Patent Publication Number: US-2021194570-A1

Title: Phased array antenna for use with low earth orbit satellite constellations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 16/425,817, filed May 29, 2019 which claims priority to U.S. Provisional Application No. 62/677,599, filed on May 29, 2018. These applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     Low Earth Orbit (“LEO”) satellite constellations have emerged as the new frontier for broadband Internet services. A LEO is an orbit around Earth with an altitude of 1,200 miles (2000 Km) or less, and an orbital period anywhere between 84 and 127 minutes. LEO satellites move very quickly relative to Earth&#39;s surface to stay in orbit. With a full trip around Earth in about 100 minutes, a single LEO satellite is likely in view of a user for about 10 minutes at a time. Providing broadband connectivity to users on Earth therefore requires a LEO satellite constellation capable of handling satellite handoffs and managing propagation losses that occur between the satellites and ground stations on Earth. 
     A ground station on Earth typically includes an antenna system to transmit and receive signals to/from the LEO satellite constellation. The antenna system may operate in the millimeter wave spectrum covering frequencies between 30 and 300 GHz or a portion thereof. Achieving high antenna performance and gain in millimeter wave frequencies poses design challenges for ground stations, gateways and user terminals. One design challenge is to steer the antenna beams into different directions to reach the LEO satellite constellation while minimizing losses and antenna squint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present application may be more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, which are not drawn to scale and in which like reference characters refer to like parts throughout, and wherein 
         FIG. 1  is a schematic diagram of an example phased array antenna for LEO satellite constellations; 
         FIG. 2  is a schematic diagram of another example phased array antenna for LEO satellite constellations; 
         FIG. 3  is a schematic diagram showing additional details for the phased array antenna of  FIG. 2 ; 
         FIG. 4  illustrates an MTS array of MTS cells in accordance with various examples; 
         FIG. 5  is a schematic diagram of a LEO MTS antenna in accordance with various examples; 
         FIG. 6  is a flowchart for transmitting signals to a LEO satellite with a phased array antenna in accordance with various examples; and 
         FIG. 7  is a flowchart for receiving signals from a LEO satellite at a phased array antenna in accordance with various examples. 
     
    
    
     DETAILED DESCRIPTION 
     Systems and methods for a phased array antenna for use with LEO satellite constellations are disclosed. The systems and methods are suitable for many different applications and can be deployed in a variety of different environments and configurations. In various examples, the systems and methods are used in ground stations, gateways or user terminals to transmit/receive RF signals to LEO satellite constellations. The ground stations, gateways, or user terminals can be stationary in fixed geographical locations in the Earth or mobile such as on top of or within airplanes, drones, yachts, vehicles, and so on. The phased array antenna is an array of antennas providing RF beams in different directions. 
       FIG. 1  illustrates a schematic diagram of an example phased array antenna for LEO satellite constellations. Phased array antenna  100  is a single antenna on a LEO satellite ground terminal  108  capable of providing RF beams in different directions, such as RF beams in the millimeter wave spectrum. Antenna  100  may be able to transmit RF signals to LEO satellites in a portion of the millimeter wave spectrum and receive RF signals from the LEO satellites in the same or another portion of the spectrum, such as the LEO satellite moving in orbit in positions  102 - 106 . Antenna  100  may be in a fixed or mobile ground station, gateway, or user terminal. 
     As shown, antenna  100  is at boresight at a distance d 1  from the LEO satellite in position  104  at a zenith angle of 0°. At boresight, the antenna  100  is at a minimum distance from the LEO satellite in position  104  and achieves its maximum gain. At a maximum slant of for example 60°, the distance from antenna  100  to the LEO is the maximum possible distance d 2 . At this maximum slant, the antenna  100  loss is at a maximum, with a 3 dB cosine loss and mismatch loss due to mutual coupling. That is, at the maximum slant, the distance is the maximum distance and the loss is the maximum loss. At d 1 , the antenna  100  is at maximum gain but at the minimum distance d 1 . Ideally, antenna  100  should be positioned such that it achieves its maximum gain at the maximum distance. 
       FIG. 2  illustrates another example phased array antenna with dynamic loss compensation. Antenna  200  is a phased array antenna with multiple antenna panels in a dome configuration. As shown, there are seven antenna panels in the phased array: a top antenna panel  202  and six side antenna panels  204 - 214  seen from the cutout perspective. In one example, the top antenna panel  202  could operate in conjunction with one of the side antenna panels  204 - 214 . One side antenna could be oriented with its boresight coinciding with the max slant direction, so as to have maximum gain from the antenna  200  at its maximum slant. Then, when the side antenna panel starts steering, the orbital distance decreases balancing out the cosine loss. Individual antenna panels can have a smaller angular span, as satellite tracking is handed over to the top antenna panel  202  or the other side antenna panels  204 - 214 , based on their location. Multiple of the antenna panels can be activated simultaneously to maximize the overall antenna gain. Each antenna panel  202 - 214  may be controlled to focus on a given angular span. Note that in this configuration, antenna  200  is able to dynamically compensate for propagation and cosine loss while having a small and compact design. In contrast, in the single antenna configuration of  FIG. 1 , each 3 dB in loss compensation results in a doubling of the antenna size. 
     It is appreciated that the proposed antenna configuration of antenna  200  optimizes the antenna performance and the overall aperture cost for tracking and communicating with LEO satellites, such as the LEO satellite moving in orbit from position  216  to position  218 . Differentiating with classical antenna design using multiple panels to reduce the large beam steering loss due to low elevation angles from ground antennas to LEOs, the proposed architecture for antenna  200  and its associated beamforming procedure make full use of the effective apertures. At least two of the antenna panels  202 - 214  receive and transmit LEO signals simultaneously. The antenna configuration of antenna  200  enables signals received from LEO satellites to be phase-aligned and power-combined, or signals transmitted to the LEO satellites to be power-divided and phase-aligned as desired. 
     For example, the signal components received by each element of one panel are phase-aligned and then power-combined, with both the top-panel  204  and the side-panels  206  and  214 . In receive, the two combined signals are phase-aligned and then power-combined again, to enhance the signal-to-noise ratio, improving the reception performance. In transmit, the transmit signals are then phase-shifted and power-divided and then fed to the two side panels  206  and  214 . Each of these signals is power-split and phase-shifted to the elements of the two panels  206  and  214 , respectively, and then sent from the two panels, improving their transmitting performance. Sometimes and more frequently, similar operations are performed with three panels, two side-panels and one top-panel. Beamforming is extended to multiple panels for performance improvement. 
     In tracking mode, the proposed multiple antenna configuration of antenna  200  also leverages all the advantages brought by its extended antenna aperture due to multiple panels involved in the operation. For example, the accuracy of angle-of-arrival (“AoA”) estimations using the proposed phased array antenna  200  with its multiple antenna panels  202 - 214 , in both orbit plane and in the perpendicular direction of the orbit plane, can be improved due to its beamsteering and phase-shifting capabilities. When the 3D location of the LEOs can be estimated with improved accuracy, its range estimation can be also improved. And as the in-orbit LEO ground speed can also be estimated or refined, based on a prediction of the LEO&#39;s position, a more accurate LEO&#39;s orbit position calculation can improve the link performance during tracking. 
     In addition, using multiple antenna panels in ground stations, instead of only one panel, in LEO communication systems, the overall amount of the hardware will be less than when only a single panel is used to achieve the same overall antenna performance. This is appreciated by observing that in the case of a single panel, the aperture must be dimensioned according to the worst case, which corresponds to the lowest elevation angle. In contrast, in the proposed configuration of antenna  200  using multiple antenna panels with say 2-3 panels working together, the beam steering loss due to low elevation angles is compensated by the different panel orientation angles. The one-panel configuration would have an unnecessary performance margin in most of the time of its operation and in particular, when LEOs are in the area near to ground station&#39;s zenith. This loss can be avoided due to the proposed antenna system configurations and the associated signal processing procedures. The proposed antenna configuration  200 , as well as the associated signal processing procedures, jointly optimize the benefits of using a multiple panel antenna configuration in communication, range and direction estimation. 
     Attention is now directed to  FIG. 3 , which illustrates a schematic diagram with additional details for the phased array antenna of  FIG. 2 . Phased array antenna  300  is controlled by a LEO antenna controller  302  and has 7 LEO antenna panels. In one example, each LEO antenna panel is a Meta-Structure (“MTS”) antenna such as LEO antenna  304 . A meta-structure, as generally defined herein, is an engineered, non- or semi-periodic structure that is spatially distributed to meet a specific phase and frequency distribution. In various examples, the LEO antenna  304  is an array of MTS cells, wherein each cell may be individually and dynamically controlled. The MTS array may be configured into subarrays that have specific characteristics, such as by having each subarray configured to reflect signals at a specific angle. 
     In operation, the antenna controller  302  is responsible for directing the LEO antenna  304  to generate RF beams with determined parameters such as beam width, transmit angle, and so on. The antenna controller  302  may also determine a voltage matrix to apply to phase shifters coupled to the LEO antenna  304  to achieve a desired phase shift profile for the antenna  304 . The phase shifters may include a varactor or a phase shift network in an RFIC  314  to provide any desired phase shift up to 360°. The phase shift network may include multiple phase shifters, each with one or more varactors, to achieve the desired phase shift. In some examples, the LEO antenna  304  is adapted to transmit a directional beam through active control of the phase shifters in RFIC  314  coupled to MTS cells that make up the LEO antenna  304 . 
     An RF signal controller  306  generates a signal for transmission and provides it to the LEO antenna  304 . The signal propagates through an impedance-matched feed network  308  to a transmission array  310  and an MTS array  312  for transmission through the air. The impedance-matched feed network  308  includes impedance matching mechanisms for respectively matching input signal parameters with the MTS cells in the MTS array  312 . The impedance matching mechanisms work in coordination with the RFIC  314  to provide phase shifting of the radiating signal(s) from the MTS array  312 . As illustrated, RFIC  314  is integrated to feed network  308  for providing phase shift control to each subarray or cell in MTS array  312 . In other examples, RFIC  314  may be integrated with the transmission array  310  such that a phase shifter is coupled to a transmission line leading to a subarray of MTS cells in the MTS array  312 . In yet other examples, the RFIC  314  may be integrated with the MTS array  312 , with each phase shifter in RFIC  314  providing phase shift control to an MTS cell or subarray of cells in MTS array  312 . 
     Note that with the illustrated dome configuration, two-dimensional phase shifting can be realized in the MTS array  312  so that each MTS cell or subarray of MTS cells can have its phase control signals combined. In various examples, a switch network can be used for variable antenna system reconfigurations. For example, when LEOs are on the zenith area, only the top-panel is used and side-panels are off. In other examples, when LEOs are at the range of 50° to 75° elevation and +30° and −30° azimuth range relative to altitude direction for the side-panels  206  and  208 , one or two side-panels and the top-panel are used. 
     It is appreciated that handovers in elevation angle can be performed according the order “side-panel(s) only” to “top-panel+side-panel(s)” and then from “top-panel+side-panel(s)” to “top-panel only”. The handover edges are 50° for handover from “side-panel(s) only” to “top-panel+side-panel(s)”, and the 75° for handover from “top-panel+side-panel(s)” to “top-panel only”. This design limits beam steering loss to cos(30°) or 0.625 dB at handover edge. With the handover, using two panels, this loss can be compensated using an additional panel&#39;s power combining gain. After the handover from 2-3 panels to one-panel, the range from LEO to ground station is shortened and beam steering loss compensation is no longer necessary until the next handover edge is reached. 
       FIG. 4  illustrates an MTS array of MTS cells in accordance with various examples. MTS array  400  contains multiple MTS cells positioned in one or more layers of a substrate and coupled to other circuits, modules and layers, as desired and depending on the application. In various examples, the MTS cells are metamaterial (“MTM”) cells. MTM cell  402  is illustrated having a conductive outer portion or loop  404  surrounding a conductive area  406  with a space in between. Each MTM cell  402  may be configured on a dielectric layer, with the conductive areas and loops provided around and between different MTM cells. 
     In the illustrated example, a voltage controlled variable reactance device  408 , e.g., a varactor, provides a controlled reactance between the conductive area  406  and the conductive loop  404 . The controlled reactance is controlled by a voltage applied from an antenna controller implemented as in  FIG. 3 , such as an applied reverse bias voltage in the case of a varactor. The change in reactance changes the behavior of the MTM cell  402 , enabling the MTM array  400  to provide focused, high gain beams directed to a specific location. In other examples, phase shifts are provided to each MTM cell or subarrays of MTM cells in array  400  by an RFIC coupled to the MTM array  400 , such as the RFIC  314  of  FIG. 3  having a phase shift network. In those examples, each MTM cell is configured without a varactor between its conductive area and conductive loop as the phase shift is provided from the RFIC instead of the varactor. It is appreciated that other configurations may include additional circuits, modules and layers integrated with the MTM array  400 . 
     As generally described herein, an MTM cell such as cell  402  is an artificially structured element used to control and manipulate physical phenomena, such as the electromagnetic (“EM”) properties of a signal including its amplitude, phase, and wavelength. Metamaterial structures behave as derived from inherent properties of their constituent materials, as well as from the geometrical arrangement of these materials with size and spacing that are much smaller relative to the scale of spatial variation of typical applications. A metamaterial is not a tangible new material, but rather is a geometric design of known materials, such as conductors, that behave in a specific way. An MTM cell may be composed of multiple microstrips, gaps, patches, vias, and so forth having a behavior that is the equivalent to a reactance element, such as a combination of series capacitors and shunt inductors. Various configurations, shapes, designs and dimensions are used to implement specific designs and meet specific constraints. In some examples, the number of dimensional freedom determines the characteristics, wherein a device having a number of edges and discontinuities may model a specific-type of electrical circuit and behave in a similar manner. In this way, an MTM cell radiates according to its configuration. Changes to the reactance parameters of the MTM cell result in changes to its radiation pattern. Where the radiation pattern is changed to achieve a phase change or phase shift, the resultant structure is a powerful antenna or radar, as small changes to the MTM cell can result in large changes to the beamform. 
     The MTM cells include a variety of conductive structures and patterns, such that a received transmission signal is radiated therefrom. In various examples, each MTM cell has some unique properties. These properties may include a negative permittivity and permeability resulting in a negative refractive index; these structures are commonly referred to as left-handed materials (“LHM”). The use of LHM enables behavior not achieved in classical structures and materials, including interesting effects that may be observed in the propagation of electromagnetic waves, or transmission signals. Metamaterials can be used for several interesting devices in microwave and terahertz engineering such as antennas, sensors, matching networks, and reflectors, such as in telecommunications, automotive and vehicular, robotic, biomedical, satellite and other applications. For antennas, metamaterials may be built at scales much smaller than the wavelengths of transmission signals radiated by the metamaterial. Metamaterial properties come from the engineered and designed structures rather than from the base material forming the structures. Precise shape, dimensions, geometry, size, orientation, arrangement and so forth result in the smart properties capable of manipulating EM waves by blocking, absorbing, enhancing, or bending waves. 
     Attention is now directed to  FIG. 5 , which shows a schematic diagram of a LEO MTS antenna in accordance with various examples. LEO MTS antenna  500  has an impedance-matched feed network  502  that may be a power divider circuit. The feed network  502  feeds a layer of transmission lines  504 , which are directed along the x-direction or length of the layer  504 . In this way, a transmission signal is provided at the input and fed to feed network  504  having multiple stages or levels. The configuration illustrated may incorporate a variety of other circuits, structures or components to enable smooth operation, such as to provide impedance matching of the various levels and distribution points, including coupling to the layer  554  and individual transmission lines formed therein. 
     The transmission layer  504  is positioned proximate MTS array  506  such that signals propagate through layer  504  and are received at MTS layer  506  for radiation in the z-direction. The feed network  502  may take any of a variety of forms, and is illustrated here as a multi-layer, equally divided structure that feeds a plurality of transmission lines of layer  504 . Each of the divided paths of feed network  502  feeds a transmission line of layer  504 . The transmission lines are arranged such that a signal propagates through from the feed network  502  to an opposite end of the layer  504 . Each transmission line may have a plurality of discontinuities positioned along its length. These discontinuities may be slots positioned within the transmission line that enable the propagating signal to travel through the slot forming electromagnetic radiation transmissions that feed layer  504  into the air or free space. The beam form and direction of the radiated signal from layer  506  is formed by an MTS array of unit cells which are controlled so as to change the resultant beamform. The layer  506  may also output multiple beamforms by allocating subarrays or by controlling the parameters of the unit cells. An RFIC as implemented in  FIG. 3  provides phase shifts to individual MTS cells or subarrays of MTS layer  506 . 
     Attention is now directed to  FIG. 6 , which is a flowchart for transmitting signals to a LEO satellite with a phased array antenna in accordance with various examples. First, in a phased array antenna having n subarrays of MTM cells, the feed network generates n transmit signal components, each transmit signal component generated for each subarray ( 600 ). Next, phase control signals are pre-conditioned and applied to all the cells within each subarray ( 602 ). The phase control signals provide voltages to the MTM cells for generating phase shifts that result in beamforming signals at the direction of the phase shift. Each transmit signal component is distributed to the MTM cells in each subarray with their phases aligned ( 604 ). The transmit signal components are then radiated as sub-beams in the subarray cells to form a single beam at a given phase ( 606 ). 
       FIG. 7  is a flowchart for receiving signals from a LEO satellite at a phased array antenna in accordance with various examples. First, signal from a LEO satellite is received at a phased array antenna having n subarrays of MTM cells ( 700 ). Next, phase control signals are pre-conditioned and applied to all the cells within each subarray ( 702 ). The phase control signals provide voltages to the MTM cells for generating phase shifts that result in beamforming signals at the direction of the phase shift. At each subarray, the signals received at their individual MTM cells are phase-aligned and power-combined ( 704 ). From each subarray, n beamformed signals are phase-aligned further and then combined coherently, ready for subsequent processing at the satellite ground terminal. As described above, the beamforming and phase control capabilities of the phased array antenna configuration disclosed herein enhance the signal-to-noise ratio and improves the overall transmit and receive performance of a LEO satellite ground terminal. 
     It is appreciated that the previous description of the disclosed examples is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these examples will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other examples without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the examples shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.