Patent Publication Number: US-9413067-B2

Title: Simple 2D phase-mode enabled beam-steering means

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application No. 61/778,097 filed Mar. 12, 2013 and entitled “Simple 2D Phase-Mode Assisted Beam-Steering Means,” which is incorporated herein by reference as if reproduced in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to antennas and electromagnetic radiation modification, and, in particular embodiments, to systems and methods for steering the direction of the main lobe of a radiation pattern effected by antennas. 
     BACKGROUND 
     Beam-steering is the angular positioning of the main lobe of a radiation pattern. This allows for greater discrimination in favor of a desired signal from a point-like source in the far field of the antenna, for sensing or information transmission and reception. When it is required to steer the beam of a planar array antenna over a limited range in 2 dimensions around the array axis (which is perpendicular to the plane of the array), it becomes difficult to fit each element with a variable phase shifter or transceiver module (TR), and incorporate them all into the feed structure as would be devised in the conventional approach. This is especially true where the wavelengths involved are small because the array elements and spacings scale with wavelength (must be in the order of half wavelength) whereas feed lines and phase shifters take up additional room and do not completely scale with wavelength, (especially TRs). In any case, the phaseshifters and TRs become very expensive for short wavelengths (e.g. millimeter-waves), so it is desirable to use as few of them as possible to achieve the necessary beam control. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment, an apparatus for beam-steering includes a first hybrid splitter/combiner connected to a 0-th phase-mode feed of an array of antenna elements, a second hybrid splitter/combiner, wherein an output of the second splitter/combiner comprises a main output beam, a first pair of variable phase shifters connecting the first hybrid splitter/combiner to the second hybrid splitter/combiner, wherein the first pair of variable phase shifters control a steering direction of the main output beam radial with respect to an array axis by adjustments of respective phases of the variable phase shifters, wherein the respective phases are equal in magnitude and opposite in sign, and wherein the array axis is perpendicular to a plane of the array, and a third variable phase shifter connecting a 1-st phase-mode feed of an array of elements to an input of the first hybrid splitter/combiner, wherein the third variable phase shifter is configured to independently control a direction of the main output beam in a direction circumferential with respect to the array axis. 
     In accordance with another embodiment, a receiving and/or transmitting system for radiation beam-steering includes a first port connected to a 0-th order phase-mode feed of an array of radiation transducer elements, a second port connected to a +1-st order phase mode feed of the array, a first pair of variable phase shifters comprising a first variable phase shifter and a second variable phase shifter, a third variable phase shifter connected to second port, a first equal-amplitude hybrid splitter/combiner connected to the first pair of variable phase shifters, to the third variable phase shifter, and to the first port, and a second equal-amplitude hybrid splitter/combiner connected to the first pair of variable phase shifters, wherein the third variable phase shifter is configured to independently control a direction of a main output beam in a direction circumferential with respect to an array axis, and wherein the first pair of variable phase shifters are configured to be controlled in equal magnitudes and opposing signs effecting steering of a radiation beam in a radial direction around an axis perpendicular to a plane of the array. 
     In accordance with yet another embodiment, an apparatus for beam-steering includes a first hybrid splitter/combiner connected to a 0-th phase-mode feed of an array of antenna elements, a second hybrid splitter/combiner, wherein an output of the second splitter/combiner comprises a resultant array beam, a first pair of variable phase shifters connecting the first hybrid splitter/combiner to the second hybrid splitter/combiner, wherein the first pair of variable phase shifters control a steering direction of a main output beam radial with respect to an array axis by adjustments of respective phases of the variable phase shifters, wherein the respective phases are equal in magnitude and opposite in sign, and wherein the array axis is perpendicular to a plane of the array, a second pair of variable phase shifters, wherein a first one of the second pair of variable phase shifters is connected to a +1-st phase-mode feed of the array of antennas and a second one of the second pair of variable phase shifters is connected to a −1-st phase-mode feed of the array of antennas, and a third hybrid splitter/combiner connected to the second pair of variable phase shifters and connected to the first hybrid splitter/combiner, wherein the second pair of variable phase shifters are configured to adjust their respective phases by an equal magnitude and an opposite sign, wherein the second pair of variable phase shifters are controllable independently from the first pair of variable phase shifters, and wherein the second pair of variable phase shifters control a scanning the steering of the main output beam in a circumferential direction with respect to the array axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings which illustrate, but do not limit the scope of the present invention to, a specific embodiment in which: 
         FIG. 1  is a graph illustrating a far-field pattern of 0-th order phase-mode P 0  of a 16 element, λ/2 spaced circular ring array; 
         FIG. 2  is a graph illustrating a far-field pattern of −1-st order phase-mode P −1  of a 16-element, λ/2 spaced circular ring array; 
         FIG. 3  is a graph of a far-field pattern of 1-st order phase-mode P 1  of a 16 element, λ/2 spaced circular ring array; 
         FIG. 4  illustrates an embodiment beam-steerer system with a variable-ratio combiner controlled by setting phaseshift φ, with phaseshift θ applied to input B; 
         FIG. 5  illustrates a plot of an example of the resultant steered-beam far-field radiation pattern at the main (M) output C from the beam-steerer system of  FIG. 4 ; 
         FIG. 6  illustrates a plot of an example of the resultant steered-beam far-field radiation pattern using both first-order phase-modes P 1  and P −1 ; and 
         FIG. 7  illustrates an embodiment of the disclosed beam-steering system; 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     Disclosed herein is a 2 dimensional (2D) phase-mode beam steering system that achieves 2D steering of the electromagnetic (EM) radiation beam of a circular or polygonal ring array of an arbitrary number of antenna elements using a fixed number of variable phase shifters and hybrid splitter/combiners. The fixed number of phase-modes are implemented in the feed structure of the antenna ring array. The different phase-modes may each use a separate concentric ring array of antennas or they may use a common ring array of antennas for any or all of the phase-modes. The number of variable phase shifters and the number of hybrid splitters/combiners is independent of the number of antenna elements used. The disclosed 2D phase-mode beam steering systems and apparatuses may be connected to a phase-mode feed network. Additional information about phase-mode feed networks and about hybrid splitters/combiners may be found in Davies, D. E. N. and Rizk, M. S. A. S., “Electronic Steering of Multiple Nulls for Circular Arrays,” Electronics Letters, Vol. 13, No. 22, pp. 669-670, 27 Oct. 1977, which is incorporated herein by reference in its entirety. 
     In an embodiment, the disclosed 2D phase-mode beam steering system achieves 2-dimensional steering of the beam of a circular ring array of any number of antenna elements using only 4 (or optionally 3) variable phase shifters and only 3 (or optionally 2) hybrid splitter/combiners. Only 3 (or optionally 2) phase-modes are required to be implemented in the feed structure of the circular ring array. They may each use a separate concentric ring array or a common one for any or all of the phase-modes. The 0-th order phase-mode may also use a filled planar polygonal array and the +1-st and −1-st phase-modes may use polygonal rings of elements on the periphery of this array; these variations of the geometry are to be understood as being implicit in the term “circular ring array” as used in this description. 
     Thus the disclosed systems and apparatuses have the potential to greatly reduce the complexity and cost of designing, fabricating and calibrating an electronically-steerable millimeter-wave array antenna. Such an antenna is desirable, for example, in a small-cell backhaul radio to enable automatic alignment of the point-to-point link, thus greatly reducing the link deployment time and cost. 
     In an embodiment, a beam-steering system includes an analog radio frequency (RF) beam-steering network connected to a circular ring array of radiating (or receiving) elements connected to a phase-mode feed network having output ports for the 0-th, 1-st and −1-st order phase-modes (P 0 , P 1  and P −1 , respectively), and to a transceiver (with optionally up to 2 additional receiver inputs) at its output ports. The phase-mode inputs to the beamsteering network may be generated from separate concentric ring arrays, or from a single common ring array, having an arbitrary number of elements. The disclosed beam-steering network includes 2 phaseshifters connected to the P 1  and P −1  phase-modes and controlled in opposite directions, θ and −θ, respectively. These are in turn connected to a hybrid splitter/combiner which forms their sum at one output, C=P 1 e jθ +P −1 e −jθ , and difference at the other output, as D=P 1 e jθ −P −1 e −jθ . Output of mode P 0  is input to a compensating and 90° phaseshift network with the same insertion delay, loss and phase as the phase-shifters and hybrid of the other two phase-modes (when set to zero variable phases). Output D and the compensated P 0  mode are then input to another sum/difference hybrid, whose outputs C and D are connected to another two oppositely-adjustable phaseshifters, φ and −φ, respectively. The outputs of these are then connected to the inputs of a third hybrid whose sum output port C gives the steered main beam for use in the main transceiver, and difference output D gives a steered auxiliary beam for use in an auxiliary receiver. Sum port output C of the first hybrid gives another independent auxiliary beam for use in a second auxiliary receiver, both auxiliary receivers being optional. 
     In an embodiment, steering of the main beam in a limited range around the array axis (direction perpendicular to the plane of array) in the radial direction is accomplished with phase-setting of φ, and in the circumferential direction with setting of θ, independently is provided. The same structure of beam-steerer can be used with ring arrays having any number of elements. 
     The disclosed beam-steerers are herein described in greater detail of their principles of operation, in the context of a steerable millimeter-wave array antenna. Specifically, in an embodiment, the antenna includes a planar ring of identical radiating (or receiving) elements connected to a phase-mode beamforming network and radiating nominally in the direction orthogonal to the plane of the array (along the array axis). 
     In the case of an electromagnetic antenna, the array elements maybe of linear or circular polarizations. In the latter case, they may be arranged with their feedpoints symmetrically around the center, so that the phase will progress linearly around the circumference by one cycle, resulting in one of the 1-st order phase modes. In an embodiment, phasing arrangements compensating for this phase-progression will form the 0-th order phase mode. Other phase-mode feed arrangements for linearly-polarized elements may be devised, such as portions of a Butler matrix or Rotman lens, spatial or guided-mode feeds and other arrangements employed by those skilled in the art. In an embodiment, the end result is a phase-mode feed structure of a circular or polygonal ring array having output ports corresponding to the 0-th, +1-st and −1-st order phase modes. 
     To help with the understanding of the operation of the invention, the far-field radiation patterns of the pertinent phase modes are illustrated in  FIGS. 1-3 .  FIG. 1  is a graph  100  illustrating a far-field pattern of 0-th order phase mode P 0  of a 16 element, λ/2 spaced circular ring array.  FIG. 2  is a graph  200  illustrating a far-field pattern of −1-st order phase-mode P −1  of a 16-element, λ/2 spaced circular ring array.  FIG. 3  is a graph  300  of a far-field pattern of 1-st order phase-mode P 1  of a 16 element, λ/2 spaced circular ring array. 
     In an embodiment, all antenna elements are assumed to be omnidirectional and linearly polarized for simplicity. In the 0-th order phase mode, P 0 , there is no phase progression in the element excitations around the circular ring array (all elements are fed in phase), so there is no phase-progression in the circumferential direction around the array (z) axis. Thus all the fields add in-phase on the array axis and form the main beam in the far field. Its normalized plot is shown in  FIG. 1  for a 16-element ring array with elements spaced half-wavelength apart around the circumference. Different shading indicates phase, with darker shading denoting −π, lighter shading denoting 0, and medium shading denoting +π radians relative to the P 0  excitation.  FIGS. 2 and 3  show similar plots for the other phase modes of the same ring array. 
     The phase progressions in the P 1  and P −1  modes&#39; far-field patterns are one complete cycle of 2π radians but in opposite directions around the z-axis, which is the same as their element excitation phase progressions. 
     Now it will be apparent that if one adds some proportion of, for example, the P 1  phase mode to the P 0  phase mode, the result will be a main lobe pointing in the direction where the two modes have the same phase (e.g. lighter shade for the above plots). The main lobe will deviate from the array axis by an amount proportional to the proportion of the P 1  mode being added. One can also vary the phase, θ, of P 1 , which will change the location on the circle where it is in phase with the original main beam P 0  thus causing the resultant main lobe to point in that direction. 
       FIG. 4  illustrates an embodiment beam-steerer system  400  with a variable-ratio combiner controlled by setting phaseshift φ, with phaseshift θ applied to input B. The system  400  is a variable-ratio combiner. The system  400  includes two hybrid splitters/combiners  402 ,  404  and two oppositely adjusted phaseshifters  406 ,  408 . Each hybrid splitter/combiner  402 ,  404  has two inputs, A and B, and two outputs, D and C. The input A for the hybrid splitter/combiner  402  is the P 0  phase mode from the far-field of an array of antennas (not shown). The input B for the hybrid splitter/combiner  402  is the P 1  phase mode from the far-field of an array of antennas, its phase shifted by phaseshifter  409 . The output D of hybrid splitter/combiner  402  is the input for phase shifter  406  and the output C of hybrid splitter/combiner  402  is the input for phase shifter  408 . The output from phase shifter  406  is the input B for the hybrid splitter/combiner  404  and the output from phase shifter  408  is the input A for the hybrid splitter/combiner  404 . The output D from the hybrid splitter/combiner  404  is the auxiliary output. The output C from the hybrid splitter/combiner  404  is the main (M) output where the steered main beam is effected. 
     In an embodiment, while the relative phaseshift of the two phase modes is simple to control using a variable phaseshifter  409 , their relative proportions of addition are achieved using the variable-ratio combiner of system  400 . In this embodiment, the two hybrid splitters/combiners  402 ,  404  and two oppositely-adjusted phaseshifters  406 ,  408  are used to realize the function described by the equation  410 . The main output, M, is described by the function
 
 M=P   0  cos φ− P   1   e   jθ  sin φ,
 
where φ is the angle of the steered beam around the array axis in the radial direction and θ is the angle of the steered beam in the circumferential direction.
 
     Strictly speaking, the mathematics of the equation  410  requires input B to hybrid splitter/combiner  402  to be shifted by a fixed 90 degrees and the (auxiliary) output D of hybrid splitter/combiner  404  also has a fixed 90 degree phaseshift, both of which are of no practical consequence as it depends on the choice of hybrid splitter/combiner in the implementation. An example of the resultant steered-beam far-field radiation pattern  500  at the main (M) output C from hybrid splitter/combiner  404  is shown in  FIG. 5  with θ=2π*0.5 and φ=2π*0.125 radians. 
     It is possible to obtain a greater steering angle and lower sidelobes in the steered beam by making use of the other first-order phase-mode, P −1 , by virtue of a simple trigonometric identities as follows. 
     Suppose the intrinsic phase of phase mode P 1  at some angle on the circumference of its main cone is α and its amplitude is ρ. Then P −1  will have the same amplitude but its phase will be −α. Applying phaseshifts θ and −θ to these, respectively, results in:
 
 P   1   e   jθ   =ρe   j(α+θ) =ρ cos(α+θ)+ j ρ sin(α+θ)
 
and
 
 P   −1   e   −jθ   =ρe   j(−α−θ) =ρ cos(−α−θ)+ j ρ sin(−α−θ)=ρ cos(α+θ)− j ρ sin(α+θ)
 
     Now, the combined, oppositely-phased first order phase modes produce:
 
 P   1   e   jθ   −P   −1   e   −jθ   =j 2ρ sin(α+θ)
 
which for any given θ reaches a maximum value of j2 where α+θ=π/2, minimum of −j2 where α is such that α+θ=−π/2, and is 0 where α+θ=π or 0. Notice that, if we compensate P 0  by multiplying it by j, it will always be in phase with the above combined and phaseshifted phase modes. By adding them all together in a variable-ratio combiner, it will result in a peak where the combined modes have a peak, a minimum where they have their minimum, and no effect where they are 0, effecting a steering of the original P 0  main lobe in the circumferential direction by roughly twice the amount as with only one 1-st order phase mode, and no “fattening” of the main beam in the directions orthogonal to the direction of steering.
 
     The effect can be seen in the  FIG. 6  for the same steering parameters as in  FIG. 5 . Note the sharper shape of the steered beam and greater tilt in the radial direction corresponding to the control by cp. Its equation is given by:
 
 M=P   0  cos φ−( P   1   e   jθ   −P   −1   e   −jθ )sin φ
 
     Note also in  FIG. 6  that there is no phase variation over the steered main beam, as in the original main beam due to P 0 . This means that the modem will not need to track carrier-phase variations as the beam is steered, unlike in the case shown in  FIG. 5 . 
       FIG. 7  illustrates an embodiment beam steering system  700 . The beam steering system  700  makes use of P 0 , P 1 , and Beam steering system  700  includes a circular array of antenna elements  718 , a phase mode feed network  720 , four phase shifters  708 ,  710 ,  712 ,  714 , three hybrid splitters/combiners  702 ,  704 ,  706 , and a delay and gain compensation module  716 . Graph  730  is a cross-section plot of the P 1  and P −1  components of the phase-mode pattern of the far-field beam received by the array  718 . Graph  740  is a cross-section plot of the P 0  component of the phase-mode pattern of the far-field beam received by the array  718 . Graph  750  is a plot of the steered far-field pattern of the resultant main output C from hybrid splitter/combiner  706 , i.e. the output M of the beam steerer  700 . 
     In an embodiment, the circular array of antenna elements  718  may be a polygonal array of antenna elements. 
     Hybrid splitter/combiner  702  has inputs A and B coupled to variable phase shifters  708  and  710 . Variable phase shifter  708  is coupled to the P 1  phase mode port of feed  720  of array  718 . Variable phase shifter  710  is coupled to the P −1  phase mode port of feed  720  of array  718 . Variable phase shifters  708 ,  710  are a phase shifter pair with the phase shift for phase shifter  710  being opposite in sign and equal in magnitude to the phase shift for phase shifter  708 . In other words, the phase shifter  708  shifts the phase by the negative of the phase shift provided by phase shifter  710 . Phase shifters  708 ,  710  control the circumferential direction of the main output beam, M. Hybrid splitter/combiner  702  has outputs C and D that are related to its inputs A and B by the following equation: 
               [         C           D         ]     =         1     2       ⁡     [         1       1           1         -   1           ]       ·     [         A           B         ]             
The output C is a second auxiliary output, A2. The output D is coupled to the B input of hybrid splitter/combiner  704 . The outputs C and D of hybrid splitter/combiner  704  are coupled to the respective inputs of phase shifters  712 ,  714 . The phase shifters  712 ,  714  are paired phase shifters where the direction of phase shift provided by phase shifter  712  is equal in magnitude and opposite in sign to that provided by phase shifter  714 . The phase shifters  712 ,  714  control the radial direction of the main output beam, M. The outputs of the phase shifters  712 ,  714  are connected to the A and B inputs of the hybrid splitter/combiner  706 .
 
     The input A of hybrid splitter/combiner  704  is coupled to a delay, gain compensation unit  716  which shifts the phase by 90 degrees and adjusts the delay and amplitude to match that caused by hybrid splitter/combiner  702 . The input of delay, gain compensation unit  716  is coupled to the P 0  phase mode of the feed  720  of array  718 . The output D of hybrid splitter/combiner  706  is an auxiliary output, A 1 , and the output C of the hybrid splitter/combiner  706  is the main output, M. The main output M, is given by the following equation:
 
 M=P   0  cos φ−( P   1   e   jθ   −P   −1   e   −jθ )sin φ
 
where P 0  is the 0-th order phase-mode input from array  718 , P 1  is the 1-st order phase-mode input of the array  718 , P −1  is the −1-st order phase-mode input of the array  718 , θ and −θ are phase shifts effected by the phase shifters  708 ,  710 , φ and −φ are phase shifts effected by the phase shifters  712 ,  714 , and j=√{square root over (−1)}.
 
     In an embodiment, each of the phase shifters  708 ,  710 ,  712 ,  714  is controllable over a range of +π to −π radians. 
     The beamsteerer principle using both P 1  and P −1  can be thought of as amplitude-directed steering because the main lobe of P 0  is steered in the direction of the peak of the sinusoidal result of the difference
 
 P   1   e   jθ   −P   −1   e   −jθ   =j 2ρ sin(α+θ)
 
whereas the principle of the beamsteerer using a single phase-mode P ±1  is phase-directed steering, as the main lobe is steered in the direction where its phase matches that of the phaseshifted P ±1  mode.
 
     The auxiliary beams at outputs A 1  and A 2  may be connected to receivers and used for adaptive nulling of co-channel interference, for spectrum monitoring outside the main beam, for feedback signals in a beam-steering algorithm, or left unused to simplify the hardware by omitting their implementation portions. 
     Assuming the variable phase shifters  708 ,  710 ,  712 ,  714  are bidirectional, the entire beam steerer system  700  can be used to transmit as well as receive. However, usually just the main beam, M, is used for both transmission (TX) and reception (RX) functions. The hybrid splitter/combiners  702 ,  704 ,  706  may be of the sum/difference type or quadrature (also known as branch-line) hybrids with suitable correction of the compensator phase at P 0  and insertion of other fixed phase-compensators at the appropriate inputs or outputs of the pertinent hybrid splitter/combiners as determined necessary by those skilled in the art. 
     In an embodiment, the auxiliary outputs A 1  and/or A 2  may be connected to transceivers (not shown in  FIG. 7 ) and may be used for spectrum monitoring and/or interference cancellation, or to provide feedback signals to a beam-steering algorithm. The total number of phase shifters  708 ,  710 ,  712 ,  714  and the hybrid splitter/combiners  702 ,  704 ,  706  is independent of the number of antenna elements in the antenna array  718 . 
     In an embodiment, the beam steerer system  700  consumes the least amount of analog hardware when implemented at the RF directly, although in principle it can be implemented at intermediary frequency (IF) stages, baseband, or even in digital form. Any implementation other than directly at RF may require coherent receivers and transmitters for each phase-mode port. 
     In another embodiment, at least the antenna array can be positioned behind the focus (the conventional feed-point) of a lens or reflector, acting as the feed of a lens-based or reflector-based antenna system in order to increase the steering angles and directivity of its beam, by virtue of the magnifying power of such an arrangement. All other components are substantially similar to their corresponding components in  FIG. 7 . 
     Although the description has been described in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of this disclosure as defined by the appended claims. Moreover, the scope of the disclosure is not intended to be limited to the particular embodiments described herein, as one of ordinary skill in the art will readily appreciate from this disclosure that processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, may perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.