Patent Application
20060022889
The present invention relates to the field of wireless communication systems, and more particularly, to a smart antenna operating in different antenna beam modes.
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 or WLAN communication system, for example. The subscriber units are provided with wireless data and/or voice services by the system operator 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.
A switched beam antenna includes an active antenna element and one or more passive antenna elements. Each passive antenna element is connected to a respective impedance load by a corresponding switch. By selectively switching the passive antenna elements to their impedance load, a desired antenna pattern is generated. When a passive antenna element is connected to an inductive load, radio frequency (RF) energy is reflected back from the passive antenna element towards the active antenna element. When a passive antenna element is connected to a capacitive load, RF energy is directed toward the passive antenna element away from the active antenna element. A switch control and driver circuit provides logic control signals to each of the respective switches.
For a switched beam antenna comprising an active antenna element and two passive antenna elements, for example, there are four different switching combinations for selecting a desired antenna beam if the switch is a single pole double throw (SPDT). Each switching combination corresponds to a different antenna beam mode, and consequently, the input impedance to the active antenna element changes between the difference modes. The efficiency of the smart antenna varies as the input impedance varies.
Similarly, in a phased array antenna, when the relative phases fed to the respective antenna elements are changed, the input impedances also vary. The phase changes are integral to the beam scanning and adaptive beam forming of a phased array antenna. This makes it difficult to match the input impedances of the various modes. To obtain a reasonable match for required beam shapes and positions, dynamic matching circuits are often used, which further add to the complexity and cost of a phased array antenna.
In view of the foregoing background, it is therefore an object of the present invention to match the input impedances of a smart antenna when operating in different antenna beam modes.
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 having a radio frequency (RF) input associated therewith, and a plurality of passive antenna elements adjacent the ground plane. A plurality of impedance elements is connected to the ground plane and is selectively connectable to the plurality of passive antenna elements for antenna beam steering. A plurality of tuning elements is adjacent the plurality of passive antenna elements for tuning thereof so that an input impedance of the RF input of the active antenna element remains relatively constant during the antenna beam steering.
The tuning elements are used to match the input impedances of the multiple antenna modes of the smart antenna by tuning the passive antenna elements. The tuning elements are essentially sub-resonant parasitic antenna elements, and are sized so that they do not interfere with the antenna patterns generated by the smart antenna. A Smith chart is used to determine the size, shape and spacing of the tuning elements, which varies between the particular applications of the smart antenna.
The tuning elements may be connected to ground. The passive antenna elements may define at least one resonant frequency, while tuning elements preferably define at least one sub-resonant frequency. The tuning elements may be positioned between the active antenna element and the passive antenna elements. At least one tuning element is adjacent a respective passive antenna element for tuning thereof.
The smart antenna may further comprise a dielectric substrate. The active antenna element, the passive antenna elements and the tuning elements may be 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, with the inductive load and the capacitive load being selectively connectable to the respective passive antenna element.
Another aspect of the present invention is directed to a mobile subscriber unit comprising a smart antenna as defined above for generating a plurality of antenna beams, a beam selector controller connected to the smart antenna for selecting one of the plurality of antenna beams, and a transceiver connected to the beam selector and to the smart antenna.
Yet another aspect of the present invention is directed to a method for matching an input impedance of a smart antenna as defined above. The method preferably comprises tuning the passive antenna elements by positioning the tuning elements adjacent thereof so that the input impedance of the RF input of the active antenna element remains relatively constant during the antenna beam steering.
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.
Referring initially to
As will be discussed in greater detail below, the tuning elements 34 are used to match the input impedances of the multiple antenna modes of the smart antenna 22 by tuning the passive antenna elements 32. The tuning elements 34 are essentially sub-resonant parasitic antenna elements, and are sized so that they do not interfere with the antenna patterns generated by the smart antenna 22. Size, shape and spacing of the tuning elements 34 vary between the particular applications of the smart antenna 22.
The smart antenna 22 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 making use of wireless local area network (WLAN) protocols.
In the exploded views of
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, 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.
Protrusion of the active and passive antenna elements 30 and 32 as well as the tuning elements 34 allows the elements to radiate freely. Although not illustrated, a protective coating or shield may optionally cover the active and passive antenna elements 30, 32 and the tuning elements 34. The illustrated shape of the active and passive antenna elements 30, 32 reduces the height of the smart antenna 22 protruding from the housing 24 of a mobile subscriber unit 20 to improve portability and appearance, as readily appreciated by those skilled in the art.
The smart antenna 22 will now be discussed in greater detail with reference to
The tuning elements 34 are parasitic antenna elements, and are sized so that they define a sub-resonant frequency that is less than the resonant frequencies defined by the passive antenna elements. This ensures that the tuning elements 34 do not interfere with the antenna patterns generated by the smart antenna 22. The illustrated tuning elements 34 are monopole antenna elements connected to ground 41.
Since the illustrated smart antenna 22 is a low profile antenna, the active antenna element 30 comprises a conductive radiator in the shape of a “T” disposed on the dielectric substrate 40. The passive antenna elements 32 are also disposed on the dielectric substrate 40 and each comprises an inverted L-shaped portion laterally adjacent the active antenna element 30. The T-shaped active antenna element 30 and the L-shaped portions of the passive antenna elements 32 advantageously reduce the height of the smart antenna 22 protruding from the housing 24 of the mobile subscriber unit 20.
Reduction in the length of protrusion of the active antenna element 30 from the housing 24 of the mobile subscriber unit 20 is accomplished by providing a top loading, and at the same time providing a slow wave structure for the body of the antenna. One of the technologies available for radiating element size reduction is meander-line technology. Other techniques can include dielectric loading, and corrugation, for example. The illustrated structure for the active antenna element 30 is a meander-line, which is illustrated as an example.
The use of the tuning elements 34 is not limited to a low-profile smart antenna 22. The active and passive antenna elements 30, 32 may be standard monopole shaped antenna elements, as readily appreciated by those skilled in the art. The active antenna element 30, the passive antenna elements 32 and the tuning elements 34 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 and the tuning elements 34 can also be disposed on a deformable or flexible substrate.
The illustrated passive antenna elements 32 each have an upper conductive segment 32(1) (including the L-shaped portion) as well as a corresponding lower conductive segment 32(2). The height of the passive antenna elements 32 is reduced by bending the top portion thereof to produce the inverted L-shape. Alternatively, top loading may be used.
The inverted L-shape is made to meet the top loading segment of the active antenna element 30, but not touching, in such a manner that more power can be coupled from the active antenna element 30 to the passive antenna elements 32 for optimum beam formation. The height of the active antenna element 30 and the upper conductive segment 32(1) of the passive antenna elements 32 shown in the figure is 0.6 inches, which corresponds to the smart antenna 22 operating at a frequency of 1.87 GHz.
Gain is expected to be reduced when the physical size of the smart antenna 22 is reduced. 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. Since the desired height reduction is in the portion of the smart antenna 22 outside the housing 24, the length of the embedded portion, i.e., the lower conductive elements 32(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 perform as reflector/director elements with controllable amplitude and phase. 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 lower conductive segment 32(2) 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), 32(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 32(2) via the inductive load 60(2), 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, i.e., the active antenna element 30.
When the upper conductive segment 32(1) is connected to a respective lower conductive segment 32(2) 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 the active antenna element 30.
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 and the tuning elements 34.
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
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.
Since a two-position switch 62 is used for each of the two passive antenna elements 32, four antenna modes are available. In other words, each switching combination corresponds to a different antenna mode. The input impedance to the active antenna element changes between the difference antenna modes. Ideally, the input impedance is 50 ohms. However, this value changes among the four different antenna modes, which in turn reduces the efficiency of the smart antenna 22. When the efficiency of the smart antenna 22 is reduced, the VSWR is increased.
The four different antenna modes for the smart antenna 22 are illustrated in
The tuning probes 34 will now be discussed in greater detail. The tuning probes 34 are miniature parasitic antenna elements that are used to fix-tune each passive antenna element 32. These miniature elements are essentially sub-resonant parasitic antennas. When monopoles are used, the sub-resonant antennas are connected to ground 41. The tuning probes 34 are sized so that they define a sub-resonant frequency so that they do not interfere with the radiation patterns generated by the passive antenna elements 32. When multiple tuned states are required by the smart antenna 22, more than one sub-resonant parasitic element may be used for each passive antenna element 32.
The tuning elements 34 are designed with the proper size, shape and spacing from their host passive antenna elements 32 to be effective. The manner that the tuning elements 34 can fit between the active antenna element 30 and the passive antenna elements 32 inside the array aperture is particularly useful for wireless applications because of the need for compactness. A valuable design aid in the design process for selecting the size/shape/spacing of the tuning elements 34 is the use of a Smith chart, wherein the loci of the Smith chart indicates the tuned condition of the passive antenna elements 32.
The loci can be generated through simulation or hardware testing. The effect of the tuning elements 34 appears as miniature loops formed in the loci. The approach for matching the various antenna modes of the smart antenna 22 is to adjust the shape, size and spacing of the tuning elements 34 so that the miniature loops can fall within the operating band. There should normally be one loop for each sub-resonant tuning element 34 unless they overlap, and there should normally be one locus trace for each passive antenna element 32.
Referring now to
In contrast, the Smith chart for the omni-directional mode, as illustrated in
Using circuit components like inductors and capacitors cannot match the input to the different antenna beam modes. This is due to the fact that circuits can vary the input impedance match only in the frequency domain, but not in the modal domain. To effect changes in the modal domain, we have to work within the radiation space, thus the parasitic probes.
The small resonant loop may be obtained through the use of the tuning probes 34 being placed adjacent the passive antenna elements 32. The tuning elements 34 are placed between the active element 30 and the passive antenna elements 32. This placement does not increase the physical size of the smart antenna 22. The inserted tuning elements 34 are kept short, and their small size limits their effect on the radiation patterns of the smart antenna 22.
Referring now to
The tuning elements 34 thus have little effect on the already well-tuned directional mode. The key point is that the small resonant loop 104 is still there, but with slight changes in location and size.
In review, the tuning elements 34 perturb the near field space of the passive antenna elements 32, and consequently, changes the input impedance so that it is more consistent for the different antenna modes. The Smith chart is a tool that is used to determine the size and shape of the tuning elements 34, as well as their spacing from the passive antenna elements 32. For example, the spacing of each tuning element 34 may vary within a range of ⅛ the wavelength of the operating frequency to 1/100 the wavelength. A nominal spacing may be on the order of about 1/20 the wavelength, for example.
The size and shape of the tuning elements 34 are selected so that the overall effect is less than ¼ the wavelength. For example, the height of each tuning elements 34 may vary within a range of 20% to 80% of the height of the passive antenna elements 32. A nominal height may be on the order of about 60%, for example. The Smith chart thus provides feedback on how the tuning elements 34 effect location of the small resonant loop 104 and 106. Once the small resonant loops 104 and 106 are located in the center of the Smith chart, the input impedance matching for the different modes will remain relatively constant.
In another embodiment, the antenna elements 30, 32 are all active elements and are combined with independently adjustable phase shifters to provide a phased array antenna, as illustrated in
Essentially, the phased array antenna 122 includes multiple antenna elements 130 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 130.
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 phased array 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.
Yet another aspect of the present invention is to provide a method for matching an input impedance of a smart antenna 22 comprising a ground plane 41; an active antenna element 30 adjacent the ground plane and having a radio frequency (RF) input associated therewith; and a plurality of passive antenna elements 32 adjacent the ground plane. A plurality of impedance elements 60 is connected to the ground plane 40 and is selectively connectable to the plurality of passive antenna elements 32 for antenna beam steering. The method comprises tuning the plurality of passive antenna elements 32 by positioning a plurality of tuning elements 34 adjacent thereof so that the input impedance of the RF input 68 of the active antenna element 30 remains relatively constant during the antenna beam steering.
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.
This application claims the benefit of U.S. Provisional Application Ser. No. 60/592,318 filed Jul. 29, 2004, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60592318 | Jul 2004 | US |
