Patent Publication Number: US-2023145768-A1

Title: Shifting a frequency band of an interference signal out of a pass band of a signal path

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/981,205 entitled “SHIFTING A FREQUENCY BAND OF AN INTERFERENCE SIGNAL OUT OF A PASS BAND OF A SIGNAL PATH” filed on Feb. 25, 2020, which is incorporated herein by reference in its entirety. 
    
    
     SUMMARY 
       FIG.  1    is a block diagram of a distributed antenna system (DAS)  10 , which is configured for coupling to one or more base stations  12 , and which includes one or more remote antenna units  14 . Typically, each base station  12  corresponds to, and is controlled by, a particular cellular operator such as T-Mobile®, Verizon®, or ATT®, and is configured to receive uplink signals, and to transmit downlink signals, via one or more of the remote antenna units  14 . For example, each base station  12  can be configured to receive uplink signals and to transmit downlink signals via all of the remote antenna units  14  such that each cellular operator is afforded the full signal-coverage area that the DAS  10  is configured to serve. 
     When the DAS  10  operates in a time-division duplexing (TDD) mode, a remote antenna unit  14  receives and conditions an uplink signal with an uplink signal path while a downlink signal path is idle (i.e., is not conditioning or transmitting a downlink signal), and conditions and transmits a downlink signal with the downlink signal path while the uplink signal path is idle (i.e., is not receiving or conditioning an uplink signal). Conditioning a signal can include, for example, one or more of upshifting the signal in a frequency domain, downshifting the signal in a frequency domain, filtering the signal, amplifying the signal, converting the signal from a digital domain to an analog domain, and converting the signal from an analog domain to a digital domain. 
     But even while the downlink signal path is idle, it may generate an interference signal, such as a noise signal, that the uplink signal path receives while also receiving an uplink signal. For example, even while the downlink signal path is idle, one or more components that form the downlink signal path may be active, such one or more components including, for example, one or more of a local oscillator, a mixer, a power amplifier, a filter, a digital-to-analog converter (DAC), and an analog-to-digital converter (ADC). Reasons that one or more components of the downlink signal path may remain active even while the downlink signal path is idle include that the time to transition a component from an inactive state to a full active state is too long to warrant deactivating the component between periods during which the downlink signal path is active and that power-supply current spikes caused by activating and deactivating one or more components can cause potentially problematic voltage fluctuations and electromagnetic interference (EMI). 
     Consequently, even though the downlink signal path is not conditioning a downlink signal for transmission while the downlink signal path is idle, the active one or more components may cause the downlink signal path to generate an interference signal that may interfere with the ability of the uplink signal path to receive and condition an uplink signal. For example, one may characterize or model such interference as noise superimposed on the received uplink signal at one or more points along the uplink signal path. 
     More generally, any transmit signal Tx, whether a downlink signal, another type of transmit information-carrying signal, or a transmit interference signal, can interfere with any receive signal Rx, whether an uplink signal or another type of receive information-carrying signal. But for clarity, hereinafter only downlink signals and downlink signal paths, and uplink signals and uplink signal paths, are described, it being understood that the below-described techniques and embodiments can be applied to systems including transmit signals and transmit paths other than downlink signals and downlink signal paths and including receive signals and receive paths other than uplink signals and uplink signal paths. 
     A technique for reducing or eliminating, from an uplink signal, interference generated by an idle downlink signal path, is to isolate, electrically, the uplink signal path from the downlink signal path. 
     But unfortunately, because the interference generated by an idle downlink signal path can have a significant level of power, circuitry configured to provide a suitable level of electrical isolation between an uplink signal path and the downlink signal path can be difficult to design, and can be too costly or too large for one or more applications. 
     Therefore, a need has arisen for a technique for reducing, to an acceptable level, interference in an uplink signal path caused by an idle downlink signal path and for configuring circuits and systems to operate according to such a technique. 
     In an embodiment, a communication circuit configured to implement such a technique includes a signal path coupled to a frequency-shifting circuit. The signal path is configured to carry, during a first period, an information signal having a frequency. The frequency-shifting circuit is configured to receive a control signal, to shift the frequency of the information signal by a first frequency in response to the control signal having a first control value, and to shift a frequency of an interference signal on the signal path during a second period by a second frequency in response to the control signal having a second control value. For example, a frequency-shifting circuit coupled to a downlink signal path is configured, during a downlink-active period, to shift a frequency of a downlink signal by a first frequency, and is configured, during a downlink-idle period, to shift a frequency of any other signal, such as an interference signal, on the downlink signal path by a second frequency. 
     Furthermore, during a downlink-active period, while the downlink signal path is conditioning and transmitting a downlink signal and while the uplink signal path is idle, coupling of the downlink signal to the uplink signal path can cause a closed loop, formed by the downlink and uplink signal paths, to oscillate. For example, such oscillation can occur due to less-than-sufficient electrical isolation between the downlink and uplink signal paths. 
     Therefore, a need has arisen for a technique for preventing, in a closed loop formed by a downlink signal path and an uplink signal path, oscillation caused by leakage of a downlink signal into an uplink signal path. 
     The above-described embodiment also can be configured to implement such an oscillation-prevention technique. For example, a frequency-shifting circuit coupled to an uplink signal path is configured, during an uplink-active period, to shift a frequency of an uplink signal by a first frequency, and, during an uplink-idle period, is configured to shift a frequency of any other signal, such as a leakage component of a downlink signal, on the uplink signal path by a second frequency. 
    
    
     
       DRAWINGS 
         FIG.  1    is a diagram of a distributed antenna system (DAS), base stations coupled to the DAS, and user equipment configured for communicating with the DAS and with the base stations via the DAS. 
         FIG.  2    is a diagram of a communication circuit of a remote antenna unit of  FIG.  1   , according to a first implementation. 
         FIG.  3    is a diagram of a communication circuit of a remote antenna unit of  FIG.  1   , according to a second implementation. 
         FIG.  4    is a diagram of a communication circuit of a remote antenna unit of  FIG.  1   , where the communication circuit is configured to implement frequency-shifting and passive isolation, according to an embodiment. 
         FIG.  5    is a plot of the frequency bands respectively occupied by a downlink signal generated by a downlink signal path during a downlink-active period, and by an interference signal generated by the downlink signal path during a downlink-idle period, according to an embodiment. 
         FIG.  6    is a plot of the frequency bands respectively occupied by an uplink and interference signals on an uplink signal path during an uplink-active period, according to an embodiment. 
         FIG.  7    is a diagram of an isolation circuit of  FIG.  4   , according to an embodiment. 
         FIG.  8    is a diagram of an isolation circuit of  FIG.  4   , according to another embodiment. 
         FIG.  9    is a diagram of a communication circuit of a remote antenna unit of  FIG.  1   , where the communication circuit is configured to implement frequency-shifting, passive, and active isolation, according to an embodiment. 
         FIG.  10    is a diagram of a communication circuit of a remote antenna unit of  FIG.  1   , where the communication circuit is configured to implement frequency-shifting isolation in the optical domain, according to an embodiment. 
         FIG.  11    is a diagram of closed loop in a communication circuit of a remote antenna unit of  FIG.  1   , where the closed loop is formed by a downlink signal path and an uplink signal path of the communication circuit, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     “Approximately,” “substantially,” and similar words, as used herein, indicate that a given quantity b can be within a range b±10% of b, or b±1 if |10% of b|&lt;1. “Approximately,” “substantially,” and similar words, as used herein, also indicate that a range |b−c| can be from |b−0.10|(c−b)| to c+0.10|(c−b)∥. Regarding the planarity of a surface or other region, “approximately,” “substantially,” and similar words, as used herein, indicate that a difference in thickness between a highest point and a lowest point of the surface/region does not exceed 0.20 millimeters (mm). 
     As described above,  FIG.  1    is a diagram of a DAS  10 , which is coupled to one or more base stations  12  and which includes one or more remote antenna units  14 . 
     In an embodiment, one or more of the remote antenna units  14  can be configured to reduce, in one or more received signals, interference generated by one or more respective downlink signal paths by frequency shifting the interference out of the frequency band(s) of the received signal(s). 
     In another embodiment, each of one or more communication circuits can be configured to increase the stability of, and, therefore, to prevent oscillations in, a respective closed loop formed by a downlink signal path and an uplink signal path of the communication circuit, where each communication circuit is partially or fully disposed on a respective one of the remote antenna units  14 . 
     The remote antenna units  14  of the DAS  10  typically are distributed within a structure (e.g., an office building, warehouse, mall, sports complex) or within an outdoor area (e.g., a stadium, a downtown, an outdoor event venue, a park, a beach) to provide wireless-communication coverage so that people can use their wireless devices (e.g., smartphones, tablets, pads, laptops) while within the structure or outdoor area. Examples of the types of wireless coverage provided by the DAS  10  can include Wi-Fi®, cellular service, and data service over one of the many available long-term-evolution (LTE) frequency bands (e.g., B1, B3, B7, B25, and B66). And the frequency range over which the DAS  10  can be configured to operate is, for example, approximately 600 MHz-71 GHz. For example, the DAS  10  can be configured to operate in the 5 th -Generation New Radio (5GNR) frequency band of approximately 3.4 GHz-3.8 GHz. 
     In addition to the remote antenna units  14 , the DAS  10  is coupled to, or includes, one or more master units  18 , which are communicatively coupled to the remote antenna units. Further in an embodiment, the DAS  10  includes a digital DAS, in which DAS traffic is distributed between the master unit(s)  18  and the remote antenna units  14  in digital form. In other embodiments, the DAS  10  is implemented, at least in part, as an analog DAS, in which DAS traffic is distributed at least part of the way between the master unit(s)  18  and the remote antenna units  14  in analog form. 
     Each master unit  18  also is communicatively coupled to the one or more base stations  12 , each of which can be co-located with the respective master unit  18  to which it is coupled (for example, where a base station  12  is dedicated to providing base-station capacity to only the DAS  10 ). Also, each of one or more of the base stations  12  can be located remotely from the respective master unit  18  to which it is coupled (for example, where the base station  12  is a macro base station providing base-station capacity to a macro cell in addition to providing capacity to the DAS  10 ). In this latter case, a master unit  18  can be coupled to a donor antenna (not shown in  FIG.  1   ) to communicate wirelessly with the remotely located base station  12 . Furthermore, one or more of the one or more base stations  12  each can be coupled to the master unit  18  using a respective network of attenuators, combiners, splitters, amplifiers, filters, cross-connects, etc., which respective network is referred to as a respective point-of-interface (POI) circuit  20 . The base stations  12  use the POI circuits  20  to couple to the master unit  18  so that, in the downstream or downlink, the desired set of RF carriers output by the base station(s)  12  can be extracted, combined, and routed to the appropriate master unit  18 , and so that, in the upstream or uplink, the desired set of carriers output by the master unit  18  can be extracted, combined, and routed to the appropriate interface of each base station  12 . 
     The base station(s)  12  each can be implemented as a respective traditional, monolithic base station. Also, the base station(s)  12  each can be implemented using a distributed base-station architecture in which a baseband unit (BBU) (not shown in  FIG.  1   ) is coupled to one or more remote radio heads (RRHs) (not shown in  FIG.  1   ), where the front haul between the BBU and the RRH uses streams of digital IQ samples. Examples of such an approach are described in the Common Public Radio Interface (CPRI) and Open Base Station Architecture Initiative (OBSAI) families of specifications, which are incorporated by reference herein. 
     The master unit(s)  18  each can be configured to use wideband interfaces or narrowband interfaces to the base station(s)  12 . Also, the master unit(s)  18  each can be configured to interface with the base station(s)  12  using analog radio-frequency (RF) interfaces or digital interfaces (for example, using a CPRI or OBSAI digital IQ interfaces). 
     Traditionally, each master unit  18  interfaces with each base station  12  using the analog radio-frequency signals that each base station  12  communicates to and from user equipment (e.g., smartphone, tablet, iPad, laptop)  22  using a suitable air-interface standard. The DAS  10  operates as a distributed repeater for such radio-frequency signals. RF signals transmitted from each base station  12  (also referred to herein as “downlink RF signals”) are received at one or more master units  18 . Each master unit  18  uses the downlink RF signals to generate a downlink transport signal distributed to one or more remote antenna units  14 . Each such remote antenna unit  14  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 at least one antenna array  16  included in, or otherwise coupled to, that remote antenna unit  14 . Each antenna array  16  can include one or more antennas. 
     A similar process is performed in the uplink direction. RF signals transmitted from the user equipment  22  (also referred to herein as “uplink RF signals”) are received at one or more remote antenna units  14 . Each remote antenna unit  14  uses the uplink RF signals to generate an uplink transport signal transmitted from the remote antenna unit  14  to a master unit  18 . Each master unit  18  receives uplink transport signals transmitted from one or more remote antenna units  14  coupled to it. The master unit  18  combines data or signals communicated via the uplink transport signals received at the master unit  18  and reconstructs a version of the uplink RF signals received at the remote antenna units  14 . The master unit  18  communicates the reconstructed uplink RF signals to one or more base stations  12 . In this way, the signal and communication coverage of the base station(s)  12  can be expanded using the DAS  10 . 
     One or more intermediate units  24  (some of which are also referred to here as “expansion units”) can be placed between the master unit(s)  18  and one or more of the remote antenna units  14 . The one or more intermediate units  24  can be used, for example, to increase the number of remote antenna units  14  that a single master unit  18  can feed, to increase the master-unit-to-remote-unit distance, and/or to reduce the amount of cabling needed to couple a master unit  18  to its associated remote antenna units  14 . 
     As noted above, the DAS  10  can be implemented as a “digital” DAS. In a “digital” DAS, signals received from and provided to the base station(s)  12  and user equipment  22  are used to produce digital in-phase (I) and quadrature (Q) samples, which are communicated between the master unit(s)  18  and the remote antenna units  14 . It is noted that this digital IQ representation of the original signals received from the base station(s)  12  and from the user equipment  22  still maintains the original modulation (that is, the change in the amplitude, phase, or frequency of a carrier) used to convey telephony or data information according to the cellular air-interface protocol used for wirelessly communicating between the base station(s)  12  and the user equipment  22 . Examples of such cellular air-interface protocols include, for example, 5GNR, the Global System for Mobile Communication (GSM), Universal Mobile Telecommunications System (UMTS), and Long-Term Evolution (LTE) air-interface protocols. Also, each stream of digital IQ samples represents or includes a portion of frequency spectrum. For example, the digital IQ samples can represent a single radio access network carrier (for example, a UMTS or LTE carrier of 5 MHz) onto which voice or data information has been modulated using a UMTS or LTE air interface. However, it is to be understood that each such stream can also represent multiple carriers (for example, in a band of frequency spectrum or a sub-band of a given band of frequency spectrum). 
     Furthermore, one or more of the master units  18  can be configured to interface with one or more base stations  12  using an analog RF interface (for example, either a traditional monolithic base station  12  or via the analog RF interface of an RRH). As described above, the base station(s)  12  can be coupled to the master units  18  using a network of attenuators, combiners, splitters, amplifiers, filters, cross-connects, etc., (sometimes referred to collectively as a “point-of-interface” or “POI” circuit  20 ). POI circuits  20  are used so that, in the downstream, the desired set of RF carriers output by the base station(s)  12  can be extracted, combined, and routed to the appropriate master unit  18 , and so that, in the upstream, the desired set of carriers output by the master unit  18  can be extracted, combined, and routed to the appropriate interface of each base station  12 . 
     Each master unit  18  can produce digital IQ samples from an analog signal received at radio-frequency (RF) by down-converting the received signal to an intermediate frequency (IF) or to baseband, digitizing the down-converted signal to produce real digital samples, and digitally down-converting the real digital samples to produce digital in-phase (I) and quadrature (Q) samples. These digital IQ samples also can 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 radio-frequency spectrum output by one or more base stations  12 . Each portion of radio-frequency spectrum can include, for example, a band of spectrum, a sub-band of a given band of spectrum, or an individual carrier. 
     Likewise, in the upstream, each master unit  18  can produce an upstream analog signal from one or more streams of digital IQ samples received from one or more remote antenna units  14  by digitally combining streams of digital IQ samples that represent the same carriers or frequency bands or sub-bands (for example, by digitally summing such digital IQ samples), digitally up-converting the combined digital IQ samples to produce real digital samples, performing a digital-to-analog process on the real samples 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. The analog signal can be produced in other ways (for example, where the digital IQ samples are provided to a quadrature digital-to-analog converter that directly produces the analog IF or baseband signal). 
     One or more of the master units  18  can be configured to interface with one or more base stations  12  using a digital interface (in addition to, or instead of) interfacing with one or more base stations  12  via an analog RF interface. For example, the master unit  18  can be configured to interact directly with one or more BBUs using the digital IQ interface used for communicating between the BBUs and an RRHs (for example, using the CPRI serial digital IQ interface). 
     In the downstream, each master unit  18  terminates one or more downstream 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 downstream streams of digital IQ samples compatible with the remote antenna units  14  used in the DAS  10 . In the upstream, each master unit  18  receives upstream streams of digital IQ samples from one or more remote antenna units  14 , digitally combining streams of digital IQ samples that represent the same carriers or frequency bands or sub-bands (for example, by digitally summing such digital IQ samples), and, if necessary, converts (by re-sampling, synchronizing, combining, separating, gain adjusting, etc.) them into upstream streams of digital IQ samples compatible with the one or more BBUs that are coupled to that master unit  18 . 
     Each master unit  18  also can be implemented in other ways. 
     In the downstream, each remote antenna unit  14  receives streams of digital IQ samples from one or more master units  18 , where each stream of digital IQ samples represents a portion of wireless radio-frequency spectrum output by one or more base stations  12 . 
     Each remote antenna unit  14  is communicatively coupled to one or more master units  18  using, e.g., one or more optical fibers or cables, one or more ETHERNET-compatible cables  26  (for example, one or more CAT-6A cables), or any other suitable coupling medium. For example, in this embodiment, each remote antenna unit  14  can be directly connected to a master unit  18  via a single ETHERNET-compatible cable  26  or indirectly via multiple ETHERNET-compatible cables  26  such as where a first ETHERNET cable connects the remote antenna unit  14  to a patch panel or expansion/intermediate unit  24  and a second optical fiber cable  28  connects the patch panel or expansion/intermediate unit  24  to the master unit  18 . Each remote antenna unit  14  can be coupled to one or more master units  18  in other ways. Also, the master unit  18  or expansion/intermediate unit(s)  24  can include one or more instances of power-supply equipment (PSE) configured to provide power to the remote antenna units  14 . 
     Still referring to  FIG.  1   , alternate embodiments of the DAS  10 , base stations  12 , and user equipment  22  are contemplated. For example, embodiments described below in conjunction with  FIGS.  2 - 11    may be applicable to the DAS  10 , base stations  12 , and/or user equipment  22  of  FIG.  1   . 
       FIG.  2    is a diagram of a communication circuit  30 , which can be partially or fully disposed on a remote antenna unit  14  of  FIG.  1   . Where the communication circuit  30  is only partially disposed on a remote antenna unit  14 , the remaining portion of the communication circuit  30  can be disposed on one or more of an expansion unit  14 , a master unit  18 , and an intermediate unit  24 . 
     The communication circuit  30  includes a downlink signal path (also called a transmit signal path)  32 , an uplink signal path (also called a receive signal path)  34 , an isolation circuit  36  common to the downlink and uplink signal paths, and an antenna  38  also common to the uplink and downlink signal paths. The communication circuit  30  also can include other components that are omitted from  FIG.  2   . 
     In addition to the isolation circuit  36  and the antenna  38 , the downlink signal path  32  includes a downlink mixer  40 , a downlink bandpass filter  42 , and a downlink amplifier  44 . Also, a downlink local oscillator  46  generates and provides a downlink oscillator signal to the downlink mixer  40 . The downlink local oscillator  46  can be considered part of or considered coupled to the downlink signal path  32 . The downlink signal path  32  can include other downlink components (not shown in  FIG.  2   ) preceding (e.g., downstream of) the downlink mixer  40 , where such other components can include a digital-to-analog converter (DAC) and can be disposed on the remote antenna unit  14  ( FIG.  1   ) or remote from the remote antenna unit  14 , such as on the master unit  18  ( FIG.  1   ) or an intermediate unit  24  ( FIG.  1   ). 
     In addition to the isolation circuit  36  and the antenna  38 , the uplink signal path  34  includes an uplink low-noise amplifier  48  and an uplink mixer  50 . Also, an uplink local oscillator  52  generates and provides an uplink oscillator signal to the uplink mixer  50 . The uplink mixer  50  and the uplink local oscillator  52  can be considered part of or considered coupled to the uplink signal path  34 . The uplink signal path  34  can include other uplink components (not shown in  FIG.  2   ) following (e.g., upstream of) the uplink mixer  50 , where such other components can include a bandpass filter and an analog-to-digital (ADC) and can be disposed on the remote antenna unit  14  ( FIG.  1   ) or remote from the remote antenna unit  14 , such as on the master unit  18  ( FIG.  1   ) or an intermediate unit  24  ( FIG.  1   ). 
     Where the communication circuit  30  is configured to operate in a TDD mode, both the downlink signal path  32  and the uplink signal path  34  are configured to operate in approximately the same frequency band. For example, where the communication circuit  30  is configured for 5GNR applications, the downlink and uplink signal paths  32  and  34  can be configured to operate in the same frequency band within an approximate range of 10 GHz-60 GHz. 
     Consequently, noise, or other interference, that the downlink signal path  32  generates while the uplink signal path  34  is actively receiving and conditioning an uplink information signal can be superimposed on the uplink signal, and, therefore, can interfere with the ability of the uplink signal path  34  to receive and condition the uplink signal and/or with the ability of a base station  12  ( FIG.  1   ) to recover information carried by the uplink signal. 
     In more detail, during a downlink active mode, the downlink mixer  40  receives, from the master unit  18  ( FIG.  1   ), a baseband or intermediate-frequency downlink signal (hereinafter intermediate downlink signal) having a center frequency f DL , and mixes the intermediate downlink signal with a local-oscillator signal generated by the downlink local oscillator  46  and having a frequency f LO_DL , which is typically higher than f DL . A result of the signal mixing is that the intermediate downlink signal is upconverted, or upshifted, to a center frequency f DL +f LO_DL . Alternatively, a result of the signal mixing may be that the intermediate downlink signal is downconverted, or downshifted, to the center frequency f DL −f LO_DL . But hereinafter it is assumed, for purposes of example, that the result of the signal mixing is that the intermediate downlink signal is upconverted, or upshifted, to the center frequency f DL +f LO_DL , it being understood that the techniques, embodiments, and principles described herein are also applicable if the intermediate signal is downconverted, or downshifted, to the center frequency f DL −f LO_DL . 
     The downlink bandpass filter  42  filters the frequency-upshifted downlink signal from the downlink mixer  40  to remove any out-of-band frequencies. 
     The downlink amplifier  44  amplifies the filtered downlink signal from the downlink bandpass filter  42 . 
     The isolation circuit  36  passes the amplified downlink signal from the downlink amplifier  44  to the antenna  38 , and electrically isolates the uplink signal path  34  from the downlink signal path  32 . 
     And the antenna  38  radiates the amplified downlink signal to one or more of the user equipment  22  (e.g., a smartphone or other device) of  FIG.  1   . 
     During a downlink-inactive, or downlink-idle, mode or period, the downlink mixer  40  receives no baseband or intermediate downlink signal from the master unit  18  ( FIG.  1   ). But the downlink local oscillator  46  is still active, and, therefore, still generates the local-oscillator signal having the frequency f LO_DL . Therefore, the downlink mixer  40  mixes noise at its input node (the node at which the downlink mixer  40  receives the baseband or intermediate downlink signal during a downlink-active mode) and any noise that the downlink mixer  40  itself generates, with the local-oscillator signal, and generates a noise, or interference, signal that is upshifted to a frequency band having a center frequency f interference +f LO_DL  (f interference  is the center frequency of the frequency band of the interference before upshifting). At the input node of the downlink mixer  40 , noise within the baseband and intermediate downlink band, respectively, has a significantly higher magnitude/power than noise of the same bandwidth but outside the intermediate downlink band. Reasons that the downlink local oscillator  46  remains active during a downlink-idle period include that the duration of a downlink-idle period may be too short for deactivating the downlink local oscillator  46  due, for example, to the startup time for the downlink local oscillator  46 , the step decrease in the supply current, and the corresponding step increase in the supply voltage, upon deactivating the downlink local oscillator  46 , and the step increase in the supply current, and the corresponding step decrease in the supply voltage, upon activating the downlink local oscillator  46 . 
     The downlink bandpass filter  42  filters the upshifted interference signal from the downlink mixer  40  to remove any out-of-band frequencies (e.g., frequencies not in the frequency band of the downlink signal that the downlink signal path  32  conditions and transmits during a downlink active mode). If the downlink bandpass filter  42  is passive, then it is effectively operational at all times. If the downlink bandpass filter  42  is active, then it may remain active during a downlink-idle period for one or more of the same reasons that the local oscillator  46  remains active during a downlink-idle period. 
     The downlink amplifier  44  amplifies the filtered interference signal from the downlink bandpass filter  42 . The downlink amplifier  44  remains active during a downlink-idle period for one or more of the same reasons that the local oscillator  46  remains active during a downlink-idle period. 
     The isolation circuit  36  passes the amplified interference signal from the downlink amplifier  44  to the antenna  38  and electrically isolates the uplink signal path  34  from the downlink signal path  32 . 
     The antenna  38  radiates the amplified interference signal. 
     While the downlink signal path  32  is idle, the uplink signal path  34  may be active, and, therefore, may receive an uplink signal from the user equipment  22  ( FIG.  1   ) and condition the received uplink signal. 
     But, because the amplified interference signal can be relatively strong, a significant (in terms of power) component of the amplified interference signal may “leak” through the isolation circuit  36  onto the uplink signal path  34 . 
     Furthermore, one or more environmental objects (not shown in  FIG.  2   ) may redirect a component of the radiated interference signal back to the antenna  38 , which receives the redirected component along with the uplink signal. 
     Therefore, the uplink signal path  34  receives a signal that is the superposition of one or more components of the interference signal on the uplink signal, which is the signal of interest. 
     Because the superimposed interference is in approximately the same frequency band as the uplink signal, the one or more filters of the uplink signal path  34  remove little or none of the superimposed interference from the uplink signal. 
     If the superimposed interference is powerful enough, it can “corrupt” the received uplink signal by rendering the uplink signal path  34  unable to accurately condition the uplink signal or rendering circuitry proceeding the uplink mixer  50  unable to recover information that the received uplink signal carries. 
     Furthermore, the uplink signal path  34  can experience the same problem, receiving an uplink signal corrupted by a superimposed interference, even when a downlink signal path of a different communication circuit generates the interference signal (the interference signal would need to be in approximately the same frequency band as the uplink signal, and the antenna  38  would need to receive a component of the interference signal). The different communication circuit can be located on the same remote antenna unit  14  ( FIG.  1   ) as the communication circuit  30  or can be located on a different remote antenna unit  14  than the communication circuit  30 . 
     A potential solution for reducing or eliminating, in the uplink signal path  34 , interference generated by the downlink signal path  32  of the same communication circuit  30 , is to set, to a suitable level, the isolation that the isolation circuit  36  provides. 
     But it can be difficult to design an isolation circuit  36  that provides a suitable level of isolation for a suitable cost and a suitable size. 
     Still referring to  FIG.  2   , consider the following example. 
     Assume that the downlink signal path  32  is configured to transmit a downlink signal with a composite power of 43 dBm with a transmit noise floor of −30 dBm/MHz. Also, assume that in the absence of the isolation circuit  36 , this transmit noise floor is present while the downlink signal path  32  is actively conditioning and transmitting a downlink signal, and is also present while the downlink signal path  32  is idle and the uplink signal path  34  is actively receiving and conditioning an uplink signal. Further assume that the receive noise figure (NF) for the uplink signal path  34  is 4 dB, and that the thermal noise floor at 25° C. at the input of the uplink signal path (e.g., at the input node of the uplink amplifier  48 ) is −174 dBm/Hz, which is, equivalently, −114 dBm/MHz. Therefore, the noise floor at the input of the uplink signal path  34  can be determined to be −114 dBm/MHz+4 dB=−110 dBm/MHz. 
     If an application calls for the noise floor of the uplink signal path  34  to be degraded no more than 0.5 dB, then the application calls for the overlaying transmit noise floor to be at least 10 dB below the receive noise floor. Consequently, the application calls for the level of isolation between the downlink signal path  32  and the uplink signal path  34  to be −30 dBm/MHz−(−110 dBm/MHz−10 dB)=90 dB. But a single isolation circuit  36 , or a single isolation technique, may be unable to provide a suitably high level of isolation between the downlink signal path  32  and the uplink signal path  34 . To achieve such a high level of isolation, one could design the communication circuit  30  to incorporate, or otherwise to implement, multiple isolator circuits or multiple other isolation techniques. 
     For example, one such technique is to reduce the in-band gain of the downlink amplifier  44  while the downlink signal path  32  is idle by shifting the bias levels, and therefore, by shifting the operating points of one or more transistors (not shown in  FIG.  2   ) internal to the downlink amplifier  44 . Such shifting of the transistor operating points can reduce the in-band gain of the downlink amplifier  44  without deactivating the downlink amplifier  44 . 
     Although such transistor-operating-point shifting can reduce the in-band gain of the downlink amplifier  44  enough so that the combination of the reduced amplifier gain and the isolation circuit  36  provides a suitable level of isolation between the downlink signal path  32  and the uplink signal path  34 . While the downlink signal path  32  is idle, such transistor-operating-point shifting can increase one or more of the cost, size, and power consumption of the downlink amplifier  44 . For example, each of one or more of the amplifier transistors may need to be made larger (e.g., made to have a thicker gate dielectric or to occupy a larger area) to survive the repeated switching cycles that shift the transistor operating points at the commencement of each downlink-active and downlink-idle period of the downlink signal path  32 . Furthermore, the addition of the circuitry configured to perform the transistor-operating-point shifting can increase the size, cost, and power consumption of the downlink amplifier  44 . 
       FIG.  3    is a diagram of a communication circuit  60  of a remote antenna unit  14  ( FIG.  1   ), and in which like numerals refer to items common to the communication circuit  30  of  FIG.  2    and the communication circuit  60  of  FIG.  3   . 
     Referring to  FIG.  3   , instead of including an isolation circuit  36  ( FIG.  2   ) to provide electrical isolation between the downlink and uplink signal paths  32  and  34 , the communication circuit  60  includes separate downlink and uplink antennas  62  and  64  to provide electrical isolation between the downlink and uplink signal paths. 
     In more detail, the downlink and uplink antennas  62  and  64  are highly directional, the downlink antenna  62  is oriented to radiate a downlink signal in a direction away from the uplink antenna  64 , and the uplink antenna  64  is oriented to receive an uplink signal from a direction away from the downlink antenna  62 . Or, said another way, the main beams of the downlink and uplink antennas  62  and  64  are oriented in different directions. Unfortunately, the described separate-antenna technique may suffer from one or more problems. 
     For example, a technician may need to orient the antennas  62  and  64  during assembly and testing of the remote antenna unit  14  ( FIG.  1   ) on which the communication circuit  60  is disposed, or a technician may need to orient or reorient the antennas during installation of the remote antenna unit  14 . Such orientation and reorientation by a technician, particularly during installation, can be time consuming and, therefore, can increase the cost of manufacturing and installation of the remote antenna and the DAS  10  ( FIG.  1   ), which includes the remote antenna unit  14 . Furthermore, adding an extra antenna (as compared to the communication circuit  30  of  FIG.  2   ) can also add extra cost and increase the complexity of assembling, testing, and installing the remote antenna unit  14 . 
     Moreover, clutter in the environment in which the communication circuit  60  is installed may render the separate downlink and uplink antennas  62  and  64  unable to provide a suitable level of electrical isolation between the downlink and uplink signal paths  32  and  34 . For example, an object (e.g., a structural beam, light fixture, wall, furniture) proximate to the communication circuit  60  may redirect, to the uplink antenna  64 , interference radiated by the downlink antenna  62 , even if the downlink antenna radiates the interference away from the uplink antenna. 
     If one or more potential interference-redirecting objects in the installation environment are stationary, then an installation technician may be able to orient, or reorient, the antennas  62  and  64  to reduce such redirections of interference to acceptable levels. But, as described above, such orientation and reorientation by a technician during installation can be time-consuming and, therefore, can increase the cost of installing the DAS  10  ( FIG.  1   ). 
     Furthermore, in an installation environment where the clutter is not stationary, orientation and reorientation of the antennas  62  and  64  may be ineffective at reducing redirections of interference to acceptable levels. 
       FIG.  4    is a diagram of a communication circuit  70 , which can form part of the DAS  10  of  FIG.  1   , according to an embodiment. For example, part or all of the communication circuit  70  can be disposed on a respective remote antenna unit  14  ( FIG.  1   ) of the DAS  10 . Furthermore, in  FIG.  4   , items common to the communication circuits  30  and  60  of  FIGS.  2  and  3   , respectively, are labeled with the same reference numerals. 
     As described in more detail below, the communication circuit  70  provides at least a portion of the electrical isolation between a downlink signal path  72  and an uplink signal path  74  by shifting interference generated by the downlink signal path  72  out of the radio-frequency (RF) band of the uplink signal path. Therefore, even if the uplink signal path  74  receives the interference superimposed on an uplink signal, the uplink signal path  74  filters out some or all of the energy of the out-of-band interference signal. 
     In addition to the downlink signal path  72  and the uplink signal path  74 , the communication circuit  70  includes an isolation circuit  36  and an antenna  38 , both of which are common to the downlink and uplink signal paths, and includes a control circuit  76 , which is coupled to both the downlink and uplink signal paths. 
     In addition to the isolation circuit  36  and the antenna  38 , the downlink signal path  72  includes a downlink mixer  40 , a downlink bandpass filter  42 , a downlink amplifier  44 , and a downlink-signal sensor  78 . A downlink-active local oscillator  80 , which is configured to generate a first downlink oscillator signal while the downlink signal path  72  is actively conditioning and transmitting a downlink signal, can be considered to be part of, or coupled to, the downlink signal path. And an optional downlink-idle local oscillator  82 , which is configured to generate a second downlink oscillator signal while the downlink signal path  72  is idle (e.g., is neither conditioning nor transmitting a downlink signal), can be considered to be part of, or coupled to, the downlink signal path. The combination of the downlink mixer  40  and the local oscillators  80  and  82  is a downlink frequency-shifting circuit. Furthermore, the downlink signal path  72  can include other downlink components (not shown in  FIG.  4   ) preceding (e.g., downstream of) the downlink mixer  40 , where such other components can include a digital-to-analog converter (DAC) and can be disposed on the remote antenna unit  14  ( FIG.  1   ) or can be disposed remote from the remote antenna unit  14 , such as on the master unit  18  ( FIG.  1   ) or an intermediate unit  24  ( FIG.  1   ). 
     In addition to the isolation circuit  36  and the antenna  38 , the uplink signal path  74  includes an uplink amplifier  48  and an uplink mixer  50 . An uplink-active local oscillator  52 , which is configured to generate a first uplink oscillator signal while the uplink signal path  74  is actively receiving and conditioning an uplink signal, can be considered to be part of, or can be considered to be coupled to, the uplink signal path  74 . And an optional uplink-idle local oscillator  84 , which is configured to generate a second uplink oscillator signal while the uplink signal path  74  is idle (e.g., the uplink signal path  74  is neither receiving nor conditioning an uplink signal), can be considered to be part of, or can be considered to be coupled to, the uplink signal path. The combination of the uplink mixer  50  and the local oscillators  52  and  84  is an uplink frequency-shifting circuit. Furthermore, the uplink signal path  74  also can include other uplink components (not shown in  FIG.  4   ) following (e.g., upstream of) the uplink mixer  50 , where such other components can include a bandpass filter and an analog-to-digital (ADC), and can be disposed on the remote antenna unit  14  or can be disposed remote from the remote antenna unit  14 , such as on the master unit  18  ( FIG.  1   ) or on an intermediate unit  24  ( FIG.  1   ). 
     Where the communication circuit  70  is configured to operate in a TDD mode, both the downlink signal path  72  and the uplink signal path  74  are configured to operate in approximately the same intermediate-frequency (IF) band (before the input to the downlink mixer  40  and after the output of the uplink mixer  50 ) and in a same radio-frequency (RF) band (after the output of the downlink mixer  40  and before the input of the uplink mixer  50 ). For example, where the communication circuit  70  is configured for 5GNR applications, the downlink and uplink signal paths  72  and  74  can be configured for operation in an approximately same RF band within an approximate range of 10 GHz-60 GHz. 
     As described above in conjunction with  FIG.  2   , the downlink bandpass filter  42  is configured to filter the upshifted downlink signal from the downlink mixer  40  to remove any out-of-band frequencies, the downlink amplifier  44  is configured to amplify the filtered downlink signal from the downlink bandpass filter  42 , and the isolation circuit  36  is configured to pass the amplified downlink signal from the downlink amplifier  44  to the antenna  38  and to isolate, electrically, the uplink signal path  74  from the downlink signal path  72 . And the antenna  38  is configured to radiate the amplified downlink signal to one or more of the user equipment  22  (e.g., a smartphone or other device) of  FIG.  1   . 
     The control circuit  76  is configured to control which of the downlink local oscillators  80  and  82  provides a downlink oscillator signal to the downlink mixer  40  and which of the uplink local oscillators  52  and  84  provides an uplink oscillator signal to the uplink mixer  50 . 
     The sensor  78  is configured to detect whether there is a downlink signal at the input node of the downlink mixer  40  and provide a sensor signal to the control circuit  76  in response to detecting the presence or absence of the downlink signal. 
     The downlink local oscillator  80  is configured to generate a downlink oscillator signal having a frequency f LO_DL_Active , and the optional downlink local oscillator  82  is configured to generate a downlink oscillator signal having a frequency f LO_DL_Idle . 
     Similarly, the uplink local oscillator  52  is configured to generate an uplink oscillator signal having a frequency f LO_UL_Active ≈f LO_DL_Active , and the optional uplink local oscillator  84  is configured to generate an uplink oscillator signal having a frequency f LO_UL_Idle . 
     As described in more detail below, to provide electrical isolation in addition to the isolation that the isolation circuit  36  is configured to provide, the downlink signal path  72  is configured, during a downlink-idle period, to shift the frequencies of an interference signal that the downlink signal path  72  generates out of the RF band of uplink signals that the uplink signal path  74  is configured to receive and to condition. Such frequency shifting allows the uplink signal path  74  to filter, from a received uplink signal, a received component of the downlink upshifted interference signal. Or, said another way, shifting the frequencies of the interference signal can reduce the level of isolation that the isolation circuit  36  needs to provide for proper operation of the communication circuit  70 , and, therefore, can reduce the complexity and cost of the isolation circuit  36 . 
       FIG.  5    is a plot  90  of a frequency band  92  of a downlink signal radiated by the antenna  38  ( FIG.  4   ) during a downlink-active period, and of a frequency band  94  of an upshifted interference signal generated by the downlink signal path  72  ( FIG.  4   ) during a downlink-idle period, according to an embodiment. 
       FIG.  6    is a plot  96  of a frequency band  98  of an uplink signal received by the antenna  38  ( FIG.  4   ) during an uplink-active period, and of the frequency band  94  of the upshifted interference signal generated by the downlink signal path  72  ( FIG.  4   ) during the uplink-active period, according to an embodiment. 
     Referring to  FIGS.  4 - 6   , operation of the communication circuit  70  is described according to an embodiment in which the communication circuit  70  includes the optional downlink local oscillator  82  but lacks the optional uplink local oscillator  84 . 
     During a downlink-active period, the downlink mixer  40  receives, from the master unit  18  ( FIG.  1   ), a baseband or intermediate-frequency downlink signal (hereinafter “intermediate downlink signal”) having a center frequency f DL . In response to the intermediate downlink signal propagating along the downlink signal path  72  to the downlink mixer  40 , the sensor  78  generates a sense signal having a first value that indicates the detection of the intermediate downlink signal by the sensor  78 . 
     In response to the sense signal having the first value, the control circuit  76  activates the downlink-active local oscillator  80  and deactivates the downlink-idle local oscillator  82 . The downlink mixer  40  mixes the intermediate downlink signal with a local-oscillator signal generated by the downlink-active local oscillator  80  and having a frequency f LO_DL_Active  to upshift the intermediate downlink signal to a frequency band having a center frequency equal to f DL +f LO_DL_Active . 
     The downlink bandpass filter  42  filters the upshifted downlink signal from the downlink mixer  40  to remove any out-of-band frequencies (e.g., frequencies not in the upshifted frequency band of the downlink signal) such that the upshifted downlink signal occupies the RF band  92 . The downlink amplifier  44  amplifies the filtered downlink signal from the downlink bandpass filter  42 . The isolation circuit  36  receives the amplified upshifted downlink signal from the downlink amplifier  44 , passes the amplified upshifted downlink signal to the antenna  38 , and electrically isolates the uplink signal path  74  from the downlink signal path  72 . And the antenna  38  radiates the amplified upshifted downlink signal to user equipment  22  ( FIG.  1   ). 
     Operation of the uplink signal path  74  while the uplink signal path  74  is idle and the downlink signal path  72  is active is described below in conjunction with  FIG.  11   . 
     Still referring to  FIGS.  4 - 6   , during a downlink idle mode, the downlink mixer  40  receives no intermediate downlink signal from the master unit  18  ( FIG.  1   ). In response to the intermediate downlink signal being absent from the downlink signal path  72 , the sensor  78  generates the sense signal having a second value that indicates the detection of a lack of an intermediate downlink signal by the sensor  78 . In response to the sense signal having the second value, the control circuit  76  deactivates the downlink-active local oscillator  80  and activates the downlink-idle local oscillator  82 . 
     For reasons similar to those described above in conjunction with the communication circuit  30  of  FIG.  2   , the downlink mixer  40  effectively mixes an interference signal on the downlink signal path  72  having a center frequency f interference  with a local-oscillator signal generated by the downlink-idle local oscillator  82  having a frequency f LO_DL_Idle  to upshift the interference signal to a center frequency equal to f interference +f LO_DL_Idle . Because f LO_DL_Idle  is greater than f LO_DL_Active , the RF band  94  of the downlink upshifted interference signal is significantly higher in frequency than the RF band  92  of the upshifted downlink signal that the downlink signal path  72  generates while the downlink signal path  72  is actively conditioning and transmitting a downlink signal. 
     Because the downlink bandpass filter  42  has a passband approximately equal to the RF band  92  of an upshifted downlink signal generated by the downlink signal path  72  during a downlink active period, and because the RF band  94  of the downlink upshifted interference signal is significantly higher in frequency than the RF band  92 , the downlink bandpass filter  42  rejects much of the power of the downlink upshifted interference signal. 
     The downlink amplifier  44  amplifies the reduced-power component of the filtered downlink upshifted interference signal that passes through the downlink bandpass filter  42 . The isolation circuit  36  passes the amplified component of the downlink upshifted interference signal from the downlink amplifier  44  to the antenna  38 , and electrically isolates the uplink signal path  74  from the downlink signal path  72 . 
     And the antenna  38  radiates the amplified component of the downlink upshifted interference signal, which, as shown in  FIGS.  5 - 6   , occupies the RF band  94 . 
     Still referring to  FIGS.  4 - 6   , during an uplink active period, the uplink signal path  74  further rejects any received downlink upshifted interference components generated by the idle downlink signal path  72 . 
     There are two sources from which the uplink signal path  74  can receive a respective downlink upshifted interference component: 1) leakage of a downlink upshifted interference component through the isolation circuit  36 , which, as described above, may provide imperfect isolation, and 2) a redirection, by an object in the environment, back to the antenna  38 , of a downlink upshifted interference component that the antenna  38  radiated. 
     For purposes of example, assume that the uplink signal path  74  receives, from both of these sources, respective downlink upshifted interference components superimposed on an uplink signal. But the below description and analysis is similar even if the uplink signal path  74  receives a downlink upshifted interference component from only one of these sources. 
     The antenna  38  receives an uplink signal on which is superimposed a redirected component of the downlink upshifted interference signal. The isolation circuit  36  passes the uplink signal on which is superimposed the redirected downlink upshifted interference signal to an input node of the uplink amplifier  48 . Furthermore, a leakage component of the downlink upshifted interference signal “leaks” through the isolation circuit  36  to the input node of the uplink amplifier  48 . Therefore, the uplink amplifier  48  receives at its input node, and amplifies, the uplink signal on which is superimposed both the redirected and leakage components of the downlink upshifted interference signal. 
     Referring to  FIG.  6   , the uplink signal occupies the RF band  98 , which is centered at a frequency f UL +f LO_UL_Active , where f LO_UL_Active ≈f LO_DL_Active , but the redirected and leakage components of the downlink upshifted interference signal occupy the RF band  94 , which is at a significantly higher frequency than the RF band  98 . For example, the separation between the center frequencies of the RF bands  94  and  98  is significantly larger than the bandwidth of the RF band  92 . Further in example, if the RF band  92  is approximately 80 MHz wide, then the separation between the center frequencies of the RF bands  94  and  98  can be approximately 100 MHz or more. 
     The uplink mixer  50  mixes the amplified uplink signal on which is superimposed the redirected and leakage components of the downlink upshifted interference signal with the oscillator signal from the uplink local oscillator  52  having a frequency approximately equal to f LO_UL_Active . 
     That is, the uplink mixer  50  downshifts the frequencies of the uplink signal and the superimposed components of the downlink upshifted interference signal such that the downshifted uplink signal occupies an IF band (not shown in  FIGS.  4 - 6   ) centered at f UL , and such that the downlink downshifted interference components occupy an IF band centered at f interference +(f LO_DL_Idle −f LO_UL_Active ). 
     Therefore, the center frequency of the IF band (not shown in  FIGS.  4 - 6   ) occupied by the downlink downshifted interference components is higher than the center frequency f UL  of the IF band (not shown in  FIGS.  4 - 6   ) occupied by the downshifted uplink signal by a frequency equal to f LO_DL_Idle −f LO_UL_Active . 
     As long as the difference frequency f LO_DL_Idle −f LO_UL_Active  is large enough, the one or more filters (not shown in  FIG.  4   ) proceeding the uplink mixer  50  pass the uplink signal centered at f UL  and reject the components of the downlink downshifted interference signal centered at f interference +(f LO_DL_Idle −f LO_UL_Active ). 
     Still referring to  FIGS.  4 - 6   , by upshifting the frequencies of interference generated by the downlink signal path  72  to an RF band that is higher than the frequency bands  92  and  98  of an upshifted downlink signal and a received uplink signal, respectively, the communication circuit  70  electrically isolates, in at least three ways, the uplink signal path  74  from interference generated by the downlink signal path  72  during a downlink-idle period. First, the downlink bandpass filter  42  rejects the downlink upshifted interference signal generated by the downlink mixer  40 . Second, the isolation circuit  36  blocks the downlink upshifted interference signal from the uplink signal path  74 . And third, one or more filters proceeding the uplink mixer  50  in the uplink signal path  74  reject the downshifted downlink upshifted interference components that are not rejected by the downlink bandpass filter  42  or blocked by the isolation circuit  36 . That is, the filtering of the downshifted downlink upshifted interference components by the one or more filters following the uplink mixer  50  is in addition to the rejection of the downlink upshifted interference components by the downlink bandpass filter  42  and the isolation circuit  36 . 
     Therefore, the communication circuit  70  can lower the level of isolation provided by the isolation circuit  36  by effectively dividing among the three above-described isolation techniques the job of providing electrical isolation between the downlink and uplink signal paths  72  and  74 . Refer to the example described above in conjunction with  FIG.  2    in which, for a given application, the isolation circuit  36  is tasked to provide 90 dB of isolation between the downlink and uplink signal paths  72  and  74 . Further to this example and assuming that there is no redirected interference component, if the combined rejection of the upshifted leakage interference signal provided by the downlink bandpass filter  42  and the one or more uplink bandpass filters is 70 dB, then the isolation circuit  36  is tasked with providing only the remaining 20 dB of isolation for the communication circuit  70  to achieve the design specification of 90 dB isolation between the downlink signal path  72  and the uplink signal path  74 . 
     Lowering the level of isolation that the isolation circuit  36  is tasked to provide can relax the design requirements of the isolation circuit  36 , and, therefore, can reduce one or more of the cost, size, and design complexity of the isolation circuit  36 . 
     Operation of the uplink signal path  74 , while the uplink signal path  74  is idle (e.g., is not receiving and conditioning an uplink signal, or otherwise while the downlink signal path  72  is conditioning and transmitting a downlink signal), is described below in conjunction with  FIG.  11   . 
     Still referring to  FIGS.  4 - 6   , alternate embodiments of the communication circuit  70  are contemplated. For example, the components of the communication circuit  70  can be distributed among a remote antenna unit  14  ( FIG.  1   ), the master unit  18  ( FIG.  1   ), one or more intermediate units  24  ( FIG.  1   ), and one or more base stations  12  ( FIG.  1   ) in any suitable manner. Furthermore, the downlink-idle local oscillator  82  can be omitted, and the downlink-active local oscillator  80  can be, for example, a voltage-controlled or a numerically controlled oscillator that is configured to generate, in response to the control circuit  76 , oscillator signals at the frequencies f LO_DL_Active  and f LO_DL_Idle ; that is, the local oscillator  80  can be configured to generate oscillator signals during both the downlink-active and the downlink-idle periods. Moreover, the communication circuit  70  can be configured to shift the downlink interference signal to a frequency band that is lower than (instead of higher than) the frequency band  92  of an upshifted downlink signal and the frequency band  98  of a received uplink signal. In addition, instead of frequency shifting the downlink interference signal with the uplink mixer  50 , the circuit  70  can shift the frequency of the downlink interference signal in another manner, such as by changing a sampling frequency (for example, changing a sampling frequency of an ADC) in the downlink signal path  72 . Furthermore, one or more of the described components can be omitted from the communication circuit  70 , and one or more other components can be added to the communication circuit  70 . For example, the isolation circuit  36  can be omitted such that each of the downlink signal path  72  and uplink signal path  74  has a respective antenna as described above in conjunction with  FIG.  3   . Moreover, the isolation circuit  36  may not be a single circuit or component, but can be a combination of multiple circuits and components. In addition, the control circuit  76  and the sensor  78  can be omitted, and one or more of a base station  12  ( FIG.  1   ), the master unit  18  ( FIG.  1   ), or an intermediate unit  24  ( FIG.  1   ) can be configured to perform the above-described functions attributed to the control circuit  76  and the sensor  78 . Furthermore, embodiments described in conjunction with  FIGS.  1 - 3  and  7 - 11    may be applicable to the communication circuit  70 . 
       FIG.  7    is a diagram of a single-pole-double-throw electronic switch circuit  100 , which can be used as, or as part of, the isolation circuit  36  of  FIG.  4   , according to an embodiment. The switch circuit  100  includes a first conduction node  102  configured for coupling to an output node of the downlink amplifier  44  ( FIG.  4   ) of the downlink signal path  72  ( FIG.  4   ), a second conduction node  104  configured for coupling to an input node of the uplink amplifier  48  ( FIG.  4   ) of the uplink signal path  74  ( FIG.  4   ), and a third conduction node  106  configured for coupling to the antenna  38  ( FIG.  4   ). Also, the switch circuit  100  can include a control node  108  configured for coupling to the control circuit  76  ( FIG.  4   ). 
     Referring to  FIGS.  4  and  7   , operation of the switch circuit  100  is described according to an embodiment. In response to the sense signal generated by the sensor  78  having the first value to indicate a downlink signal is present at an input node of the downlink mixer  40 , the control circuit  76  causes the switch circuit  100  to couple the downlink signal path  72  to the antenna  38 , and to uncouple the antenna  38  from the uplink signal path  74 . Conversely, in response to the sense signal generated by the sensor  78  having the second value to indicate a downlink signal is absent from an input node of the downlink mixer  40  (e.g., to indicate that the downlink signal path  72  is idle), the control circuit  76  causes the switch circuit  100  to uncouple the downlink signal path  72  from the antenna  38 , and to couple the antenna  38  to the uplink signal path  74 . 
     Still referring to  FIG.  7   , alternate embodiments of the switch circuit  100  are contemplated. For example, embodiments described in conjunction with  FIGS.  1 - 6  and  8 - 11    may be applicable to the switch circuit  100 . 
       FIG.  8    is a diagram of a circulator circuit  110 , which can be used as, or as part of, the isolation circuit  36  of  FIG.  4   , according to an embodiment. The circulator circuit  110  includes a downlink node  112  configured for coupling to an output node of the downlink amplifier  44  ( FIG.  4   ) of the downlink signal path  72  ( FIG.  4   ), an uplink node  114  configured for coupling to an input node of the uplink amplifier  48  ( FIG.  4   ) of the uplink signal path  74  ( FIG.  4   ), and an antenna node  116  configured for coupling to the antenna  38  ( FIG.  4   ). 
     Referring to  FIGS.  4  and  8   , operation of the circulator circuit  110  is described according to an embodiment. During a downlink-active period, the circulator circuit  110  couples a downlink signal from the downlink signal path  72  to the antenna  38 , and electrically isolates the uplink signal path  74  from the downlink signal path. Conversely, during a downlink-idle period, the circulator circuit  110  couples an uplink signal received by the antenna  38  to the uplink signal path  74 , and electrically isolates the uplink signal path  74  from the downlink signal path  72 . 
     Still referring to  FIG.  8   , alternate embodiments of the circulator circuit  110  are contemplated. For example, embodiments described in conjunction with  FIGS.  1 - 7  and  9 - 11    may be applicable to the circulator circuit  110 . 
       FIG.  9    is a diagram of a communication circuit  120  of a remote antenna unit  14  ( FIG.  1   ), according to an embodiment and in which like numerals reference components common to  FIGS.  4 , 7 , and  8   . 
     In addition to including an isolation circuit (here, the circulator circuit  110  of  FIG.  8   ) and to being configured to implement the downlink-interference-frequency-shifting technique described above in conjunction with  FIGS.  4 - 6   , the communication circuit  120  is configured to dynamically cancel, from the uplink signal path  74 , a second downlink interference signal that is, after upshifting by the downlink mixer  40  during the downlink idle mode and after downshifting by the uplink mixer  50  during the uplink active mode, in the same intermediate-frequency (IF) band as the uplink signal. 
     As described above in conjunction with  FIG.  4   , while the downlink signal path  72  is idle, the downlink signal path  72  may generate, on the input side of the downlink mixer  40 , a first downlink interference signal that is within the IF passband of the downlink and uplink signals (assuming that the DL_ active  oscillator  80  ( FIG.  4   ) and the UL_ active  oscillator  52  ( FIG.  4   ) have approximately the same frequency, as they do in the above-described embodiment). But the downlink mixer  40  is configured to shift (i.e., upshift or downshift) the frequencies of the first downlink interference signal out of the radio-frequency (RF) passbands of the downlink and uplink signals as described above in conjunction with  FIGS.  4 - 6   . Consequently, the downlink bandpass filter  42  rejects most or all of the frequency-shifted, out-of-band first downlink interference signal, and any component of the frequency-shifted, out-of-band first downlink interference signal that the downlink bandpass filter  42  does not reject is rejected by the uplink bandpass filter  126 . Said another way, the uplink bandpass filter  126  further rejects the frequency-shifted, out-of-band first downlink interference signal that the downlink bandpass filter  42  already rejected. 
     But while frequency shifting the first downlink interference signal out of the RF passbands of the downlink and uplink signals, the mixer  40  may also shift a second downlink interference signal into the RF passbands of the downlink and uplink signals. The downlink signal path  72  may generate, at the input side of the downlink mixer  40 , both the first and the second downlink interference signals. As described above and in conjunction with  FIGS.  4 - 6   , the first downlink interference signal is within the IF passbands of the downlink and uplink signals. However, the second downlink interference signal, which typically has a lower power than the first downlink interference signal, is outside of the IF passbands of the downlink and uplink signals. Unfortunately, while the downlink mixer  40  is frequency shifting the first downlink interference signal out of the RF passbands of the downlink and uplink signals, the downlink mixer  40  also may be frequency shifting the second downlink interference signal into the RF passbands of the downlink and uplink signals. 
     Consequently, as described below, the communication circuit  120  is configured not only to reduce, in the uplink signal, interference caused by the first downlink interference signal, but is also configured to reduce, in the uplink signal, interference caused by the second downlink interference signal. 
     In addition to the downlink mixer  40 , downlink bandpass filter  42 , and downlink amplifier  44 , the downlink signal path  72  includes a DAC  122  and a signal coupler  124 . For clarity, the downlink-active and downlink-idle local oscillators  80  and  82  ( FIG.  4   ) are omitted from  FIG.  9   , but at least the oscillator  80  is present in the communication circuit  120 . 
     In addition to the uplink amplifier  48  and the uplink mixer  50 , the uplink signal path  74  includes an uplink bandpass filter  126 , an uplink ADC  128 , and an uplink summer  130 . For clarity, the uplink-active and uplink-idle local oscillators  52  and  84  ( FIG.  4   ) are omitted from  FIG.  9   , but at least the oscillator  52  is present in the communication circuit  120 . 
     The communication circuit  120  also includes an interference-cancellation (also called an interference-compensation) path  132 , which includes an interference mixer  134  matched to the uplink mixer  50 , an interference bandpass filter  136  matched to the uplink bandpass filter  126 , a cancellation ADC  138  matched to the uplink ADC  128 , and an adjustable-gain-and-phase circuit  140 . 
     Operation of the communication circuit  70  during contemporaneous downlink idle and uplink active periods is described below, according to an embodiment. 
     As described above in conjunction with  FIG.  4   , the downlink signal path  72  generates, at an output node of the downlink amplifier  44 , a frequency-upshifted first downlink interference signal, which is outside of the RF passbands of the downlink and uplink signals, and a frequency-upshifted second downlink interference signal, which is within the RF passbands of the downlink and uplink signals. 
     Also as described above in conjunction with  FIG.  4   , the uplink signal path  74  receives, at an input node of the uplink amplifier  48 , an uplink signal on which is superimposed leakage components (received via leakage through the circulator circuit  110 ) and redirected components (received by the antenna  38  via a redirection of the first and second downlink upshifted interference signals by an environmental object) of the first and the second downlink upshifted interference signals, respectively. 
     Further as described in conjunction with  FIG.  4   , the frequency upshifting, by the downlink mixer  40 , of the first downlink interference signal allows the uplink bandpass filter  126  to reject the leakage and redirected components of the first downlink upshifted interference signal, because, as described above, the downlink mixer  40  shifts the frequencies of the first downlink interference signal outside of the RF passband of the uplink signal, and, therefore, outside of the passband of the uplink bandpass filter  126 . 
     But the uplink bandpass filter  126  may be unable to reject the leakage and redirected components of the second downlink upshifted interference signal because, as described above, the downlink mixer  40  shifts the frequencies of the second downlink interference signal inside of the RF passband of the uplink signal. Therefore, the components of the second downlink upshifted interference signal pass through the uplink bandpass filter  126  and to and through the ADC  128 . 
     It is the components of the second downlink interference signal that the interference-cancellation path  132  cancels, partially or fully, from the portion of the uplink signal path  74  following the ADC  128 . 
     The coupler  124  couples the second downlink upshifted interference signal at the output of the downlink amplifier  44  to the cancellation mixer  134 , which is matched (e.g., in one or more signal-mixing characteristics) to, and which receives the same oscillator signal as, the uplink mixer  50 . 
     Therefore, the cancellation mixer  134  downshifts the frequencies of the second downlink upshifted interference signal to a frequency band having approximately the same center frequency as the downshifted uplink signal at the input to the uplink bandpass filter  126 . 
     Next, the cancellation bandpass filter  136 , which is matched (e.g., in bandwidth, phase shift, and gain/attenuation) to the uplink bandpass filter  126 , filters the downshifted second downlink interference signal such that the downshifted second downlink interference signal at the output of the cancellation bandpass filter occupies approximately the same frequency band as the filtered downshifted uplink signal at the output node of the uplink bandpass filter. 
     Then, the cancellation ADC  138 , which is matched (e.g., in resolution, sample rate, dynamic range) to the uplink ADC  128 , converts the filtered analog second downlink interference signal into a digital interference signal that has approximately the same characteristics (e.g., quantization error, resolution, frequency band) as the digital uplink signal that the uplink ADC generates in response to the filtered analog uplink signal from the uplink bandpass filter  126 . 
     So, from the input nodes of the uplink and cancellation mixers  50  and  134  to the output nodes of the uplink and cancellation ADCs  128  and  138 , the uplink signal with components of the second downlink interference signal superimposed thereon, and the second downlink interference signal that the coupler  124  couples from the downlink signal path  72 , experience approximately the same signal conditioning due to the correspondence and matching of the mixers  50  and  134 , filters  126  and  136 , and ADCs  128  and  138  in the uplink and cancellation paths  74  and  132 , respectively. 
     But the components of the second downlink interference signal superimposed on the uplink signal have experienced additional conditioning by virtue of being coupled to the uplink signal path  74  via leakage through the isolation circulator circuit  110  and via redirection to and through the antenna  38  and the isolation circulator circuit  110 . 
     Therefore, the adjustable-gain-and-phase circuit  140  is configured to impart, to the digital second downlink interference signal from the cancellation ADC  138 , a phase adjustment and a gain adjustment that effectively mimic the result of the additional conditioning experienced by the components of the second downlink interference signal superimposed on the uplink signal such that, at least ideally, the phase-and-gain-adjusted digital second downlink interference signal equals the sum of the components of the digital second downlink interference signal superimposed on the digital uplink signal. The adjusted digital second downlink interference signal that the circuit  140  generates can be called a digital cancellation signal. Furthermore, in an embodiment the frequency response of the circuit  140  is configurable to replicate more accurately the channel that the uplink signal and superimposed components of the second downlink interference signal experience. 
     The summer  130  subtracts the digital cancellation signal from the digital uplink signal with the components of the second downlink interference signal superimposed thereon. 
     If the digital cancellation signal equals the sum of the components of the second downlink interference signal superimposed on the uplink signal, then the signal resulting from the subtraction and appearing at an output node of the summer  130  is the uplink signal with the components of the first and second downlink interference signals removed; that is, at least ideally, the components of the first and second downlink interference signals are no longer superimposed on the uplink signal at the output of the summer. 
     Even if the digital cancellation signal does not equal the sum of the components of the second downlink interference signal superimposed on the uplink signal, the magnitudes of these interference components superimposed on the uplink signal output from the summer  130  are typically less than the magnitudes of these same interference components superimposed on the digital uplink signal input to the summer. Therefore, even if the cancellation path  132  does not provide perfect cancellation, the cancellation path still provides a level of isolation in addition to the isolation provided by the frequency upshifting of the first downlink interference signal by the downlink signal path  72  and by the RF circulator circuit (or other isolation circuit)  110 . For example, such additional isolation that the cancellation path  132  provides can be in an approximate range of 25 dB to 30 dB. 
     The phase and gain adjustments that the adjustable-gain-and-phase circuit  140  makes to the digital second downlink interference signal from the cancellation ADC  138  can be set, for example, during installation of the DAS  10  ( FIG.  1   ) and can remain fixed thereafter, or the communication circuit  120 , in cooperation with other circuitry, can be configured to dynamically alter the gain and phase adjustments that the circuit  140  imparts to the digital second downlink interference signal from the ADC  138 . For example, the adjustable-gain-and-phase circuit  140  can be part of a negative-feedback loop (not shown in  FIG.  9   ) that dithers one or both of the phase and gain adjustments, determines the magnitude of the digital uplink signal after it propagates from the summer  130 , and moves one or both of the gain and phase adjustments to respective levels that minimize a magnitude of the post-summer digital uplink signal. The theory of this dithering technique is that the magnitude of the post-summer uplink signal is at a minimum in response to maximum cancellation of the components of the second downlink interference signal superimposed on the digital uplink signal. 
     A communication circuit that includes a cancellation path similar to the cancellation path  132  is described in U.S. Pat. No. 10,205,481 (see, for example,  FIG.  7   ), which is incorporated by reference. 
     Still referring to  FIG.  9   , alternate embodiments of the communication circuit  120  are contemplated. For example, the communication circuit  120  can include an isolation circuit other than, or in addition to, the RF circulator circuit  110 . Furthermore, embodiments described in conjunction with  FIGS.  1 - 8  and  10 - 11    may apply to the communication circuit  120 . 
       FIG.  10    is a diagram of a communication circuit  150  of a remote antenna unit  14  ( FIG.  1   ), according to an embodiment and in which like numerals reference components common to  FIGS.  4  and  7 - 9   . Like the communication circuits  70  of  FIG.  4  and  120    of  FIG.  9   , the communication circuit  150  shifts (e.g., upshifts) the frequencies of a downlink interference signal during idle periods of the downlink signal path  152  to isolate the uplink signal path  170  from the downlink interference signal. But unlike the communication circuits  70  of  FIG.  4  and  120    of  FIG.  9   , the communication circuit  150  performs at least some functions in the optical domain instead of in the electrical domain. 
     In addition to the isolation circuit  36 , the antenna  38 , and the downlink amplifier  44 , a downlink signal path  152  of the communication circuit  150  includes laser diodes  154 ,  156 , and  158 , photodiode  160 , a switch circuit  162 , an optical-signal combiner  164 , and current sources  165 ,  166 , and  167 . Although omitted from  FIG.  10    for clarity, the downlink signal path  152  can also include the downlink bandpass filter  42  ( FIG.  4   ) coupled between a cathode of the photodiode  160  and an input node of the downlink amplifier  44 . 
     And in addition to the uplink amplifier  48 , an uplink signal path  170  of the communication circuit  150  includes laser diodes  172 ,  174 , and  176 , photodiode  178 , a switch circuit  180 , an optical-signal combiner  182 , and current sources  181 ,  183 , and  184 . In addition to the downlink signal path  152  and the uplink signal path  170 , the communication circuit  150  includes the control circuit  76  and the sensor  78 . 
     Still referring to  FIG.  10   , operation of the communication circuit  150  is described according to an embodiment. 
     During a downlink-active mode, the laser diode  154  receives, from the master unit  18  ( FIG.  1   ), an electrical intermediate downlink signal having a center frequency f DL , and converts the intermediate downlink signal into an optical downlink signal. 
     In response to the electrical intermediate downlink signal propagating along the downlink signal path  152  to the laser diode  154 , the sensor  78  generates a sense signal having a first value that indicates the detection of the intermediate downlink signal by the sensor  78 . 
     In response to the sense signal having the first value, the control circuit  76  causes the switch circuit  162  to couple the laser diode  156  to the downlink combiner  164  (the control circuit  76  controls the switch circuit  162  via a control node  188  of the switch circuit  162 ). 
     The laser diode  156 , acting as a local optical oscillator, generates an optical oscillator signal having a frequency such that the difference between the frequencies of the laser diode  154  and the laser diode  156  approximately equals f LO_DL_Active . 
     The optical-signal combiner  164  combines the optical downlink and optical oscillator signals from the laser diodes  154  and  156 , respectively, onto a single optical path. 
     The current source  166  generates a bias current to the photodiode  160 , which, due to its nonlinear characteristics, effectively mixes the optical intermediate downlink signal with the optical local-oscillator signal to generate a frequency-upshifted electrical downlink signal having a center frequency equal to f DL +f LO_DL_Active . 
     If the downlink bandpass filter  42  ( FIG.  4   ) is present, then it filters the upshifted downlink signal from the photodiode  160  to remove any out-of-band frequencies (e.g., frequencies not in the frequency band of the upshifted downlink signal) such that the upshifted electrical downlink signal occupies the frequency band  92  of  FIG.  5   . 
     The downlink amplifier  44  amplifies the filtered downlink signal from the downlink bandpass filter  42  (or, if the downlink bandpass filter is absent, the downlink amplifier  44  amplifies the upshifted downlink signal from the photodiode  160 ). The isolation circuit  36  passes the amplified downlink signal from the downlink amplifier  44  to the antenna  38 , and electrically isolates the uplink signal path  170  from the downlink signal path  152 . And the antenna  38  radiates the amplified downlink signal. 
     Operation of the uplink signal path  170  while the downlink signal path  152  is active is described below in conjunction with  FIG.  11   . 
     Still referring to  FIG.  10   , during a downlink-idle period, the laser diode  154  receives no intermediate downlink signal from the master unit  18  ( FIG.  1   ) In response to the intermediate downlink signal being absent from the portion of the downlink signal path  152  preceding the laser diode  154 , the sensor  78  generates the sense signal having the second value that indicates the detection of a lack of an intermediate downlink signal by the sensor  78 . 
     In response to the sense signal having the second value, the control circuit  76  causes the switch circuit  162  to uncouple the laser diode  156  from the optical combiner  164  and to couple the laser diode  158  to the optical combiner  164 . 
     The optical combiner  164  combines an optical interference signal from the laser diode  154  and the optical downlink-idle-oscillator signal from the laser diode  158  onto a single optical path that is coupled to the photodiode  160 . 
     For reasons similar to those described above in conjunction with the communication circuit  30  of  FIG.  2   , the photodiode  160  (biased by the current source  166 ) effectively mixes an optical interference signal on the downlink signal path  152  and having a modulated center frequency f interference  with the optical local-oscillator signal having a frequency such that the difference of frequencies between the laser  154  and the laser  158  corresponds to f LO_DL_Idle , and generates an electrical interference signal that is upshifted to a center frequency equal to f noise +f LO_DL_Idle . Because f LO_DL_Idle  is greater than f LO_DL_Active , the frequency band  94  ( FIG.  5   ) of the interference signal is significantly higher in frequency than the frequency band  92  ( FIG.  5   ) of an upshifted downlink signal that the downlink signal path  152  generates while the downlink signal path  152  is actively conditioning and transmitting a downlink signal. 
     If a downlink bandpass filter  42  is present between the cathode of the photodiode  160  and an input node of the downlink amplifier  44 , the downlink bandpass filter  42  ( FIG.  4   ) has a passband approximately equal to the frequency band  92  ( FIG.  5   ) of an upshifted downlink signal generated by the downlink signal path  152  during a downlink active period. Because the frequency band  94  ( FIG.  5   ) of the interference signal is significantly higher in frequency than the frequency band  92 , the downlink bandpass filter  42  rejects a significant portion of the power of the upshifted interference signal. 
     The downlink amplifier  44  amplifies the reduced-power filtered electrical interference signal that passes from the photodiode  160  through the downlink bandpass filter  42 . The isolation circuit  36  passes the amplified interference signal from an output node of the downlink amplifier  44  to the antenna  38 , and electrically isolates the uplink signal path  170  from the downlink signal path  152 . Also, the antenna  38  radiates the amplified interference signal, which, as shown in  FIG.  5   , occupies the frequency band  94 . 
     Still referring to  FIG.  10   , during an uplink active mode, the uplink signal path  170  further rejects any received downlink upshifted interference components generated by the idle downlink signal path  152 . 
     There are two sources from which the uplink signal path  170  can receive a respective component of the downlink interference signal: 1) leakage of downlink interference through the isolation circuit  36 , which, as described above, may provide imperfect isolation, and 2) a redirection, by an object in the environment back to the antenna  38 , of a component of the downlink interference that the antenna  38  radiated. 
     For example purposes, it is assumed that the uplink signal path  170  receives, from both of these sources, respective leakage and redirected downlink interference components superimposed on an uplink signal. But the below description and analysis is similar even if the uplink signal path  170  receives a downlink interference component from only one of these sources. 
     The antenna  38  receives an uplink signal on which is superimposed a redirected component of the downlink interference signal. The isolation circuit  36  passes the uplink signal on which is superimposed the redirected component of the downlink interference signal to an input node of the uplink amplifier  48 . Furthermore, a leakage component of the downlink interference signal “leaks” through the isolation circuit  36  to the input node of the uplink amplifier  48 . Therefore, the uplink amplifier  48  receives, at its input node, and amplifies, an uplink signal on which is superimposed both the redirected and leakage components of the downlink interference signal. 
     Referring to  FIG.  6   , the uplink signal occupies the frequency band  98 , which is centered at a frequency f UL +f LO_UL_Active , where f LO_UL_Active ≈f LO_DL_Active ), but the redirected and leakage components of the downlink upshifted interference signal occupy the frequency band  94 , which is at a significantly higher frequency than the frequency band  98 . For example, the separation between the center frequencies of the bands  94  and  98  is 2.8 GHz. 
     For clarity, although it is not shown, in  FIG.  10   , the UL signal, centered at f UL +f LO_UL_active , is down shifted to f UL  before it is converted into an optical signal by the laser diode  172 . Although not shown in  FIG.  10   , the communication circuit  150  can include circuitry for performing other possible processing, such as predistortion processing, of the wanted RF signals. 
     The laser diode  172  converts the amplified and down shifted uplink signal on which is superimposed the redirected and leakage components of the downlink interference signal into an optical uplink signal. The laser diode  174  generates an optical uplink-active oscillator signal, and the switch circuit  180 , in response to a control signal from the control circuit  76  on a control node  190 , couples the optical uplink-active oscillator signal to the optical-signal combiner  182 . The combiner  182  combines the optical uplink signal from the laser diode  172  and the optical uplink-active oscillator signal from the laser diode  174  (via the switch circuit  180 ) onto a single optical path. 
     The use of the laser diode  174  allows generation, at the output of the photodiode  178 , an upshifted electrical signal at the original center frequency f UL +f LO_UL_active . This can be accomplished by setting the frequencies of the lasers  172  and  174  such that the difference between these frequencies approximately equals f LO_UL_active . That is, the photodiode  178  effectively upshifts the frequencies of the uplink signal and the components of the downlink interference signal such that the upshifted uplink signal occupies a frequency band  98  ( FIG.  6   ) centered at f UL +f LO_UL_active , and such that the upshifted interference components occupy a frequency band  94  ( FIG.  6   ) centered at f interference +f LO_DL_Idle  Therefore, the center frequency of the frequency band (shown in  FIG.  6   ) occupied by the upshifted interference components is higher than the center frequency f UL +f LO_UL_active  of the frequency band  98  ( FIG.  6   ) occupied by the upshifted uplink signal by a frequency difference f LO_DL_Idle −f LO_DL_Active . As long as the difference frequency f LO_DL_Idle −f LO_UL_Active  is large enough, the one or more filters (not shown in  FIG.  4   ) following the photodiode  178  pass the upshifted uplink signal centered at f UL +f LO_UL_active  and reject the components of the downlink interference signal centered at f interference +f LO_DL_Idle . 
     Still referring to  FIG.  10   , by upshifting the frequencies of an interference signal generated by the downlink signal path  152  to a frequency band that is higher than the frequency bands of an upshifted downlink signal and of a received uplink signal, the communication circuit  150  isolates the uplink signal path  170  from interference components generated by the downlink signal path  152  during a downlink-idle period in a manner similar to that described above in conjunction with the communication circuit  70  of  FIG.  4   . Such isolation also can provide the communication circuit  150  with advantages similar to those described above in conjunction with the communication circuit  70 . Furthermore, the use of optical components, instead of electrical components, to perform some functions, such as oscillator-signal generation and signal mixing, can further reduce the noise and other interference generated by the downlink signal path  152  and present on the uplink signal path  170 . 
     Operation of the uplink signal path  170  while the uplink signal path  170  is idle (e.g., is not receiving an uplink signal while the downlink signal path  152  is conditioning and transmitting a downlink signal) is described below in conjunction with  FIG.  11   . 
     Still referring to  FIG.  10   , alternate embodiments of the communication circuit  150  are contemplated. For example, the communication circuit  150  can perform in the optical domain one or more of the functions described as being performed in the electrical domain, and can perform in the electrical domain one or more of the functions described as being performed in the optical domain. Furthermore, although the photodiode  160  is described as upshifting the downlink signal in frequency, the photodiode  160  can downshift the downlink signal and noise in frequency by generating respective differences between the frequencies of the optical signals generated by the laser diodes  154  and  156  and generated by the laser diodes  154  and  158 . For example, where the wavelengths of the signals generated by the laser diodes  154  and  156  are 1309.84 nanometers (nm) and 1310 nm, respectively, the downlink signal can be downshifted or upshifted by approximately 28 GHz). Furthermore, embodiments described in conjunction with  FIGS.  1 - 9  and  11    may be applicable to the communication circuit  150 . 
       FIG.  11    is a representative circuit diagram of a communication circuit  200  and of portions  202  and  204  of the downlink and uplink signal paths  206  and  208 , respectively, that are external to the communication circuit  200 , according to an embodiment; in  FIG.  11    like numerals reference components common to the communication circuit of  FIG.  11    and the communication circuits of  FIGS.  4 ,  9 , and  10   . For example, the circuit diagram of  FIG.  11    represents one or more of the communication circuits  70 ,  120 , and  150  of  FIGS.  4 ,  9 , and  10   , respectively, and represents other portions of the respective downlink and uplink signal paths  72  and  74 , and  152  and  170  of  FIGS.  4 ,  9 , and  10   , respectively. 
     Further below, operation of the uplink signal path  208  during a period in which the downlink signal path  206  is active and the uplink signal path  208  is idle is described, according to an embodiment. 
     In addition to the downlink mixer  40 , downlink bandpass filter  42 , downlink amplifier  44 , isolation circuit  36 , and antenna  38 , the downlink signal path  206  includes, external to the communication circuit  200 , another isolator circuit  210 , another downlink amplifier  212 , and another downlink mixer  214 , and includes a signal transport layer  216 , which interfaces the portion  202  of the downlink signal path  206  external to the communication circuit  200  with a portion  218  of the downlink signal path  206  that forms part of the communication circuit  200 . The signal transport layer  216  can be external to the communication circuit  200 , can form part of the communication circuit  200 , or can include both one or more components external to the communication circuit  200  and one or more components that form part of the communication circuit  200 . Furthermore, the downlink local oscillators  80  and  82  are disposed on the communication circuit  200 , and may be considered part of the downlink signal path  206  or separate therefrom. 
     Also, in addition to the antenna  38 , the isolation circuit  36 , the low-noise uplink amplifier  48 , and the uplink mixer  50 , the uplink signal path  208  includes the signal transport layer  216  and includes, external to the communication circuit  200 , another uplink mixer  220 , an uplink bandpass filter  222 , another uplink amplifier  224 , and the other isolator circuit  210 . The signal transport layer  216  interfaces the portion  204  of the uplink signal path  208  external to the communication circuit  200  with a portion  226  of the uplink signal path  208  that forms part of the communication circuit  200 . As described above, the signal transport layer  216  may be external to the communication circuit  200 , may form part of the communication circuit  200 , or may include both one or more components external to the communication circuit  200  and one or more components that form part of the communication circuit  200 . Furthermore, the uplink local oscillators  52  and  84  are disposed on the communication circuit  200 , and may be considered part of the uplink signal path  208  or separate therefrom. 
     Furthermore, each of the control circuit  76  and the sensor  78  may be disposed on, or external to, the communication circuit  200 , and the sensor  78  may be considered part of, or external to, the downlink signal path  206 . 
     Due to signal leakage through the isolation circuits  36  and  210 , and due to redirection, by an object, of a radiated downlink signal back to the antenna  38 , the downlink and uplink signal paths  206  and  208  effectively form a closed loop, which, if not properly compensated, can be unstable and oscillate or ring. For example, if, at a particular frequency, the closed loop has a gain equal to or greater than unity, and has a phase shift of 360°, then the loop may oscillate at the particular frequency. 
     During a downlink-active period in which the downlink signal path  206  is actively conditioning and transmitting a downlink signal and during a contemporaneous uplink-idle period in which the uplink signal path  208  is inactive, and, therefore, not receiving and conditioning an uplink signal, coupling of the downlink signal to the uplink signal path  208 , due to, for example, the above-described downlink-signal leakage and redirection, can cause the closed downlink-path-uplink-path loop to oscillate if the loop gain is too high. 
     One technique for reducing the loop gain while the uplink signal path  208  is inactive is to shift the frequencies of the components of the downlink signal received by the uplink signal path  208  out of the passband of the uplink signal path. 
     Referring to  FIGS.  5  and  6   , the frequency band  92  of the transmitted downlink signal is approximately the same as the frequency band  98  of a received uplink signal. 
     Therefore, during an uplink-idle period while the uplink signal path  208  is not receiving and conditioning an uplink signal, if the uplink local oscillator  52  remains active and generates an oscillator signal having a frequency f LO_UL_Active , then the uplink mixer  50  effectively downshifts the received leakage and redirected components of the downlink signal to the passband of the uplink signal path  208 . 
     If the power of the received leakage and redirected components of the downlink signal is high enough to render the loop gain unity or greater, then the loop formed by the downlink and uplink signal paths  206  and  208  may oscillate. 
     But to compensate for the potential increase in loop gain that the leakage and redirected components of the downlink signal can cause, the control circuit  76 , in response to the sensor  78  detecting a downlink signal at the input node of the downlink mixer  40 , deactivates the uplink-active local oscillator  52  and activates the uplink-idle local oscillator  84 . 
     The uplink-idle local oscillator  84  generates an oscillator signal having a frequency f LO_UL_Idle , which is different than f LO_UL_Active ; for example, referring to  FIG.  6   , the uplink-idle local oscillator  84  can generate the local oscillator signal having a frequency f LO_UL_Idle =f LO_DL_Idle . 
     The result of the oscillator signal having a frequency f LO_UL_Idle , which is different from f LO_UL_Active , is that the uplink mixer  50  shifts the frequencies of the received leakage and redirected components of the downlink signal down, up, or both down and up, and, therefore, out of the passband of the portion of the uplink signal path  208  following the uplink mixer  50 . 
     And, as described above, shifting the frequencies of the received leakage and redirected components of the downlink signal out of the passband of the uplink signal path  208  effectively reduces the gain of the loop formed by the downlink signal path  206  and the uplink signal path  208 , and, therefore, can increase the stability of the loop by lowering the loop gain to below unity. 
     Still referring to  FIG.  11   , alternate embodiments of the communication circuit  200  are contemplated. For example, embodiments described in conjunction with  FIGS.  1 - 10    may be applicable to the communication circuit  200 . 
     The methods and techniques described herein may be implemented in analog electronic circuitry, digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor, a general-purpose processor such as a computer, a microprocessor, or microcontroller) or other circuit (for example, an FPGA), firmware, software, or in combinations of them. Apparatuses 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). Furthermore, although described in conjunction with uplink and downlink signals for purposes of explanation, the embodiments, techniques, and concepts disclosed herein are applicable to other transmit and receive signals. 
     Example Embodiments 
     Example 1 includes a communication circuit, comprising: a first signal path configured to carry, during a first period, a first information signal having a first frequency; and a first frequency-shifting circuit coupled to the first signal path and configured to: receive a first control signal; shift the first frequency of the first information signal by a second frequency in response to the first control signal having a first control value; and shift a third frequency of a first interference signal on the first signal path during a second period by a fourth frequency in response to the first control signal having a second control value. 
     Example 2 includes the communication circuit of Example 1, wherein: the first signal path includes a downlink signal path; and the first period includes a transmit period. 
     Example 3 includes the communication circuit of any of Examples 1-2, wherein: the first signal path includes a transmit signal path; and the first period includes a transmit period. 
     Example 4 includes the communication circuit of any of Examples 1-3, wherein the first frequency-shifting circuit includes a signal mixer that forms part of the first signal path. 
     Example 5 includes the communication circuit of any of Examples 1-4, wherein the first frequency-shifting circuit includes: a signal mixer; and an oscillator configured to: receive the control signal, provide, to the mixer, a mixing signal having the second frequency in response to the control signal having the first control value, and provide, to the mixer, the mixing signal having the fourth frequency in response to the control signal having the second control value. 
     Example 6 includes the communication circuit of any of Examples 1-5, wherein the first frequency-shifting circuit includes: a signal mixer; a first oscillator configured to: receive the control signal, and provide, to the mixer, a mixing signal having the second frequency in response to the control signal having the first control value; and a second oscillator configured to: receive the control signal, and provide, to the mixer, a mixing signal having the fourth frequency in response to the control signal having the second control value. 
     Example 7 includes the communication circuit of any of Examples 1-6, wherein: the first frequency-shifting circuit includes an output node configured to carry the frequency-shifted first information signal during the first period and the frequency-shifted interference signal during the second period; wherein the first signal path includes: a bandpass filter having an input node coupled to the output node of the first frequency-shifting circuit and having an output node; an amplifier having an input node coupled to the output node of the bandpass filter and having an output node; and an antenna coupled to the output node of the amplifier. 
     Example 8 includes the communication circuit of any of Examples 1-7, further comprising: a second signal path configured to carry, during the second period, a second information signal having the first frequency; and an isolation circuit coupled between the first and second signal paths. 
     Example 9 includes the communication circuit of any of Examples 1-8, further comprising: a second signal path configured to carry, during the second period, a second information signal having the first frequency; and a second frequency-shifting circuit coupled to the second signal path and configured to: receive a second control signal; shift the first frequency of the second information signal by a fifth frequency in response to the second control signal having a third control value; and shift a sixth frequency of a second interference signal on the second signal path during the first period by a sixth frequency in response to the second control signal having a fourth control value. 
     Example 10 includes the communication circuit of Example 9, wherein the second frequency-shifting circuit includes: a signal mixer; a first oscillator configured to: receive the second control signal, and provide, to the mixer, a mixing signal having the fifth frequency in response to the control signal having the third control value; and a second oscillator configured to: receive the second control signal, and provide, to the mixer, a mixing signal having the sixth frequency in response to the second control signal having the fourth control value. 
     Example 11 includes the communication circuit of any of Examples 1-10, further comprising: a second signal path configured to carry, during the second period, a second information signal having the first frequency; and a second frequency-shifting circuit coupled to the second signal path and configured to: receive a second control signal; shift the first frequency of the second information signal by a fifth frequency in response to the second control signal having a third control value; and shift a sixth frequency of a signal on the second signal path during the first period by a sixth frequency in response to the second control signal having a fourth control value to increase stability of a loop formed by the first and second signal paths. 
     Example 12 includes the communication circuit of any of Examples 1-11, wherein: the first signal path includes: an antenna configured to generate the first information signal during the first period, and an amplifier having an output node coupled to the antenna and having an input node; wherein the first frequency-shifting circuit includes an output node coupled to the input node of the amplifier and the output node configured to provide the frequency-shifted first information signal during the first period and the frequency-shifted interference signal during the second period. 
     Example 13 includes the communication circuit of any of Examples 1-12, further comprising: a second signal path configured to carry, during the second period, a second information signal having the first frequency and a leakage signal from the first signal path; and a compensation circuit coupled to the first signal path and configured to reduce a magnitude of the leakage signal. 
     Example 14 includes the communication circuit of any of Examples 1-13, wherein: the first frequency-shifting circuit is further configured to: convert the first information signal into an optical first information signal; shift the first frequency of the optical first information signal by a second optical frequency in response to the first control signal having the first control value; and shift the third frequency of an optical first interference signal on the first signal path during the second period by a fourth optical frequency in response to the first control signal having the second control value. 
     Example 15 includes the communication circuit of any of Examples 1-14, further comprising: a second signal path configured to carry, during the second period, a second information signal having the first frequency; and a second frequency-shifting circuit coupled to the second signal path and configured to: receive a second control signal; convert the second information signal into an optical second information signal; shift the first frequency of the optical second information signal by a fifth frequency in response to the second control signal having a third control value; and shift a sixth frequency of a second interference signal on the second signal path during the first period by a sixth frequency in response to the second signal having a fourth control value. 
     Example 16 includes the communication circuit of any of Examples 1-15, wherein the first interference signal includes a first noise signal. 
     Example 17 includes a remote antenna unit, comprising: a first signal path configured to carry, during a first period, a first information signal having a first frequency; and a first frequency-shifting circuit coupled to the first signal path and configured to: receive a first control signal; shift the first frequency of the first information signal by a second frequency in response to the first control signal having a first control value; and shift a third frequency of a first interference signal on the first signal path during a second period by a fourth frequency in response to the first control signal having a second control value. 
     Example 18 includes a distributed antenna system, comprising: a master unit; and at least one remote antenna unit each coupled to the master unit and including: a first signal path configured to carry, during a first period, a first information signal having a first frequency, and a first frequency-shifting circuit coupled to the first signal path and configured to: receive a first control signal; shift the first frequency of the first information signal by a second frequency in response to the first control signal having a first control value; and shift a third frequency of a first interference signal on the first signal path during a second period by a fourth frequency in response to the first control signal having a second control value. 
     Example 19 includes the distributed antenna system of Example 18, further comprising an intermediate unit coupled between the master unit and at least one of the at least one remote antenna unit. 
     Example 20 includes a communication system, comprising: a distributed antenna system, including: a master unit; at least one remote antenna unit each coupled to the master unit and including: a first signal path configured to carry, during a first period, a first information signal having a first frequency; and a first frequency-shifting circuit coupled to the first signal path and configured to: receive a first control signal; shift the first frequency of the first information signal by a second frequency in response to the first control signal having a first control value; and shift a third frequency of a first interference signal on the first signal path during a second period by a fourth frequency in response to the first control signal having a second control value; and at least one base station coupled to the distributed antenna system. 
     Example 21 includes a method, comprising: shifting a frequency of a signal on a signal path by a first amount; and in absence of the signal on the signal path, shifting a frequency of another signal on the signal path by a second amount. 
     Example 22 includes the method of Example 21, wherein: the signal includes a downlink signal; and the signal path includes a downlink transmitting path. 
     Example 23 includes the method of any of Examples 21-22, wherein: the signal includes an uplink signal; and the signal path includes an uplink receiving path. 
     Example 24 includes the method of any of Examples 21-23, wherein: the signal includes a transmit signal; and the signal path includes a transmitting path. 
     Example 25 includes the method of any of Examples 21-24, wherein: the signal includes a receive signal; and the signal path includes a receiving path. 
     Example 26 includes the method of any of Examples 21-25, wherein the other signal includes an interference signal. 
     Example 27 includes the method of any of Examples 21-26, wherein the other signal includes a noise signal. 
     Example 28 includes the method of any of Examples 21-27, wherein the other signal includes a downlink signal generated by another signal path. 
     Example 29 includes the method of any of Examples 21-28, wherein the other signal includes a downlink signal generated by another signal path that is part of a same communication circuit as the signal path. 
     Example 30 includes the method of any of Examples 21-29, wherein the other signal includes a transmit signal generated by another signal path. 
     Example 31 includes the method of any of Examples 21-30, wherein the other signal includes a transmit signal generated by another signal path that is part of a same communication circuit as the signal path. 
     Example 32 includes a tangible, non-transitory readable medium storing instructions that, when executed by one or more computing circuits, cause at least one of the one or more computing circuits, or another circuit controlled by at least one of the one or more computing circuits: to shift a frequency of a signal on a signal path by a first amount; and in response to an absence of the signal on the signal path, to shift a frequency of another signal on the signal path by a second amount. 
     A number of embodiments 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.