System and method for end-to-end beamforming
Methods and systems are described for providing end-to-end beamforming. For example, an end-to-end beamforming system include a relay satellite and a ground network to provide communications to user terminals located in user beam coverage areas. The ground network includes geographically distributed access nodes and a central processing system (CPS). Beamformers of the ground network generate forward uplink signals from appropriately weighted combinations of user data streams that, after relay by the satellite, produce forward downlink signals that combine to form forward user beams.
1. A method of operation by a satellite communications system for forming forward user beams, for serving user terminals in corresponding forward user beam coverage areas, the method comprising:
transmitting channel sounding probes from a plurality of geographically distributed access nodes, wherein an associated satellite includes a plurality of cooperating transmit signal paths that each relays a unique superposition of the channel sounding probes;
receiving amplitude and phase feedback from each designated terminal among a plurality of designated user terminals, the amplitude and phase feedback from each designated user terminal based on reception by the designated user terminal of the channel sounding probes as relayed via the plurality of cooperating transmit signal paths and indicative of an end-to-end forward channel between each access node and the designated user terminal;
calculating forward beam weights for weighting a plurality of forward uplink signals synchronously transmitted from the plurality of access nodes, wherein the forward beam weights are calculated in dependence on end-to-end forward gains estimated from the amplitude and phase feedback, so that superpositions of the forward uplink signals relayed by the satellite form a plurality of forward user beams, each forward user beam centered on a respective one among the plurality of designated user terminals; and
synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes according to the forward beam weights.
2. The method according to claim 1 , further comprising maintaining the forward beam weights on an ongoing basis for ongoing transmission of the plurality of forward uplink signals, based on repeatedly transmitting channel sounding probes, receiving corresponding amplitude and phase feedback, and recalculating the forward beam weights.
3. The method according to claim 1 , wherein each access node transmits a respective one among the plurality of forward uplink signals based on receiving a respective access-node specific forward signal from a Central Processing System (CPS) in a ground network of the satellite communications system, and wherein synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes comprises the CPS performing delay equalization by sending the respective access-node specific forward signals at respective time-domain offsets, the respective time-domain offsets accounting for path delay differences among the plurality of access nodes.
4. The method according to claim 1 , wherein each access node transmits a respective one among the plurality of forward uplink signals based on receiving a respective access-node specific forward signal from a Central Processing System (CPS) in a ground network of the satellite communications system, and wherein synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes comprises the CPS performing delay equalization by sending respective time-domain offsets to be applied by corresponding ones among the plurality of access nodes for transmission of the respective forward uplink signals, the respective time-domain offsets accounting for path delay differences among the plurality of access nodes.
5. The method according to claim 1 , wherein synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes comprises each access node determining a transmission timing for the forward uplink signal transmitted by the access node based on the access node based on aligning a beacon signal transmitted by the access node with a corresponding relay beacon signal transmitted by the satellite.
6. The method according to claim 5 , wherein the forward uplink signal transmitted by each access node corresponds to a respective access-node specific forward signal received from a Central Processing System (CPS) of the satellite communications system, the access-node specific forward signal containing time stamps indicating desired signal arrival times at the satellite, and wherein the access node determines the transmission timing for the forward uplink signal transmitted by the access node based on aligning signal samples contained in the access-node specific forward signal with the beacon signal according to the time stamps.
7. The method according to claim 1 , wherein calculating the forward beam weights for weighting the plurality of forward uplink signals comprises generating a M×K forward beam weight matrix, where M is the number of access nodes in the plurality of access nodes and K is the number of forward user beams in the plurality of forward user beams, such that each column of the M×K forward beam weight matrix corresponds to one of the K forward user beams and comprises a vector of M access-node specific weights calculated such that simultaneous transmission of a signal from the M access nodes according to the vector of M access-node specific weights results in the plurality of cooperating transmit signal paths of the satellite relaying superpositions of the signal that form the corresponding one among the K forward user beams.
8. The method according to claim 7 , wherein synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes according to the forward beam weights comprises:
splitting each forward beam signal among K forward beam signals into a corresponding set of M duplicate signals, such that there are K sets of M duplicate signals, and wherein each one of the K forward beam signals corresponds to a respective one among the K forward user beams and carries data for user terminals served by the respective forward user beam;
for each one of the K sets of M duplicate signals, forming a corresponding set of weighted signals by weighting each duplicate signal in the set by the corresponding element from the corresponding column of the M×K forward beam weight matrix;
forming M access-node specific signals by, for each one of the M access nodes, combining the weighted signals across the K sets of M weighted signals that correspond to the access node; and
distributing the M access-node specific signals to the M access nodes.
9. The method according to claim 1 , wherein the end-to-end forward gains estimated from the amplitude and phase feedback account for forward gains from the plurality of access nodes to the plurality of cooperating signal paths onboard the satellite, forward gains of the cooperating signal paths, and forward gains from the plurality of cooperating signal paths to the designated terminals.
10. A satellite communications system configured to form forward user beams, for serving user terminals in corresponding forward user beam coverage areas, the satellite communications system comprising:
a ground network comprising a Central Processing System (CPS) communicatively coupled to a plurality of geographically distributed access nodes;
a satellite comprising a plurality of cooperating transmit signal paths; and
wherein the ground network is configured to:
transmit channel sounding probes via the plurality of access nodes, wherein each one among the plurality of cooperating transmit signal paths onboard the satellite relays a unique superposition of the channel sounding probes;
receive amplitude and phase feedback via the plurality of access nodes, respective amplitude and phase feedback received from each designated terminal among a plurality of designated user terminals, the amplitude and phase feedback received from each designated user terminal based on reception by the designated user terminal of the channel sounding probes as relayed via the plurality of cooperating transmit signal paths and indicative of an end-to-end forward channel between each access node and the designated user terminal;
calculate forward beam weights for weighting a plurality of forward uplink signals synchronously transmitted from the plurality of access nodes, wherein the forward beam weights are calculated in dependence on end-to-end forward gains estimated from the amplitude and phase feedback, so that superpositions of the forward uplink signals relayed by the satellite form a plurality of forward user beams, each forward user beam centered on a respective one among the plurality of designated user terminals; and
synchronously transmit the plurality of forward uplink signals from the plurality of access nodes according to the forward beam weights.
11. The satellite communications system according to claim 10 , wherein the CPS includes circuitry that is configured to maintain the forward beam weights on an ongoing basis for ongoing transmission of the plurality of forward uplink signals, based on repeatedly transmitting channel sounding probes, receiving corresponding amplitude and phase feedback, and recalculating the forward beam weights.
12. The satellite communications system according to claim 10 , wherein each access node is configured to transmit a respective one among the plurality of forward uplink signals based on receiving a respective access-node specific forward signal from the CPS, and wherein, for synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes, the CPS includes circuitry configured to perform delay equalization, based on sending the respective access-node specific forward signals at respective time-domain offsets, the respective time-domain offsets accounting for path delay differences among the plurality of access nodes.
13. The satellite communications system according to claim 10 , wherein each access node is configured to transmit a respective one among the plurality of forward uplink signals based on receiving a respective access-node specific forward signal from the CPS, and wherein, for synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes, the CPS includes circuitry configured to perform delay equalization, based on sending respective time-domain offsets to be applied by corresponding ones among the plurality of access nodes for transmission of the respective forward uplink signals, the respective time-domain offsets accounting for path delay differences among the plurality of access nodes.
14. The satellite communications system according to claim 10 , wherein, for synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes, each access node includes circuitry configured to determine a transmission timing for the forward uplink signal transmitted by the access node, based on aligning a beacon signal transmitted by the access node with a corresponding relay beacon signal transmitted by the satellite.
15. The satellite communications system according to claim 14 , wherein the forward uplink signal transmitted by each access node corresponds to a respective access-node specific forward signal received from the CPS, the access-node specific forward signal containing time stamps indicating desired signal arrival times at the satellite, and wherein the access node is configured to determine the transmission timing for the forward uplink signal transmitted by the access node based on aligning signal samples contained in the access-node specific forward signal with the beacon signal, according to the time stamps.
16. The satellite communications system according to claim 10 , wherein the CPS includes circuitry configured to calculate the forward beam weights as a M×K forward beam weight matrix, where M is the number of access nodes in the plurality of access nodes and K is the number of forward user beams in the plurality of forward user beams, such that each column of the M×K forward beam weight matrix corresponds to one of the K forward user beams and comprises a vector of M access-node specific weights calculated such that simultaneous transmission of a signal from the M access nodes according to the vector of access-node specific weights results in the plurality of cooperating transmit signal paths of the satellite relaying superpositions of the signal that form the corresponding one of the K forward user beams.
17. The satellite communications system according to claim 16 , wherein, for synchronously transmitting the plurality of forward uplink signals from the plurality of access nodes according to the forward beam weights, the CPS includes circuitry configured to:
split each forward beam signal among K forward beam signals into a corresponding set of M duplicate signals, such that there are K sets of M duplicate signals, and wherein each one of the K forward beam signals corresponds to a respective one among the K forward user beams and carries data for user terminals served by the respective forward user beam;
for each one of the K sets of M duplicate signals, form a corresponding set of weighted signals by weighting each duplicate signal in the set by a corresponding element from the corresponding column of the M×K forward beam weight matrix;
form M access-node specific signals by, for each one of the M access nodes, combining the weighted signals across the K sets of M weighted signals that correspond to the access node; and
distribute the M access-node specific signals to the M access nodes.
18. The satellite communications system according to claim 10 , wherein the end-to-end forward gains estimated from the amplitude and phase feedback account for forward gains from the plurality of access nodes to the plurality of cooperating signal paths onboard the satellite, forward gains of the cooperating signal paths, and forward gains from the plurality of cooperating signal paths to the designated terminals.