Abstract:
An antenna assembly includes at least two active or main radiating omni-directional antenna elements arranged with at least one beam control or passive antenna element used as a reflector. The beam control antenna element(s) may have multiple reactance elements that can electrically terminate it to adjust the input or output beam pattern(s) produced by the combination of the active antenna elements and the beam control antenna element(s). More specifically, the beam control antenna element(s) may be coupled to different terminating reactances to change beam characteristics, such as the directivity and angular beamwidth. Processing may be employed to select which terminating reactance to use. Consequently, the radiator pattern of the antenna can be more easily directed towards a specific target receiver/transmitter, reduce signal-to-noise interference levels, and/or increase gain. A Multiple-Input, Multiple-Output (MIMO) processing technique may be employed to operate the antenna assembly with simultaneous beam patterns.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. application Ser. No. 10/664,413, filed Sep. 17, 2003, which claims the benefit of U.S. Provisional Application No. 60/411,570 (Attorney&#39;s Docket No. 2479.2171-000), filed on Sep. 17, 2002. The entire teachings of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     It is becoming increasingly important to reduce the size of radio equipment to enhance its portability. For example, the smallest available cellular telephone handset today can conveniently fit into a shirt pocket or small purse. In fact, so much emphasis has been placed on obtaining small size for radio equipment that corresponding antenna gains are extremely poor. For example, antenna gains of the smallest handheld phones are only −3 dBi or even lower. Consequently, the receivers in such phones generally do not have the ability to mitigate interference or reduce fading.  
         [0003]     Some prior art systems provide multiple element beam formers for these purposes. These antenna systems are characterized by having at least two radiating elements and at least two receivers that use complex magnitude and phase weighting filters. These functions can be implemented either by discrete analog components or by digital signal processors. The problem with this type of antenna system is that performance is heavily influenced by the spatial separation between the antenna elements. If the antennas are too close together or if they are arranged in a sub-optimum geometry with respect to one another, then the performance of the beam forming operation is severely limited. This is indeed the case in many compact wireless electronic devices, such as cellular handsets, wireless access points, and the like, where it is very difficult to obtain sufficient spacing or proper geometry between antenna elements to achieve improvement.  
         [0004]     Indoor multipaths, mostly outside the main beam, interfere with the main beam signal and create fading. The indoor multi paths also create standing wave nulls that prevent reception if the directive antenna is situated at these nulls. For a traditional array, if one element of the array is at the null, the received signal is still significantly reduced. Reciprocity makes this effect hold true for the transmit direction, too.  
       SUMMARY OF THE INVENTION  
       [0005]     This invention relates to an adaptive antenna array for a wireless communications application that optionally uses multiple receivers. The invention provides a low cost, compact antenna system that offers high performance with the added advantage of providing multiple isolated spatial antenna beams or effecting an aggregate antenna beam. It can be used for multiple simultaneous receive and transmit functions, suitable for Multiple-Input, Multiple Output (MIMO) applications.  
         [0006]     Devices that can benefit from the technology underlying the invention include, but are not limited to, cellular telephone handsets such as those used in Code Division Multiple Access (CDMA) systems such as IS-95, IS-2000, CDMA 2000 and the like, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, wireless local area networking equipment such as IEEE 802.11 or WiFi access equipment, and/or military communications equipment such as ManPacks, and the like.  
         [0007]     In one embodiment, an antenna assembly includes at least two active or main radiating antenna elements arranged with at least one beam control or passive antenna element electromagnetically disposed between them. The beam control antenna element(s), referred to herein as beam control or passive antenna element(s), is/are not used as active antenna element(s). Rather, the beam control antenna element(s) is/are used as a reflector by terminating its/their signal terminal(s) into fixed or variable reactance(s). As a result, a system using the antenna assembly can adjust the input or output beam pattern produced by the combination of at least one main radiating antenna elements and the beam control antenna element(s). More specifically, the beam control antenna element(s) may be connected to different terminating reactances, optionally through a switch, to change beam characteristics, such as the directivity and angular beamwidth, or the beam control antenna element(s) may be directly attached to ground. Processing may be employed to select which terminating reactance to use.  
         [0008]     Consequently, the radiator pattern of the antenna can be more easily directed towards a specific target receiver/transmitter, reduce signal-to-noise interference levels, and/or increase gain. The radiation pattern may also be used to reduce multipath effects, including indoor multipath effects. One result is that cellular fading can be minimized.  
         [0009]     In one embodiment, at least one beam control antenna element is positioned to lie along a common line with the two active antenna elements, referred to as a one-dimensional array or curvi-linear array. However, the degree to which the active and beam control antenna elements lie along the same line can vary, depending upon the specific needs of the application. In another embodiment, more than two active antenna elements are arranged in a predetermined shape, such as a circle, with at least one beam control antenna element electromagnetically coupled to the active antenna elements. Shapes beyond the one-dimensional array or curvi-linear array are generally referred to as a two-dimensional array.  
         [0010]     The spacing of the active antenna elements with respect to the beam control antenna elements can also vary upon the application. For example, the beam control antenna element can be positioned about one-quarter wavelength from each of the two active antenna elements to enhance beam steering capabilities. This may translate to a spacing to between approximately 0.5 and 1.5 inches for use in certain compact portable devices, such as cellular telephone handsets. Such an antenna system will work as expected, even though such a spacing might be smaller than one-quarter of a corresponding radio wavelength at which the antennas are expected to operate.  
         [0011]     The invention has many advantages over the prior art. For example, the combination of active antenna elements with the beam control antenna element(s) can be employed to adjust the beam width of an input/output beam pattern. Using few components, an antenna system using the principles of the present invention can be easily assembled into a compact device, such as in a portable cellular telephone or Personal Digital Assistant (PDA). Consequently, this steerable antenna system can be inexpensive to manufacture. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0013]      FIG. 1  is a schematic diagram of a prior art beam former antenna system with two active antenna elements;  
         [0014]      FIG. 2  is a schematic diagram of a beam former antenna system with an antenna assembly including two active antenna elements and one beam control antenna element according to the principles of the present invention;  
         [0015]      FIG. 3  is a diagram of another embodiment of the antenna assembly of  FIG. 2 ;  
         [0016]      FIG. 4A  is a generalized wave diagram related to the antenna assembly of  FIG. 1 ;  
         [0017]      FIG. 4B  is a wave diagram related to the antenna assemblies of  FIGS. 2 and 3 ;  
         [0018]      FIG. 5  is a top view of a beam pattern formed by another embodiment of the beam former system of  FIG. 2 ;  
         [0019]      FIG. 6  is a diagram of another embodiment of the antenna assembly of  FIG. 2 ;  
         [0020]      FIG. 7  is a schematic diagram of another embodiment of the beam former system of  FIG. 2 ;  
         [0021]      FIG. 8A  is a diagram of a user station in an 802.11 network using the beam former system of  FIG. 7  with external antenna assembly;  
         [0022]      FIG. 8B  is a diagram the user station of  FIG. 8A  using an internal antenna assembly;  
         [0023]      FIG. 9  is a diagram of another embodiment of the antenna assembly of  FIG. 2 ;  
         [0024]      FIGS. 10A-10D  are antenna directivity patterns for the antenna assembly of  FIG. 9 ;  
         [0025]      FIG. 10E  is a diagram of the antenna assembly of  FIG. 9  represented on x, y, and z coordinate axes;  
         [0026]      FIGS. 11A-11C  are antenna directivity patterns for the antenna assembly of  FIG. 9 ;  
         [0027]      FIGS. 11D-11F  are antenna directivity patterns for the antenna assembly of  FIG. 9 ; and  
         [0028]      FIGS. 12A-12C  are three-dimensional antenna directivity patterns for the antenna assembly of  FIG. 9 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     A description of preferred embodiments of the invention follows.  
         [0030]      FIG. 1  illustrates prior art multiple element beam former. Such systems are characterized by having at least two active or radiating antenna elements  100 - 1 ,  100 - 2  that have associated omni-directional radiating patterns  101 - 1 ,  101 - 2 , respectively. The antenna elements  100  are each connected to a corresponding radio receiver, such as down-converters  110 - 1  and  110 - 2 , which provide baseband signals to a respective pair of Analog-to-Digital (A/D) converters  120 - 1 ,  120 - 2 . The digital received signals are fed to a digital signal processor  130 . The digital signal processor  130  then performs baseband beam forming algorithms, such as combining the signals received from the antenna elements  100  with complex magnitude and phase weighting functions.  
         [0031]     One difficulty with this type of system is that performance is heavily influenced by the spatial separation and geometry of the antenna elements  100 . For example, if the antenna elements  100  are spaced too close together, then performance of the beam forming operation is reduced. Furthermore, the antenna elements  100  themselves must typically have a geometry that is of an appropriate type to provide not only the desired omni-directional pattern but also operate within the geometry for the desired wavelengths. Thus, this architecture is generally not of desirable use in compact, hand held wireless electronic devices, such as cellular telephones and/or low cost wireless access points or stations (sometimes referred to as a client device or station device), where it is difficult to obtain sufficient spacing between the elements  100  or to manufacture antenna geometries at low cost.  
         [0032]     In contrast to this, one aspect of the present invention is to form directional multiple fixed antenna beams, such as a semi-omni or so called “peanut” pattern in a very small space. Specifically, referring to  FIG. 2 , there is the same pair of active antenna elements  100 - 1 ,  100 - 2  as in the prior art of  FIG. 1 ; however, according to the principles of the present invention, a passive or beam control antenna element  115  is inserted between the active antenna elements  100 . In a receive mode, received signals are fed to the corresponding pair of down converters  110 - 1 ,  110 - 2 , A/D converters  120 - 1 ,  120 - 2 , and Digital Signal Processor (DSP)  130 , as in the prior art.  
         [0033]     With this arrangement, two beams  180 - 1 ,  180 - 2  may be formed simultaneously in opposite directions when the beam control antenna element  115  is switched or fed to a first terminating reactance  150 - 1 . The first terminating reactance  150 - 1  is specifically selected to cause the beam control antenna element  115  to act as a reflector in this mode. Since these two patterns  180 - 1 ,  180 - 2  cover approximately one-half of a hemisphere, they are likely to provide sufficient directivity performance for a useable antenna system.  
         [0034]     In an optional configuration, if different antenna patterns are required, such as a “peanut” pattern  190  illustrated by the dashed line, then a multiple element switch  170  can be utilized to electrically connect a second terminating reactance  150 - 2  with the beam control antenna element  115 . The multiple element switch  170  may be used to select among multiple reactances  150  to achieve a combination of the different patterns, resulting in one or more “peanut” patterns  190 .  
         [0035]     Thus, it is seen how the center beam control antenna element  115  can be connected either to a fixed reactance or switched into different reactances to generate different antenna patterns  180 ,  190  at minimal cost. In the preferred embodiment, at least three antenna elements, including the two active antenna elements  100  and single passive element  115 , are disposed in a line such that they remain aligned in parallel. However, it should be understood that in certain embodiments they may be arranged at various angles with respect to one another.  
         [0036]     Various other numbers and configurations of the antenna elements  100 , switch  170 , and passive beam control antenna element(s)  115  are possible. For example, multiple active antenna elements  100  (e.g., sixteen) may be used with four passive beam control antenna elements  115  interspersed among the active antenna elements  100 , where each passive beam control antenna element  115  is electromagnetically coupled to a subset of the active antenna elements  100 , where a subset may be as few as two or as many as sixteen, in the example embodiment.  
         [0037]     Another embodiment of an antenna assembly according to the principles of the present invention is now discussed in reference to an antenna assembly  300  depicted in  FIG. 3 . The antenna assembly  300  uses a reflector or beam control antenna element  305 , or multiple reflector antenna elements (not shown), and a phased array of active antenna elements  310 . The antenna elements  305 ,  310  are, in this embodiment, mechanically disposed on a ground plane  315 . The reflector antenna element  305  is used to create its own multi-path.  
         [0038]     This multi-path is simple and is inside the active antenna elements  310 . Because of the close proximity of the reflector antenna element  305  to the active antenna elements  310 , its presence overrides other multi-paths and remove the nulls created by them. The new multi-path has a predictable property and is thus controllable. The phased array can be used to focus its beam on a signal, and the combination of reflector antenna element  305  and active antenna elements  310  removes fading and signal path misalignment, which creates “ghosts” often seen in TV receptions.  
         [0039]     In this embodiment, the reflector  305  is cylindrical and is situated in the center of the circular array  300  of active antenna elements  310 . This distance between the active antenna elements  310  and the conducting surface of the reflector antenna elements  305  may be kept at a quarter wave length or less. The presence of the cylindrical reflector antenna element  305  prevents any wave from propagating through the array  300  of active antenna elements  310 . It thus prevents the formation of standing waves created by the interfering effect of oppositely traveling waves  405 , as indicated by the arrows  415  in  FIG. 4A . The result is that the indoor nulls  410  are removed from the vicinity of the array elements  310 . However, the beam control antenna element  305  creates its own standing waves, as depicted in  FIG. 4B .  
         [0040]     Referring now to  FIG. 4B , the traveling wave  405  travels toward (i.e., arrow  415 ) a reflector  420 . The reflector  420  forms a node  410  at the reflector  420  and standing wave  405  having a peak at the antenna elements  310  surrounding the reflector antenna element  305  as a result of the quarter wave spacing. So, with this arrangement, the nulls from the environment are removed, and, at the same time, this arrangement confines the signal peaks to the active antenna elements  310 , which are ready to be phased into a beam that points to the strongest signal path, as determined by a processor (e.g.,  FIG. 2 , DSP  130 ) coupled to the antenna array  300 .  
         [0041]      FIG. 5  is a top view of example antenna beam patterns  500  formed by the linear antenna assembly of  FIG. 2 . In this embodiment, the beam control antenna element  115  is electrically connected to reactance components (e.g.,  FIG. 2 , reactance components  150 - 1 ,  150 - 2 ) that creates respective effective reflective rings  505 - 1 ,  505 - 2 . For example, the more inductance, the smaller the effective diameter of the ring  505  about the beam control antenna element  115 .  
         [0042]     Responsively, the antenna beam patterns  510 ,  515  produced by the antenna assembly  500 , arranged in a linear array, are kidney shaped, as depicted by dash lines. As should be understood, the smaller the diameter of the reflection rings  505 , the narrower the beam and, consequently, more gain, that is provided to the active antenna elements  100  in a perpendicular direction to the axis of the linear array. Note that the uncoupled antenna beam patterns  510 ,  515  do not form a “peanut” pattern as in  FIG. 2 , which is caused in part by the selection of the reactance components  150 .  
         [0043]     A secondary advantage of having this active/beam control/active antenna element arrangement is that the beam control antenna element  115  tends to isolate the two active antenna elements  100 , so there is a potential to reduce the size of the array. It should be understood that the active antenna elements  100  may be spaced closer to one another or farther apart from one another, depending on the application. Further, the reflective antenna element  115  electromagnetically disposed between the active antenna elements  100  reduces losses due to mutual coupling. However, loading on the beam control antenna element  115  may make it directive instead of reflective, which increases coupling between the active antenna elements  100  and coupling losses due to same. So, there is a range of reactances that can be applied to the beam control antenna element  115  that is appropriate for certain applications.  
         [0044]     Continuing to refer to  FIG. 5 , there are two basic modes of operation of the antenna array: (1) dual beam high gain (i.e., non-omnidirectional) mode, where the beam control antenna element  115  is reflective and (2) dual near-omni mode with low mutual coupling, where the center antenna element  115  is short enough but not too short so each active antenna element  100  sees the kidney-shaped beam  510 ,  515 , as shown. The reason this is near-omni is because the antenna array is not circular, so it is not a true omni-directional mode. As discussed above, changing the reactance electrically connected to the beam control antenna element  115  changes the mode of operation of the antenna array  500 .  
         [0045]     Examples of the reactances that may be applied to this center passive antenna element  115  are between about −500 ohms and 500 ohms. Also the height of the active antenna elements  100  may be about 1.2 inches, and the height of the passive antenna element  115  may be about 1.45 inches at an operating frequency of 2.4 GHz. It should be understood that these reactances and dimensions are merely exemplary and can be changed by proportionate or disproportionate scale factors.  
         [0046]      FIG. 6  is a mechanical diagram of a circular antenna assembly  600 . The circular antenna assembly  600  includes a subset of active antenna elements  610   a  separated by multiple beam control antenna elements  605  from another subset of active antenna elements  610   b . The active antenna elements  610   a ,  610   b , form a circular array. The beam control antenna elements  605  form a linear array.  
         [0047]     The beam control antenna elements  605  are electrically connected to reactance elements (not shown). Each of the beam control antenna elements  605  may be selectably connected to respective reactance elements through switches, where the respective reactance elements may include sets of the same range of reactance or reactance values so as to increase the dimensions of a rectangular-shaped reflector  620 , which surrounds the beam control antenna elements  605 , by the same amount along the length of the beam control antenna elements  605 . By changing the dimensions of the rectangular reflector  620 , the shape of the beams produced by the active antenna elements  610   a ,  610   b  can be altered, and secondarily, the mutual coupling between the active antenna element  610   a ,  610   b  can be increased or decreased for a given application. It should be understood that more or fewer beam control antenna elements  605  can be employed for use in different applications depending on shapes of beam patterns or mutual coupling between active antenna element  610   a ,  610   b  desired. For example, instead of a linear array of beam control antenna elements  605 , the array may be circular or rectangular in shape.  
         [0048]      FIG. 7  is another embodiment of an antenna system  700  that includes an antenna assembly  702  with a beam control antenna element  705  and multiple active antenna elements  710  disposed on a reflective surface  707  in a circular arrangement and electromagnetically coupled to at least one beam control antenna element  705 . As discussed above, the beam control antenna element  705  is electrically connected to an reactance or reactance, such as an inductor  750   a , delay line  750   b , or capacitor  750   c , which are electrically connected to a ground. Other embodiments may include a lumped reactance, such as a (i) capacitor and inductor or (ii) variable reactance element that is set through the use of digital control lines. The reactive elements  750 , in this embodiment, are connected to feed line  715  via a single-pole, multiple-throw switch  745 . The feed line  715  connects the beam control antenna element  705  to the switch  745 .  
         [0049]     A control line  765  is connected to the ground  755  or a separate signal return through a coil  760  that is magnetically connected to the switch  745 . Activation of the coil  760  causes the switch to connect the beam control antenna element  705  to ground  755  through a selected reactance element  750 . In this embodiment, the switch  745  is shown as a mechanical switch. In other embodiments, the switch  745  may be a solid state switch or other type of switch with a different form of control input, such as optical control. The switch  745  and reactance elements  750  may be provided in a various forms, such as hybrid circuit  740 , Application Specific Integrated Circuit (ASIC)  740 , or discrete elements on a circuit board.  
         [0050]     A processor  770  may sequence outputs from the antenna array  702  to determine a direction that maximizes a signal-to-noise ratio (SNR), for example, or maximizes another beam direction related metric. In this way, the antenna assembly  702  may provide more signal capacity than without the processor  770 . With the MIMO  735 , the antenna system  700  can look at all sectors at all times and add up the result, which is a form of a diversity antenna with more than two antenna elements. The use of the MIMO  735 , therefore, provides much increase in information throughput. For example, instead of only receiving a signal through the antenna beam in a primary direction, the MIMO  735  can simultaneously transmit or receive a primary signal and multi-path signal. Without being able to look at all sectors at all times, the added signal strength from the multi-path direction is lost.  
         [0051]      FIG. 8A  is a diagram of an example use in which the directive antenna array  502   a  may be employed. In this example, a station  800   a  in an 802.11 network, for example, or a subscriber unit in a CDMA network, for example, may include a portable digital system  820  such as a personal computer, personal digital assist (PDA), or cellular telephone that uses a directive antenna assembly  502 . The directive antenna assembly  502  may include multiple active antenna elements  805  and a beam control antenna element  806  electromagnetically coupled to the active antenna elements  805 . The directive antenna assembly  502   a  may be connected to the portable digital system  820  via a Universal System Bus (USB) port  815 .  
         [0052]     In another embodiment, a station  800   b  of  FIG. 8B  includes a PCMCIA card  825  that includes a directive antenna assembly  502   b  on the card  825 . The PCMCIA card  825  is installed in the portable digital device  820 .  
         [0053]     It should be understood that the antenna assembly  502  in either implementation of  FIG. 8A  or  8 B may be deployed in an Access Point (AP) in an 802.11 network or base station in a wireless cellular network. Further, the principles of the present invention may also be employed for use in other types of networks, such as a Bluetooth network and the like.  
         [0054]      FIGS. 9-11  represent an antenna assembly  900  and associated simulated antenna beam patterns produced thereby.  
         [0055]     Referring first to  FIG. 9 , the antenna assembly  900  includes four active antenna elements  910  deployed along a perimeter of a circle and a central beam control antenna element  905 . The antenna elements  905 ,  910  are mechanically connected to a ground plane  915 .  
         [0056]     In this embodiment, the active antenna elements  910  have dimensions 0.25″ to 3.0″ W×0.5″ to 3.0″ H, which are optimized for the 2.4 GHz ISM band (802.11b). The beam control antenna element  905  has dimensions 0.2″ W×1.45″ H. The height of the beam control antenna element  905  is longer in this embodiment to provide more reflectance and is not as wide to reduce directional characteristics.  
         [0057]      FIGS. 10A-10D  are simulated beam patterns for the antenna assembly  900  of  FIG. 9 . The antenna assembly  900  has been redrawn with x, y, and z axes as shown in  FIG. 10E . The simulated beam patterns of  FIGS. 10A-10D  are for individual active antenna elements  910 . The simulation is for 802.11b with a carrier frequency of 2.45 GHz. The beam patterns are shown for azimuth (x-y plane) at Phi=0 degs to 360 degs and elevation=30 degrees, or theta=60 degrees. The simulated beam pattern of  FIG. 10A  corresponds to the active antenna element  910  that lies along the +x axis. The null in the 180 degree direction represents the interaction between the active antenna element  910  and the beam control antenna element  905 . Similarly, the simulated beam pattern of  FIG. 10B  corresponds to the active antenna element that lies along the +y axis; the simulated beam pattern of  FIG. 10C  corresponds to the active antenna element  910  that lies along the −x axis; and the simulated beam pattern of  FIG. 10D  corresponds to the active antenna element  910  that lies along the −y axis. The nulls in simulated beam patterns of  FIGS. 10B-10D  correspond to the respective active antenna elements  910  and beam control antenna element  905  interactions.  
         [0058]     Referring now to  FIGS. 11A-11C , these simulated antenna directivity (i.e., beam) patterns correspond to the antenna beams produced by the active antenna  910  in the antenna assembly  900  that lies along the +x axis. Each of  FIGS. 11A-11C  have three antenna directivity curves for theta=30, 60, and 90 degrees, where the angles are degrees from zenith (i.e, zero degrees points along the +z axis. The simulations of  FIGS. 11A-11C  are for 2.50, 2.45, and 2.40 GHz, respectively.  
         [0059]      FIGS. 11D-11F  are simulated antenna directivity patterns for the elevation direction corresponding to the simulated antenna directivity (i.e., beam) patterns of  FIGS. 11A-11C . The three curves correspond to Phi=0, 45, and 90 degrees, where the angles are degrees from zenith.  
         [0060]      FIGS. 12A-12C  are three-dimensional plots corresponding to the cumulative plots of  FIGS. 11A-11F .  
         [0061]     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.