Method and apparatus for diversity transmission in a satellite-based communication system

Abstract

A satellite communications system uses multiple optical feeder links between its ground segment and its space segment and employs spatial diversity transmission, which splits each user data stream into substreams transmitted over two or more of the optical feeder links to allow recovery of the full stream at the receiving end despite impairments affecting the individual feeder links along which the stream is split. Furthermore, the system applies beamforming to the feeder links individually, meaning that transmission time alignment between each one of the substreams is not required. Spatial diversity transmission may occur in the forward or return direction, or both, and may be conditionally employed and dynamically adjusted for individual user terminals, groups of user terminals, or the entire population of user terminals.

Description

[Technical Field]

[0001] Satellite communication systems provide spatially diverse transmission of user data streams in one or both of the forward and return link directions using multiple free-space optical links supported by respective ground stations. [Background technology]

[0002] Many challenges arise in the design, deployment, and operation of satellite communications networks, and capacity limitations and spectral efficiency represent recurring problems with no easy solutions. The increasing data rates required for the delivery of richer media and the desire to reduce latency exacerbate these problems.

[0003] One approach to addressing bandwidth limitations involves the use of free-space optical "feeder links" between satellites and terrestrial gateway stations that send forward traffic to the satellites and receive return traffic from them. Certain satellite communication systems also use optical links for inter-satellite communications, which can improve overall capacity or provide additional coverage and traffic routing flexibility.

[0004] While optical feeder links offer significant bandwidth gains and concomitant improvements in feeder link capacity compared to radio frequency (RF) feeder links, they suffer from the disadvantage of being susceptible to degradation due to cloud coverage and atmospheric effects such as beam wandering and scintillation. Scintillation arises from refractive index variations due to small temperature changes in the propagation medium, resulting in fluctuations in received optical power. These degradations make optical feeder links more susceptible to severe signal degradation or complete signal loss than RF feeder links.

[0005] While the use of diverse optical links alleviates the problems associated with using a single optical link, diversity transmission over multiple optical links presents unique challenges in terms of how the multiple links are used to transmit the information of interest. Additional challenges arise in the context of underlying technologies such as terrestrial beamforming, in which the ground segment of a satellite communications system performs or controls the signal weighting used to form forward or return beams used to serve user terminals at different locations. Summary of the Invention

[0006] A satellite communications system uses multiple optical feeder links between its ground segment and its space segment and employs spatial diversity transmission, which splits each user data stream into substreams transmitted over two or more of the optical feeder links to allow recovery of the full stream at the receiving end despite impairments affecting the individual feeder links along which the stream is split. Furthermore, the system applies beamforming independently on the feeder links, meaning that transmission time alignment is not required between each one of the substreams. Spatial diversity transmission may occur in the forward or return direction, or both, and may be conditionally employed and dynamically adjusted for individual user terminals, groups of user terminals, or the entire population of user terminals.

[0007] An exemplary embodiment includes a method of operating a satellite communications system having a ground segment and a space segment. The method includes providing a plurality of forward user beams for transmitting forward user traffic to respective user terminals. Each forward user beam has a corresponding forward user beam coverage area, and at least one subset of the forward user beams is arranged as a forward spatial diversity beam subset including two or more forward user beams having a unique combination of signal frequency and polarization and having respective forward user beam coverage areas that overlap by more than a threshold amount. The method further includes, for each forward spatial diversity beam subset, transmitting the forward user traffic mapped to each forward user beam included in the forward spatial diversity beam subset from the ground segment to the space segment via different optical forward uplink signals originating from different ones of two or more geographically dispersed ground stations.

[0008] Still further, for at least one user terminal located within the overlapping forward beam coverage area of ​​a given forward spatial diversity beam subset, the method includes (a) dividing an inbound user data stream targeted at the user terminal into two or more forward user data substreams by block-coding the inbound user data stream and partitioning the resulting coded blocks so that each forward user data substream carries a different subset of coded data from the coded blocks, and (b) using diversity forward transmission by mapping each forward user data substream to a different one of the different forward user beams included in the given forward spatial diversity subset. With this approach, each forward user data substream undergoes ground-based beamforming separately from the other substreams, thereby eliminating the need for transmission time alignment between respective substreams via associated ground stations for coherent beamforming.

[0009] A method according to a further embodiment includes receiving, at a processing node of the ground segment, inbound user data streams, each inbound user data stream targeted to a respective user terminal served by the satellite communications system, the method further including using forward spatial diversity transmission for one or more of the inbound user data streams.

[0010] Using forward spatial diversity transmission includes, for each inbound user data stream, forming a corresponding set of two or more forward user data substreams by block coding the inbound user data stream and dividing each coded data block into different subsets of coded data, mapping each forward user data substream to a respective one of two or more forward beam signals respectively corresponding to two or more forward user beams of the satellite communications system, the respective forward user beam coverage areas surrounding the location of the user terminal targeted by the inbound user data stream, and transmitting each forward beam signal from the ground segment to the space segment via different optical forward uplink signals originating from different ground stations of a plurality of geographically dispersed ground stations included in the ground segment, each such forward uplink signal multiplexing multiple forward optical channel signals carrying a respective copy of the forward beam signal weighted for beamforming transmission from respective antenna elements of a targeted antenna array in the space segment for far-field formation of the corresponding forward user beam.

[0011] In the context of the aforementioned method, the satellite communications system provides a plurality of forward user beams via one or more satellites included in the space segment, each forward user beam having a respective combination of downlink signal frequency and polarization based on a corresponding forward user beam signal. Accordingly, the method may further include receiving, at each of one or more ground stations among the plurality of ground stations, two or more forward beam signals corresponding to two or more forward user beams having the same respective combination of downlink signal frequency and polarization. For each such ground station, the method includes: (a) generating a set of radio frequency signals, each radio frequency signal corresponding to an antenna element of a target satellite antenna array, modulated by a forward beam signal, thereby forming a respective set of forward beam element signals for each of two or more forward beam signals, and weighting the sets of forward beam element signals with corresponding forward beam weights from a corresponding set of forward beam weights calculated to form a corresponding forward user beam in the far field upon simultaneous transmission of the sets of forward beam element signals from the target satellite antenna; (b) combining the respective sets of forward beam element signals to form a set of combined forward beam element signals; (c) modulating each optical carrier of a plurality of optical carriers of different wavelengths with a respective one of the set of combined forward beam element signals to obtain a plurality of forward optical channel signals; (d) multiplexing the plurality of forward optical channel signals in the optical domain to form corresponding optical forward uplink signals; and (e) transmitting the corresponding optical forward uplink signals toward a satellite having the target satellite antenna array.

[0012] The above-described ground station processing may occur for two or more sets of forward beam signals, each set representing a corresponding set of forward user beams. For each set of forward beam signals, the ground station forms a set of combined forward beam element signals and modulates a corresponding set of optical carriers to obtain a corresponding set of forward optical channel signals. Each set of forward optical channel signals resides in a different segment of the optical spectrum, such that multiple sets of forward optical channel signals may be "stacked" in the optical domain to form a corresponding optical forward uplink signal having an overall bandwidth spanning each chunk of the optical spectrum occupied by the individual sets of forward optical channel signals.

[0013] Another embodiment includes a method of operating with a satellite communications system including a ground segment and a space segment, the method including receiving two or more return user data substreams via one or more satellites included in the space segment. The two or more return user data substreams are transmitted by the same user terminal that block-encodes the return user data stream and divides the resulting block-encoded data into two or more return user data streams, each return user data substream carrying a different portion of the encoded data from each encoded block. The method further includes transmitting each return user data substream to the ground segment via a different optical return downlink signal, each optical return downlink signal being received at a different ground station among a plurality of geographically dispersed ground stations included in the ground segment, and receiving, at a processing node in the ground segment, each of the two or more return user data substreams from a respective ground station that received one of the two or more return user data substreams. The method further includes, by the processing node, reconstructing the return user data stream from the return user data stream for forwarding toward a target destination. In one or more embodiments or variations, such operations include performing return beamforming in the ground segment to enhance the return uplink signal for a return user beam coverage area that may correspond to part or all of the forward user beam coverage area.

[0014] It is to be understood that 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]

[0015] [Figure 1] FIG. 1 is a block diagram of a satellite communications system that provides spatial transmit diversity over a free-space optical link between a ground station and a satellite, according to one embodiment. [Figure 2] FIG. 2 is a block diagram of a detailed example of a spatial diversity transmit processing configuration, according to one embodiment. [Figure 3] FIG. 3 is a block diagram of a detailed example of a ground station, according to one embodiment. [Figure 4] FIG. 4 is a block diagram of a detailed example of a ground station, according to one embodiment. [Figure 5] FIG. 5 is a block diagram of a detailed example of an optical transmitter, according to one embodiment. [Figure 6] FIG. 6 is a diagram of an exemplary multiplexing in the optical domain, according to one embodiment. [Figure 7] FIG. 7 is a block diagram of a detailed example of an optical transmitter, according to one embodiment. [Figure 8] FIG. 8 is a block diagram of a detailed example of a satellite, according to one embodiment. [Figure 9] FIG. 9 is a block diagram of a detailed example of a user terminal, according to one embodiment. [Figure 10] FIG. 10 is a block diagram of a detailed example of a user terminal, according to one embodiment. [Figure 11] FIG. 11 is a block diagram of a detailed example of a user terminal, according to one embodiment. [Figure 12] FIG. 12 is a block diagram of a more detailed example of the satellite communications system introduced in FIG. [Figure 13] FIG. 13 is a logic flow diagram of a method of operation by a satellite communications system, according to an exemplary embodiment. [Figure 14] FIG. 14 is a logic flow diagram of a method of operation by a user terminal, according to an exemplary embodiment. [Figure 15] FIG. 15 is a logic flow diagram of a method of operation by a user terminal, according to an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] 1 illustrates a satellite communications system (SCS) 10 that provides multiple forward user beams 12, each having a corresponding forward user beam coverage area 14 and resulting from beamforming transmission of a corresponding forward downlink signal 16. Each forward downlink signal 16 carries scheduled forward user traffic for a user terminal (UT) 18 served by the corresponding forward user beam 12. For example, each forward downlink signal 16 carries scheduled forward user traffic for multiple UTs 18 according to a time division multiple access (TDMA) configuration.

[0017] The overall aggregation of forward user beam coverage areas 14 corresponds to a potentially large geographic region that is an aggregate coverage area 20 over which SCS 10 provides communication services. For example, aggregate coverage area 20 may span at least a portion of North America or other geographic region. Aggregate coverage area 20 may also be referred to as a satellite service area.

[0018] Among the multiple forward user beams 12, there are one or more forward spatial diversity beam subsets 22. Each forward spatial diversity beam subset 22 includes a subset of two or more forward user beams 12 having respective forward user beam coverage areas 14 that overlap by more than a threshold amount, i.e., that intentionally overlap such that the SCS 10 can serve UTs 18 in the overlapping coverage areas via two or more forward user beams 12.

[0019] Serving a UT 18 via two or more forward user beams 12 is done based on forward user traffic targeted to that UT 18 being divided across two or more forward downlink signals 16 that are beamformed to generate respective forward user beams 12, each such beam providing coverage with respect to the location of the UT 18. Each beam coverage area 14 may be defined by an effective isotropic radiated power (EIRP) contour, e.g., a respective 3 dB contour.

[0020] The forward user beams 12 within each forward spatial diversity beam subset 22 have forward beam coverage areas 14 that are coextensive or substantially overlap, e.g., overlap by more than 50 percent. In at least one embodiment, the forward user beams 12 within a forward spatial diversity beam subset 22 have the same nominal forward user beam coverage area 14, although there may be differences in actual coverage area.

[0021] With respect to beam signal separability within the overlapping forward beam coverage area 14 of a given forward spatial diversity beam subset 22, the forward user beams 12 within a given forward spatial diversity beam subset 22 may be in different downlink frequency bands, have different polarizations, or both. In other words, the forward user beams 12 included in any given forward spatial diversity beam subset 22 have unique combinations of downlink signal frequencies and polarizations. Of course, in one or more embodiments, the SCS 10 may employ frequency and polarization reuse across the aggregation region 20, with each forward user beam 12 being assigned a frequency and polarization according to a reuse pattern that avoids or minimizes inter-beam interference across the aggregate coverage area 20. Thus, among the multiple forward user beams 12 provided by the SCS 10, there may be multiple forward user beams 12 that use the same downlink signal frequency and polarization combination, but these beams are not overlapping. Conversely, the forward user beams in the forward spatial diversity beam subset 22 overlap at least to some extent, but are distinct from one another according to their respective frequency / polarization combinations. The same configuration may be used in the return direction for return user beam coverage areas that subdivide the satellite service area.

[0022] The SCS 10 may provide forward spatial diversity coverage across all portions of the aggregation region 20. Alternatively, some portions of the aggregation region 20 may have forward spatial diversity coverage via corresponding forward spatial diversity beam subsets 22, while other portions may have only non-diversity coverage provided by a respective single forward user beam 12.

[0023] The SCS 10 is communicatively coupled to one or more external networks 24, such as the public switched telephone network and the Internet or other packet data network. User traffic entering the SCS 10 for delivery to each UT 18 includes, for example, a respective inbound user data stream 26. Each inbound user data stream 26 includes, for example, data packets having a destination address that identifies the target UT 18. Meanwhile, the SCS 10 maintains information indicating the location of each UT 18 or otherwise indicating which forward user beam(s) 12 are used or can be used to serve each UT 18. In this manner, the SCS 10 knows which forward user beam(s) 12 can be used to carry the forward user traffic of a given UT 18 and uses that knowledge to map the scheduled forward user traffic of each UT 18 to a respective one of multiple forward user beams 12 provided by the SCS 10.

[0024] An advantageous processing function performed by the SCS 10 in one or more embodiments with respect to an inbound user data stream 26 is determining whether to use forward spatial diversity transmission. Additionally, in at least one embodiment, the SCS 10 determines the "degree" of forward spatial diversity to apply, where "degree" refers to the number of forward user beams 12 to use for forward spatial diversity transmission of a particular inbound user data stream 26. Two forward user beams 12 represent a minimum degree of diversity, while three, four, or more beams represent higher degrees. For each inbound user data stream 26 transmitted using forward spatial diversity, the inbound user data stream 26 is split into a number of forward user data substreams equal to the number of forward user beams 12 used for diversity.

[0025] As an example, assume that a given inbound user data stream 26 targets a UT 18 that is located in a location covered by five forward user beams 12, each having a different forward downlink frequency and / or polarization. Thus, the “maximum” forward transmit diversity for a given inbound user data stream 26 is a five-fold division of the inbound user data stream 26 across the five forward downlink signals 16 corresponding to the five forward user beams 12. Therefore, for this user data stream 26, the SCS 10 may use forward non-diversity transmission, in which a single forward user beam 12 transmits the inbound user data stream 26 targeted at the UT 18, or the SCS 10 may use forward spatial diversity transmission, in which two or more forward user beams 12 each transmit a respective forward user data sub-stream formed by dividing the inbound user data stream 26. For forward spatial diversity transmission, the SCS 10 in one or more embodiments may dynamically select the number of forward user beams 12 to include in the diversity forward transmission.

[0026] The decision to use forward spatial diversity transmission or forward non-diversity transmission may be made for individual inbound user data streams 26 or for groups or classes of inbound user data streams 26. As an example, the decision per user data stream depends on a user subscription or other service agreement. As a particular example, the SCS 10 uses forward diversity transmission for inbound user data streams 26 that are prioritized or deemed more important pursuant to the subscription. The decision may additionally or alternatively take into account the communication service or service type involved, for example, based on throughput requirements, importance, or other quality of service (QoS) considerations. Additionally or alternatively, the decision may depend on atmospheric conditions related to the reliability of the forward uplink 30 connecting the ground segment 32 of the SCS 10 to the involved satellite(s).

[0027] In this regard, FIG. 1 simplifies the explanation by showing a single satellite 34 providing multiple forward user beams 12. However, it is understood that the SCS 10 includes one or more satellites 34, each providing a potentially large number of forward user beams 12. In at least one embodiment, the SCS 10 includes a constellation of one or more satellites 34. Each such satellite 34 is, in at least one embodiment, a geostationary satellite, and the corresponding forward user beam coverage area 14 is at least nominally fixed. In one or more embodiments, each such satellite 34 is a bent-pipe satellite that uses an unprocessed signal path to relay user traffic in the forward and / or return directions. The "unprocessed" signal path may include conversion between the electrical and optical domains and may include filtering, amplification, and frequency shifting, but excludes demodulation and regeneration of user traffic.

[0028] As long as a given UT 18 is within the overlapping forward beam coverage area 14 of two or more forward user beams 12, i.e., its location is within the coverage provided by the forward spatial diversity beam subset 22, the SCS 10 may choose to serve the given UT 18 using a single forward user beam 12 or two or more forward user beams 12.

[0029] An important aspect of forward spatial diversity, as the term is used herein, is that each forward user data substream split off from any given inbound user data stream 26 is carried from the ground segment of the SCS 10 to the space segment of the SCS 10 over a different optical feeder link 30. This approach, combined with the encoding and splitting used to form each forward user data substream, allows the target UT 18 to recover the complete user data stream even in the event of a temporary failure of an individual optical feeder link 30 involved in carrying each forward user data substream. In this regard, because different ground stations 36 within the ground segment provide different optical feeder uplinks 30, and because the different ground stations 36 are geographically dispersed, atmospheric-related impairments affecting one optical feeder uplink 30 are not correlated with atmospheric-related impairments affecting other optical feeder uplinks 30.

[0030] The ability to use forward spatial diversity provides numerous advantages, particularly in the context of using optical feeder uplinks 30 between the satellite(s) 34 of the SCS 10 and their respective ground stations 36. Because the free-space optical feeder uplinks 30 are individually susceptible to temporary impairments, the problem of dropped or interrupted communications is mitigated by distributing the inbound user data streams 26 across multiple optical feeder uplinks 30 using a data partitioning approach that allows forward error correction (FEC)-based recovery of the complete stream at the target UT 18 even when fewer than all of the forward user data substreams are successfully received.

[0031] 1, each ground station 36 is labeled "OGS" to stand for "optical ground station." Each ground station 36 includes one or more optical transmitters ("OT") 38, each of which anchors a corresponding one of the optical feeder uplinks 30, each of which targets a particular satellite 34. More specifically, each optical transmitter 38 in each ground station 36 transmits a respective optical forward uplink signal 40 carrying one or more forward beam signals. Each forward beam signal carries scheduled forward user traffic corresponding to an individual UT 18 served by the forward user beam 12 corresponding to the forward user beam signal.

[0032] Using forward spatial diversity transmission means that each forward user data substream split from a given inbound user data stream 26 is carried by a different forward beam signal, and further means that each forward user beam signal is transmitted by a different ground station 36, so that optical impairments interfering with the transmission of one of the forward beam signals are not correlated with optical impairments interfering with the transmission of one or more other of the forward beam signals. Per the above definition, "forward feeder link spatial diversity" is an equivalent term describing the process by which forward user data substreams are split from a given inbound user data stream 26 and transmitted from the ground segment to the space segment using different optical feeder uplinks 30 provided by different ground stations 36. Here, the geographic separation between the ground stations 36 provides at least a portion of the "spatial diversity." Further spatial diversity occurs when different satellites 34 in the space segment are used to transmit different forward user beams 12 carrying each forward user data substream.

[0033] The communications and control subsystem (CCS) 42 included in the ground segment 32 of the SCS 10 includes one or more computer servers or other physical computing platforms configured to perform certain forward transmission processing, including generating forward beam signals 44 that are distributed to respective ground stations 36.

[0034] As such, the CCS 42 may be understood to include processing nodes. References herein to a "processing node" should be interpreted broadly to encompass single-node or multi-node embodiments, such as physically separated but communicatively linked nodes operating in concert. A processing node includes, for example, processing circuitry and one or more types of communication interfaces for exchanging signaling with other entities within the SCS 10, such as respective ground stations 36. In at least one embodiment, the CCS 42 includes one or more microprocessors specially adapted based on the execution of stored computer program instructions to perform the CCS functions described herein. In such embodiments, the CCS 42 includes storage, including one or more types of computer-readable media for storing computer program instructions.

[0035] Each ground station 36 corresponds to a particular forward downlink frequency and polarization combination. More specifically, each ground station 36 transmits one or more sets or pluralities of forward beam signals in each optical forward uplink signal 40 that it transmits. Each set or pluralities of forward beam signals corresponds to a set or pluralities of forward user beams 12 having the same downlink signal frequency and polarization and are carried in a different segment of the overall optical spectrum occupied by the forward uplink signal 40. For clarity, the same reference number "44" is used for all forward beam signals, but it should be understood that the user traffic carried in one forward beam signal 44 is different from the user traffic carried in another forward beam signal 44 and that each forward beam signal 44 corresponds to a different one of the overall plurality of forward user beams 12 provided by the SCS 10.

[0036] According to the illustrated embodiment of the SCS 10, the CCS 42 receives inbound user data streams 26 from the external network(s) 26 and performs forward diversity processing. Such processing includes determining whether to use forward spatial diversity transmission or forward non-diversity transmission. In at least one such embodiment, the decision-making further includes determining the degree of forward spatial diversity to use. These decisions may be made on a stream-by-stream basis, for groups of inbound user data streams 26, or for all inbound user data streams 26.

[0037] The CCS 42 may make these decisions based on current conditions, such as whether weather impairments have been detected or are predicted on one or more of the optical forward uplinks 30. For example, in at least one embodiment, the SCS 10 operates modally by selecting between a diversity mode that employs forward spatial diversity for at least some inbound user data streams 26 and a non-diversity mode that does not employ forward spatial diversity. Further, in at least one embodiment, the SCS 10 omits the decision-making operation and employs forward spatial diversity for all inbound user data streams 26. Thus, the SCS 10 "employing forward spatial diversity for at least one inbound user data stream 26" should be understood to mean that, under at least some conditions, or in at least one embodiment, the SCS 10 employs forward spatial diversity to transmit one or more of the inbound user data streams 26.

[0038] 1, for any inbound user data streams 26 transmitted without forward spatial diversity, the CCS 42 passes them as forward user data streams 46 to a forward beam mapping function implemented via processing circuitry in the CCS 42. Inbound user data streams 26 transmitted with forward spatial diversity are split into respective forward user data substreams 48 that are passed to the forward beam mapping function. It should be noted here that the forward user data streams 46 may include block-coded versions of the corresponding inbound user data streams based on applying a defined block code having a defined block length, and for any inbound user data streams 26 transmitted using forward spatial diversity, the corresponding forward user data stream 46 may be split such that different portions of the coded data from each coded block form respective forward user data substreams 48.

[0039] Each forward user data stream 46 targets a specific UT 18 and is mapped to a forward user beam 12 used to serve that UT 18. Similarly, each forward user data substream 48 is mapped to a respective forward user beam 12 in the forward spatial diversity subset 22 associated with serving the targeted UT 18. Each forward user beam 12 may carry a mixture of forward user data streams 48 for UTs 18 served via non-diversity forward transmissions and forward user data substreams 48 for UTs 18 served via diversity forward transmissions. In a broad sense, all such traffic may be referred to as forward user traffic or scheduled forward user traffic, although it is understood that forward user traffic is beam-specific. Notably, each forward user data substream 48 used to serve any particular UT 18 via forward spatial diversity is transmitted by a different forward user beam 12, meaning that they are mapped to different forward beam signals 44 by the ground segment.

[0040] Traffic-to-beam mapping is performed continuously for the data flows that make up each inbound user data stream 26, with the forward beam mapping function of CCS 42 logically grouping the forward user data streams 46 and forward user data substreams 48 according to the respective forward user beams 12 over which they are transmitted. From there, the forward user scheduling function continuously multiplexes the forward user data streams 46 and forward user data substreams 48 for each forward user beam 12 according to a user scheduling algorithm.

[0041] The forward beam signal generation function implemented via the processing circuitry of the CCS 42 forms a plurality of forward beam signals 44, each of which is represented diagrammatically by signal line "44" output from the forward beam signal generation function. Each forward beam signal 44 corresponds to a specific one of the forward user beams 12. That is, each forward downlink signal 16 results from a beamforming transmission of a respective one of the forward beam signals 44. Here, "beamforming" transmission refers to the transmission of a set of forward beam element signals from a target satellite antenna array, each forward beam element signal in the set being modulated according to that forward beam signal 44 and weighted according to a respective forward beam weight from a corresponding set of forward beam weights calculated from channel state information (CSI), e.g., propagation channel estimates, describing a forward path from the target antenna array to one or more UTs 18 within the forward user beam coverage area 14 of the forward user beam 12 corresponding to the forward beam signal 44, for transmission from a respective antenna element in the antenna array.

[0042] For each forward beam user signal 44, the simultaneous transmission of a corresponding set of forward beam element signals from the target satellite antenna array can be understood as a beamforming transmission of a corresponding forward downlink signal 16, with the weighting of the element signals resulting in a constructive and destructive superposition pattern of the forward downlink signal 16 in the far field resulting in a corresponding forward user beam 12, where "far field" refers to the electromagnetic field region where radiation behavior dominates.

[0043] FIG. 1 illustrates this detail by showing each forward beam signal 44 as having a corresponding set of forward beam weights 50 determined by a forward beam weight calculation function implemented via processing circuitry in the CCS 42. Note that the reference numeral "50" in FIG. 1 is used in the plural, meaning that there is a set of forward beam weights 50 for each forward beam signal 44. The forward beam weights 50 are calculated for each forward user beam 12, for example, using channel feedback from one or more UTs 18 operating within the corresponding forward user beam coverage area 14. Thus, the forward beam weight calculation function in one or more embodiments uses forward channel estimates to calculate a corresponding set of forward beam weights 50 for each forward beam signal 44.

[0044] A forward beam signal distribution function implemented via the processing and communication interface circuitry of the CCS 42 distributes the forward beam signals 44 and additional corresponding information, such as a set of forward beam weights 50, to each ground station 36. In FIG. 1, the distribution of the forward beam signals 44 with additional corresponding information is indicated using the reference numeral 52. That is, each "signal 52" will be understood to be one or more sets of forward beam signals 44 with corresponding sets of forward beam weights 50. Furthermore, each "set" of forward beam signals 44 transmitted to each ground station 36 represents one or more forward user beams 12 having the same downlink signal frequency and polarization and transmitted from the same satellite antenna array.

[0045] In an exemplary configuration, each ground station 36 serves one or more sets of forward beam signals 44 corresponding to one or more sets of forward user beams 12 from the overall plurality of forward user beams 12 provided by the SCS 10. Distribution of each forward beam signal 44 to the ground stations 36 by the CCS 42 may be based on such association. However, such association may change over time, for example, to account for a failed ground station 36 or maintenance, or other availability or load balancing considerations, and the distribution is updated to reflect the change. However, for any given forward spatial diversity beam subset 22, at least when forward spatial diversity is employed, each included forward beam 12 is served by a different ground station 12.

[0046] In at least one embodiment, one or more of the ground stations 36 may transmit two or more optical forward uplink signals 40 using their respective optical transmitters 38. In any case, each optical forward uplink signal 40 carries one or more sets of forward beam signals 44, each such set representing a corresponding set of forward user beams 12 and carried within a respective segment of the overall optical spectrum spanned by the forward uplink signal 40. More specifically, in at least one embodiment, each "set" of forward beam signals 44 carried in any given optical forward uplink signal 40 transmitted by any given ground station 36 corresponds to a forward user beam 12 having the same downlink signal frequency and polarization. Furthermore, each such set of forward beam signals 44 targets the same antenna array onboard the satellite targeted by the forward uplink signal 40 and is carried in a respective segment of the overall optical spectrum of the forward uplink signal 40. This approach reduces the complexity of beamforming at the ground station 36 and simplifies the electrical-to-optical conversion and multiplexing used to form the optical forward uplink signals 40, and the optical bandwidth of each forward uplink signal 40 means that each forward uplink signal 40 can carry a set of multiple forward beam signals 44 corresponding to a potentially large number of forward user beams 12, e.g., hundreds of forward user beams 12.

[0047] 2 illustrates, in one embodiment, the implementation of forward diversity processing by CCS 42. This diagram illustrates the processing for a given inbound user data stream 26, with the understanding that CCS 42 includes additional signal paths and processing to apply the same treatment to all inbound user data streams 26. In other words, CCS 42 is configured to process multiple inbound user data streams 26 in parallel.

[0048] The forward transmit diversity controller 100 determines whether to apply forward spatial diversity to the inbound user data streams 26 and accordingly routes each inbound user data stream 26 to either a non-diversity processing path 102 or a diversity processing path 104. The diversity processing path 104 includes a block encoder 106 that performs block coding of the inbound user data streams 26 and outputs a corresponding forward stream of coded blocks 108.

[0049] A divider 110 divides each coded block 108 into respective sub-blocks containing different subsets of the coded data contained within the coded block 108. This operation creates a corresponding stream of sub-blocks 112, each containing different coded data, which in turn comprise forward user data sub-streams 48 used for forward spatial diversity transmission of the associated inbound user data stream 26. The number of forward user data sub-streams 48 formed from a given inbound user data stream 26 represents the degree of diversity, i.e., the number of distinct forward user beams 12 used to transmit the inbound user data stream 26 to the target UT 18.

[0050] In at least one embodiment, the CCS 42 applies the same block coding to each inbound user data stream 26, regardless of whether the inbound user data stream 26 is transmitted using forward spatial diversity. That is, the CCS 42 performs block coding on all inbound user data streams 26 to form corresponding forward user data streams 46, and then, for each forward user data stream 46 transmitted using forward spatial diversity, the CCS 42 performs coding block segmentation to generate a corresponding set of forward user data substreams 48. Thus, forward diversity processing by the CCS 42 outputs a forward user data stream 46 for inbound user data streams 26 that are not transmitted with diversity, and outputs a corresponding set of forward user data substreams 48 for inbound user data streams 26 that are transmitted with diversity.

[0051] 3 shows an example configuration of a ground station 36 including a communications interface 120 configured to receive a signal 52 from a CCS 42, the signal 52 including one or more sets of forward beam signals 44 and a corresponding set of forward beam weights 50. The ground station 36 may include two or more optical transmitters 38, and the incoming signal 52 from the CCS 42 may include one or more sets of forward beam signals 44 for transmission from each respective optical transmitter 38.

[0052] The communications interface 120 includes physical layer receiver circuitry, along with timing and communications processing, and outputs forward beam signals 44 transmitted via a particular optical transmitter 38 to corresponding forward path circuits 122. There is a population of forward path circuits 122 associated with each optical transmitter 38 included in the ground station, the circuitry configured to provide signal processing for a particular set of one or more forward beam signals 44 transmitted by the associated optical transmitter 38. The population of forward path circuits 122 associated with each optical transmitter 38 generates a set of combined forward beam element signals 124 for each set of forward beam signals 44 carried in the forward signal 40 output by the optical transmitter 38. It will thus be understood that each population of forward path circuits 122 receives one or more sets of forward beam signals 44, each such set including at least one forward beam signal 44, and a corresponding set of forward beam weights 50 is received for each such forward beam signal 44.

[0053] FIG. 4 shows example details of the forward path circuitry 122 associated with each optical transmitter 38, according to one embodiment. More specifically, FIG. 4 shows the circuitry used for each respective set of forward beam signals 44 transmitted in the same forward uplink signal 40. For each forward beam signal 44 in each set, there is a forward beam element signal generator 126. While FIG. 4 suggests three forward beam element signal generators 126 for the exemplary set of three individual forward beam signals 44, it will be understood that for each optical transmitter 38 included in each ground station 36, there may be a prescribed number of forward beam element signal generators 126 corresponding to the maximum number of forward beam signals 44 that can be transmitted via the corresponding forward uplink signal 40.

[0054] Each forward beam element signal generator 126 operates on a respective forward beam signal 44 in the associated set and includes a plurality of radio frequency modulators 128, or equivalently, RF modulators and signal splitters. Each forward beam element signal generator 126 is configured to output a set of RF signals 130. Each RF signal 130 corresponds to a respective antenna element of the target satellite antenna array used to form the forward user beam 12 corresponding to the forward beam signal 44 being processed. The RF signals 130 are formed by modulating an RF carrier according to the forward beam signal 44, and they may all be at the same frequency, e.g., a given intermediate frequency. A weighting circuit 132 applies a corresponding set of forward beam weights 50 to the set of RF signals 130 to form a set of forward beam element signals 134. The corresponding set of forward beam weights 50 is calculated so that simultaneous transmission of the set of forward beam element signals 134 from corresponding antenna elements of the target satellite antenna array causes a superposition of signals forming the corresponding forward user beam 12 in the far field.

[0055] As shown, a set of forward beam element signals 134 is generated for each forward beam signal 44 in each set of forward beam signals 44 transmitted by optical transmitter 38. Combining circuit 136 combines a corresponding one of each set of forward beam element signals 134 to obtain a corresponding set of combined forward beam element signals 124. This combining is performed element-by-element, such that forward beam element signals 134 from each of the sets that map to the same antenna element in the target antenna array are combined. These linear combinations are possible because the forward beam signals 12 corresponding to the sets of forward beam element signals 134 being combined all have the same downlink signal frequency and polarization.

[0056] For example, assume there are four forward beam signals 44 in a given set of forward beam signals 44 being processed by the ground station 36 for a particular optical transmitter 38 within the ground station 36, and there are 100 antenna elements in the target satellite antenna array. For each forward beam signal 44 being processed, the ground station 36 generates 100 forward beam element signals 134 based on the corresponding set of 100 forward beam weights 50. For the ith antenna element of the target satellite antenna array, there are four ith forward beam element signals 134, one for each forward beam signal 44, and these four forward beam element signals 130 are summed to generate the corresponding combined forward beam element signal 124 for the ith antenna element. Of course, if there is only one forward beam signal 44 in the "set" of forward beam signals 44, then the set of combined forward beam element signals 124 output from the combining circuit 136 is just one set of forward beam element signals 134 generated for the one forward beam element signal 44.

[0057] 4, each forward uplink signal 40 carries one or more sets of forward beam signals 44. Each such set of forward beam signals 44 is represented by a set of combined forward beam element signals 124. The set of combined forward beam element signals 124 is obtained by combining the individual sets of forward beam element signals 134 generated for the individual forward beam signals 44 included in the set of forward beam signals 44.

[0058] 5 illustrates an optical transmitter 38 according to one embodiment. For ease of illustration, the illustration assumes that there are three sets of transmitted forward beam signals 44, each represented by a respective set of combined forward beam element signals 124 entering the optical transmitter 38. Of course, there may be many such sets of forward beam signals 44, each set containing one or more forward beam signals 44, thereby efficiently utilizing the overall optical spectrum of the forward uplink signal 40.

[0059] Each set of combined forward beam element signals 124 is provided to a respective set of optical modulators 144. Within each such optical modulator 144, each one of the combined forward beam element signals 124 is used to modulate a corresponding optical carrier 142 from a plurality of optical carriers 142 provided by a plurality of light sources 140. For each set of combined forward beam element signals 124, the corresponding set of optical carriers 142 are at different optical wavelengths that define a respective optical channel. Furthermore, each respective set of light sources 140 outputs their respective set of optical carriers 142 in a different portion of the optical spectrum; i.e., each set of optical carriers 142 corresponds to a different set of optical channels. Thus, each set of transmitted forward beam signals 44 is represented by a different set of combined forward beam element signals 124, and each such set of combined forward beam element signals 144 is carried in a different set of optical channel signals 146.

[0060] Each optical channel signal 146 in each set of optical channel signals 146 carries a respective one of the corresponding set of combined forward beam element signals 124 used to generate the set of optical channel signals. Because the combined forward beam element signals 124 are RF signals, one approach is to intensity modulate the optical carriers 142, such that intensity variations in each resulting forward optical channel signal 146 cause a corresponding combined forward beam element signal 124 to be carried. In at least one embodiment, phase modulation is used, where the phase of each optical carrier 142 is modulated according to a respective one of the combined forward beam element signals 124. Generally, each forward optical channel signal 146 carries user traffic contained in the forward beam signal(s) 44 from which the corresponding combined forward beam element signal 124 was formed.

[0061] 6 illustrates an exemplary configuration for forming forward uplink signal 40 as a multiplexed optical signal. In this example, assume there are three sets of forward beam signals 44 transmitted in forward uplink signal 40, each such set including one or more forward beam signals 44, and that the forward beam signals 44 included in each such set correspond to forward user beams 12 having the same downlink signal frequency and polarization. Each set of forward beam signals 44 is used to generate a corresponding set of combined forward beam element signals 124, and each set of combined forward beam element signals 124 is transmitted in a corresponding set of forward optical channel signals 146.

[0062] 6 contains a first set of forward optical channel signals 146 and transmits a first set of combined forward beam element signals 124 corresponding to a first set of forward beam signals 44 representing a first set of forward user beams 12. Segment 2 of the optical spectrum contains a second set of forward optical channel signals 146 and transmits a second set of combined forward beam element signals 124 corresponding to a second set of forward beam signals 44 representing a second set of forward user beams 12. Segment 3 of the optical spectrum contains a third set of forward optical channel signals 146 and transmits a third set of combined forward beam element signals 124 corresponding to a third set of forward beam signals 44 representing a third set of forward user beams 12. There may be as many additional sets of forward optical channel signals 146 as will fit within the overall spectrum allocated to the forward uplink signal 40.

[0063] In this way, the forward uplink signal 40 effectively "stacks" in the optical frequency domain each set of combined forward beam element signals 124 using dense wavelength division multiplexing (DWDM). This approach allows, but does not require, all combined forward beam element signals 124 within each such set to be at the same RF frequency, which may be the downlink signal frequency used by the forward user beam 12 represented by the set of combined forward beam element signals 124, or may be some intermediate frequency, e.g., 3.5 GHz.

[0064] As shown in FIG. 5 , an optical multiplexer 148 within the optical transmitter 38 forms a forward uplink signal 40 as an aggregation of this set of forward optical channel signals 146, and an optical head unit 150 focuses or otherwise guides the forward uplink signal 40 for transmission toward the target satellite 34. As previously described, the head unit 150 focuses or otherwise directs the forward uplink signal 40 for free-space transmission toward an optical receiver at the target satellite 34. As an example, the head unit 150 includes one or more mirrors. As another example, the head unit includes one or more prisms. In at least one embodiment, the head unit 150 is steerable in response to a steering command signal, allowing the head unit 150 to be adjusted for alignment with a target optical receiver on the target satellite 34.

[0065] Figure 7 shows a further detailed example of optical transmitter 38, according to one embodiment. Figure 7 is simplified to show implementation details for only a single set of combined forward beam element signals 124 carrying one corresponding set of forward beam signal(s) 44 transmitted in forward uplink signal 40 output by optical transmitter 38. However, it should be understood that optical transmitter 38 includes modulation circuitry for each set of combined forward beam element signals 124 addressed by optical transmitter 38.

[0066] 7, a first combined forward beam element signal 124 of the illustrated combined forward beam element signals 124 serves as a modulation input for a first optical modulator 144-1, and a second combined forward beam element signal 124 of the illustrated combined forward beam element signals 124 serves as a modulation input for a second optical modulator 144-2, and so forth. Each optical modulator 144 includes, for example, a bias circuit 152 that applies a DC bias to the corresponding input combined forward beam element signal 124, which is then applied to a modulator 154. The modulator 154 is, for example, a Mach-Zehnder modulator (MZM) that modulates an optical carrier 142 output from a light source 140, such as a laser diode that outputs light of a particular optical wavelength. As previously mentioned, the modulation in one or more embodiments is phase modulation. Intensity modulation may also be used.

[0067] Returning to FIG. 1 , each satellite 34 is equipped with one or more optical receivers (“OR”) 200. Each optical receiver 200 is configured to receive a respective forward uplink signal 40 from a respective optical transmitter 38 at a respective ground station 36. It should be understood that, like the optical transmitters 38, the optical receivers 200 are steerable in one or more embodiments, such that a given optical receiver 200 on a given satellite 34 may be aligned with different optical transmitters 38 in the same or different ground stations 36 at different times. Steering may be used for load balancing, to accommodate ground station maintenance, failures, or weather conditions, or for other reasons.

[0068] Each optical receiver 200 outputs a set of recovered RF signals 202 corresponding to each set of combined forward beam element signals 124 transmitted in the received forward uplink signal 40. That is, for each set of forward beam signals 44 transmitted in the forward beam signal 40, the optical receiver 200 recovers a corresponding set of RF signals 202 that includes recovered versions of the set of combined forward beam element signals 124 generated from the set of forward beam signals 44 at the transmitter. It should be understood that each signal line in FIG. 1 labeled "202" represents a set of recovered RF signals 202.

[0069] Such operation is based on demultiplexing the received forward uplink signal 40 in the optical domain to recover a respective set of forward optical channel signals 146 carried by the forward uplink signal 40. Each recovered forward optical channel signal 146 is then demodulated using a photodetector to track, for example, the phase modulation of the recovered forward optical channel signal 146 to generate a corresponding one of the recovered RF signals 202. The use of reference numeral 202 in the receiver context, rather than numeral 124 used in the transmitter context, merely emphasizes the distinction between the transmitter and receiver contexts. In the absence of disturbances or corruption, each set of RF signals 202 recovered from the received forward uplink signal 40 is identical in terms of information content to the corresponding set of combined forward beam element signals 124 multiplexed into the forward uplink signal 40 at the corresponding optical transmitter 38.

[0070] A forward transmit (TX) subsystem 204 couples each set of recovered RF signals 202 to one of one or more antenna arrays 210 onboard the satellite 34. Each forward TX subsystem 204 takes in a respective set of recovered RF signals 202 and outputs a corresponding set of forward antenna element signals 206. It should be understood that each signal line in FIG. 1 labeled "206" represents a set of forward antenna element signals 206.

[0071] Each set of forward antenna element signals 206 differs from its corresponding set of recovered RF signals 202 in any one or more of amplification, filtering, and frequency conversion. In at least one embodiment, each set of recovered RF signals 202 is at IF, and the forward TX subsystem 204 corresponding to each such set converts those signals to the downlink signal frequencies used by the forward user beam(s) 12 represented in the set. Thus, in one or more embodiments, each forward TX subsystem 204 is associated with a particular antenna array 210 and / or a particular set of antenna input feeds having associated downlink signal frequencies and polarizations, and each forward TX subsystem 204 includes a set of forward analog signal paths that provide at least power amplification for each set of recovered RF signals 202 for transmission from the respective array elements of the associated antenna array 210.

[0072] The satellite 34 may have a single antenna array 201 with different sets of input feeds associated with different downlink signal frequencies and polarizations, such that forward user beams 12 of different downlink signal frequencies and polarizations are transmitted from the same antenna array 210. For example, each optical receiver 200 receives a respective forward uplink signal 40 carrying one or more sets of forward beam signals 44, each such set associated with one or more forward user beams 12 having a particular downlink signal frequency and polarization combination, and all forward TX subsystems 204 apply corresponding sets of forward antenna element signals 206 to different sets of input feeds of the same antenna array 210. Alternatively, the satellite 34 may include multiple antenna arrays 210, each associated with one or more particular downlink signal frequency and polarization combinations, and each forward TX subsystem 204 is associated with a respective one of the antenna arrays 210 according to the frequency-polarization relationship.

[0073] Generally, with each set of forward antenna element signals 206 corresponding to one or more forward user beams 12 having a particular downlink carrier frequency and / or polarization, each forward TX subsystem 204 couples its output set of forward antenna element signals 206 to a set of antenna feeds corresponding to that particular frequency and / or polarization.

[0074] 8 illustrates an example of detailed configuration of a given optical receiver 200 onboard a given satellite 34 for receiving corresponding forward uplink signals 40. An optical head 220, e.g., one or more lenses and / or mirrors, receives the forward uplink signals 40 from the respective ground stations 36, and an optical demultiplexer 222 uses wavelength division demultiplexing to recover one or more sets of forward optical channel signals 224 corresponding to the set of forward optical signals 146 multiplexed into the received forward uplink signals 40. For simplicity, FIG. 8 illustrates the recovery of a single set of forward optical channel signals 224 corresponding to one set of combined forward beam element signals 124, which in turn represents one set of forward beam signals 44.

[0075] 6 illustrates that multiple spectral segments within forward uplink signal 40 contain respective sets of forward optical channel signals 146, whereas FIG. 8 can be understood to illustrate the recovery and demultiplexing of one such set. Accordingly, it will be appreciated that additional similar circuitry and additional forward TX subsystems 204 are included within optical receiver 200 for recovering additional sets of forward optical channel signals 146 from the received forward uplink signal 40, and correspondingly, for recovering corresponding sets of RF signals 202 and generating corresponding sets of forward antenna element signals 206. Each set of forward antenna element signals 206 represents a corresponding set of one or more forward user beams 12, each such beam carrying the traffic contained in a corresponding forward beam signal 44.

[0076] The use of the reference number "224" in the receiver context for the forward optical channel signals 146 rather than the number "146" is merely to emphasize the distinction between the receiver and transmitter contexts. In the absence of disturbances or corruption, each set of forward optical channel signals 224 multiplexed at the optical receiver 200 is identical in information content to the corresponding set of forward optical channel signals 146 multiplexed at the corresponding optical transmitter 38.

[0077] A secondary lens or mirror 226 may be used to direct each optical channel signal 224 to a corresponding photodiode 228, e.g., a first one of the optical channel signals 224 is directed via lens 226-1 to photodiode 228-1, a second one of the optical channel signals 224 is directed via lens 226-2 to photodiode 228-2, and so on. Each photodiode 228 outputs an electrical signal responsive to the phase or intensity of the corresponding optical channel signal. These output electrical signals are the recovered RF signal 202 described above.

[0078] The forward TX subsystem 204 in the illustrated embodiment includes an analog forward signal path for each RF signal 202. Each forward signal path includes, for example, a low noise amplifier (LNA) 230, a frequency converter (FC) 232, and a power amplifier (PA) 234. The FC 232 converts the RF signal 202 from an intermediate frequency to a downlink carrier frequency associated with the target forward user beam(s) 12. The FC 232 may be implemented as an upconverter or a downconverter, depending on the frequencies involved.

[0079] The PA 234 provides power amplification of the frequency-converted RF signal 202, and the power-amplified signals output from the PA 234 are referred to as a set of forward antenna element signals 206. The set of forward antenna element signals 206 are applied to a set of input feeds 236 of an antenna array 210 onboard the satellite 34, each of which corresponds to a respective antenna element 238 of the antenna array 210. Each antenna element 238 radiates a respective forward beam element signal 206 applied to its corresponding input feed 236, and the collective transmission of these per-element signals can be viewed as the transmission of corresponding forward downlink signal(s) 16, and the far-field superposition of the per-element signals generates corresponding forward user beam(s) 12.

[0080] To understand these results, recall that the forward uplink signal 40 received by the optical receiver 200 carries one or more sets of combined forward beam element signals 124, each set of combined forward beam element signals 124 formed by combining two or more sets of forward beam element signals 134. Each such set of forward beam element signals 134 is weighted by a corresponding set of forward beam weights 50, calculated such that simultaneous transmission of the set of forward beam element signals 134 from each antenna element 238 of the target satellite antenna array 210 results in a far-field signal superposition that forms a particular one of the forward user beams 12 provided by the SCS 10. Thus, transmitting a set of forward antenna element signals 206 formed from satellite-recovered versions of the set of combined forward beam element signals 124 results in a respective forward user beam 12 represented by the set of combined forward beam element signals 124.

[0081] FIG. 9 illustrates an exemplary embodiment of a UT 18, which includes one or more transmit / receive antennas 240 and associated communications circuitry 242. The communications circuitry 242 includes two or more transceiver signal chains 244, each including a satellite radio receiver and / or transmitter, and a baseband processor 246 for processing and controlling the transmit and receive signals. The exemplary UT 18 further includes a system processor 248 that controls the overall operation of the UT and, for example, executes one or more applications that implement the intended functionality of the UT 18. Exemplary functionality includes telecommunications services, broadband multimedia delivery, etc. The UT 18 may include additional circuitry 250 to support its intended functionality.

[0082] The system processor 248 and / or the baseband processor 246 implement substream processing functionality 252 that provides processing of forward user data substreams in a diversity forward transmission context and / or processing of return user data substreams in a diversity return transmission context. The system processor 248 and the baseband processor 246 include one or more microprocessors, digital signal processors, FPGAs, ASICs, SoCs, or other digital processing circuits, as well as supporting clock circuits, computer-readable storage media, etc.

[0083] FIG. 10 illustrates exemplary sub-stream processing at the UT 18 for diversity forward transmission, while FIG. 11 illustrates exemplary sub-stream processing at the UT 18 for diversity return transmission.

[0084] 10 uses an example in which a forward user data stream 46 targeted for UT 18 is split into three forward user data sub-streams 48, designated as 48-1, 48-2, and 48-3. Recalling with reference to FIG. 1, forward user data stream 46 may be understood as encoded, beam-mapped, and scheduled forward user traffic from inbound user data stream 26.

[0085] In one or more embodiments, the exemplary UT 18 uses separate receiver circuits RX1, RX2, and RX3 to receive three different forward downlink signals 16, designated 16-1, 16-2, and 16-3. Each forward downlink signal 16-1, 16-2, and 16-3 is at a different downlink signal frequency and / or polarization, and each forward downlink signal 16 carries a respective one of forward user data substreams 48-1, 48-2, and 48-3. Each forward downlink signal 16 is transmit beamformed to generate a respective forward user beam 12.

[0086] In one or more other embodiments, some or all of the receiver circuitry within the receiver chain of the UT 18 may be shared. For example, in order for the UT 18 to receive two diversity carriers at different frequencies, e.g., 18 GHz and 19 GHz, the UT 18 may use the same LNA, RF converter, analog-to-digital converter (ADC), etc., and then separately demodulate the “combined” signal in the digital domain for the two carriers. Furthermore, at least some of the DSP or ASIC resources used for demodulation and other signal processing for the different received diversity carriers, e.g., buffers, demodulators, etc., may be shared. Because such processing ultimately produces separately extracted information from each received diversity carrier, the UT 18 may be considered to have a functionally separate receiver chain for each diversity carrier, even though it still shares some or all of the physical circuitry used to receive two or more diversity carriers, i.e., different forward downlink signals 16.

[0087] 10, it should be understood that transmission of forward downlink signal 16-1 by the involved satellite 34 forms forward user beam 12-1 having forward user beam coverage area 14-1 surrounding the location of UT 18, transmission of forward downlink signal 16-2 forms forward user beam 12-2 having forward user beam coverage area 14-2 surrounding the location of UT 18, and transmission of forward downlink signal 16-3 forms forward user beam 12-3 having forward user beam coverage area 14-31 surrounding the location of UT 18. In other words, UT 18 is within the overlapping coverage area formed by the three forward user beams 12-1, 12-2, and 12-3, and the three forward user beams 12-1, 12-2, and 12-3 function as a forward spatial diversity beam subset 22 for UT 18.

[0088] One or more of the forward user data substreams 48 have header information that indicates the reconstruction order of the forward user data substreams 48 to recover the corresponding stream of coded blocks. The UT 18 can be understood to use the header information to perform reconstruction of the coded blocks and thus recover the forward user data stream 46. The UT 18 performs block decoding to recover the original inbound user data stream 26 and provides it for higher layer processing, e.g., application layer processing at the UT 18. Configured processing circuitry, such as the baseband processor 246 and / or the system processor 248, performs all of the functions shown in FIG. 10.

[0089] It should be noted that the UT 18 may additionally or alternatively be served using non-diversity forward transmission, meaning that the UT 18 is forward served using a single forward user beam 12 resulting from the beamformed transmission of a single forward downlink signal 16 carrying a single forward user data stream 46 targeted to the UT 18. In such a case, receive processing at the UT 18 relies on a single receiver circuit.

[0090] 11 illustrates an exemplary configuration of the UT 18 in a return transmission context. High-level processing in the UT 18 generates an outbound user data stream 258 for return transmission back to the CCS 42. The UT 18 block encodes the outbound user data stream 258, for example, according to a defined transport block size, to obtain a corresponding return user data stream 260. In at least one embodiment, the processing circuitry of the UT 18 is configured to determine whether to use return spatial diversity processing for the return user data stream 260. If return spatial diversity is not employed, the UT 18 transmits the return user data stream 260 via a corresponding return uplink signal 264, which is a radio transmission on a defined return uplink frequency.

[0091] When the UT 18 employs return spatial diversity, the return user data stream 260 is split block by block to form two or more return user data substreams 262; Figure 11 shows an exemplary scenario of three return user data substreams 262-1, 262-2, and 262-3. Separate transmit signal chains TX1, TX2, and TX3 are used to transmit each return user data substream 262-1, 262-2, and 262-3 in respective return uplink signals 264-1, 264-2, and 264-3. At least in the case of simultaneously transmitting multiple return user data substreams 262, the multiple return uplink signals 264 are distinguished from one another in terms of signal frequency and / or polarization.

[0092] The illustrated "TX" block is understood to be a transmit circuit including modulation, upconversion, and amplification. Similar to the previous discussion regarding the possibility of sharing circuitry to receive different diversity carriers, at least some transmit circuitry, including the digital and / or analog domains, may be shared for transmitting diverse return carriers. Here, "return carrier" refers to the return uplink signal 264 transmitted by the UT 18 using a particular carrier signal frequency and / or polarization.

[0093] In some embodiments, the UT 18 determines, based on control signaling transmitted from the SCS 10, for example, whether to use return spatial diversity transmission for the return user data stream 260. If return spatial diversity is used, the UT 18 embeds header information in one or more of the return user data substreams 262 formed from the return user data stream 260 in order to reconstruct the return user data stream 260 at the CCS 42.

[0094] 12 illustrates the SCS 10 in a return direction context. The aggregate coverage area 20 may be partitioned using multiple return user beams 300 with corresponding return user beam coverage areas 302. That is, the overall satellite service area may be logically divided into multiple return user beam coverage areas, with each return user beam coverage area 302 representing a corresponding return user beam 300. One or more return spatial diversity subsets 304 may exist, each subset including two or more return user beams 300 with respective return user beam coverage areas 302 that overlap by more than a threshold amount, such that UTs 18 within the overlapping area may be served in the return direction via two or more of the associated return user beams 300. With respect to such service, the SCS 10 may be understood to provide return spatial transmit diversity.

[0095] Each UT 18 covered by a return spatial diversity subset 304 need not operate in a return spatial diversity transmission mode. Indeed, individual UTs 18 or groups thereof may be controlled to operate in a return spatial diversity transmission mode, while other UTs 18 within the same return user beam coverage overlap may be controlled individually or in groups to operate in a return non-spatial diversity transmission mode. A UT 18 operating in a return spatial diversity transmission mode splits data comprising a return user data stream into a set of two or more return user data substreams, each of which is transmitted as a separate radio transmission using an uplink signal frequency and / or polarization associated with a respective return user beam 300 within the return spatial diversity subset 304 associated with the location of the UT 18. A UT 18 operating in a return non-spatial diversity transmission mode does not split any of the one or more return user data streams it transmits.

[0096] In one or more embodiments, the return user beams 300 are realized a posteriori based on return beamforming in the CCS 42, rather than based on any transmit beamforming performed by the UTs 18 or any receive beamforming applied at the satellites 34. Thus, while FIG. 12 shows return user beams 300 in free space, such beams may exist only from a signal processing perspective based on signal weighting applied to the recovered signals at the CCS 42. In particular, the CCS 42, in one or more embodiments, calculates a set of return beam weights representing each return user beam coverage area 302, the set of return beam weights being calculated to maximize the signal-to-noise ratio (SNR) of return uplink signals 264 originating from UTs 18 operating within that return user beam coverage area 302. In this manner, CCS42 generates respective return user beam signals, each of which corresponds to a specific one of the return user beams 300 and carries a return uplink signal 264 transmitted by a UT18 located within the return user beam coverage area 302 corresponding to that specific return user beam 300.

[0097] The return beam coverage area 302 may or may not be the same as the forward beam coverage area 14. However, in at least one embodiment, the return beam coverage area 302 and the forward beam coverage area 14 are at least nominally the same.

[0098] One or more antenna arrays 320 on the satellite 34 receive the inbound return uplink signals 264 from respective ones of the UTs 18. The one or more antenna arrays 320 may be the same as or separate from the one or more antenna arrays 210 shown in FIG.

[0099] Individual antenna elements of each antenna array 320 receive a superposition of return uplink signals 264 from multiple UTs 18, potentially across multiple return beam coverage areas 302. Respective return uplink signals 264 from different UTs 18 are transmitted according to return link traffic scheduling by the CCS 42. UTs 18 operating in spatial diversity return transmission each transmit multiple return uplink signals 264. Each one of the multiple return uplink signals 264 corresponds to a different return user beam 300, the different beams being distinguished by uplink signal frequency and / or polarization, and each such return user beam 300 carries one return user data substream 262 split from the corresponding return user data stream 260. UTs 18 operating in non-diversity return transmission transmit return uplink signals 264 carrying an unsplit return user data stream 260.

[0100] One or more antenna arrays 320 onboard the satellite 34 output respective sets of return antenna element signals 322, each such set corresponding to a different combination of return uplink signal frequency and polarization. Each signal line in FIG. 12 labeled "322" should be understood to represent a respective set of return antenna element signals 322. Each return antenna element signal 322 within each set of return antenna element signals 322 may be understood as a composite of the return uplink signals 264 received at the corresponding antenna element for the associated combination of return uplink signal frequency and polarization.

[0101] Each return TX subsystem 324 of two or more return TX subsystems 324 onboard the satellite 34 receives a respective set of return antenna element signals 322 and outputs a corresponding set of combined return beam element signals 326. Each signal line in FIG. 12 labeled "326" will be understood to represent a set of combined return beam element signals 326. Each set of combined return beam element signals 326 contains a corresponding return uplink signal 264 received via the antenna array(s) 320 for a corresponding combination of uplink signal frequency and polarization, and represents one or more return user beams 300 using that frequency and polarization combination. In at least one embodiment, each set of combined return beam element signals 326 is a set of radio signals that have been amplified, filtered, and frequency shifted compared to the corresponding set of return antenna element signals 322.

[0102] To implement return spatial diversity, the satellite 34 has two or more optical transmitters 328. The return signal paths and processing within the satellite 34 are configured such that the return uplink signals 264 associated with each respective return user beam 300 included in a given return diversity beam subset are relayed to the ground segment 32 of the SCS 10 using separate optical transmitters 328, each directed to a different ground station 36. That is, at least one subset of the return user beams 300 is configured as a return spatial diversity beam subset including two or more return user beams 300 having a unique combination of signal frequency and polarization and having respective return user beam coverage areas 302 that overlap by more than a threshold amount.

[0103] Each optical transmitter 328 is coupled to a different ground station 36 via a respective optical return feeder link 330. As a result, for example, for a UT 18 that splits its return user data stream 260 into three return user data substreams 262 and transmits each such return user data substream 262 in a different return uplink signal 264, each such return uplink signal 264 is carried back to the CCS 42 via a different optical return feeder link 330. This configuration provides spatial diversity in the return direction of the different return user data substreams 262, enabling the CCS 42 to recover the return user data stream 260 even during a temporary fade or interruption of the individual optical return feeder links 330 used for return spatial diversity transmission.

[0104] Thus, each optical transmitter 328 onboard the satellite 34 targets a different ground station 36 and transmits an optical return downlink signal 332 containing a plurality of multiplexed return optical channel signals, each such return optical channel signal being modulated based on a respective one of the set of transmitted combined return beam element signals 326. Although lighter in weight for satellite use, with reference to FIG. 7 , each optical transmitter 328 may be configured similarly to the optical transmitter 38 described for the ground station 36. The multiplexing used for the optical return downlink signal 332 may be configured similarly to that shown and described in detail for the forward uplink signal 40. For example, any given optical transmitter 328 may be used to transmit one or more sets of recovered RF signals 202 based on forming respective sets of return optical channel signals, each occupying a respective segment of the optical spectrum. Each optical channel signal in each such set of return optical channel signals is modulated with a respective one of the corresponding set of RF signals 202, and each set of return optical channel signals is aggregated via DWDM to form a corresponding optical return downlink signal 332.

[0105] Each ground station 36 includes one or more optical receivers 334, each receiving a return downlink signal 332 from a respective optical transmitter 328 onboard a respective satellite 34 at any given time. Each such optical receiver 334 may be configured similarly to the optical receiver 200 onboard the satellite 200. See FIG. 8. In this manner, each optical receiver 334 demultiplexes return optical channel signals from the received return downlink signal 332, each return optical channel signal being at a different optical wavelength, and the demultiplexing is performed based on wavelength-based filtering to recover the individual return optical channel signals.

[0106] A respective photodetector, such as a photodiode, is used in the ground station 36 to demodulate each return optical channel signal, thereby recovering the corresponding transmitted combined return beam element signal 326. That is, the photodetector outputs an analog domain radio signal that serves as a recovered version of the corresponding combined return beam element signal 326. In other words, each optical receiver 334 includes an optical demultiplexer for recovering each set of return optical channel signals multiplexed into the received return downlink signal 332, and also includes multiple photodetectors for demodulating each recovered set of return optical channel signals to recover the corresponding set of combined return beam element signals 326 carried by the return downlink signal 332. Such operations are performed within each ground station 36 for each included optical receiver 334 therein, or at least for each included optical receiver 334 during active operation and while receiving the corresponding return downlink signal 334.

[0107] Each ground station 36 sends a return signal 336 to the CCS 42, carrying the reconstructed set(s) of combined return beam element signals acquired by the ground station 36. Because each reconstructed set of combined return beam element signals corresponds to one or more return user beams 300 having a particular return uplink signal frequency and polarization, the CCS 42 performs return beamforming on each such set. In particular, for each reconstructed set of combined return beam element signals, the CCS 42 individually applies a respective set of return beam weights 338. By individually applying, we mean independently applying each set of return beam weights 338 to a separate copy of the reconstructed set of combined return beam element signals. Each such weight set is calculated to produce an enhanced SNR and directional sensitivity for return uplink signals 264 originating from UTs 18 within the return user beam coverage area 302 corresponding to the return user beam 300 for which the weight set is calculated.

[0108] CCS 42 applies such processing to all recovered sets of combined return beam element signals coming into CCS 42 from ground stations 36, resulting in (continuously) generating respective return beam signals 340. Each return beam signal 340 corresponds to one of the return user beams 300, and CCS 42 recovers and forwards the return user data streams 260 transmitted in each return beam signal 340 as outbound user traffic 342, for example, via one or more external networks 24, to their target destination addresses.

[0109] For each UT 18 using diversity return transmission, recovery of the corresponding return user data stream 260 is based on the CCS 42 reconstructing the multiple return user data substreams 262, for example, using reconstruction information carried by one or more of the multiple return user data substreams 262. The reconstruction produces a block-coded version of the return user data stream 260, and the CCS 42 performs block decoding to obtain the return user data stream 260.

[0110] 13 illustrates an overall method 1300 of operation by SCS 10, according to one example embodiment. Method 1300 is performed in a loop or continuously, meaning that one or more of the illustrated operations reflect ongoing actions taken on a data stream inbound to SCS 10 for transmission.

[0111] The method 1300 includes the SCS 10 receiving inbound user data streams 26 (block 1302), each targeted at a particular UT 18. The method 1300 further includes per-stream processing (block 1304), including determining, for each inbound user data stream 26, whether to use forward spatial diversity (block 1304A). If forward spatial diversity is used ("Yes" in block 1304A), the method 1300 optionally includes determining a degree of forward spatial diversity (block 1304B) and further includes splitting the inbound user data stream into two or more forward user data substreams 48 (block 1304C) and mapping each forward user data substream 48 to a respective forward beam signal 44 corresponding to a respective forward user beam 12 covering the location of the target UT 18 (block 1304D).

[0112] If "No" at block 1304A, forward spatial diversity transmission is not used for the inbound user data stream 26, and processing continues by mapping the forward user data stream 48 corresponding to the inbound user data stream 26 to a respective forward beam signal 44 corresponding to a forward user beam 12 covering the location of the target UT 18. Such processing may include, for example, block coding the inbound user data stream 26, and a particular forward user beam 12 may be selected as part of a load balancing or user scheduling operation.

[0113] Further operations in method 1300 include performing continuous distribution of the forward beam signals 44 to corresponding ground stations 36 (block 1306). Block 1308 refers to operations for each ground station, including, at each ground station 36, (A) forming a set of forward beam element signals 134 for each forward beam signal 44 transmitted by the ground station 36, (B) multiplexing the forward beam element signals 134 onto an optical carrier, and (C) transmitting the resulting optical forward uplink signal 40 to the target satellite 34. Returning momentarily to FIG. 4 , the ground stations 36 form respective forward uplink signals 40 carrying one or more sets of forward beam signals 44.

[0114] Such operations include, for each set of forward beam signals 44 transmitted in forward uplink signal 40, forming a set of forward beam element signals 134 for each forward beam signal in the set, and combining those sets of forward beam element signals 134 to form a set of combined forward beam element signals 124. Each set of combined forward beam element signals 124 is used to modulate a respective set of optical carriers 142 to form a corresponding set of forward optical channel signals 146, which are multiplexed to form optical forward uplink signal 40.

[0115] Block 1310 refers to operations for each satellite, including (A) recovering one or more sets of combined forward beam element signals 124 from each received forward uplink signal 40, and (B) transmitting a corresponding set of forward antenna element signals 206 to form a corresponding forward user beam 12.

[0116] 14 illustrates a method 1400 of operation performed by the UT 18, according to one embodiment. This figure assumes forward spatial diversity reception at the UT 18, in which the SCS 10 generates a forward user data stream 46 based on block coding the inbound user data stream 26 for transmission and divides each coded block into distinctive subsets of coded data to form two or more forward user data substreams 48, which are forward transmitted by the SCS 10 using respective forward user beams 12 corresponding to forward beam signals 44 transmitted via respective forward uplink signals 40 on geographically separated forward optical feeder uplinks 30.

[0117] Thus, method 1400 includes the UT 18 receiving (block 1402) two or more forward user data substreams 48 on different forward user beams 12. The UT 18 reconstructs (block 1404) a forward user data stream from the received forward user data substreams 48, which includes recovering missing information associated with a temporary interruption to each one of the forward user data substreams 48 based on forward error correction (FEC) coding applied in generating the forward user data stream 46. Further, method 1400 includes passing the reconstructed inbound user data stream 26 to upper layer processing at the UT 18.

[0118] FIG. 15 illustrates a method 1500 performed by the UT 18 in one embodiment with respect to return spatial diversity. Processing begins with receiving an outbound user data stream 258 (block 1502). For example, an application executing on the UT 18 generates outbound packet data targeted to a remote device or system. Therefore, "receiving" herein refers to an internal operation within the UT 18, in which the UT 18 determines whether to use return spatial diversity for the outbound user data stream 258. If return spatial diversity is not to be used ("No" at block 1504), processing continues with the UT 18 obtaining a return user data stream 260 by encoding the outbound user data stream and transmitting the return user data stream 260 via a return uplink signal 264 associated with a single return user beam 300 and conveyed to the CCS 42 via a single optical return feeder link 330 (block 1506).

[0119] If the UT 18 uses return spatial diversity (“Yes” at block 1504), in at least one embodiment, the process continues by determining the degree of diversity (block 1508), which means determining the number of return user data substreams 262 into which the return user data stream 260 is divided. In other embodiments or other operating scenarios, the number of return user data substreams 262 is a default or predefined number. In at least some embodiments, the SCS 10 is configured to determine the degree of return spatial diversity used by an individual UT 18, a group of UTs 18, or the entire population of UTs 18. Such a determination is made based on, for example, the number of supported UTs 18, e.g., per return user beam, and loading, such as the amount(s) or type of traffic originating from an individual UT 18 or group of UTs 18, e.g., within the respective return user beam coverage area 302. Control signaling sent by the SCS 10 to the UT 18 configures whether or to what extent a given UT 18 uses return spatial diversity.

[0120] The process continues with the UT 18 splitting the return user data stream 260 into two or more return user data substreams 262 (block 1510). Splitting refers to dividing each coded block included in the return user data stream 260 into a unique subset of coded data, with the resulting flow of data subsets representing corresponding return user data substreams 262. The UT 18 then transmits each return user data substream 262 via a different return uplink signal 264 (block 1512). Each return uplink signal 264 corresponds to a different return user beam 300 and is transmitted back to the CCS 42 via a different optical return feeder link 330.

[0121] It should be noted that 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 particular 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 method of operating a satellite communications system including a ground segment and a space segment, the method comprising: receiving, at one or more processing nodes of the ground segment, inbound user data streams, each inbound user data stream targeted to a respective user terminal served by the satellite communications system; for each of the inbound user data streams: block-encoding the inbound user data stream and dividing each coded data block into different subsets of the coded data to form a corresponding set of two or more forward user data sub-streams; mapping each forward user data substream to a respective one of two or more forward beam signals respectively corresponding to two or more forward user beams of the satellite communications system, the forward beam beams having respective forward user beam coverage areas surrounding the location of the user terminal targeted by the inbound user data stream; employing forward spatial diversity transmission for one or more of the inbound user data streams by transmitting each forward beam signal to a different one of a plurality of geographically dispersed ground stations, each transmitting to the space segment using a respective optical forward uplink signal; A method comprising:

2. 10. The method of claim 1, further comprising: at each ground station of the plurality of geographically dispersed ground stations, forming a respective forward uplink signal by multiplexing a plurality of forward optical channel signals carrying respective copies of the forward beam signals received by the ground station weighted for beamforming transmission from respective antenna elements of a target antenna array in the space segment for far-field forming of the corresponding forward user beam.

3. 3. The method of claim 1, further comprising, for each inbound user data stream for which forward spatial diversity is employed, embedding stream reconstruction information in at least one of the forward user data substreams in a corresponding set of two or more forward user data substreams, wherein the reconstruction information results in an ordered reconstruction of the inbound user data stream at the target user terminal.

4. 4. A method according to any one of claims 1 to 3, wherein the forward spatial diversity transmission is selectively used, and the method further comprises, for any inbound user data stream for which the forward spatial diversity transmission is not used, using forward non-diversity transmission in which the inbound user data stream is block coded and mapped to a single forward beam signal corresponding to each forward user beam.

5. 5. The method of claim 1, wherein the satellite communications system provides a plurality of forward user beams via one or more satellites included in the space segment, and among the plurality of forward user beams there are one or more forward spatial diversity beam subsets, each forward spatial diversity beam subset including two or more forward user beams having a unique combination of signal frequency and polarization and having respective forward user beam coverage areas that overlap by more than a threshold amount, and wherein using forward spatial diversity transmission for one or more of the inbound user data streams includes mapping, for each of the inbound user data streams, the corresponding two or more forward user data substreams to respective forward beam signals corresponding to forward user beams that are members of a forward spatial diversity beam subset that provides coverage for the location of the respective user terminal.

6. 6. The method of claim 5, wherein forward user traffic mapped to each forward user beam included in each forward spatial diversity beam subset is transmitted from the ground segment to the space segment via different optical forward uplink signals originating from different ones of two or more of the geographically dispersed ground stations.

7. 7. The method of claim 1, further comprising: determining whether to use forward spatial diversity transmission for any one or more of the inbound user data streams based on a corresponding user subscription, whereby forward spatial diversity transmission is used for a given inbound user data stream depending on the corresponding user subscription.

8. 7. The method of claim 1, further comprising continuously determining whether to use forward spatial diversity transmission for any one or more of the inbound user data streams based on any one or any combination of the number of ground stations available to transmit forward beam signals to the space segment, detected failures of any one or more of the optical forward uplink signals used to transmit the forward beam signals from the ground segment to the space segment, and loading of the respective forward user beams.

9. the satellite communications system provides a plurality of forward user beams via one or more satellites included in the space segment, each forward user beam having a respective combination of downlink signal frequency and polarization based on a corresponding forward user beam signal, and the method comprises: receiving, at each of one or more ground stations of the plurality of ground stations, two or more forward beam signals corresponding to two or more forward user beams having respective combinations of the same downlink signal frequency and polarization, wherein at each such ground station: generating a set of radio frequency signals, each corresponding to an antenna element of a target satellite antenna array, modulated by said forward beam signals to form a respective set of forward beam element signals for each of two or more forward beam signals, weighted with a corresponding forward beam weight from a set of corresponding forward beam weights calculated to form said corresponding forward user beam in the far field by simultaneous transmission of the sets of forward beam element signals from said target satellite antenna; combining said respective sets of forward beam element signals to form a set of combined forward beam element signals; modulating each optical carrier of a plurality of optical carriers of different wavelengths with a respective one of the set of combined forward beam element signals to obtain a plurality of forward optical channel signals; multiplexing the plurality of forward optical channel signals in the optical domain to form a corresponding optical forward uplink signal; The method of any one of claims 1 to 8, further comprising transmitting the corresponding optical forward uplink signal towards a satellite having the targeted satellite antenna array.

10. 1. A satellite communications system including a ground segment, the ground segment comprising: an interface circuit configured to receive inbound user data streams, each inbound user data stream targeted to a respective user terminal served by said satellite communications system; for each of the inbound user data streams: block-encoding the inbound user data stream and dividing each coded data block into different subsets of coded data to form a corresponding set of two or more forward user data sub-streams; mapping each forward user data substream to a respective one of two or more forward beam signals respectively corresponding to two or more forward user beams of the satellite communications system, the forward beam beams having respective forward user beam coverage areas surrounding the location of the user terminal targeted by the inbound user data stream; a processing circuit configured to use forward spatial diversity transmission for one or more of the inbound user data streams based on the forward beam signals being configured to transmit each forward beam signal to a different one of a plurality of geographically dispersed ground stations, each configured to transmit using a respective optical forward uplink signal to a space segment of the satellite communications system; satellite communication systems, including

11. 11. The satellite communications system of claim 10, further comprising: a plurality of geographically dispersed ground stations, each of said plurality of geographically dispersed ground stations configured to form said respective forward uplink signals by multiplexing a plurality of forward optical channel signals carrying respective copies of said forward beam signals received by said ground stations weighted for beamforming transmission from respective antenna elements of a target antenna array in said space segment for far-field forming of said corresponding forward user beams.

12. 12. A satellite communications system according to claim 10 or 11, wherein for each inbound user data stream for which forward spatial diversity is employed, the processing circuitry in the ground segment is configured to embed stream reconstruction information in at least one of the forward user data substreams in a corresponding set of two or more forward user data substreams, the reconstruction information resulting in an ordered reconstruction of the inbound user data stream at the target user terminal.

13. A satellite communications system as claimed in any one of claims 10 to 12, wherein forward spatial diversity transmission is selectively used and the processing circuitry included in the ground segment is configured to use forward non-diversity transmission for any inbound user data stream for which forward spatial diversity transmission is not used, in which the inbound user data stream is block coded and mapped to a single forward beam signal corresponding to each forward user beam.

14. 14. The satellite communications system of claim 10, wherein the satellite communications system provides a plurality of forward user beams via one or more satellites included in the space segment, and among the plurality of forward user beams there are one or more forward spatial diversity beam subsets, each forward spatial diversity beam subset including two or more forward user beams having a unique combination of signal frequency and polarization and having respective forward user beam coverage areas that overlap by more than a threshold amount, and to use forward spatial diversity transmission for one or more of the inbound user data streams, the processing circuitry included in the ground segment is configured to map, for each of the inbound user data streams, the corresponding two or more forward user data substreams to respective forward beam signals corresponding to forward user beams that are members of a forward spatial diversity beam subset that provides coverage for the location of the respective user terminal.

15. 15. The satellite communications system of claim 14, wherein the ground segment is configured such that forward user traffic mapped to each forward user beam included in each forward spatial diversity beam subset is transmitted from the ground segment to the space segment via different optical forward uplink signals originating from different ones of two or more geographically dispersed ground stations.

16. 16. A satellite communications system according to any one of claims 10 to 15, wherein the processing circuitry included in the ground segment is configured to determine whether to use forward spatial diversity transmission for any one or more of the inbound user data streams based on a corresponding user subscription, whereby forward spatial diversity transmission is used for a given inbound user data stream depending on the corresponding user subscription.

17. 17. The satellite communications system of claim 10, wherein the processing circuitry included in the ground segment is configured to continuously determine whether to use forward spatial diversity transmission for any one or more of the inbound user data streams based on any one or any combination of the number of ground stations available to transmit forward beam signals to the space segment, detected failures of any one or more of the optical forward uplink signals used to transmit the forward beam signals from the ground segment to the space segment, and loading of the respective forward user beams.

18. the satellite communications system provides a plurality of forward user beams via one or more satellites included in the space segment, each forward user beam having a respective combination of downlink signal frequency and polarization based on a corresponding forward user beam signal, and each ground station: an interface circuit configured to receive two or more forward beam signals corresponding to two or more forward user beams having respective combinations of the same downlink signal frequency and polarization; A processing and transmission circuit, comprising: generating a set of radio frequency signals, each corresponding to an antenna element of a target satellite antenna array, modulated by said forward beam signals to form a respective set of forward beam element signals for each of two or more forward beam signals, weighted with a corresponding forward beam weight from a corresponding set of forward beam weights calculated to form said corresponding forward user beam in the far field by simultaneous transmission of the sets of forward beam element signals from said target satellite antenna array; combining said respective sets of said forward beam element signals to form a set of combined forward beam element signals; modulating each optical carrier of a plurality of optical carriers of different wavelengths with a respective one of the set of combined forward beam element signals to obtain a plurality of forward optical channel signals; multiplexing the plurality of forward optical channel signals in the optical domain to form a corresponding optical forward uplink signal; a processing and transmitting circuit configured to transmit the corresponding optical forward uplink signal toward a satellite having the target satellite antenna array; A satellite communication system according to any one of claims 10 to 17, comprising:

19. 1. A method of operation by a processing node of a satellite communications system, comprising: receiving a plurality of return signals, each return signal received from a respective one of a plurality of geographically dispersed ground stations included in a ground segment of the satellite communications system, each return signal derived from a respective optical return downlink signal received by the respective ground station from a space segment of the satellite communications system, each return signal carrying return user traffic transmitted by a user terminal within one or more respective return user beam coverage areas associated with the return signal; forming, for each return signal, a return beam signal for each of the one or more respective user beam coverage areas associated with the return signal by applying a corresponding set of return user beam weights to the return signal, the corresponding set of return user beam weights representing a return user beam corresponding to the respective return user beam coverage area and calculated to maximize a signal-to-noise ratio (SNR) of a return uplink signal transmitted by a user terminal located in the respective return user beam coverage area; recovering two or more return user data substreams that together form a set of return user data substreams transmitted by a user terminal at a location surrounded by two or more of the return user beam coverage areas and relayed by the satellite communications system using return spatial diversity transmission, wherein each return user data substream is relayed to a different one of the plurality of earth stations using a respective optical return downlink signal, whereby each return user data substream in the set of return user data substreams is recovered from the return signal incoming from a different one of the plurality of geographically dispersed earth stations, recovering two or more return user data substreams corresponding to a respective one of the return user beams that correspond to the two or more return user beam coverage areas surrounding the location of the user terminal; reconstructing the set of return user data substreams to obtain a corresponding return user data stream; transmitting an outbound user data stream corresponding to said return user data stream towards an external network; A method comprising:

20. 20. The method of claim 19, wherein reconstructing the return user data substream includes using reconstruction information included by the user terminal in at least one of the two or more return user data substreams.

21. 21. The method of claim 19 or 20, wherein each return user data substream is transmitted by the user terminal using a different combination of return uplink signal frequency and polarization, each of the combinations corresponding to a respective one of the two or more return user beam coverage areas surrounding the location of the user terminal.

22. 22. The method of claim 19, wherein the satellite communications system serves a population of user terminals within a satellite service area logically divided into a plurality of return user beam coverage areas, each return user beam coverage area being associated with a corresponding return uplink signal frequency and polarization combination, each return signal transmitting a unique set of combined return beam element signals representing one or more non-overlapping return user beams having the same corresponding return uplink signal frequency and polarization combination, and each combined return beam element signal in each unique set of combined return beam element signals corresponding to a respective element of a corresponding satellite antenna array used to receive return uplink signals transmitted by the population of user terminals.

23. 23. The method of claim 22, comprising forming, for each return signal, a return beam signal for each return user beam represented by a unique set of the combined return beam element signals carried by the return signal.

24. 24. The method of claim 23, further comprising: using channel state information (CSI) determined for one or more reference user terminals in each return user beam coverage area to calculate corresponding return user beam weights for forming the corresponding return user beam signals from the CSI.

25. 25. The method of claim 19, wherein a satellite service area is logically divided into a plurality of return user beam coverage areas, each of the return user beams representing a respective return user beam of a corresponding plurality of return user beams, among the plurality of return user beams there are one or more return spatial diversity beam subsets, each return spatial diversity beam subset including two or more return user beams having a unique combination of signal frequency and polarization and having respective return user beam coverage areas that overlap by more than a threshold amount, the location of the user terminal being within the overlap of one of the return spatial diversity beam subsets, and wherein using return spatial diversity transmission for the user terminal comprises transmitting each return user data substream corresponding to the return user data stream from the space segment to the ground segment via a different return optical downlink signal received at a different one of the ground stations.

26. 26. The method of claim 19, further comprising: at each ground station, receiving the respective optical return downlink signals; optically demultiplexing the respective optical return downlink signals to obtain a plurality of optical channel signals at respective optical wavelengths; and generating from the plurality of optical channel signals the unique set of combined return beam element signals carried by the return signal output from the ground station.

27. 27. The method of claim 19, further comprising controlling the user terminal to selectively operate in a return spatial diversity transmission mode rather than a return non-spatial diversity transmission mode in which the user terminal transmits the return user data streams rather than forming and transmitting the set of return user data sub-streams.

28. 28. The method of claim 27, wherein controlling the user terminals to selectively operate in the return spatial diversity transmission mode comprises determining whether the user terminals, as individual user terminals or as one of a larger group of user terminals, operate in the return spatial diversity transmission mode or the return non-spatial diversity transmission mode depending on any one or any combination of: a number of earth stations available to receive their respective optical return downlink signals; detected failures of any one or more optical return downlink signals; and loading of the respective return user beams.

29. a processing node, the processing node comprising: an interface circuit configured to receive a plurality of return signals, each return signal received from a respective ground station of a plurality of geographically dispersed ground stations, each return signal derived from a respective optical return downlink signal received by the respective ground station from a corresponding satellite, each return signal carrying return user traffic transmitted by a user terminal within one or more respective return user beam coverage areas associated with the return signal; A processing circuit, for each return signal, forming a return beam signal for each of the one or more respective user beam coverage areas associated with the return signal by applying a corresponding set of return user beam weights to the return signal, the corresponding set of return user beam weights representing a return user beam corresponding to the respective return user beam coverage area and calculated to maximize a signal-to-noise ratio (SNR) of a return uplink signal transmitted by a user terminal located in the respective return user beam coverage area; recovering two or more return user data substreams that together form a set of return user data substreams transmitted by a user terminal at a location surrounded by two or more of the return user beam coverage areas and relayed by the satellite communications system using return spatial diversity transmission, each return user data substream being relayed to a different one of the plurality of earth stations using a respective optical return downlink signal, whereby each return user data substream in the set of return user data substreams is recovered from the return signal incoming from a different one of the plurality of geographically dispersed earth stations and corresponds to a respective one of the return user beams that correspond to the two or more return user beam coverage areas surrounding the location of the user terminal; reconstructing the set of return user data substreams to obtain a corresponding return user data stream; a processing circuit configured to transmit an outbound user data stream corresponding to the return user data stream toward an external network; satellite communication systems, including

30. 30. The satellite communications system of claim 29, wherein the processing circuitry is configured to reconstruct the set of return user data substreams using reconstruction information included by the user terminal in at least one of the two or more return user data substreams.

31. 31. A satellite communications system as claimed in claim 29 or 30, wherein each return user data sub-stream is transmitted by the user terminal using a different combination of return uplink signal frequency and polarization, each of said combinations corresponding to a respective one of said two or more return user beam coverage areas surrounding the location of the user terminal.

32. 32. A satellite communications system as claimed in any one of claims 29 to 31, wherein the satellite communications system serves a population of user terminals within a satellite service area logically divided into a plurality of return user beam coverage areas, each return user beam coverage area being associated with a corresponding return uplink signal frequency and polarization combination, each return signal transmitting a unique set of combined return beam element signals representing one or more non-overlapping return user beams having the same corresponding return uplink signal frequency and polarization combination, and each combined return beam element signal in each unique set of combined return beam element signals corresponding to a respective element of a corresponding satellite antenna array used to receive return uplink signals transmitted by the population of user terminals.

33. 33. The satellite communications system of claim 32, wherein the processing circuitry is configured to form, for each return signal, a return beam signal for each return user beam represented by a unique set of the combined return beam element signals carried by the return signal.

34. 34. The satellite communications system of claim 33, wherein the processing circuitry is configured to use channel state information (CSI) determined for one or more reference user terminals within each return user beam coverage area to calculate corresponding return user beam weights for forming the corresponding return user beam signals from the CSI.

35. 35. The satellite communications system of claim 29, wherein a satellite service area is logically divided into a plurality of return user beam coverage areas, each of the return user beams representing a respective return user beam of a corresponding plurality of return user beams, wherein among the plurality of return user beams there are one or more return spatial diversity beam subsets, each return spatial diversity beam subset including two or more return user beams having a unique combination of signal frequency and polarization and having respective return user beam coverage areas that overlap by more than a threshold amount, wherein the location of the user terminal is within the overlap of one of the return spatial diversity beam subsets, and wherein the satellite communications system employs return spatial diversity transmission to the user terminal by transmitting each return user data substream corresponding to the return user data stream from the space segment to the ground segment via a different return optical downlink signal received at a different one of the ground stations.

36. 36. A satellite communications system as claimed in any one of claims 29 to 35, further comprising a plurality of geographically dispersed ground stations, each configured to receive the respective optical return downlink signals, optically multiplex the respective optical return downlink signals to obtain a plurality of optical channel signals at respective optical wavelengths, and generate from the plurality of optical channel signals the unique set of combined return beam element signals carried by the return signal output from the ground station.

37. 37. A satellite communications system according to any one of claims 29 to 36, wherein the processing circuitry is configured to control the user terminal to selectively operate in a return spatial diversity transmission mode rather than a return non-spatial diversity transmission mode, in which the user terminal transmits the return user data stream rather than forming and transmitting the set of return user data sub-streams.

38. 38. The satellite communications system of claim 37, wherein the processing circuitry is configured to determine whether the user terminal, as an individual user terminal or one of a larger group of user terminals, operates in the return spatial diversity transmission mode or the return non-spatial diversity transmission mode depending on any one or any combination of the number of earth stations available to receive the respective optical return downlink signal, detected failures of any one or more optical return downlink signals, and loading of the respective return user beam.

39. 1. A method of operating a satellite in a satellite communications system, comprising: receiving two or more return user substreams from a user terminal at a location surrounded by two or more return user beam coverage areas, each return user substream being received as a separate wireless transmission from the user terminal and separated from a return user data stream at the user terminal; transmitting each return user substream to a ground segment of the satellite communications system via a respective optical return downlink signal received at a different one of a plurality of geographically dispersed ground stations included in the ground segment of the satellite communications system; A method comprising:

40. 40. The method of claim 39, wherein each return user data substream is received as a respective return uplink signal, each of the two or more return uplink signals having a unique combination of return uplink signal frequency and polarization.

41. 41. The method of claim 40, wherein the two or more return user uplink signals are received via corresponding antenna arrays onboard the satellite, the corresponding antenna arrays including a plurality of antenna elements and having a plurality of antenna outputs for outputting a corresponding plurality of array element signals.

42. 42. The method of claim 40 or 41, wherein the corresponding antenna array is identical for each return user data substream, the corresponding antenna array having a respective plurality of antenna outputs for each unique combination of return uplink signal frequency and polarization.

43. 43. The method of claim 41 or 42, further comprising: forming each optical return downlink signal by modulating a respective one of a plurality of optical channel carriers at a respective optical wavelength in accordance with a respective one of the corresponding plurality of array element signals; and then multiplexing the resulting plurality of optical channel signals in the frequency domain to form the optical return downlink signal.

44. 44. The method of any one of claims 39 to 43, wherein transmitting each return user sub-stream to a ground segment of the satellite communications system via a respective optical return downlink signal comprises transmitting each optical return downlink signal towards a respective one of the plurality of geographically dispersed ground stations.

45. 45. The method of any one of claims 39 to 44, further comprising receiving control signaling that controls which earth stations the satellite targets to receive the respective optical return downlink signals.

46. 46. ​​The method of any one of claims 39 to 45, wherein the satellite is a bent-pipe satellite that includes an unprocessed return signal path.

47. 1. A method of operating a satellite in a satellite communications system, comprising: receiving two or more optical forward uplink signals, each optical forward uplink signal received from a respective one of two or more geographically dispersed ground stations within a ground segment of the satellite communications system, each optical forward uplink signal containing a respective forward user data substream within a set of two or more forward user data substreams subdivided at the ground segment from a forward user data stream targeted at a user terminal located at a position encompassed by two or more forward user beam coverage areas; transmitting each forward user substream for the user terminal via a respective one of two or more forward user beams corresponding to the two or more forward user beam coverage areas; A method comprising:

48. 48. The method of claim 47, wherein transmitting each forward user substream for the user terminal via the respective one of the two or more forward user beams corresponding to the two or more forward user beam coverage areas comprises, for each forward user substream, transmitting a corresponding plurality of antenna element signals from a corresponding plurality of antenna elements of a corresponding antenna array onboard the satellite, the antenna element signals being weighted such that a superposition in the far field of the antenna element signals forms the respective forward user beam.

49. 49. The method of claim 48, wherein the method includes demultiplexing each optical forward uplink signal to obtain a corresponding plurality of optical channel signals of different optical wavelengths, each optical channel signal carrying a respective combined forward beam element signal of a set of combined forward beam element signals, each combined forward beam element signal being weighted for beamforming transmission from a respective element in the corresponding antenna array, and the corresponding plurality of antenna element signals being the set of combined forward beam element signals or being derived from the set of combined forward beam element signals.

50. 50. The method of claim 49, wherein each combined set of forward beam element signals carries forward user traffic for user terminals associated with one or more forward user beams of a plurality of forward user beams defined by the satellite communications system, the one or more forward user beams being non-overlapping and having the same user downlink signal frequency and polarization.

51. The method of any one of claims 47 to 50, further comprising receiving control signaling that controls from which earth stations the satellite receives the respective optical forward uplink signals.

52. 52. The method of any one of claims 47 to 51, wherein the satellite is a bent-pipe satellite including an unprocessed return signal path.

53. 1. A satellite configured for operation in a satellite communications system, comprising: one or more antenna arrays configured to receive two or more return user substreams from a user terminal at a location surrounded by two or more return user beam coverage areas, each return user substream being received as a separate wireless transmission from the user terminal and separated from a return user data stream at the user terminal; two or more optical transmitters, each configured to transmit a respective return user sub-stream to a ground segment of the satellite communications system via a respective optical return downlink signal received at a different one of a plurality of geographically dispersed ground stations included in the ground segment of the satellite communications system; Including satellites.

54. 54. The satellite of claim 53, wherein each return user data substream is received as a respective return uplink signal, each of said two or more return uplink signals having a unique combination of return uplink signal frequency and polarization.

55. 55. The satellite of claim 54, wherein said one or more antenna arrays include a single antenna array, said single antenna array having a plurality of array elements for receiving said separate radio transmissions.

56. 56. A satellite as claimed in claim 55, wherein said one antenna array has a plurality of antenna outputs, each for a unique combination of return uplink signal frequency and polarization.

57. 57. A satellite as claimed in claim 55 or 56, wherein each optical transmitter is configured to form the respective optical return downlink signal by modulating a respective one of a plurality of optical carriers at a respective optical wavelength in accordance with a respective one of the corresponding plurality of array element signals, and then multiplexing the resulting plurality of optical channel signals in the frequency domain to form the optical return downlink signal.

58. 58. A satellite as claimed in any one of claims 53 to 57, wherein the satellite is configured to transmit each optical return downlink signal towards a respective one of the plurality of geographically dispersed ground stations.

59. A satellite as claimed in any one of claims 53 to 58, wherein the satellite is configured to receive control signalling for controlling which earth stations the satellite targets for receiving the respective optical return downlink signals.

60. A satellite according to any one of claims 53 to 59, wherein the satellite is a bent-pipe satellite including an unprocessed return signal path connecting the one or more antenna arrays to the two or more optical transmitters.

61. 1. A satellite configured for operation in a satellite communications system, comprising: two or more optical receivers, each configured to receive a respective one of two or more optical forward uplink signals, each optical forward uplink signal received from a respective one of two or more geographically dispersed ground stations within a ground segment of said satellite communications system, each optical forward uplink signal containing a respective forward user data substream within a set of two or more forward user data substreams subdivided at said ground segment from a forward user data stream targeted at user terminals located at locations encompassed by two or more forward user beam coverage areas; radio frequency (RF) transmit circuitry associated with one or more antenna arrays configured to transmit each forward user substream for the user terminal via a respective one of two or more forward user beams corresponding to the two or more forward user beam coverage areas; Including satellites.

62. 62. The satellite of claim 61, wherein the satellite is configured to transmit a corresponding plurality of antenna element signals from a corresponding plurality of antenna elements of a corresponding antenna array on board the satellite to transmit each forward user sub-stream to the user terminal via the respective one of the two or more forward user beams corresponding to the two or more forward user beam coverage areas, the antenna element signals being weighted such that a superposition in the far field of the antenna element signals forms the respective forward user beam.

63. 63. The satellite of claim 62, wherein each optical receiver is configured to demultiplex the optical forward uplink signal received by the optical receiver to obtain a corresponding plurality of optical channel signals of different optical wavelengths, each optical channel signal carrying a respective combined forward beam element signal of a set of combined forward beam element signals, each combined forward beam element signal being weighted for beamforming transmission from a respective element in the corresponding antenna array, and the corresponding plurality of antenna element signals are the set of combined forward beam element signals or are derived from the set of combined forward beam element signals.

64. 64. The satellite of claim 63, wherein each combined set of forward beam element signals carries forward user traffic for user terminals associated with one or more forward user beams of a plurality of forward user beams defined by a satellite communications system, the one or more forward user beams being non-overlapping and having the same user downlink signal frequency and polarization.

65. 65. A satellite as claimed in any one of claims 61 to 64, wherein the satellite is configured to receive control signalling and align the two or more optical receivers accordingly to control from which ground station the satellite receives the respective optical forward uplink signals.

66. A satellite according to any one of claims 61 to 65, wherein the satellite is a bent-pipe satellite including an unprocessed return signal path connecting the two or more optical receivers to the one or more antenna arrays.

67. 1. A method of operating with satellite communications including a ground segment and a space segment, comprising: receiving, via one or more satellites included in the space segment, two or more return user data substreams transmitted by the same user terminal that block-coded the return user data stream and divided the resulting block-coded data into the two or more return user data streams, each return user data substream carrying a different portion of the coded data from each coding block; transmitting each return user data substream to the ground segment via a different optical return downlink signal, each optical return downlink signal being received at a different ground station of a plurality of geographically dispersed ground stations included in the ground segment; receiving, at a processing node of the ground segment, each of the two or more return user data substreams from a respective ground station from which the two or more return user data substreams were received; reconstructing the return user data stream from the return user data stream for forwarding towards a target destination; A method comprising:

68. 1. A satellite communications system, comprising: a ground segment including a plurality of geographically dispersed ground stations, the ground segment further including an interface and processing circuitry, the interface and processing circuitry comprising: receiving, at a processing node of the ground segment, inbound user data streams each targeted to a respective user terminal served by the satellite communications system; for each of the inbound user data streams: block-encoding the inbound user data stream and dividing each coded data block into different subsets of coded data to form a corresponding set of two or more forward user data sub-streams; employing forward spatial diversity transmission for one or more of the inbound user data streams by mapping each forward user data substream to a respective one of two or more forward beam signals respectively corresponding to two or more forward user beams of the satellite communications system having respective forward user beam coverage areas surrounding the location of the user terminal targeted by the inbound user data stream; 1. A satellite communications system configured to transmit each forward beam signal from the ground segment to the space segment via a different optical forward uplink signal originating from a different ground station of the plurality of geographically dispersed ground stations, each forward uplink signal multiplexing a plurality of forward optical channel signals carrying a respective copy of the forward beam signal weighted for beamforming transmission from a respective antenna element of a target antenna array in the space segment for far-field forming of the corresponding forward user beam.

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