Method and apparatus for satellite beam centering control for user scheduling

Abstract

A satellite communication system (SCS) (20) is disclosed that implements a beam recentering function (32) that dynamically recenters one or more spot beams (10) relative to their corresponding nominal beam coverage areas (40) as part of user scheduling. Updating a beam center target (46) of a spot beam (10) may be understood that beamforming is adjusted by the SCS (20) to move a beam center (14) of the spot beam (10) such that the maximum signal power of the beam corresponds over time to different positions within the same nominal beam coverage area. In one example embodiment, the SCS (20) uses dynamic beamcentering with respect to a plurality of forward user beams (100), where the underlying beamforming is ground-based beamforming, such as end-to-end beamforming, or satellite-based beamforming, such as end-to-end beamforming, or satellite-based beamforming, such as end-to-end beamforming, or satellite-based beamforming. This may be supported by ground-based or satellite-based calculations of dynamically varying beamforming solutions.

Description

Technical Field

[0001] The methods and apparatus disclosed herein embody techniques for beam centering control in satellite communication systems as an integral part of user scheduling. Background Art

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

[0003] A beamforming satellite communication system performs beamforming in a forward link to serve a corresponding group of user terminals in a corresponding forward user beam coverage area. Additionally or alternatively, the satellite communication system performs beamforming in a return link to serve a corresponding group of user terminals in a corresponding return user beam coverage area. The return user beam coverage area may correspond to the forward user beam coverage area.

[0004] The forward link direction involves the satellite communication system forming a plurality of forward user beams, which are directional radio signals. Specifically, the forward user beams can be "spot beams," where the power of each such spot beam is concentrated to achieve focused coverage of a corresponding limited geographic area.

[0005] The entire geographic area may be divided into a plurality of nominal forward user beam coverage areas, wherein the satellite communication system is configured to generate a corresponding plurality of forward user beams shaped and sized to illuminate the respective nominal forward user beam coverage areas. The satellite communication system reuses a combination of signal frequency and polarization across the plurality of beams to maximize utilization of the limited available spectrum, and performs user scheduling across the plurality of beams. Summary of the Invention

[0006] A satellite communication system (SCS) implements a beam re-centering function that dynamically re-centers one or more spot beams relative to their corresponding nominal beam coverage areas as part of user scheduling. Updating the beam center target of a spot beam can be understood as adjusting beamforming by the SCS to move the beam center of the spot beam so that the maximum signal power of the beam corresponds to different locations within the same nominal beam coverage area over time. In an example embodiment, the SCS uses dynamic beam centering relative to multiple forward user beams, where 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 dynamically changing beamforming solutions.

[0007] One example embodiment includes a method for satellite beam steering 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 relative to the corresponding nominal beam coverage areas as part of user scheduling performed by the SCS. The movement of the beam centers is based on selecting a beam center target to be used for centering the spot beam, relative to each spot beam and relative to each beam centering control interval in a series of beam centering control intervals. Here, the beam center target corresponds to a location within the corresponding nominal beam coverage area.

[0008] Another example embodiment includes an SCS comprising a ground segment having one or more satellite access nodes and a space segment having one or more satellites. 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 SCS is configured to move beam centers of the plurality of spot beams relative to the corresponding nominal beam coverage areas as part of user scheduling performed by the SCS. Movement of the beam centers is based on, relative to each spot beam and relative to each beam centering control interval in a series of beam centering control intervals, the processing circuitry being configured to select a beam center target to be used for centering the spot beam. As noted above, 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. In fact, those skilled in the art will recognize additional features and advantages after reading the following detailed description and viewing the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1is a block diagram of a satellite communication system (SCS) according to an example embodiment, shown in the context of a plurality of spot beams provided by the SCS for serving a corresponding plurality of user terminals.

[0011] Figure 2 is a schematic diagram of nominal beam coverage areas, each nominal beam coverage area includes a corresponding plurality of user terminals, and each user terminal is illuminated by a beam centered at a selected location.

[0012] Figure 3 is a schematic diagram of an example selection of a new beam center target for a corresponding spot beam.

[0013] Figure 4 is a logic flow diagram of a method of dynamic beam centering according to an example embodiment.

[0014] Figure 5 is a logic flow diagram of further example details of a method of dynamic beam centering.

[0015] Figure 6 is a block diagram of an SCS according to another example embodiment, wherein the SCS implements end-to-end beamforming as a type of ground-based beamforming (GBBF).

[0016] Figure 7 is a block diagram of example traffic processing and signal formation for end-to-end beamforming in the forward direction.

[0017] Figure 8 is a block diagram of an SCS according to another embodiment, wherein a satellite performs onboard beamforming and dynamic beam centering.

[0018] Figure 9 is a block diagram of an SCS according to another embodiment, in which the ground segment implements another type of GBBF.

[0019] Figure 10 FIG. 1 is a schematic diagram of sectorizing beam areas according to one embodiment.

[0020] Figure 11 is a logic flow diagram of a method of converting selected coverage locations into corresponding beam centers according to an example embodiment. DETAILED DESCRIPTION

[0021] Figure 1 A plurality of spot beams 10 are shown, where "beam" refers to the radiation pattern of electromagnetic signal energy and the word "spot" refers to a focused beam, thereby illuminating a larger service area using a potentially large number of individual spot beams 10. Each spot beam 10 has a cross-sectional beam area 12 and a beam center 14. Note that Figure 1The circular shape of the beam area 12 is depicted as an example for ease of explanation. The actual shape may not be circular.

[0022] A corresponding satellite communication system (SCS) 20 provides a plurality of spot beams 10. The depicted example 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 plurality of spot beams 10 are provided. In at least one embodiment, the one or more satellites 28 include one or more geostationary (GEO) satellites, such as a GEO satellite constellation.

[0023] With respect to user traffic, such as data packets, carried by the plurality of spot beams 10, the ground segment 22 includes a user scheduling function 30 responsible for scheduling transmissions to and from respective user terminals or groups of user terminals within the population of user terminals served by the plurality of spot beams 10. Here, and elsewhere in this specification, the word "function" refers to a specific activity or group of related activities performed by corresponding physical processing circuitry to implement an overall logical operation. Thus, the user scheduling function 30 includes circuitry configured to decide which user terminals to serve during each of a series of scheduling intervals.

[0024] The SCS 20 also includes a dynamic beam centering function 32 that operates as an integral part of the user scheduling function 30 . Figure 1 The following two options are presented: a first option is to implement the dynamic beam centering functionality 32 in the ground segment 22, and a second option is to implement the dynamic beam centering functionality 32 in the space segment 26. As a further alternative, a hybrid implementation involves implementing a portion of the functionality in the ground segment 22 and a portion of the functionality in the space segment 26. Regardless of the details of implementing the beam centering functionality 32, dynamic beam centering means moving the beam center 14 of one or more of the plurality of spot beams 10 as part of user scheduling.

[0025] For example, the beam center 14 of the spot beam 10 is occasionally moved to change the focus position of the spot beam 10 within the nominal beam coverage area. As a specific example, assuming that the user terminals are distributed within the nominal beam coverage area, the beam center 14 is moved to reflect which user terminals or subsets of user terminals are being served by the spot beam 10 during any given scheduling interval. Moving the beam center 14 can be understood as adjusting the beamforming parameters to improve the signal-to-noise ratio (SNR) of the scheduled user terminals. In the example of geosynchronous orbit, where the spot beam 10 is generally fixed, dynamic beam centering applies a slight movement or offset of the beam focus based on user scheduling to improve the signal conditions for the scheduled users.

[0026] The beam area 12 of a given spot beam 10 can be defined by a contour line reflecting the radiated power level. For example, the perimeter of the spot beam 10 is defined by the -3dB contour line. Here, -3dB represents the attenuation of the radiated power relative to the maximum signal power within the beam area 12. The power can 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 reaches a maximum. 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.

[0027] Figure 2 An example arrangement is depicted that includes two nominal beam coverage areas 40, which are defined geographical areas. 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.

[0028] Each spot beam 10 has a corresponding beam coverage area on the surface of the earth, which depends on the beam area 12 and the beam angle. The beam coverage area can also be called the beam coverage area, and correspondingly, the "nominal beam coverage area 40" can be understood as the default or predefined ground area for which the illumination by the corresponding spot beam 10 is performed.

[0029] A plurality of spot beams 10 may be used to illuminate a larger geographic service area, with each spot beam 10 at least nominally oriented to illuminate a corresponding one of a plurality of nominal beam coverage areas 40 that subdivide the overall geographic service area. For example, where the satellite 28 operates as a GEO satellite, each spot beam 10 may be a nominally stationary or fixed beam—absent perturbations or systematic errors—to provide corresponding consistent illumination of the respective nominal beam coverage area 40.

[0030] For example, each nominal beam coverage area 40 includes one or more user terminals 42. At different times, different locations 44 within each nominal beam coverage area 40 may be selected for aiming the beam center 14 of the corresponding spot beam 10. For example, it is possible that there are "clusters" (geographical groupings) of user terminals 42 within the nominal beam coverage area 40, and the user scheduling function 30 imposes a time multiplexing scheme in which the 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 respective cluster during the scheduling interval during which the spot beam 10 carries the traffic of the respective cluster. As another example, each nominal beam coverage area 40 is subdivided into sectors, and at different times, the center coordinates of different sectors are selected as the selected locations 44 for defining the beam center target for the spot beam 10 in question.

[0031] For any given selected location 44 , the geographic coordinates of the selected location 44 may be converted into a beamforming coordinate system, which may be based on azimuth and elevation angles, to define the beam center target for the spot beam 10 in question. Figure 3 An example scenario is shown in which a beam center target 46 is offset from a current beam center 14, and in which the beamforming solution—the 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 position 44, relative to the corresponding nominal beam coverage area 40, results in a recalculation of the beamforming weights for the involved spot beam 10 to “move” the beam center 14 to the beam center target 46 corresponding to the newly selected position 44.

[0032] As indicated, each nominal beam coverage area 40 includes a plurality of user terminals 42 served by the spot beam 10 corresponding to the nominal beam coverage area 40. The distribution pattern of the user terminals 42 may be different 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 stationary terminals or mobile terminals, or a mixture of both. However, with respect to the mobile beam center 14, it is assumed that the mobile terminals change position relatively slowly compared to the rate at which the beam center changes.

[0033] Figure 4 An embodiment is shown that includes a method 400 for satellite beam steering for an SCS 20 that includes 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 areas 40 as part of user scheduling performed by the SCS 20. Moving the beam centers 14 includes selecting, relative to each spot beam 10 and relative to each beam centering control interval in a series of beam centering control intervals, a beam center target 46 to be used for centering the spot beam 10, the beam center target 46 corresponding to a location 44 in the corresponding nominal beam coverage area 40.

[0034] The "providing" step (block 402) can be understood as operating the SCS 20 in a manner that forms the spot beams 10. As an example, "providing" means that the SCS 20 continuously performs beamforming. Correspondingly, the "moving" step (block 404) can be understood as a continuous or repetitive operation, such as updating the beamforming weights used in the SCS 20 to provide the plurality of spot beams 10.

[0035] 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. In each beam centering control interval, the beam center target 46 of each spot beam 10 corresponds to a selected one of the sectors. At least one such embodiment includes selecting the sectors on a recurring basis and, with respect to each recurring selection period, controlling the length of time that each sector remains selected based on the communication needs of user terminals 42 located in the respective sectors.

[0036] In at least one embodiment, selecting a beam center target 46 for centering each spot beam 10 with respect to each beam centering control interval includes selecting a location 44 within the corresponding nominal beam coverage area 40 based on at least one of: a spatial distribution of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40; or respective communication demands of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40. In at least one embodiment, a pattern of location selection within each corresponding nominal beam coverage area 40 varies within a plurality of beam centering control intervals in the beam centering control interval as a user scheduling algorithm implemented by the SCS 20 varies. For example, the method 400 may include identifying clusters of user terminals 42 within each nominal beam coverage area 40 and, over time, selecting different locations 44 corresponding to different identified clusters as the beam center targets 46 for the spot beam 10 corresponding to the nominal beam coverage area 40.

[0037] In one or more embodiments, the method 400 includes aligning the boundaries of the beam centering control interval with the boundaries of the transmission time slots used by the SCS 20 to transmit user traffic. This ensures that movement of the beam center 14 of any of the plurality of spot beams 10 occurs only on the transmission time slot boundaries. As an example, the SCS 20 organizes transmissions based on a frame structure, where each frame in a series of frames includes a defined number of subframes, each subframe includes one or more time slots, and the one or more time slots serve as a transmission time interval (TTI) representing the smallest allocatable time unit for scheduling transmissions to or from the corresponding user terminal 42.

[0038] In at least one embodiment, moving the beam centers 14 of the plurality of spot beams 10 includes selecting, during any current beam centering control interval, a next beam center target 46 for each spot beam 10 relative to a 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 beamforming performed by the SCS 20 at the beginning 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.

[0039] In one or more embodiments, moving the beam center 14 of any one of the plurality of spot beams 10 includes calculating a new value for a corresponding set of beamforming weights used by the SCS 20 to produce a superposition of radiated signals resulting in the spot beams 10. The SCS 20 performs beamforming, for example, to provide a plurality of spot beams 10, each of which has a corresponding set of beamforming weights used by the SCS 20. As a specific example, the SCS 20 performs end-to-end beamforming in a forward direction, wherein the plurality of spot beams 10 include a plurality of forward user beams implemented using end-to-end beamforming. For details on end-to-end beamforming, see, for example, U.S. Patent No. 10,720,988B2 issued on July 21, 2020.

[0040] Other methods of 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. Still other methods include transmitting forward beam signals with corresponding weighting information from the ground segment 22 for application of weights via beamforming circuitry onboard the satellite 28. At least one embodiment relies on a fully onboard implementation in which the satellite 28 calculates the beamforming weights and applies them to the forward beam signals for transmission from the onboard phased array antenna.

[0041] In any case, for each forward user beam, the corresponding beam center 14 is moved to a new beam center target 46 for each forward user beam, and the method 400 includes calculating new values ​​for the beamforming weights involved. Here, 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, for any given beam center target 46 corresponding to a selected location 44 corresponding to the spot beam 10 involved within the nominal beam coverage area 40, achieving the beam center target 46 means calculating beamforming weights that optimize the signal-to-noise ratio (SNR) for user terminals at or near the selected location 44.

[0042] In one or more embodiments, for any beam centering control interval, for any beam centering control interval, one or more beam centers 14 are moved, and the method 400 includes calculating new values ​​for all beamforming weights corresponding to all spot beams 10 to account for the change in inter-beam interference caused by the moved beam center 14. That is, there is a beamforming weight corresponding to each spot beam 10, and all such weights collectively represent the overall beamforming solution implemented by the SCS 20, and the SCS 20 performs a joint optimization of the beamforming solutions for any given set of beam center targets 46 determined for a given beam centering control interval. The joint optimization limits inter-beam interference and correspondingly maximizes the SNR of the respective beam signals at the respective beam center targets 46.

[0043] 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 SCS 20. The boundaries of the beam centering control interval are aligned with the boundaries of the user scheduling interval so that movement of beam center 14 is constrained in a temporal sense to transition from one user scheduling interval to the next.

[0044] Although the recalculation of the overall beamforming solution can be a joint function of the overall set of beam center targets 46 to be used for any given beam centering control interval, each beam center target 46 can be selected independently. However, in at least one embodiment, the method 400 includes applying spatial constraints to the selection of the beam center targets 46 to constrain spot beam overlap between adjacent spot beams 10 at the same frequency.

[0045] Figure 5 A method 500 of operations performed by the beam centering function 32 is shown and may be understood as Figure 4 The method 500 may be executed in a loop or otherwise on an ongoing basis, and generally speaking, Figure 4 and Figure 5 The operations detailed in may be performed on an ongoing basis along with other operations performed by SCS20. Figure 4 and Figure 5 It may be included in or performed in conjunction with user scheduling, in which the SCS schedules transmissions to and / or from a corresponding user terminal 42 or group of user terminals 42. Such scheduling may be performed on a per-beam basis, but scheduling may also take into account overall system capacity and bandwidth limitations, which may involve sharing certain system resources across spot beams 10.

[0046] The operations in the method 500 include selecting (block 502) a beam center target 46 for each spot beam 10 based on a centering metric for each beam centering control interval. The example centering metric may be determined by the beam centering function 32 or the user scheduling function 30 and may include information indicating a corresponding location 44 in the nominal beam coverage area 40 to be used as the beam center target 46 in dynamic beam recentering. Other example metrics include any one or more of the following: information about the spatial distribution of user terminals 42 in each nominal beam coverage area 40, information about a communication service or service type associated with a corresponding user terminal 42 in each nominal beam coverage area 40, and information about communication needs or communication statistics of the corresponding user terminal 42 in each nominal beam coverage area 40. Such information includes, for example, quality of service (QoS) requirements, such as minimum throughput, etc. Generally speaking, the beam center target 46 can be moved within consecutive beam centering control intervals according to a user scheduling algorithm, such as a proportional fair scheduling algorithm, which decides which user terminals 42 to schedule based on one or more weighted parameters of the control proportion, subject to some lower limit or baseline constraint to prevent "unfairness" or to meet some minimum requirements.

[0047] In at least one embodiment, the centering metric includes information indicating selected locations 44 of a plurality of nominal beam coverage areas 40 for the next beam centering control interval, and the beam centering function 32 uses this information to calculate a new beam center target 46. It is possible that the "new" beam center target 46 selected for any given spot beam 10 for any given upcoming beam centering control interval is the same as the beam center target used for the current beam centering control interval. In such a case, for such spot beam 10, the beam center target 46 will not change when the next beam centering control interval begins. In other words, the beam center target 46 does not necessarily change for every spot beam 10 in every single beam centering control interval.

[0048] With the new beam center target 46 selected for one or more of the spot beams 10, the method 500 continues by calculating (block 504) new values ​​for respective beamforming weights corresponding to the spot beams 10. Calculating new beamforming weights for any one or more of the spot beams 10 may be referred to as "updating" or "adjusting" a beamforming solution, where, as noted, the term "beamforming solution" refers to the overall set of beamforming weights or beamforming weights used to achieve the plurality of spot beams 10.

[0049] In at least one embodiment, there are predefined sets of beam center targets 46 for multiple spot beams 10, and these sets are indexed or mapped to corresponding precomputed beamforming solutions, so that the beamforming solutions do not need to be calculated on the fly, but are retrieved from a lookup table or other stored data structure. Because such solutions are less flexible than those calculated on the fly, they can be more advantageous in embodiments where the nominal beam coverage area 40 is sectorized according to a known partitioning scheme and where the selected locations 44 are constrained to defined sectors.

[0050] Regardless of whether the new value is pre-calculated or calculated on the fly, the method 500 continues by applying (block 506) the new value at the next beam centering control interval. As noted, in one or more embodiments, movement of any beam center 14 of any spot beam 10 involves recalculating the beamforming weights for all spot beams 10. Such recalculation of the beamforming solution reflects a joint optimization of the beamforming weights for all spot beams 10 for reducing inter-beam interference and correspondingly maximizing the SNR at the respective beam center targets 46 for all spot beams 10.

[0051] Figure 6 An SCS 20 using end-to-end beamforming is shown according to an example embodiment, wherein the ground segment includes one or more SANs 24 supported by communication processing circuitry 60, which may be implemented in one or more nodes (e.g., one or more computer servers). The communication processing circuitry 60 interfaces with one or more external networks 62, such as the Internet or other packet data networks (PDNs), public switched telephone networks (PSTNs), etc. User traffic destined for respective user terminals 42 served by the SCS 20 flows from the external networks 62 into the communication processing circuitry 60, and user traffic originating from respective user terminals 42 served by the SCS 20 flows from the communication processing circuitry 60 out to the external networks 62.

[0052] Communication processing circuitry 60 includes or interfaces with user scheduling circuitry 64 and beam centering control circuitry 66. Further, communication processing circuitry 60 includes or interfaces with beamforming circuitry 68. Beamforming circuitry 68 calculates and applies beamforming weights 70 for the forward link direction toward user terminal 42, for the return link direction from user terminal 42, or for both directions. Beamforming circuitry 68 calculates beamforming weights 70 based on channel estimates. For example, beamforming weights 70 include a set of forward beamforming weights and a set of return beamforming weights.

[0053] Each SAN 24 includes interface circuitry 72 for communicating with communication processing circuitry 60 in both the forward and return directions. Interface circuitry 72 includes circuitry configured for physical layer signal reception and transmission via a wired or wireless medium and may include higher layer circuitry for protocol processing, synchronization, etc. Further, each SAN 24 includes transmitter / receiver circuitry 74. In at least one embodiment, transmitter / receiver circuitry 74 includes a radio frequency (RF) transmitter and receiver for providing RF-based feeder uplinks and downlinks between each SAN 24 and satellite 28.

[0054] The example satellite 28 includes a plurality of transponders 80, each of which provides a corresponding signal path through the satellite 28. There may be a transponder 80 dedicated to the forward link direction, which provides a forward link signal path for relaying forward user traffic from the ground segment 22 toward the user terminal 42; and a plurality of transponders 80 dedicated to the return link direction, which provides 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 forward link signal paths and return link signal paths on a time-multiplexed, switched basis. In other arrangements, the plurality of transponders 80 include at least some transponders with switchable connections, allowing them to be used in either the forward or return direction.

[0055] Using the forward link direction shown in the example 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 onboard the satellite 28 dedicated to reception in the forward direction and / or the return direction, as well as further antenna subsystems dedicated to transmission in the forward direction and / or the return direction. In the return direction in at least the end-to-end beamforming context, the "input" end of the transponder 80 receives a superposition of return uplink signals from user terminals 42 operating in one or more return user beam coverage areas, which may or may not coincide with the nominal forward user beam coverage area. Correspondingly, the "output" end of the transponder transmits the received superposition of return uplink signals as corresponding return downlink signals received at two or more of the SANs 24.

[0056] For end-to-end beamforming in the forward direction, communication processing circuitry 60 forms forward user streams under the control of user scheduling circuitry 64. Based on the operation of user scheduling circuitry 64, each forward user stream multiplexes forward user traffic for a corresponding user terminal 42 within a specific one of nominal beam coverage areas 40. Thus, each forward user stream may be understood as conveying forward user traffic for transmission via a corresponding spot beam from among the plurality of spot beams 10.

[0057] The beamforming circuitry 68 uses the end-to-end channel estimate in the forward direction to calculate the beamforming weights for the forward beamforming as an M x K beamforming weight matrix. Here, M equals the number of SANs 24 participating in the 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, meaning there are K forward beam signals, each of which carries forward user traffic for transmission in a corresponding one of the K forward user beams.

[0058] The beamforming circuitry 68 applies the values ​​of the M x K beam weight matrix to each of the K forward beam signals to generate M access node-specific forward signals. Each of the access node-specific forward signals corresponds to a specific one of the M SANs 24, and each SAN includes 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. Accordingly, each forward weighting and summing module receives all K forward beam signals. Here, "module" refers to a configured circuitry.

[0059] Furthermore, the circuitry within the beamforming circuitry 68 operates as a forward beam weight generator module that generates an M x K forward beam weight matrix. In one or more embodiments, the forward beam weight matrix is ​​generated based on a channel matrix in which the elements are estimates of the end-to-end forward gain for each of the K x M end-to-end forward multipath channels used to form the forward channel matrix. The estimates of the end-to-end forward gain are obtained in the channel estimator module.

[0060] Thus, in the forward link direction, each SAN 24 receives one of the M access node-specific forward signals 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 the forward uplink signals 92, where each such superposition involves forward uplink signals 92 from two or more of the M SANs 24. The superposition is unique because the SANs 24 are geographically distributed, resulting in a different uplink channel between each SAN 24 and each receive antenna element 82 on the satellite 28.

[0061] 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 the input of each of the transponders 80 receives a unique forward composite uplink signal 94. Each transponder 80 operates as an unprocessed bent-pipe transponder that couples the corresponding received forward composite uplink signal 94 to the user downlink side of the satellite 28 for transmission from a corresponding one of the transmit antenna elements 84 as a forward user downlink signal 96. The forward user downlink signal 96 is the corresponding forward composite uplink signal 94 that has been filtered, amplified, and, in one or more embodiments, frequency converted from an uplink signal frequency to a downlink signal frequency.

[0062] The plurality of transmit antenna elements 84 are configured such that respective forward user downlink signals 96 transmitted from different transmit antenna elements 84 superimpose in the far field—that is, at a distance from the transmit antenna elements 84 where the radiation behavior of the electromagnetic signals dominates. These superpositions form a plurality of forward user beams 100, each of which is a beamformed transmission of a forward beam signal 102 and each having a corresponding forward user beam coverage area 104. The aggregate of the forward user beams 100 illuminates an overall forward user service area 106. It should be understood that the forward user beams 100 are examples of the spot beams 10 discussed herein.

[0063] Each forward user beam 100 illuminates a corresponding forward user beam coverage area 104, and there may be predefined or default geographic coordinates or boundaries that nominally define the specific area on the surface of the Earth that is illuminated - that is, there may be a defined nominal beam coverage area 40 for each forward user beam 100. Accordingly, in the context of this example, the dynamic beam re-centering described herein may be understood as dynamically re-centering one, some, or all of the forward user beams 100 relative to each beam centering control interval. Each forward user beam 100 may be understood as a beamformed transmission of a forward beam signal 102 that multiplexes user traffic for the user terminals 42 served by the forward user beam 100 according to ongoing user scheduling.

[0064] The communication processing circuitry 60 includes or is communicatively associated with user scheduling circuitry 64, beam centering control circuitry 66, and beamforming circuitry 68. All of such circuitry may include fixed circuitry or programmatically configured circuitry, or a mixture of both. For example, at least some of the beamforming circuitry 68 may include digital signal processing (DSP) hardware configured to perform beamforming calculations. Further, Figure 6 At least some of the depicted circuitry includes one or more microprocessors or DSPs or other programmatically configured digital processing circuitry specifically adapted to perform the described functions based on the execution of computer program instructions stored in a computer-readable medium. For example, the communication processing circuitry 60 includes or is associated with one or more types of storage devices (such as RAM for operating program execution and FLASH for non-volatile storage of program instructions). Such storage devices are also used to store beamforming weights 70 and channel estimates.

[0065] Channel estimates can be determined from channel state information (CSI). One approach to providing CSI feedback relies on one or more user terminals 42 operating as "reference terminals" (RTs) or "designated terminals" (DTs) in each nominal beam coverage area 40. For example, one or more user terminals 42 located at or near the geographic center of the nominal beam coverage area 40 can serve as RTs. Such an approach involves transmitting a unique reference signal per SAN 24 and receiving downlink channel estimates for each nominal user coverage area 40 from the corresponding RT for use in channel estimation at the RT.

[0066] Figure 7An example of end-to-end beamforming in the forward direction is presented based on example functionality. Incoming forward user traffic 90 is scheduled, for example, based on determining the identity of the user terminal 42 for which a given portion of the traffic is intended, determining the nominal beam coverage area 40 associated with the intended 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., by applying an M x K forward weight matrix as described above, to create a set of M access node-specific forward signals 99, each of which is generated for a specific one of the SANs 24 participating in the end-to-end beamforming. Note that the beamforming weights 70 are adapted with respect to each beam centering control interval to reflect the beam centering target 46 applicable to each beam centering control interval.

[0067] Figure 8 An alternative embodiment of beamforming is shown. Here, the beamforming is based on a satellite 28 carrying a phased array antenna 140 comprising a plurality of antenna elements 142 arranged in a feed plane. Although not shown, there may be a reflector onboard the satellite 28 associated with the phased array antenna 142. In this arrangement, there are a plurality of antenna element signals 144, one for each antenna element. The antenna element signals 144 are weighted so that their transmissions produce a superposition of signals that form a forward user beam 100, as an example of the spot beam 10 discussed herein.

[0068] In one embodiment, satellite 28 includes an antenna subsystem 150 through which the satellite receives a forward uplink signal 152 including one or more forward beam signals. Antenna subsystem 150 couples the received forward uplink signal 152 to forward transmit circuitry 154 including beamforming circuitry 156.

[0069] The beamforming circuitry 156 forms the antenna element signals 124 based on dividing each forward beam signal into N unweighted element signals, where N is equal to the number of antenna elements 122, and then applying a corresponding set of beamforming weights. Each such set provides for the formation of a respective one of the forward user beams 100 and includes a respective beam weight—phase and / or amplitude—for each antenna element 142.

[0070] There may be multiple phased array antennas 140 associated with different downlink signal frequencies and / or polarizations, for example, or the phased array antenna 140 may include multiple sets of input antenna feeds corresponding to different downlink signal frequencies and / or polarizations. By way of further illustration, 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. In one or more embodiments, the beamforming circuitry 156 is configured to calculate beamforming weights 70 and may incorporate the beam centering functionality 32 so that 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 for the beam centering control interval, for transmission to the satellite 28 and corresponding application by the beamforming circuitry 156 onboard the satellite.

[0071] Figure 9 Another embodiment is shown in which the beamforming weights 70 are calculated and applied on the ground. By this method, the ground segment 22: forms forward user beams, each forward user beam corresponding to one of the forward user beams 100; and splits each forward user beam into N beam element signals, each beam element signal corresponding to one antenna element 142 in the phased array antenna 140 onboard the satellite 28 and weighted such that the beam element signals from the phased array antenna 140 are transmitted simultaneously to form the corresponding forward user beam 100.

[0072] The SAN 24 forms a forward uplink signal 160 that transmits the forward beam element signal toward the satellite 28, which includes an antenna subsystem 162. The forward transmit circuitry 164 provides filtering, amplification, and, in one or more embodiments, frequency conversion. The forward transmit circuitry 164 couples the forward beam element signal to the corresponding antenna element 142 for transmission.

[0073] Figure 10 An example of logically dividing the nominal beam coverage area 40 into a plurality of sectors 170 is shown. Each sector 170 may be represented by defined geographic coordinates representing a sector center or other reference point associated with the sector. For any given nominal beam coverage area 40, different ones of the reference points may be individually selected as the location 44 for calculating the beam center target 46 for the associated spot beam 10 within successive beam centering control intervals.

[0074] Figure 11A method 1100 is shown for converting any given selected location 44 into a beam center target 46. The 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 selected for a given beam centering control interval, where the geographic coordinates of the selected location 44 are known or determined on the fly. The method 1100 then proceeds to convert (block 1104) the geographic coordinates into beam coordinates, e.g., angular values ​​for recalculating corresponding beam weights to achieve recentering of the spot beam 10 involved.

[0075] With the above example in mind, the SCS 20 in the example embodiment includes a ground segment 22 comprising one or more SANs 24 and a space segment 26 comprising one or more satellites 28. The one or more SANs 24 and the one or more satellites 28 are configured to cooperate to provide a plurality of spot beams 10. Each spot beam 10 serves a corresponding plurality of user terminals 42 and has a corresponding nominal beam coverage area 40.

[0076] The processing circuit system included in the SCS20 is configured to select a beam center target 46 to be used for centering the spot beam 10 relative to the corresponding nominal beam coverage area 40 based on moving the beam center 14 of multiple spot beams 10 relative to each spot beam 10 and relative to each beam centering control interval in a series of beam centering control intervals as part of user scheduling performed by the SCS20, the beam center target 46 corresponding to the position 44 in the corresponding nominal beam coverage area 40.

[0077] In one or more embodiments, such processing circuitry resides in the surface segment 22, see, for example, Figure 6 In one or more other embodiments, such processing circuitry resides in the spatial segment 26, e.g., see Figure 8 The beamforming circuitry 136 in the ground segment 22 implements the dynamic beam centering functionality 32 discussed herein in one or more embodiments. In still other embodiments, the dynamic beam centering functionality 32 is implemented collaboratively 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.

[0078] In one or more embodiments, the one or more satellites 28 include geostationary satellites that provide the plurality of spot beams 10 as nominally stationary spot beams corresponding to the nominal beam coverage area 40. The corresponding nominal beam coverage area 40 of each spot beam 10 is logically divided into a plurality of sectors 170, and the beam center target 46 of each spot beam 10 in each beam centering control interval corresponds to a selected one of the sectors 170. Accordingly, in at least one embodiment, the processing circuitry in the SCS 20 that performs dynamic beam centering is configured to select a sector 170 on a recurring basis and, with respect to each recurring selection period, control the length of time that each of the sectors 170 remains selected based on the communication needs of the user terminals 42 located in the respective sectors 170.

[0079] Relative to each beam centering control interval and selection of a beam center target 46 to be used for centering each spot beam, in one or more embodiments, the processing circuit system involved in the SCS20 is configured to select a location in the corresponding nominal beam coverage area 40 based 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 corresponding communication requirements of the corresponding plurality of user terminals 42 within the corresponding nominal beam coverage area 40.

[0080] For example, the processing circuit system involved is configured to follow a location selection pattern within each corresponding nominal beam coverage area 40 over multiple beam centering control intervals in the beam centering control interval, where the pattern changes as a user scheduling algorithm implemented by SCS20 changes.

[0081] In one or more embodiments, the processing circuit system involved is configured to identify clusters of user terminals 42 within each nominal beam coverage area 40 and, over time, select different locations 44 corresponding to different identified clusters as beam center targets 46 for the spot beam 10 corresponding to the nominal beam coverage area 40.

[0082] In at least one embodiment, the processing circuitry is configured to align the boundaries of the beam centering control interval with the boundaries of the transmission slots used by the SCS to transmit user traffic. Doing so constrains movement of the beam center 14 of any of the plurality of spot beams 10 to occur only on the boundaries of the transmission slots.

[0083] In one or more embodiments, the processing circuit system involved is configured to move the beam center 14 of multiple spot beams 10 by: during any current beam centering control interval, selecting a next beam center target 46 for each spot beam 10 relative to a next beam centering control interval, and for any spot beam 10 in 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 beginning 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.

[0084] To move the beam center 14 of any one of the plurality of spot beams 10, in one or more embodiments, the processing circuitry involved is configured to calculate new values ​​for a corresponding set of beamforming weights used by the SCS 20 to produce a signal superposition that results in the spot beams 10. Thus, in one or more embodiments, the SCS 20 is configured to provide the plurality of spot beams 10 using beamforming, wherein the SCS 20 calculates and applies the beamforming weights to its user traffic transmissions to achieve the corresponding spot beams 10. As noted, in at least such embodiments, the SCS 20 is configured to perform end-to-end beamforming in a forward direction, wherein the plurality of spot beams 10 includes a plurality of forward user beams 100.

[0085] In at least one embodiment, the one or more satellites 28 include a satellite 28 having a phased array antenna 140 for providing a plurality of forward user beams 100. Corresponding beamforming weights are calculated onboard the satellite 28 or calculated in the ground segment 22. For example, processing circuitry for implementing the dynamic beam centering functionality 32 may be implemented in the ground segment 22 and configured to transmit the dynamically calculated beamforming weights to the satellite 28 or cause the dynamically calculated beamforming weights to be transmitted to the satellite as a forward uplink transmission from one of the one or more SANs 24.

[0086] In at least one embodiment, for each forward user beam 100, the processing circuitry is configured to calculate new values ​​for a corresponding set of beamforming weights for each forward user beam that moves the 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. In one or more embodiments, the processing circuitry is configured to account for changes in inter-beam interference caused by the moved beam center 14 when calculating the new values ​​for the beamforming weights.

[0087] Broadly speaking, the present disclosure details techniques for dynamically re-centering spot beams 10 such that, over time, different locations within the corresponding nominal beam coverage area 40 experience maximum beam signal power. For example, the SCS 20 uses beam centering to improve the signal-to-noise ratio (SNR) of scheduled user terminals 42. In the context of fixed coverage areas where the beam center of a conventional beam remains fixed—absent unintended perturbations—dynamically re-centering the beam as part of user scheduling yields significant improvements in beam / system throughput.

[0088] In one example, the SCS 20 implements end-to-end beamforming in the forward direction using geostationary satellites 28 as end-to-end relays between a plurality of geographically distributed SANs 24 and a population of user terminals 42 distributed over an aggregate service area 106 illuminated by a plurality of forward user beams 100. Processing circuitry 66 in the ground segment 22 of the SCS 20 dynamically selects a location 44 within the forward user beam coverage area 104 corresponding to the forward user beam 100 for use in determining a corresponding beam center target 46 for an upcoming beam centering control interval, and updates the beamforming solution used by the SCS 20 to implement the forward user beam 100.

[0089] In another example of ground-based beamforming, the ground segment 22 of the SCS 20 forms beam signals corresponding to forward user beams 100 and splits each such forward beam signal into a plurality of forward beam element signals. The forward beam element signals are weighted for transmission from corresponding antenna elements 142 of a phased array antenna 140 onboard the satellite 28. Accordingly, the ground segment 22 transmits one or more forward uplink signals 132 that convey the forward beam element signals of the various forward beam signals to the satellite 28 for recovery and corresponding transmission from the phased array antenna 140. This transmission generates the desired plurality of forward user beams 100.

[0090] In yet another alternative, the satellite receives the forward beam signal and generates corresponding forward beam element signals for transmission from one or more onboard phased array antennas 140. As a further variation of this embodiment, the beamforming solution used to generate the forward beam element signals can be calculated in the ground segment 22 and transmitted to the satellite 28, or the satellite 28 can calculate the beamforming solution. That is, in at least one embodiment, the processing circuitry configured to implement the beam centering function 32 is onboard the satellite 28, and the satellite 28 updates the beamforming solution to reflect the updated beam center target 46.

[0091] It is noted that modifications and other embodiments of the disclosed invention will occur to those skilled in the art having the benefit of the teachings presented in the foregoing description and the associated drawings. Therefore, it should be understood that the invention is not limited to the specific embodiments disclosed, and modifications and other embodiments are intended to be included within the scope of this 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. A method for satellite beam steering in a satellite communication system, wherein the satellite communication system comprises one or more satellite access nodes and one or more satellites, the 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; as well as Selecting a beam center target to be used for centering the spot beams based on moving the beam centers of the plurality of spot beams relative to each spot beam and relative to each beam centering control interval in a series of beam centering control intervals relative to the corresponding nominal beam coverage area as part of user scheduling performed by the satellite communication system, the beam center target corresponding to a location within the corresponding nominal beam coverage area.

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

3. A method according to claim 1 or 2, wherein the corresponding nominal beam coverage area of ​​each spot beam is logically divided into multiple sectors, and wherein the beam center target of each spot beam in each beam centering control interval corresponds to a selected one of the sectors.

4. The method according to claim 3, further comprising: The sectors are selected on a round-robin basis, and with respect to each round-robin selection period, the length of time that the respective sectors remain selected is controlled based on the communication needs of user terminals located in the respective ones of the sectors.

5. The method of any one of claims 1 to 3, wherein selecting the beam center target to be used for centering each spot beam relative to each beam centering control interval comprises: The position within the corresponding nominal beam coverage area is selected based on at least one of: the spatial distribution of the corresponding multiple user terminals within the corresponding nominal beam coverage area; or the corresponding communication requirements of the corresponding multiple user terminals within the corresponding nominal beam coverage area.

6. A method according to claim 5, wherein the position selection pattern within each corresponding nominal beam coverage area on multiple beam centering control intervals in the beam centering control interval changes with the change of the user scheduling algorithm implemented by the satellite communication system.

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

8. The method according to any one of claims 1 to 7, further comprising: Boundaries of the beam centering control interval are aligned with boundaries of transmission time slots used by the satellite communication system to transmit user traffic, so that movement of the beam center of any of the plurality of spot beams occurs only on transmission time slot boundaries.

9. The method according to any one of claims 1 to 8, wherein moving the beam centers of the plurality of spot beams comprises: During any current beam centering control interval, a next beam center target is selected for each spot beam relative to a next beam centering control interval, and for any spot beam where 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, beamforming performed by the satellite communication system at the beginning of the next beam centering control interval is adjusted to move the beam center of the spot beam to the next beam center target.

10. The method according to any one of claims 1 to 9, wherein moving the beam center of any one of the plurality of spot beams comprises: New values ​​for a corresponding set of beamforming weights used by the satellite communication system are calculated to produce a superposition of signals resulting in the spot beams.

11. The method of any one of claims 1 to 10, 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. The method of claim 11, wherein the satellite communication system performs end-to-end beamforming in a forward direction, wherein the plurality of spot beams comprises a plurality of forward user beams.

13. The method of claim 11 , wherein the one or more satellites comprises a satellite having a phased array antenna for providing the plurality of spot beams as a plurality of forward user beams, and wherein the corresponding beamforming weights are computed onboard the satellite or in a terrestrial segment of the satellite communication system and transmitted to the satellite as a forward uplink transmission from one of the one or more satellite access nodes.

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

15. A method according to claim 14, wherein for any beam centering control interval, one or more beam centers are moved for said any beam centering control interval, and the method includes calculating new values ​​of all beamforming weights to take into account the change in inter-beam interference caused by the moved beam centers.

16. A method according to any one of claims 1 to 15, wherein the duration of the beam centering control interval is an integer multiple of the duration of the user scheduling interval used by the satellite communication system, and wherein the boundaries of the beam centering control interval are aligned with the boundaries of the user scheduling interval.

17. The method of any one of claims 1 to 16, wherein the beam center target of each of the plurality of spot beams is independently selected.

18. The method according to any one of claims 1 to 17, further comprising: Spatial constraints are applied to the selection of beam center targets to constrain spot beam overlap between adjacent spot beams at the same frequency.

19. A satellite communication system, comprising: a ground segment, the ground segment including one or more satellite access nodes; as well as a space segment comprising one or more satellites; wherein 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; and The processing circuit system included in the satellite communication system is configured to select a beam center target to be used for centering the spot beam relative to the corresponding nominal beam coverage area based on moving the beam center of the multiple spot beams relative to each spot beam and relative to each beam centering control interval in a series of beam centering control intervals as part of user scheduling performed by the satellite communication system, the beam center target corresponding to a position in the corresponding nominal beam coverage area.

20. The satellite communication system of claim 19, wherein the one or more satellites comprise geostationary satellites configured to provide the plurality of spot beams as nominally stationary spot beams corresponding to the nominal beam coverage areas.

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

22. A satellite communication system according to claim 21, wherein the processing circuit system is configured to select the sectors on a round-robin basis and, relative to each round-robin selection period, control the length of time that the respective sectors remain selected based on the communication needs of user terminals located in the respective ones of the sectors.

23. A satellite communication system according to any one of claims 19 to 21, wherein, relative to each beam centering control interval and selecting the beam center target to be used to center each spot beam, the processing circuit system is configured to select the position within the corresponding nominal beam coverage area based on at least one of: the spatial distribution of the corresponding multiple user terminals within the corresponding nominal beam coverage area; or the corresponding communication requirements of the corresponding multiple user terminals within the corresponding nominal beam coverage area.

24. A satellite communication system according to claim 23, wherein the processing circuit system is configured to follow a position selection pattern within each corresponding nominal beam coverage area on a plurality of beam centering control intervals in the beam centering control interval, and the position selection pattern changes with changes in a user scheduling algorithm implemented by the communication system.

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

26. A satellite communication system according to any one of claims 19 to 25, wherein the processing circuit system is configured to align the boundaries of the beam centering control interval with the boundaries of the transmission time slot used by the satellite communication system to transmit user services, so that the movement of the beam center of any spot beam among the multiple spot beams occurs only on the transmission time slot boundary.

27. A satellite communication system according to any one of claims 19 to 26, wherein the processing circuit system is configured to move the beam centers of the multiple spot beams by: during any current beam centering control interval, selecting a next beam center target for each spot beam relative to a next beam centering control interval, and for any spot beam in 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 the beamforming performed by the satellite communication system at the beginning of the next beam centering control interval to move the beam center of the spot beam to the next beam center target.

28. The satellite communication system of any one of claims 19 to 27, wherein to move the beam center 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 communication system to produce a signal superposition that results in the spot beam.

29. The satellite communication system of any one of claims 19 to 28, wherein the satellite communication 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 communication system.

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

31. The satellite communications system of claim 29, wherein the one or more satellites comprises a satellite having a phased array antenna for providing the plurality of forward user beams, and wherein the corresponding beamforming weights are calculated onboard the satellite or via the processing circuitry included in the ground segment, in which case the processing circuitry is configured to cause the corresponding beamforming weights to be transmitted to the satellite as a forward uplink transmission from one of the one or more satellite access nodes.

32. A satellite communication system according to claim 30 or 31, wherein the processing circuit system is configured to: for each forward user beam, move the corresponding beam center to a new beam center target for each forward user beam, calculate new values ​​of the corresponding beamforming weight set, wherein the new values ​​are calculated to optimize the signal-to-noise ratio (SNR) at the new beam center target and minimize other beam interference at the new beam center target.

33. A satellite communication system according to claim 32, wherein for any beam centering control interval, one or more beam centers are moved for any beam centering control interval, and the processing circuit system is configured to calculate new values ​​of all beamforming weights to take into account the change in inter-beam interference caused by the moved beam centers.

34. A satellite communication system according to any one of claims 19 to 33, wherein the duration of the beam centering control interval is an integer multiple of the duration of the user scheduling interval used by the satellite communication system, and wherein the boundaries of the beam centering control interval are aligned with the boundaries of the user scheduling interval.

35. The satellite communications system of any one of claims 19 to 34, wherein the processing circuitry is configured to independently select the beam center target for each of the plurality of spot beams.

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 beam centre targets to constrain spot beam overlap between adjacent spot beams at the same frequency.

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