System and method for return end-to-end beamforming
Methods and systems are described for providing end-to-end beamforming. For example, end-to-end beamforming systems include end-to-end relays and ground networks to provide communications to user terminals located in user beam coverage areas. The ground segment can include geographically distributed access nodes and a central processing system. Return uplink signals, transmitted from the user terminals, have multipath induced by a plurality of receive/transmit signal paths in the end to end relay and are relayed to the ground network. The ground network, using beamformers, recovers user data streams transmitted by the user terminals from return downlink signals. The ground network, using beamformers generates forward uplink signals from appropriately weighted combinations of user data streams that, after relay by the end-end-end relay, produce forward downlink signals that combine to form user beams.
1. A method of operation by a satellite communication system, the method comprising:
receiving a plurality of composite return downlink signals, with each composite return downlink signal provided from a respective one among a plurality of geographically distributed access nodes, with each composite return downlink signal being a unique received superposition of return downlink signals transmitted by respective ones among a plurality of cooperating receive/transmit paths of a satellite of the satellite communication system, with each return downlink signal transmitted by the satellite being a relayed version of a composite return uplink signal received by the corresponding receive/transmit path, and with each composite return uplink signal being a unique received superposition of return uplink signals transmitted by respective user terminals among a plurality of user terminals located within a user coverage area that is logically subdivided into a plurality of return user beam coverage areas;
calculating return beam weights in dependence on end-to-end return gains estimated from phases and amplitudes of known signals as received by the access nodes, wherein one or more designated terminals in each return user beam coverage area transmit respective ones of the known signals for relayed transmission to the plurality of access nodes via the plurality of receive/transmit signal paths of the satellite;
applying the return beam weights to the plurality of composite return downlink signals, to obtain a plurality of return user beam signals, each return user beam signal corresponding to a respective one among the return user beam coverage areas and increasing a signal-to-noise ratio of the return uplink signals originating from user terminals in the corresponding return user beam coverage area; and
recovering return user traffic originating from respective user terminals in each return user beam coverage area, from the return user beam signal corresponding to the return user beam coverage area.
2. The method according to claim 1 , wherein applying the return beam weights to the plurality of composite return downlink signals, to obtain the plurality of return user beam signals, comprises forming each return user beam signal as a correspondingly weighted combination of the plurality of composite return downlink signals.
3. The method according to claim 2 , further comprising synchronizing the plurality of composite return downlink signals before combining them.
4. The method according to claim 3 , wherein synchronizing the plurality of composite return downlink signals comprises accounting for respective path delay differences among the plurality of geographically distributed access nodes with respect to the satellite.
5. The method according to claim 4 , further comprising determining the respective path delay differences among the plurality of geographically distributed access nodes according to a beacon signal received from the satellite by one or more of the access nodes.
6. The method according to claim 1 , wherein calculating the return beam weights comprises calculating a set of return beam weights that account for multi-path return channels through the satellite from each designated terminal to each access node.
7. The method according to claim 1 , wherein the satellite comprises L cooperating receive/transmit paths, wherein there are K designated terminals, each one corresponding to a respective one among K return user beam coverage areas, wherein there are M distributed access nodes, and wherein calculating the return beam weights comprises calculating a M×K return channel matrix that models the L end-to-end return channels from each one of the K designated terminals to each one of the M distributed access nodes, and computing the return beam weights from the M×K return channel matrix.
8. The method according to claim 7 , wherein the M×K return channel matrix is computed as C t ×E ret ×A r , where E ret is a L×L payload matrix representing the L cooperating receive/transmit paths, Ar is a L×K return uplink radiation matrix relating each one of the K designated terminals to each one of the L cooperating receive/transmit paths, and C t is a M×L return downlink radiation matrix that relates each one of the L cooperating receive/transmit paths to each one of the M distributed access nodes.
9. The method according to claim 7 , wherein the return beam weights comprise a K×M return beam weight matrix, with each row corresponding to the k-th one of the K return user beams and each column corresponding to the m-th one of the M distributed access nodes, and wherein obtaining the return beam signal corresponding to the k-th return user beam coverage area comprises weighting the composite return downlink signal from each of the M distributed access nodes by the corresponding matrix element in the k-th row of the K×M return beam weight matrix and combining the resulting M weighted composite return downlink signals.
10. A central processing system (CPS) of a satellite communication system, the CPS comprising:
an interface configured to receive a plurality of composite return downlink signals, with each composite return downlink signal provided by a respective one among a plurality of geographically distributed access nodes and being a unique received superposition of return downlink signals transmitted by respective ones among a plurality of cooperating receive/transmit paths of a satellite of the satellite communication system, with each return downlink signal transmitted by the satellite being a relayed version of a composite return uplink signal received by the corresponding receive/transmit path, and with each composite return uplink signal being a unique received superposition of return uplink signals transmitted by respective user terminals among a plurality of user terminals located within a user coverage area that is logically subdivided into a plurality of return user beam coverage areas; and
processing circuitry configured to:
calculate return beam weights in dependence on end-to-end return gains estimated from phases and amplitudes of known signals as received by the access nodes, wherein one or more designated terminals in each return user beam coverage area transmit respective ones of the known signals for relayed transmission to the plurality of access nodes via the plurality of receive/transmit signal paths of the satellite;
apply the return beam weights to the plurality of composite return downlink signals, to obtain a plurality of return user beam signals, each return user beam signal corresponding to a respective one among the return user beam coverage areas and increasing a signal-to-noise ratio of the return uplink signals originating from user terminals in the corresponding return user beam coverage area; and
recover return user traffic originating from respective user terminals in each return user beam coverage area, from the return user beam signal corresponding to the return user beam coverage area.
11. The CPS according to claim 10 , wherein, with respect to applying the return beam weights to the plurality of composite return downlink signals to obtain the plurality of return user beam signals, the processing circuitry is configured to form each return user beam signal as a correspondingly weighted combination of the plurality of composite return downlink signals.
12. The CPS according to claim 11 , wherein the processing circuitry is configured to synchronize the plurality of composite return downlink signals before combining them.
13. The CPS according to claim 12 , wherein the processing circuitry is configured to synchronize the plurality of composite return downlink signals to account for respective path delay differences among the plurality of geographically distributed access nodes with respect to the satellite.
14. The CPS according to claim 13 , wherein the processing circuitry is configured to determine the respective path delay differences among the plurality of geographically distributed access nodes according to a beacon signal received from the satellite by one or more of the access nodes.
15. The CPS according to claim 10 , wherein, to calculate the return beam weights, the processing circuitry is configured to calculate a set of return beam weights that account for multi-path return channels through the satellite from each designated terminal to each access node.
16. The CPS according to claim 10 , wherein the satellite comprises L cooperating receive/transmit paths, wherein there are K designated terminals, each one corresponding to a respective one among K return user beam coverage areas, wherein there are M distributed access nodes, and wherein, to calculate the return beam weights, the processing circuitry is configured to calculate a M×K return channel matrix that models the L end-to-end return channels from each one of the K designated terminals to each one of the M distributed access nodes and compute the return beam weights from the M×K return channel matrix.
17. The CPS according to claim 16 , wherein the M×K return channel matrix is computed as C t ×E ret ×A r , where E ret is a L×L payload matrix representing the L cooperating receive/transmit paths, A r is a L×K return uplink radiation matrix relating each one of the K designated terminals to each one of the L cooperating receive/transmit paths, and C t is a M×L return downlink radiation matrix that relates each one of the L cooperating receive/transmit paths to each one of the M distributed access nodes.
18. The CPS according to claim 16 , wherein the return beam weights comprise a K×M return beam weight matrix, with each row corresponding to the k-th one of the K return user beams and each column corresponding to the m-th one of the M distributed access nodes, and wherein, to obtain the return beam signal corresponding to the k-th return user beam coverage area, the processing circuitry is configured to weight the composite return downlink signal from each of the M distributed access nodes by the corresponding matrix element in the k-th row of the K×M return beam weight matrix and combine the resulting M weighted composite return downlink signals.