Patent Publication Number: US-2007109211-A1

Title: Antennas Supporting High Density of Wireless Users in Specific Directions

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a divisional of and claims priority, under 35 U.S.C. §120, to U.S. patent application Ser. No. 10/757,987 filed Jan. 16, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to base stations used in wireless communications, and more specifically to antennas supporting high density of users in specific directions.  
      2. Related Art  
      An antenna generally refers to a component which is used to send and receive wireless signals. In a typical configuration, the antenna is contained in a base station which transmits and receives voice, data, video, etc., in the form of electromagnetic waves. The base station enables wireless users (or wireless devices, in general) to communicate with various other (mobile devices) users, as is well known in the relevant arts.  
      Base stations are often employed to ensure that the areas (or users within that area) sought to be covered are within the range of wireless signals transmitted by the corresponding antennas. A wireless signal is generally characterized by illumination (energy intensity) at each point of the covered area, and a threshold level of illumination is generally necessary for coverage to extend to the corresponding point. In addition, higher illumination leads to better signal-to-noise-ratio (SNR), which in turn lead to advantages such as high data transfer rates.  
      A prior base station may provide for a hemispherical coverage area, with approximately uniform illumination in each segment of the area. Such an approach ensures that users at any portion of the hemispherical area are consistently provided connectivity. However, such a solution may not be suitable in areas where users are present in only small portions of the coverage area (e.g., in rural areas) since electrical power is wasted illuminating portions of the covered area in which users are not present. The antennas used in such base stations are generally referred to as ‘normal antennas’ (as opposed to smart antennas described below).  
      Another prior base station may overcome some of such disadvantages by using a ‘smart antenna’. Such base stations generally operate by ‘learning’ the specific direction in which active users are present, and beaming (transmitting) only in the directions in which such users are present. The direction of beaming is often controlled by controlling the phase of the signals emitted by each antenna element (contained in the antenna). By transmitting only in the directions in which active users are present, unneeded wastage of power may be avoided. Thus, smart antennas are generally suitable in coverage areas having low density of subscribers.  
      One problem with smart antenna-based base stations is that the required electronic circuitry (e.g., to control the antenna elements) may be complex, expensive and/or subject to a poor performance due to manufacturing imperfections, particularly as the base stations working at millimeter wave bands needs to support higher transfer rates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be described with reference to the following accompanying drawings, described briefly below.  
       FIG. 1  is an example environment in which the present invention can be implemented.  
       FIG. 2A  is a diagram containing array elements of an antenna in one embodiment.  
       FIG. 2B  is a diagram illustrating the manner in which a lens is used in combination with an antenna according to an aspect of the present invention.  
       FIG. 3  is a block diagram illustrating an example device implemented according to an aspect of the present invention.  
       FIG. 4  is a flow-chart describing the manner in which a lens may be designed according to an aspect of the present invention.  
    
    
      In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      1. Overview  
      According to an aspect of the present invention, a lens is provided associated with antenna elements to collimate the corresponding beam in a specific direction in which wireless users are expected to be present. Such a solution is particularly useful in situations such as road intersections where higher density of wireless users can be expected in the direction of the roads.  
      By decreasing the need for expensive and/or unreliable electronics circuits (such as in the case with smart antennas) for achieving the desired collimation, the overall cost of implementation of the antenna (and thus the base station) may be reduced. The lens can conveniently cover all the array elements as well, thereby serving as a radome which provides physical security for the antenna elements.  
      Another aspect of the present invention provides a design methodology using which lens can be designed to provide a desired operation at least in situations such as that noted above. The lens may be designed taking into consideration that each array element would be located spatially at a different location (during operation of the antenna). The radiation pattern of each array element is determined in a specific coordinate system having a corresponding origin, but transformed into a common coordinate system (having the corresponding common origin) before determining a composite radiation of the antennas.  
      The shape of the lens is then determined based on the determined composite radiation and the desired collimation characteristics. Shaping of lens based on such considerations potentially results in accurate beam shaping in the desired pre-specified directions.  
      Several aspects of the invention are described below with reference to examples for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One skilled in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention.  
      2. Example Environment  
       FIG. 1A  is a diagram illustrating the desired illumination coverage in an example scenario. The diagram is shown containing antenna  150  located at the intersection of two cross roads. As may be appreciated, it would be expected that a large number of mobile users will be present along the direction of the road (at least during peak traffic hours). Accordingly, it may be desirable to collimate the beam generated by the antenna to cover areas  110 -A through  110 -D. In addition, it may be desirable to provide at least some coverage between the roads, as shown by areas  120 -A through  120 -D.  
       FIG. 1B  is a diagram illustrating the desired illumination coverage in an alternative scenario. This diagram also contains antenna  150  located at the intersection of two cross roads. Assuming that one road (covered by areas  160 -D and  160 -B) has a substantial amount of traffic and another road (covered by areas  160 -A and  160 -C) has limited traffic, the illumination intensity in each of the areas  160 -D and  160 -B is higher compared to each of areas  160 -A and  160 -C. Assuming that substantial coverage in not required except adjacent to the roads (other than close to the intersection), illumination intensity in each of areas  170 -A through  170 -D is shown lower compared to each of areas  160 -A and  160 -B, as shown.  
      The manner in which such coverage (of  FIGS. 1A and 1B ) can be achieved according to various aspects of the present invention, is described below in further detail.  
      3. Solution  
       FIGS. 2A and 2B  together illustrate the general principle using which an antenna may be provided according to various aspects of the present invention.  FIG. 2A  contains four array elements  210 - 1  through  210 - 4  mounted in the form of a 2×2 array. Antenna elements  210 - 1  through  210 - 4  may be implemented in a known way.  
       FIG. 2B  shows a top view with lens  250  covering the four array elements (radiators) to form antenna  200 . The area within lens  250  is shown shaded to illustrate that the array elements are entirely covered by lens  250  in the embodiment, thereby potentially serving as a radome as well. The radius of curvature of the lens at each point may be determined by the approach(es) described in sections below.  
      Lens  250  may be implemented using robust material (or multiple layers) such that it serves also as a radome for the antenna elements  210 - 1  through  210 - 4 . Lens  250  may be implemented using various approaches. While only a single lens is shown for illustration, it should be appreciated that multiple lenses may be provided for corresponding collimations in different directions.  
      Lens  250  may be shaped such that the transmissions of the array elements are collimated in a desired manner as described above with reference to  FIGS. 1A and 1B . For example, with reference to  FIG. 1A , the radius of curvature needs to be more in the direction of each of areas  110 -A through  110 -D compared to in the direction of each of areas  120 -A through  120 -D. Similarly, with respect to  FIG. 1B , the radius of curvature along each of directions  160 -B and  160 -D needs to be more than the radius of curvature along of each of directions  160 -A and  160 -C, which in turn needs to be more than the radius of curvature along each of directions  170 -A through  170 -D.  
      Accordingly, lens  250  generally needs to be designed taking into account various considerations. One example approach to designing such lens based on some example considerations is described in sections below. First, an example device using the solution is described below.  
      4. Example Device  
       FIG. 3  is a block diagram illustrating the details of an example device implemented according to an aspect of the present invention. Base station  300  is shown containing antenna  200  (represented in the form of a lens), processing block  310 , phase shifters  320 -A through  320 -D, attenuators  330 -A through  330 -D, division block  340 , summing block  350 , transmitter  360 , and receiver  380 . It may be appreciated that base station  300  is described as being used in the intersection of  FIG. 2 , and thus antenna  200  is shown as being a part of base station  300 .  
      Antenna  200  receives radio signals, generates the corresponding electrical signals, and provides the electrical signals on paths  321 -A through  321 -D (to control blocks containing phase shifters and attenuators). Only four paths are shown assuming that antenna  200  contains four antenna elements. Similarly, antenna  200  receives electrical signals on paths  321 -A through  321 -D, and generates the corresponding electromagnetic waves for transmitting the contained application data (voice, data, video etc.). As may be appreciated, direction of the beam and the range of illumination is determined based on the signals provided to antenna elements and collimation provided by the lens.  
      Processing system  310  is shown containing DOA (direction of arrival) block  315  and ABF (adaptive beam formation) block  318 . DOA block  315  examines the signals on paths  321 -A through  321 -D to estimate the direction of the active mobile nodes, and also the extent of calibration required due to the different spatial coordinates as well as imperfections of each antenna element (which causes signals originating from the same location to be received at different times). The estimates are provided to ABF block  318 . ABF block  318  determines the phase and attenuation factor values that need to be provided to phase shifters ( 320 -A through  320 -D) and attenuators ( 330 -A through  330 -D) based on the estimates provided by DOA block  315 . DOA  315  and ABF  318  may be implemented in a known way.  
      Summing block  350  receives signals (representing each active channel) on paths  314 -A through  314 -D, and generates a broadband signal containing all the active channels. The broadband signal is forwarded to receiver  380 . Receiver  380  processes the broadband signal to perform operations such as down-conversion and amplification, and the resulting baseband signal is forwarded to a corresponding application (e.g., voice switch). Receiver  380  and summing block  350  may be implemented in a known way.  
      Transmitter  360  receives a baseband signal corresponding to an application and generates a broadband signal in the frequency range suitable for transmission by the antenna elements. Division block  340  generates electrical signals suitable for processing by antenna elements from the broadband signal, and the generated signals are passed via corresponding phase shifter and attenuator circuits. Transmitter  360  and division block  340  may be implemented in a known way.  
      Each of attenuators  330 -A through  330 -D attenuates the signal received from division block  340  (under the control of ABF block  318 ) and provides the attenuated signal to a corresponding one of phase shifters  320 -A through  320 -D. Each phase shifter  320 -A through  320 -D shifts the corresponding input signal by an amount determined ABF block  318 , and the shifted signal is provided to the corresponding antenna element.  
      The antenna elements contained in antenna  200  generate a beam corresponding to the signals received on paths  321 -A through  321 -D. The beam is collimated by the lens as described above with reference to  FIGS. 1A, 1B , and  2 B. As noted above, the lens needs to be designed carefully for the proper operation of antenna  200 . The manner in which the lens can be implemented is described below in further detail.  
      5. Implementing the Lens  
       FIG. 4  is a flow-chart describing the manner in which a lens can be designed according to an aspect of the present invention. For illustration, the method is described with reference to  FIGS. 1A, 1B  and  2 B. However, the approach may be used in various other environments without deviating from the scope and spirit of several aspects of the present invention. The method begins in step  401  in which control immediately passes to step  410 .  
      In step  410 , radiation pattern of each array element in the absence of lens is characterized with reference to a corresponding coordinate system (i.e., having a corresponding origin). In one embodiment, the radiation pattern caused due to each array element is modeled as a spherical modal expansion. As is well known in the relevant arts, the radiation pattern contains electric (E) and magnetic (H) components. The radiation pattern of an array element at any given point is then given according to the following equations:  
               Ei   ⁡     (     R   ,   θ   ,   ϕ     )       =     ∑     (         a   in     ⁢     M   mn       +       b   in     ⁢     N   mn         )               Equation   ⁢           ⁢     (   1   )                   Hi   ⁡     (     R   ,   θ   ,   ϕ     )       =       -   j     ⁢           ⁢     Y   0     ⁢       ∑   n     ⁢     (         a   in     ⁢     N   mn       +       b   in     ⁢     M   mn         )                 Equation   ⁢           ⁢     (   2   )               
 
      wherein Ei represents the electric radiation in the absence of the lens, Hi represents the magnetic radiation in the absence of the lens, a in  and b in  represent the coefficients according to the spherical modal expansion for an array element, R represents a radial distance from the array element to a point at which field strength is determined, θ and φ respectively represent elevation and azimuth angles of a coordinate system of the array element as is well known in the relevant arts, Y 0  represents characteristic admittance of free space, j represents the complex number equaling square root of minus 1, ‘m’ represents the azimuthal modal index, ‘n’ represents the modal harmonic number, and M mn  and N mn  are representations of the orthogonal spherical modal harmonics and can be determined in a known way.  
      The radiation pattern at multiple points in the near field is measured (e.g., using a probe) or computed by accurate theoretical means in a known way. The measured field may then be used to compute the expansion coefficients (a in  and b in ), and thus determine the radiation pattern of each array element in the corresponding coordinate system.  
      In step  420 , the radiation pattern of each array element is computed with reference to a common coordinate system. That is, a common coordinate system is chosen, and the values computed in step  410  are transformed to correspond to the common coordinate system. The same common coordinate system may be used with respect to lens as well. The computations may be performed, for example, by using translation and rotation derivations for the spherical harmonics computed in step  410 , as described below.  
      First, a translation derivation is performed on the coefficients of radiator and the resulting electrical field (Et) and magnetic field (Ht) is shown in Equations (3) and (4) below.  
               Et   ⁡     (     R   ,   θ   ,   ϕ     )       =       ∑   n     ⁢     (         A   in     ⁢     M   mn       +       B   in     ⁢     N   mn         )               Equation   ⁢           ⁢     (   3   )                   Ht   ⁡     (     R   ,   θ   ,   ϕ     )       =       -   j     ⁢           ⁢     Y   0     ⁢       ∑   n     ⁢     (         A   in     ⁢     N   mn       +       B   in     ⁢     M   mn         )                 Equation   ⁢           ⁢     (   4   )               
 
      wherein A in  and B in  are the translated radiator coefficients and are respectively represented by Equations (5) and (6).  
               A   in     =       ∑   v     ⁢     (         A   v   n     ⁢     a   iv       +       B   v   n     ⁢     b   iv         )               Equation   ⁢           ⁢     (   5   )                   B   in     =       ∑   v     ⁢     (         B   v   n     ⁢     a   iv       +       A   v   n     ⁢     b   iv         )               Equation   ⁢           ⁢     (   6   )               
 
      wherein A v   n  and B v   n  are the translation coefficients, and a iv =a in  and b iv =b in  are the radiator coefficients in the original untranslated coordinate system of step  410 .  
      The translated coefficients can be further rotated using rotational derivations, and Equations (7) and (8) shown below respectively represent the translated and rotated electrical component (Etr) and magnetic component (Htr).  
             Etr   =       ∑   n   α     ⁢       ∑     m   =     -   n       n     ⁢     {         F   mn     ⁢       M   mn     ⁡     (     R   ,   θ   ,   ϕ     )         +       G   mn     ⁢       N   mn     ⁡     (     R   ,   θ   ,   ϕ     )           }                 Equation   ⁢           ⁢     (   7   )                 Htr   =       (       K   /     -   j       ⁢           ⁢     Y   o       )     ⁢       ∑   n   α     ⁢       ∑     m   =     -   n       n     ⁢     {         F   mn     ⁢       N   mn     ⁡     (     R   ,   θ   ,   ϕ     )         +       G   mn     ⁢       M   mn     ⁡     (     R   ,   θ   ,   ϕ     )           }                   Equation   ⁢           ⁢     (   8   )               
 
 wherein F mn  and G mn  respectively represent the radiators translated-rotated coefficients referred to the common coordinate system with origin at O and are respectively shown in Equations (9) and (10).  
               F   mn     =       ∑     v   =   1     α     ⁢       ∑     μ   =     -   v       v     ⁢     {         A     μ   ⁢           ⁢   v       ⁢     C   ⁡     (     μ   ,     v   /   m     ,   n     )         +       B     μ   ⁢           ⁢   v       ⁢     D   ⁡     (     μ   ,     v   /   m     ,   n     )           }                 Equation   ⁢           ⁢     (   9   )                   G   mn     =       ∑     v   =   1     α     ⁢       ∑     μ   =     -   v       v     ⁢     {         A     μ   ⁢           ⁢   v       ⁢     D   ⁡     (     μ   ,     v   /   m     ,   n     )         +       B     μ   ⁢           ⁢   v       ⁢     C   ⁡     (     μ   ,     v   /   m     ,   n     )           }                 Equation   ⁢           ⁢     (   10   )               
 
      wherein A&#39;s, B&#39;s represent translation coefficients, C&#39;s and D&#39;s represent rotation coefficients, μm,ν respectively represent the modal indices.  
      In step  430 , composite radiation pattern (CRP) of antenna  200  is determined based on the computed radiations patterns with reference to the common coordinate system. CRP can be represented as a function (f) of coefficients a in  and b in  and is represented as shown in Equation (11) below. 
 
CRP=f1(a in , b in )  Equation (11)
 
      In step  440 , the desired collimation pattern (DCP) is characterized. In one embodiment, collimation pattern may be characterized by electromagnetic field in the far field. Such far field representation (R=constant,θ,φ) may be represented by the scattered spherical modal expansion coefficients (X mn  and Y mn ) external to dielectric lens (determined by applying the appropriate boundary conditions). The collimation may be represented by Equation (12) below. 
 
DCP=f2(X mn , Y mn )  Equation (12)
 
      In step  470 , the shape of the lens is determined from the characterized collimation pattern and composite radiation pattern. For example, assuming that the desired shape of the lens is given by Equation (13) below, inverse scattering technique well known in the relevant arts can be applied in accordance with equations (14) and (15) to determine the desired shape of the lens. 
 
Shape= f 3 (R,θ,φ)  Equation (13) 
 
 f 3( R ,θ,φ)× f 1( a   in   , b   in )= f 2( X   mn   , Y   mn )  Equation (14) 
 
 f 3( R ,θ,φ)= f 1 −1 ( a   in   , b   in )× f 2( X   mn   , Y   mn )  Equation (15)
 
      In step  480 , lens is designed based on shape (f 3 ) determined in step  470 . In an embodiment, lens parameters such as permittivity, permeability and loss tangent of the dielectric material, are embedded in computation of f 2 . The values are entered for known dielectrics with constant permeability, and low loss tangent. The method ends in step  499 .  
      Thus, using the principles described above, a lens that will collimate the beam in the direction of high density of subscribers may be implemented.  
      6. Conclusion  
      While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.