Patent Publication Number: US-2022231415-A1

Title: Antenna calibration device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. patent application Ser. No. 16/423,312, filed May 28, 2019, which in turn claims priority under 35 U.S.C. § 119 to Chinese Patent Application No. 201810548253.0, filed May 31, 2018, the entire content of each of which is incorporated herein by reference as if set forth in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to antennas and, more particularly, to a multi-channel calibration device for an antenna array. 
     DESCRIPTION OF RELATED ART 
     Adaptive antenna arrays that include a plurality of radiating elements are widely used in modern mobile communication systems. These antenna arrays are often referred to as “smart” antennas. By adjusting the amplitude and phase characteristics of radio frequency (“RF”) signals transmitted to the radiating elements, a smart antenna can generate spatially directional radiation patterns or “antenna beams” that can be directed to cover a selected area or to point at individual users. The use of smart antennas may significantly improve the capacity and link quality of a communication system. 
     Smart antennas include beamforming networks that adjust the amplitude and/or phase of the RF signals that are passed to the radiating elements. In a base station including a smart antenna, signals transmitted by an RF transceiver enter the beamforming network through an RF port. In the beamforming network, the signals are divided into sub-components that travel along multiple paths (which are also referred to as “channels”) that correspond to multiple radiating elements or to multiple groups (e.g., columns) of radiating elements. The signals in each path undergo independent adjustments of their amplitude and phase characteristics (which is referred to as “beamforming”). The beamformed signals along the multiple paths are passed to the corresponding radiating elements or corresponding groups of radiating elements. Each radiating element or each group of radiating elements generates an independent, spatially directional beam. After interference and superimposition of the independent beams, the resulting composite antenna beam may exhibit good directivity. 
     In order to form a desired antenna beam, it is desirable to make sure that transmission/reception paths from the RF transceiver to the radiating elements or columns of radiating elements are uniform in terms of their amplitude and phase characteristics, so that the composite antenna beam may have the shape and characteristics of the antenna beam that is expected to be obtained by adjustment of the beamforming network. However, in practice it can be difficult to ensure that the characteristics of the multiple transmission/reception paths are uniform, and any differences between the paths may seriously degrade the performance of the smart antenna. Thus, calibration devices may be used to identify and compensate for differences between the multiple paths. 
     Calibration devices are known in the art that collect respective portions of the signals traversing each transmission/reception path and output the collected energy at a calibration port of the antenna.  FIG. 1  is a schematic diagram illustrating the basic structure and principle of such a prior art calibration device  20 . The calibration device  20  in  FIG. 1  is implemented in an antenna having an antenna array that includes eight radiating units  10 . Each radiating unit  10  may include one radiating element or a vertical column of radiating elements. For an antenna having eight radiating units  10 , the calibration device  20  may include eight signal input ports  11 . Each signal input port  11  is connected to a corresponding RF transceiver (not shown) via a beamforming network. In a transmission mode, each RF transceiver of the antenna may output a subcomponent of the RF signal to be transmitted. The subcomponent of the RF signal arrives at a corresponding input port  11  via the beamforming network to be input to the calibration device  20 . At the calibration device  20 , each subcomponent of the RF signal is transferred to a corresponding output port  12  through a corresponding transmission line  13  that pass the subcomponents to the corresponding radiating units  10 . 
     In order to collect a portion of the signals passing through the transmission lines  13  from the signal input ports  11  to the radiating units  10 , the calibration device  20  may include eight directional couplers  22 . Each directional coupler  22  is a four-port device. The calibration device  20  further includes cascaded power combiners  24  that combine the outputs of the eight directional couplers  22  and connect those outputs to a single calibration port  25 . The directional couplers  22 , together with the cascaded power combiners  24 , form a calibration network between the radiating units  10  and the calibration port  25 . In a calibration operation for, e.g., a transmission path, a calibration test signal may be transmitted by the RF transceivers. The calibration test signal is transferred through the beamforming network and input to the calibration device  20  at corresponding input ports  11 . A small part of the power of each subcomponent of the calibration test signal is output by a corresponding directional coupler  22  through its coupling port  23 , and is then transferred to the cascaded power combiners  24 . For an antenna having eight radiating units  10 , the cascaded power combiners  24  may be a 3-stage cascade Wilkinson power divider circuit that is configured to combine the coupling signals output by the coupling ports  23  in pairs and ultimately combine the signals as a composite calibration test signal. The composite calibration test signal is output from the calibration device  20  via the calibration port  25 . The calibration port  25  may be connected to a calibration transceiver. The calibration transceiver may compare the composite test signal with a reference test signal so as to be capable of detecting the uniformity of amplitude/phase of various transmission paths. Based on the comparison, the amplitude and/or phase characteristics of the signal components on the transmission paths can be adjusted to compensate for the difference in amplitude/phase between the transmission paths so as to achieve a desired radiation pattern. 
     When there is a large number of radiating units, the prior art calibration device includes a large number of couplers and power combiners. Thus the size of the calibration device may be very large, making the calibration device hard to satisfy the requirement of highly integration and miniaturization of the antennas. 
     Moreover, the calibration network, which is composed of couplers and power combiners, may itself introduce non-uniformities in amplitude/phase between paths. When a non-uniformity is observed at the calibration port, it may not be possible to determine whether the non-uniformity is caused by the beamforming and feeding network between the RF transceiver and the radiating units or by the calibration network between the radiating units and the calibration port. 
     When radiating units in an antenna array are used for services in different frequency bands (i.e., two or more frequency bands), the beamforming networks for different frequency bands need to be calibrated separately. Thus it is necessary to scale up a single calibration port into a plurality of calibration ports to calibrate the beamforming networks for the different frequency bands separately. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a coupling device, an antenna calibration device, and an antenna device that overcomes at least one of the deficiencies in the prior art. 
     According to a first aspect of the present disclosure, a coupling device is provided. This coupling device includes a plurality of couplers. Each coupler includes a transmission main line and a coupling subline. The transmission main line is used for providing an input end and an output end. The coupling subline is coupled with the transmission main line to provide two coupling ends. The coupling subline includes first and second parts located on opposite sides of the transmission main line and a third part connected between the first and second parts. The coupling sublines of the plurality of couplers are connected in series to provide a first coupling output port and a second coupling output port. The first part of the coupling subline of at least one coupler has a shared section with the third part of the coupling subline of an adjacent coupler. 
     According to a second aspect of the present disclosure, an antenna calibration device is provided. The antenna calibration device includes a first power allocating part and a coupling device. This coupling device includes a plurality of couplers. Each coupler includes a transmission main line and a coupling subline. The transmission main line is used for providing an input end and an output end. The coupling subline is coupled with the transmission main line to provide two coupling ends. The coupling subline includes first and second parts located on opposite sides of the transmission main line and a third part connected between the first and second parts. The coupling sublines of the plurality of couplers are connected in series to provide a first coupling output port and a second coupling output port. The first part of the coupling subline of at least one coupler has a shared section with the third part of the coupling subline of an adjacent coupler. The coupling device is connected to a first power allocating part via the first coupling output port. The first power allocating part provides a plurality of first calibration ports. 
     According to a third aspect of the present disclosure, an antenna calibration device is provided. The antenna calibration device includes a plurality of directional couplers and a coupling device. The coupling device includes a plurality of couplers. Each coupler includes a transmission main line and a coupling subline. The transmission main line is used for providing an input end and an output end. The coupling subline is coupled with the transmission main line to provide two coupling ends. The coupling subline includes first and second parts located on opposite sides of the transmission main line and a third part connected between the first and second parts. The coupling sublines of the plurality of couplers are connected in series to provide a first coupling output port and a second coupling output port. The first part of the coupling subline of at least one coupler has a shared section with the third part of the coupling subline of an adjacent coupler. An input end of at least one coupler is connected to a coupling end of a corresponding directional coupler of a plurality of directional couplers. 
     According to a fourth aspect of the present disclosure, an antenna device is provided. The antenna device includes a plurality of antenna radiating elements, a plurality of RF ports and a coupling device coupled between the plurality of antenna radiating elements and the plurality of RF ports. The coupling device includes a plurality of couplers. Each coupler includes a transmission main line and a coupling subline. The transmission main line is used for providing an input end and an output end. The coupling subline is coupled with the transmission main line to provide two coupling ends. The coupling subline includes first and second parts located on opposite sides of the transmission main line and a third part connected between the first and second parts. The coupling sublines of the plurality of couplers are connected in series to provide a first coupling output port and a second coupling output port. The first part of the coupling subline of at least one coupler has a shared section with the third part of the coupling subline of an adjacent coupler. Input ends of the couplers of the coupling device are connected to the corresponding ones of a plurality of RF ports, and output ends of the couplers are connected to the corresponding ones of the plurality of antenna radiating elements. 
     According to a fifth aspect of the present disclosure, an antenna device is provided. This antenna device includes a plurality of antenna radiating elements, a plurality of RF ports, and a plurality of directional couplers and a coupling device which are coupled between a plurality of radiating units and the plurality of RF ports. The plurality of radiating units, the plurality of RF ports and the plurality of directional couplers correspond to each other on a one-to-one basis. This coupling device includes a plurality of couplers. Each coupler includes a transmission main line and a coupling subline. The transmission main line is used for providing an input end and an output end. The coupling subline is coupled with the transmission main line to provide two coupling ends. The coupling subline includes first and second parts located on opposite sides of the transmission main line and a third part connected between the first and second parts. The coupling sublines of the plurality of couplers are connected in series to provide a first coupling output port and a second coupling output port. The first part of the coupling subline of at least one coupler has a shared section with the third part of the coupling subline of an adjacent coupler. The input end of each of the plurality of directional couplers is connected to a corresponding RF port of the plurality of RF ports, and the output end of each directional coupler is connected to a corresponding radiating unit of the plurality of radiating units, and the coupling end of each directional coupler is connected to the input end of a corresponding coupler of the coupling device. 
     According to a sixth aspect of the present disclosure, a coupling circuit is provided. The coupling circuit includes a first coupling device. The coupling device includes a transmission line and two coupling lines. The transmission line has a first end to receive signals. The two coupling lines are located on opposite sides of the transmission line and are coupled to the transmission line. An end of each coupling line close to the first end of the transmission line provides a coupled component of the signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the basic structure and principle of a prior art calibration device; 
         FIG. 2  is a schematic structural diagram of an antenna that includes a coupling device according to an embodiment of the present invention; 
         FIG. 3  is a schematic structural diagram of a coupling device according to an embodiment of the present invention; 
         FIG. 4  is an enlarged view of a coupler included in the coupling device of  FIG. 3 ; 
         FIG. 5  is a schematic structural diagram of a calibration device according to another embodiment of the present invention; and 
         FIG. 6  is a schematic structural diagram of a calibration device which uses dual-directional couplers for scaling up the calibration ports. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described as follows with reference to the accompanying drawings, in which certain embodiments of the present invention are shown. However, it is to be understood that the present invention may 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. It will also be appreciated that the embodiments disclosed herein can be combined in any way to provide many additional embodiments. 
     Like numbers refer to like elements throughout. In the figures, the thickness of certain lines, layers, components, elements or features may be exaggerated for clarity. Well-known functions or constructions may not be described in details for brevity. 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “on”, “attached” to, “connected” to, “coupled” with, “contacting”, etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, “directly on”, “directly attached” to, “directly connected” to, “directly coupled” with or “directly contacting” another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature may have portions that overlap or underlie the adjacent feature. 
     Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper”, “lateral”, “left”, “right” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is inverted, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the descriptors of relative spatial relationships used herein interpreted accordingly. 
     Embodiments of the present invention provide coupling devices for calibrating an antenna. The coupling device can provide two independent coupling output ports via appropriate connections between a plurality of couplers. The calibration devices according to embodiments of the present invention do not include a plurality of individual directional couplers and cascaded power combiners as in the prior art device of  FIG. 1 , which reduces the size and cost of the calibration device. Moreover, the two independent coupling output ports can be used for eliminating the path difference caused by the calibration device itself, and thus can detect non-uniformities in amplitude/phase along the transmission/reception paths more accurately. 
     Referring now to  FIG. 2 , a schematic structural diagram of an antenna  200  that includes a coupling device according to an embodiment of the present invention is shown. As illustrated in  FIG. 2 , the antenna  200  includes an antenna array  210  and a calibration device  220 . 
     The antenna  210  includes a plurality of (e.g., n) radiating units. Each radiating unit  210 - 1 ,  210 - 2 , . . .  210 - n  may include a single radiating element or a plurality of radiating elements (e.g., a column of radiating elements). Each radiating element completes radiation of a transmit signal and performs front-end reception of a receive signal. In the following text, radiating units  210 - 1 ,  210 - 2 , . . .  210 - n  may also be collectively referred to as “radiating unit  210 ”. 
     The calibration device  220  includes a plurality of signal input ports  211 , a plurality of signal output ports  212  that correspond to the signal input ports  211  on a one-to-one basis, and a coupling device  230  that is connected between the signal input ports  211  and the signal output ports  212 . The coupling device  230  includes a first coupling output port  225   a  and a second coupling output port  225   b  which can provide independent outputs. 
     The coupling device  230  may operate bi-directionally. At the time of transmission, RF signals (e.g., from a beamforming network or other feeding network) enter the coupling device  230  via the individual signal input ports  211 , and are then passed to the corresponding radiating units  210  via the corresponding signal output ports  212 . The coupling device  230  collects a portion of the energy of the RF signals transmitted between each signal input port  211  and its corresponding signal output port  212 . The collected signal energy can be output from the calibration device  220  via the first coupling output port  225   a  and the second coupling output port  225   b . Preferably, the collected energy is a small part of the energy of the input signals. During normal reception, RF signals from the radiating units  210  may enter the coupling device  230  through the signal output ports  212  and be output from the signal input ports  211 ; during reception calibration, the calibration test signal may enter the coupling device  230  via the first coupling output port  225   a  or the second coupling output port  225   b  and be output from the signal input ports  211 . Ideally, there is certain isolation between the signal output ports  212  and the coupling output ports  225   a  and  225   b.    
     As is further shown in  FIG. 2 , the antenna  200  may further include a beamforming network  240 , RF transceiver(s)  250  and an antenna interface unit  260 . 
     The antenna interface unit  260  includes processors for receiving a digital signal from a baseband unit (not shown) and providing a digitized received signal to the baseband unit. The baseband unit may be a part of a main control system of a base station (not shown). The RF transceivers  250  may perform various signal processing, such as, but not limited to, digital processing, digital-to-analog or analog-to-digital conversion, baseband/intermediate frequency (IF)/RF conversion, low noise amplification and filtering. Beamforming network  240  may comprise phase-shifters or feeding power allocating circuits for applying predetermined gain/phase adjustment to signals of different paths from the RF transceivers and feeding the signals to respective radiating units  210 , or for receiving signals from respective radiating units  210  and transferring the signals to respective RF transceivers  250 . 
     When the antenna  200  operates in a transmission mode, antenna interface unit  260  receives a signal to be transmitted from the baseband unit, divides the signal into n identical subcomponents (for example, the number of subcomponents may equal to the number of radiating units  210 ), applies gain and phase adjustments to each subcomponent, and provides the subcomponents to corresponding RF transceivers  250 . The subcomponents are converted into analog RF signals by the RF transceivers  250 , and arrive at corresponding radiating units  210  via the beamforming network  240 . The RF signals are radiated by the radiating units  210  and the radiated RF signals are interfered and superimposed together to form a desired radiation pattern. 
     When the antenna  200  operates in a reception mode, each radiating unit  210  receives different subcomponents of the received RF signal. Each subcomponent of the received RF signal is provided to a corresponding RF transceiver  250 , converted into a digital subcomponent at the RF transceiver  250  and delivered to antenna interface unit  260 . Antenna interface unit  260  applies gain and phase adjustments to the received digital subcomponents, and combines the digital subcomponents to form a composite received signal. 
     Antenna  200  may perform complicated beamforming manipulation by controlling the gains and phase shifts. For example, the antenna  200  may electronically adjust the beam width, beam shape and pointing direction of the antenna beam by changing the gains (via the power allocating circuits) and the phase shifts (via the phase shifters) in the beamforming network  240 . 
     However, the transmission paths through the antenna  200  may have different propagation characteristics, such that even if the desired gains and phase shifts are applied to the signals traversing each path, the resulting antenna beam may differ from the expected (desired) antenna beam. Moreover, during operation of the smart antenna, relative changes may occur between the different paths. Therefore, it may be necessary to detect, calibrate and compensate for the differences and changes in the transmission characteristics between the paths. 
     By collecting a portion of the signals on the transmission and reception paths, the calibration device  220  (mainly including a coupling device  230 ) may be used to calibrate the paths to compensate for non-uniformities in amplitude/phase between individual transmission and reception paths. In some embodiments, the antenna  200  may also include a calibration transceiver  270 . The coupling output ports  225   a  and  225   b  of the calibration device  220  are coupled to the calibration transceiver  270  via, for example, a RF cable. The calibration device  220  and the calibration transceiver  270  may be used for monitoring the values of the gains and the phases of all the transmission and reception paths so as to be capable of adjusting them. The calibration device  220  may perform initial calibration and continuous monitoring and adjustment during normal use of the antenna  200 . 
     To calibrate the transmission channels, a calibration test signal is transmitted from the antenna interface unit  260  to the radiating units  210 . A portion of the energy of the calibration test signal is extracted through the coupling device  230  and is output through the coupling output ports  225   a  and  225   b  to the calibration transceiver  270 . The calibration transceiver  270  performs an operation similar to the operation of the RF transceiver(s)  250  and measures a composite calibration test signal. The calibration transceiver  270  and/or the antenna interface unit  260  implement an algorithm to determine the necessary adjustments of the gain and the phase of the signals on the transmission paths. Antenna interface unit  260  then carries out the adjustment. 
     Various algorithms for calibration are already known to a person skilled in the art, and thus will not be further described herein. 
     In order to calibrate the reception paths, the calibration transceiver  270  transmits the test signal to the coupling output ports  225   a  and  225   b . A portion of the energy of each subcomponent of the calibration test signal is transmitted, via the coupling device  230 , to the respective reception paths where the subcomponents are processed by the respective RF transceivers  250  and provided to antenna interface unit  260 . Antenna interface unit  260  receives different versions of the calibration test signals from the reception paths and uses a suitable algorithm to change the gain and phase of the signals received from the reception paths in order to form an appropriate antenna beam. 
       FIG. 3  is a schematic structural diagram of a coupling device  300  according to an embodiment of the present invention. The coupling device  300  in  FIG. 3  may be used, for example, to implement the coupling device  230  of  FIG. 2 . The coupling device  300  includes a plurality of couplers. For a smart antenna having eight radiating units  210 , the coupling device  300  may include eight couplers  302 - 1 ,  302 - 2  . . .  302 - 8  that correspond to the eight radiating units  210  on a one-to-one basis. For simplicity, couplers  302 - 1 ,  302 - 2  . . .  302 - 8  hereinafter may be individually and/or collectively referred to as “coupler(s)  302 ”. In other embodiments, the number of couplers may be more or less than eight couplers  302 .  FIG. 4  is an enlarged view of one of the couplers  302  that is included in the coupling device  300  of  FIG. 3 . 
     As illustrated in  FIG. 4 , coupler  302  includes a main transmission line  303  and a coupling subline  305 . The main transmission line  303  may be used for transmitting signals between a beamforming network (e.g., beamforming network  240  of  FIG. 2 ) and a corresponding radiating unit  210 . The input end  304   a  of the main transmission line  303 , which corresponds to signal input port  211  in  FIG. 2 , may be connected to the beamforming network  240 . The output end  304   b  of the main transmission line  303 , which corresponds to signal output port  212  in  FIG. 2 , may be connected to a radiating unit  210  that corresponds to the particular coupler  302 . 
     The coupling subline  305  includes first and second coupling portions  307   a  and  307   b  which each are configured to couple with the main transmission line  303 . The first and second coupling portions  307   a  and  307   b  may be located on opposite sides of the main transmission line  303 . The first and second coupling portions  307   a  and  307   b  and the main transmission line  303  may be implemented, for example, using coaxial lines, rectangular waveguides, circular waveguides, strip lines, microstrip transmission lines and/or any other transmission lines. Coupling between the first and second coupling portions  307   a ,  307   b  and the main transmission line  303  may be achieved by various known coupling technologies, including but not limited to aperture coupling, parallel coupling, branch coupling and matching double-T. In some embodiments, the coupling subline  305  and the main transmission line  303  may be implemented as edge-coupled microstrip transmission lines, so that the structure of the calibration device is more compact and meets the requirements for miniaturization. In the three-line coupling structure formed by the first and second coupling portions  307   a  and  307   b  and the main transmission line  303  that is shown in  FIG. 4, 304   a  is an input end,  304   b  is an output end,  306   a  and  306   b  are coupling ends, and  308   a  and  308   b  are isolation ends. A third connection portion  307   c  of the coupling subline  305  connects the first and second coupling portions  307   a  and  307   b . The shape of the third connection portion  307   c  is not limited. The electrical length of the connection portion  307   c  may be designed so that its impedance matches the impedance of the three-line coupling structure. As a result, when a signal is input at the input end  304   a , the coupled components of the signal are only transmitted through the coupling ends  306   a  and  306   b , and no power is output on the third connection portion  307   c  between the isolation ends  308   a  and  308   b.    
     Returning to  FIG. 3 , the same reference signs in  FIG. 3  and  FIG. 4  represent the same components. Firstly, coupling sublines  305  of couplers  302 - 1  to  302 - 8  are connected in series and the series connection may be terminated on either end by a first coupling output port  325   a  and a second coupling output port  325   b  (corresponding to  225   a  and  225   b  in  FIG. 2 ). By connecting the coupling sublines  305  of a plurality of couplers  302  in series, the coupling device  300  may have the following ability: when a signal is injected at the input end  304   a  of any individual coupler  302 - 1  to  302 - 8 , two coupled components of the injected signal can always be obtained from the two coupling output port  325   a  and  325   b . The two coupled components are obtained by coupling and transferring the injected signal through two different paths that are independent of each other. For example, when a signal is injected at the input end  304   a  of coupler  302 - 5 , coupled components are obtained at coupling ends  306   a  and  306   b  of the coupler  302 - 5 , respectively. Thereafter, according to the coupling theory, the coupled component at the coupling end  306   a  sequentially passes through the coupling sublines of couplers  302 - 4 ,  302 - 3 ,  302 - 2  and  302 - 1  to arrive at the coupling output port  325   a ; the coupled component at the coupling end  306   b  sequentially passes through the coupling sublines of couplers  302 - 6 ,  302 - 7  and  302 - 8  to arrive at the coupling output port  325   b.    
     For an individual coupler  302 , a first path from the input end  304   a  to the first coupling output port  325   a  and a second path from the input end  304   a  to the second coupling output port  325   b  will impart different amplitude/phase changes to the signals transferred thereon. However, for all the couplers  302 , a sum of the amplitude/phase changes of the first path and the amplitude/phase changes of the second path is fixed. For example, in  FIG. 3 , the sum approximately equals an amount of change in amplitude/phase caused by the electrical length of the overlapped coupling sublines of the eight couplers. In this way, for the individual transmission paths connected to the corresponding couplers, the amplitude/phase uniformity between these transmission paths can be determined by comparing the difference between the sum values of the coupling output ports  325   a  and  325   b . For the individual couplers  302  in the coupling device  300 , it is not necessary to ensure that they have identical circuit structures, since the difference in circuit structures can be eliminated in the calibration algorithm based on the two coupled components at the coupling output ports  325   a  and  325   b . According to the characteristics of signal propagation in the circuit structure, a person skilled in the art knows how to modify the calibration algorithm in the prior art. For example, it is possible to modify the direct comparison of the difference between the output signals from the calibration ports of the individual transmission paths in the prior art as comparison between the sum values of signals from the coupling output ports  325   a  and  325   b  of the individual transmission paths. Other details of the calibration algorithm will not be described here. 
     Secondly, a coupling subline  305  of at least one coupler  302  has a shared section with a coupling subline  305  of an adjacent coupler  302 . Specifically, one of the two coupling portions  307   a ,  307   b  of the coupling subline  305  of at least one coupler  302  has a shared section with a connection portion  307   c  of the coupling subline  305  of an adjacent coupler  302 . For example, as shown in  FIG. 3 , coupling portion  307   b  of the coupling subline  305  of coupler  302 - 1  serves as at least one part of the connection portion  317   c  of adjacent coupler  302 - 2 . Since the coupling sublines  305  of adjacent couplers  302  are sequentially connected in series, that is, in an order of  307   c ,  307   b ,  317   a ,  317   c , when  307   b  and  317   c  have a shared section,  307   c  and  317   a  also has a shared section. For example, in  FIG. 3 , coupling portion  317   a  serves as a section  307   c - 1  of connection portion  307   c . Accordingly, an isolation end  308   a  of coupler  302 - 1  is connected to a coupling end  316   a  of coupler  302 - 2 , and the other isolation end  308   b  of coupler  302 - 1  is connected to the isolation end  318   a  of coupler  302 - 2 , and a coupling end  306   b  of coupler  302 - 1  is connected to an isolation end  318   b  of coupler  302 - 2  via the connection portion  317   c . In this way, adjacent couplers  302 - 1  and  302 - 2  present an interleaved arrangement. That is, the input end  304   a  of coupler  302 - 1  and the input end  314   a  of coupler  302 - 2  are remote from each other, while the output ends  304   b  and  314   b  are close to each other. 
     When a signal is injected at an input end  304   a  of coupler  302 - 1 , coupled components are output from coupler  302 - 1  at the coupling ends  306   a  and  306   b . The coupled component at coupling end  306   a  may pass to the first coupling output port  325   a . The coupled component at coupling end  306   b  passes to the coupling end  316   b  of coupler  302 - 2  along the coupling subline  305  of coupler  302 - 2 , and continues to pass along the serially connected coupling sublines  305  of couplers  302 - 3  to  302 - 8  in the same manner so that the coupled component at coupling end  306   b  of  202 - 1  is ultimately output through the second coupling output port  325   b.    
     Due to the shared section between the coupling sublines  305  of at least one coupler  302  and its adjacent coupler  302 , as compared to using individual couplers, a path from the input end of one coupler to the coupling output port is reduced, which decreases the insertion loss of the coupling output port relative to the input end. This arrangement of couplers is also more compact, further reducing the size of the coupling device  300 . A simulation for a specific embodiment of the coupling device  300  indicates that, as compared to using individual couplers, the size of the coupling device  300  is reduced by 44%, and the insertion loss of the coupling ports is reduced by 1.6 dB. In addition, the coupling device  300  can also have improved return loss and coupling flatness. 
     It should be appreciated that, although  FIG. 3  shows that each coupler  302  shares a section of the coupling subline  305  with an adjacent coupler  302 , in other embodiments, it is also possible that only one coupler  302  shares a section of the coupling subline  305  with an adjacent coupler  302  while the other couplers  302  still only have their coupling sublines  305  connected in series. Such embodiments also fall within the scope of the present invention. 
     Returning to  FIG. 2 , in some embodiments, a first power allocating part  231  is connected to the first coupling output port  225   a . The first power allocating part  231  has a plurality of first calibration ports (e.g., calibration ports  232 - 1  and  232 - 2  shown in  FIG. 2 ). The first power allocating part  231  may equally divide and/or unequally divide the power of the signals output at the first coupling output port  225   a . The number of first calibration ports included in the first power allocating part  231  is not limited to two as shown in  FIG. 2 . The first power allocating part  231  may be implemented, for example, using any of the following techniques: a 1 to N Wilkinson-type or other type of power splitter, a select-1-from-N switch array, or a circuit or device having similar functionality. In order to reduce the return loss of the first calibration ports relative to the signal input ports  211  of coupling device  230 , the first power allocating part  231  may also include a power attenuator. 
     In a further embodiment, the first power allocating part  231  may include a frequency demultiplexing element. The frequency demultiplexing element splits signals output through the first coupling output port  225   a  based on frequency such that at least two of the plurality of first calibration ports  232  may output signals in different frequency bands. For example, the first calibration ports  232 - 1  and  232 - 2  in  FIG. 2  may correspond to different frequency bands, respectively. The frequency demultiplexing element may include filters of various known technologies. In a specific example, a signal from the first coupling output port  225   a  may be subdivided into two parts by a power divider in the first power allocating part at first. At least one of the output branches may be provided with band pass (or other suitable) filters for different frequency bands. Dividing the signals output by the coupling device by frequency is beneficial when the radiating units in the antenna array are used to support service in multiple frequency bands because it can prevent mutual interference between signals in different frequency bands that are output by the coupling device which might occur during subsequent signal processing. To ensure the uniformity of calibration channels for signals in different frequency bands in the calibration device  220 , it is preferable to provide uniform amplitude/phase characteristics to signals transferred on paths from the first coupling output port  225  to respective first calibration ports used for different frequency bands. For example, in  FIG. 2 , when calibration ports  232 - 1  and  232 - 2  are used for different frequency bands, a first path from  225   a  to  232 - 1  and a second path from  225   a  to  232 - 2  may have mirror-symmetrical circuit arrangements. 
     A second power allocating part  233  may be coupled to the second coupling output port  225   b  to be scaled up into a plurality of second calibration ports (e.g., calibration ports  234 - 1  and  234 - 2  illustrated in  FIG. 2 ). In some embodiments, the second power allocating part  233  may have a circuit arrangement which is mirror-symmetrical with the first power allocating part  231 . When the first power allocating part  231  includes a frequency demultiplexing element, the second power allocating part  233  may also include the same frequency demultiplexing element, such that the plurality of second calibration ports correspond to the respective plurality of first calibration ports. A first calibration port may be used in a pair with a corresponding second calibration port (e.g.,  232 - 1  and  234 - 1  are used in a pair) for calibrating reception/transmission paths for a particular frequency band. In other embodiments, the second power allocating part  233  may be different from the first power allocating part  231 . A first calibration port (such as  232 - 1 ) and a second calibration port (such as  234 - 1 ) can be used as a pair for calibrating the corresponding reception/transmission paths as long as they are used for the same frequency band. 
     In some embodiments, the calibration device may also include a plurality of directional couplers in addition to the coupling device. Each directional coupler may include a transmission line and a coupling line that are coupled to each other. The transmission lines may be feed lines for transferring signals between corresponding RF ports of a beamforming network and corresponding radiating elements. A coupling line is used to extract a portion of the signal energy transferred on the feed line and transfer it to the coupling device according to the embodiments of the present invention as described with reference to  FIGS. 2-4 . 
       FIG. 5  is a schematic structural diagram of a calibration device  520  according to a further embodiment of the present invention. Similar to  FIG. 2 , the calibration device  520  is connected to an antenna array  510  that includes a plurality of radiating units. The specific structure of the antenna array  510  is similar to the antenna array in  FIG. 2 , and will not be described in detail herein. The calibration device  520  includes a coupling device  530  and a plurality of directional couplers  522 . Each directional coupler  522  may be a four-port device, which includes a transmission line  513  and a coupling line that is coupled to the transmission line  513 . The two ends of the transmission line  513  are an input end and an output end, respectively. The end of the coupling line that is close to the input end is a coupling end  523 , while the opposite end is an isolation end  526 . Each directional coupler  522  may be connected to a beamforming network via a signal input port  511  as an input end and with a corresponding radiating unit  510  via a signal output end  512  as an output end. Each transmission line  513  extends between a respective one of the signal input ports  511  and a corresponding signal output port  512 , and is a part of a transmission path extending between a RF transceiver and the corresponding radiating unit. The transmission line  513  may be implemented as, for example, a microstrip RF transmission line. Each coupling end  523  is connected to an input port of the coupling device  530  (e.g., connected to an input end  304   a  of each coupler  302  shown in  FIG. 3 ) for transferring a small part of the energy of the calibration test signal transmitted through each transmission path. The isolation end  526  is connected to a matched load, which may be, for example, a 50 Ohm resistor. 
     Similar to the coupling output ports  225   a  and  226   b  in  FIG. 2 , the coupling device  530  has a first coupling output port  525   a  and a second coupling output port  525   b . The coupling device  530  may adopt the structure as discussed above with reference to  FIGS. 3-4 . Each output port  531  of the coupling device (e.g., an output end  304   b  of each coupler  302  shown in  FIG. 3 ) is connected to a matched load, which may also be, for example, a 50 Ohm resistor. 
     When there is a reflection signal from a radiating unit  510 , since there is a certain degree of isolation between an output port  512  and a coupling end  523  of a directional coupler  522 , it is possible to effectively suppress the reflection signal from entering the coupling device  530  and from being output from the coupling output ports  525   a  and  525   b . Therefore, as compared with the coupling device  230  of the calibration device  220  in  FIG. 2  which is directly connected between the beamforming network and the radiating units, the calibration device  520  in  FIG. 5  can use directional couplers to absorb reflection signals from the radiating units. In addition, since the directional couplers can be located proximate the radiating units, the calibration device  520  can calibrate the entire feed network at the input ports of the radiating units, thereby making the calibration more accurate and effective. In a specific example of a simulated calculation, as compared with the calibration device  220 , the coupling accuracy of the coupling device  520  is increased from ±4.85 dB to ±2 dB, and the phase accuracy is increased from ±21° to ±3°. 
     According to another aspect of the present invention, as a substituted for the Wilkinson power divider, the first power allocating part  231  and/or the second power allocating part  233  of  FIG. 2  may use a dual-directional coupler to scale up the coupling output port  225   a  or  225   b  of the coupling device  230  into a plurality of calibration ports. When radiating units in the smart antenna are used to support services in two or more frequency bands, the plurality of calibration ports, which are obtained by scaling up, can be used for calibration of the different frequency bands. 
       FIG. 6  is a schematic structural diagram of a calibration device  620  which employs dual-directional couplers to scale up a calibration port. The calibration device  620  includes a coupling device  630  similar to the coupler device  230  in  FIG. 2 , which includes a plurality of input ports  611 , a plurality of output ports  612  and coupling output ports  625   a  and  625   b . The coupling device  630  may adopt the structure as discussed above with reference to  FIGS. 3-4 . The calibration device  620  also includes a power distribution portion  631  for scaling up the coupling output port  625   a  into a plurality of calibration ports  632 - 1  and  632 - 2 . The power distribution portion  631  includes a dual-directional coupler  635 . The dual-directional coupler  635  includes a transmission line that is connected to the coupling output port  625   a  and two coupling lines that are coupled to the transmission line on opposite sides thereof, thereby having six ports. The through port  637  of the transmission line may be grounded via a matched load end so as to reduce reflection signals that enter the coupling device  630 . The matched load may be, for example, a 50 Ohm resistor. In some embodiments, isolation ports  636   a  and  636   b  that are adjacent port  637  on two coupling lines may be grounded via a matched load end to avoid interference of the reflection signals. In some embodiments, as illustrated in  FIG. 6 , isolation ports  636   a  and  636   b  may be connected to respective T-shaped bias circuits. The bias signal ports  638 - 1  and  638 - 2  of the T-shaped bias circuits are used for applying respective DC bias signals. The ports  639   a  and  639   b  may be terminated with fan-shaped traces via matched loads (e.g., 50 Ohm resistors) to allow DC signals to pass and to make high frequency signals virtually grounded. Accordingly, when a RF signal is input from the port  625   a  and a DC bias signal is applied at the bias signal port  638 - 1 , a composite signal of a coupled component of the RF signal and the DC bias signal will be obtained from the coupling port  632 - 1 . Likewise, a composite signal of a coupled component of the RF signal and the DC bias signal applied via the bias signal port  638 - 2  will be obtained from the coupling port  638 - 1 . 
     When a Wilkinson power divider is used, there is a direct connection between the calibration ports  632 - 1  and  632 - 2 , and DC bias signals added to different calibration ports may cause mutual interference. Therefore, a DC block is usually needed between two calibration ports in practice. In contrast, the use of a dual-directional coupler  635  can prevent DC connections between different calibration ports and achieve better performance than a DC block at a lower cost. In addition, the dual-directional coupler  635  achieves better return loss than the Wilkinson power divider and reduces the size of the calibration device. 
     It will be appreciated that although  FIG. 5  shows that the power distribution circuit  631  is coupled to a coupling device providing two independent coupling output ports according to an embodiment of the present invention, in other embodiments, the power distribution circuit  631  may be connected with calibration ports of various calibration devices in the prior art for scaling up the number of calibration ports. 
     The foregoing is illustrative of the present disclosure and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art should readily appreciate that many variations and modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such variations and modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.