Abstract:
A base station clustered adaptive antenna array includes a plurality of clusters of antenna elements. Each cluster is spaced away from an adjacent cluster by a first predetermined spacing related to receive-mode beamforming and includes a plurality of transmit-receive antenna elements. Each element within the cluster is spaced away from an adjacent element by a second predetermined spacing related to transmit-mode beamforming. In order to reduce the visual impact of the antenna array, each cluster is included within a single exterior housing or radome. A medial receive-only antenna element may be provided between adjacent clusters to enhance beamforming for reverse link reception.

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relates to wireless information communications systems. More particularly, the present invention relates to an adaptive antenna array of a base station formed as a plurality of spaced apart clusters of antenna elements lying generally within a common horizontal plane. 
     2. Related Art 
     Wireless data and voice communications services are proliferating throughout the world. One popular service is the so-called “cellular” telephone service. In cellular telephone service, service areas are divided up into “cells”, where each cell covers a specific geographical area and services mobile units located in, or passing through, the service area. Typically, radio frequencies are used in the ultra high frequency spectrum, and more typically in the 800 MHz or higher frequency range. The nature of radio wave propagation at these relatively short wavelengths limits the maximum effective distance between the mobile and the base, frequently to several miles. This propagation limit enables reuse of the same frequencies or bands within non-adjacent cells of the cellular network. Since the service range of each base station is limited to a radius of e.g. several miles, it is necessary to provide a number of base stations within a service area in order to provide effective wireless service throughout the area. 
     One known way to increase the number of mobile stations that may be served within a cell is to divide the cell into sectors, such as three sectors, spaced apart by 120° about the compass rose. In such an arrangement, each sector is provided with its own 120°-wide transmit beam from the base station. 
     Further increase in the number of mobile stations that may be simultaneously served within a cell or sector is to employ base station antenna arrays having plural elements. Embedding adaptive antenna array technology into the existing cellular telephone infrastructure potentially provides very significant capacity increases. This technology offers the ability to eliminate same cell interference for mobile stations being served simultaneously. It offers the prospect of a reduction of inter-cell interference. It also increases the signal-to-noise ratio of a particular mobile station being served and therefore enables an increase in user data rate. These benefits and advantages result in either higher data throughputs, or the ability to service more mobile stations simultaneously, within a given cell or service infrastructure. With spatially separated elements, beamforming becomes practical for both transmit and receive modes. Focusing radiant energy in the direction of a mobile station reduces the amount of overall power needed to be generated by the base station in order to maintain a given service quality. Antenna array technology can be used to focus power coming from the mobile station to the base station via a reverse link or an uplink, as well as from the base station to the mobile station via a forward link or downlink. 
     Usually, during transmit mode, a wide transmit beam is desired so that the transmit beam, and its associated pilot, reaches all of the mobiles within the service area or sector, since the base station does not initially know where any particular mobile would be within that area. In transmit mode, relatively wide transmit beams may be formed by using phased antenna elements of an array wherein the elements are spaced relatively closely apart, with spacing between adjacent elements being on the order of between one half and one wavelength at the transmit frequencies. At the cellular frequency bands in the range of 800 MHz, one wavelength equals 0.375 meters, or 14.775 inches, with one half wavelength being half of these linear values. After a particular mobile station is located within the service area of the base station, narrower transmit beams may be employed to divide and concentrate limited base station power among all of the mobile stations being served simultaneously. 
     In base station receive mode, very narrow beams are highly desirable in order to provide multiple beam diversities and concentrate the signal energy from a particular one of the mobiles operating within a particular one of the available service channels and to exclude or reduce signal energies from other mobiles within the same service area using other ones of the available service channels. Beamforming narrow beams in receive mode requires that the phased receive antenna elements be placed relatively farther apart than the transmit elements. Phased adjacent receive elements are most preferably placed apart by approximately three wavelengths. At 800 MHz, three wavelengths equals 1.125 meters or 44.325 inches. From these desirable spacings, it becomes immediately apparent that base station receive mode antenna arrays may become relatively quite large and visually noticeable at the base station locations within the neighborhoods of the various cellular communications service areas. Since the highest service requirements occur in the most highly populated areas, large base station antenna arrays become the subject of observation and complaint by a relatively large part of the population as a whole. One popular misconception held by some members of the public at large is that the larger the antenna array, the greater will be the exposure level to electromagnetic radiation at the vicinity of the array. Also, members of the public may object to what is perceived to be a negative visual impact or blight upon the environment of a particular neighborhood presented by large antenna arrays providing wireless communications services. 
     For example, FIG. 1 shows a conventional three-sector cellular antenna array  10  mounted at desired elevation above ambient terrain upon a triangular support tower  12 . A triangular support tower is frequently employed in wireless communications because it provides considerable strength with minimal material and takes advantage of the inherent strength of three-leg, triangle geometry in the horizontal plane and triangle bracing in the vertical planes of each tower face. The antenna array  10  is designed to serve three service sectors  14 ,  16  and  18 . For sector  14 , a transmit-receive element  20  is located at one corner of the tower  12 , and a receive-only element  22  is located at another corner of the tower  12  at a spacing selected to enable effective diversity reception. The antenna elements  20  and  22  are enclosed and protected from the weather by radomes  24 , typically formed of radio-wave-transparent material such as molded fiberglass or plastic. 
     The transmit-receive element  20  is adapted to broadcast a service beam throughout the sector  14 , and the element  20  may also be simultaneously used to receive at a different frequency or band with the inclusion of conventionally available duplexer filter technology, or may be used in a time division multiplex arrangement, with one time increment operating in transmit mode and a next time increment operating in receive mode. The receive mode element  22  provides spatial diversity reception for signals arising within the service sector  14 . Similarly, the service sector  16  includes transmit-receive element  26  and receive-only element  28 , and the service sector  18  includes transmit-receive element  30  and receive-only element  32 . While the arrangement of antenna array  10  in FIG. 1 enables some beamforming, very narrow receive-mode beams with additional array gains of about 5 dB with respect to a single antenna element are not achievable with only two spatially diverse receive antenna elements. 
     Narrow beamforming creating very narrow beams with high antenna array gains at the base station for both receive and transmit modes typically requires more antenna elements. FIG. 2A presents a more recently proposed antenna array for wireless cellular communications service which employs a relatively large multi-element receive antenna array  50  and a relatively small multi-element transmit antenna array  52 . While FIG. 2A shows the transmit array  52  to the side of the receive array  50 , the transmit array  52  may also be mounted concentrically with the receive array  50  on a tower, provided that a different elevation is used to prevent the transmit array  52  from being blocked by the receive array  50 . 
     The receive array  50  includes e.g. 16 separate receive elements  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 , and  84  disposed along a circular locus formed by a support ring  85 . The spacing between adjacent elements of the receive array  50  is preferably on the order of three wavelengths (3λ). Each element  54 - 84  is provided with its own radome  86 . As shown in the rectangular coordinate graph of FIG. 2B, the receive array  50  is capable of forming a relatively very narrow receive beam  88  in a particular direction within the service area relative to the array  50  with nearest adjacent side lobes  89  separated in phase from the beam  88  by approximately 22°. The receive array antenna beam pattern shown directed to 180° shown in FIG. 2B is a typical beam pattern that can be formed using the receive array  50 . The rectangular coordinate graph of FIG. 2C shows a receive array beam pattern directed to 90° and represents a typical beam pattern that can be formed using the receive array  50 . In the graph of FIG. 2C, the nearest adjacent side lobes  89  are shown separated in phase from the main lobe  88  by approximately 22°. 
     The transmit array  52  includes e.g. 16 separate transmit elements  90 ,  92 ,  94 ,  96 ,  98 ,  100 ,  102 ,  104 ,  106 ,  108 ,  110 ,  112 ,  114 ,  116 ,  118  and  120 , also disposed about a circular locus formed by a support ring  121 . The spacing between adjacent elements of the transmit array is preferably on the order of one-half wavelength (½λ) to one wavelength (1λ). Because of the relatively close spacing, all of the transmit elements  90 - 120  may be enclosed within a common radome  122 . As previously noted, the transmit array  52  may be located to the side of the receive array  50 , or preferably above or below the receive array  50  in a concentric arrangement shown in dashed outline relative to the receive array  50  in FIG.  2 A. The transmit antenna array  52  is arranged and operated to provide simultaneous transmit (downlink) signals for e.g. three sectors  124 ,  126  and  128 . 
     FIG. 2D depicts a typical, relatively narrow transmit beam pattern having a main lobe  130  focused at a direction of 180° with a maximum antenna gain (a main lobe 3 dB beamwidth at 17°, and side lobes  131  separated by 100°) formed using the transmit array  52  of FIG.  2 A. FIG. 2E depicts the relatively narrow transmit beam pattern formed by the transmit array  52  at a direction of 90°. FIG. 2F shows a typical relatively wide transmit beam having a single lobe  132  directed at 180° which may be formed by the transmit array  52 . The beam of FIG. 2F has a 3 dB beamwidth of 120°, and is typically used for transmitting common pilot and broadcast channel information for the particular sector being serviced. With the transmit array  52  shown in FIG. 2A, each sector can be provided with a relatively narrow transmit beam  130  (FIG. 2E) and a relatively wide transmit beam  132  (FIG.  2 F). The receive array  50  provides receive-mode (uplink) beamforming for all three sectors  124 ,  126  and  128 , in the present example. 
     Spatial diversity multiple access methods employing adaptive antenna arrays are described in U.S. Pat. Nos. 5,471,647 and 5,634,199 to Gerlach et al., and methods and structures for providing rapid beamforming for both uplink and downlink channels using adaptive antenna arrays are described in commonly assigned U.S. patent application Ser. No. 08/929,638 to Scherzer, entitled “Practical Space-Time Radio Method for CDMA Communication Capacity Enhancement”, all of which are incorporated herein by reference in their entirety. 
     While a number of benefits including increased service capacity can be realized by using adaptive antenna arrays, such arrays have heretofore been objected to by land use planning regulators because of concerns relating to perceived electromagnetic radiation hazards and concerns relating to objectionable or negative visual impact. Thus, an unsolved need has remained for a multi-element antenna array that provides such benefits while presenting a reduced visual impact at the base station location. 
     SUMMARY 
     A general object of the present invention is to provide a base station multi-element adaptive antenna array which manifests a reduced visual impact at the base station location while enabling effective forward link and reverse link beam forming. 
     Another object of the present invention is to group multiple receive-transmit antenna elements of an adaptive antenna array into a plurality of spatially separated clusters and to provide a single exterior housing or radome for each cluster. 
     A further object of the present invention is to provide additional receive only antenna elements medially between adjacent clusters of antenna elements of an adaptive antenna array. 
     In accordance with principles of the present invention, a base station clustered adaptive antenna array includes a plurality of clusters of antenna elements. Each cluster is spaced away from an adjacent cluster by a first predetermined spacing related to receive-mode beamforming. One such first spacing is equal to approximately ten wavelengths of the receive frequency or band. Each cluster includes a plurality (e.g., four) of transmit-receive antenna elements. Each element within the cluster is spaced away from an adjacent element by a second predetermined spacing related to transmit-mode beamforming. One such second spacing is equal to approximately between one-half and one wavelength of the transmit frequency or band. In order to reduce the visual impact of the antenna array, each cluster, in one embodiment, is included within a single exterior housing or radome. In one embodiment, the clustered adaptive antenna array includes three clusters mounted to corners of a support structure supporting a generally triangular frame with horizontal side dimensions approximating at least the first predetermined spacing of ten wavelengths of the receive frequency or band in one embodiment. The support frame may be an integral part of a triangular tower, or it may be a frame mounted to and supported at operational elevation by any suitable tower or structure, whether of triangular, circular, or other suitable cross-sectional geometry. 
     In accordance with a related aspect of the present invention, the clustered adaptive antenna array may include at least one receive antenna pod mounted medially between an adjacent pair of the plurality of clusters of antenna elements. In one embodiment of the triangular support tower, three medial receive antenna pods are provided, with a receive antenna pod being mounted medially between adjacent ones of the three clusters of antenna elements within a horizontal plane including the clusters. 
     The present invention will be more fully understood when taken in light of the following detailed description taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a known cellular telephone base station antenna array serving three sectors of a service cell; 
     FIG. 2A is a top plan view of a known multi-element circular receive array and a multi-element circular transmit array providing beamforming for receive mode (uplink) and transmit mode (downlink) base station communications within a service cell; 
     FIG. 2B depicts a typical receive beam pattern of the receive array of FIG. 2A at a direction of 180°; 
     FIG. 2C shows a typical receive beam pattern of the receive array of FIG. 2A at a direction of 90°; 
     FIG. 2D shows a typical transmit beam pattern with a maximum gain that can be formed using the circular transmit array of FIG. 2A focused at a direction of 180°; 
     FIG. 2E shows a typical transmit beam pattern with maximum gain using the circular transmit array of FIG. 2A focused at a direction of 90°; 
     FIG. 2F shows a typical transmit beam pattern having an effective 120° width that can be formed using the circular transmit array of FIG. 2A; 
     FIG. 3 is a top plan view of a base station clustered adaptive antenna array in accordance with one embodiment of the present invention; 
     FIG. 4 is a rectangular coordinate graph that depicts a relatively narrow forward link (transmit) beam pattern formed by one of the antenna clusters of the clustered adaptive array in FIG. 3, at a direction of 180°; 
     FIG. 5 depicts a relatively wide forward link beam pattern formed by one of the antenna clusters of the array in FIG. 3, at a direction of 180°; 
     FIG. 6 depicts a relatively narrow reverse link (receive) beam pattern formed by the clustered adaptive array including medial receive elements shown in FIG. 3, focused at a direction of 180°; and 
     FIG. 7 graphs a relatively narrow reverse link (receive) beam pattern formed by the clustered adaptive array including medial receive elements shown in FIG. 3, focused at a direction of 90°. 
    
    
     Use of the same or similar reference numbers in different figures indicates same or like elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a top view of a base station clustered adaptive antenna array  150  in accordance with one embodiment of the present invention. The array  150  may be mounted to and supported at a useful service height by a conventional support structure, such as a triangular metal tower  12  of the type illustrated in FIG.  1 . In this embodiment, the antenna array  150  is intended for use within the 1.9 GHz cellular telephone service. However, the principles of the present invention also apply to other land mobile wireless services and bands. 
     As shown in FIG. 3, the antenna array  150  includes three clusters  152 ,  154  and  156  of antenna elements lying generally within a common horizontal plane parallel to the surface of the earth. Each cluster is located at a corner of a triangular support structure  159 . Thus, in the present illustration, array cluster  152  is located at a corner  153 , array cluster  154  is located at a corner  155 , and array cluster  156  is located at a corner  157 . The antenna support structure  159  may be coextensive with a triangular tower, or it may be attached to, or extend from, another suitable support, such as a cylindrical column tower, for example. Each cluster  152 ,  154  and  156  can include a single radome enclosure  158  of radio-transparent material. The tower  12 , the support structure  159  and each radome enclosure  158  may be imparted with a dull color having a hue and tone selected to blend in with the environment, thereby minimizing visual impact of the antenna array  150  at the vicinity of the base station and tower  12 . 
     Each cluster may be mounted on an extension arm  163  which extends from each corner of the support structure  159  for a predetermined distance, to position each cluster outwardly from a center  161  of the support structure by a predetermined amount, such as approximately 78 cm. The length of each extension arm  163  is adjustable, in one embodiment, over a range of −15 cm to +30 cm relative to the center  161 . 
     Multiple closely spaced antenna elements are provided within each cluster. For example, in cluster  152 , four transmit-receive antenna elements  160 ,  162 ,  164  and  166  are provided. Adjacent ones of the elements  160 ,  162 ,  164  and  166  are angled apart from the extension arm by a mounting arm  167  in order to achieve a desired transmit array spacing of between one half wavelength and one wavelength at the operating frequency or band. In one embodiment, the length of the mounting arm  167  is approximately 15 cm and enables a −5 cm to +10 cm radial adjustment at the distal end of arm  163 . The angular spacing of adjacent receive-transmit elements within each cluster  152 ,  154 ,  156  is approximately 36° in one embodiment. In this manner, a relatively broad transmit (forward link) beam, as well as a relatively narrow transmit (forward link) beam may be transmitted to a service sector  170 , there being three 120° sectors  170 ,  172 , and  174  served by the antenna array  150 . 
     As shown in FIG. 3, antenna cluster  154  includes four transmit-receive antenna elements  180 ,  182 ,  184 , and  186  and provides wide/narrow beam forward link service to mobile stations in the sector  172 . Antenna cluster  156  includes four transmit-receive antenna elements  190 ,  192 ,  194 ,  196  and provides wide/narrow beam forward link service to mobile stations in the sector  174 . 
     FIG. 4 depicts a relatively narrow forward link beam pattern comprising a main lobe  220  directed to 180°, and having side lobes  222  at approximately ±90° formed by one of the clusters  152 ,  154 , or  156 . The main lobe  220  has a 3-dB beamwidth of 35°. The relatively narrow beam pattern of FIG. 4 is generally used for forward link traffic data transmissions. FIG. 5 graphs a relatively wide antenna beam pattern which may be formed by each one of the clusters  152 ,  154 ,  156 . The beam pattern of FIG. 5 includes a single lobe  224  shown directed at 180° and having a 3-dB beamwidth of 100°. The relatively wide beam pattern of FIG. 5 is generally used for forward link common pilot and broadcast channel transmissions. 
     In receive (reverse link) mode, all of the antenna elements  160 ,  162 ,  164 ,  166 ,  180 ,  182 ,  184 ,  186 ,  190 ,  192 ,  194  and  196  are used. Since there are three antenna clusters  152 ,  154 , and  156  in this example, narrow reverse link beamforming can be achieved by taking advantage of the spatial separation of the three clusters  152 ,  154 , and  156 . 
     Referring back to FIG. 3, further improvements in reverse link beamforming may be realized by adding single-receive element pods  200 ,  202  and  204  between the array clusters  152 - 154 ,  154 - 156 , and  156 - 152 , respectively, within the common horizontal plane of the array  150 . The pod  200  includes a receive element  201 , the pod  202  includes a receive element  203 , and the pod  204  includes a receive element  205 . These antenna elements can be of the same type as used in the array clusters. Each pod  200 ,  202 , and  204  is positionally mounted to the support structure  159  by a support arm  207 . In one embodiment, each support arm  207  is about 17 cm long. Each support arm  207  is offset from a center of a leg of the structure  159 , e.g., by approximately 20 cm, there being a range of adjustment of ±25 cm from the center of the leg in one embodiment. In this arrangement, one of the elements of each of the clusters  152 ,  154  and  156  becomes a transmit only element, and its receive function is redirected to a respective one of the medial receive elements  201 ,  203  and  205 . Small, visually minimized radomes  206  are used to enclose the medial receive elements  201 ,  203  and  205 , thereby protecting such element from exposure to the external ambient weather and atmospheric conditions. These radomes  206  may be provided with a color or finish treatment consistent with that applied to the radomes  158  and support structure  159  in order to minimize visual impact of the antenna array  150 . 
     FIG. 6 depicts a narrow reverse link antenna beam pattern that can be formed using the array  150  with the three medial receive elements  201 ,  203  and  205 . In FIG. 6, a main lobe  230  has a narrow beam width from 0 dB to −5 dB and becomes somewhat broader at −5 dB. The pattern of FIG. 6 is shown directed to 180°. FIG. 7 shows a beam pattern having a main lobe  232  directed to 90° which can be formed using the adaptive antenna array  150  with the three medial receive elements  201 ,  203  and  205 . The beam pattern shown in FIG. 7 is very similar the pattern achieved in the 180° direction shown in FIG.  6 . 
     The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as fall within the true spirit and scope of this invention.