Patent Publication Number: US-7224321-B2

Title: Broadband smart antenna and associated methods

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/592,084 filed Jul. 29, 2004, the entire contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of wireless communication systems, and more particularly, to a broadband smart antenna. 
   BACKGROUND OF THE INVENTION 
   In wireless communication systems, portable or mobile subscriber units communicate with a centrally located base station within a cell. The wireless communication systems may be a CDMA2000, GSM and WLAN communication system, for example. The subscriber units are provided with wireless data and/or voice services and can connect devices such as, for example, laptop computers, personal digital assistants (PDAs), cellular telephones or the like through the base station to a network. 
   Each subscriber unit is equipped with an antenna. To increase the communications range between the base station and the mobile subscriber units, and for also increasing network throughput, smart antennas may be used. Smart antennas may also be used with access points and client stations in WLAN communication systems. A smart antenna includes a switched beam antenna or a phased array antenna, for example, and generates directional antenna beams. 
   Example smart antennas are disclosed in U.S. Pat. Nos. 6,369,770 and 6,480,157. Both of these patents are assigned to the current assignee of the present invention, and are incorporated herein by reference in their entirety. Antennas in general have limited bandwidth, and smart antennas also exhibit this same behavior. 
   With the emergence of new wireless applications, there is a demand for smart antennas having a wider bandwidth than had been previously developed. A wider bandwidth often requires a more complex design, which could increase antenna loss. Alternatively, reactive components can be added to increase the bandwidth, but this adds to the cost of a smart antenna. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing background, it is therefore an object of the present invention to increase the bandwidth of a smart antenna with minimum increases in antenna loss and costs. 
   This and other objects, features, and advantages in accordance with the present invention are provided by a smart antenna comprising a ground plane, an active antenna element adjacent the ground plane, and a plurality of passive antenna elements adjacent the ground plane. The passive antenna elements may have different sizes for defining a plurality of different resonant frequencies for increasing a bandwidth of the smart antenna. A plurality of impedance elements may be connected to the ground plane, and may be selectively connectable to the plurality of passive antenna elements for antenna beam steering. 
   The different sizes of the plurality of passive antenna elements may correspond to passive antenna elements with different heights. The different sizes of the plurality of passive antenna elements may also correspond to passive antenna elements with different widths. 
   The different size passive antenna elements are thus stagger-tuned passive antenna elements, which creates a series of different resonant frequencies for increasing a bandwidth of the smart antenna. A wider bandwidth is advantageously achieved while minimizing additional antenna loss and production costs. 
   The smart antenna may further comprise a dielectric substrate, and the active antenna element and the plurality of passive antenna elements are carried by the dielectric substrate. The smart antenna may also further comprise a plurality of switches for selectively connecting the plurality of passive antenna elements to the plurality of impedance elements. 
   Each impedance element may be associated with a respective passive antenna element. Each impedance element may comprise an inductive load and a capacitive load. The inductive load and the capacitive load may be selectively connectable to the respective passive antenna element. 
   In lieu of the different size passive antenna elements (i.e., the passive antenna elements are the same size), each passive antenna element may comprise a dielectric layer thereon, with the dielectric layers having different dielectric constants for defining a plurality of different resonant frequencies for increasing a bandwidth of the smart antenna. The dielectric layers having different dielectric constants may also be used on different size passive antenna elements. Alternatively, the spacing between the active elements may be varied to each of the passive elements. 
   Another aspect of the present invention is directed to a mobile subscriber unit comprising a smart antenna for generating a plurality of antenna beams, and a beam selector controller connected to the smart antenna for selecting one of the plurality of antenna beams. A transceiver may be connected to the beam selector and to the smart antenna. The smart antenna is as defined above. 
   Yet another aspect of the present invention is directed to a method for making a smart antenna as defined above with either the different size passive antenna elements, and/or the dielectric layers with different dielectric constants on the passive antenna elements for defining a plurality of different resonant frequencies for increasing a bandwidth of the smart antenna. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a mobile subscriber unit with a smart antenna in accordance with the present invention. 
       FIG. 2  is an exploded view illustrating integration of the smart antenna in the mobile subscriber unit shown in  FIG. 1 . 
       FIG. 3  is a schematic diagram of the smart antenna shown in  FIG. 1  internal the mobile subscriber unit. 
       FIG. 4  is an exploded view illustrating integration of the smart antenna in the mobile subscriber unit shown in  FIG. 3 . 
       FIG. 5  is a schematic diagram of the smart antenna shown in  FIGS. 1–4 . 
       FIG. 6  is a schematic diagram of the smart antenna shown in  FIG. 5  on a dielectric substrate in close proximity to other handset circuitry. 
       FIG. 7  is a graph illustrating the operating bandwidth for the smart antenna in accordance with the present invention. 
       FIG. 8  is a schematic diagram of another embodiment of the smart antenna shown in  FIG. 5 . 
       FIG. 9  is a schematic diagram of yet another embodiment of the smart antenna shown in  FIG. 5 . 
       FIG. 10  is a schematic diagram of the switch and impedance elements for the passive antenna elements in accordance with the present invention. 
       FIG. 11  is a perspective view of another embodiment of a smart antenna in accordance with the present invention. 
       FIG. 12  is a perspective view of yet another embodiment of a smart antenna in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime and double prime notations are used to indicate similar elements in alternative embodiments. 
   Referring initially to  FIGS. 1–4 , the illustrated mobile subscriber unit  20  includes in  FIGS. 1 and 2  a smart antenna  22  that protrudes from the housing  24  of the mobile subscriber unit  20 , and in  FIGS. 3 and 4  a smart antenna that is internal the housing  24 . In both cases, the smart antenna  22  includes an active antenna element  30  and a plurality of passive antenna elements  32 . 
   The passive antenna elements  32  have different sizes for defining different resonant frequencies for the smart antenna  22 . By defining different resonant frequencies, the bandwidth of the smart antenna  22  is advantageously increased. The different sizes of the passive antenna elements  32  may be due to different heights, widths and/or thicknesses. 
   As an alternative, dielectric materials having different dielectric constants may coat “same-size” passive antenna elements in order to define different resonant frequencies for the smart antenna  22 . The different dielectric constants change the electrical characteristics of the passive antenna elements as if their heights, widths and/or thicknesses were changed. Of course, another configuration for defining different resonant frequencies for the smart antenna  22  is to coat “different-size” passive antenna elements  32  with dielectric materials having different dielectric constants. 
   The smart antenna  22  in accordance with the present invention provides for directional reception and transmission of radio communication signals with a base station in the case of a cellular handset, or from an access point in the case of a wireless data unit by making use of wireless local area network (WLAN) protocols. 
   In the exploded view of  FIGS. 2 and 4  illustrating integration of the smart antenna  22  into the mobile subscriber unit  20 , the smart antenna is formed on a printed circuit board and placed within a rear housing  24 ( 1 ) of the mobile subscriber unit. A center module  26  may include electronic circuitry, radio reception and transmission equipment, and the like. An outer housing  24 ( 2 ) may serve as, for example, a front cover of the mobile subscriber unit  20 . When the rear and outer housings  24 ( 1 ),  24 ( 2 ) are connected together, they form the housing  24  of the mobile subscriber unit  20 . 
   The printed circuit board implementation of the smart antenna  22  can easily fit within a handset form factor. In an alternate embodiment, the smart antenna  22  may be formed as an integral part of the center module  26  or of part of  24 ( 1 ) or  24 ( 2 ), resulting in the smart antenna and the center module being fabricated on the same printed circuit board. The ground portion  41  of the smart antenna  22  is embedded inside the housing  24 . 
   The smart antenna  22  may be disposed on a dielectric substrate  40  such as a printed circuit board, including the center active antenna element  30  and the outer passive antenna elements  32 , as illustrated in  FIGS. 5 and 6 . Each of the passive antenna elements  32  can be operated in a reflective or directive mode. 
   The active antenna element  30  comprises a conductive radiator disposed on the dielectric substrate  40 . The passive antenna elements  32  are also disposed on the dielectric substrate  40  and are laterally adjacent the active antenna element  30 . 
   To increase or broaden the bandwidth of the smart antenna  22 , the heights of the passive antenna elements  32  are selected so that they are different from one another. The heights of the passive antenna elements  32  are approximately one-quarter the wavelength of the operating frequency of the smart antenna  22 , which is the height of the active antenna element  30 . When the passive antenna elements  32  all have the same height, they in turn all have the same resonant frequency. 
   In the illustrated embodiment, the heights of the passive antenna elements  32  are selected so that the resonant frequencies of the passive antenna elements are different from one another. Slight variations in the resonant frequencies cause the bandwidth of the smart antenna  22  to increase. 
   The heights of the passive antenna elements  32  may be varied in multiples of one-eighth or one-sixteenth the wavelength of the operating frequency, for example. Of course, other multiples may be selected as long as different resonant frequencies are defined. 
   For example, the height of the passive antenna element  32 ( 1 ) is slightly less than the height of the active antenna element  30 , whereas the height of the passive antenna element  32 ( 2 ) is slightly greater than the height of the active antenna element. As an example, the height of the passive antenna element  32 ( 1 ) is three-eighths the wavelength of the operating frequency, and the height of the passive antenna element  32 ( 2 ) is five-eighths the wavelength of the operating frequency. The height of the active antenna element  30  is one-fourth the wavelength of the operating frequency. 
   If there was a third passive antenna element, then its height could be the same as the active antenna element  30 . Alternatively, the height of a third passive antenna element may be less than three-eighths or greater than five-eighths the wavelength of the operating frequency. 
   By changing the height of the passive antenna elements  32 , the corresponding resonant frequency for each passive antenna element is also changed. The passive antenna elements  32  in turn affect the induced resonance of the active antenna element  30 . By staggering the resonant frequencies of the passive antenna elements  32 , the overall bandwidth of the smart antenna  22  is broadened. 
   The measured result of a smart antenna  22  operating over a frequency range of 1.5 to 2 GHz within the PCS frequency band with stagger-tuned passive antenna elements  32  is provided in  FIG. 7 . The bandwidth of the smart antenna  22  as indicated by markers  51 ,  53  and  55  was increased by 70%. The stagger-tuned passive antenna elements  32  create a series of low-return loss dips in the frequency sweep of the smart antenna  22  to broaden the bandwidth, as indicated by line  47 . A wider bandwidth is advantageously achieved without incurring additional antenna loss, nor increasing production costs. 
   In lieu of varying the height of the passive antenna elements  32 , other changes include changing the widths/thicknesses while the heights remain the same, as illustrated in  FIG. 8 . The width/thickness of the passive antenna element  32 ( 1 )′ is a narrow as compared to the width/thickness of the active antenna element  30 ′. The width/thickness of the passive antenna element  32 ( 2 )′ is a thick as compared to the width/thickness of the active antenna element  30 ′. A combination of different heights and widths/thicknesses may also be selected for changing the resonant frequencies of the passive antenna elements  32 ′, as readily appreciated by those skilled in the art. 
   Yet another embodiment for changing the resonant frequencies of the passive antenna elements is to coat or place adjacent the passive elements  32 ″ a dielectric material  72 ″ in which different dielectric constant materials are used. Dielectric materials having different dielectric constants are readily known by those skilled in the art. The different dielectric constants change the electrical characteristics of the passive antenna elements  32 ″ without actually changing their sizes. 
   Alternatively, dielectric materials with different dielectric constants may also be used with different size passive antenna elements  32 ″. The material loading of the dielectric material  72 ″ of the passive antenna elements  32 ″ thus causes property changes of the passive antenna elements so that they alter the passband characteristics of the smart antenna  22 ″ or induce additional resonance that broaden the total band by adding to the original bandwidth. 
   The smart antenna  22  will now be discussed in greater detail while referring to  FIGS. 5 and 10 , for example. The active antenna element  30  and the passive antenna elements  32  are preferably fabricated from a single dielectric substrate such as a printed circuit board with the respective elements disposed thereon. The antenna elements  30 ,  32  can also be disposed on a deformable or flexible substrate 
   The passive antenna elements  32  each have an upper conductive segment  32 ( 1 ),  32 ( 2 ) as well as a corresponding lower conductive segment  82 ( 1 ),  82 ( 2 ). Capacitive and inductive loads  60 ( 1 ),  60 ( 2 ) are at the feed points of the passive antenna elements  32  for antenna beam steering. 
   The lower conductive segments  82 ( 1 ) and  82 ( 2 ) can also be adjusted to provide staggered-tuning. In other words, the length, width, thickness and dielectric loading can be changed to create an offset resonant frequency for staggered-tuning just like the staggered-tuning of the upper conductive segments  32 ( 1 ) and  32 ( 2 ). 
   Gain is expected to be reduced or increased when the height of the upper half of a passive antenna element is other than one-quarter the wavelength of the operating frequency. In some size constrained cases, this gain reduction may be acceptable to meet packaging requirements. However, a variety of techniques can be used to reduce this loss. In particular, the length of the embedded portion, i.e., the lower conductive elements  82 ( 1 ) and  82 ( 2 ), can be increased to compensate for the reduced height. 
   This in effect turns the passive antenna elements  32  into offset fed dipoles. The passive antenna elements  32  are used to perform as a reflector/director element with controllable amplitude and phase. There is no input impedance for a reactive load  60  to match. In fact, a lossless mismatch is desired so the length change and offset feeding do not hinder performance of the smart antenna  22 , as long as the loads  60  are low loss and the mismatch phase can be controlled. 
   For a passive antenna element  32  to operate in either a reflective or directive mode, the upper conductive segment  32 ( 1 ) is connected to the corresponding lower conductive segment  82 ( 1 ) via at least one impedance element  60 . The at least one impedance element  60  comprises a capacitive load  60 ( 1 ) and an inductive load  60 ( 2 ), and each load is connected between the upper and lower conductive segments  32 ( 1 )/ 82 ( 1 ) and  32 ( 2 )/ 82 ( 2 ) via a switch  62 . The switch  62  may be a single pole, double throw switch, for example. 
   When the upper conductive segment  32 ( 1 ) is connected to a respective lower conductive segment  82 ( 1 ) via the inductive load  60 ( 1 ), the passive antenna element  32  operates in a reflective mode. This results in radio frequency (RF) energy being reflected back from the passive antenna element  32  towards its source. 
   When the upper conductive segment  32 ( 1 ) is connected to a respective lower conductive segment  82 ( 1 ) via the capacitive load  60 ( 2 ), the passive antenna element  32  operates in a directive mode. This results in RF energy being directed toward the passive antenna element  32  away from its source. 
   A switch control and driver circuit  64  provides logic control signals to each of the respective switches  62  via conductive traces  66 . The switches  62 , the switch control and driver circuit  64  and the conductive traces  66  may be on the same dielectric substrate  40  as the antenna elements  30 ,  32 . 
   As noted above, electronic circuitry, radio reception and transmission equipment, and the like may be on the center module  26 . Alternatively, this equipment may be on the same dielectric substrate  40  as the smart antenna  22 . As illustrated in  FIG. 6 , this equipment includes a beam selector  70  for selecting the antenna beams, and a transceiver  72  coupled to a feed  68  of the active antenna element  30 . 
   An antenna steering algorithm module  74  runs an antenna steering algorithm for determining which antenna beam provides the best reception. The antenna steering algorithm operates the beam selector  70  for scanning the plurality of antenna beams for receiving signals. 
   Different embodiments of the smart antenna will now be discussed with reference to  FIGS. 11–12 . One embodiment of the smart antenna  122  comprises four antenna elements  190 ,  192  placed on a planar triangular ground plane  194 , as illustrated in  FIG. 11 . Three of the antenna elements  190 ( 1 ),  190 ( 2 ) and  190 ( 3 ) are placed on the corners of the triangular ground plane  194  and one of the antenna elements  192  is placed at the center point of the triangular ground plane. The illustrated shape of the ground plane  194  and the illustrated number of antenna elements  190 ,  192  may vary depending on the intended applications, as readily appreciated by those skilled in the art. 
   In one form of a switched beam antenna, the 3 outer antenna elements  190 ( 1 ),  190 ( 2 ) and  190 ( 3 ) are passive and the center antenna element  192  is active. The passive elements  190 ( 1 ),  190 ( 2 ) and  190 ( 3 ) act together with the active element  192  to form an array. In accordance with the present invention, the height of at least two of the passive antenna elements are different from one another in order to stagger the resonant frequencies of the illustrated smart antenna  122 . 
   To alter the radiation pattern, the termination impedances of the passive elements  190 ( 1 ),  190 ( 2 ) and  190 ( 3 ) are switchable to change the current flowing in these elements. The passive elements  190 ( 1 ),  190 ( 2 ) and  190 ( 3 ) become reflectors when shorted to the ground plane  194  using pin diodes, for example. When the passive elements  190 ( 1 ),  190 ( 2 ) and  190 ( 3 ) are not shorted to the ground pane  194 , they have little effect on the antenna characteristics. 
   In another embodiment, the antenna elements  190 ,  192  are all active elements and are combined with independently adjustable phase shifters to provide a phased array antenna. In this embodiment, multiple directional beams as well as an omni-directional beam in the azimuth direction can be generated. 
   Essentially, the phased array antenna includes multiple antenna elements and a like number less one of adjustable phase shifters, each respectively coupled to one of the antenna elements. The phase shifters are independently adjustable (i.e., programmable) to affect the phase of respective downlink/uplink signals to be received/transmitted on each of the antenna elements. 
   A summation circuit is also coupled to each phase shifter and provides respective uplink signals from the subscriber device to each of the phase shifters for transmission from the subscriber device. The summation circuit also receives and combines the respective downlink signals from each of the phase shifters into one received downlink signal provided to the subscriber device  20 . 
   The phase shifters are also independently adjustable to affect the phase of the downlink signals received at the subscriber device  20  on each of the antenna elements. By adjusting phase for downlink link signals, the smart antenna  122  provides rejection of signals that are received and that are not transmitted from a similar direction as are the downlink signals intended for the subscriber device  20 . 
   Another embodiment of the smart antenna  122 ′ is illustrated in  FIG. 12  where the three antenna elements  190 ( 1 )′,  190 ( 2 )′ and  190 ( 3 )′ placed at the corners of the triangular ground plane  194 ′ have independently adjustable reactive load elements in the upper and lower halves of the antenna elements. The upper halves of the antenna elements are represented by references  190 ( 1 )′,  190 ( 2 )′ and  190 ( 3 )′, wherein the corresponding lower halves are represented by references  200 ( 1 )′,  200 ( 2 )′ and  200 ( 3 )′. Such an embodiment can provide a plurality of beams that are directional in azimuth and/or elevation. 
   The independently adjustable reactive load elements include varactors or mechanically insertable RF choke elements, for example, to provide asymmetrical loading on the antenna elements. This results in antenna beams being formed that are directional in elevation. 
   Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.