Abstract:
A planar antenna that facilitates directional communication to a mesh network. The antenna is housed in a relatively small, planar package that can easily be attached to a window pane to enable the antenna to communicate with a neighboring rooftop mounted node of the mesh network. The package contains an M by N element phased array, where M and N are integers greater than one. The array is driven by microwave signals supplied from a P-angle phase shifting circuit, where P is an integer greater than one. Thus, the antenna synthesizes a single main beam and the antenna&#39;s main beam can be electrically “pointed” in one of P directions. In one embodiment of the invention, the array comprises 40 physical elements (8×5 elements) and has three selectable directions (i.e., the phase shifters provide +90, 0 and −90 degree shifts that move the beam left 45 degrees, center and right 45 degrees).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to wireless networks, and more particularly to antennas for wireless networks.  
           [0003]    2. Description of the Related Art  
           [0004]    Consumer appetite for access to information continues to grow along with growth of the Internet. Corresponding to such growth, new information is added to the Internet constantly. With respect to multimedia content in particular, much of this information comes at a significant cost in bandwidth.  
           [0005]    Telephone dial-up service is being replaced with broader bandwidth systems such as satellite, digital subscriber line (DSL), and cable modem. Unfortunately, these systems are not presently available to a significant portion of the population. Moreover, acquisition and installation costs associated with these systems make them less appealing.  
           [0006]    Accordingly, wireless connectivity is on the rise. Wireless systems may be deployed more rapidly with less cost than their wired counterparts. Systems using cellular phone technologies are directed at providing mobile wireless Internet connectivity. Unfortunately, such systems are bandwidth limited.  
           [0007]    Alternatives to cellular telephone technologies are point to multi-point (PMP) cellular architectures providing high speed, data only services. Benefits of wireless systems for delivering high-speed services include rapid deployment without overhead associated with installation of local wired distribution networks. Unfortunately, PMP systems rely upon long-range transmissions and a sophisticated customer premise installation.  
           [0008]    Another alternative system that provides a fixed wireless solution with bandwidth comparable to DSL and cable modem technologies that is less complex to install and less costly is a mesh network architecture. As described in U.S. patent application Ser. No. 10/122,886, filed Apr. 15, 2002 (Attorney Docket No. SKY/004-1) and application Ser. No. 10/122,762, filed Apr. 15, 2002 (Attorney Docket No. SKY/005-1), which are both incorporated herein by reference, a mesh network comprises a plurality of wirelessly connected nodes that communicate data traffic across a wide area at bandwidths exceeding DSL or cable. The nodes of the mesh communicate with one another using radio or microwave communications signals that are transceived using a roof mounted, directional antenna. Directional antennas are useful in a mesh network because they extend the maximum distance between the mesh nodes and reduce the effects of interfering signals from other nodes and other sources. The disclosed antenna structure uses antenna array technology to provide an antenna that has switched directionality. The antenna&#39;s main beam or beams may be pointed in a variety of different directions covering 360 degrees. Such roof top directional antennas are very effective in connecting to neighboring nodes (other roof top antennas) without obstruction.  
           [0009]    Although the rooftop antennas provide an optimal solution for interconnecting mesh nodes, in some instances, rooftop access is not available or the user is incapable of installing the antenna on the roof.  
           [0010]    Therefore, there is a need in the art for an antenna that enables a user to join a mesh using a non-rooftop mounted antenna, i.e., a window mount or wall mount antenna. Desired features of the window/wall mount antenna include a thin form factor for unobtrusive installation, substantial directivity for long range connectivity, the ability to point the antenna beam to increase signal power or reject interference.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a planar antenna that facilitates directional communication to a mesh network. The antenna is housed in a relatively small, thin, planar package that can easily be attached to a window pane or wall to enable the antenna to communicate with at least one neighboring rooftop mounted node of the mesh network. The package contains an M by N element phased array, where M and N are integers greater than one. The array elements are driven by microwave signals supplied from amplitude and phase shifting circuits. These circuits provide P combinations of phase and amplitude shifts at each element, where P is an integer greater than one, to optimally combine the signals impinging upon each element (or transmitted from each element). Thus, the antenna synthesizes a single main beam and the antenna&#39;s main beam can be electrically “pointed” in one of P directions.  
           [0012]    Residential communication services require the use of low cost equipment to be economically feasible. The cost of amplitude and phase shifting circuits has prohibited the use of electronically steered antennas in this application. An important feature of this embodiment is its low cost. Low cost has been achieved by minimizing the number of unique amplitudes and unique phase shifts required to synthesize P beams. Further, this embodiment uses phase shifts of +90° and −90° that are easily produced in analog circuitry.  
           [0013]    In one embodiment of the invention, the array comprises 40 physical elements (8×5 elements) and has three selectable directions (i.e., left 45 degrees, center and right 45 degrees). These states are accomplished by using fixed amplitudes on each of the 5 columns of antenna elements, and phase shift states of 0°, +90° and −90°. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.  
         [0015]    It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0016]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is a network diagram depicting an exemplary portion of a network in accordance with an aspect of the present invention;  
         [0018]    [0018]FIG. 2A depicts an azimuth plan view of a beam produced by the antenna of the present invention;  
         [0019]    [0019]FIG. 2B depicts an elevation plan view of a beam produced by the antenna of the present invention;  
         [0020]    [0020]FIG. 3 depicts a block diagram of drive circuitry for the antenna array elements;  
         [0021]    [0021]FIG. 4 depicts a plan view of the antenna array elements;  
         [0022]    [0022]FIG. 5 depicts a vertical, cross sectional view of the antenna; and  
         [0023]    [0023]FIG. 6 depicts an azimuth pattern produced by a planar antenna of the present invention; and  
         [0024]    [0024]FIG. 7 depicts a schematic diagram of a phase shifter that is used in the drive circuitry of FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 1 is a network diagram depicting an exemplary portion of a mesh network  100  as described in commonly assigned U.S. patent application Ser. No. 10/122,886, filed Apr. 15, 2002 (Attorney Docket No. SKY/004-1) and application Ser. No. 10/122,762, filed Apr. 15, 2002 (Attorney Docket No. SKY/005-1), which are herein incorporated by reference in its entirety. Network  100  comprises network access concentrators (SNAPs)  103 , network access points (NAPs)  101  and network access nodes  102 . Network traffic may be routed from a network access node  102  to a neighboring network access node  102 . Such a neighboring network access node  102  may route such traffic to one of its neighboring network access nodes  102  and so on until a NAP  101  or a final destination network access node  102  is reached. Notably, nodes  102  may be in communication with one another but not with any node  101  to form a private wireless network.  
         [0026]    SNAPs  103  may be coupled to various backhauls  105 , which backhauls  105  may be coupled to network  106 . Network  106  may be coupled to an operations center (OC)  104 . Backhauls  105  may form a part of network  106 . Network  106  may comprise a portion of the Internet, a private network, or the like. By private network, it is meant a network not connected to the Internet.  
         [0027]    NAPs  101  may be in communication with SNAPs  103  or network  106  via backhaul communication links  107 . It should be understood that backhauls may be wired or wireless. In particular, backhauls coupled to NAPs  101  may have a wireless backhaul. In an embodiment, point-to-point communication is used as between a SNAP  103  and a NAP  101  in the Unlicensed National Information Infrastructure (UNII) band (e.g., using a frequency of about 5.8 Ghz). Though, at locations where wired connectivity is available, wired connectivity may be used.  
         [0028]    Network access nodes  102  are in wireless communication with at least one NAP  101  or node  102 . It should be understood that nodes  102  or NAPs  101  may be configured for any of or some combination of broadcasting, point-to-point communication, and multicasting. By broadcasting, it is meant transmitting without singling out any particular target recipient among a potential audience of one or more recipients. By point-to-point communication, it is meant transmitting with singling out a particular target recipient among a potential audience of one or more recipients. By multicasting, it is meant transmitting with singling out a plurality of particular target recipients among a potential audience of recipients. For purposes of clarity, communication between nodes  102 , between NAPs  101 , or between a NAP  101  and a node  102 , described below is done in terms of point-to-point communication.  
         [0029]    In one embodiment, this is accomplished using radio communication in the UNII band. However, other known bands may be used. Nodes  102  form, at least in part, a Wide Area Network (WAN) using in part wireless interlinks  108 . More particularly, IEEE 802.11a physical and link layer standards may be employed for communication in a range of 9 to 54 megabits per second (Mbits/s).  
         [0030]    Communication slots as described herein are time slots with associated frequencies. However, one of ordinary skill in the art will understand that other types of communication spaces may be used, including without limitation codes, channels, and the like.  
         [0031]    The nodes of  102  may utilize both rooftop antennas  112  or a panel mount antenna  110  (i.e., a substantially planar antenna that is adapted to be mounted to a wall or window. The panel mount antenna  100  is capable of communicating with any mesh node  102  that is within line-of-sight to mounting location of the antenna  110 .  
         [0032]    [0032]FIG. 2A depicts a top plan view of the panel mount antenna  110  communicating with neighboring nodes  102 A,  102 B and  102 C. While this figure shows communications with a signal neighbor node in each of the three possible beams, more than one neighbor node may reside in any of the beams. FIG. 2B depicts a side view of panel mount antenna  110  communicating with rooftop node  102 B. As shall be described below, the panel mount antenna  110  synthesizes a single, directional beam that may be switched in a multitude of directions to connect to various nodes  102  within the neighborhood as well as avoid interference sources that may exist in the neighborhood. For example, panel mount antenna  110  may communicate with node  102 B using a beam that is directed perpendicular from the face of the antenna  110 . In other instances, the beam may be shifted to communicate with other neighboring nodes  102 A or  102 C as described below.  
         [0033]    In one embodiment of the invention, the panel mount antenna  110  does not actively control the elevation of the beam, i.e., the elevation of the beam is fixed to point at a right angle from the face of the antenna. However, the neighboring rooftop nodes are typically at a slight elevation relative to the panel mount antenna. Although the panel mount antenna has a vertical beamwidth that is sufficient to receive signals from nodes at a slight elevation relative to the panel mount antenna, to maximize the signal strength coupled to a rooftop mounted antenna, the panel mount antenna  110  may be tilted either physically or electrically. Empirical study indicates that an elevation of approximately five degrees is sufficient. In alternative embodiment, the beam elevation may be electronically controlled in the same manner as the azimuth direction is controlled, as described below.  
         [0034]    [0034]FIG. 3 depicts a block diagram of the antenna  110 . The antenna  110  comprises a power delivery circuit  300  coupled to a plurality of array elements  302 . The power delivery circuit  300  is mounted on one side of a circuit board and the array elements are mounted on the opposite side of the circuit board. FIG. 4 depicts a top plan view of the array elements  302 . FIG. 5 depicts a vertical, cross sectional view of the antenna  110 . To best understand the invention, the reader should simultaneously view FIGS. 3, 4, and  5  while reading the following description of the invention.  
         [0035]    The power delivery circuit  300  comprises a power divider  304 , a plurality of attenuators  306 ,  308 ,  310 ,  312  and  314 , and a pair of phase shifters  316  and  318 . The input power to the array is applied to terminal  312 , which has, for example, a 50-ohm input impedance. In one embodiment of the invention, the antenna operates at approximately 5.8 GHz (e.g., frequencies in the UNII band). The power from port  312  is divided by the power divider  304  into five paths  305 A-E, (i.e., a 1:5 power splitter). To ensure proper side lobe attenuation relative to the main beam of the antenna  110 , each output from the power divider contains attenuation (a thinning of the stripline) to adjust the relative amplitudes of the signals. To maintain a low cost, the attenuation is produced in this fixed manner. Four of the signals are then applied to phase shifters  316 ,  318 ,  320  and  322 . The center signal (path  305 C) is not phase shifted and forms a phase reference for the other paths  305 A, B, D, E.  
         [0036]    To provide a low cost antenna, the phase shifters  316 ,  318 ,  320  and  322  operate by shifting the signals in discrete quantities using PIN diodes to vary the coupling within a hybrid coupler. FIG. 7 depicts a schematic diagram of one of the phase shifters  316 . The other phase shifters  318 ,  320  and  322  have the same structure. The phase shifter  316  comprises a hybrid coupler  700  and four PIN diodes  702 A,  702 B,  702 C,  702 D (collectively diodes  702 ). The diodes are spaced from one another alng the branches  706 A and  706 B by an eighth of a wavelength and spaced from the cross arms  704 A and  704 B of the coupler  700  by an eighth of a wavelength. The diodes  702  can be selectively biased by control signals to form a short to ground. In one embodiment of the invention, the phase shifters utilize the four PIN diodes  702  to shift the signal +90°, −90° or 0°. To facilitate phase shift selection, a control circuit  320  provides a bias voltage to the PIN diodes  702 . When no bias is applied and the diodes form open circuits, the phase shift from input to output of the coupler  700  is −90 degrees. When diodes  702 B and  702 C are shorted to ground by biasing them, the phase shift through the coupler  700  is +90 degrees and, when diodes  702 A and  702 D are shorted to ground by biasing them, the phase shift through the coupler  700  is 0 degrees. These three discrete phase shifts may be applied to each of the four signal paths  305 A, B, D, E. The shifted signals are applied to the array elements  302  through vias in the circuit board (see FIG. 5 below).  
         [0037]    [0037]FIG. 4 depicts one embodiment of an arrangement for the antenna elements within the array  302 . This embodiment comprises five active columns  400 ,  402 ,  404 ,  406  and  408 . Each column  400 ,  402 ,  404 ,  406 , and  408  comprises eight elements  400 A-H,  402 A-H,  404 A-H,  406 A-H, and  408 A-H. Each element is a radiating patch. The number of elements in the column determines the vertical beam width of the antenna. More or less than 8 elements may be used in a column. Furthermore, in other embodiments of the invention, another type of radiating element, such as a slot, dipole or other aperture, could be used. Each element in a column is connected to a neighboring element by a conductor  410 . Microwave power is coupled to/from each column using a via  514  that is centrally located along the columns  402 ,  404 ,  406 ,  408 . In the embodiment of the invention, each column is spaced one half wavelength from an adjacent column. Other column spacings could be used with some degradation in the beam pattern side-lobes, one half wavelength spacing provides the optimum side-lobe levels.  
         [0038]    Though five columns are used, the embodiment can logically be considered to be a seven-column array where the “phantom” columns between  400  and  402  or between  406  and  408  have infinite attenuation and are not printed on the panel. This provides the performance of a seven-column antenna using the complexity and cost of a five-column circuit.  
         [0039]    In an embodiment of the invention used in the UNII band, column  400  is spaced about 5.17 cm from column  402 , while columns  402 ,  404  and  406  are spaced from one another by about 2.59 cm and column  408  is spaced from column  406  by about 5.17 cm. The elements within each column are equally spaced from one another by about 3.1 cm. Each element has the dimensions of about 0.9 cm by 1.4 cm. The size of each patch and the spacing between patches is wavelength dependent and would be scaled to design an antenna to other frequency bands.  
         [0040]    The phase shifters  316  and  318  control the phase of the signal applied to each of the columns such that the antenna beam may be shifted in the horizontal plane (azimuth), but is fixed in the vertical plane (elevation). As described above, to facilitate maximizing the signal strength coupled to rooftop nodes, the vertical spacing between the elements may be adjusted to provide a slight inclination to the main beam of the antenna pattern.  
         [0041]    [0041]FIG. 5 depicts a vertical, cross sectional view of the antenna  110 . The antenna  110  comprises an enclosure  500  having a thickness of about 3 cm that houses a substrate, e.g., a multi-layer circuit board  502 . The enclosure may be less than 3 cm thick depending upon the circuit configuration. Within the circuit board  502 , the first layer  504  of metallization comprises the antenna elements  302 , the second layer  506  of metallization comprises a ground plane and the third layer  508  comprises the driver circuit  300 . A via  514  conductively couples each column of antenna elements  302  to their respective driver circuits  300 . The third layer  508  also could support the transceiver and modem circuits  510 . As such, the antenna sends and receives microwave communications signals via the antenna elements, processes the signals within the transceiver/modem circuits and provides data input and output at port  512 . The antenna  110  can be affixed to a window  516  via suction cups  518  or other form of adhesive. In a wall-mounted configuration, the antenna may be affixed to a wall using screws or bolts. The technique used to mount the planar antenna  110  can be adapted to any type of mounting configuration.  
         [0042]    The material and thickness between layers  504  and  506  and between  508  and  506  are important to the antenna performance (i.e., the spacing of the antenna elements and microwave circuits from the ground plane effects the operation of the circuits and the pattern of the antenna). In one embodiment of the invention, the circuit board material is a low loss material useful for fabricating microwave circuits. One type of low cost material is available from Roger&#39;s Corporation as Material RO4003. This material provides a dielectric constant such that the circuit board for operation in the UNII band is 0.032 inches thick, as measured from the ground plane to the antenna elements. The total circuit board thickness is 0.065 inches. The total circuit board size is 7 inches by 10 inches. As such, the enclosure  500  has the approximate dimensions of 3 cm thick by 25 cm tall by 20 cm wide—a size that, when installed in a window, may easily be hidden behind a curtain.  
         [0043]    In an alternative embodiment, the antenna elements  302  of the first layer  504  may be separated from the ground plane  506  by a foam core or by an air gap. The drive circuitry can then be assembled on a conventional printed circuit board and mounted to the ground plane on the opposite side of the antenna elements. Such a foam core or air gap based circuit construction will further lower the cost of the panel mount antenna.  
         [0044]    In the final design of the antenna structure, the spacing of the elements in the horizontal and vertical planes as well as the amplitude attenuation provided by the attenuators within the drive circuitry are adjusted to compensate for the impedance of the glass (or other material) against which the antenna is mounted.  
         [0045]    In the embodiment where the phase shifters provide +90, −90 and 0 degree phase shifts, the single main beam of the antenna can be switched +/−45° as well as the center. As such, the antenna can be actively pointed toward the neighboring nodes to communicate with specific nodes as well as avoid unwanted interference from nodes that it is currently not communicating with as well as other microwave sources of interference.  
         [0046]    [0046]FIG. 6 depicts the azimuth pattern  600  of the planar antenna  110  having the configuration described above for operation in the UNII band. The pattern  600  comprises a center beam  602 , a right beam  604  and a left beam  606 . The antenna  110  has a directive gain of 18.5 dBi with an elevation beamwidth of about 10 degrees and a azimuth beamwidth of about 47 degrees. The bandwidth of the antenna is 150 MHz.  
         [0047]    While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.