to the illustrated embodiment. Finally, “exemplary” indicates the description is used as an example, rather than implying that it is an ideal. 
     A number of embodiments of the invention defined by the following claims have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the spirit and scope of the claimed invention. Accordingly, other embodiments are within the scope of the following claims. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.