Patent Publication Number: US-10317634-B2

Title: Capacitive-loaded jumper cables, shunt capacitance units and related methods for enhanced power delivery to remote radio heads

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 120 as a continuation of U.S. patent application Ser. No. 15/671,226, filed Aug. 8, 2017, which is a continuation application of U.S. patent application Ser. No. 14/619,211, filed Feb. 11, 2015, which in turn claims priority under 35 U.S.C. § 120 as a continuation-in-part of U.S. patent application Ser. No. 14/487,329, filed Sep. 16, 2014, which in turn claims priority from U.S. Provisional Patent Application Ser. No. 61/878,821, filed Sep. 17, 2013, the disclosure of each of which is hereby incorporated herein by reference as if set forth in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to remote radio heads, and more particularly to delivering power to remote radio heads at the top of antenna towers and/or in other locations that are remote from a power supply. 
     BACKGROUND 
     Cellular base stations typically include, among other things, a radio, a baseband unit, and one or more antennas. The radio receives digital information and control signals from the baseband unit and modulates this information into a radio frequency (“RF”) signal that is then transmitted through the antennas. The radio also receives RF signals from the antenna and demodulates these signals and supplies them to the baseband unit. The baseband unit processes demodulated signals received from the radio into a format suitable for transmission over a backhaul communications system. The baseband unit also processes signals received from the backhaul communications system and supplies the processed signals to the radio. A power supply is provided that generates suitable direct current (“DC”) power signals for powering the baseband unit and the radio. The radio is often powered by a (nominal) −48 Volt DC power supply. 
     In order to increase coverage and signal quality, the antennas in many cellular base stations are located at the top of a tower, which may be, for example, about fifty to two hundred feet tall. In early cellular systems, the power supply, baseband unit and radio were all located in an equipment enclosure at the bottom of the tower to provide easy access for maintenance, repair and/or later upgrades to the equipment. Coaxial cable(s) were routed from the equipment enclosure to the top of the tower that carried signal transmissions between the radio and the antennas. However, in recent years, a shift has occurred and the radio is now more typically located at the top of the antenna tower and referred to as a remote radio head (“RRH”). Using remote radio heads may significantly improve the quality of the cellular data signals that are transmitted and received by the cellular base station, as the use of remote radio heads may reduce signal transmission losses and noise. In particular, as the coaxial cable runs up the tower may be 100-200 feet or more, the signal loss that occurs in transmitting signals at cellular frequencies (e.g., 1.8 GHz, 3.0 GHz, etc.) over the coaxial cable may be significant. Because of this loss in signal power, the signal-to-noise ratio of the RF signals may be degraded in systems that locate the radio at the bottom of the tower as compared to cellular base stations where remote radio heads are located at the top of the tower next to the antennas (note that signal losses in the cabling connection between the baseband unit at the bottom of the tower and the remote radio head at the top of the tower may be much smaller, as these signals are transmitted at baseband frequencies or as optical signals on a fiber optic cable and then converted to RF frequencies at the top of the tower). 
       FIG. 1  schematically illustrates a conventional cellular base station  10  in which the radios are implemented as remote radio heads. As shown in  FIG. 1 , the cellular base station  10  includes an equipment enclosure  20  and a tower  30 . The equipment enclosure  20  is typically located at the base of the tower  30 , and a baseband unit  22  and a power supply  26  are located within the equipment enclosure  20 . The baseband unit  22  may be in communication with a backhaul communications system  28 . A plurality of remote radio heads  24  and a plurality of antennas  32  (e.g., three sectorized antennas  32 ) are located at the top of the tower  30 . While the use of tower-mounted remote radio heads  24  may improve signal quality, it also requires that DC power be delivered to the top of the tower  30  to power the remote radio heads  24 . 
     A fiber optic cable  38  connects the baseband unit  22  to the remote radio heads  24 , as fiber optic links may provide greater bandwidth and lower loss transmissions. A power cable  36  is also provided for delivering the DC power signal up the tower  30  to the remote radio heads  24 . The power cable  36  may include a first insulated power supply conductor and a second insulated return conductor. The fiber optic cable  38  and the power cable  36  may be provided together in a hybrid power/fiber optic cable  40  (such hybrid cables that carry power and data signals up an antenna tower are commonly referred to as “trunk” cables). The trunk cable  40  may include a plurality of individual power cables that each power a respective one of the remote radio heads  24  at the top of the antenna tower  30 . The trunk cable  40  may include a breakout enclosure  42  at one end thereof (the end at the top of the tower  30 ). Individual optical fibers from the fiber optic cable  38  and individual conductors of the power cable  36  are separated out in the breakout enclosure  42  and connected to the remote radio heads  24  via respective breakout cords  44  (which may or may not be integral with the trunk cable  40 ) that run between the remote radio heads  24  and the breakout enclosure  42 . Stand-alone breakout cords  44  are typically referred to as “jumper cables” or “jumpers.” Coaxial cables  46  are used to connect each remote radio head  24  to a respective one of the antennas  32 . 
     The DC voltage of a power signal that is supplied to a remote radio head  24  from the power supply  26  over a power cable  36  and breakout cord  44  may be determined as follows:
 
 V   RRH   =V   PS   −V   Drop   (1)
 
where V RRH  is the DC voltage of the power signal that is delivered to the remote radio head  24 , V PS is the DC voltage of the power signal that is output by the power supply  26 , and V Drop  is the decrease in the DC voltage that occurs as the DC power signal traverses the power cable  36  and breakout cord  44  that connect the power supply  26  to the remote radio head  24 . V Drop  may be determined according to Ohm&#39;s Law as follows:
 
 V   Drop   =I   RRH   *R   Cable   (2)
 
where R Cable  is the cumulative electrical resistance (in Ohms) along the power supply and the return conductors of the power cable  36  and breakout cord  44  that connect the power supply  26  to the remote radio head  24 , and I RRH  is the average current (in Amperes) flowing through the power cable  36  and breakout cord  44  to the remote radio head  24 . As is readily apparent from Equation 2, as the current I RRH  drawn by a remote radio head  24  increases, the voltage drop V Drop  along the power cable  36  will increase as well. The voltage drop V Drop  of Equation 2 is also referred to herein as the I*R voltage drop.
 
     The power cables  36  and breakout cords  44  employed in cellular base stations typically use copper power supply and return conductors (or alloys thereof) that have physical properties which are familiar to those skilled in the art. One important property of these conductors is their electrical resistance. The electrical resistance of a conductor of the power cable  36  (or breakout cord  44 ) is inversely proportional to the diameter of the conductor (assuming a conductor having a circular cross-section). Thus, the larger the diameter of the conductors (i.e., the lower the gauge of the conductor), the lower the resistance of the power cable  36 . Copper resistance is specified in terms of unit length, typically milliohms (mΩ)/ft; as such, the cumulative electrical resistance R Cable  of the power cable  36  and the breakout cord  44  increases with the lengths of the cable  36  and the breakout cord  44 . Typically, the breakout cords  44  are much shorter than the power cables  36 , and hence the power cable  36  is the primary contributor to the cumulative resistance. Thus, the longer the power cable  36 , the higher the voltage drop V Drop . This effect is well understand and is typically accounted for by engineering and the system architects. 
     Remote radio heads  24  are typically designed to operate properly if supplied with a DC power signal having a voltage within a specified range. Conventionally, the power supply  26  at the base of the tower  30  will be set to output a DC power signal having a fixed voltage V PS . As V Drop  is a function of the current I RRH  that is supplied to the remote radio head  24  (see Equation 2 above), the voltage V RRH  of the power signal that is delivered to the remote radio head  24  will change with variation in the current I RRH  drawn by the remote radio head  24  due to variation in the voltage drop V Drop . If V Drop  becomes too large, then the voltage of the power signal that is supplied to the remote radio head  24  may fall below the minimum voltage that is necessary to properly power the remote radio head  24 . 
     SUMMARY 
     Some embodiments of the present invention are directed to jumper cables for a cellular base station. These jumper cables include a cable segment that has a power supply conductor and a return conductor that are enclosed within a cable jacket and electrically insulated from each other, and first and second connectors that are terminated onto opposing first and second ends of the cable segment. These jumper cables further include a shunt capacitance unit that is connected between the power supply conductor and the return conductor, the shunt capacitance unit including at least one capacitor that is coupled between the power supply conductor and the return conductor. 
     In some embodiments, wherein the at least one capacitor may be a non-polar electrolytic capacitor or at least two polar electrolytic capacitors. The at least one capacitor may have a capacitance of at least 400 microfarads. The shunt capacitance unit may be configured to reduce a voltage drop at the remote radio head due to a spike in a direct current power supply signal carried by the jumper cable. 
     In some embodiments, the shunt capacitance unit may include a housing that has first and second apertures that the cable segment extends through, and the at least one capacitor may be mounted within the housing. In such embodiments, the housing may be filled with epoxy that is configured to provide environmental protection to the at least one capacitor and the power supply and return conductors. In other embodiments, the shunt capacitance unit may be included in at least one of the first connector and the second connector. In still other embodiments, the shunt capacitance unit may be enclosed within the cable jacket. 
     In some embodiments, the jumper cable may also include a fuse circuit that is coupled in series with the at least one capacitor between the power supply conductor and the return conductor. The jumper cable may also include at least one optical fiber within the jacket. The jumper cable may be used at a cellular base station to connect a breakout enclosure that terminates a trunk cable that is routed up an antenna tower to a remote radio head. 
     Pursuant to further embodiments of the present invention, methods of operating a cellular base station are provided in which a direct current (“DC”) power signal is output from a power supply, and this DC power signal is supplied to a remote radio head that is mounted remotely from the power supply over a trunk cable and a jumper cable that are connected in series, the jumper cable including a power supply conductor, a return conductor and a shunt capacitance unit that is coupled between the power supply conductor and the return conductor. A voltage level of the DC power signal that is output from the power supply is adjusted so that the DC power signal that is delivered to the remote radio head has a substantially constant voltage notwithstanding variation in a current level of the DC power signal that is output from the power supply 
     In some embodiments, the power supply may be a programmable power supply, and information may be input to the power supply from which the voltage level of the DC power signal that is output from the power supply can be determined that will provide the DC power signal at the first end of the power cable that has the substantially constant voltage. The method may further include measuring the current level of the DC power signal that is output from the power supply, where the voltage level of the DC power signal that is output by the power supply is automatically adjusted in response to changes in the measured current level of the DC power signal that is output from the power supply to provide the DC power signal at the first end of the power cable that has the substantially constant voltage. The method may also include determining a resistance or an impedance of the power cabling connection between the power supply and the shunt capacitance unit by transmitting an alternating current signal over the power cabling connection and through the shunt capacitance unit. The substantially constant voltage may be a voltage that exceeds a nominal power signal voltage of the remote radio head and which is less than a maximum power signal voltage of the remote radio head. 
     Pursuant to still further embodiments of the present invention, shunt capacitance units for cellular base stations are provided that include a housing, a first connector coupled to the housing, the first connector including a first power supply conductor and a first return conductor, and a second connector coupled to the housing, the second connector including a second power supply conductor that is electrically connected to the first power supply conductor and a second return conductor that is electrically connected to the first return conductor. These shunt capacitance units also include at least one capacitor electrically coupled between the first power supply conductor and the first return conductor 
     In some embodiments, the shunt capacitance unit may further include a fuse circuit coupled in series with the at least one capacitor between the first power supply conductor and the first return conductor. The at least one capacitor may be a non-polar electrolytic capacitor or at least two polar electrolytic capacitors. The at least one capacitor may have a capacitance of at least 400 microfarads. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a simplified, schematic view of a conventional cellular base station in which several remote radio heads are located at the top of an antenna tower. 
         FIGS. 2A and 2B  are graphs illustrating the DC voltage and current, respectively, of a DC power signal as function of time at a remote radio head under steady state conditions. 
         FIG. 3A  is a graph illustrating the current of a DC power signal as function of time at a remote radio head during a current spike event.  FIG. 3B  is a graph illustrating how the DC voltage of the power signal varies as function of time at the remote radio head due to the I*R voltage drop in response to the current spike of  FIG. 3A .  FIG. 3C  is a graph illustrating the DC voltage of the DC power signal as function of time at the remote radio head in response to the current spike of  FIG. 3A  when both the I*R and the dI/dt voltage drops are taken into account. 
         FIG. 4  is a graph illustrating the DC voltage of the DC power signal as function of time at a remote radio head in response to the current spike of  FIG. 3A  when both the I*R and the dI/dt voltage drops are taken into account when a shunted capacitance is used to dampen the effect of the dI/dt drop. 
         FIGS. 5A through 5C  are circuit diagrams that illustrate example locations where a shunt capacitance unit may be place along a power cable that delivers a DC power signal up an antenna tower to a remote radio head. 
         FIGS. 6A and 6B  are schematic diagrams of cellular base stations that illustrate example locations where shunt capacitance unit may be located at the top of the antenna tower. 
         FIG. 7  is a schematic diagram illustrating a cellular base station according to further embodiments of the present invention that uses a power cable having shunt capacitance units built into the power cable. 
         FIG. 8  is a perspective view of an end portion of a hybrid power/fiber optic cable that may be used to implement the trunk cable of  FIG. 7 . 
         FIG. 9  is a schematic block diagram of a cellular base station according to further embodiments of the present invention. 
         FIG. 10  is a schematic drawing illustrating how a jumper cable may be used to connect a junction enclosure such as a breakout box of a trunk cable or a stand-alone enclosure to a remote radio head. 
         FIGS. 11A and 11B  are a partially-exploded perspective view and a perspective view, respectively, of a shunt capacitance unit according to certain embodiments of the present invention that may be included in the jumper cable of  FIG. 10 . 
         FIG. 11C  is a circuit diagram illustrating how the shunt capacitance unit of the jumper cable of  FIGS. 11A-B  is electrically connected between the power supply and return conductors of the jumper cable. 
         FIGS. 12A and 12B  are side views of capacitive-loaded jumper cables according to further embodiments of the present invention. 
         FIG. 13  is a schematic view of an inline connector that includes a shunt capacitance unit according to certain embodiments of the present invention. 
         FIG. 14  is a block diagram of portions of a cellular base station according to embodiments of the present invention that includes a capacitive-loaded jumper cable that is used to reduce voltage drop and to measure the resistance of the power cabling connection in order to set the output of a variable power supply that powers a remote radio head. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed above, when remote radio heads are used in a cellular base station, a voltage drop occurs along the power cables that connect a power supply at the base of the antenna tower to the remote radio heads at the top of the antenna tower. This voltage drop may cause several problems, as explained below. 
     First, as the current drawn by one of the remote radio heads increases, the voltage drop V Drop  on the individual power cable(s) that connect the power supply to the remote radio head likewise increases. Consequently, the voltage of the power signal that is supplied to the remote radio head may, if the voltage drop becomes too large, fall below the minimum voltage that is necessary to properly power the remote radio head. Thus, for a power cable having copper conductors of a fixed size, the voltage drop V Drop  may effectively limit the length of the power cable that may be used. While this limitation on the length of the power cable may be overcome by using larger conductors in the power cable, the use of larger conductors results in increased material and installation costs, increased loading on the tower and various other disadvantages. 
     Second, the voltage drop along the power cable also increases the cost of running the remote radio head, as power is lost when delivering the power signal to the remote radio head, and the amount of power lost is a function of the current running through the power cable. In particular, the power that is lost (P Loss ) in delivering the power signal to the remote radio head over a power cable may be calculated as follows:
 
 P   Loss   =V   Drop   *I   RRH   (3)
 
where V Drop  =the average voltage drop in Volts along the power cable. Since antenna towers for cellular base stations may be hundreds of feet tall and the voltage and currents required to power each remote radio head may be quite high (e.g., about 50 Volts at about 20 Amperes of current), the power loss that may occur along the hundreds of feet of cabling may be significant.
 
     Third, another physical property of the power cable that can result in a voltage drop is the inductance per unit length of the conductors of the cable. In particular, the cumulative inductance of the conductors of the power cable can produce a voltage drop that is expressed as:
 
 V   dI/dt Drop   =L* ( dI/dt )  (4)
 
where L is the cumulative inductance of the conductors and dI/dt is the rate of change in current flowing through the conductors with respect to time. Herein, V dI/dt Drop  is referred to as the “dI/dt voltage drop.” Accordingly, not only is the voltage drop impacted by changes in current (see Equation 2), it is also impacted by how quickly the current changes (see Equation 4). Examples of scenarios where the current drawn by a remote radio head may change quickly (a “current spike”) such that the DI/dt voltage drop can impact performance are (1) when multiple handsets connect and demand high speed data simultaneously and (2) when the remote radio head is turned off or on, or from idle to operational. While this voltage drop typically only lasts for a period on the order of about 1-20 milliseconds, this period is long enough such that rapid current spikes may lead to large V dI/dt Drop  values (e.g., as much as 5 Volts) which have the potential to impact the performance of a remote radio head.
 
     Pursuant to embodiments of the present invention, various methods are provided that may reduce the impact of the above-described voltage drops. These techniques may be used individually or together to improve the performance of cellular base stations that use remote radio heads that are mounted atop antenna towers. It will also be appreciated that cellular base stations exist where the remote radio heads and antennas are mounted in locations remote from the baseband equipment and power supply other than towers such as, for example, remote radio heads and antennas that are mounted on rooftops, atop utility poles, in subway tunnels and the like. It will be appreciated that the techniques described herein are equally applicable to these “non-tower” remote locations for the remote radio heads. Thus, while embodiments of the present invention are described below with reference to tower-mounted remote radio heads, it will be appreciated that all of the embodiments described below may be implemented in cellular base stations that place the remote radio heads in other locations such as on rooftops, atop utility poles and in tunnels or other locations that are remote from the power supply and baseband equipment. 
     For example, in some embodiments, a shunt capacitance unit such as, for example, a capacitor, may be provided between the two conductors of a power cable that is used to provide a DC power signal to a remote radio head. This shunt capacitance unit may reduce the dI/dt voltage drop that would otherwise occur in response to current spikes. In some embodiments, these shunt capacitance units may be integrated into a power cable or a trunk cable that includes a plurality of individual power cables that are used to provide power to a plurality of remote radio heads on an antenna tower. In other embodiments, the shunt capacitance unit may be incorporated into jumper cables that connect remote radio heads to a junction enclosure or to a breakout enclosure of a power cable. In still other embodiments, the shunt capacitance unit may be incorporated into an inline connector that may be connected to, for example, a jumper cable. The shunt capacitance units may, in some embodiments, be sized based on, for example, the length of the power cable and the resistance per unit length of the power supply and return conductors included in the power cable. 
     In some embodiments, the shunt capacitance units may be used in conjunction with a programmable power supply that is configured to (1) sense the current being drawn by the remote radio head (or another suitable parameter) and (2) adjust the voltage of the power signal that is output by the power supply to substantially maintain the voltage of the power signal that is supplied to the remote radio head at or near a desired value. This desired voltage value may be, for example, a value that is close to the maximum voltage for the power signal that may be input to the remote radio head. In some embodiments, the programmable power supply may set the voltage of the DC power signal that is output by the power supply based on the resistance of the power cable and the current of the power signal that is output from the power supply so that the voltage of the power signal at the top of the tower will be substantially maintained at a desired level. The resistance of the power cable may, for example, be input to the power supply, calculated based on information that is input to the power supply, or measured. As the current drawn by the remote radio head varies, the programmable power supply may adjust the voltage of its output power signal to a voltage level that will deliver a power signal to the remote radio head that is at or near the maximum voltage for a power signal that may be input to the remote radio head. As shown by Equation (5) below, which expands Equation (3), the power loss varies as the square of the current drawn by the remote radio head. By increasing the voltage of the signal that is delivered to the remote radio head, the current I RRH of the power signal is correspondingly reduced, thereby reducing the power loss. As a typical remote radio head may require about a kilowatt of power and may run 24 hours a day, seven days a week, and as a large number of remote radio heads may be provided at each cellular base station (e.g., three to twelve), the power savings may be significant.
 
 P   Loss   =V   Drop   *I   RRH =( I   RRH   *R   Cable )* I   RRH   =I   RRH   2   *R   Cable   (5)
 
     While the above-discussed programmable power supply will alter the voltage of the power signal that it outputs in response to the current drawn by the remote radio head, the change in the voltage of the output power signal may lag behind the change in current. Thus, even when programmable power supplies are used, dI/dt losses may still arise that may degrade the performance of the remote radio head. Thus, in some embodiments, both programmable power supplies and shunt capacitance units may be used to counter the negative effects of both I*R and dI/dt voltage drops. 
     Using a capacitor to help maintain the voltage level of a signal during a spike in current is known in the art. Moreover, commercially available remote radio heads may include an internal bulk capacitance across the input terminals for the power supply leads that is used to reduce ripple and noise on the DC power signal line. However, it is not believed that appreciation of the impact of dI/dt voltage drops on remote radio head performance is well understood, nor is it believed to be understood the benefits that may accrue by providing, for example, a capability to provide a variable amount of shunt capacitance between the leads of a power cable for a remote radio head where the shunt capacitance unit may be sized based on the length of the power cable, the cumulative resistance of the conductors of the power cable and/or various other factors such that the shunt capacitance unit may be designed to overcome the problem of dI/dt voltage drops. According to some embodiments, the shunt capacitance units may be integrated directly into the power cable, into a jumper cable, or into an inline connector that is connected to the jumper cable. 
     Embodiments of the present invention will now be discussed in more detail with reference to  FIGS. 2-14 , in which example embodiments of the present invention are shown. 
     As noted above, pursuant to embodiments of the present invention, power cables, jumper cables and inline connectors having relatively large shunt capacitance units may be provided that may be used to maintain the voltage of the power signal delivered to a remote radio head at or above a desired minimum level during current spikes. The effect of the inclusion of a large shunt capacitance on the voltage of the power signal delivered to the remote radio head is illustrated in  FIGS. 2-4 . 
       FIGS. 2A and 2B  illustrate the power signal received at a remote radio head under steady state conditions. In particular,  FIG. 2A  is a graph plotting the DC voltage of the power signal (V RRH ) delivered to a remote radio head as function of time when the remote radio head is operating under steady state conditions, and  FIG. 2B  is a graph plotting the current of the power signal (I RRH ) drawn by the remote radio head as function of time during such steady state conditions. As shown in  FIGS. 2A-2B , under such steady state conditions, the voltage V RRH  and the current I RRH  may remain constant. 
       FIGS. 3A-3C  illustrate how the voltage and current of the power signal at the remote radio head change in response to a current spike. In particular,  FIG. 3A  illustrates a current spike which may occur in the power signal when the current requirements of the remote radio head are increased. As shown in  FIG. 3A , the current spike may be approximated as a step function. Such a current spike may result, for example, if multiple carriers key up transmission simultaneously. Assuming that the power supply is outputting a power signal V PS  having a constant voltage,  FIG. 3B  shows the effect that the increased current draw will have on the voltage of the power signal V RRH  at the remote radio head. Specifically, as shown in  FIG. 3B , the increased current draw will result in a decrease in the voltage of the power signal V RRH  at the remote radio head based on Ohm&#39;s law. The graph of  FIG. 3C  illustrates how the dI/dt drop may further impact the voltage of the power signal V RRH  supplied to the remote radio head. As shown in  FIG. 3C , the dI/dt voltage drop may result in a temporary deep decrease in the voltage V RRH  that gradually recovers to the new steady state voltage that applies. The broken line in  FIG. 3C  indicates the voltage level where the power signal may be inadequate to properly power the remote radio head. As shown in  FIG. 3C , the dI/dt voltage drop may be sufficient in some cases to cause a remote radio head to temporarily malfunction due to an insufficient voltage level for the power signal. 
       FIG. 4  is a graph illustrating how a shunt capacitance unit may be used to dampen the effect of the dI/dt voltage drop that is shown in  FIG. 3C . As shown in  FIG. 4 , the shunt capacitance unit may dampen the effect of the current spike on the voltage V RRH . The voltage spike of  FIG. 3C  is largely dissipated by the shunt capacitance unit, such that the voltage level V RRH  does not drop below the broken line that indicates operational problems with the remote radio head. The shunt capacitance unit effectively acts as an auxiliary power supply that helps maintain the voltage by discharging the stored charges during the current spike event. Once steady state conditions are reached, the shunt capacitance unit may recharge to be available to dampen the effect of the next current spike. By including the shunt capacitance unit, undesirable events (e.g., the remote radio head shutting down) that the unwanted voltage spike might otherwise cause can be reduced or prevented. 
       FIGS. 5A-5C  are circuit diagrams that illustrate example locations where a shunt capacitance unit  48  may be placed along a power cable  36  that delivers a power signal up an antenna tower  30  to a remote radio head  24 . In each of  FIGS. 5A-5C , a power supply  26  is connected via a power cable  36  to a remote radio head  24  that is mounted on an antenna tower  30 . As shown in each of  FIGS. 5A-5C , the power cable  36  for each remote radio head  24  may comprise a power supply conductor  36 - 1  and a return conductor  36 - 2 . The conductors  36 - 1 ,  36 - 2  may each be modelled as a plurality of inductors that are disposed in series. In the embodiment of  FIG. 5A , the shunt capacitance unit  48  is inserted between the power supply conductor  36 - 1  and the return conductor  36 - 2  near the power supply  26  (i.e., at the base of the tower  30 ). As an alternative,  FIG. 5B  shows that shunt capacitance unit  48  can be inserted as a series of shunt capacitors  48  that are interposed at different points along the power supply conductor  36 - 1  and the return conductor  36 - 2 .  FIG. 5C  shows that shunt capacitance unit  48  can be inserted between the power supply conductor  36 - 1  and the return conductor  36 - 2  near the remote radio head  24  at or near the top of the tower  30 . While  FIGS. 5A-5C  show a single power cable  36  connecting the power supply  26  to the remote radio head  24  to provide a simplified example, it will be appreciated that more typically the end of the power cable  36  at the top of the antenna tower  30  terminates into a junction enclosure or includes an integrated breakout enclosure, and jumper cables are connected between this enclosure and each remote radio head  24  that carry power and data signals between the enclosure and each remote radio head  24 . 
     In some embodiments such as the embodiment shown in  FIG. 5C , the shunt capacitance unit  48  is placed very close to the remote radio head  24 .  FIGS. 6A and 6B  identify two locations near a remote radio head  24  which may be considered both accessible and functional as locations for the shunt capacitance unit  48 . As shown in  FIG. 6A , the tower  30  may include a junction enclosure  50  (which is sometimes referred to as a “breakout box”) that is positioned adjacent a remote radio head  24  near the top of the tower  30 . The junction enclosure  50  typically includes a bus bar, fiber breakout units, and the like, and has externally accessible connectors. The remote radio head  24  is connected to the breakout box  50  via a breakout cord  44 . A power supply  26  is connected to the breakout box  50  via the power cable  36  (which may be a hybrid power/fiber optic trunk cable  40 ). As shown in  FIG. 6A , one exemplary location for the shunt capacitance unit  48  is inside or at the breakout box  50 , which would be located at a relatively long distance up the tower  100 . In  FIG. 6B , a cellular base station having the same components as the cellular base station of  FIG. 6A  is shown, with the exception that the shunt capacitance unit  48  is connected at the input of the remote radio head  24 . In the embodiment of  FIG. 6B , shunt capacitance unit  48  may, for example, be included in an inline connector module.  FIG. 13  illustrates such an inline connector module according to embodiments of the present invention. The example locations for the shunt capacitance units  48  illustrated in  FIGS. 6A and 6B  are relatively accessible to technicians. 
       FIG. 7  is a schematic block diagram illustrating a cellular base station  100  according to further embodiments of the present invention that includes shunt capacitance units in a power cable  136  that supplies DC power signals from a power supply  26  to a plurality of remote radio heads  24 . In the example of  FIG. 7 , a total of three remote radio heads  24  are mounted on an antenna tower  30 . The power cable  136  includes three pairs of insulated copper conductors  136 A- 136 C (i.e., three individual power cables  136 A- 136 C are included in the composite power cable  136 ) that are used to deliver the DC power signals from the power supply  26  to the respective remote radio heads  24 . Each pair of insulated copper conductors  136 A- 136 C includes a power supply conductor  136 - 1  and a return conductor  136 - 2 . The power supply  26  includes three outputs  27 A- 27 C. One end of each of the individual power cables  136 A- 136 C is connected to a respective one of the outputs  27 A- 27 C on power supply  26 , while the other end of each of the individual power cables  136 A- 136 C is connected to a respective one of the remote radio heads  24 . The three outputs  27 A- 27 C on the power supply  26  are independent from each other and may each deliver a power signal that meets the power needs of a respective one of the remote radio heads  24 . Thus, through the outputs  27 , the power supply  26  may provide three independent power signals having voltage and current characteristics that are suitable for meeting the instantaneous power requirements of each of the remote radio heads  24 . 
       FIG. 8  is a schematic diagram illustrating a trunk cable assembly  200  that may used, for example, to implement the power cable  136  (as well as the fiber optic cable  38 ) of  FIG. 7 . The trunk cable assembly  200  of  FIG. 8  comprises a hybrid power/fiber optic cable  210 , a first breakout canister  230  and a second breakout canister  250 . The hybrid power/fiber optic cable  210  has nine individual power cables  212  (see the callout in  FIG. 8 , which depicts three of the individual power cables  212 ), which may be stranded together to form a composite power cable  218 , and a fiber optic cable  220  that includes thirty-six optical fibers  222 . The fiber optic cable  220  may comprise a jacketed or unjacketed fiber optic cable of any appropriate conventional design. The composite power cable  218  and the fiber optic cable  220  may be enclosed in a jacket  224 . While one example hybrid power/fiber optic cable  210  is shown in  FIG. 8 , it will be appreciated that any conventional hybrid power/fiber optic cable may be used, and that the cable may have more or fewer power cables and/or optical fibers. An exemplary hybrid power/fiber optic cable is the HTC-24SM-1206-618-APV cable, available from CommScope, Inc. (Hickory, N.C.). 
     The first breakout canister  230  comprises a body  232  and a cover  236 . The body  232  includes a hollow stem  234  at one end that receives the hybrid power/fiber optic cable  210 , and a cylindrical receptacle at the opposite end. The cover  236  is mounted on the cylindrical receptacle to form the breakout canister  230  having an open interior. The hybrid power/fiber optic cable  210  enters the body  232  through the stem  234 . The composite power cable  218  is broken out into the nine individual power cables  212  within the first breakout canister  230 . Each individual power cable  212  includes a power supply conductor  214  and a return conductor  216 . The nine individual power cables  212  are routed through respective sockets  238  in the cover  236 , where they are received with respective protective conduits  240  such as a nylon conduit that may be sufficiently hardy to resist damage from birds. Thus, each individual power cable  212  extends from the first breakout canister  230  within a respective protective conduit  240 . The optical fibers  222  are maintained as a single group and are routed through a specific socket  238  on the cover  236 , where they are inserted as a group into a conduit  242 . Thus, the first breakout canister  230  is used to singulated the nine power cables  212  of composite power cable  218  into individual power cables  212  that may be run to respective remote radio heads  24 , while passing all of the optical fibers  222  to a separate breakout canister  250 . 
     As shown in the inset of  FIG. 8 , a plurality of shunt capacitance units in the form of ceramic capacitors  248  are provided within the first breakout canister  230 . Each capacitor  248  is connected between the power supply conductor  214  and the return conductor  216  of a respective one of the individual power cables  212 . The breakout canister  230  may include a plurality of sockets that each receive one of the capacitors  248 . Each individual power cable may be physically and electrically connected to these sockets. For low frequency signals such as a DC power signal, the shunt capacitors  248  appear as an open circuit, and thus the DC power signal that is carried on each individual power cable  212  will pass by the respective shunt capacitor  248  to the remote radio heads  24 . However, as discussed above, during periods where the current carried by an individual power cable  212  spikes in response to an increased current requirement at the remote radio head  24 , the shunt capacitor  248  may act akin to an auxiliary power supply to reduce the magnitude of the dI/dt voltage drop on the DC power signal. 
     As noted above, the optical fibers  222  pass through the first breakout canister  230  as a single unit in conduit  242  which connects to the second breakout canister  250 . In the second breakout canister  250 , the thirty-six optical fibers  222  are separated into nine optical fiber subgroups  252 . The optical fiber subgroups  252  are each protected within a respective conduit  254 . The second breakout canister  250  may be similar to the first breakout canister  230 , and hence will not be discussed in further detail. The second breakout canister  250 , however, does not include the shunt capacitors  248 . 
     As is known to those of skill in the art, commercially available remote radio heads may have capacitances across the leads that receive a power cable that powers the remote radio head. However, this capacitance is typically small and may not be sufficient to dampen the dI/dt voltage drop. By providing power cables such as the hybrid power/fiber optic cable assembly  200  of  FIG. 8  that have shunt capacitors  248  integrated into each individual power cable  212 , it is possible to ensure that a sufficient shunt capacitance is provided in each instance. For example, as discussed above, voltage drop becomes a more significant issue the longer the power cable, as I*R based voltage drops increase linearly with the length of the power cable. Thus, longer power cables are more likely to experience the situation illustrated in  FIG. 3C  above where the combination of the I*R voltage drop and the dI/dt voltage drop may cause the voltage of the power signal to temporarily dip below some minimum required voltage level, thereby disrupting operation of the remote radio head. By implementing the shunt capacitances within the power cables—which may be sold in known lengths—the shunt capacitances may be appropriately pre-sized to provide a sufficient amount of capacitance while not providing excess capacitance that may increase the cost, size and/or weight of the power cable. 
     Additionally, by implementing the shunt capacitances within the power cable and, more particularly, within a breakout enclosure of the power cable, it is possible to have shunt capacitances that may be sized appropriately at the time of installation. For example, in some embodiments, the shunt capacitance may be plug-in or screw-in capacitors that are connected across the conductors of each power cable so that the appropriately-sized capacitor may be installed in the breakout canister based on the specific requirements of the cellular base station. Additionally, since the breakout canisters may be opened up, if necessary, after installation, defective or damaged capacitors may be replaced if needed. 
     Those of skill in this art will appreciate that the shunt capacitances used in embodiments of the present invention may be provided in any number of forms. For example, a shunt capacitance unit may be in the form of individual components, such as one or more capacitors, or in the form of other physical structures such as parallel conductors separated by an air gap that may act like a capacitor. The amount of shunt capacitance provided may vary depending on a number of factors including, for example, the length of the conductors and the diameter of the conductors. Generally speaking, the amount of shunt capacitance may be on the order of hundreds, thousands, tens of thousands, or hundreds of thousands of microfarads in some embodiments. 
     The benefits that may accrue from using shunt capacitances in the manner described herein may include the following. From a system perspective, the conductors of a power cable for a remote radio head need not be oversized or overdesigned to compensate for large dI/dt voltage drops; consequently, longer conductor runs utilizing less conductor material are possible. The use of less conductor material also allows for lighter power cable assemblies, which can also be advantageous because increased current demand at the top of the tower is a rapidly developing issue. Moreover, existing tower architectures may be retrofitted with this approach with minimal impact to mounted hardware should dI/dt voltage drops arise as an issue. In particular, embodiments of this invention can allow for a variable amount of required capacitance based on, for example, conductor length, while also allowing the ability to retrofit an existing tower, power cable, or remote radio head architecture. 
     Pursuant to further embodiments of the present invention, power cables that include shunt capacitances may be used in cellular base stations that employ programmable power supplies that are used to reduce the I*R voltage drop by maintaining the voltages of the power signals at the remote radio heads at or near a maximum voltage for the power signal that is specified for each remote radio head. Examples of such an embodiment will now be described with reference to  FIG. 9 . 
     In particular,  FIG. 9  is a schematic block diagram of a cellular base station  300  according to embodiments of the present invention. As shown in  FIG. 3 , the cellular base station  300  includes an equipment enclosure  20  and a tower  30 . A baseband unit  22 , a first power supply  326  and a second power supply  328  are located within the equipment enclosure  20 . A plurality of remote radio heads  24  and plurality of antennas  32  are mounted on the tower  30 . 
     Each remote radio head  24  receives digital information (data) and control signals from the baseband unit  22  over a fiber optic cable  38 . Typically, the fiber optic cable  38  will include a plurality of optical fibers, with two (or more optical fibers) provided for each remote radio head  24 . Each remote radio head  24  modulates the data signals received over its respective “uplink” optical fiber into a radio frequency (“RF”) signal at the appropriate cellular frequency that is then transmitted through an antennas  32 . Each remote radio head  24  also receives RF signals from an antenna  32 , demodulates these signals, and supplies the demodulated signals to the baseband unit  22  over its respective “downlink” optical fiber that is included in the fiber optic cable  38 . The baseband unit  22  processes the demodulated signals received from the remote radio heads  24  and forwards the processed signals to the backhaul communications system  28 . The baseband unit  22  also processes signals received from the backhaul communications system  28  and supplies these signals to the remote radio heads  24 . Typically, the baseband unit  22  and the remote radio heads  24  each include optical-to-electrical and electrical-to-optical converters that couple the digital information and control signals to and from the fiber optic cable  38 . 
     The first power supply  326  generates direct current (“DC”) power signals. The second power supply  328  is a DC-to-DC converter that accepts the DC power signal output by the first power supply  326  as an input and outputs a DC power signal having a different voltage. A power cable  336  is connected to the output of the second power supply  328  and routed up the tower  30 . The power cable  336  may be a composite power cable that includes multiple individual power cables, namely an individual power cable for each remote radio head  24 . In some embodiments, the fiber optic cable  38  and the power cable  336  may be implemented together as a hybrid power/fiber optic cable  340  which may be implemented, for example, using the hybrid power/fiber optic cable assembly  200  of  FIG. 8 . While the first power supply  326  and the second power supply  328  are illustrated as separate power supply units in the embodiment of  FIG. 9 , it will be appreciated that the two power supplies  326 ,  328  may be combined into a single power supply unit in other embodiments. 
     The power supply  328  is a programmable power supply that receives input DC power signals from power supply  326  and outputs DC power signals to each of the individual power cables within the power cable  336 . The voltage of the DC power signal output by the power supply  328  may vary in response to variations in the current of the DC power signal drawn from the power supply  328  by the remote radio heads  24  that are connected to each individual power cable. The voltage of the DC power signal output by the power supply  328  may be set so that the voltage of the DC power signal at the far end of each individual power cable in power cable  336  (i.e., the end adjacent the remote radio heads  24 ) is maintained at or near the maximum specified voltage for the power signal of the remote radio heads  24 . This may reduce the power loss associated with supplying the DC power signal to the remote radio head  24 , since, for a given power level, a higher voltage for the DC power signal corresponds to a lower current, and lower current values result in reduced power loss. In some embodiments, the programmable power supply  328  may be designed to maintain the voltage of the DC power signal at or near a maximum operating voltage for the power signal that may be supplied to the remote radio head  24 . 
     In some embodiments, the voltage of the DC power signal at the far end of the individual power cables in power cable  336  (i.e., V RRH ) may be maintained at or near a predetermined value by setting the voltage level of the power signal output by power supply  328  based on (1) the current of the DC power signal drawn from the power supply (note from Equations 1 and 2 that V RRH  is a function of I RRH ) and (2) the resistance R Cable  of the power cable. The programmable power supplies according to embodiments of the present invention may be configured to measure, estimate, calculate or receive both values. U.S. patent application Ser. No. 14/321,897 (“the &#39;897 application”), filed Jul. 2, 2014, describes various programmable power supplies that may be used to implement the programmable power supply  328 . The &#39;897 application is incorporated by reference herein in its entirety, and hence further description of regarding implementation of these programmable power supplies entire will be omitted. Note that either the resistance or the impedance of the power cable may be used to set the voltage level of the power signal output by power supply  328 , and references herein to the “resistance” of the power cable are intended to cover both the resistance and/or the impedance. In other embodiments, a feedback loop may be used to control the voltage of the DC power signal output by the DC power supply so that the voltage of the DC power signal at the far end of the power cable that connects the power supply  328  and a remote radio head  24  is maintained at a desired level. The use of such feedback loops is also discussed in the &#39;897 application. 
     The use of such programmable power supplies may both reduce power losses and also reduce the I*R voltage drop, as supplying power signals having voltages that are maintained near a maximum acceptable value to each remote radio head reduces the average current of the power signals, thereby reducing I*R voltage drop. Additionally, the cellular base stations according to embodiments of the present invention may also employ shunt capacitances on each individual power cable in the manner described above with reference to  FIGS. 2-8  to reduce the impact of dI/dt voltage drops. 
     Pursuant to further embodiments of the present invention, the shunt capacitance unit may be incorporated into a jumper cable that connects to a breakout enclosure of a trunk cable or a separate junction enclosure to a remote radio head, or may be included in, for example, an inline connector unit that is directly connected to such jumper cables. Such approaches may have advantages over including the shunt capacitance unit in the trunk cable in certain situations. 
     As discussed above, trunk cables are often used to transmit power from a power supply and data signals from a baseband unit that are located adjacent the bottom of an antenna tower of a cellular base station to a junction enclosure (or other breakout enclosure or canister) that is mounted near the top of the antenna tower. Typically, the trunk cable includes a plurality of pairs of power conductors and a plurality of pairs of optical fibers, where each pair of power conductors is provided to deliver a power signal to a respective one of the remote radio heads mounted at the top of the antenna tower, and each pair of optical fibers is provided to carry the uplink and downlink traffic to a respective one of the remote radio heads. These pairs of power conductors and optical fibers are terminated into connectors that are provided in the junction enclosure. Individual jumper cables may be connected between the respective connectors of the junction enclosure and respective remote radio heads in order to complete the connections between each remote radio head and the power supply and baseband equipment at the base of the antenna tower. In some cases, separate power jumper cables and fiber optic jumper cables are provided, while in other cases composite jumper cables that include both optical fibers and power conductors (which are separately connectorized) may be used to connect each remote radio head to the junction enclosure. 
     The jumper cables are much shorter in length than the trunk cables, as the junction enclosure is typically located only a few feet from the remote radio heads, whereas the trunk cable is routed tens or hundreds of feet up the antenna tower. Additionally, the jumper cables include far fewer components. For example, a power jumper cable may comprise a 6-foot cable having two insulated conductors and connectors on either end thereof. In contrast, for an antenna tower with nine remote radio heads, which is an increasingly common configuration, the trunk cable may be 250 feet long, include eighteen insulated conductors, and also include eighteen optical fibers, along with connectors for each power conductor and optical fiber. As such, trunk cables are typically far more expensive than jumper cables. 
     If the shunt capacitance is implemented in the trunk cable, then retrofit installations may require replacement of an existing trunk cable, which may be very expensive in terms of both the cost of the trunk cable and the costs associated with replacing a trunk cable in terms of man hours, equipment rental, etc. As noted above, the shunt capacitance units according to embodiments of the present invention may also be subject to failure, particularly as the power cabling on cellular antennas may be subject to lightning strikes and/or other voltage surges. When such failures occur, it again may be necessary to replace the cabling or enclosure in which the shunt capacitance unit is contained. This may be very expensive when the shunt capacitance unit is contained in the trunk cable or a breakout enclosure thereof. While replacing shunt capacitance units that are contained in a junction enclosure may not be as expensive from a capital cost viewpoint, opening junction enclosures at the top of cellular towers is generally discouraged for a wide variety of reasons such as safety, environmental sealing concerns and the like. Thus, while placing shunt capacitance units in a trunk cable, trunk cable breakout enclosure and/or junction enclosure may have various advantages, such as locating the shunt capacitance unit at a short distance from the associated remote radio head and sizing the capacitance based on the length of the trunk cable, there may also be various disadvantages with this approach. 
     Providing jumper cables that have associated shunt capacitance units may provide a more efficient and cost-effective way of retrofitting existing cellular base stations to include shunt capacitance units. As noted above, jumper cables are much less expensive than trunk cables, and hence can be replaced at less cost, even though it may be necessary to replace a plurality of jumper cables (as a separate jumper cable connects each remote radio head to the junction enclosure or breakout enclosure of the trunk cable). Moreover, jumper cables may be easily replaced by a technician as they are designed to be connected and disconnected, and jumper cable replacement does not raise environmental sealing concerns as does opening a junction enclosure. As any labor performed at the top of an antenna tower is expensive, the ease with which a retrofit may be performed using jumper cables having associated shunt capacitance units may represent a significant advantage. The advantages that jumper cables with associated shunt capacitance units provide for retrofit applications are equally applicable in situations where the capacitor of a shunt capacitance unit burns out and must be replaced. 
     Additionally, locating the shunt capacitance unit in a jumper cable may also place the shunt capacitance unit closer to the remote radio head. As discussed above, this may provide improved performance. Moreover, maintaining an inventory of jumper cables having shunt capacitance units may be far more efficient than maintaining an inventory of trunk cables with such shunt capacitance units. 
     Jumper cables having associated shunt capacitance units according to embodiments of the present invention will now be discussed with reference to  FIGS. 10-12 and 14 . 
       FIG. 10  is a schematic drawing illustrating how a jumper cable having an associated shunt capacitance unit according to embodiments of the present invention may be used to connect a junction enclosure such as a breakout box of a trunk cable or a stand-alone enclosure to a remote radio head. As shown in  FIG. 10 , a trunk cable  410  is terminated into a junction enclosure  420  at, for example, the top of an antenna tower (not shown). A jumper cable  430 - 1  connects the junction enclosure  420  to a remote radio head  440 . The jumper cable  430 - 1  includes a cable segment  431 . The cable segment  431  may include a power supply conductor  432  and a return conductor  433  that are electrically insulated from each other (see  FIG. 11A ). In some embodiments, the power supply conductor  432  and the return conductor  433  may each comprise an 8-gauge to a 14-gauge copper or copper alloy wire. The wire may be a solid wire or may be stranded. Stranded wires may be preferred in some embodiments as they may increase the flexibility of the jumper cable  430 - 1 . In some embodiments, two stranded 10-gauge wires may be stranded together to form the power supply and/or return conductors  432 ,  433 . The use of two smaller wires that are stranded together to form the power supply and/or return conductor  432 ,  433  may further enhance the flexibility of the jumper cable  430 - 1 . 
     A protective jacket  434  may enclose the power supply and return conductors  432 ,  433 . First and second connectors  435 ,  436  are terminated onto either end of the cable segment  431 . The first connector  435  is configured to connect to a mating connector  422  on the junction enclosure  420 , and the second connector  436  is configured to connect to a mating connector  442  of the remote radio head  440 . Typically, the connector  422  on the junction enclosure  420  and the connector  442  on the remote radio head  440  are identical so that either of connectors  435  and  436  may be connected to either of the connectors  422 ,  442 . The jumper cable  430 - 1  may comprise a power cable that only includes the power supply and return conductors  432 ,  433 , or alternatively may be a hybrid fiber optic-power cable that includes both the power supply and return conductors  432 ,  433  along with two or more optical fibers. The jumper cable  430 - 1  may include an associated shunt capacitance unit  438  that may be implemented in a variety of locations. Configuration and operation of the shunt capacitance unit  438  will be described in further detail below with reference to  FIGS. 11A-12B . 
     In some embodiments, the shunt capacitance unit  438  may be implemented on the jumper cable  430 - 1  as a sealed unit  500  that is interposed along the cable segment  431 .  FIGS. 11A and 11B  are a broken-line perspective view and a partially-exploded perspective view, respectively, of an example embodiment of a sealed unit  500  that may be used to implement the shunt capacitance unit  438 . As shown in  FIGS. 11A-11B , the sealed unit  500  may have a housing  510  that includes housing pieces  520 ,  530 . Each housing piece  520 ,  530  includes a respective cable aperture  522 ,  532  that allows the cable segment  431  to pass through the housing  510 . The housing pieces  520 ,  530  may be formed of, for example, a thermoplastic material or anodized aluminium. In the embodiment of  FIGS. 11A-11B , the shunt capacitance is implemented using a pair of electrolytic capacitors  540 - 1 ,  540 - 2  that are connected in series between the power supply conductor  432  and the return conductor  433 . The capacitors  540  may have a cylindrical shape and may be physically positioned so that a longitudinal axis of each capacitor  540  extends along the longitudinal axis of the cable segment  431 . In example embodiments, the capacitor(s)  540  may have a total capacitance of between 400 and 2500 microfarads. The housing  510  may include a pair of openings  524 ,  534 . The first of these openings  524  may be used to inject an environmental sealing element such as an epoxy into the interior of the housing  510 , and the second of these openings  534  may allow air to escape during the epoxy-fill operation. The epoxy (not shown) may be injected as a gel and may fill the interior of the housing  510 . The epoxy may dry upon exposure to air into a hardened, air and water impermeable solid that fills the housing  510 . Caps (not shown) may be placed over the openings  524 ,  534  in some embodiments, while in other embodiments the openings  524 ,  534  may remain uncovered. 
     The capacitors  540  may comprise, for example, non-polar electrolytic capacitors. The use of non-polar capacitors  540  may allow the jumper cable  430 - 1  to be installed in either direction between the junction enclosure  420  and the remote radio head  440 . In other words, if the jumper cable  430 - 1  is implemented using non-polar capacitors  540 , then connector  435  of jumper cable  430 - 1  may be mated to either the mating connector  422  on the junction enclosure  420  or to the mating connector  442  of the remote radio head  440  and the jumper cable  430 - 1  will operate properly. In contrast, if a polar capacitor was used instead, then connector  435  would always need to be connected to the mating connector  422  on the junction enclosure  420  in order to prevent damage to the capacitors. As technicians may not appreciate that capacitors could be damaged or destroyed if the jumper cable  430 - 1  were installed in the wrong direction, the use of non-polar capacitors  540  may prevent installation errors and damage to the jumper cables. In some embodiments, at least two polar electrolytic capacitors may be used instead of a non-polar electrolytic capacitor. 
     The electrolytic capacitors  540  (or other capacitors used to implement the shunt capacitance) may fail for a variety of reasons. For example, voltage surges resulting from, for example, lightning strikes may damage these capacitors  540 . Additionally, electrolytic capacitors may have defects as manufactured that may not be identified during testing at the factory but which may result in premature failure of the capacitor in the field. The jumper cables according to some embodiments of the present invention may have circuitry that is designed to ensure, or at least increase the likelihood, that if one of the capacitors  540  fails during use, the capacitor  540  will exhibit an open circuit between the power supply and ground conductors  432 ,  433 . As long as such an open circuit is presented, the jumper cable  430 - 1  will continue to perform like an ordinary jumper cable that does not include a shunt capacitance unit  500 . If the capacitors  540  fail to an open circuit, the jumper cable  430 - 1  may be more vulnerable to dI/dt voltage drops, but the jumper cable  430 - 1  will otherwise continue to operate and the failure of the capacitors  540  will not result in a general link failure. 
     In order to ensure that the capacitor  540 , when it fails, will exhibit an open circuit between the supply and ground conductors  432 ,  433 , a fuse circuit  550  may be provided that creates an open circuit in the event of failure of the capacitor  540 . The fuse circuit  550  may be internal to the capacitor  540  or may be a separate circuit that is external to the capacitor  540 . In some embodiments, the fuse circuit  550  may be implemented using a fuse  552  that is inserted along the shunt path between the power supply and return conductors  432 ,  433 . If the capacitor  540  fails in a manner that results in a short circuit between the power supply and return conductors  432 ,  433 , the fuse  552  will “blow” (i.e., create an open circuit) in result to the increased current across the shunt path. The fuse  552  may be designed to conduct the relatively small currents that will flow through the capacitors  540  in response to a dl/dt voltage drop, but the fuse  552  will blow in response to the much larger currents that will pass in the event that the capacitors  540  fail and create a short circuit between the power supply and return conductors  432 ,  433 . The fuse circuit  550  may be any appropriate circuit that creates an open circuit along the shunt capacitance path in the event that the capacitors  540  fail in such a manner to create a short circuit between the power supply and return conductors  432 ,  433 . 
       FIG. 11C  is a circuit diagram illustrating how the shunt capacitance unit  500  is interposed between the power supply and return conductors  432 ,  433 . As shown in  FIG. 11C , the capacitors  540 - 1  and  540 - 2  are connected in series between the power supply conductor  432  and the return conductor  433 . The fuse circuit  550  is located in series between the capacitors  540 - 1  and  540 - 2 . In other embodiments, the fuse circuit  550  may be positioned, for example, between the power supply conductor and the first capacitor  540 - 1  or between the return conductor  433  and the second capacitor  540 - 2 . 
       FIGS. 12A and 12B  are side views of capacitive-loaded jumper cables  430 - 2  and  430 - 3  according to further embodiments of the present invention. As discussed above with reference to  FIGS. 10, 11A and 11B , in some embodiments, the shunt capacitance unit  438  of jumper cable  430 - 1  may be implemented as a sealed unit  500  that is interposed along cable segment  431 . As is schematically shown in  FIG. 12A , in other embodiments, jumper cables  430 - 2  may be provided in which the shunt capacitance unit  438  may be implemented within the cable jacket  434  in order to eliminate the need for the housing  510 . In such embodiments, the cable jacket  434  may provide environmental protection to the shunt capacitance unit  438 . As is schematically shown in  FIG. 12B , in still further embodiments, jumper cables  430 - 3  may be provided in which the shunt capacitance unit  438  may be implemented within one of the connectors  435 ,  436  of the jumper cable  430 - 3 , which again eliminates the need for a separate housing  510 . In such embodiments, the connectors  435 ,  436  may provide environmental protection to the shunt capacitance unit  438 . In the jumper cables  430 - 2  and  430 - 3  of both  FIGS. 12A and 12B , the shunt capacitance unit  438  may have the circuit configuration illustrated in  FIG. 11C . 
     It will also be appreciated that in still further embodiments, the shunt capacitance unit may be implemented as a stand-alone unit that may be connected, for example, between the junction enclosure  420  and a conventional jumper cable or between the remote radio head  440  and a conventional jumper cable. By way of example,  FIG. 13  illustrates how a stand-alone shunt capacitance unit may be provided in the form of an inline connector module  600  that may be connected between a conventional jumper cable  430  and either the junction enclosure  420  or the remote radio head  440 . As shown in  FIG. 13 , the inline connector module  600  includes a housing  610  and first and second connectors  620 ,  622 . The connector  620  may be configured to connect to one of the connectors  435 ,  436  of the conventional jumper cable  430  and the connector  622  may be configured to mate with a connector of the junction enclosure  420  or the remote radio head  440 . Power supply and ground conductors  632 ,  633  may be provided within the inline connector module  600 , and a shunt capacitance unit  640  may be provided that includes one or more capacitors and, optionally, a fuse circuit, that are connected in series along a shunt path between the power supply and ground conductors  632 ,  633 . The power supply and ground conductors  632 ,  633 , and the capacitors and the fuse circuit of the shunt capacitance unit  640  may be electrically arranged with respect to each other as shown in the circuit diagram of  FIG. 11C . 
     As discussed above with reference to  FIG. 9 , in some embodiments, shunt capacitance units may be used in cellular base stations that employ programmable power supplies that are used to reduce the I*R voltage drop by maintaining the voltages of the power signals at the remote radio heads at or near a maximum voltage for the power signal that is specified for each remote radio head. It will be appreciated that the shunt capacitance units may be implemented in the trunk cable, in the jumper cables, or using stand-alone units as discussed in the various embodiments described above. 
     As is discussed in detail in the aforementioned &#39;897 application, in some embodiments, the programmable power supplies may be configured to measure, estimate, calculate or receive the resistance of the power cabling connection that is interposed between the power supply and the remote radio head (this connection typically includes a trunk cable and a jumper cable). Pursuant to further embodiments of the present invention, the shunt capacitance units may be used to measure the impedance of the power cabling connection. 
     In particular,  FIG. 14  as a schematic block diagram illustrating how the shunt capacitance units according to embodiments of the present invention may be used to facilitate measuring the resistance of the power cabling between the power supply and, for example, a remote radio head. As shown in  FIG. 14 , a programmable power supply  700  may be provided that delivers DC power to a remote radio head  760  over the power supply and return conductors  751 ,  752  of a power cabling connection  750 . The power cabling connection  750  may comprise one or more cables. For example, the power cabling connection  750  may comprise the power supply portion of a trunk cable and a jumper cable that are connected in series between the programmable power supply  700  and the remote radio head  760 . As shown in  FIG. 14 , a shunt capacitance unit  753  is provided along the power cabling connection  750  adjacent the remote radio head  760 . The shunt capacitance unit  753  may be implemented, for example, in a jumper cable that connects the power supply and return conductors of a trunk cable to the remote radio head  760 . 
     The programmable power supply  700  includes a DC power generator  710  that provides a DC power signal that is used to power the remote radio head  760 . The programmable power supply  700  further includes a pulse generator  720  that is configured to generate an alternating current (“AC”) control signal that may also be transmitted onto the power cabling connection  750 . This AC signal may be, for example, a 100 Hz to 100 kHz voltage pulse. The frequency of the voltage pulse may be selected so that the voltage pulse will pass through the capacitors of the shunt capacitance unit  753 . The frequency of the voltage pulse may also be selected to be lower than the RF data signals that are transmitted by the remote radio head  760  to reduce or minimize any potential interference between the voltage pulse and the RF data signals. 
     As the shunt capacitance unit  753  will appear as a short circuit to the voltage pulse, the voltage pulse will not pass to the remote radio head  760 , but instead will flow through the shunt capacitance unit  753  and back to the programmable power supply  700 . Ohm&#39;s law may then be used to determine the resistance of the power cable  750  based on the current/voltage characteristics of the voltage pulse that is received at the programmable power supply  700 . The programmable power supply  700  may include control circuitry  740  that is used to measure the voltage and or current levels of the return voltage pulse and control logic  730  which calculates the resistance of the power cabling connection  750  based thereon. 
     Thus, in the embodiment of  FIG. 14 , so long as the shunt capacitance unit operates properly, it may be used as a bypass path for the voltage pulse in order to allow the programmable power supply  700  to measure the resistance of the power cable  750  and dynamically vary the output of the power supply  700  based thereon. Moreover, if a capacitor in the shunt capacitance unit  753  fails, the voltage pulse will no longer be received at the power supply  700 , as the shunt capacitance unit  753  will fail to an open circuit. When this occurs, the power supply  700  may be configured to issue an alarm so that the shunt capacitance unit may be replaced. 
     The present invention has been described with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments that are pictured and described herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the specification and drawings. It will also be appreciated that the embodiments disclosed above can be combined in any way and/or combination to provide many additional embodiments. For example, the shunt capacitance units described herein may be used in any of the example embodiments disclosed in the above-described &#39;897 application. 
     It will be understood that, although the terms first, second, etc. are used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Unless otherwise defined, all technical and scientific terms that are used in this disclosure have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the above description is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in this disclosure, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that when an element (e.g., a device, circuit, etc.) is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof. 
     In the drawings and specification, there have been disclosed typical embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.