Method and apparatus for satellite beam centering control for user scheduling

Abstract

A satellite communications system (SCS) (20) implements a beam recentering function (32) that dynamically recenters one or more spot beams (10) with respect to their corresponding nominal beam coverage areas (40) as a component of user scheduling. Updating the beam center target (46) of a spot beam (10) adjusts beamforming by the SCS (20) to move the beam center (14) of the spot beam (10), thereby realizing that the beam's maximum signal power corresponds to a different location within the same nominal beam coverage area over time. In an exemplary embodiment, the SCS (20) uses dynamic beam centering with respect to multiple forward user beams (100), where the underlying beamforming is ground-based beamforming, such as end-to-end beamforming, or satellite-based beamforming, which may be supported by ground-based or satellite-based computation of a dynamically changing beamforming solution.

Description

[Technical Field]

[0001] The methods and apparatus disclosed herein embody techniques for beam centering control in satellite communication systems as a component of user scheduling. [Background technology]

[0002] "User scheduling" refers to the handling of user traffic for multiple "users," and in particular the scheduling of transmission resources to carry traffic for individual users in a manner that meets one or more scheduling objectives, such as maximizing throughput, ensuring proportional fairness among users, etc. User scheduling applies to one or both of the forward link direction, in which the participating communication systems carry traffic toward users, and the return link direction, in which the communication systems carry traffic from users.

[0003] A beamforming satellite communications system performs beamforming on the forward link to serve distinct groups of user terminals in distinct forward user beam coverage areas. Additionally or alternatively, the satellite communications system performs beamforming on the return link to serve distinct groups of user terminals in distinct return user beam coverage areas, which may correspond to the forward user beam coverage areas.

[0004] The forward link direction involves the satellite communication system forming multiple forward user beams, which are directional radio signals. In particular, the forward user beams may be "spot beams," each of which has concentrated power to provide intensive coverage over a corresponding limited geographic area.

[0005] The overall geographic area may be divided into a plurality of nominal forward user beam coverage areas, and the satellite communications system is configured to generate a corresponding plurality of forward user beams having shapes and sizes to illuminate the respective nominal forward user beam coverage areas. The satellite communications system reuses signal frequency and polarization combinations across the plurality of beams and performs user scheduling across the plurality of beams to maximize use of the limited spectrum available. Summary of the Invention

[0006] A satellite communications system (SCS) implements a beam recentering function that dynamically recenters one or more spot beams with respect to their corresponding nominal beam coverage areas as a component of user scheduling. Updating a beam center target for a spot beam can be understood to adjust beamforming by the SCS to move the beam center of the spot beam so that the beam's maximum signal power corresponds to a different location within the same nominal beam coverage area over time. In an exemplary embodiment, the SCS uses dynamic beam centering for multiple forward user beams, and the underlying beamforming is ground-based beamforming, such as end-to-end beamforming, or satellite-based beamforming, which can be supported by ground-based or satellite-based computation of a dynamically changing beamforming solution.

[0007] An example embodiment includes a satellite beam control method for an SCS comprising one or more satellite access nodes and one or more satellites. The method includes providing a plurality of spot beams, each spot beam serving a corresponding plurality of user terminals and having a corresponding nominal beam coverage area, and moving beam centers of the plurality of spot beams with respect to the corresponding nominal beam coverage area as a component of user scheduling by the SCS. The beam center movement is based on selecting, for each spot beam and for each beam centering control interval of successive beam centering control intervals, a beam center target for use in centering the spot beam, where the beam center target corresponds to a location within the corresponding nominal beam coverage area.

[0008] Another exemplary embodiment includes an SCS including a ground segment including one or more satellite access nodes and a space segment including one or more satellites. The one or more satellite access nodes and the one or more satellites are configured to cooperate to provide multiple spot beams, each spot beam serving a corresponding plurality of user terminals and having a corresponding nominal beam coverage area. Processing circuitry included in the SCS is configured to move beam centers of the multiple spot beams with respect to the corresponding nominal beam coverage area as a component of user scheduling by the SCS. The beam center movement is based on the processing circuitry being configured to select, for each spot beam and for each beam centering control interval of successive beam centering control intervals, a beam center target for use in centering the spot beam. As previously described, the beam center target corresponds to a location within the corresponding nominal beam coverage area.

[0009] Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram of a satellite communications system (SCS) in accordance with an exemplary embodiment, shown in the context of multiple spot beams provided by the SCS to serve a corresponding number of user terminals. [Figure 2] FIG. 2 is a diagram of nominal beam coverage areas, each of which includes a corresponding number of user terminals, each illuminated by a beam centered on a selected location. [Figure 3] FIG. 3 illustrates an example of the selection of a new beam center target for a corresponding spot beam. [Figure 4] FIG. 4 is a logic flow diagram of a dynamic beam centering method in accordance with an example embodiment. [Figure 5] FIG. 5 is a logic flow diagram of a more detailed example of a dynamic beam centering method. [Figure 6] FIG. 6 is a block diagram of an SCS according to another example embodiment, where the SCS implements end-to-end beamforming as a type of ground-based beamforming (GBBF). [Figure 7] FIG. 7 is a block diagram illustrating example traffic processing and signaling for end-to-end beamforming in the forward direction. [Figure 8] FIG. 8 is a block diagram of an SCS in accordance with another embodiment, in which the satellite performs on-board beamforming and dynamic beam centering. [Figure 9] FIG. 9 is a block diagram of an SCS in accordance with another embodiment, where the ground segment implements another type of GBBF. [Figure 10] FIG. 10 is a diagram of beam area sectorization according to one embodiment. [Figure 11]FIG. 11 is a logic flow diagram of a method for converting a selected coverage location to a corresponding beam center, according to an example embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] FIG. 1 illustrates multiple spot beams 10, where "beam" refers to a radiation pattern of electromagnetic signal energy and the term "spot" refers to a focused beam, such that a larger service area is illuminated using potentially many individual spot beams 10. Each spot beam 10 has a beam cross-sectional area 12 and a beam center 14. It should be noted that the circular shape of the beam areas 12 depicted in FIG. 1 is a convenient example for illustrative purposes; the actual shape may not be circular.

[0012] A corresponding satellite communications system (SCS) 20 provides multiple spot beams 10. The depicted exemplary embodiment of the SCS 20 includes a ground segment 22 that includes one or more satellite access nodes (SANs) 24. A space segment 26 includes one or more satellites 28 by or through which the multiple spot beams 10 are provided. In at least one embodiment, the one or more satellites 28 include one or more geostationary orbit (GEO) satellites, such as a constellation of GEO satellites.

[0013] With respect to user traffic, such as data packets, carried by the multiple spot beams 10, the ground segment 22 includes a user scheduling function 30 that is responsible for scheduling transmissions between individual user terminals or groups of user terminals among the population of user terminals served by the multiple spot beams 10. In this and other parts of this specification, the term "function" refers to a particular activity or group of related activities performed by corresponding physical processing circuitry to achieve an overall logical operation. Thus, the user scheduling function 30 includes circuitry configured to determine, in successive scheduling intervals, which user terminals to serve during each scheduling interval.

[0014] The SCS 20 also includes a dynamic beam centering function 32 that operates as a component of the user scheduling function 30. Two options are shown in Figure 1: a first option is to implement the dynamic beam centering function 32 in the ground segment 22, and a second option is to implement the dynamic beam centering function 32 in the space segment 26. As yet another alternative, a hybrid implementation involves implementing part of the function in the ground segment 22 and part of the function in the space segment 26. Regardless of the implementation details of the beam centering function 32, dynamic beam centering refers to moving the beam center 14 of one or more of the multiple spot beams 10 as a component of user scheduling.

[0015] For example, the beam center 14 of the spot beam 10 may be moved from time to time to change where the spot beam 10 is focused within the nominal beam coverage area. As a specific example, assuming user terminals are distributed within the nominal beam coverage area, the beam center 14 may be moved to reflect which user terminal(s) or subset of user terminals are being served by the spot beam 10 during any given scheduling interval. Moving the beam center 14 may be understood to adjust beamforming parameters with the goal of improving the signal-to-noise ratio (SNR) of the scheduled user terminals. In the example of a geostationary orbit where the spot beam 10 is typically fixed, dynamic beam centering involves slight movement or displacement of the beam focus in conjunction with user scheduling with the goal of improving signal conditions for the scheduled users.

[0016] The beam area 12 of a given spot beam 10 may be defined according to contours reflecting radiated power levels, with the perimeter of the spot beam 10 defined, for example, as the -3 dB contour, where -3 dB represents the drop in radiated power relative to the maximum signal power within the beam area 12. The power may be expressed in terms of equivalent isotropic radiated power or EIRP. The beam center 14 represents the location within the beam cross-section where the radiated power is greatest. Depending on the beam shape and implementation details, the beam center 14 is not necessarily the geometric center or centroid of the beam cross-section.

[0017] 2 shows an exemplary arrangement that includes two nominal beam coverage areas 40, which are defined geographic regions. Of course, in practice there may be many nominal beam coverage areas 40, and dynamic beam centering may be used for one or more of them.

[0018] Each spot beam 10 has a corresponding beam footprint on the Earth's surface that depends on the beam area 12 and the beam angle. The beam footprint may also be referred to as the beam coverage area, and correspondingly, a "nominal beam coverage area 40" may be understood as a predetermined or predefined area on the ground that is covered by the corresponding spot beam 10.

[0019] A large geographic service area may be illuminated using multiple spot beams 10. Each spot beam 10 is at least nominally oriented to illuminate a corresponding one of multiple nominal beam coverage areas 40 that subdivide the overall geographic service area. For example, if satellite 28 is operating as a GEO satellite, each spot beam 10 may be a nominally stationary or fixed beam that provides corresponding consistent illumination of a respective nominal beam coverage area 40 in the absence of perturbations or systematic errors.

[0020] Each nominal beam coverage area 40 includes, for example, one or more user terminals 42. At different times of the day, different locations 44 within each nominal beam coverage area 40 may be selected to target the beam center 14 of the corresponding spot beam 10. For example, there may be “clusters” (geographical groupings) of user terminals 42 within the nominal beam coverage area 40, and the user scheduling function 30 may impose a time-division multiplexing scheme in which user traffic associated with a particular cluster is grouped in time. Correspondingly, the beam centering function 32 dynamically moves the beam center 14 of the corresponding spot beam 10 to focus the spot beam 10 on each individual cluster during one or more scheduling intervals, during which the spot beam 10 carries traffic for the individual cluster. As another example, each nominal beam coverage area 40 may be subdivided into sectors, and the center coordinates of different sectors may be selected at different times of the day as the selected locations 44 used to define the beam center target of the associated spot beam 10.

[0021] For any given selected location 44, the geographic coordinates of the selected location 44 may be transformed into a beamforming coordinate system, which may be based on azimuth and elevation angles, to define a beam center target for the participating spot beam 10. Figure 3 illustrates an exemplary scenario in which the beam center target 46 is offset from the current beam center 14, and the beamforming solution, i.e., beamforming weights, used to form the illustrated spot beam 10 are adjusted to move the beam center 14 to the beam center target 46. Each selection of a new location 44 for the corresponding nominal beam coverage area 40 results in a recalculation of the beamforming weights for the participating spot beam 10 to "move" the beam center 14 to the beam center target 46 corresponding to the newly selected location 44.

[0022] As previously mentioned, each nominal beam coverage area 40 includes multiple user terminals 42 served by the spot beam 10 corresponding to the nominal beam coverage area 40. The distribution pattern of user terminals 42 may differ in each nominal beam coverage area 40 and may change over time. For example, the user terminals 42 located within any given nominal beam coverage area 40 may include fixed terminals, mobile terminals, or a combination of both. However, with respect to movement of the beam center 14, mobile terminals are expected to change location relatively slowly compared to the rate at which beam centering changes can be determined.

[0023] 4 illustrates an embodiment including a method 400 of satellite beam control for an SCS 20 including one or more satellite access nodes 24 and one or more satellites 28. The method 400 includes providing (block 402) a plurality of spot beams 10, each spot beam 10 serving a corresponding plurality of user terminals 42 and having a corresponding nominal beam coverage area 40, and moving (block 404) beam centers 14 of the plurality of spot beams 10 relative to the corresponding nominal beam coverage area 40 as a component of user scheduling by the SCS 20. Moving the beam centers 14 includes selecting, for each spot beam 10 and for each beam centering control interval of successive beam centering control intervals, a beam center target 46 for use in centering the spot beam 10, the beam center target 46 corresponding to a location 44 within the corresponding nominal beam coverage area 40.

[0024] The "providing" step (block 402) may be understood as operating the SCS 20 in a manner to form spot beams 10. As an example, "providing" refers to the continuous performance of beamforming by the SCS 20. Correspondingly, the "moving" step (block 404) may be understood as a continuous or recurring operation, such as updating the beamforming weights used by the SCS 20 to provide multiple spot beams 10.

[0025] According to one or more embodiments, the corresponding nominal beam coverage area 40 of each spot beam 10 is logically divided into a plurality of sectors. The beam center target 46 for each spot beam 10 in each beam centering control interval corresponds to a selected one of the sectors. At least one such embodiment includes selecting sectors on a round-robin basis and controlling the length of time that individual ones of the sectors remain selected for each round-robin selection cycle based on the communication needs of user terminals 42 located in the individual sectors.

[0026] Selecting a beam center target 46 to use for centering each spot beam 10, for each beam centering control interval, in at least one embodiment, includes selecting locations 44 within the corresponding nominal beam coverage area 40 depending on at least one of the spatial distribution of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40 or the individual communication needs of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40. In at least one embodiment, the pattern of location selection within each corresponding nominal beam coverage area 40 across multiple ones of the beam centering control intervals is a function of a user scheduling algorithm implemented by the SCS 20. For example, the method 400 may include identifying clusters of user terminals 42 within each nominal beam coverage area 40 and selecting different locations 44 corresponding to the identified different clusters over time as beam center targets 46 for the spot beams 10 corresponding to the nominal beam coverage area 40.

[0027] In one or more embodiments, the method 400 includes aligning boundaries of the beam centering control interval with boundaries of transmission slots used by the SCS 20 for transmitting user traffic. Doing so ensures that movement of the beam center 14 of any one of the plurality of spot beams 10 occurs only on transmission slot boundaries. As an example, the SCS 20 organizes transmissions based on a frame structure, where each frame of a plurality of consecutive frames includes a defined number of subframes, and each subframe includes one or more time slots that function as a transmission time interval (TTI), representing the smallest unit of time allocatable for scheduling transmissions between individual user terminals 42.

[0028] In at least one embodiment, moving the beam centers 14 of the multiple spot beams 10 includes selecting, during any current beam centering control interval, a next beam center target 46 for each spot beam 10 for the next beam centering control interval, and, for any spot beam 10 for which the next beam center target 46 selected for the next beam centering control interval is different from the current beam center target 46 selected for the current beam centering interval, adjusting the beamforming performed by the SCS 20 at the start of the next beam centering control interval to move the beam center 14 of the spot beam 10 to the next beam center target 46.

[0029] Moving the beam center 14 of any one of the multiple spot beams 10, in one or more embodiments, includes calculating new values ​​for a corresponding set of beamforming weights used by the SCS 20 to generate the superposition of emitted signals that results in the spot beam 10. The SCS 20, for example, performs beamforming to provide the multiple spot beams 10, each having a corresponding set of beamforming weights used by the SCS 20. As a specific example, the SCS 20 performs end-to-end beamforming in the forward direction, and the multiple spot beams 10 include multiple forward user beams realized using end-to-end beamforming. For a detailed example regarding end-to-end beamforming, see U.S. Patent No. 10,720,988 B2, issued July 21, 2020.

[0030] Other approaches to ground-based beamforming may be used, such as transmitting weighted beam element signals to the satellite 28 for transmission from corresponding antenna elements of an onboard phased array antenna. Yet another approach includes transmitting forward beam signals with corresponding weighting information from the ground segment 22 for application of the weights via beamforming circuitry onboard the satellite 28. At least one embodiment relies on a completely onboard implementation in which the satellite 28 calculates beamforming weights and applies them to the forward beam signals for transmission from the onboard phased array antenna.

[0031] In any event, for each forward user beam that moves its corresponding beam center 14 to a new beam center target 46, the method 400 includes calculating new values ​​for the associated beamforming weights, where the new values ​​are calculated to optimize the signal-to-noise ratio (SNR) at the new beam center target 46 and minimize other-beam interference at the new beam center target 46. In other words, if any given beam center target 46 corresponds to a selected location 44 within the nominal beam coverage area 40 corresponding to the associated spot beam 10, achieving the beam center target 46 means calculating beamforming weights that optimize the SNR for the user terminal(s) at or near the selected location 44.

[0032] For any beam centering control interval that moves one or more beam centers 14, method 400, in one or more embodiments, includes calculating new values ​​for all beamforming weights corresponding to all spot beams 10 to account for changes in inter-beam interference resulting from the moved beam center(s) 14. That is, there is a beamforming weight corresponding to each spot beam 10, and the sum of all such weights represents the overall beamforming solution implemented by SCS 20, which performs joint optimization of the beamforming solution for any given collection of beam center targets 46 determined for a given beam centering control interval. The joint optimization limits inter-beam interference and correspondingly maximizes individual beam signal SNRs at the individual beam center targets 46.

[0033] In at least one embodiment, the duration of the beam centering control interval is an integer multiple of the duration of the user scheduling interval used by the SCS 20. The boundaries of the beam centering control interval coincide with the boundaries of the user scheduling interval, and movement of the beam center 14 is limited in time to the transition from one user scheduling interval to the next.

[0034] The recalculation of the overall beamforming solution may be a joint function of the overall set of beam center targets 46 used for any given beam centering control interval, although individual beam center targets 46 may be selected independently. However, in at least one embodiment, the method 400 includes applying spatial constraints to the selection of beam center targets 46 to limit inter-spot beam overlap between adjacent spot beams 10 that are the same frequency.

[0035] FIG. 5 illustrates a method 500 of operation by beam centering function 32 and can be understood as an example of detailed implementation of block 402 of FIG. 4. Method 500 may be looped or otherwise performed continuously, and in general, the operations detailed in FIGS. 4 and 5 may be performed continuously, along with other operations by SCS 20. FIGS. 4 and 5 may be subsumed within or performed in conjunction with user scheduling, in which the SCS schedules transmissions to and / or from individual user terminals 42 or groups of user terminals 42. While such scheduling may be performed on a beam-by-beam basis, scheduling may also take into account system-wide capacity and bandwidth limitations, which may involve sharing certain system resources across spot beams 10.

[0036] Operation of method 500 includes selecting a beam center target 46 for each spot beam 10 based on a centering indicator for each beam centering control interval (block 502). An exemplary centering indicator may be determined by beam centering functionality 32 or user scheduling functionality 30 and may include information indicative of individual locations 44 within a nominal beam coverage area 40 for use as a beam center target 46 in dynamic beam recentering. Other exemplary indicators include any one or more of information about the spatial distribution of user terminals 42 within each nominal beam coverage area 40, information about communication services or service types associated with individual user terminals 42 within each nominal beam coverage area 40, and information about communication needs or communication statistics of individual user terminals 42 within each nominal beam coverage area 40. Such information may include, for example, quality of service (QoS) requirements, such as minimum throughput. In general, the beam center target 46 may be moved over successive beam centering control intervals according to a user scheduling algorithm, such as a proportional fair scheduling algorithm, which determines which user terminals 42 to schedule depending on one or more weighting parameters that control proportionality, subject to some lower or baseline limit to prevent "fairness" or to meet some minimum requirement.

[0037] In at least one embodiment, the centering indicator includes information indicating selected locations 44 of multiple nominal beam coverage areas 40 for the next beam centering control interval, and the beam centering function 32 uses that information to calculate a new beam center target 46. For any given future beam centering control interval, the “new” beam center target 46 selected for any given spot beam 10 may be the same as the one used for the current beam centering control interval. In such a case, for such spot beam 10, the beam center target 46 does not change when the next beam centering control interval begins. In other words, the beam center target 46 does not necessarily change for every single spot beam 10 in every single beam centering control interval.

[0038] If a new beam center target 46 is selected for one or more of the spot beams 10, the method 500 proceeds to calculating new values ​​for the individual beamforming weights corresponding to the spot beams 10 (block 504). Calculating new beamforming weights for any one or more of the spot beams 10 may be referred to as "updating" or "adjusting" the beamforming solution, and as mentioned above, the term "beamforming solution" refers to the entire set or plurality of beamforming weights used to realize the multiple spot beams 10.

[0039] In at least one embodiment, there is a predefined set of beam center targets 46 for the multiple spot beams 10 that are indexed or mapped to corresponding pre-calculated beamforming solutions, thereby eliminating the need to compute the beamforming solutions on the fly, but instead retrieved from a look-up table or other stored data structure. Because such solutions are less flexible than on-the-fly computations, they may be more advantageous in embodiments where the nominal beam coverage area 40 is sectorized according to a known division scheme and the selected locations 44 are restricted to the defined sectors.

[0040] Whether the new values ​​were pre-calculated or calculated on the fly, method 500 proceeds to apply the new values ​​in the next beam centering control interval (block 506). As previously mentioned, in one or more embodiments, movement of any beam center 14 of any spot beam 10 involves recalculation of the beamforming weights for all spot beams 10. Such recalculation of the beamforming solution reflects a joint optimization of the beamforming weights of all spot beams 10 to reduce inter-beam interference and correspondingly maximize the SNR at the individual beam center targets 46 of all spot beams 10.

[0041] 6 illustrates an SCS 20 according to an exemplary embodiment, which employs end-to-end beamforming, where the ground segment includes one or more SANs 24 supported by communications processing circuitry 60, which may be implemented in one or more nodes, e.g., one or more computer servers. The communications processing circuitry 60 interfaces with one or more external networks 62, such as the Internet, other packet data networks (PDNs), or public switched telephone networks (PSTNs). User traffic targeted to individual user terminals 42 served by the SCS 20 flows into the communications processing circuitry 60 from the external network(s) 62, and user traffic originating from individual user terminals 42 served by the SCS 20 flows out of the communications processing circuitry 60 to the external network(s) 62.

[0042] The communications processing circuitry 60 includes or interfaces with a user scheduling circuitry 64 and a beam centering control circuitry 66. Additionally, the communications processing circuitry 60 includes or interfaces with a beamforming circuitry 68. The beamforming circuitry 68 calculates and applies beamforming weights 70 for a forward link direction toward the user terminal 42, a return link direction from the user terminal 42, or both. The beamforming circuitry 68 calculates the beamforming weights 70 based on channel estimates. The beamforming weights 70 include, for example, a set of forward beamforming weights and a set of return beamforming weights.

[0043] Each SAN 24 includes interface circuitry 72 for communicating with communications processing circuitry 60 in the forward and return directions. Interface circuitry 72 includes circuitry configured to send and receive physical layer signals over wired or wireless media and may include higher layer circuitry for protocol processing, synchronization, etc. Additionally, each SAN 24 includes transmitter / receiver circuitry 74. In at least one embodiment, transmitter / receiver circuitry 74 comprises a radio frequency (RF) transmitter and receiver for providing RF-based feeder uplinks and downlinks between each SAN 24 and satellites 28.

[0044] The exemplary satellite 28 includes multiple transponders 80, each providing a separate signal path through the satellite 28. There may be a transponder 80 dedicated to the forward link direction, providing a forward link signal path for relaying forward user traffic from the ground segment 22 to the user terminal 42, and a separate plurality of transponders 80 dedicated to the return link direction, providing a return link signal path for relaying return user traffic from the user terminal 42 to the ground segment 22. In other embodiments, the same plurality of transponders 80 provide the forward link signal path and the return link signal path on a time-division multiplexed and switching basis. In other arrangements, the plurality of transponders 80 includes at least some with switchable connections, allowing their use in either the forward or return direction.

[0045] Using the illustrated forward link direction for an exemplary context, each transponder 80 has an input (receive) end associated with a receive antenna element 82 and an output (transmit) end associated with a transmit antenna element 84. There may be corresponding antenna subsystems dedicated to forward and / or return leg reception onboard the satellite 28, and additional antenna subsystems dedicated to forward and / or return leg transmission. For the return leg, at least in the context of end-to-end beamforming, the “input” end of the transponder 80 receives a superposition of return leg uplink signals from user terminals 42 operating within one or more return leg beam coverage areas, which may or may not coincide with the forward beam coverage areas. Correspondingly, the “output” end of the transponder transmits the received superposition of return leg uplink signals as corresponding return leg downlink signals received at two or more of the SANs 24.

[0046] For end-to-end beamforming in the forward direction, the communications processing circuitry 60 forms forward user streams under the control of the user scheduling circuitry 64. Each forward user stream multiplexes forward user traffic for individual user terminals 42 in a particular one of the nominal beam coverage areas 40, in accordance with the operation of the user scheduling circuitry 64. Thus, each forward user stream can be understood as carrying forward user traffic for transmission via a corresponding one of the plurality of spot beams 10.

[0047] The beamforming circuitry 68 uses the end-to-end channel estimates for the forward direction to calculate beamforming weights for forward beamforming as an M x K matrix of beamforming weights, where M equals the number of SANs 24 participating in end-to-end beamforming and K equals the number of forward user beams. Each forward user stream is used to form a forward beam signal. This means that there are K forward beam signals, each carrying forward user traffic for transmission in a separate one of the K forward user beams.

[0048] The beamforming circuitry 68 applies the values ​​of the M×K beam weight matrix to each of the K forward beam signals to generate M access node-specific forward signals. Each access node-specific forward signal corresponds to a specific one of the M SANs 24 and each comprises K weighted forward beam signals. The beamforming circuitry 68 may include a splitting module and M forward weighting and summing modules. The splitting module splits (e.g., replicates) each of the K forward beam signals into M groups of K forward beam signals, one group for each of the M forward weighting and summing modules. Thus, each forward weighting and summing module receives all K forward beam signals. Here, "module" refers to the configured circuitry.

[0049] Additionally, circuitry within beamforming circuitry 68 operates as a forward beam weight generator module that generates an M×K forward beam weight matrix. In one or more embodiments, the forward beam weight matrix is ​​generated based on a channel matrix whose elements are estimates of the end-to-end forward gain for each of the K×M end-to-end forward multipath channels to form the forward channel matrix. The end-to-end forward gain estimation is performed in the channel estimator module.

[0050] Thus, in the forward link direction, each SAN 24 receives one of the M access node-specific forward signals 136 for transmission by the SAN 24 as a forward uplink signal 92. Each receive antenna element of the satellite 28 receives a unique superposition of forward uplink signals 92, each such superposition involving forward uplink signals 92 from two or more of the M SANs 24. The superpositions are unique because, due to the geographic distribution of the SANs 24, the uplink channel between each SAN 24 and each receive antenna element 82 on the satellite 28 is consequently different.

[0051] The unique superposition of forward uplink signals 92 received at each receive antenna element 82 may be referred to as a forward composite uplink signal 94, meaning that each input of transponder 80 receives a unique forward composite uplink signal 94. Each transponder 80 operates as an unprocessed bent-pipe transponder that combines its corresponding received forward composite uplink signal 94 with the user downlink side of satellite 28 for transmission as a forward user downlink signal 96 from a corresponding one of transmit antenna elements 84. Forward user downlink signal 96 is the corresponding forward composite uplink signal 94, which has undergone filtering, amplification, and, in one or more embodiments, frequency translation from the uplink signal frequency to the downlink signal frequency.

[0052] The multiple transmit antenna elements 84 are configured such that individual forward user downlink signals 96 transmitted from different transmit antenna elements 84 overlap in the far field (i.e., the distance from the transmit antenna elements 84 where the radiation behavior of the electromagnetic signal governs). These overlaps form multiple forward user beams 100, each a beamformed transmission of a forward beam signal 102, each having a corresponding forward user beam coverage area 104. The collection of forward user beams 100 illuminates an entire forward user service area 106. It should be understood that the forward user beam 100 is an example of a spot beam 10 discussed herein.

[0053] Each forward user beam 100 illuminates a distinct forward user beam coverage area 104, and there may be predefined or pre-defined geographic coordinates or boundaries that nominally define the specific region on the Earth's surface that is illuminated. That is, there may be a nominal beam coverage area 40 defined for each forward user beam 100. Correspondingly, dynamic beam recentering as described herein may be understood, in the context of this example, as dynamically recentering one, some, or all forward user beams 100 for each beam centering control interval. Each forward user beam 100 may be understood as a beamformed transmission of forward beam signals 102 that multiplex user traffic for user terminals 42 served by the forward user beam 100 according to ongoing user scheduling.

[0054] The communications processing circuitry 60 includes or is communicatively associated with a user scheduling circuitry 64, a beam centering control circuitry 66, and a beamforming circuitry 68. All such circuits may include fixed or programmably configured circuitry, or a combination of both. In one example, at least a portion of the beamforming circuitry 68 includes digital signal processing (DSP) hardware configured to perform beamforming calculations. Additionally, at least a portion of the circuitry depicted in FIG. 6 includes one or more microprocessors or DSPs or other programmably configured digital processing circuits that are specially adapted to perform the described functions based on the execution of computer program instructions stored in a computer-readable medium. For example, the communications processing circuitry 60 includes or is associated with one or more types of memory devices (e.g., RAM for executing operating programs and FLASH for non-volatile storage of program instructions). Such memory devices are also used to store beamforming weights 70 and channel estimates.

[0055] Channel estimates may be determined from channel state information (CSI). One approach to providing CSI feedback relies on one or more user terminals 42 operating in each nominal beam coverage area 40 as “reference terminals” (RTs) or “designated terminals” (DTs). For example, one or more user terminals 42 at or near the geographic center of the nominal beam coverage area 40 may be RTs. Such an approach involves receiving downlink channel estimates from separate RTs for each nominal user coverage area 40 based on each SAN 24 transmitting a unique reference signal for use in channel estimation at the RTs.

[0056] FIG. 7 illustrates an example of end-to-end beamforming in the forward direction with exemplary functions. Forward user traffic 90 input is scheduled, for example, based on determining the identity of a user terminal 42 targeted by a given portion of the traffic, determining a nominal beam coverage area 40 associated with the targeted terminal, and forming a corresponding forward stream signal 92. A forward beam signal generation function 94 outputs a forward beam signal 96 corresponding to the forward stream signal 92, and a beamforming function 98 applies beamforming weights 70 to the forward beam signal 98, e.g., applying an M×K forward weight matrix as described above, to create a set of M access node-specific forward signals 99, each generated for a specific one of the SANs 24 participating in the end-to-end beamforming. Note that the beamforming weights 70 are adapted for each beam centering control interval to reflect the beam center target 46 applicable to each beam centering control interval.

[0057] 8 illustrates an alternative embodiment of beamforming. Here, the beamforming is based on a satellite 28 carrying a phased array antenna 140 including multiple antenna elements 142 arranged in a feeder plane. Although not shown, the satellite 28 may also carry a reflector associated with the phased array antenna 142. In this arrangement, there are multiple antenna element signals 144, each corresponding to a separate antenna element. For the example spot beam 10 considered herein, the antenna element signals 144 are weighted such that their transmission results in a superposition of signals that form the forward user beam 100.

[0058] In one embodiment, satellite 28 includes an antenna subsystem 150 for receiving forward uplink signals 152 that include one or more forward beam signals. Antenna subsystem 150 couples the received forward uplink signals 152 to forward transmission circuitry 154, which includes beamforming circuitry 156.

[0059] The beamforming circuitry 156 splits each forward beam signal into N unweighted element signals, where N is equal to the number of antenna elements 122, and then forms the antenna element signals 124 based on applying a corresponding set of beamforming weights. Each such set provides the formation of a separate one of the forward user beams 100 and includes separate beam weights (phase and / or amplitude) for each antenna element 142.

[0060] For example, there may be multiple phased array antennas 140 associated with different downlink signal frequencies and / or polarizations, or the phased array antenna 140 may include multiple sets of input antenna feeds corresponding to different downlink signal frequencies and / or polarizations. Of further note, the beamforming circuitry 156 may form a set of antenna element signals 144 for each forward beam signal and combine those sets corresponding to forward user beams 100 having the same downlink signal frequency and polarization. The beamforming circuitry 156 in one or more embodiments is configured to calculate the beamforming weights 70 and may incorporate a beam centering function 32, whereby the beamforming weights 70 are adjusted for dynamic movement of the beam center 14. Alternatively, the communications processing circuitry 60 may incorporate processing circuitry configured as a beamforming weight calculator that calculates the beamforming weights 70, including dynamic adjustments to the beam centering interval, for transmission to the satellite 28 and corresponding application by the beamforming circuitry 156 onboard the satellite.

[0061] 9 illustrates another embodiment in which the beamforming weights 70 are calculated and applied on the ground. In this approach, the ground segment 22 forms forward user beams, each corresponding to one of the forward user beams 100, and divides each forward user beam into N beam element signals, each corresponding to an antenna element 142 of a phased array antenna 140 carried on the satellite 28, and weights the simultaneous transmission of the beam element signals from the phased array antenna 140 to form a corresponding forward user beam 100.

[0062] The SAN 24 forms a forward uplink signal 160 that conveys the forward beam element signals toward the satellite 28, which includes an antenna subsystem 162. The forward transmission circuitry 164 provides filtering, amplification, and, in one or more embodiments, frequency conversion. The forward transmission circuitry 164 combines the forward beam element signals to the individual antenna elements 142 for transmission.

[0063] 10 illustrates an example of logically dividing a nominal beam coverage area 40 into multiple sectors 170. Each sector 170 may be represented by defined geographic coordinates representing a sector center or other reference point associated with the sector. Over successive beam centering control intervals, different reference points for any given nominal beam coverage area 40 may be individually selected as locations 44 for calculating beam center targets 46 for the associated spot beams 10.

[0064] 11 illustrates a method 1100 used to convert any given selected location 44 into a beam center target 46. Method 1100 includes determining (block 1102) geographic coordinates representing the beam center target, i.e., a location 44 within a given nominal beam coverage area 40 is selected for a given beam centering control interval, and the geographic coordinates of the selected location 44 are known or immediately determined. Method 1100 then proceeds to converting (block 1104) the geographic coordinates into beam coordinates, e.g., angular values, which are used to recalculate the corresponding beam weights to achieve recentering of the involved spot beam 10.

[0065] With the above example in mind, the SCS 20 in the exemplary embodiment includes a ground segment 22 that includes one or more SANs 24 and a space segment 26 that includes one or more satellites 28. The one or more SANs 24 and the one or more satellites 28 are configured to cooperate to provide multiple spot beams 10. Each spot beam 10 serves a corresponding number of user terminals 42 and has a corresponding nominal beam coverage area 40.

[0066] The processing circuitry included in the SCS 20 is configured to, as a component of user scheduling by the SCS 20, move the beam centers 14 of the multiple spot beams 10 relative to the corresponding nominal beam coverage area 40 based on selecting, for each spot beam 10 and for each beam centering control interval of successive beam centering control intervals, a beam center target 46 to use for centering the spot beam 10, the beam center target 46 corresponding to a location 44 within the corresponding nominal beam coverage area 40.

[0067] In one or more embodiments, such processing circuitry resides in the ground segment 22. See, for example, the beam centering control circuit 66 of FIG. 6. In one or more other embodiments, such processing circuitry resides in the space segment 26. See, for example, the beamforming circuitry 136 of FIG. 8. This circuitry, in one or more embodiments, implements the dynamic beam centering function 32 discussed herein. In yet other embodiments, the dynamic beam centering function 32 is realized cooperatively between the ground segment 22 and the space segment 26, such as by recalculating beamforming solutions in the ground segment 22 for dynamic beam centering and applying those beamforming solutions in the space segment 26.

[0068] In one or more embodiments, one or more satellites 28 include geostationary satellites used to provide multiple spot beams 10 as nominally stationary spot beams corresponding to nominal beam coverage areas 40. The corresponding nominal beam coverage areas 40 of each spot beam 10 are logically divided into multiple sectors 170, and the beam center target 46 for each spot beam 10 in each beam centering control interval corresponds to a selected one of the sectors 170. Correspondingly, in at least one embodiment, processing circuitry within SCS 20 that implements dynamic beam centering is configured to select sectors 170 on a round-robin basis and, for each round-robin selection cycle, control the length of time that individual ones of the sectors 170 remain selected based on the communication needs of user terminals 42 located in those sectors 170.

[0069] With respect to each beam centering control interval and the selection of the beam center target 46 to use for centering each spot beam, the associated processing circuitry of the SCS 20 is configured, in one or more embodiments, to select a location within the corresponding nominal beam coverage area 40 depending on at least one of the spatial distribution of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40 or the individual communication needs of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40.

[0070] For example, the processing circuitry involved is configured to follow a pattern of location selection within each corresponding nominal beam coverage area 40 over multiple ones of the beam centering control intervals, the pattern being a function of a user scheduling algorithm implemented by the SCS 20.

[0071] In one or more embodiments, the processing circuitry involved is configured to identify clusters of user terminals 42 within each nominal beam coverage area 40 and to select different locations 44 corresponding to the different identified clusters over time as beam center targets 46 for the spot beams 10 corresponding to the nominal beam coverage areas 40.

[0072] The processing circuitry involved is configured, in at least one embodiment, to cause the boundaries of the beam centering control interval to coincide with the boundaries of transmission slots used by the SCS for the transmission of user traffic, thereby restricting movement of the beam center 14 of any one of the plurality of spot beams 10 to occur only on the boundaries of the transmission slots.

[0073] In one or more embodiments, the processing circuitry involved is configured to move the beam centers 14 of the multiple spot beams 10 by selecting, during any current beam centering control interval, a next beam center target 46 for each spot beam 10 for the next beam centering control interval, and, for any spot beam 10 for which the next beam center target 46 selected for the next beam centering control interval is different from the current beam center target 46 selected for the current beam centering interval, adjusting the beamforming performed by the SCS 20 at the start of the next beam centering control interval to move the beam center 14 of the spot beam 10 to the next beam center target 46.

[0074] To move the beam center 14 of any one of the multiple spot beams 10, the involved processing circuitry is configured, in one or more embodiments, to calculate new values ​​for a corresponding set of beamforming weights used by the SCS 20 to generate the superposition of signals that results in the spot beam 10. Thus, in one or more embodiments, the SCS 20 is configured to provide multiple spot beams 10 using beamforming, and the SCS 20 calculates and applies beamforming weights to transmissions of user traffic to realize the individual spot beams 10. As previously mentioned, in at least such embodiments, the SCS 20 is configured to perform end-to-end beamforming in the forward direction, and the multiple spot beams 10 include multiple forward user beams 100.

[0075] In at least one embodiment, one or more satellites 28 include a satellite 28 having a phased array antenna 140 used to provide multiple forward user beams 100. Corresponding beamforming weights are calculated onboard the satellite 28 or are calculated in the ground segment 22. For example, the processing circuitry used to implement the dynamic beam centering function 32 may be implemented in the ground segment 22 and configured to transmit, or cause to be transmitted, the dynamically calculated beamforming weights to the satellite 28 as a forward uplink transmission from one of the one or more SANs 24.

[0076] In at least one embodiment, the processing circuitry involved is configured to calculate new values ​​for a corresponding set of beamforming weights for each forward user beam 100 that moves its corresponding beam center 14 to a new beam center target 46. The new values ​​are calculated to optimize the SNR at the new beam center target 46 and minimize other beam interference at the new beam center target 46. The processing circuitry in one or more embodiments is configured to take into account changes in inter-beam interference resulting from the moved beam center 14 when calculating the new values ​​of the beamforming weights.

[0077] Broadly speaking, this disclosure details techniques for dynamically recentering spot beams 10 such that different locations within a corresponding nominal beam coverage area 40 maximize beam signal power over time. For example, the SCS 20 uses beam centering to improve the SNR of scheduled user terminals 42. In the context of a fixed coverage area where the beam center of a conventional beam remains fixed, dynamically recentering the beam as a component of user scheduling significantly improves beam / system throughput, provided there are no unintended perturbations.

[0078] As an example, the SCS 20 implements end-to-end forward beamforming using geostationary orbiting satellites 28 as end-to-end relays between multiple geographically dispersed SANs 24 and a population of user terminals 42 dispersed across an aggregated service area 106 illuminated by multiple forward user beams 100. Processing circuitry 66 in the ground segment 22 of the SCS 20 dynamically selects locations 44 within the forward user beam coverage areas 104 corresponding to the forward user beams 100 for use in determining corresponding beam center targets 46 for upcoming beam centering control intervals and updates the beamforming solutions used by the SCS 20 to realize the forward user beams 100.

[0079] In another example of ground-based beamforming, the ground segment 22 of the SCS 20 forms beam signals corresponding to the forward user beams 100 and splits each such forward beam signal into multiple forward beam element signals. The forward beam element signals are weighted for transmission from individual antenna elements 142 of a phased array antenna 140 carried on the satellite 28. Correspondingly, the ground segment 22 transmits one or more forward uplink signals 132 conveying the forward beam element signals for the various forward beam signals to the satellite 28 for retrieval from and corresponding transmission to the phased array antenna 140. The transmissions generate the desired multiple forward user beams 100.

[0080] In yet another alternative, the satellite receives the forward beam signals and generates corresponding forward beam element signals for transmission from one or more onboard phased array antennas 140. As a further variation on this embodiment, the beamforming solutions used to create the forward beam element signals may be calculated on the ground segment 22 and transmitted to the satellite 28, or the satellite 28 may calculate the beamforming solutions. That is, in at least one embodiment, processing circuitry configured to implement the beam centering function 32 is onboard the satellite 28, and the satellite 28 updates the beamforming solutions to reflect the updated beam center target 46.

[0081] In particular, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. It is therefore to be understood that the invention(s) is not limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the present disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. 1. A satellite beam control method for a satellite communications system including one or more satellite access nodes and one or more satellites, said method comprising: providing a plurality of spot beams, each spot beam serving a corresponding plurality of user terminals and having a corresponding nominal beam coverage area; moving beam centers of the plurality of spot beams relative to the corresponding nominal beam coverage area based on selecting, for each spot beam and for each beam centering control interval of successive beam centering control intervals, a beam center target for use in centering the spot beam as a component of user scheduling by the satellite communications system, the beam center target corresponding to a location within the corresponding nominal beam coverage area; A method comprising:

2. 10. The method of claim 1, wherein the one or more satellites include a geostationary satellite used to provide the plurality of spot beams as nominally stationary spot beams corresponding to the nominal beam coverage area.

3. 3. The method of claim 1, wherein the corresponding nominal beam coverage area of ​​each spot beam is logically divided into a plurality of sectors, and the beam center target for each spot beam in each beam centering control interval corresponds to a selected one of the sectors.

4. 4. The method of claim 3, further comprising: selecting the sectors on a round-robin basis; and, for each round-robin selection cycle, controlling the length of time that individual ones of the sectors remain selected based on communication needs of user terminals located in the individual sectors.

5. 4. The method of claim 1, wherein for each beam centering control interval, selecting the beam center target to use for centering each spot beam comprises selecting a location within the corresponding nominal beam coverage area depending on at least one of a spatial distribution of the corresponding plurality of user terminals within the corresponding nominal beam coverage area or individual communication needs of the corresponding plurality of user terminals within the corresponding nominal beam coverage area.

6. 6. The method of claim 5, wherein a pattern of location selection within each corresponding nominal beam coverage area over a plurality of the beam centering control intervals is a function of a user scheduling algorithm implemented by the satellite communications system.

7. 7. The method of claim 5 or 6, further comprising identifying clusters of user terminals within each nominal beam coverage area, and selecting, over time, different locations corresponding to different identified clusters as the beam center targets for the spot beams corresponding to the nominal beam coverage areas.

8. 8. The method of claim 1, further comprising: matching boundaries of the beam centering control interval with boundaries of transmission slots used by the satellite communications system for transmitting user traffic, whereby movement of the beam center of any spot beam among the plurality of spot beams occurs only on boundaries of transmission slots.

9. 9. The method of claim 1, wherein moving the beam centers of the plurality of spot beams comprises: selecting, during any current beam centering control interval, a next beam center target for each spot beam for a next beam centering control interval; and, for any spot beam for which the next beam center target selected for the next beam centering control interval is different from the current beam center target selected for the current beam centering interval, adjusting beamforming performed by the satellite communications system at the start of the next beam centering control interval to move the beam center of the spot beam to the next beam center target.

10. 10. The method of claim 1, wherein moving the beam center of any one of the plurality of spot beams comprises calculating new values ​​for a corresponding set of beamforming weights used by the satellite communications system to generate a superposition of signals that results in the spot beam.

11. 11. The method of claim 1, wherein the satellite communication system performs beamforming to provide the plurality of spot beams, each spot beam having a corresponding set of beamforming weights used by the satellite communication system.

12. 12. The method of claim 11, wherein the satellite communications system implements end-to-end beamforming in a forward direction, and the plurality of spot beams comprises a plurality of forward user beams.

13. 12. The method of claim 11, wherein the one or more satellites include a satellite having a phased array antenna used to provide the plurality of spot beams as a plurality of forward user beams, and the corresponding beamforming weights are calculated on board the satellite or are calculated in a ground segment of the satellite communications system and communicated to the satellite as a forward uplink transmission from one of the one or more satellite access nodes.

14. 14. The method of claim 12 or 13, wherein for each forward user beam whose corresponding beam center is moved to a new beam center target, the method includes calculating new values ​​for the corresponding set of beamforming weights, the new values ​​being calculated to optimize a signal-to-noise ratio (SNR) at the new beam center target and minimize other-beam interference at the new beam center target.

15. 15. The method of claim 14, wherein for any beam centering control interval in which one or more beam centers are moved, the method includes calculating new values ​​for all beamforming weights to account for changes in inter-beam interference resulting from the moved beam centers.

16. 16. The method of claim 1, wherein the duration of the beam centering control interval is an integer multiple of the duration of a user scheduling interval used by the satellite communications system, and wherein the boundaries of the beam centering control interval coincide with the boundaries of the user scheduling interval.

17. The method of any preceding claim, wherein the beam center targets for each of the plurality of spot beams are independently selected.

18. The method of any one of claims 1 to 17, further comprising applying spatial constraints to the selection of the beam center target to limit spot beam overlap between adjacent spot beams of the same frequency.

19. 1. A satellite communications system, comprising: a terrestrial segment including one or more satellite access nodes; a space segment including one or more satellites; Including, the one or more satellite access nodes and the one or more satellites are configured to cooperate to provide a plurality of spot beams, each spot beam serving a corresponding plurality of user terminals and having a corresponding nominal beam coverage area; processing circuitry included in the satellite communications system is configured to, as a component of user scheduling by the satellite communications system, move beam centers of the plurality of spot beams relative to the corresponding nominal beam coverage areas based on selecting, for each spot beam and for each beam centering control interval of successive beam centering control intervals, a beam center target for use in centering the spot beam, the beam center target corresponding to a location within the corresponding nominal beam coverage area; Satellite communication system.

20. 20. The satellite communications system of claim 19, wherein the one or more satellites include a geostationary satellite used to provide the plurality of spot beams as nominally stationary spot beams corresponding to the nominal beam coverage area.

21. 21. The satellite communications system of claim 19 or 20, wherein the corresponding nominal beam coverage area of ​​each spot beam is logically divided into a plurality of sectors, and the beam center target for each spot beam in each beam centering control interval corresponds to a selected one of the sectors.

22. 22. The satellite communications system of claim 21, wherein the processing circuitry is configured to select the sectors on a round-robin basis and, for each round-robin selection cycle, to control the length of time that individual ones of the sectors remain selected based on the communications needs of user terminals located in the individual sectors.

23. 22. A satellite communications system as claimed in any one of claims 19 to 21, wherein for each beam centering control interval and for selecting the beam center target for use in centering each spot beam, the processing circuitry is configured to select a location within the corresponding nominal beam coverage area depending on at least one of a spatial distribution of the corresponding plurality of user terminals within the corresponding nominal beam coverage area or individual communications needs of the corresponding plurality of user terminals within the corresponding nominal beam coverage area.

24. 24. The satellite communications system of claim 23, wherein the processing circuitry is configured to follow a pattern of location selection within each corresponding nominal beam coverage area over a plurality of the beam centering control intervals that is a function of a user scheduling algorithm implemented by the satellite communications system.

25. 25. A satellite communications system as claimed in claim 23 or 24, wherein the processing circuitry is configured to identify clusters of user terminals within each nominal beam coverage area and to select, over time, different locations corresponding to different identified clusters as the beam centre targets for the spot beams corresponding to the nominal beam coverage areas.

26. 26. A satellite communications system as claimed in any one of claims 19 to 25, wherein the processing circuitry is configured to align boundaries of the beam centering control interval with boundaries of transmission slots used by the satellite communications system for transmitting user traffic, whereby movement of the beam center of any spot beam among the plurality of spot beams occurs only on boundaries of transmission slots.

27. 27. The satellite communications system of claim 19, wherein the processing circuitry is configured to move the beam centers of the plurality of spot beams by: selecting, during any current beam centering control interval, a next beam center target for each spot beam for a next beam centering control interval; and, for any spot beam for which the next beam center target selected for the next beam centering control interval is different from the current beam center target selected for the current beam centering interval, adjusting beamforming performed by the satellite communications system at the start of the next beam centering control interval to move the beam center of the spot beam to the next beam center target.

28. 28. A satellite communications system as claimed in any one of claims 19 to 27, wherein to move the beam centre of any one of the plurality of spot beams, the processing circuitry is configured to calculate new values ​​for a corresponding set of beamforming weights used by the satellite communications system to generate the superposition of signals that result in the spot beam.

29. 29. A satellite communications system according to any one of claims 19 to 28, wherein the satellite communications system is configured to use beamforming to provide the plurality of spot beams, each spot beam having a corresponding set of beamforming weights used by the satellite communications system.

30. 30. The satellite communications system of claim 29, wherein the satellite communications system is configured to perform end-to-end beamforming in a forward direction, and wherein the plurality of spot beams comprises a plurality of forward user beams.

31. 30. The satellite communications system of claim 29, wherein the one or more satellites include a satellite having a phased array antenna used to provide the plurality of forward user beams, and wherein the corresponding beamforming weights are calculated on board the satellite or via the processing circuitry included in the ground segment, wherein the processing circuitry is configured to cause the satellite to transmit the corresponding beamforming weights as a forward uplink transmission from one of the one or more satellite access nodes.

32. 32. The satellite communications system of claim 30 or 31, wherein the processing circuitry is configured to calculate new values ​​for the corresponding set of beamforming weights for each forward user beam whose corresponding beam center is moved to a new beam center target, the new values ​​being calculated to optimize a signal-to-noise ratio (SNR) at the new beam center target and minimize other beam interference at the new beam center target.

33. 33. The satellite communications system of claim 32, wherein for any beam centering control interval in which one or more beam centers are moved, the processing circuitry is configured to calculate new values ​​for all beamforming weights to account for changes in inter-beam interference resulting from the moved beam centers.

34. A satellite communications system as claimed in any one of claims 19 to 33, wherein the duration of the beam centering control interval is an integer multiple of the duration of a user scheduling interval used by the satellite communications system, and the boundaries of the beam centering control interval coincide with the boundaries of the user scheduling interval.

35. A satellite communications system according to any one of claims 19 to 34, wherein the processing circuitry is configured to independently select the beam centre targets for individual ones of the plurality of spot beams.

36. 36. A satellite communications system according to any one of claims 19 to 35, wherein the processing circuitry is configured to apply spatial constraints to the selection of the beam centre targets to limit spot beam overlap between adjacent spot beams that are the same frequency.

Images