Patent Publication Number: US-2003227420-A1

Title: Integrated aperture and calibration feed for adaptive beamforming systems

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates generally to beam forming antennas, and more particularly to calibration of such antennas.  
       BACKGROUND OF THE INVENTION  
       [0002] Many antennas used for cellular communications today are constructed by mounting multiple radiating elements in vertical columns. The width of the total aperture determines the width of the beam formed by the antenna. The spacing between the columns determines the antenna&#39;s ability to be scanned, e.g. the ability to point the beam of the antenna.  
       [0003] The beam of these antennas is controlled by varying the amplitude and phase of the signals feeding the columns. A system that varies the amplitude/phase signal to each of the columns is often referred to as an “adaptive beamformer.” 
       [0004] In certain applications, individual amplifiers are used to power each of the columns in an antenna. These amplifiers often vary in response, i.e. phase and amplitude. Couplers, cabling, splitters, etc. used between these amplifiers and the columns can also vary in response. Thus, if a beamformer is placed before the amplifiers, the beamwidth is often not varied as selected by the beamformer due to variation in the response of the amplifiers, as well as other devices between the amplifiers and the columns.  
       [0005] One method of dealing with these variations is to do what is referred to as “calibrating” the antenna. One method involves the process of driving a first column of the antenna and simultaneously monitoring a second column to sample the radiation of the first column. This process of driving a first column and sampling a second column is typically repeated for each column until a calibration factor is determined for each column. Once calculated, the calibration factors for each column may then be added to any desired signal in the beamformer to properly form a beam during the normal operation of the antenna.  
       [0006] One shortcoming of this method is due to the variation of the radiating elements used in the second column to sample the radiation of the first column. Due to element variation in the second column, e.g., amplitude and phase, losses, etc., a calibration factor calculated for the first column may be affected by the variation of the radiating elements in the second column. Later, when the calibration factor for the first column is used for beamforming, the beam is not formed as accurately as it might have been had variation from the radiating elements from the second column not been introduced into the calibration for the first column. Another shortcoming of this method is that in order to perform the calibration, it may be necessary to reconfigure the cabling to the antenna. Another shortcoming is that this technique is affected by the external environment due to the fact that the signal is radiated by one column and received by another.  
       [0007] Therefore, a need exists for a manner of calibrating an antenna to be used in conjunction with a beamforming system that is easy to use, relatively simple in design and that can be manufactured at a relatively low cost. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] The invention and further objectives and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:  
     [0009]FIG. 1 is a perspective view of an antenna incorporating an integrated calibration feed consistent with the present invention, with a portion thereof shown in phantom.  
     [0010]FIG. 2 is a cross section of the substrate of a portion of the antenna shown in FIG. 1, taken along lines  2 - 2 .  
     [0011]FIG. 3 is an illustration of the inner conductive layer of a portion of the antenna shown in FIGS. 1 and 2, with repetitive portions shown in phantom, and showing the relative location of the dual polarized radiators relative thereto.  
     [0012]FIG. 4 is an enlarged perspective view of a dual polarized radiator shown in FIGS. 1 and 3.  
     [0013]FIG. 5 is an enlarged perspective view of a dual polarized radiator mounting area shown in FIGS. 1 and 3.  
     [0014]FIG. 6 is a schematic diagram of another antenna consistent with the present invention.  
     [0015]FIG. 7 is a flow chart illustrating the steps of a receive calibration process using the antenna of FIG. 6.  
     [0016]FIG. 8 is a flow chart illustrating the steps of a transmit calibration process using the antenna of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0017] The present invention provides an integrated aperture and calibration feed for adaptive beamforming systems that is easy to use, relatively simple in design and that can be manufactured at a relatively low cost. Such an integrated aperture and calibration feed often eliminates the use of a second column in calibrating a first column in an antenna, thereby providing a manner of calibrating the first column independent any variation in the radiating elements of a second column. Furthermore, in many embodiments the integrated aperture and calibration feed may facilitate improved beamforming without requiring any cable reconfiguration or errors due to external environment.  
     [0018] With reference to FIGS.  1 - 5 , there is shown one embodiment  10  of an antenna in accordance with the principles of the present invention. Antenna  10  comprises a substrate, divided into two sections at reference numerals  12   a ,  12   b , and a plurality of dual polarized radiators  14  coupled to the substrate  12   a ,  12   b . Antenna  10  is approximately two feet wide and four feet high. The substrate can be a single piece, or may be formed from multiple sections as is shown in FIG. 1. Although antenna  10  is constructed using a divided substrate, antennas using a unitary, or one piece, substrate may be constructed without departing from the spirit of the present invention.  
     [0019] Dual polarized radiators  14  are arranged into a plurality of rows  16   a - h ,  16   a ′- 16   h ′ and columns  18   a - h . Dual polarized radiators  14  in rows  16   a - h  in columns  18   a ,  18   c ,  18   e ,  18   g  are offset with respect to the dual polarized radiators in rows  16   a ′- h ′ in columns  18   b ,  18   d ,  18   f ,  18   h , so as to equally space the dual polarized radiators  14  diagonally. The offset and accompanying equal spacing reduces the mutual coupling between adjacent dual polarized radiators  14  in adjacent columns  18   a - h , improving performance and cross-polarization isolation. Other relative spacings of radiators may be used in the alternative.  
     [0020] Since each portion  12   a ,  12   b  contains an equal number and like spacing of the radiators  14 , and like feed networks, as will be shown in FIG. 3, the details of substrate  12   b  are not shown. One skilled in the art will readily appreciate that substrate  12   b  may be configured to function like substrate  12   a , or may be differently configured in some applications.  
     [0021] Although the embodiment  10  of FIGS.  1 - 5  contains eight columns and eight rows, and each column  18   a - h  and each row  16   a,a ′- 16   h - h ′ contains eight radiators  14 , other embodiments of the invention may be constructed using any number of columns or rows containing any number of radiators. Further, while the embodiment of FIGS.  1 - 5  uses dual polarized dipole radiators with a common phase center to realize dual slant forty-five degree (45°) polarization with close column spacing, those skilled in the art will recognize that other embodiments of the present invention may be configured with other radiating elements, such as vertically or horizontally oriented dipoles, etc.  
     [0022] Referring now to FIG. 2, a cross section of a portion of the substrate  12   a ,  12   b  shown in FIG. 1 is illustrated. Substrate  12   a ,  12   b  comprises an inner layer  20  etched or deposited on a dielectric material  22  located between upper and lower sheets of dielectric material  24 ,  26 , the outer surfaces of which have upper and lower ground planes  28 ,  30  etched or deposited thereon. Inner layer  20  may be about one once (1 oz.) finished copper. Dielectric material  22  may be about 0.004″ thick Rogers material. Dielectric materials  24 ,  26  may be about 0.032″ thick Rogers material. Ground planes  28 ,  30  may be about two ounce (2 oz.) finished copper, making substrate  12   a ,  12   b  about 0.068″ plus or minus (+/−) 0.005″ thick when assembled. As is common practice in the art, vias  21  may be used to connect the inner layer  20  to conductive materials in the upper and lower ground planes  28 ,  30 , e.g., to provide an external connection point and/or to interface with a radiator  14  (as will be described below). Those skilled in the art will appreciate that the forgoing is merely exemplary of the possible materials, layouts, manufacturing processes, etc. that could be used for substrate  12   a ,  12   b . As such, the present invention is not intended to be limited in the type of substrate used for various embodiments.  
     [0023] Referring to FIG. 3, inner layer  20  of substrate portion  12   a  of antenna  10  shown in FIGS. 1 and 2 is illustrated in greater detail. Columns  18   a ,  18   b  including rows  16   a - d  and  16   a ′- d ′, respectively, are shown for purposes of illustration, whereas columns  18   c - h  are shown in phantom line due to redundancy. The locations of the radiators  14  are also shown in phantom line. Further, only substrate portion  12   a  is illustrated, as substrate portion  12   b  essentially mirrors substrate portion  12   a , as will be discussed.  
     [0024] For each column  18   a - h  of substrate  12   a ,  12   b , a pair of stripline distribution, or corporate feed, networks  32   a ,  32   b  are disposed in inner layer  20  and coupled to dual polarized radiators  14 . Moreover, for each column, networks  32   a  and  32   b  are routed on opposite sides of the column. For column  18   a , for example, corporate feed  32   a  is coupled to the first radiating elements  34   a  and corporate feed  32   b  is coupled to the second radiating elements  34   b  (shown in FIGS. 4 and 5) of the dual polarized radiators  14 . Corporate feed network  32   a  extends along a first side  36  of column  18   a , and corporate feed network  32   b  extends along a second side  38  of column  18   a.    
     [0025] The portions of the corporate feed networks  32   a ,  32   b  on substrate portions  12   a  and  12   b  are connected together at reference numeral  42 . At locations  42 , a portion of upper and lower dielectric  24 ,  26  is relieved so that portions of the corporate feed networks  32   a ,  32   b  on substrates  12   a ,  12   b  may be soldered together.  
     [0026] Electrical connectivity with the corporate feed networks  32   a ,  32   b  may be provided through connectors located at reference numeral  40 . As illustrated, connectors at locations  40  for columns  18   a ,  18   c ,  18   e ,  18   g  are on substrate  12   a  while the connectors  40  for columns  18   b ,  18   d ,  18   f ,  18   h  are on substrate  12   b.    
     [0027] Substrate portions  12   a ,  12   b  also include a stripline calibration feed network  44 . Stripline calibration feed network  44  includes calibration feed traces  48  that extend along the columns  18   a - h  of dual polarized radiators  14  intermediate the first and second corporate feed networks  32   a ,  32   b , terminating in couplers  46   a  and  46   b . As illustrated, the calibration feed traces  48  are aligned with the common phase centers of the dual polarized dipole radiators  14 , realizing dual slant forty-five degree (45°) polarization. Couplers  46   a ,  46   b  for columns  18   a ,  18   c ,  18   e ,  18   g  and their feed traces  48  are disposed on substrate portion  12   a  while couplers  46   a ,  46   b  and their feed traces  48  for columns  18   b ,  18   d ,  18   f ,  18   h  are disposed on substrate portion  12   b . Stripline calibration feed network  44  also includes a location  42  for soldering portions of the stripline calibration feed network on substrate portions  12   a ,  12   b  together. A connector location  41  is also advantageously provided for the stripline calibration feed network  44  on substrate  12   a . Those skilled in the art will appreciate that other connector locations are possible without departing from the spirit of the present invention.  
     [0028] The calibration feed traces  48  and couplers  46   a ,  46   b  alternate between columns  18   a ,  18   c ,  18   e ,  18   g  on substrate portion  12   a  and columns  18   b ,  18   d ,  18   f ,  18   h  on substrate portion  12   b  so that mutual coupling between adjacent columns  18   a - h  and the calibration feed network  44  is reduced. The calibration feed network  44  includes portions proximate the ends of each column where the calibration feed traces are joined to connector location  41 . For example, as illustrated in FIG. 3, for substrate portion  12   a , columns  18   a  and  18   c  and columns  18   e  and  18   g  are joined together. The junctions of columns  18   a  and  18   c  and columns  18   e  and  18   g  are then joined together and connected to connector location  41 . Substrate  12   b  includes a similar arrangement at the other end of the columns  18   a - h  for columns  18   b ,  18   d ,  18   f ,  18   h , respectively.  
     [0029] Those skilled in the art will appreciate that calibration feed network  44 , calibration feed traces  48  and couplers  46   a ,  46   b  could be located elsewhere on substrate portions  12   a  and  12   b  without departing from the spirit of the present invention. Such a calibration feed network  44 , calibration feed traces  48  and couplers  46   a ,  46   b  could be located on the inner layer  20  of either substrate portion  12   a  or  12   b  solely, without departing from the spirit of the present invention. Further, such a calibration feed network  44  could also be located in another layer of substrate  12   a ,  12   b  without departing from the spirit of the present invention. However, such a configuration of substrate  12   a ,  12   b  may increase costs.  
     [0030] Couplers  46   a ,  46   b  are formed by adjacent portions of the corporate feed networks  32   a ,  32   b  and the end of feed traces  48 , the end most portions being configured as loads. Such a physical arrangement on inner layer  20 , as indicated at  46   a  and  46   b , allows bidirectional coupling of an electrical signal between the corporate feed networks  32   a ,  32   b  and the distribution network  48 . Those skilled in the art will appreciate that other types of proximity couplers, with or without loading, may be used without departing form the spirit of the present invention.  
     [0031] Referring to FIG. 4, an enlarged view of a dual polarized radiator  14  is shown. Dual polarized radiator  14  comprises a first radiating element  34   a  and a second radiating element  34   b . Radiating elements  34   a ,  34   b  are deposited or etched on dielectric material  50   a ,  50   b , as is well known in the art. Dielectric material  50   a ,  50   b  include a tab portion  52   a ,  52   b  for mounted the dual polarized radiator to substrate portions  12   a ,  12   b.    
     [0032] Referring to FIG. 5, an enlarged view of a dual polarized radiator  14  mounting area is illustrated. Dual polarized radiator  14  is mounted to a substrate portion  12   a ,  12   b  by inserting tabs  52   a ,  52   b  into corresponding slots  54   a ,  54   b  in substrate  12   a ,  12   b  and soldering the radiating elements  34   a ,  34   b  to lands  56   a ,  56   b  etched out of the ground plane  28 . Connection of the radiating elements  34   a ,  34   b  to corporate feed networks  32   a ,  32   b  on inner layer  20  may be made through vias  58   a ,  58   b , respectively.  
     [0033] In operation, to calibrate the antenna  10  for transmission, a signal at a desired transmission frequency is applied to each connector location  40  for each of the columns  18   a - h . The signal may be applied to the columns  18   a - h  sequentially or simultaneously, e.g., if a code division multiple access (CDMA) code may be used for each column  18   a - h . The signal, in each column  18   a - h , couples through locations  46  to calibration feed traces  48  in the stripline calibration feed network  44 . The coupled signal may then be measured at connector location  41  for the stripline calibration feed network  44  and a calibration factor calculated for each column  18   a - h , so that the radiation for each column  18   a - h  is equal after application of the calibration factor. A beamformer used with the antenna  10  may then multiply the signal for each column by the column transmit calibration factor to properly form a transmit beam, independent an adjacent column&#39;s radiators.  
     [0034] Similarly, to calibrate the antenna  10  for reception, a signal at a desired reception frequency is applied to connector location  41  of the stripline calibration feed network  48 . The applied signal travels down the calibration feed traces  48  and couples through locations  46  to distribution feed networks  32   a ,  32   b  and connector locations  40  for columns  18   a - h . The signal at columns  18   a - h  may then be measured and a calibration factor calculated for each column  18   a - h , so that the signal from each column  18   a - h  is equal. A beamformer used with the antenna  10  may then multiply the signal received by each column by the column receive calibration factor to properly form a receive beam, independent an adjacent column&#39;s radiators.  
     [0035] Referring to FIG. 6, and for the purposes of further illustrating a calibration method consistent with the invention, a schematic diagram of an antenna  60  is illustrated. Antenna  60  comprises a plurality of radiators  62  arranged into a plurality of columns  64   a - d , corresponding to receive/transmit (RX/TX) columns  1  through  4 . Each column  64   a - d  includes a distribution network  66   a - d . Antenna  60  further comprises a calibration feed network  68  having a calibration port (CAL) and including calibration feed traces  70   a - d , each terminated in a load  72   a - d . Mutual coupling occurs between calibration feed traces  70   a - d  and distribution networks  66   a - d  in areas  74   a - d , respectively.  
     [0036] Referring now to FIGS. 6 and 7, FIG. 7 is a flow chart of a receive calibration routine  80  for the antenna  60  of FIG. 6. In order to calibrate the four reception paths, denoted as RX 1 - 4 , a calibration signal, at a desired reception frequency, is applied to the calibration port (CAL) at step  82 .  
     [0037] The calibration signal traverses the calibration feed network  44  and the calibration feed traces  48   a - d  coupling through areas  46   a - d  into reception paths RX 1 - 4 . The coupled signal is then sampled for each path, or column  18   a - h , at step  84 .  
     [0038] In step  86 , a receive calibration factor for each path is calculated so that RX 1 =RX 2 =RX 3 =RX 4 . The receive calibration factors for RX 1 - 4  calibration are then exported for use, such as by a beamformer, in step  88 .  
     [0039] Referring now to FIGS. 6 and 8, FIG. 8 is a flow chart of a transmit calibration routine  90  for the antenna  60  of FIG. 6. At step  92 , a calibration signal, at a desired transmit frequency, is applied to TX 1  and sampled at the calibration port (CAL) in step  94 . This process is repeated for TX 2 - 4 , as shown at  96 , until a sample is made for each transmit path, TX 1 - 4 . Once a sample is made for each transmit path, a transmit calibration factor for each path is calculated so that TX 1 =TX 2 =TX 3 =TX 4  at step  98 . The transmit calibration factors for TX 1 - 4  are then exported for use, such as by a beamformer, in step  100 .  
     [0040] By virtue of the foregoing, there is thus provided a integrated aperture and calibration feed for a beamforming system for use in varying the beamwidth of an antenna that is easy to use, relatively simple in design and that can be manufactured at a relatively low cost.  
     [0041] Various other modifications may be made to the herein-described embodiments without departing from the spirit and scope of the invention. For example, it will be appreciated that a wide variety of alternate antenna arrangements, including various alternate electronic components, layouts and the like, may be used consistent with the invention. Alternate routings of traces and/or positioning of connectors, e.g., at one end of the substrate or columns, etc., also may be used without departing from the spirit of the present invention. Therefore, the invention lies in the claims hereinafter appended.