Patent Publication Number: US-11658634-B1

Title: Radio frequency port impedance detection using concurrent radios

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 16/744,947, filed Jan. 16, 2020, the entire contents are hereby incorporated by reference. 
    
    
     BACKGROUND 
     A large and growing population of users is enjoying entertainment through the consumption of digital media items, such as music, movies, images, electronic books, and so on. The users employ various electronic devices to consume such media items. Among these electronic devices (referred to herein as endpoint devices, user devices, clients, client devices, or user equipment) are electronic book readers, cellular telephones, Personal Digital Assistants (PDAs), portable media players, tablet computers, netbooks, laptops, and the like. These electronic devices wirelessly communicate with a communications infrastructure to enable the consumption of the digital media items. In order to communicate with other devices wirelessly, these electronic devices include one or more antennas. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present inventions will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the present invention, which, however, should not be taken to limit the present invention to the specific embodiments, but are for explanation and understanding only. 
         FIG.  1 A  is a block diagram of a wireless device with an impedance detection circuit where a first radio transmits a first signal according to one embodiment. 
         FIG.  1 B  is a block diagram of the wireless device with the impedance detection circuit where a second radio transmits a second signal according to one embodiment. 
         FIG.  2    is a block diagram of a wireless device  200  with an impedance detection circuit  202  with a butler matrix according to one embodiment. 
         FIG.  3    is a block diagram of a wireless device with an impedance detection circuit in a frequency domain reflectometry mode according to one embodiment. 
         FIG.  4    illustrates operation of a bi-directional RF coupler in a frequency domain reflectometry mode according to one embodiment. 
         FIG.  5    illustrates a frequency domain reflectometer visualization of the bi-directional RF coupler according to one embodiment. 
         FIG.  6 A  is a graph of S-parameter measurements for a bi-directional coupler according to one embodiment. 
         FIG.  6 B  is a graph of S-parameter measurements for a bi-directional coupler with an attenuator at a coupler (CPL) port according to one embodiment. 
         FIG.  7    is a block diagram of a wireless device with an impedance detection circuit with a pair of reflective RF switches (SW_R) in an RSSI mode according to one embodiment. 
         FIG.  8    is a block diagram of a wireless device with an impedance detection circuit with a pair of reflective RF switches (SW_R) in a reflectometry mode according to one embodiment. 
         FIG.  9    is a flow diagram of a method of determining a reflection distance of an impedance mismatch condition according to one embodiment. 
         FIG.  10    is a flow diagram of a method of determining an impedance mismatch condition according to one embodiment. 
         FIG.  11    is a timing diagram illustrating a wireless device transmitting a frame to reserve a medium and a data frame for impedance detection according to one embodiment 
         FIG.  12    is a block diagram of an electronic device that can be configured to detect impedance mismatch conditions and a physical distance to a location where the impedance mismatch condition occurs as described herein according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Technologies directed to a wireless device with Radio Frequency (RF) port impedance detection using concurrent radios are described. Many wireless devices used in a fixed wireless infrastructure use external antennas to maintain flexibility in coverage and system gain for each use case. However, antennas may be disconnected from the radio or damage to cabling, antenna, or both may occur. Under these conditions, a network operations center (NOC) may only see changes in radio link-level parameters, including receive signal strength indicator (RSSI) and throughput parameters. Degradation due to channel deterioration may be indistinguishable from a device issue where the antenna is disconnected or there is damage to an RF cable or the antenna itself. Degradation can also be caused by a physical attribute of the RF cable that causes malfunction of the RF cable. Traditionally, there is no direct feedback from devices in the fixed wireless infrastructure indicating a condition of an RF signal path external to the device. Network scalability and maintainability can be significantly impacted by these types of impedance mismatch conditions. In some cases, when a wireless device with concurrent radios is deployed and yet only a single antenna is connected, a second radio should be shut off to reduce power consumption and potential interference. This may not be done properly at staging (i.e., pre-deployment configuration) by an installer since it is difficult for the installer to know which radio to shut off, even with visual identification of the external RF ports. The manual configuration of selecting a radio to shut down by the installer is error prone. 
     Aspects of the present disclosure overcome the deficiencies of traditional wireless devices by providing an impedance detection circuit for concurrent, co-channel radios and an RSSI based algorithm to detect RF port impedance. Aspects of the present disclosure overcome the deficiencies of traditional wireless devices by providing a channelized frequency-domain reflectometry based algorithm to detect a physical distance to an impedance mismatch. The physical distance is between an RF port and a location where the impedance mismatch condition occurs. At any impedance mismatch, some of the energy of an incident signal is reflected backward toward the source as a reflected signal. If these reflections are excessive, the antenna system does not operate properly. Reflectometry is the use of radiation and reflections of radiation in the forms of electromagnetic pulses that are used to determine a location of an impedance mismatch condition at or beyond the RF port, such as in an RF cable coupled to the RF port. The circuitry described herein can be placed in a reflectometry mode in order to determine a physical distance between the RF port and the location where the impedance mismatch condition occurs. If the antenna is disconnected, the physical distance will be zero. If a physical attribute of the RF cable is causing the malfunction, the circuitry in the reflectometry mode can determine the physical distance to the location causing the impedance mismatch. In time-domain reflectometry, the circuit can determine the characteristics of the RF cable by observing reflected waveforms. The characterization can be used to locate the faults or discontinuities in a connector, in the RF cable, or the like. In frequency-domain reflectometry, the circuitry generates a sweep across a frequency range (e.g., frequencies of a channel) as an input into the transmission link. The circuitry (e.g., receiver) measures the interference pattern generated when the swept RF source output signal adds and subtracts with reflected signals from faults and other reflective characteristics within the tested transmission line. The circuitry can calculate a distance to a location where the impedance mismatch condition occurs based on the measured reflected signals across the frequency range. 
     Aspects of the present disclosure overcome the deficiencies of traditional wireless devices by providing a multi-mode impedance detection circuit with RSSI and frequency-domain reflectometry modes. Aspects of the present disclosure overcome the deficiencies of traditional wireless devices by providing airtime reservation during RSSI detection. The aspects of the present disclosure can implement an RF port impedance detection technique that uses concurrent radios, capable of co-channel operation, to detect reflected energy from an impedance mismatch caused by a disconnected RF port or damage to an RF cable or antenna itself. The aspects of the present disclosure can implement an impedance detection circuit, with an RF coupler, can intentionally couple the radios to enable sensing the impedance of the RF connector termination. An RSSI threshold condition, such as a mismatch threshold, may be set in software based on calibration measurements. In addition to an RSSI based algorithm and impedance detection circuit to detect RF port impedance, the application also includes a channelized frequency-domain reflectometry based algorithm to detect a physical distance to a location where the impedance mismatch occurs and airtime reservation during RSSI detection. Details of the channelized frequency-domain reflectometry based algorithm are described below with respect to  FIGS.  3 - 5    and  FIG.  9   . 
     As described herein, concurrent radios capable of co-channel operation may be used to detect reflected energy from an impedance mismatch such as a disconnected RF port. Wireless devices can include wireless local area network (WLAN) radios that operate in the 2.4 GHz and 5 GHz bands and utilize various WLAN protocols, such as the Wi-Fi® protocols (e.g., 802.11n, 802.11ac, or the like). For example, many Wi-Fi® chipsets support dual concurrency on either 2.4 GHz ISM or 5 GHz U-NII bands. Typically, RF circuitry for each radio is designed to maximize isolation to avoid desensing the receivers especially for asynchronous operation (e.g., carrier-sense multiple access with collision avoidance (CSMA-CA)). However, intentional coupling between the radios enables sensing the impedance of the RF connector termination. Radio-to-radio coupling over circuit board traces (e.g., printed circuit board (PCB) traces) may be used if the RSSI level is sufficiently high. However, there may be many coupling paths interacting in complex ways. This may impact the RSSI level and stability under conditions unrelated to port impedance, including device placement relative to building structures. Therefore, in some embodiments described herein, a dedicated impedance detection circuit is used. 
     Various devices are described herein that include WLAN radios operate in the 2.4 GHz and 5 GHz frequency bands and utilize various WLAN protocols, such as the Wi-Fi® protocols (e.g., 802.11n, 802.11ac, or the like). The radios can utilize 2×2 spatial multiplexing Multiple-input-multiple-output (MIMO) and channel bandwidths from 5 MHz to 160 MHz. The radios can see all 5.x GHz channels, including Dynamic Frequency Selection (DFS) channels and can operates at an Equivalent Isotropic Radiated Power (EIRP) up to 36 dBmi, depending on the channel. Alternatively, other types of radios can be used to determine impedance mismatch conditions using the RF port impedance detection technologies described herein. 
     In one embodiment of dual concurrent radios, an impedance detection-circuit includes an RF switch matrix, a bi-directional RF coupler, and RFFE signal paths. During an impedance measurement, the RFFE circuitry is bypassed. This eliminates potential signal attenuation from amplifier reverse isolation. A second antenna (RX antenna) can be disconnected from the bi-directional RF coupler to reduce unwanted leakage between RF ports. Absorptive switches (SW_A) can be used to maintain the impedance match on coupler ports of the bi-directional RF coupler and the RFFE circuitry. When a first radio (TX radio) transmits a signal, the signal is reflected at the unmatched RF port (i.e. no antenna) and is routed through the bi-directional RF coupler back toward a second radio (RX radio). The second radio measures an RSSI value of the reflected signal. During normal radio operation, the bi-directional RF coupler is bypassed to avoid receiver desense and reduce signal loss. An example of dual concurrent radios and an impedance detection circuit is described and illustrated below with respect to  FIGS.  1 A- 1 B . In another embodiment, the coupling can be done on a PCB without a bi-directional coupler. Also, in other embodiments, the switch matrix can be simplified from the illustrated embodiments. 
       FIG.  1 A  is a block diagram of a wireless device  100  with an impedance detection circuit  102  where a first radio  106  transmits a first signal according to one embodiment. The wireless device  100  includes a processing device  104 , a first radio  106 , a second radio  108 , and the impedance detection circuit  102 . In general, the impedance detection circuit  102 , under control by the processing device  104 , is used to measure one or more RSSI values of a reflected signal, the reflected signal being caused by an impedance mismatch condition caused by an antenna being disconnected from a first RF port (labeled as “no antenna  110 ) or vi) damage to an RF cable coupled between the antenna and the first RF port (not illustrated in  FIG.  1 A ). The RF ports are also referred to as antenna ports. 
     In the depicted embodiment, the first radio  106  is a first WLAN radio and the second radio  108  is a second WLAN radio, both of which are capable of co-channel operation. The first radio  106  is coupled to the processing device  104  and the processing device  104  can control the first radio  106  over a first radio control interface  101 . The second radio  108  is coupled to the processing device  104  and the processing device  104  can control the second radio  108  over a second radio control interface  103 . The impedance detection circuit  102  is coupled to the processing device  104  and the processing device  104  can control the impedance detection circuit  102  over a switch control interface  105 . 
     The impedance-detector circuit  102  includes a first RF front-end (RFFE) circuitry  112 , second RFFE circuitry  114 , a bi-directional RF coupler  116 , and switching circuitry, including a first switch  118 , a second switch  120 , a third switch  122 , and a fourth switch  124  in the depicted embodiment. It should be noted that other configurations of switches can be used for the switching circuitry of the impedance detection circuit  102 . The bi-directional RF coupler  116  includes a first port, a second port, a third port, and a fourth port. The switching circuitry, in a coupler mode, i) couples the first radio  106  to the first port, a first RF port  126  to the third port, and the second radio  108  to the second port, and ii) decouples a second RF port  128  from the fourth port. The switching circuitry, in a normal mode, iii) decouples the bi-directional RF coupler  116  from each of the first radio  106 , the second WLAN radio  108 , the first RF port  126 , and the second RF port  128  and iv) couples the first radio  106  to the first RF port  126  via the first RFFE circuitry  112  and the second radio  108  to the second RF port  128  via the second RFFE circuitry  114 . The processing device  104  can receive a first message from a device at a network operations center (NOC)  130 . The device at the NOC  130  can be a remote server that manages devices in the network. The first message can include a request to check for an impedance mismatch condition caused by the first antenna being disconnected from the first RF port (no antenna  110 ) or vi) damage to an RF cable coupled between the first antenna and the first RF port  126 . The processing device  104  sends a control signal over the switch control interface  105  to the impedance detection circuit  102  that causes the impedance detection circuit  102  to switch from the normal mode to the coupler mode. The processing device  104  instructs the first radio  106  over the first radio control interface  101  to send a first signal via the first RF port  126 . The processing device  104  instructs the second radio  108  over the second radio control interface  103  to measure a first RSSI value of a first reflected signal at the second port of the bi-directional RF coupler  116 . The first reflected signal  138  is caused by an impedance mismatch condition caused by the first antenna not being coupled to the first RF port  126  (“no antenna”  110 ) or damage to an RF cable coupled between the first antenna and the first RF port  126 . The processing device  104  determines whether the first RSSI value exceeds a mismatch threshold  132 . The mismatch threshold  132  represents the impedance mismatch condition. When the RSSI value exceeds the mismatch threshold  132 , there is a high reflection between the first radio  106  and the second radio  108 . The high reflection causes the first reflected signal  138 , as well as leakage at the first RF port  126 . Responsive to the processing device determining that the first RSSI value exceeds the mismatch threshold  132 , the processing device sends a second message to the device at the NOC  130  (also referred to herein as a remote server that manages devices in a network). The second message includes a response with the impedance mismatch condition detected (e.g. information that identifies the first antenna not being coupled to the first RF port  126  (“no antenna”  110 ) or damage to an RF cable coupled between the first antenna and the first RF port  126 ). The processing device receives a third message from the device at the NOC  130 , the third message including a command to disable the first radio  106 . 
     In one embodiment, responsive to the first RSSI value exceeding the threshold, the processing device  104  instructs the first radio  106  to send the same signal via the first antenna in each channel in a set of channels in a frequency-domain reflectometry mode. The processing device  104  instructs the second radio  108  to measure a RSSI value for each channel in the set of channels in the frequency-domain reflectometry mode. The processing device  104  determines a physical distance between the first RF port and a location where the impedance mismatch condition occurs using the RSSI values for the set of channels. The processing device  104  sends a fourth message to the device at the NOC  130 . The fourth message includes a value representing the physical distance. Alternatively, the value representing the physical distance can also be reported in the second message described above that includes an indication of the impedance mismatch condition. 
     The processing device  104  can include one or more Central Processing Units (CPUs), microcontrollers, field programmable gate arrays, or other types of processors or processing devices. The processing device  104  can implement processing logic that comprises hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, firmware, or a combination thereof. The processing logic can configure the wireless device  100  to operate according to any of the processes described herein. The processing device  104  can communicate with other devices over the wired interfaces, the wireless interfaces, or any combination thereof. The wireless device  100  can also include other components, such as one or more memory devices, additional radios, wired interfaces, or the like. 
     The impedance detection circuit  102  can include the first RF port  126 , the second RF port  128 , a first terminal  134  coupled to the first radio  106 , and a second terminal  136  coupled to the second radio  108 . The switching circuitry is coupled to the first RF port  126 , the second RF port  128 , the first terminal  134 , the second terminal  136 , and the bi-directional RF coupler  116 . The processing device  104  controls the switching circuitry to i) couple the first terminal  134  to the first port of the bi-directional RF coupler  116 , ii) couple the first RF port  126  to the third port of the bi-directional RF coupler  116 , iii) couple the second terminal to the second port of the bi-directional RF coupler  116 , and iv) decouple the second RF port  128  from the fourth port of the bi-directional RF coupler  116 . This can be done in response to the processing device  104  receiving a command from the NOC  130  to put the radios in a coupler mode. The processing device  104  causes the first radio  106  to send a first signal (TX signal) via the first terminal  134 . The processing device  104  causes the second radio  108  to measure a first RSSI value of a first reflected signal  138  at the second terminal  136 . The first reflected signal  138  is a reflected signal of the first signal (TX signal). The reflected signal  138  is caused by the impedance mismatch at the first RF port  110  because there is no antenna  110 . Because there is no antenna  110  at the first RF port  126 , there is leakage by the first radio  106 . The reflected signal  138  passes from the first RF port  126  through the bi-directional RF coupler  116  from the third port to the second port that is coupled to the second radio  108 . The processing device  104  receives the first RSSI value from the second radio  108  and determines that the first RSSI value exceeds the mismatch threshold  132 . The processing device sends a message indicative of the impedance mismatch condition to a second device at the NOC  130 . 
     In a further embodiment, the processing device  104  receives a first message from a device at the NOC  130 . The first message include a request to check for an impedance mismatch condition caused by i) a first antenna being disconnected from the first RF port  126  or ii) damage to an RF cable between the first antenna and the first RF port  126 . In response, the processing device sends a control signal to the switching circuitry to switch from a normal mode to the coupler mode. As described above, in the coupler mode, the switching circuitry i) couples the first radio  106  to the first port of the bi-directional RF coupler  116 , the first RF port  126  to the third port of the bi-directional RF coupler  116 , and the second radio  108  to the second port of the bi-directional RF coupler  116 , and ii) decouples the second RF port  128  from the fourth port of the bi-directional RF coupler  116 . In the normal mode, the switching circuitry i) decouples the bi-directional RF coupler  116  from the RF paths and ii) couples the first radio  106  to the first RF port  126  via the first RFFE circuitry  112  and the second radio  108  to the second RF port  128  via the second RFFE circuitry  114 . The switching circuitry decouples the bi-directional RF coupler  116  from the RF paths by decoupling the first radio  106 , the second radio  108 , the first RF port  126 , and the second RF port  128 . 
     In the depicted embodiment, the switching circuitry includes the first switch  118  that is coupled to the first terminal  134 , the first RFFE circuitry  112 , and the first port of the bi-directional RF coupler  116 . The switching circuitry includes the second switch  120  that is coupled to the second terminal  136 , the second RFFE circuitry  114 , and the second port of the bi-directional RF coupler  116 . The switching circuitry includes the third switch  122  that is coupled to the first RFFE circuitry  112 , the third port of the bi-directional RF coupler  116 , and the first RF port  126 . The switching circuitry includes the fourth switch  124  that is coupled to the second RFFE circuitry  114 , the fourth port of the bi-directional RF coupler  116 , and the second RF port  128 . The switches  118 - 124  can be controlled by one or more control signals from the processing device  104  over the switch control interface  105 . 
     In a further embodiment, the processing device  104  causes the second radio  108  to send a second signal via the second terminal  136 , such as illustrated and described below with respect to  FIG.  1 B . 
       FIG.  1 B  is the block diagram of the wireless device  100  with the impedance detection circuit  102  where the second radio  108  transmits a second signal according to one embodiment. The wireless device  100  in  FIG.  1 B  is similar to the wireless device  100  in  FIG.  1 A , as noted by the same reference numbers, but the second radio  108  is transmitting instead of the first radio  106 . For example, the processing device  104  causes the second radio  108  to send the second signal via the second terminal  136  and causes the first radio  106  to measure a second RSSI value of a second reflected signal  140  at the first terminal  116 . The second reflected signal  140  is a reflected signal of the second signal. The processing device  104  receives the second RSSI value from the first radio  106  and determines whether the second RSSI value exceeds the mismatch threshold  132 . The mismatch threshold  132  represents the impedance mismatch condition. In this case, the processing device  104  determines that the second RSSI value does not exceed the mismatch threshold  132 . When the RSSI value does not exceed the mismatch threshold  132 , there is a low reflection between the first radio  106  and the second radio  108 . The low reflection is caused because a second antenna  142  is coupled to the second RF port  128  and is well matched. The second antenna  142  may not be perfectly matched, so there can still be the second reflected signal  140  that passes from the second RF port  128  through the bidirectional RF coupler  116  from the fourth port to the first port that is coupled to the first radio  106 . The matched antenna load with low reflection causes efficient radiation by the second antenna  142 . The matched antenna load with low reflection causes the second reflected signal  140  to be lower than the first reflected signal  138 , as well as lower than the mismatch threshold  132 . The processing device  104  receives the second RSSI value from the first radio  106  and determines that the second RSSI value does not exceed the mismatch threshold  132 . In one embodiment, the processing device  104  sends a message with an indication that there is not an impedance mismatch condition at the second RF port  128 . In another embodiment, the processing device  104  determines the impedance mismatch condition when the first RSSI value exceeds the mismatch threshold  132  and the second RSSI value does not exceed the mismatch threshold  132 . That is, the processing device  104  can send the indication of the impedance mismatch condition to the second device at the NOC  130  responsive to the first RSSI value exceeding the threshold and the second RSSI value not exceeding the threshold. As described above, the impedance mismatch condition is caused by i) a first antenna being disconnected from the first RF port  126  (i.e., no antenna  110 ) or ii) a physical attribute of an RF cable that is coupled between the first antenna and the first RF port  126 . The physical attribute can be a damaged portion of the RF cable, poor shielding, or the like. 
     It should be noted that there is very little reflected energy coupled to the first radio  106  when the second radio  108  is properly impedance matched by the second antenna  140 , as shown in  FIG.  1 B . The difference in RSSI between matched and mismatched can be set to be in the tens of dB at the receiving radio. An RSSI threshold condition may be set in software based on calibration measurements at the factory. 
     As illustrated in  FIGS.  1 A- 1 B , the impedance detection circuit  102  can be used to detect an impedance mismatch condition using the two radios: first radio  106  and second radio  108 . In other embodiments, the wireless device  100  includes one or more additional radios and the switching circuitry of the impedance detection circuit  102  can connect the bi-directional RF coupler  116  between any pair of the three or more radios. Alternatively, the processing device  104  can control the switching circuitry in other manners to transmit a signal and measure an RSSI value at each of the other radios, for example. 
     The technology described above with respect to two antennas can be implemented to devices with more than two concurrent radios. In one embodiment, each pair of radios uses the same impedance detection circuit described above. In another embodiment, all radios can be coupled through a Butler matrix, as described below. In a four-radio example, such as illustrated in  FIG.  2   , a single RF port is routed to the Butler matrix using an RF switch. All other RF ports are disconnected from the Butler matrix. Cascaded hybrid couplers are used to distribute the reflected signals to all radios in receive mode. Each radio can receive a similar RSSI level, which can be compared, averaged, or otherwise processed in parallel to determine whether there is an impedance mismatch condition at an RF port. 
     In one embodiment, the processing device  104  controls the first radio  106  to be coupled to the first terminal  134  during a first time period and during a second time period. The processing device  104  controls the second radio  108  to be coupled to the second terminal  136  during the first time period and a third radio (not illustrated in  FIGS.  1 A- 1 B ) to be coupled to the second terminal  136  during the second time period. During the first time period, the processing device  104  causes the first radio to send a third signal and causes the second radio to measure a third RSSI value of a third reflected signal at the second terminal  136 . During the second time period, the processing device  104  causes the first radio to send a fourth signal and causes the third radio to measure a fourth RSSI value of a fourth reflected signal at the second terminal  136 . The processing device  104  receives the third RSSI value from the second radio and the fourth RSSI value from the third radio. The processing device  104  determines whether the third RSSI value or the fourth RSSI value exceed the mismatch threshold  132 . The processing device sends one or more indications of any impedance mismatch conditions to the device at the NOC  130 . For example, the impedance mismatch condition is caused by a first antenna not being connected to the first RF port  126  or damage to the RF cable. Alternatively, the impedance mismatch condition can be caused by a second antenna not being connected to the second RF port  128 . In another embodiment, the first radio sends the third signal and both the second radio and the third radio each receive a reflected signal and each measure a RSSI value. 
     In another embodiment, the first radio is coupled to the first terminal during a first time period, the second radio is coupled to the second terminal during the first time period, a third radio is coupled to the first terminal during a second time period, and a fourth radio is coupled to the second terminal during the second time period. The processing device  104 , during the first time period, causes the first radio to send the first signal and causes the second radio to measure the first RSSI value. The processing device  104 , during the second time period, causes the third radio to send a second signal and causes the fourth radio to measure a second RSSI value of a second reflected signal at the second terminal. The processing device  104  receives the second RSSI value from the fourth radio and determines that the second RSSI value exceeds the mismatch threshold. The processing device sends a second indication of a second impedance mismatch condition to the second device responsive to the second RSSI value exceeding the threshold. The impedance mismatch condition can be caused by i) a first antenna being disconnected from the first RF port during the first time period or ii) damage to an RF cable that is coupled between the first antenna and the first RF port during the first time period. In this embodiment, the second antenna is connected to the second RF port during the first time period. The second impedance mismatch condition can be caused by i) a third antenna being disconnected from the first RF port during the second time period or ii) damage to a second RF cable that is coupled between the third antenna and the first RF port during the second time period. In this case, a fourth antenna is connected to the second RF port during the second time period. 
     In another embodiment, the switching circuitry includes a butler matrix, such as illustrated and described with respect to  FIG.  2   . 
       FIG.  2    is a block diagram of a wireless device  200  with an impedance detection circuit  202  with a butler matrix according to one embodiment. The wireless device  200  includes a processing device  204 , a first radio  206 , a second radio  208 , a third radio  246 , a fourth radio  248 , and the impedance detection circuit  202 . In general, the impedance detection circuit  202 , under control by the processing device  204 , is used to measure one or more RSSI values of one or more reflected signals, the reflected signals being caused by an impedance mismatch condition caused by an antenna being disconnected from a first RF port  226  (labeled as “no antenna  210 ) or vi) damage to an RF cable coupled between the antenna and the first RF port  226  (not illustrated in  FIG.  2   ). 
     In the depicted embodiment, the first radio  206  is a first WLAN radio, the second radio  208  is a second WLAN radio, the third radio  246  is a third WLAN radio, and the fourth radio  248  is a fourth WLAN radio, each of which is capable of co-channel operation. The first radio  206  is coupled to the processing device  204  and the processing device  204  can control the first radio  206  over a first radio control interface  201 . The second radio  208  is coupled to the processing device  204  and the processing device  204  can control the second radio  208  over a second radio control interface  203 . The third radio  246  is coupled to the processing device  204  and the processing device  204  can control the third radio  246  over a third radio control interface  207 . The fourth radio  248  is coupled to the processing device  204  and the processing device  204  can control the fourth radio  248  over a fourth radio control interface  209 . The impedance detection circuit  202  is coupled to the processing device  204  and the processing device  204  can control the impedance detection circuit  202  over a switch control interface  205 . The impedance-detector circuit  202  includes a first RFFE circuitry  212 , second RFFE circuitry  214 , third RFFE circuitry  252 , fourth RFFE circuitry  254 , the butler matrix, and switching circuitry, including a first switch  218 , a second switch  220 , a third switch  222 , a fourth switch  224 , a fifth switch  258  coupled to a third terminal  274 , a sixth switch  260  coupled to a fourth terminal  276 , a seventh switch  262  coupled to a third RF port  266 , and an eighth switch  264  coupled to a fourth RF port  268 , as set forth in the depicted embodiment. The switches  218 - 224  and  258 - 264  can be controlled by one or more control signals from the processing device  204  over the switch control interface  205 . It should be noted that other configurations of switches can be used for the switching circuitry of the impedance detection circuit  202 . 
     As illustrated in the depicted embodiment, the butler matrix includes: a i) bi-directional RF coupler  216  that includes a first port, a second port, a third port, and a fourth port; ii) a second bi-directional RF coupler  256  that includes a first port, a second port, a third port, and a fourth port; iii) a first phase shifter  217 ; iv) a second phase shifter  257 ; v) a third bi-directional RF coupler  219 ; and vi) a fourth bi-directional RF coupler  259 . 
     In one embodiment, the switching circuitry, in a coupler mode, i) couples the first radio  206  to the first port of the first bi-directional RF coupler  216  via the first switch  218 , a first RF port  226  to the third port of the first bi-directional RF coupler  216  via the first phase shifter  217 , the third bi-directional RF coupler  219 , and the second switch  222 . In the coupler mode, the switching circuitry also ii) couples the second radio  2018  to the second port of the first bi-directional RF coupler  216  via the second switch  220 , and the second radio  208  to the second port, and iii) decouples a second RF port  228  from the fourth port of the third bi-directional RF coupler  219 . The fourth port of the first bi-directional RF coupler  216  is coupled to a first port of the fourth bi-directional RF coupler  259 . A third port of the second bi-directional RF coupler  256  is coupled to a second port of the third bi-directional RF coupler  219 . In the coupler mode, the switching circuitry also iv) couples the third radio  246  to the first port of the second bi-directional RF coupler  256  via the fifth switch  258 . The switching circuitry also v) decouples a third RF port  266  from the second port of the fourth bi-directional RF coupler  259  and a fourth RF port  268  from a fourth port of the fourth bi-directional RF coupler  259 . 
     The switching circuitry, in a normal mode, i) decouples the first bi-directional RF coupler  216 , the second bi-directional RF coupler  256 , the third bi-directional RF coupler  219 , and the fourth bi-directional RF coupler  259  from each of the radios and each of the RF ports; and ii) couples each of the respective RFFE circuitry  212 ,  214 ,  252 , and  254  to each of the radios  206 ,  208 ,  246 , and  248 , respectively and to each of the RF ports  226 ,  228 ,  266 , and  268 , respectively. 
     In the normal mode, the processing device  204  can receive a first message from a device at a NOC  230 . The first message can include a request to check for an impedance mismatch condition caused by the first antenna being disconnected from the first RF port (no antenna  210 ) or vi) damage to an RF cable coupled between the first antenna and the first RF port  226 . The processing device  204  sends a control signal over the switch control interface  205  to the impedance detection circuit  202  that causes the impedance detection circuit  202  to switch from the normal mode to the coupler mode as set forth above. The processing device  204  instructs the first radio  206  over the first radio control interface  201  to send a first signal via the first RF port  226 . The processing device  204  instructs the second radio  208  over the second radio control interface  203  to measure a first RSSI value of a first reflected signal  238  at the second port of the first bi-directional RF coupler  216 . The processing device  204  instructs the third radio  246  over the third radio control interface  207  to measure a second RSSI value of a second reflected signal  278  at the first port of the second bi-directional RF coupler  256 . The processing device  204  instructs the fourth radio  248  over the fourth radio control interface  209  to measure a third RSSI value of a third reflected signal  282  at the second port of the second bi-directional RF coupler  256 . The first reflected signal  238 , the second reflected signal  278 , and the third reflected signal  282  are caused by an impedance mismatch condition caused by the first antenna not being coupled to the first RF port  226  (“no antenna”  210 ) or damage to an RF cable coupled between the first antenna and the first RF port  226 . 
     The processing device  204  receives the first RSSI value from the second radio  208 , the second RSSI value from the third radio  246 , and the third RSSI value from the fourth radio  248 . The processing device  204  determines whether each of the first, second, and third RSSI values exceed a mismatch threshold  232 . The mismatch threshold  232  represents the impedance mismatch condition. When an RSSI value exceeds the mismatch threshold  232 , there is a high reflection between the receiving radio and the transmitting radio. The high reflection causes the reflected signals, as well as leakage at the first RF port  226 . Responsive to the processing device determining that one or more of the RSSI values exceed the mismatch threshold  232 , the processing device  204  sends a second message to the device at the NOC  230 . The second message includes a response with the impedance mismatch condition detected (e.g. information that identifies the first antenna not being coupled to the first RF port  226  (“no antenna”  210 ) or damage to an RF cable coupled between the first antenna and the first RF port  226 ). 
     In other embodiments, the processing device  204  can control transmissions by the first radio  206  over a set of channels in a frequency-domain reflectometry mode. In the frequency-domain reflectometry mode, the processing device can determine a physical distance of the mismatch impedance condition using the RSSI values over the set of channels. As described herein, the physical distance is between an RF port  226  and a location  225  where the impedance mismatch condition occurs. The processing device  204  can receive a command that instructs the processing device  204  to enter the frequency-domain reflectometry mode after determining that an impedance mismatch condition is detected. In the frequency-domain reflectometry mode, the processing device  204  can perform a channel sweep and measure RSSI measurements of the reflected signals during the channel sweep. The processing device computes a reflection distance using the RSSI measurements, such as illustrated in  FIG.  5   , the reflection distance being between the RF port and a location where the impedance mismatch condition occurs. The processing device  204  sends a third message to the device at the NOC  230 . The third message includes the physical distance of the impedance mismatch condition. The physical distance can also be reported in the second message described above that includes the impedance mismatch condition. 
     Like the processing device  104 , the processing device  204  can include one or more CPUs, microcontrollers, field programmable gate arrays, or other types of processors or processing devices. The processing device  204  can implement processing logic that comprises hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, firmware, or a combination thereof. The processing logic can configure the wireless device  200  to operate according to any of the processes described herein. The processing device  204  can communicate with other devices over the wired interfaces, the wireless interfaces, or any combination thereof. The wireless device  200  can also include other components, such as one or more memory devices, additional radios, wired interfaces, or the like. 
     In other embodiments, the processing device  204  can check each of the other RF ports  228 ,  266 , and  268  in a similar fashion as the first RF port  226  as described above. In those embodiments, the processing device  204  sends different control signals to the respective radios to transmit and receive and measure the RSSI values of the reflected signals. The processing device  204  determines whether the impedance mismatch conditions occur on the respective RF port using the measured RSSI values. Similarly, the processing device  204  can determine a physical distance in the frequency-domain reflectometry modes for each of the RF ports. 
     In one embodiment, the switching circuitry of the impedance detection circuit  202  includes: the first switch  218  that is coupled to the first terminal  234 , the first RFFE circuitry  212  and the first port of the first bi-directional RF coupler  216 ; the second switch  220  that is coupled to the second terminal  236 , the second RFFE circuitry  214  and the second port of the first bi-directional RF coupler  212 ; the third switch  222  that is coupled to the first RFFE circuit  212 , the first RF port  226 , and a third port of the third bi-directional RF coupler  219 , a first port of the third bi-directional RF coupler  219  being coupled to a second port of the first bi-directional RF coupler  216  via a first phase shifter  217 ; the fourth switch  224  that is coupled to a fourth port of the second bi-directional RF coupler  256 , the second RF port  228 , and the second RFFE circuitry  214 ; the fifth switch  258  that is coupled to a third terminal  274 , the third RFFE circuitry  252 , and a first port of the second bi-directional RF coupler  256 ; the sixth switch  260  that is coupled to a fourth terminal  276 , a fourth RFFE circuitry  254 , and a second port of the second bi-directional RF coupler  256 , a third port of the second bi-directional RF coupler  256  being coupled to a second port of the third bi-directional RF coupler  219 ; the seventh switch  262  that is coupled to the third RFFE circuitry  252 , a third RF port  266 , and a second port of a fourth bi-directional RF coupler  259 , a first port of the fourth bi-directional RF coupler  259  being coupled to a fourth port of the first bi-directional RF coupler  216  and a second port of the fourth bi-directional RF coupler  259  being coupled to a fourth port of the second bi-directional RF coupler  256  via a second phase shifter  257 ; and the eighth switch  265  that is coupled to a fourth port of the fourth bi-directional RF coupler  259 , a fourth RF port  268 , and the fourth RFFE circuitry  254 . The processing device  204  can control the switching circuitry to be in this configuration for a coupled mode. The processing device  204  can control the switching circuitry to switch between a normal mode and the coupled mode. In the normal mode, each of the respective RFFE circuitry is switched between a terminal and an RF port, removing the bi-directional RF couplers and the phase shifters out of the RF paths. 
     In a first mode, the processing device  204  can determine RSSI values and an impedance mismatch condition using a RSSI-based impedance detection algorithm. In a second mode (e.g., frequency-domain reflectometry mode), the processing device can determine RSSI values and a physical distance of the impedance mismatch condition using a RSSI-based frequency-domain reflectometry algorithm. In the second mode, the processing device  204  can cause the first radio  206  to i) send a set of signals, including the first signal, and ii) cause the second radio to measure a first set of RSSI values, including the first RSSI value, the third radio to measure a second set of RSSI values, and the fourth radio to measure a third set of RSSI values, wherein each of the set of signals has a different frequency of a channel. Each of the first set of RSSI values corresponds to each of the different frequencies of the channel. Each of the second set of RSSI values corresponds to each of the different frequencies of the channel. Each of the third set of RSSI values corresponds to each of the different frequencies of the channel. The processing device  204  determines a reflection distance to a location where the impedance mismatch condition occurs, such as at or beyond the first RF port, using the first set of RSSI values, the second set of RSSI values, and the third set of RSSI values. The processing device sends a value representing the reflection distance to the second device at the NOC  230 . 
     As described herein, the impedance detection circuit can operate as a frequency domain reflectometer (also referred to herein as operating in a “frequency domain reflectometry” mode). The RSSI level can be used to determine the level of reflected energy from a particular RF port. However, this information alone does not indicate a physical distance (e.g., a location) of the impedance mismatch. For example, the impedance mismatch condition may actually be due to reflections from scatterers near an antenna, such as illustrated in  FIG.  3   . A nearby scatterer is a nearby object that refracts or diffracts electromagnetic radiation irregularly to diffuse in many directions. Once the physical distance is determined by the wireless device in the frequency domain reflectometry mode, the wireless device can send this information to the NOC. There are circumstances where it will be useful at the NOC to know if signal degradation is occurring at the RF ports or beyond the antenna. 
       FIG.  3    is a block diagram of the wireless device  100  with the impedance detection circuit  102  in a frequency domain reflectometry mode according to one embodiment. The wireless device  100  is similar to the wireless device  100  described above with respect to  FIGS.  1 A- 1 B , as noted by similar components with the same reference numbers. In the frequency domain reflectometry mode, the processing device  104  can measure RSSI values of reflected signals due to a nearby scatter  310 . As illustrated in  FIG.  3   , the first radio  134  can measure a signal  338  that passes through the bi-directional RF coupler  116  from the second RF port  128 . The signal  338  can be representative of reflection caused by the nearby scatterer  310 . The RSSI values measured in the frequency domain reflectometry mode can be used to determine a physical location of the impedance mismatch condition from frequency spacing between reflection nulls. The coupler-based frequency domain reflectometer shown in  FIG.  3    can rely on constructive and destructive interference between reflected and coupled signals, such as illustrated and described below with respect to  FIG.  4     
       FIG.  4    illustrates operation of a bi-directional RF coupler  400  in a frequency domain reflectometry mode according to one embodiment. The bi-directional RF coupler  400  in the frequency domain is similar to the bi-directional RF couplers described herein, such as the bi-directional RF coupler  102  of  FIGS.  1 A- 1 B . The bi-directional RF coupler  400  relies on constructive and destructive interference between reflected and coupled signals. An incident transmitted signal  401 , at a first input port  402 , is coupled to an unmatched coupler port (CPL)  404  and is used as a reflectometer reference signal  403 . The open circuit impedance mismatch reflects a reflected signal  405  through the bi-directional RF coupler  400  to a test port  406  (TEST). A reference signal level  407  is equal to the transmit power of the incident transmitted signal  401  minus a coupling coefficient (i.e. 10 dB coupler) and insertion losses. The incident-transmitted signal  401  is routed through the bi-directional coupler  400  to an antenna port  408  as a test signal  409 . Depending on antenna port impedance, some of the test signal  409  is reflected back toward the bi-directional RF coupler  400  as a reflected test signal  411 . The reflected test signal  411  is then coupled to the test port  406  as a test signal  413  and combined with the reference signal  407 . This produces frequency domain nulls whenever the test signal  413  and reference signal  407  are out of phase by 180 degrees (180°), as illustrated in  FIG.  5   . 
     As described herein, the bi-directional RF coupler  400  in a frequency domain reflectometry mode can determine a reflection distance  415  between the RF port  408  and a location  417  where the impedance mismatch condition occurs. Here, the impedance mismatch condition occurs at location  417  because of a disconnected antenna port. In other cases, the location  417  could be at other locations along the transmission line (RF cabling  410 ) between the RF port  408  and the antenna port due to some physical attribute that causes malfunction of the RF cabling  410 . 
       FIG.  5    illustrates a frequency domain reflectometer visualization  500  of the bi-directional RF coupler  400  of  FIG.  4    according to one embodiment. The test signal  413  accumulates phase as it propagates along RF cabling  410  in both forward (incident) and reverse directions (reflection). For simplicity, assume the phase of the reference signal  407  at the test port  406  is 0°. Nulls will occur when the phase difference equals an odd multiple of 180° as set forth in equation (1):
 
φ n =φ t −φ r =(2 n −1)π−,  (1)
 
where φ t  is the test signal phase, φ r =0° is the reference signal phase and n=1,2,3, . . . .
 
     The phase difference in terms of propagation coefficient and distance are set forth in equations (2) and (3): 
     
       
         
           
             
               
                 
                   
                     β 
                     n 
                   
                   = 
                   
                     
                       4 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         f 
                         n 
                       
                     
                     
                       v 
                       p 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     φ 
                     n 
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       
                         β 
                         n 
                       
                       ⁢ 
                       R 
                     
                     = 
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             n 
                           
                           - 
                           1 
                         
                         ) 
                       
                       ⁢ 
                       π 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Differentiating with respect to n and solving for distance R gives the simple relation between null spacing and distance to impedance mismatch, as set forth in equation (4): 
                   R   =       v   p       (     2   ⁢   πΔ   ⁢   f     )               (   4   )               
where v p  is the phase velocity of the transmission line.
 
     Minimum distance resolution for this reflectometer type is expressed in equation (5): 
                     Δ   ⁢     R   ⁡   (   m   )       =       (       1   .   5     ×   1   ⁢     0   8       )     ⁢       v   p     BW               (   5   )               
where v p  is the phase velocity of the transmission line and the BW is the total bandwidth available.
 
     The maximum detectable range is expressed in equation (6):
 
 R ( m )= N×ΔR   (6)
 
where N is the number of sample points within the available bandwidth.
 
     For example, consider the approximate frequency spectrum (e.g., ˜600 MHz) in the 5.170 to 5.825 GHz U-NII band. Assuming a phase velocity, v p =0.7, the minimum detectable distance is ≈0.2 m. Maximum distance detection is ≈20 m when measured on approximately one-hundred 5 MHz channels. The difference between disconnected antenna (&lt;5 m) and nearby reflections (&gt;5 m) are both detectable within this range. 
       FIG.  6 A  is a graph  600  of S-parameter measurements for a bi-directional coupler according to one embodiment. The graph  600  shows the measured null spacing for different unterminated antenna port cable lengths. For example, a 2-meter cable sample has 50 MHz null spacing. This result scales to the predicted 20 meters maximum cable length when measured using 5 MHz channels. One interesting observation is that the null depths decrease as cable length increases. This is because the reflected test signal amplitude reduces as the cable loss increases. The reference signal remains unchanged resulting in partial signal cancellation at the test port. One method to improve the null depth is to match the test and reference signal amplitudes using an attenuator at the coupler (CPL) port. A 5 dB improvement in null depth is shown in  FIG.  6 B . 
       FIG.  6 B  is a graph  650  of S-parameter measurements for a bi-directional coupler with an attenuator at a coupler (CPL) port according to one embodiment. The graph  650  shows the null depth dependence on CPL port impedance load. 
     In addition to operating as a frequency domain reflectometer, the impedance detection circuit can be used in a multi-mode port impedance circuit, as described and illustrated below with respect to  FIGS.  7 - 8   . RSSI and reflectometer functionality require different CPL port terminations. For example, RSSI level measurements have flat frequency response and highest accuracy when CPL port is match terminated. In contrast, reflectometry is best suited with a reflective (open or short circuit) or partial attenuation at the CPL port. Therefore, a modified impedance detection circuit may be used. In this embodiment, a pair of reflective RF switches (SW_R) is added to the impedance detection circuit as shown and described below with respect to  FIG.  7    and  FIG.  8   . This allows the CPL port to be switched between RSSI (matched CPL load) and reflectometry (reflective or partially attenuated CPL load). 
       FIG.  7    is a block diagram of a wireless device  700  with an impedance detection circuit  702  with a pair of reflective RF switches (SW_R)  722 ,  724  in an RSSI mode according to one embodiment. The wireless device  700  is similar to the wireless device  700  described above with respect to  FIGS.  1 A- 1 B , as noted by similar components with the same reference numbers. The impedance detection circuit  702  is similar to the impedance detection circuit  102  described above with respect to  FIGS.  1 A- 1 B , as noted by similar components with the same reference numbers. In the RSSI mode, the processing device  104  controls the pair of reflective RF switch  722 ,  724 . In particular, the processing device  104  connects the reflective switch  722  between the second port of the bi-directional RF coupler  116  and the third switch  122 , which is coupled to the first RF port  126 . The processing device  104  connects the reflective switch  724  between the fourth port of the bi-directional RF coupler  116  and the fourth switch  124 , which is coupled to the second RF port  126 . The reflective switch  724  is coupled to a matched port  726 , causing a matched CPL load on the bi-directional RF coupler  116 . 
       FIG.  8    is a block diagram of the wireless device  700  with the impedance detection circuit  702  with a pair of reflective RF switches (SW_R) in a reflectometry mode according to one embodiment. In the frequency domain reflectometry mode, the processing device  104  connects the reflective switch  722  between the second port of the bi-directional RF coupler  116  and the third switch  122 , which is coupled to the first RF port  126 . The processing device  104  connects the reflective switch  724  between the fourth port of the bi-directional RF coupler  116  and the fourth switch  124 , which is coupled to the second RF port  126 . The reflective switch  724  is coupled to a reflective/attenuated port  826 , causing a reflective or partially attenuated CPL load CPL load on the bi-directional RF coupler  116 . 
       FIG.  9    is a flow diagram of a method  900  of determining a reflection distance of an impedance mismatch condition according to one embodiment. The method  900  may be performed by processing logic that comprises hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, firmware, or a combination thereof. In one embodiment, the method  900  may be performed by any of the processing device devices described herein and illustrated with respect to  FIGS.  1 - 8   . 
     Referring back to  FIG.  9   , the method  900  begins by the processing logic receiving a first message from a second device, such as a device  901  at a NOC, the first message including a request to check for an impedance mismatch condition caused by i) a first antenna being disconnected from a first RF port of the wireless device or ii) damage to an RF cable coupled between the first antenna and the first RF port, and a reflection distance to a location where the impedance mismatch condition occurs (block  902 ). The processing logic determines whether a reflected signal is detected (block  904 ). The reflected signal can be whether there RSSI values at the second radio are measured. If there is no reflected signal, the processing logic can transition to a normal radio operation (block  906 ). If there are RSSI values for the reflected signal, the processing logic can instruct the first radio to be in a transmit (TX) mode (block  908 ) and the second radio to be in a receive (RX) mode (block  910 ). The first radio transmits an RF signal in the TX mode and the second radio receives a reflected signal in the RX mode. The processing logic also instructs the switching circuitry to switch into a coupler mode (block  912 ). The processing logic receives RSSI measurement(s) (e.g., RSSI values) from the second radio (block  914 ). The processing logic determines whether the RSSI measurement(s) exceed a mismatch threshold corresponding to an impedance mismatch condition (block  916 ). If the RSSI measurement(s) do not exceed the mismatch threshold at block  916 , the processing logic can transition to the normal radio operation at block  906 . If the RSSI measurement(s) exceed the mismatch threshold at block  918 , the processing logic can transition from an RSSI mode to a reflectometry mode in which the processing logic performs a channel sweep using the first radio (block  918 ). The processing device receives RSSI measurement(s) from the second radio (block  920 ). The processing logic determines whether additional channels still need to be swept (block  922 ). If more channels need to be swept, the processing logic returns to block  918 . If the channel sweep is done on the last channel at block  922 , the processing logic computes a reflection distance of the impedance mismatch condition (block  924 ), and the processing logic sends the impedance mismatch condition and the reflection distance back to the device  901  at the NOC (block  926 ); and the method  900  ends. 
       FIG.  10    is a flow diagram of a method  1000  of determining an impedance mismatch condition according to one embodiment. The method  1000  may be performed by processing logic that comprises hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, firmware, or a combination thereof. In one embodiment, the method  1000  may be performed by any of the processing device devices described herein and illustrated with respect to  FIGS.  1 - 8   . 
     Referring back to  FIG.  10   , the method  1000  begins by the processing logic receiving a first message from a second device (block  1002 ). The first message comprises a request to check for an impedance mismatch condition caused by i) a first antenna being disconnected from a first RF port of the wireless device or ii) damage to an RF cable coupled between the first antenna and the first RF port. The processing logic couples a first radio to a first port of a coupler of the wireless device (block  1004 ). The processing logic couples a second radio to a second port of the coupler (block  1006 ). The processing logic couples a first RF port to a third port of the coupler (block  1008 ). The processing logic decouples a second RF port from a fourth port of the coupler (block  1010 ). The processing logic causes the first radio to send a first signal via the first terminal (block  1012 ). The processing logic causes the second radio to measure a first RSSI value of a first reflected signal at the second terminal (block  1014 ). The processing logic receives the first RSSI value from the second radio (block  1016 ) and determines that the first RSSI value exceeds a threshold (block  1018 ). The threshold represents an impedance mismatch condition at or beyond the first RF port. The processing logic sends a response to the second device (block  1020 ), the response including an indication of the impedance mismatch condition; and the method  1000  ends. 
     In a further embodiment, the first message includes a scheduled time for the check. The processing logic sets the first radio to operate in a TX mode and the second radio to operate in a receive (RX) mode responsive to the scheduled time. The processing logic sets the switching circuitry to operate in a coupler mode in which the switching circuitry is to i) couple the first radio to the first port, ii) couple the second radio to the second port, iii) couple the first RF port to the third port, and iv) decouple the second antenna from the fourth port. The processing logic causes the first radio to send the first signal and causes the second radio to measure the first RSSI value while the switching circuitry is set to operate in the coupler mode. In another embodiment, the processing logic sets the switching circuitry to operate in a normal radio mode. In the normal radio mode, the switching circuitry decouples the first radio from the first port, decouples the second radio from the second port, decouples the first RF port from the third port, and couples the first radio to the first RF port and the second radio to the second RF port. This bypasses the coupler. 
     In another embodiment, the processing logic causes the first radio to send a set of signals in a channel sweep. Each signal in the set of signals has a different frequency in the channel. The processing logic causes the second radio to measure a set of RSSI values, each of the set of RSSI values corresponding to a reflected signal associated with each of the set of signals. The processing logic determines a reflection distance to a location where the impedance mismatch condition occurs using the set of RSSI values. The processing logic sends the reflection distance to the second device. 
     In another embodiment, the processing logic causes the second radio to send a second signal via the second terminal. The processing logic causes the first radio to measure a second RSSI value of a second reflected signal at the first terminal. The processing logic receives the second RSSI value from the first radio. The processing logic determines that the second RSSI value does not exceed the threshold. The processing logic can send the indication responsive to the first RSSI value exceeding the threshold and the second RSSI value not exceeding the threshold. In another embodiment, the processing logic determines that the second RSSI value does exceed the threshold. The processing logic can send the indication responsive to the first RSSI value and the second RSSI value exceeding the threshold. 
     In another embodiment, the coupling of the first radio to the first port, the coupling of second radio to the second port, and the coupling of first RF port to the third port, and the decoupling the second RF port from the fourth port are performed during a first time period. During a second time period that is different than the first period, the processing logic: couples a third radio to the first port; couples a fourth radio to the second port; couples a third RF port to the third port; and decouples a fourth RF port from the fourth port. The processing logic causes the third radio to send a second signal and causes the fourth radio to measure a second RSSI value of a second reflected signal at the second terminal. The processing logic receives the second RSSI value from the fourth radio and determines that the second RSSI value exceeds the threshold. The processing logic sends a second indication of a second impedance mismatch condition to the second device responsive to the second RSSI value exceeding the threshold. The second impedance mismatch condition can be caused by i) a third antenna being disconnected from the third RF port during the second time period or ii) damage to a second RF cable that is coupled between the third antenna and the third RF port during the second time period. It should be noted that a second antenna is connected to the second RF port during the first time period and a fourth antenna is connected to the fourth RF port during the second time period. 
     In another embodiment, the processing logic causes the first radio to send a frame that reserves a channel for a duration of time. The processing logic causes the first radio to send the first signal as a data frame after sending the frame. The data frame includes a destination address set to an address of the wireless device itself. The processing logic causes the second radio measure the first RSSI value during the duration of time. 
     In one embodiment, to perform an accurate RSSI measurement, the wireless device can transmit a clear to send to self (CTS-to-Self) frame to reserve a wireless medium for up to a duration of time (e.g., 32 milliseconds) that the RSSI measurement is taking place. After the CTS-to-Self frame, the wireless device sends a data frame (e.g., 1500-byte data frame) with a destination address set as itself and uses the data frame for the impedance detection, such as illustrated in  FIG.  11   . 
       FIG.  11    is a timing diagram  1000  illustrating a wireless device transmitting a first frame  1102  to reserve a medium (e.g., a channel) and a data frame for impedance detection according to one embodiment. The processing device causes the first radio to send a first frame  1102  (e.g., CTS-to-self) that reserves a wireless medium for a duration of time  1104 . The processing device sends a second frame  1106  (e.g., data frame) with a destination address set to an address of the wireless device. The processing device causes the second radio to measure a RSSI value during the duration of time  1104  (also referred to as the detection period  1104 ). 
     In one embodiment, the first frame  1102  can be sent after an idle timeout period  1108  that follows normal traffic  1110  during a service period  1112 . The idle timeout period  1108  occurs during a silent period  1114 . After the detection period  1104 , normal traffic  1116  can resume during a service period  1118 . 
     The embodiments described herein include an RSSI based algorithm to detect RF port impedance. The embodiments described herein can include a channelized frequency domain reflectometry based algorithm to detect a physical distance to impedance mismatch. The embodiments described herein can include a dedicated RSSI based impedance detection circuit for concurrent, co-channel radios. The embodiments described herein can include a multi-mode impedance detection circuit with a RSSI mode and a frequency domain reflectometry mode. The embodiments described herein can include an airtime reservation during RSSI detection. Alternatively, any combination of these features can be used together. 
       FIG.  12    is a block diagram of an electronic device  1200  that can be configured to detect an impedance mismatch condition and a physical distance to a location where the impedance mismatch condition occurs as described herein according to one embodiment. The electronic device  1200  may correspond to the electronic devices described above with respect to  FIGS.  1 - 11   . In one embodiment, the electronic device  1200  is the wireless devices described herein and includes an impedance detection circuit  1201 . The impedance detection circuit  1201  can be the impedance detection circuit  102  of  FIGS.  1 A,  1 B,  3   , the impedance detection circuit  202  of  FIG.  2   , or the impedance detection circuit  702  of  FIG.  7 ,  8   . Alternatively, the electronic device  1200  is coupled to the impedance detection circuit  1201 . The impedance detection circuit  1201  can be the impedance detection circuit  102  of  FIGS.  1 A,  1 B,  3   , the impedance detection circuit  202  of  FIG.  2   , or the impedance detection circuit  702  of  FIG.  7 ,  8   . Alternatively, the electronic device  1200  may be other electronic devices, as described herein. 
     The electronic device  1200  includes one or more processor(s)  1230 , such as one or more CPUs, microcontrollers, field programmable gate arrays, or other types of processors. The electronic device  1200  also includes system memory  1206 , which may correspond to any combination of volatile and/or non-volatile storage mechanisms. The system memory  1206  stores information that provides operating system component  1208 , various program modules  1210 , program data  1212 , and/or other components. In one embodiment, the system memory  1206  stores instructions of methods to control operation of the electronic device  1200 . The electronic device  1200  performs functions by using the processor(s)  1230  to execute instructions provided by the system memory  1206 . In one embodiment, the program modules  1210  may include RSSI based impedance detection logic  1203  and RSSI based reflectometry logic  1205  that may perform some or all of the operations described herein, such as the method  900 , the method  1000 , or any combination thereof. The RSSI based impedance detection logic  1203  may perform some or all of the operations described herein to detect an impedance mismatch condition. The RSSI based reflectometry logic  1205  may perform some or all of the operations described herein to determine a physical distance to a location where the impedance mismatch condition occurs. 
     The electronic device  1200  also includes a data storage device  1214  that may be composed of one or more types of removable storage and/or one or more types of non-removable storage. The data storage device  1214  includes a computer-readable storage medium  1216  on which is stored one or more sets of instructions embodying any of the methodologies or functions described herein. Instructions for the program modules  1210  (e.g., RSSI based impedance detection logic  1203  and RSSI based reflectometry logic  1205 ) may reside, completely or at least partially, within the computer-readable storage medium  1216 , system memory  1206  and/or within the processor(s)  1230  during execution thereof by the electronic device  1200 , the system memory  1206  and the processor(s)  1230  also constituting computer-readable media. The electronic device  1200  may also include one or more input devices  1218  (keyboard, mouse device, specialized selection keys, etc.) and one or more output devices  1220  (displays, printers, audio output mechanisms, etc.). 
     The electronic device  1200  further includes a modem  1222  to allow the electronic device  1200  to communicate via wireless connections (e.g., such as provided by the wireless communication system) with other computing devices, such as remote computers, an item providing system, and so forth. The modem  1222  can be connected to one or more radio frequency (RF) modules  1286 . The RF modules  1286  may be a WLAN module, a WAN module, wireless personal area network (WPAN) module, Global Positioning System (GPS) module, or the like. The antenna structures (antenna(s)  1284 ,  1285 ,  1287 ) are coupled to the front-end circuitry  1290 , which is coupled to the modem  1022 . The front-end circuitry  1290  may include radio front-end circuitry, antenna switching circuitry, impedance matching circuitry, or the like. The antennas  1284  may be GPS antennas, Near-Field Communication (NFC) antennas, other WAN antennas, WLAN or PAN antennas, or the like. The modem  1222  allows the electronic device  1200  to handle both voice and non-voice communications (such as communications for text messages, multimedia messages, media downloads, web browsing, etc.) with a wireless communication system. The modem  1222  may provide network connectivity using any type of mobile network technology including, for example, Cellular Digital Packet Data (CDPD), General Packet Radio Service (GPRS), EDGE, Universal Mobile Telecommunications System (UMTS), Single-Carrier Radio Transmission Technology (1×RTT), Evaluation Data Optimized (EVDO), High-Speed Down-Link Packet Access (HSDPA), Wi-Fi®, Long Term Evolution (LTE) and LTE Advanced (sometimes generally referred to as 4G), etc. 
     The modem  1222  may generate signals and send these signals to antenna(s)  1284  of a first type (e.g., WLAN 5 GHz), antenna(s) 1285 of a second type (e.g., WLAN 2.4 GHz), and/or antenna(s)  1287  of a third type (e.g., WAN), via front-end circuitry  1290 , and RF module(s)  1286  as descried herein. Antennas  1284 ,  1285 ,  1287  may be configured to transmit in different frequency bands and/or using different wireless communication protocols. The antennas  1284 ,  1285 ,  1287  may be directional, omnidirectional, or non-directional antennas. In addition to sending data, antennas  1284 ,  1285 ,  1287  may also receive data, which is sent to appropriate RF modules connected to the antennas. One of the antennas  1284 ,  1285 ,  1287  may be any combination of the antenna structures described herein. 
     In one embodiment, the electronic device  1200  establishes a first connection using a first wireless communication protocol, and a second connection using a different wireless communication protocol. The first wireless connection and second wireless connection may be active concurrently, for example, if an electronic device is receiving a media item from another electronic device via the first connection) and transferring a file to another electronic device (e.g., via the second connection) at the same time. Alternatively, the two connections may be active concurrently during wireless communications with multiple devices. In one embodiment, the first wireless connection is associated with a first resonant mode of an antenna structure that operates at a first frequency band and the second wireless connection is associated with a second resonant mode of the antenna structure that operates at a second frequency band. In another embodiment, the first wireless connection is associated with a first antenna structure and the second wireless connection is associated with a second antenna. 
     Though a modem  1222  is shown to control transmission and reception via antenna ( 1284 ,  1285 ,  1287 ), the electronic device  1200  may alternatively include multiple modems, each of which is configured to transmit/receive data via a different antenna and/or wireless transmission protocol. 
     In the above description, numerous details are set forth. It will be apparent, however, to one of ordinary skill in the art having the benefit of this disclosure, that embodiments may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the description. 
     Some portions of the detailed description are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to convey the substance of their work most effectively to others skilled in the art. An algorithm is used herein, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the above discussion, it is appreciated that throughout the description, discussions utilizing terms such as “inducing,” “parasitically inducing,” “radiating,” “detecting,” determining,” “generating,” “communicating,” “receiving,” “disabling,” or the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (e.g., electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Embodiments also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, Read-Only Memories (ROMs), compact disc ROMs (CD-ROMs) and magnetic-optical disks, Random Access Memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct a more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present embodiments are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the present embodiments as described herein. It should also be noted that the terms “when” or the phrase “in response to,” as used herein, should be understood to indicate that there may be intervening time, intervening events, or both before the identified operation is performed. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the present embodiments should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.