Patent Publication Number: US-2022224366-A1

Title: Indirect reflection detection for receiver circuitry protection in tdd transceivers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 63/137,255, filed Jan. 14, 2021, and titled “INDIRECT REFLECTION DETECTION FOR RECEIVER CIRCUITRY PROTECTION IN TDD TRANSCEIVERS,” which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Reflection measurements are commonly used at the output of systems in order to determine return loss and monitor whether the system is sufficiently well matched to a following stage (for example, an antenna or cable coupled to an antenna). During normal operation, the cable and the antenna coupled to a system only cause minor reflections. However, when the cable is broken or the antenna is shorted, the reflection coefficient at the output of the system increases significantly and a large amount of transmit radio frequency (RF) power is reflected. The reflected transmit RF power can damage components of the receiver circuitry (for example, low-noise amplifier (LNA), RF switches, terminations, etc.) in time-division duplexing (TDD) systems, frequency-division duplexing (FDD), and in-band full-duplex (FD) systems. 
     SUMMARY 
     In an example, a communications device is provided. The communications device includes a transmit signal path and a receive signal path. The communications device further includes an isolation device coupled to the transmit signal path and the receive signal path. The isolation device is configured to provide an analog transmit signal from the transmit signal path to an antenna port of the isolation device and to provide an analog receive signal from the antenna port to the receive signal path. The communications device further includes a coupler coupled to the receive signal path between the isolation device and an amplifier of the receive signal path. The communications device further includes a power detector coupled to the coupler and configured to measure a power level of a transmit leakage signal in the receive signal path. The communications device further includes a comparator configured to compare the measured power level of the transmit leakage signal to a first threshold value and output a first alarm signal indicating that the measured power level of the transmit leakage signal exceeds the first threshold value. The communications device is configured to reduce a transmit output power in response to the first alarm signal. 
     In another example, a method of protecting receiver circuitry is provided. The method includes measuring a power level of a transmit leakage signal in a receive signal path. The method further includes comparing the measured power level of the transmit leakage signal to a threshold voltage. The method further includes outputting an alarm signal indicating that the measured power level of the transmit leakage signal exceeds the threshold voltage. The method further includes reducing a transmit output power in response to the alarm signal. 
     In an example, a receiver protection circuit is provided. The receiver protection circuit includes a directional coupler configured to be coupled to a receive signal path of a communications device between an isolation device and an amplifier of the receive signal path. The receiver protection circuit further includes a power detector coupled to the directional coupler and configured to measure a power level of a transmit leakage signal in the receive signal path. The receiver protection circuit further includes a comparator configured to compare the measured power level of the transmit leakage signal in the receive signal path to a threshold value and output an alarm signal indicating that the measured power level of the transmit leakage signal exceeds the threshold value. 
    
    
     
       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 through the use of the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an example TDD transceiver that includes a receiver protection circuit; 
         FIG. 2  is graph of isolation between a transmit signal path and receive signal path in a time-division-duplexing (TDD) transceiver including the receiver protection circuit of  FIG. 1 ; 
         FIG. 3A  is a block diagram of an example compensation circuit; 
         FIG. 3B  is a block diagram of an example compensation circuit; 
         FIG. 4  is a block diagram illustrating an example distributed antenna system utilizing a receiver protection circuit; 
         FIG. 5  is a block diagram illustrating an example repeater system utilizing a receiver protection circuit; and 
         FIG. 6  is a block diagram illustrating an example radio access network utilizing a receiver protection circuit. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. 
     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 logical, mechanical, and electrical changes may be made. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Return loss generally describes how well an impedance is matched to the characteristic impedance and is defined by the logarithmic ratio between the reflected wave and the transmitted wave at the point of measurement. Common return loss measurement systems include a directional coupler at or near the antenna, power detectors, and a processing unit. The directional coupler couples and separates the transmitted wave and the reflected wave, and the power detectors measure the power levels of the coupled portions of the transmitted wave and the reflected wave. The power detectors output voltages proportional to the power levels, which are converted to digital signals and used by the processing unit to calculate the return loss. 
     There are several disadvantages of the common return loss measurement systems. First, the common return loss measurement systems are not able to accurately measure low return losses where there is a significant amount of reflected transmit RF power, which can lead to false alarms or damaged receiver circuitry. Further, the insertion loss of the directional coupler used in the common return loss measurement systems can degrade the maximum output power of the RF system. Also, the processing unit used in common return loss measurement systems is relatively expensive and requires a sometimes unacceptably large amount of time to detect reflection, assert an alarm, and take further action to protect the receiver circuitry from damage. There is a need for a mechanism that addresses the problems with common return loss measurement systems. 
       FIG. 1  illustrates a block diagram of an example TDD transceiver  100  that includes a receiver protection circuit  113 . The TDD transceiver  100  includes a transmit signal path  101 , a receive signal path  102 , and a receiver protection circuit  113  coupled to the receive signal path  102 . It should be understood that the particular components and configuration of the components of the TDD transceiver  100  can vary depending on requirements. 
     In the example shown in  FIG. 1 , the transmit signal path  101  includes a transmit input  103 , a driver stage  104 , and a power amplifier  106 . In the example shown in  FIG. 1 , the transmit signal path  101  of the TDD transceiver  100  is configured to receive an analog transmit signal via the transmit input  103  and amplify the analog transmit signal using the driver stage  104  and the power amplifier  106  in the transmit signal path  101 . In some examples, the transmit signal path  101  can include one or more pre-amplifiers, one or more additional driver stages, and/or one or more additional power amplifiers. In the example shown in  FIG. 1 , the transmit signal path  101  is coupled to an isolation device  108 , which is configured to direct the amplified, analog transmit signal to the antenna  112  for radiation to one or more mobile devices in the coverage area of the TDD transceiver  100 . 
     In the example shown in  FIG. 1 , the receive signal path  102  is also coupled to the isolation device  108 , which is configured to provide an analog receive signal from the antenna  112  to the receive signal path  102 . In the example shown in  FIG. 1 , the receive signal path  102  includes a switch  120  coupled to a termination  121 , a low-noise amplifier  122 , and a receive output  124 . For a transmit mode of TDD operation, the TDD transceiver  100  is configured to couple the switch  120  to the termination  121  in order to provide additional isolation between the transmit signal path  101  and the receive signal path  102 . For a receive mode of TDD operation, the TDD transceiver  100  is configured to couple the switch  120  to the low-noise amplifier  122  to enable reception and processing of the analog receive signals in the receive signal path  102 . The receive signal path  102  is configured to amplify the analog receive signal using the low-noise amplifier  122  and output the amplified, analog receive signal via the receive output  124  for further processing. 
     In some examples, the isolation device  108  is a circulator where the transmit signal path  101  is coupled to a first port of the circulator, the antenna  112  is coupled to a second port of the circulator, and the receive signal path  102  is coupled to a third port of the circulator. In some such examples, the TDD transceiver  100  also includes a bandpass filter  110  between the circulator and the antenna  112 . In some examples, the isolation device  108  is a device other than a circulator (for example, a hybrid coupler). In some examples, the isolation device  108  can also be coupled to multiple separate antennas rather than a single antenna  112  as shown in  FIG. 1 . It should be understood that different configurations of the isolation device  108  and antennas  112  can also be used. 
     During operation, the majority of the amplified, analog transmit signal should be transmitted via the antenna  112 . However, a small portion of the amplified, analog transmit signal can leak into the receive signal path  102  due to finite transmitter-to-receiver isolation of the isolation device  108 . In order to protect the circuitry in the receive signal path  102  from damage caused by the finite isolation and reflected RF power, the TDD transceiver  100  further includes a receiver protection circuit  113  coupled to the receive signal path  102 . In the example shown in  FIG. 1 , the receiver protection circuit  113  is coupled between the isolation device  108  and the switch  120  in the receive signal path  102 . 
     In the example shown in  FIG. 1 , the receiver protection circuit  113  includes a directional coupler  114 , a power detector  116 , and a comparator  118 . The directional coupler  114  is coupled to the receive signal path  102  between the isolation device  108  and the components to be protected (for example, the switch  120 , the termination  121 , and the low-noise amplifier  122 ). The directional coupler  114  is configured to couple a portion of a transmit leakage signal from the receive signal path  102  to the power detector  116 . 
     The power detector  116  is configured to measure a power level of the transmit leakage signal coupled from the receive signal path  102 . In some examples, the power detector  116  is a peak power detector. In other examples, the power detector  116  is an average power detector or a different type of power detector. The power detector  116  is configured to provide a voltage that is proportional to the measured power level of the transmit leakage signal to the comparator  118 . While the power detector  116  and the comparator  118  are shown as discrete components in  FIG. 1 , it should be understood that the power detector  116  and comparator  118  can be combined in a single component in other examples. 
     The comparator  118  is configured to compare the signal from the power detector  116  to a threshold value (for example, a threshold voltage) and output an alarm signal indicating that the measured power level of the transmit leakage signal exceeds the threshold value. In some examples, the threshold value is determined or selected based on a known relationship between return loss and the isolation between the transmit signal path  101  and the receive signal path  102  for the TDD transceiver  100 .  FIG. 2  illustrates an example graph showing the isolation between the transmit signal path and the receive signal path at room temperature. It can be seen that the insertion loss (y-axis) for the transmit leakage signal is dependent on the return loss at the antenna port of the isolation device  108 , so the isolation provided between the transmit signal path  101  and the receive signal path  102  is dependent on the return loss at the antenna port of the isolation device  108 . In the example shown in  FIG. 2 , the isolation between the transmit signal path  101  and the receive signal path  102  is lowest when the return loss is 0 dB and highest when the return loss is 20 dB, so the isolation between the transmit signal path  101  and the receive signal path  102  increases as the return loss increases. The behavior of the TDD transceiver  100  can be observed in order to determine power levels of transmit leakage signal that are correlated with particular return losses at the antenna port of the isolation device  108 . The threshold value can be set such that an alarm signal is only output by the comparator  118  when a particular return loss is inferred in order to avoid false alarms and degraded performance. 
     In some examples, the comparator  118  is configured to compare the signal from the power detector  116  to multiple threshold values (for example, threshold voltages) and output different alarm signals indicating that the measured power level of the transmit leakage signal exceeds a particular threshold value. In some examples, each of the multiple threshold values are determined or selected based on a known relationship between return loss and the isolation between the transmit signal path  101  and the receive signal path  102  for the TDD transceiver  100 . The different threshold values are determined or selected to correspond to different risk levels for the circuitry in the receive signal path  102 . For example, a first threshold value can correspond to a power level that is harmful to the circuitry of the receive signal path  102  and a second threshold value can correspond to a power level that is critical, but would not damage the circuitry of the receive signal path  102 . More than two threshold values can used by the comparator  118  depending on the desired level of protection. 
     The TDD transceiver  100  is configured to reduce the output signal power level in response to the comparator  118  outputting the alarm signal(s). The particular actions taken to reduce the output signal power level and protect the components of the receive signal path  102  for the TDD transceiver  100  can be different depending on the particular threshold value that the measured power level of the transmit leakage signal exceeds. 
     In some examples, the TDD transceiver  100  is configured to reduce the output signal power level by shutting down the driver stage  104  in response to the comparator  118  outputting an alarm signal (for example, indicating that the measured power level exceeds the first threshold value discussed above). In some examples, the comparator  118  is an analog component that is coupled to a switch  125  in a power supply  126  of the driver stage  104 . In such examples, the alarm signal output by the comparator  118  is used to control the switch  125  to shut down the power supply  126  of the driver stage  104 . In other examples, the comparator  118  is coupled to a controller of the TDD transceiver  100  or a controller of a device that includes the TDD transceiver  100 , and the controller is configured to shut down the driver stage  104  (for example, by controlling the switch  125  of power supply  126 ). 
     In some examples, the TDD transceiver  100  is configured to reduce the output signal power level by reducing the supply voltage of the driver stage  104  in response to the comparator  118  outputting an alarm signal (for example, indicating that the measured power level exceeds the second threshold value discussed above). For example, the TDD transceiver  100  can reduce the supply voltage of the driver stage  104  from 5 V to 3.3 V. It should be understood that a different supply voltage (other than 5 V) and/or a different reduction (other than 1.7 V) could be used to reduce the output signal power level. In some examples, the alarm signal output by the comparator  118  can be used to control the supply voltage provided by the power supply  126  of the driver stage  104 . In other examples, the comparator  118  is coupled to a controller of the TDD transceiver  100  or a controller of a device that includes the TDD transceiver  100 , and the controller is configured to reduce the supply voltage of the driver stage  104  (for example, by controlling the power supply  126 ). Reducing the supply voltage of the driver stage  104  could be performed in addition to (for example, prior to shutting down the driver stage  104 ) or instead of shutting down the driver stage  104 . 
     In some examples, the TDD transceiver  100  is configured to reduce the output signal power level by shutting down the power amplifier  106  in response to the comparator  118  outputting an alarm signal (for example, indicating that the measured power level exceeds the first threshold value discussed above or a different threshold value). In some examples, the biasing of the power amplifier  106  can be switched off in response to the alarm signal. In some examples, the comparator  118  is an analog component that is coupled to a switch in a power supply (not shown) of the power amplifier  106 . In such examples, the alarm signal output by the comparator  118  is used to control the switch to shut down the power supply of the power amplifier  106 . In other examples, the comparator  118  is coupled to a controller of the TDD transceiver  100  or a controller of a device that includes the TDD transceiver  100 , and the controller is configured to shut down the power amplifier  106  (for example, by controlling the switch of power supply of the power amplifier). Shutting down the power amplifier  106  could be performed in addition to or instead of modifying operation of the driver stage  104 . 
     In some examples, the TDD transceiver  100  is configured to reduce the output signal power level by reducing the supply voltage of the power amplifier  106  in response to the comparator  118  outputting an alarm signal (for example, indicating that the measured power level exceeds the second threshold value discussed above or a different threshold value). For example, the TDD transceiver  100  can reduce the supply voltage of the power amplifier  106  from 32 V to 23 V. It should be understood that a different supply voltage (other than 32 V) and/or a different reduction (other than 9 V) could be used to reduce the output signal power level. In some examples, the alarm signal output by the comparator  118  can be used to control the supply voltage provided by the power supply (not shown) of the power amplifier  106 . In other examples, the comparator  118  is coupled to a controller of the TDD transceiver  100  or a controller of a device that includes the TDD transceiver  100 , and the controller is configured to reduce the supply voltage of the power amplifier  106  (for example, by controlling the power supply of the power amplifier). Reducing the supply voltage of the power amplifier  106  could be performed in addition to shutting down the power amplifier  106  (for example, prior to shutting down the power amplifier) or instead of shutting down the power amplifier  106 . Reducing the supply voltage of the power amplifier  106  could also be performed in addition to or instead of modifying operation of the driver stage  104 . 
     By reducing the supply voltage to and/or shutting down the driver stage  104  and/or power amplifier  106 , the output power of the amplified, analog transmit signal is reduced to a non-harmful level and the components (for example, the switch  120 , termination  121 , and low-noise amplifier  122 ) in the receive signal path  102  are protected from being damaged. 
     One or more additional (or alternative) components in the TDD transceiver  100  can also be used to reduce the output power of the amplified, analog transmit signal. In some examples, an attenuator is included in the transmit signal path  101  and the attenuator is configured to increase attenuation of a signal traversing the transmit signal path  101  in response to the alarm signal. In some examples, the switch  120  in the receive signal path  102  can be coupled to the termination  121  in response to the alarm signal in order to protect the low-noise amplifier  122 . The variety of shutdown options enables the TDD transceiver  100  to use a cascaded switching pattern that allows the output power of the amplified, analog transmit signal to be reduced to a required level to protect the receiver circuitry. 
     In some examples, the characteristics of the isolation device  108  and other components of the TDD transceiver  100  can vary depending on environmental conditions. For some applications, the accuracy of measurements and alarm determinations needs to be maintained over large variations in temperature and humidity. Therefore, in some examples, the threshold values(s) used by the comparator  118  are compensated based on variation in temperature and humidity.  FIGS. 3A-3B  depict example circuits  300 ,  350 , which are configured to compensate the threshold value(s) for the comparator  118 . 
     In the example shown in  FIG. 3A , the compensation circuit  300  is passive and includes a voltage divider to compensate the threshold value(s). In the example shown in  FIG. 3A , the compensation circuit  300  includes a resistor  302  and a thermistor  304 . In some examples, the thermistor  304  is a negative temperature coefficient thermistor. In other examples, the thermistor  304  is a positive temperature coefficient thermistor. The threshold value(s) used by the comparator  118  are modified based on the output signal from the compensation circuit  300 . 
     In the example shown in  FIG. 3B , the compensation circuit  350  is active and includes a controller to compensate the threshold value(s). In the example shown in  FIG. 3B , the compensation circuit  350  includes a humidity sensor  352  and a temperature sensor  354  coupled to a controller  356 . In some examples, the controller  356  is a PIC microcontroller. The controller  356  is configured to receive humidity and temperature measurements from the humidity sensor  352  and temperature sensor  254 , respectively, and adjust the threshold value(s) for the comparator  118  based on the humidity and temperature measurements. 
     Compared to the common return loss measurement systems, the receiver protection circuit  113  described herein provides significant improvements in shutdown and alarming time. For example, one implementation of the receiver protection circuit  113  was shown to output an alarm signal and shut down the driver stage in the range of 100 ns. Further, the receiver protection circuit  113  described herein delivers better accuracy for determining low return losses and does not degrade the maximum transmit output power by locating the directional coupler in the receive signal path rather than at the antenna. Moreover, the receiver protection circuit  113  does not require a processing unit, so the receiver protection circuit  113  can be implemented at a lower cost than common return measurement systems. 
     The receiver protection circuit  113  described above can be used in conjunction with a number of RF circuits and system architectures such as, but not limited to: wireless network access points, distributed antenna systems, RF repeaters, cellular communications base stations, and small cell base stations. 
       FIG. 4  is a block diagram of an example distributed antenna system (DAS)  400  that includes the receiver protection circuit  113  in one or more components of the DAS  400 . In the example of  FIG. 4 , the DAS  400  includes one or more master units  402  (also referred to as “host units” or “central area nodes” or “central units”) and one or more remote antenna units  404  (also referred to as “remote units” or “radiating points”) that are communicatively coupled to the one or more master units  402 . In this example, the DAS  400  comprises a digital DAS, in which DAS traffic is distributed between the master units  402  and the remote antenna units  404  in digital form. The DAS  400  can be deployed at a site to provide wireless coverage and capacity for one or more wireless network operators. The site may be, for example, a building or campus or other grouping of buildings (used, for example, by one or more businesses, governments, or other enterprise entities) or some other public venue (such as a hotel, resort, amusement park, hospital, shopping center, airport, university campus, arena, or an outdoor area such as a ski area, stadium or a densely-populated downtown area). 
     The master unit  402  is communicatively coupled to the plurality of base stations  406 . One or more of the base stations  406  can be co-located with the respective master unit  402  to which it is coupled (for example, where the base station  406  is dedicated to providing base station capacity to the DAS  400 ). Also, one or more of the base stations  406  can be located remotely from the respective master unit  402  to which it is coupled (for example, where the base station  406  is a macro base station providing base station capacity to a macro cell in addition to providing capacity to the DAS  400 ). In this latter case, a master unit  402  can be coupled to a donor antenna using an over-the-air repeater in order to wirelessly communicate with the remotely located base station. 
     The base stations  406  can be implemented in a traditional manner in which a base band unit (BBU) is deployed at the same location with a remote radio head (RRH) to which it is coupled, where the BBU and RRH are coupled to each other using optical fibers over which front haul data is communicated as streams of digital IQ samples (for example, in a format that complies with one of the Common Public Radio Interface (CPRI), Open Base Station Architecture Initiative (OBSAI), and Open RAN (O-RAN) families of specifications). Also, the base stations  406  can be implemented in other ways (for example, using a centralized radio access network (C-RAN) topology where multiple BBUs are deployed together in a central location, where each of BBU is coupled to one or more RRHs that are deployed in the area in which wireless service is to be provided. Also, the base station  406  can be implemented as a small cell base station in which the BBU and RRH functions are deployed together in a single package. 
     The master unit  402  can be configured to use wideband interfaces or narrowband interfaces to the base stations  406 . Also, the master unit  402  can be configured to interface with the base stations  406  using analog radio frequency (RF) interfaces or digital interfaces (for example, using a CPRI, OBSAI, or O-RAN digital interface). In some examples, the master unit  402  interfaces with the base stations  406  via one or more wireless interface nodes (not shown). A wireless interface node can be located, for example, at a base station hotel, and group a particular part of a RF installation to transfer to the master unit  402 . 
     Traditionally, a master unit  402  interfaces with one or more base stations  406  using the analog radio frequency signals that each base station  406  communicates to and from a mobile device  408  (also referred to as “mobile units” or “user equipment”) of a user using a suitable air interface standard. Although the devices  408  are referred to here as “mobile” devices  408 , it is to be understood that the devices  408  need not be mobile in ordinary use (for example, where the device  408  is integrated into, or is coupled to, a sensor unit that is deployed in a fixed location and that periodically wirelessly communicates with a gateway or other device). The DAS  400  operates as a distributed repeater for such radio frequency signals. RF signals transmitted from each base station  406  (also referred to herein as “downlink RF signals”) are received at the master unit. In such examples, the master unit  402  uses the downlink RF signals to generate a downlink transport signal that is distributed to one or more of the remote antenna units  404 . Each such remote antenna unit  404  receives the downlink transport signal and reconstructs a version of the downlink RF signals based on the downlink transport signal and causes the reconstructed downlink RF signals to be radiated from an antenna  414  coupled to or included in that remote antenna unit  404 . 
     In some aspects, the master unit  402  is directly coupled to the remote antenna units  404 . In such aspects, the master unit  402  is coupled to the remote antenna units  404  using cables  421 . For example, the cables  421  can include optical fiber or Ethernet cable complying with the Category 5, Category 5e, Category 6, Category 6A, or Category 7 specifications. Future communication medium specifications used for Ethernet signals are also within the scope of the present disclosure. 
     A similar process can be performed in the uplink direction. RF signals transmitted from mobile devices  408  (also referred to herein as “uplink RF signals”) are received at one or more remote antenna units  404  via an antenna  414 . Each remote antenna unit  404  uses the uplink RF signals to generate an uplink transport signal that is transmitted from the remote antenna unit  404  to a master unit  402 . The master unit  402  receives uplink transport signals transmitted from one or more remote antenna units  404  coupled to it. The master unit  402  can combine data or signals communicated via the uplink transport signals from multiple remote antenna units  404  (for example, where the DAS  400  is implemented as a digital DAS  400 , by digitally summing corresponding digital samples received from the various remote antenna units  404 ) and generates uplink RF signals from the combined data or signals. In such examples, the master unit  402  communicates the generated uplink RF signals to one or more base stations  406 . In this way, the coverage of the base stations  406  can be expanded using the DAS  400 . 
     As noted above, in the example shown in  FIG. 4 , the DAS  400  is implemented as a digital DAS. In some examples of a “digital” DAS, real digital signals are communicated between the master unit  402  and the remote antenna units  404 . In some examples of a “digital” DAS, signals received from and provided to the base stations  406  and mobile devices  408  are used to produce digital in-phase (I) and quadrature (Q) samples, which are communicated between the master unit  402  and remote antenna units  404 . It is important to note that this digital IQ representation of the original signals received from the base stations  406  and from the mobile units still maintains the original modulation (that is, the change in the instantaneous amplitude, phase, or frequency of a carrier) used to convey telephony or data information pursuant to the cellular air interface standard used for wirelessly communicating between the base stations  406  and the mobile units. Examples of such cellular air interface standards include, for example, the Global System for Mobile Communication (GSM), Universal Mobile Telecommunications System (UMTS), High-Speed Downlink Packet Access (HSDPA), Long-Term Evolution (LTE), Citizens Broadband Radio Service (CBRS), and fifth generation New Radio (5G NR) air interface standards. Also, each stream of digital IQ samples represents or includes a portion of the frequency spectrum. For example, the digital IQ samples can represent a single radio access network carrier (for example, a 5G NR carrier with 40 MHz or 400 MHz signal bandwidth) onto which voice or data information has been modulated using a 5G NR air interface. However, it is to be understood that each such stream can also represent multiple carriers (for example, in a band of the frequency spectrum or a sub-band of a given band of the frequency spectrum). 
     In the example shown in  FIG. 4 , the master unit  402  can be configured to interface with one or more base stations  406  using an analog RF interface (for example, via the analog RF interface of an RRH or a small cell base station). In some examples, the base stations  406  can be coupled to the master unit  402  using a network of attenuators, combiners, splitters, amplifiers, filters, cross-connects, etc., which is referred to collectively as a point-of-interface (POI)  407 . This is done so that, in the downlink, the desired set of RF carriers output by the base stations  406  can be extracted, combined, and routed to the appropriate master unit  402 , and so that, in the uplink, the desired set of carriers output by the master unit  402  can be extracted, combined, and routed to the appropriate interface of each base station  406 . In other examples, the POI  407  can be part of the master unit  402 . 
     In the example shown in  FIG. 4 , in the downlink, the master unit  402  can produce digital IQ samples from an analog signal received at certain radio frequencies. These digital IQ samples can also be filtered, amplified, attenuated, and/or re-sampled or decimated to a lower sample rate. The digital samples can be produced in other ways. Each stream of digital IQ samples represents a portion of the frequency spectrum output by one or more base stations  406 . 
     Likewise, in the uplink, the master unit  402  can produce an uplink analog signal from one or more streams of digital IQ samples received from one or more remote antenna units  404  by digitally combining streams of digital IQ samples that represent the same carriers or frequency bands or sub-bands received from multiple remote antenna units  404  (for example, by digitally summing corresponding digital IQ samples from the various remote antenna units  404 ), performing a digital-to-analog process on the real samples in order to produce an IF or baseband analog signal, and up-converting the IF or baseband analog signal to the desired RF frequency. The digital IQ samples can also be filtered, amplified, attenuated, and/or re-sampled or interpolated to a higher sample rate, before and/or after being combined. 
     In the example shown in  FIG. 4 , the master unit  402  can be configured to interface with one or more base stations  406  using a digital interface (in addition to, or instead of) interfacing with one or more base stations  406  via an analog RF interface. For example, the master unit  402  can be configured to interact directly with one or more BBUs using the digital IQ interface that is used for communicating between the BBUs and an RRHs (for example, using the CPRI serial digital IQ interface). 
     In the downlink, the master unit  402  terminates one or more downlink streams of digital IQ samples provided to it from one or more BBUs and, if necessary, converts (by re-sampling, synchronizing, combining, separating, gain adjusting, etc.) them into downlink streams of digital IQ samples compatible with the remote antenna units  404  used in the DAS  400 . In the uplink, the master unit  402  receives uplink streams of digital IQ samples from one or more remote antenna units  404 , digitally combining streams of digital IQ samples that represent the same carriers or frequency bands or sub-bands received from multiple remote antenna units  404  (for example, by digitally summing corresponding digital IQ samples received from the various remote antenna units  404 ), and, if necessary, converts (by re-sampling, synchronizing, combining, separating, gain adjusting, etc.) them into uplink streams of digital IQ samples compatible with the one or more BBUs that are coupled to that master unit  402 . 
     In the downlink, each remote antenna unit  404  receives streams of digital IQ samples from the master unit  402 , where each stream of digital IQ samples represents a portion of the radio frequency spectrum output by one or more base stations  406 . Each remote antenna unit  404  generates, from the downlink digital IQ samples, one or more downlink RF signals for radiation from the one or more antennas coupled to that remote antenna unit  404  for reception by any mobile devices  408  in the associated coverage area. In the uplink, each remote antenna unit  404  receives one or more uplink radio frequency signals transmitted from any mobile devices  408  in the associated coverage area, generates one or more uplink streams of digital IQ samples derived from the received one or more uplink radio frequency signals, and transmits them to the master unit  402 . 
     Each remote antenna unit  404  can be communicatively coupled directly to one or more master units  402  or indirectly via one or more other remote antenna units  404  and/or via one or more intermediate units  416  (also referred to as “expansion units” or “transport expansion nodes”). The latter approach can be done, for example, in order to increase the number of remote antenna units  404  that a single master unit  402  can feed, to increase the master-unit-to-remote-antenna-unit distance, and/or to reduce the amount of cabling needed to couple a master unit  402  to its associated remote antenna units  404 . The expansion units are coupled to the master unit  402  via one or more cables  421 . 
     In the example DAS  400  shown in  FIG. 4 , a remote antenna unit  404  is shown having another co-located remote antenna unit  405  (also referred to herein as an “extension unit”) communicatively coupled to it. Subtending a co-located extension remote antenna unit  405  from another remote antenna unit  404  can be done in order to expand the number of frequency bands that are radiated from that same location and/or to support MIMO service (for example, where different co-located remote antenna units radiate and receive different MIMO streams for a single MIMO frequency band). The remote antenna unit  404  is communicatively coupled to the “extension” remote antenna units  405  using a fiber optic cable, a multi-conductor cable, coaxial cable, or the like. In such an implementation, the remote antenna units  405  are coupled to the master unit  402  of the DAS  400  via the remote antenna unit  404 . 
     In some examples, one or more components of the DAS  400  include the receiver protection circuit  113  as described above. For example, the remote antenna units  404 ,  405  can include the receiver protection circuit  113  in order to prevent damage to the receiver circuitry in the remote antenna units  404 ,  405 . In some examples, the receiver protection circuit  113  is coupled to the uplink path in one or more remote antenna units  404 ,  405 . 
     Other types of radio frequency distribution systems can also benefit from the receiver protection circuit  113  described above.  FIG. 5  illustrates an example of a single-node repeater  500  that includes one or more receiver protection circuits  113  as discussed above. 
     In the exemplary embodiment shown in  FIG. 5 , the single-node repeater  500  is coupled to one or more base stations  502  using a donor antenna  530 . 
     The single-node repeater  500  includes a first isolation device  506  having a common port that is coupled to the donor antenna  530  via a cable  532 , a downlink port that is coupled to the downlink circuitry  508 , and an uplink port that is coupled to the uplink circuitry  510 . 
     In general, the single-node repeater  500  is configured to receive one or more downlink signals from one or more base stations  502 . Each base station downlink signal includes one or more radio frequency channels used for communicating in the downlink direction with user equipment  514  over the relevant one or more wireless air interfaces. The downlink circuitry  508  is configured to amplify the downlink signals received at the repeater  500  and re-radiate the amplified downlink signals via the coverage antenna  516 . As a part of doing this, the downlink circuitry  508  can be configured to filter the downlink signals to separate out the individual channels, individually amplify each filtered downlink channel signal, combine the individually amplified downlink channel signals, and re-radiate the resulting combined signal. 
     Similar processing is performed in the uplink. The single-node repeater  500  is configured to receive one or more uplink signals from mobile device  514 . Each mobile device uplink signal includes one or more radio frequency channels used for communicating in the uplink direction with one or more base stations  502  over the relevant one or more wireless air interfaces. The uplink circuitry  510  is configured to amplify the uplink signals received at the repeater  500  and re-radiate the amplified uplink signals via the donor antenna  530 . As a part of doing this, the uplink circuitry  510  can be configured to filter the uplink signals to separate out the individual channels, individually amplify each filtered uplink channel signal, combine the individually amplified uplink channel signals, and re-radiate the resulting combined signal. 
     The single-node repeater  500  can be configured to implement one or more features to provide sufficient isolation between the donor antenna  530  and the coverage antenna  516 . These features can include gain control circuitry and adaptive cancellation circuitry. Other features can be implemented. These features can be implemented in one or more of the downlink circuitry  508  and/or the uplink circuitry  510 . These features can also be implemented in separate circuitry. 
     In some examples, the single-node repeater  500  can include at least one receiver protection circuit  113  as described above in order to protect receiver circuitry from damage. For example, the single-node repeater  500  can include a receiver protection circuit  113  coupled to the uplink path between the donor antenna  530  and the uplink circuitry  510  and/or a receiver protection circuit  113  coupled to the downlink path between the downlink circuitry  508  and the coverage antenna  516 . 
     The various circuitry and features of the single-node repeater  500  can be implemented in analog circuitry, digital circuitry, or combinations of analog circuitry and digital circuitry. The downlink circuitry  508  and uplink circuitry  510  can comprise one or more appropriate connectors, attenuators, combiners, splitters, amplifiers, filters, duplexers, 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 circuitry  508  and uplink circuitry  510  may share common circuitry and/or components. 
     Another example of a telecommunication system in which the receiver protection circuit  113  described above can be used is shown in  FIG. 6 .  FIG. 6  is a block diagram illustrating one exemplary embodiment of a radio access network (RAN) system  600  in which the receiver protection circuit  113  described above can be used. The RAN system  600  shown in  FIG. 6  implements a base station. The RAN system  600  can also be referred to here as a “base station” or “base station system.” 
     In the example shown in  FIG. 6 , the system  600  is implemented at least in part using a centralized or cloud RAN (C-RAN) architecture that employs, for each cell (or sector)  601  served by the system  600 , at least one distributed unit (DU)  604  and one or more remote units (RUs)  606 . The system  600  is also referred to here as a “C-RAN system”  600 . The one or more RUs  606  are remotely located from each DU  604  serving it. Also, in some examples, at least one of the RUs  606  is remotely located from at least one other RU  606  serving that cell  602 . It should be understood that the C-RAN implementation of the RAN system  600  is only one way of implementing the RAN system  600  and the architecture of the RAN system  600  can be implemented in other ways. 
     The RAN system  600  can be implemented in accordance with one or more public standards and specifications. For example, the RAN system  600  can be implemented using a RAN architecture and/or RAN fronthaul interfaces defined by the O-RAN Alliance. In such an O-RAN example, the DU  604  and one or more RUs  606  can be implemented as O-RAN distributed units (DUs) and one or more O-RAN remote units (RUs), respectively, in accordance with the O-RAN specifications. More specifically, the DU  604  and the one or more RUs  606  are configured to use the O-RAN fronthaul specification. While multiple RUs  606  are shown in  FIG. 6 , it should be understood that the RAN system  600  can be implemented with one DU  604  and one RU  606 , which is more common for O-RAN implementations. 
     The one or more RUs  606  include or are coupled to one or more antennas  608  via which downlink RF signals are radiated to various items of user equipment (UE)  610  and via which uplink RF signals transmitted by UEs  610  are received. 
     In some examples, the system  600  is coupled to a core network of the associated wireless network operator over an appropriate backhaul (such as the Internet). Also, each DU  604  is communicatively coupled to the one or more RUs  606  served by it using a fronthaul  612 . Each of the DU  604  and the one or more RUs  606  include one or more network interfaces (not shown) in order to enable the DU  604  and the one or more RUs  606  to communicate over the fronthaul  612 . 
     In one implementation, the fronthaul  612  that communicatively couples the DU  604  to the one or more RUs  606  is implemented using a switched ETHERNET network  614 . In such an implementation, each DU  604  and one or more RUs  606  includes one or more ETHERNET interfaces for communicating over the switched ETHERNET network  614  used for the fronthaul  612 . However, it is to be understood that the fronthaul between each DU  604  and the one or more RUs  606  served by it can be implemented in other ways. 
     Generally, for each cell  602  implemented by the RAN system  600 , each DU  604  serving the cell  602  performs the LAYER-3 and LAYER-2 functions for the particular wireless interface used for that cell  602 . Also, for each cell  602  implemented by the RAN system  600 , each corresponding DU  604  serving the cell  602  performs some of the LAYER-1 functions for the particular wireless interface used for that cell  602 . Each of the one or more RUs  606  serving that cell  602  perform the LAYER-1 functions not performed by the DU  604  as well as implementing the basic RF and antenna functions. 
     Each DU  604  and RU  606  (and the functionality described as being included therein), as well as the system  600  more generally, and any of the specific features described here as being implemented by any of the foregoing, can be implemented in hardware, software, or combinations of hardware and software, and the various implementations (whether hardware, software, or combinations of hardware and software) can also be referred to generally as “circuitry” or a “circuit” or “circuits” configured to implement at least some of the associated functionality. When implemented in software, such software can be implemented in software or firmware executing on one or more suitable programmable processors or configuring a programmable device (for example, processors or devices included in or used to implement special-purpose hardware, general-purpose hardware, and/or a virtual platform). Such hardware or software (or portions thereof) can be implemented in other ways (for example, in an application specific integrated circuit (ASIC), etc.). Also, the RF functionality can be implemented using one or more RF integrated circuits (RFICs) and/or discrete components. Each DU  604 , RU  606 , and the system  600  more generally, can be implemented in other ways. 
     In some examples, one or more components of the RAN system  600  include the receiver protection circuit  113  as described above. For example, one or more RUs  606  can include the receiver protection circuit  113  in order to protect receiver circuitry from damage. In some examples, a receiver protection circuit  113  is coupled to the uplink path in one or more RUs  606 . 
     In various aspects, system elements, method steps, or examples described throughout this disclosure (such as the TDD transceiver, DAS, single-node repeater, RAN system, or components thereof, for example) may be implemented on one or more computer systems, field programmable gate array (FPGA), application specific integrated circuit (ASIC) or similar devices comprising hardware executing code to realize those elements, processes, or examples, said code stored on a non-transient data storage device. These devices include or function with software programs, firmware, or other computer readable instructions for carrying out various methods, process tasks, calculations, and control functions, used in a distributed antenna system. 
     These instructions are typically stored on any appropriate computer readable medium used for storage of computer readable instructions or data structures. The computer readable medium can be implemented as any available media that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device. Suitable processor-readable media may include storage or memory media such as magnetic or optical media. For example, storage or memory media may include conventional hard disks, Compact Disk-Read Only Memory (CD-ROM), volatile or non-volatile media such as Random Access Memory (RAM) (including, but not limited to, Synchronous Dynamic Random Access Memory (SDRAM), Double Data Rate (DDR) RAM, RAMBUS Dynamic RAM (RDRAM), Static RAM (SRAM), etc.), Read Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), and flash memory, etc. Suitable processor-readable media may also include transmission media, which are provided by communication networks, wired, and/or wireless. 
     The methods and techniques described here may be implemented in digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor or a general-purpose processor such as a computer) firmware, software, or in combinations of them. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed by a programmable processor executing a program of instructions to perform desired functions by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs). 
     Example Embodiments 
     Example 1 includes a communications device, comprising: a transmit signal path; a receive signal path; an isolation device coupled to the transmit signal path and the receive signal path, wherein the isolation device is configured to provide an analog transmit signal from the transmit signal path to an antenna port of the isolation device, wherein the isolation device is configured to provide an analog receive signal from the antenna port to the receive signal path; a coupler coupled to the receive signal path between the isolation device and an amplifier of the receive signal path; a power detector coupled to the coupler and configured to measure a power level of a transmit leakage signal in the receive signal path; a comparator configured to compare the measured power level of the transmit leakage signal to a first threshold value and output a first alarm signal indicating that the measured power level of the transmit leakage signal exceeds the first threshold value; and wherein the communications device is configured to reduce a transmit output power in response to the first alarm signal. 
     Example 2 includes the communications device of Example 1, wherein the communications device is configured to reduce the transmit output power by: turning off a driver stage in the transmit signal path; reducing a supply voltage of the driver stage in the transmit signal path; reducing a supply voltage of a power amplifier in the transmit signal path; attenuating a signal traversing the transmit signal path; and/or switching off a bias signal for a power amplifier in the transmit signal path. 
     Example 3 includes the communications device of any of Examples 1-2, further comprising: a resistor and a thermistor that form a passive compensation circuit; wherein the communications device is configured to adjust the first threshold value based on an output of the passive compensation circuit. 
     Example 4 includes the communications device of any of Examples 1-2, further comprising: a temperature sensor; a humidity sensor; and a controller configured to adjust the first threshold value based on a temperature measurement from the temperature sensor and a humidity measurement from the humidity sensor. 
     Example 5 includes the communications device of any of Examples 1-4, further comprising a switch in the receive signal path, wherein the switch is positioned between the coupler and the amplifier, wherein the switch is configured to provide the analog receive signal to the amplifier in a first state, wherein the switch is coupled to a termination in a second state; wherein the communications device is configured to control the switch to be in the first state or the second state based on a time-division duplexing schedule. 
     Example 6 includes the communications device of Example 5, wherein the communications device is configured to control the switch to be in the second state in response to the first alarm signal. 
     Example 7 includes the communications device of any of Examples 1-6, wherein the first threshold value is determined based on a known relationship between return loss at the antenna port and isolation between the transmit signal path and the receive signal path. 
     Example 8 includes the communications device of any of Examples 1-7, wherein the isolation device is a circulator. 
     Example 9 includes the communications device of any of Examples 1-8, wherein the power detector is a peak power detector. 
     Example 10 includes the communications device of any of Examples 1-9, wherein the communications device is one of: a remote unit of a distributed antenna system; a radio frequency repeater; a radio point for a small cell; an access point; or a remote radio head of a base station. 
     Example 11 includes the communications device of any of Examples 1-10, wherein the comparator is configured to compare the measured power level of the transmit leakage signal to a second threshold value that is different than the first threshold value and output a second alarm signal indicating that the measured power level of the transmit leakage signal exceeds the second threshold value; wherein the communications device is configured to reduce the transmit output power in response to the second alarm signal. 
     Example 12 includes a method of protecting receiver circuitry, comprising: measuring a power level of a transmit leakage signal in a receive signal path; comparing the measured power level of the transmit leakage signal to a threshold voltage; outputting an alarm signal indicating that the measured power level of the transmit leakage signal exceeds the threshold voltage; and reducing a transmit output power in response to the alarm signal. 
     Example 13 includes the method of Example 12, wherein reducing the transmit output power includes: turning off a driver stage in a transmit signal path; reducing a supply voltage of the driver stage in the transmit signal path; reducing a supply voltage of a power amplifier in the transmit signal path; attenuating a signal traversing the transmit signal path; and/or switching off a bias signal for a power amplifier in the transmit signal path. 
     Example 14 includes the method of any of Examples 12-13, further comprising controlling a switch in the receive signal path to be in a first state or a second state based on a time-division duplexing schedule, wherein the switch is configured to provide an analog receive signal to an amplifier of the receive signal path in the first state, wherein the switch is coupled to a termination in the second state. 
     Example 15 includes the method of Example 14, further comprising controlling the switch to be in the second state in response to the alarm signal. 
     Example 16 includes a receiver protection circuit, comprising: a directional coupler configured to be coupled to a receive signal path of a communications device between an isolation device and an amplifier of the receive signal path; a power detector coupled to the directional coupler and configured to measure a power level of a transmit leakage signal in the receive signal path; and a comparator configured to compare the measured power level of the transmit leakage signal in the receive signal path to a first threshold value and output a first alarm signal indicating that the measured power level of the transmit leakage signal exceeds the first threshold value. 
     Example 17 includes the receiver protection circuit of Example 16, further comprising: a resistor and a thermistor that form a passive compensation circuit; wherein the first threshold value used by the comparator is configured to be adjusted based on an output of the passive compensation circuit. 
     Example 18 includes the receiver protection circuit of Example 16, further comprising: a temperature sensor; a humidity sensor; and a microcontroller configured to adjust the first threshold value based on a temperature measurement from the temperature sensor and a humidity measurement from the humidity sensor. 
     Example 19 includes the receiver protection circuit of any of Examples 16-18, wherein the power detector is a peak power detector. 
     Example 20 includes the receiver protection circuit of any of Examples 16-19, wherein the comparator is configured to compare the measured power level of the transmit leakage signal to a second threshold value that is different than the first threshold value and output a second alarm signal indicating that the measured power level of the transmit leakage signal exceeds the second threshold value; wherein the communications device is configured to reduce a transmit output power in response to the second alarm signal. 
     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.