Methods and apparatuses for supporting an optical feeder link in a satellite communications system

Abstract

A satellite communications system (SCS) uses one or more optical phase locked loops (OPLLs) and one or more optical frequency combs (OFCs) for the transmission and / or reception of one or more pluralities of optical communication signals going between an optical ground station (OGS) and a satellite. Use of a pilot tone with an associated OPLL / OFC arrangement reduces the number of individual laser modules or sources needed for generation of the optical communication signals, which is particularly advantageous at the satellite, and it provides an advantageous mechanism for locking the respective carrier frequency and phase each optical communication signal to a high-quality source.

Description

METHODS AND APPARATUSES FOR SUPPORTING AN OPTICAL FEEDER LINK IN A SATELLITE COMMUNICATIONS SYSTEMTECHNICAL FIELD

[0001] Methods and apparatuses disclosed herein relate to optical feeder links in a satellite communications system.BACKGROUND

[0002] Optical feeder links (OFLs) offer significant advantages for satellite communications systems (SCSs), not least being wide bandwidths supporting high data rates. Use of OFLs, however, brings multiple challenges. For example, generating multiple optical signals at different wavelengths oftentimes relies on the use of a corresponding number of laser sources, which consumes significant space and power, and increases cost. Similar disadvantages exist in the reception context, where a conventional approach to receiving multiple optical communication signals relies on having multiple laser sources corresponding to the different wavelengths of the received optical communication signals.SUMMARY

[0003] A satellite communications system (SCS) uses one or more optical phase locked loops (OPLLs) and one or more optical frequency combs (OFCs) for the transmission and / or reception of one or more pluralities of optical communication signals going between an optical ground station (OGS) and a satellite. Use of a pilot tone with an associated OPLL / OFC arrangement reduces the number of individual laser modules or sources needed for generation of the optical communication signals, which is particularly advantageous at the satellite, and it provides an advantageous mechanism for locking the respective carrier frequency and phase of each optical communication signal to a high-quality source.

[0004] One embodiment disclosed herein comprises a method of operation at a satellite of a SCS. The method includes the satellite: receiving a plurality of optical communication signals via free space propagation from a ground segment of the SCS, each optical communication signal having a corresponding optical carrier frequency and conveying one or more information signals for relaying via the satellite; receiving an optical pilot signal in conjunction with the plurality of optical communication signals, the optical pilot signal conveying a reference tone; generating a plurality of optical reference carriers at the corresponding optical carrier frequencies of the plurality of optical communication signals by recovering the reference tone from theoptical pilot signal, locking a local laser to a carrier phase and frequency of the optical pilot signal, and inputting light from the locked local laser into an optical frequency comb modulated via a local oscillator (LO) signal derived from the reference tone; and recovering the information signals by performing coherent detection on each optical communication signal among the plurality of optical communication signals, using the corresponding optical reference carrier.

[0005] A related embodiment comprises a satellite configured for operation in a SCS. The satellite includes a laser communication terminal (LCT) that is configured to receive a plurality of optical communication signals via free space propagation from a ground segment of the SCS. Each optical communication signal has a corresponding optical carrier frequency and conveys one or more information signals for relaying via the satellite. The LCT is further configured to receive an optical pilot signal in conjunction with the plurality of optical communication signals, where the optical pilot signal conveys a reference tone.

[0006] The satellite further includes an optical receiver configured to: generate a plurality of optical reference carriers at the corresponding optical carrier frequencies of the plurality of optical communication signals by recovering the reference tone from the optical pilot signal, locking a local laser to a carrier phase and frequency of the optical pilot signal, and inputting light from the locked local laser into an optical frequency comb modulated via a local oscillator (LO) signal derived from the reference tone; and recover the information signals by performing coherent detection on each optical communication signal among the plurality of optical communication signals, using the corresponding optical reference carrier.

[0007] Another embodiment disclosed herein comprises a method of operation at a satellite of a SCS. The method includes the satellite: receiving RF uplink signals from respective ones among a plurality of terminals operating in a satellite service area associated with the satellite, the RF uplink signals impinging on an antenna system of the satellite, with the antenna system outputting corresponding antenna-received signals; generating a plurality of optical reference carriers by inputting light from a local laser into an optical frequency comb, with the optical frequency comb modulated via a local oscillator (LO) signal derived from a reference tone conveyed by an optical pilot signal received from an OGS of the SCS; impressing respective ones among a plurality of information signals onto respective ones of the optical reference carriers, to form a plurality of optical communication signals, each information signal comprising or derived from one or more respective ones among the antenna signals; and transmitting the plurality of optical communication signals to the OGS.

[0008] A related embodiment comprises a satellite that is configured to operate in a SCS. The satellite includes an antenna system configured to receive RF uplink signals from respective ones among a plurality of terminals operating in a satellite service area associated with the satellite, the antenna system operative to output corresponding antenna-received signals. Further, the satellite includes an optical transmitter configured to: generate a plurality of optical reference carriers by inputting light from a local laser into an optical frequency comb, with the optical frequency comb modulated via a local oscillator (LO) signal derived from a reference tone conveyed by an optical pilot signal received from an OGS of the SCS; impress respective ones among a plurality of information signals onto respective ones of the optical reference carriers, to form a plurality of optical communication signals, each information signal comprising or derived from one or more respective ones among the antenna-received signals; and output the plurality of optical communication signals for transmission via a laser communication terminal (LCT) of the satellite, to the OGS.

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

[0010] Figure 1 is a block diagram of a satellite communications system (SCS), according to a one embodiment.

[0011] Figure 2 is a block diagram of details for a satellite, according to one embodiment.

[0012] Figure 3 is a block diagram of details for an optical ground station (OGS), according to one embodiment.

[0013] Figure 4 is a block diagram of further details for a satellite, according to one embodiment.

[0014] Figures 5A and 5B are block diagrams of an optical phase locked loop (OPLL) and associated optical frequency comb (OFC), according to example embodiments.

[0015] Figure 6 is a block diagram of further details for a satellite, according to one embodiment.

[0016] Figures 7 and 8 are logic flow diagrams illustrating methods of operation by a satellite, according to one or more embodiments.

[0017] Figures 9 and 10 are block diagrams of further details for a satellite, according to one or more embodiments.DETAILED DESCRIPTION

[0018] Figure 1 illustrates an example satellite communications system (SCS) 10 configured to provide one or more communication services to user terminals (UTs) 12. For example, the SCS 10 operates as an access network for communicatively coupling the UTs 12 to one or more external networks 14, such as the Internet.

[0019] The SCS 10 includes a ground segment 16 and a space segment 18. The ground segment 16, also referred to as a ground network, includes one or more optical ground stations (OGSs) 20. Only one OGS 20 appears for convenience of illustration and discussion, but the SCS 10 in one or more embodiments includes a plurality of geographically distributed OGSs 20. With geographical distribution, each OGS 20 experiences its own local weather and atmospheric conditions. Similarly, although Figure 1 depicts only one satellite 22 comprised in the space segment 18, the SCS 10 may include multiple such satellites 22, which may operate as one or more constellations of satellites. In at least one embodiment, the SCS 10 includes one or more satellites 22 in geosynchronous orbit. In at least one other embodiment, the SCS 10 includes a plurality of satellites 22 operating as a non-geosynchronous constellation of satellites, e.g., a low-earth orbit (LEO) constellation. In yet other embodiments, the SCS 10 includes a mix of satellites 22, in GEO and non-GEO orbits.

[0020] Having multiple OGSs 20 in different locations helps to ensure that one or more OGSs 20 at any given time are weather uncorrelated and can support an optical feeder link (OFL) 24 with the satellite 22. With respect to the illustrated OGS 20 and satellite 22, the OFL 24 includes one or more optical uplink signals 26 going from the OGS 20 to the satellite 22, and one or more optical downlink signals 28 going from the satellite 22 to the OGS 20. The optical uplink signals 26 comprise, for example, one or more optical communication signals carrying forward traffic — e.g., data packets — for respective UTs 12 and one or more other optical signals, such as pilot or other reference signals and, possibly, signals for optical alignment or control. Similarly, the optical downlink signals 28 may comprise one or more optical communication signals carrying return traffic — e.g., data packets — from respective UTs 12, along with one or more other optical signals, e.g., for reference, alignment, control, etc.

[0021] The satellite 22 in one or more embodiments is configured to provide communications coverage to a potentially large satellite service area 30, such as the continentalUnited States or other large land area. In at least one such embodiment, the satellite 22 is configured to use a plurality of user beams 32 to cover respective portions — sub-areas — of the satellite service area 30. Each user beam 32 has a corresponding user beam coverage area 34, with each user beam 32 serving a respective subset of UTs 12 among an overall population of UTs 12 distributed over the satellite service area 30. More particularly, UTs 12 that are in a particular user beam coverage area 34 are assigned to the corresponding user beam 32. In this sense, different user beams 32 serve different subsets of UTs 12. The particular user beam 32 used to carry traffic to or from a given UT 12 may be referred to as a “serving beam” for that given UT 12. Likewise, particular satellite 22 associated with the serving beam may be referred to as the “serving satellite.”

[0022] In the transmit context, a user beam 32 may be understood as a directional or shaped transmission of a radio signal bearing information targeting one or more UTs 12 served by the beam. In the receive context, a user beam 32 may be understood as a directional or shaped reception of radio signals bearing information from one or more UTs 12. In one or more embodiments, with an overall population of UTs 12 transmitting return uplink signals, the SCS 10 forms a given return user beam by processing the combination of return uplink signals received at the satellite 22 to enhance the signal-to-noise ratio (SNR) of those return uplink signals originating from the UTs 12 operating in a particular user beam coverage area 34. In such implementations, the return user beams exist only in the signal processing domain — i.e., the signal weighting occurs within the digital processing domain, rather than in the analog domain involving the antenna-received signals.

[0023] Although Figure 1 does not label the user beams 32 as forward user beams or return user beams, it should be understood that the SCS 10 in one or more embodiments uses forward user beams for transmitting forward traffic to an overall population of UTs 12 and uses return user beams for receiving return traffic from that population. In at least one embodiment of the SCS 10, the user beam coverage areas 34 are the same for forward and return user beams. In at least one other embodiment, the forward and return user beam coverage areas differ. Thus, the depicted user beams 32 provide a generalized, example representation of a respective set of forward or return user beams.

[0024] The satellite 22 includes an OFL interface 40 for reception of the one or more optical uplink signals 26 and transmission of the one or more optical downlink signals 28. The satellite 22 further includes a payload 42 for generating the radiofrequency (RF) downlink signalstransmitted via the user antenna system(s) 44 for the UTs 12, and receiving the RF uplink signals incoming to the user antenna system(s) 44 from the UTs 12.

[0025] In one or more embodiments, the optical uplink signals 26 include an incoming composite optical signal comprising a plurality of forward optical channel signals according to wavelength division multiplexing (WDM). Each forward optical channel signal corresponds to a respective optical wavelength and carries one or more respective forward communication signals, i.e., forward traffic for UTs 12. In at least one embodiment, the involved OGS 20 forms each forward optical channel signal by modulating an optical carrier according to a respective one among multiple forward communication signals carrying forward data streams for different UTs 12 or groups of UTs 12. The satellite 22 recovers each forward communication signal via detection of the optical modulation.

[0026] In at least one embodiment, the forward communication signals are recovered from the composite optical signal as electrical domain signals and are retransmitted by the payload 42 without digital processing. That is, in at least one embodiment, the payload 42 is a bent-pipe payload that filters, amplifies, and, optionally, frequency-translates, the forward communication signals recovered from the optical channel signals, for transmission by the user antenna system(s) 44 as radiofrequency (RF) downlink signals for reception by the UTs 12.

[0027] The user antenna system(s) 44 comprise, for example, directional radiofrequency (RF) antennas, such as feed horns and reflector assemblies, for beam-based transmission of forward RF downlink signals to respective UTs 12 and reception of return RF uplink signals from respective UTs 12. In at least one embodiment, the user antenna system(s) 44 comprise one or more phased array antennas for forward and / or return beamforming. Other elements onboard the satellite 22 include the satellite bus comprising power and spacecraft control systems, with such details being well understood in the art and not shown in the figure.

[0028] On the ground “end” or “side” of the OFL 24, the OGS 20 includes an OFL interface 50, transceiver circuitry 52, and a core network (CN) interface 54 that communicatively couples the OGS 20 to the CN 56. Communications processing by the one or more computer servers or other processing nodes comprised in the CN 56 includes forming forward data streams for respective UTs 12 and distributing those forward data streams to the OGS(s) 20 that have OFL coupling to the satellite(s) 22 used for serving the respective UTs 12. In the return direction, a given OGS 20 having OFL coupling to a given satellite 22 receives return communication signals conveying return data streams from respective UTs 12 or groups of UTs 12.

[0029] In at least one embodiment, the one or more optical downlink signals 28 include a composite optical signal comprising a plurality of return optical channel signals according to a WDM arrangement. Each return optical channel signal conveys one or more return communication signals, with the OGS 20 recovering these return communication signals from the plurality of return optical channel signals and transmitting them, or the contained return data streams, to the CN 56. In such embodiments, the OFL interface 50 provides the transceiver circuitry with the return communication signals. In turn, the CN interface 54 sends the return communication signals, or packetized versions thereof, back to the CN 56 for processing and forwarding to the external network(s) 14.

[0030] In one or more embodiments, transmission and reception of the one or more optical uplink signals 26 and the one or more optical downlink signals 28 is based on the use of pluralities of optical reference carriers at corresponding optical carrier frequencies. In at least one such embodiment, the OFL interface 40 onboard the satellite 22 includes one or more optical phase locked loops (OPLLs) 60 and one or more optical frequency combs (OFCs) 62 that provide one or more such pluralities of optical reference carriers 64 to an optical transceiver 66 included in the OFL interface 40. Among the various advantages of such arrangements is the dramatic reduction in the number of onboard laser modules needed by the satellite 22, and the ability to slave the plurality or pluralities of optical reference carriers 64 to a high-quality laser source in the OGS 20.

[0031] At least some of the same advantages carry over to the OGS 20, which in one or more embodiments includes one or more OPLLs 70 and one or more OFCs 72 that generate one or more pluralities of optical reference carriers 74. An optical transceiver 76 included in the OFL interface 50 of the OGS 20 uses the one or more pluralities of optical reference carriers 74 in transmission of the one or more optical uplink signals 26 and / or in the reception of the one or more optical downlink signals 28.

[0032] Figure 2 illustrates example details for the satellite 22, where a laser communication terminal (LCT) 80 is configured for reception of the one or more optical uplink signals 26 and transmission of the one or more optical downlink signals 28. An optical receiver 82 and an optical transmitter 84 represent the optical transceiver 66 shown in Figure 1 and, collectively, the LCT 80, the optical receiver 82, and the optical transmitter 84 represent the OFL interface 40 shown in Figure 1. Further, the forward (FWD) transponders 86 and the return (RET) transponders 88 represent the payload 42 shown in Figure 1. Here, “transponder” refers to theseries of interconnected units that form a communications channel or pathway between the OFL interface 40 and the user antenna system(s) 44.

[0033] In an example relating to the forward direction, the LCT 80 includes one or more telescopes for receiving the optical uplink signal(s) 26, and further includes an optical path comprising, for example, one or more mirrors, beam splitters, collimators, etc., for directing the received optical uplink signal(s) 26 into the optical receiver 82 as one or more received optical signals 90. For example, the LCT 80 filters and collimates the optical uplink signal(s) 26 and directs them into a fiber optic cable coupling the LCT 80 with the optical receiver 82. In at least one embodiment, the one or more received optical signals 90 comprise a composite of multiple optical channel signals occupying different optical wavelengths, and the optical receiver 82 outputs one or more corresponding electrical domain received signals 92. The forward transponders 86 act on the one or more electrical domain received signals 92 to form one or more transmit signals 94 that are applied to the user antenna system(s) 44 for transmission as RF downlink signals 96. These RF downlink signals 96 may be transmitted as, or in such a way as to form, the user beams 32 depicted in Figure 1.

[0034] In an example relating to the return direction, the user antenna system(s) 44 is / are configured to receive RF uplink signals 98 from UTs 12 operating at various locations within the overall satellite service area 30 shown in Figure 1. The user antenna system(s) 44 output corresponding antenna-received signals 100, which are acted on by the return transponders 88, to produce one or more electrical domain transmit signals 102. The optical transmitter 84 forms one or more optical transmit signals 104, for emission from the LCT 80 as the one or more optical downlink signals 28. In at least one embodiment, the one or more optical transmit signals 104 comprise a composite optical signal containing a multiplexed plurality of transmit optical channel signals at respective optical wavelengths. The LCT 80 includes one or more optical paths for directing the optical transmit signal(s) 104 for output from an optical telescope of the LCT 80 as the one or more optical downlink signals 28.

[0035] As shown in Figure 2, the optical receiver 82 in one or more embodiments includes at least one OPLL 60 and at least one OFC 62, which are used to produce a plurality of optical reference carriers — not explicitly shown in the diagram — for use in generating or otherwise recovering the electrical domain received signal(s) 92 from the received optical signal(s) 90. The optical transmitter 84 in one or more embodiments includes one or more OFCs 62, for producing a plurality of optical reference carriers that are used forming the optical transmit signal(s) 104 incorrespondence with the electrical domain transmit signals 102. The one or more OFCs 62 may use a reference RF tone derived from an optical pilot signal provided by the OGS 20. The signals 102 may be referred to as information signals 102, to denote the fact that they carry return traffic from the UTs 12.

[0036] Functionally similar arrangements may be used at the OGS 20, such as the arrangements illustrated in Figure 3. A LCT 110 is configured for reception of the one or more optical downlink signals 28 and transmission of the one or more optical uplink signals 26. The LCT 110 may be configured similarly to the LCT 80, as described above. An optical receiver 112 and an optical transmitter 114 represent the optical transceiver 76 shown in Figure 1 and, collectively, the LCT 1 10, the optical receiver 1 12, and the optical transmitter 1 14 represent the OFL interface 50 shown in Figure 1. Further, the RX circuitry 116 and the TX circuitry 118 represent the transceiver 52 shown in Figure 1.

[0037] In an example embodiment relating to operation in the forward direction, the CN interface 54 receives forward signals 120 from the CN 56, with the CN interface 54 providing forward data streams 122, e.g., such as by de-packetizing digital data values or otherwise recovering the forward data streams 122 from the incoming forward signals 120. The TX circuitry 118 outputs electrical domain transmit signals 124 corresponding to the forward data streams 122, and the optical transmitter 114 outputs one or more outgoing optical signals 126, for output from the LCT 110 as the one or more optical uplink signals 26.

[0038] In an example relating to the return direction, the LCT 110 includes one or more telescopes for receiving the optical downlink signal(s) 28, and further includes an optical path comprising, for example, one or more mirrors, beam splitters, collimators, etc., for directing the received optical downlink signal(s) 28 into the optical receiver 112 as one or more received optical signals 130. For example, the LCT 110 filters and collimates the optical downlink signal(s) 28 and directs them into a fiber optic cable coupling the LCT 110 with the optical receiver 112. In at least one embodiment, the one or more received optical signals 130 comprise a composite of multiple optical signals occupying different optical wavelengths, and the optical receiver 112 outputs one or more corresponding electrical domain received signals 132. The RX circuitry 116 acts on the one or more electrical domain received signals 132 to form one or more return signals 134 that are processed or otherwise formatted by the CN interface 54 for transmission to the CN 56 as return signals 136.

[0039] Figure 4 provides implementation details for the satellite 22 in one or more embodiments and relates to the return link direction where the satellite receives return uplink signals and relays them via optical carrier to the ground. The diagram assumes that two pairs of RF segments — four RF segments — are being transported per optical carrier, e.g., with two segments at one RF frequency and two at another RF frequency, such as Ka and V bands. Depending upon SNR requirements and available transmit power, more RF segments may be used per optical carrier. Further, the use of smaller segments — less bandwidth per RF segment — would allow for carrying more segments per optical carrier.

[0040] RF uplink signals 98 impinge on the user antenna system(s) 44, which outputs the corresponding antenna-received signals 100 to the return transponders 88. The RF uplink signals 98 comprise signals from various user terminals. The antenna-received signals 100 comprise, for example, outputs from respective elements of a phased array antenna.

[0041] In the depicted example, the return transponders 88 are bent-pipe transponders, which comprise analog electrical signal paths that perform no digitization and regeneration of the antenna-received signals 100. Here, the return transponders 88 include a RF segmenter 140 that dispatches different frequencies used in the return uplink, along with corresponding RF hybrids 141 and associated sets of frequency converters (FCs) 142, RF multiplexers 144, and RF amplifiers 146. The FCs 142 translate from respective ones of the RF uplink frequencies to an intermediate frequency (IF), for example. However, the FCs 142 are omitted in one or more embodiments.

[0042] The return transponders 88 output a plurality of electrical domain transmit signals 102, which can be understood as frequency-translated, multiplexed, and amplified versions of the antenna-received signals 100. The electrical domain transmit signals 102 are input to the optical transmitter 84, as respective modulation inputs to a plurality of optical in-phase / quadrature (I / Q) modulators 150. Each optical I / Q modulator 150 modulates a respective one among a plurality of optical reference carriers 64 according to the corresponding electrical domain transmit signal 102 input to it, to form a respective return optical channel signal 152. Each return optical channel signal 152 is at the optical wavelength of the particular optical reference carrier 64 that was modulated to form it. That is, the respective electrical domain transmit signals 102 can be understood as being impressed on or otherwise conveyed by respective ones among the plurality of return optical channel signals 152.

[0043] An optical multiplexer 154 performs wavelength division multiplexing of the plurality of return optical channel signals 152 to form a composite optical signal 156. An optical power amplifier (PA) amplifies the composite optical signal 156 to form the optical transmit signal(s) 104 shown in Figure 2. The optical transmit signal(s) 104 pass through one or more optical paths of the LCT 80 and exits a telescope of the LCT 80 as the one or more optical downlink signals 28. Here, it should be understood that the one or more optical downlink signals 28 may also include dedicated optical alignment signals for LCT-to-LCT alignment, which are at non-interfering wavelengths and which are not shown in the diagram.

[0044] Advantageously, the satellite 22 generates the plurality of optical reference carriers 64 using one or more OPLLs 60 and one or more OFCs 62. Particularly, there may be a practical limit regarding the number of optical reference carriers generated by a single OFC 62 and, to the extent that the total number of optical reference carriers 64 needed exceeds that limit, respective OFCs 62 among two or more OFCs 62 may be used to output respective subsets of the total number of optical reference carriers 64. All OFCs 62 may be driven by the same OPLL 60, or more than one OPLL 60 may be used.

[0045] The illustrated arrangement depicts an example OPLL 60 that outputs a reference laser signal 160 that is frequency and phase locked to a source laser beam 162. The OFC 62 generates a plurality of optical reference carriers 64, spaced equidistantly over a defined portion of the optical spectrum. The spacing of the optical reference carriers 64 is defined by a LO signal 166 that is input to the OFC 62. In at least one embodiment, the source laser beam 162 and a reference tone — e.g., a RF tone — are recovered from an optical signal received from an OGS 20. For example, in one embodiment, the one or more optical uplink signals 26 comprise a composite optical signal in which a plurality of forward optical channel signals are multiplexed. The forward optical channel signals include a plurality of optical communication signals carrying forward traffic for UTs 12, along with one or more optical pilot signals. Each optical pilot signal serves as a source laser beam 162 and may be modulated with a RF tone for extraction and use in deriving the LO signal 166 for use by the OFC 62.

[0046] In more detail, the OPLL 60 in one or more embodiments includes a local laser module 164 that outputs the reference laser signal 160, with the OPLL 60 including optoelectronic circuitry that locks the frequency and phase of the reference laser signal 160 to the source laser beam 162. Among the various advantages of this approach, the OGS 20 may be equipped with a relatively expensive, high quality laser module for stable, accurate generation ofthe laser signal that is received and used by the satellite 22 as the source laser beam 162, meaning that the local laser module 164 used in the satellite 22 may be relatively inexpensive and small in comparison.

[0047] Consider further example details where the one or more optical uplink signals 26 comprise a plurality of optical channel signals based on wavelength division multiplexing, where each optical channel signal is modulated to carry one or more RF segments to be transmitted as corresponding downlink user beams. In a particular example, each optical channel signal incoming as the one or more optical uplink signals carries a pair of RF segments offset to one side of the optical carrier frequency, and another pair of RF segments offsets to the other side of the optical carrier frequency. Because all the RF segments are locked in phase to their corresponding optical carrier, all the reference optical carriers generated by the same OFC 62 are locked to the corresponding source laser beam 162; aligning the possibly few laser sources is easier than aligning all the optical carriers individually, and results in an efficient alignment of the full communication spectrum.

[0048] Figure 5A illustrates example details for the OPLL 60, where the OPLL 60 includes a 180-degree hybrid coupler 170 that applies the source laser signal 162 and a portion of the reference laser signal 160 to a differential photodetector (PD) 172. With the illustrated connectivity for first and second photodiodes 174A and 174B of the differential photodetector 172, illumination of the first photodiode 174A with the source laser signal 162 and illumination of the second photodiode 174B with the reference laser signal 160 produces a differential current signal 176 that is a function of the frequency / phase difference between the two laser signals 160 and 162.

[0049] A transimpedance amplifier (TIA) 178 produces an error signal 180 in dependence on the differential current signal, with the error signal 180 also referred to as a feedback control signal. The error signal 180 in this example implementation is a voltage-mode signal having a beat frequency that is a function of the frequency / phase difference between the source laser signal 162 and the reference laser signal 160.

[0050] Thus, to the extent that the source laser signal 162 is received or otherwise derived from a high-quality, stable ground-based laser signal, the OPLL 60 operates to lock the reference laser signal 160 to the ground-based laser signal. Particularly, the local laser module 164 includes a frequency / phase adjustment control input 182 to which the error signal 180 is applied. Locking the reference laser signal 160 to a ground-based reference of the OGS 20 means that theplurality of optical reference carriers 64 generated by the OFC 62 are also locked to the ground- based reference, which improves the optical reception and / or transmission performance at the satellite 22 without requiring multiple, high-quality laser sources onboard the satellite 22.

[0051] In other words, the arrangement can be understood in one or more embodiments as locking a space-based laser to a ground originated optical pilot tone, e.g., for coherent optical detection and / or optical signal generation onboard the satellite 22. Also note that use of the source laser signal 162 as a reference for generation of the reference laser signal 160 improves robustness with respect to fading affecting reception by the satellite 22 of the optical pilot tone that serves as or otherwise provides the source laser signal 162. That is, the OPLL 60 has a drift error that remains within some defined error bounds over a corresponding interval, meaning that instantaneous or short losses of the optical pilot tone do not upset operation of the OPLL 60.

[0052] An optical pilot carries a RF LO reference tone in one or more embodiments. In one or more other embodiments, the optical pilot does not carry a RF LO reference, and is used for channel estimation only. In yet another embodiment, the optical pilot carries a RF LO reference, which is extracted for use, but the optical pilot itself is not used to slave any local seed laser.

[0053] Turning back to Figure 5A, the diagram further illustrates an example arrangement for generation of the LO signal 166. In particular, the source laser signal 162 in one or more embodiments comprises an optical pilot signal as one optical channel signal among a plurality of optical channel signals that are frequency-domain multiplexed in a composite optical signal received by the satellite 22 as the one or more optical uplink signals 26. The optical pilot signal is generated in the OGS 20 via a high-quality, stable laser source and it carries a stable, frequency-accurate RF tone. The OPLL 60 includes or is associated with an extraction circuit 184 that is configured to extract the RF tone 168 for application to a RF local oscillator (LO) 186 of the OPLL 60, for generation of the LO signal 166 that defines the wavelength spacing between the optical reference carriers 64.

[0054] Broadly, the optical pilot could come from the OGS through the forward uplink, in the same overall spectrum as the communications signals. However, in one or more other embodiments, the optical pilot signal is generated independently from the ground. Also, the reference tone for OFC operations in the OGS may be derived or reused from the forward link.

[0055] Figure 5B provides further example details for the OFC 62 as an overall implementation, which includes a plurality of cascaded optical phase and amplitude modulators 181, which produce a multi- wavelength output optical signal that is filtered into respectiveoptical wavelengths — reference optical carriers — by a WDM filter / demultiplexer 183. Outputs from the WDM filter / demultiplexer 183 are optical reference carriers at respective frequencies, e.g., for use in coherent detection. Additional circuitry comprises stabilization and biasing control circuitry 185, and a RF driver amplifier 187, which takes as its input the LO signal 166.

[0056] Figure 6 illustrates example details for the optical receiver 82 onboard the satellite 22. In an example embodiment, the one or more optical uplink signals 26 incoming to the LCT 80 from the LCT 110 of the OGS 20 via free space propagation comprise a composite optical signal comprising a plurality of forward optical channel signals at respective optical wavelengths. Thus, the corresponding one or more received optical signals 90 output from the LCT 80 comprise a composite optical signal 192 and the optical receiver 82 includes an optical demultiplexer 194 that demultiplexes the composite optical signal 192, to recover the plurality of forward optical channel signals 196.

[0057] The recovered plurality of forward optical channel signals 196 includes, for example, a plurality of optical communication signals 198, each carrying one or more information signals 200 for transmission to UTs 12 — i.e., the information signals 200 convey forward traffic for UTs 12 served by the satellite 22. In an example arrangement, each information signal 200 itself comprises a set of RF signals centered around a respective optical carrier frequency — e.g., respective RF signal pairs offset on either side of the optical carrier frequency.

[0058] The recovered plurality of forward optical channel signals 196 in one or more embodiments also includes at least one optical pilot signal 202. In at least one embodiment, the optical pilot signal 202 is applied to a carrier extractor 204, which correspondingly outputs the source laser signal 162 to the OPLL 60 in the optical domain, and extracts a RF tone 206 that is used by the RF LO 186 to produce the LO signal 166 used by the OFC 62. In this way, the frequency and phase of the plurality of optical reference carriers 64 produced by the OFC 62 are frequency and phase-locked to the ground-based frequency and phase of the optical pilot signal 202. Assuming that the plurality of optical communication signals 198 is also referenced to that same ground-based frequency and phase, the plurality of optical reference carriers 64 is optimized for use in a plurality of optical phase detectors 208 in performing coherent detection on the plurality of optical communication signals 198. Each optical phase detector 208 operates as a coherent receiver and outputs an electrical domain received signal 92 that can be understood as a recovered version of the information signal(s) 200 carried by the optical communication signal 198 applied to the optical input of the optical phase detector 208. As noted, in one or moreembodiments, each information signal 200 comprises a set of RF signals centered around a respective optical carrier frequency — e.g., respective signal pairs offset on either side of the optical carrier frequency. For such embodiments, each electrical-domain received signal 92 comprises a recovered pair of RF signals, which are subsequently demultiplexed into individual RF signals for retransmission in the forward transponders 86.

[0059] In one or more embodiments, the OGS 20 receives forward traffic streams and generates corresponding analog IF or RF signals in the electrical domain, and uses each such IF or RF signal to modulate a respective one among a plurality of optical reference carriers to generate a plurality of optical communication signals which are then multiplexed to form the one or more optical uplink signals 26 as the composite optical signal described above. In this regard, the OGS 20 may employ one or more OPLLs 70 and a corresponding one or more OFCs 72 for advantageous generation of the plurality of optical reference carriers. The OPLL(s) 70 and OFC(s) 72 may be implemented substantially as detailed for the example OPLL 60 and example OFC 62. Correspondingly, the optical transmitter 114 of the OGS 20 may be implemented substantially as detailed for the optical transmitter 84 of the satellite 22, albeit with different power levels, size, etc.

[0060] The OGS 20 in one or more embodiments uses one or more OFC(s) 72 for coherent detection of return optical channel signals contained in a composite optical signal received by the OGS 20 as the one or more optical downlink signals 28. See, e.g., the discussion of the OFC used in the optical receiver 82 of the satellite 22.

[0061] Figure 7 illustrates a method 700 of operation at a satellite 22. The method 700 includes the satellite 22: receiving (Block 702) a plurality of optical communication signals via free space propagation from a ground segment 16 of a SCS 10, each optical communication signal having a corresponding optical carrier frequency and conveying one or more information signals for relaying via the satellite 22; receiving (Block 704) an optical pilot signal in conjunction with the plurality of optical communication signals, the optical pilot signal conveying a reference tone; generating (Block 706) a plurality of optical reference carriers at the corresponding optical carrier frequencies of the plurality of optical communication signals by recovering the reference tone from the optical pilot signal, locking a local laser to a carrier phase and frequency of the optical pilot signal, and inputting light from the locked local laser into an OFC 62 that is modulated via a local oscillator (LO) signal derived from the reference tone; and recovering (Block 708) the information signals by performing coherent detection on each opticalcommunication signal among the plurality of optical communication signals, using the corresponding optical reference carrier.

[0062] In one or more embodiments, the plurality of optical communication signals is received as a composite optical signal from a respective OGS 20 in the ground segment 16, wherein the composite optical signal is formed via WDM, and wherein receiving the plurality of optical communication signals includes optically demultiplexing the plurality of optical communication signals from the composite optical signal.

[0063] The composite optical signal is a first composite optical signal among two or more composite optical signals received by the satellite 22, for example, and wherein the method 700 includes performing coherent detection on the information signals conveyed in each composite optical signal, based on generating a respective plurality of optical reference carriers for each composite optical signal. A single OFC 62 is used to generate all respective pluralities of optical reference carriers or, there are two or more OFCs 62, with each such OFC 62 used to generate one or more pluralities of optical reference carriers corresponding to one or more composite optical signals received by the satellite 22. In at least one such embodiment, the same optical pilot signal is used for each of the two or more OFCs 62.

[0064] In at least one embodiment, each optical communication signal is modulated to convey two or more information signals at respective RF or IF carrier frequencies, and the satellite 22 performs coherent detection on each optical communication signal by generating one or more electrical-domain signals responsive to detected modulation of the optical communication signal and performs electrical-domain frequency demultiplexing to separate the two or more information signals.

[0065] In at least one embodiment, each information signal comprises a forward beam signal conveying information for one or more UTs 12 operating in a user beam coverage area 34 associated with the forward beam signal. In one or more embodiments, the satellite 22 performs bent-pipe transmission of each forward beam signal.

[0066] In one or more other embodiments, the information signals are weighted for beamforming transmission from respective antenna elements comprised in an antenna array onboard the satellite 22. In such embodiments, the method 700 includes the satellite 22 transmitting respective RF downlink signals from the respective antenna elements, each RF downlink signal comprising or derived from a corresponding one of the information signals, with resulting superpositions of the transmitted RF downlink signals forming spot beams in the farfield, with each spot beam corresponding to a respective user beam coverage area 34 within an overall satellite service area 30 associated with the satellite 22.

[0067] Figure 8 illustrates another method 800 of operation by a satellite 22 of a SCS 10. The method 800 includes the satellite 22: receiving (Block 802) RF uplink signals from respective ones among a plurality of UTs 12 operating in a satellite service area 30 associated with the satellite 22, the RF uplink signals impinging on a user antenna system 44 of the satellite 22, with the antenna system 44 outputting corresponding antenna-received signals; generating (Block 804) a plurality of optical reference carriers by inputting light from a local laser into an OFC 62, with the OFC 62 modulated via a local oscillator (LO) signal derived from a reference tone conveyed by an optical pilot signal received from an OGS 20 of the SCS 10; impressing (Block 806) respective ones among a plurality of information signals onto respective ones of the optical reference carriers, to form a plurality of optical communication signals, each information signal comprising or derived from one or more respective ones among the antenna signals; and transmitting (Block 808) the plurality of optical communication signals to the OGS 20.

[0068] In the context of the method 800, transmitting the plurality of optical communication signals to the OGS 20 comprises, for example, the satellite 22 forming a composite optical signal via wavelength division multiplexing of the plurality of optical communication signals and transmitting the composite optical signal towards the OGS 20. In at least one embodiment, the local laser that feeds the OFC 62 is locked to the carrier frequency and phase of the optical pilot signal, which is incoming to the satellite 22 from the OGS 20 via free space propagation.

[0069] In one or more embodiments the user antenna system(s) 44 comprises or includes a phased array antenna comprising a plurality of antenna elements. In a return direction example, the satellite 22 in one or more embodiments is configured to receive RF uplink signals from the UTs 12 in the satellite service area 30 via the phased array antenna, with the phased array antenna outputting corresponding per-element received signals. In at least one such embodiment, the return transponders 88 are bent-pipe electrical pathways that amplify and optionally frequency translate the per-element received signals to form the aforementioned information signals 200, which are impressed (modulated) onto respective ones among a plurality of optical reference carriers 64 to form a plurality of return optical channel signals. These return optical channel signals are then multiplexed to form a composite optical signal that is output by the LCT 80 for free-space propagation to the LCT 110 of the OGS 20 as one of the one or more optical downlink signals 28. Figure 9 illustrates such an arrangement with reference back to Figure 3.

[0070] A phased array antenna 210 comprises or is included in the user antenna system(s) 44 and it includes a plurality of antenna elements 212. RF uplink signals from UTs 12 in the satellite service area 30 impinge on the phased array antenna 210, with each antenna element 212 outputting a corresponding per-element received signal 214 representing a composite of the RF uplink signals as received on that particular antenna element 212. The signals 214 may be understood as one example of the signals 100 shown in Figure 2. Each per-element received signal 214 passes through a bent-pipe signal path provided by a respective one among the return transponders 88, shown here as individual transponders 88-1 through 88-N. There may be as many return transponders 88 as there are antenna elements 212.

[0071] The information signals 102 output by the return transponders 88 comprise analog electrical signals at IF or RF, for example. Each information signal 102 is used to modulate a respective one among a plurality of optical reference carriers 64, to form a respective optical communication signal. With this arrangement, each one of the optical communication signals 152 output by the involved plurality of optical modulators 150 conveys a respective one of the per-element received signals 214.The optical multiplexer 154 multiplexes the plurality of optical communication signals 152, possibly with one or more optical control or reference signals, and outputs a corresponding composite optical signal 156. An optical power amplifier (PA) 158 amplifies the composite optical signal 156 to form an optical transmit signal 104 that is coupled into the LCT 80 for transmission as an optical downlink signal 28.

[0072] The foregoing arrangement allows, for example, the application of return beam weights in the ground segment 16, for formation of return user beams in the digital signal processing domain. That is, the ground segment 16 recovers the per-element received signals 214 and applies return beam weights, for return beamforming in the digital domain.

[0073] Figure 10 illustrates a complementary or similar details for implementation in the forward direction. In at least one embodiment, the one or more incoming optical uplink signals 26 comprise or include a composite optical signal containing a plurality of forward optical channel signals, including a plurality of optical communication signals. Each such optical communication signal conveys a per-element transmit signal for transmission from a particular one among the plurality of antenna elements 222 contained in a phased array antenna 220 that comprises or is included in the user antenna system(s) 44. The phased array antenna 220 may be the phased array antenna 210 depicted in Figure 9 or may be distinct from it.

[0074] The optical receiver 82 of the satellite 22 includes the aforementioned low-noise optical amplifier (LNOA) 190, optical demultiplexer 194, and optical phase detectors 208, for recovery of the per-element transmit signals conveyed in the received composite optical signal, with these recovered signal versions shown as signals 92 — see Figure 2 for reference. In at least one embodiment, the forward transponders 86 provide a bent-pipe electrical signal path for each signal 92, e.g., with filtering, amplification, and frequency translation, with the forward transponders 86 correspondingly outputting per-element transmit signals 224, each for transmission from a particular one of the antenna elements 222. These per-element transmit signals 224 can be understood as an example of the signals 94 shown in Figure 2. With ground- based weighting applied to the per-element signals, simultaneous transmission of the per-element transmit signals 224 from the phased array antenna 220 results in far-field signal superpositions that yield the desired forward user beams for serving respective subsets among the population of UTs 12 served by the satellite 22.

[0075] Notably, 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. Therefore, it is to be understood that the invention(s) is / are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

CLAIMSWhat is claimed is:

1. A method of operation at a satellite of a satellite communications system (SCS), the method comprising: receiving a plurality of optical communication signals via free space propagation from a ground segment of the SCS, each optical communication signal having a corresponding optical carrier frequency and conveying one or more information signals for relaying via the satellite; receiving an optical pilot signal in conjunction with the plurality of optical communication signals, the optical pilot signal conveying a reference tone; generating a plurality of optical reference carriers at the corresponding optical carrier frequencies of the plurality of optical communication signals by recovering the reference tone from the optical pilot signal, locking a local laser to a carrier phase and frequency of the optical pilot signal, and inputting light from the locked local laser into an optical frequency comb modulated via a local oscillator (LO) signal derived from the reference tone; and recovering the information signals by performing coherent detection on each optical communication signal among the plurality of optical communication signals, using the corresponding optical reference carrier.

2. The method according to claim 1 , wherein the plurality of optical communication signals are received as a composite optical signal from a respective optical ground station (OGS) in the ground segment, the composite optical signal formed via wavelength division multiplexing (WDM), and wherein receiving the plurality of optical communication signals includes optically demultiplexing of the plurality of optical communication signals from the composite optical signal.

3. The method according to claim 2, wherein the composite optical signal is a first composite optical signal among two or more composite optical signals received by the satellite, and wherein the method includes performing coherent detection on the information signals conveyed in each composite optical signal, based on generating a respective plurality of optical reference carriers for each composite optical signal.

4. The method according to claim 3, wherein the optical frequency comb is used to generate all respective pluralities of optical reference carriers.

5. The method according to claim 3, wherein the optical frequency comb is a first one among two or more optical frequency combs, each optical frequency comb generating one or more pluralities of optical reference carriers corresponding to one or more composite optical signals received by the satellite.

6. The method according to claim 5, wherein the optical pilot signal is used commonly for each of the two or more optical frequency combs.

7. The method according to any one of claims 1-6, wherein each optical communication signal is modulated to convey two or more information signals at respective radiofrequency (RF) or intermediate frequency (IF) carrier frequencies, and wherein performing coherent detection on each optical communication signal comprises generating one or more electrical-domain signals responsive to detected modulation of the optical communication signal and performing electrical-domain frequency demultiplexing to separate the two or more information signals.

8. The method according to claim 7, wherein each information signal comprises a forward beam signal conveying information for one or more terminals operating in a beam coverage area associated with the forward beam signal.

9. The method according to claim 8, further comprising performing bent-pipe transmission of each forward beam signal.

10. The method according to any one of claims 1-7, wherein the information signals are weighted for beamforming transmission from respective antenna elements comprised in a phased array antenna onboard the satellite, and wherein the method includes transmitting respective radiofrequency (RF) downlink signals from the respective antenna elements, each RF downlink signal comprising or derived from a corresponding one of the information signals, with resulting superpositions of the transmitted RF downlink signals forming spot beams in the far field, witheach spot beam corresponding to a respective beam coverage area within an overall satellite service area associated with the satellite.

11. A method of operation at a satellite of a satellite communications system (SCS), the method comprising: receiving radiofrequency (RF) uplink signals from respective ones among a plurality of terminals operating in a satellite service area associated with the satellite, the RF uplink signals impinging on an antenna system of the satellite, with the antenna system outputting corresponding antenna-received signals; generating a plurality of optical reference carriers by inputting light from a local laser into an optical frequency comb, with the optical frequency comb modulated via a local oscillator (LO) signal derived from a reference tone conveyed by an optical pilot signal received from an optical ground station (OGS) of the SCS ; impressing respective ones among a plurality of information signals onto respective ones of the optical reference carriers, to form a plurality of optical communication signals, each information signal comprising or derived from one or more respective ones among the antenna signals; transmitting the plurality of optical communication signals to the OGS.

12. The method according to claim 11, wherein transmitting the plurality of optical communication signals to one or more OGSs comprises forming a composite optical signal via wavelength division multiplexing (WDM) of the plurality of optical communication signals and transmitting the composite optical signal towards the OGS.

13. The method according to claim 11 or claim 12, wherein the antenna system is a phased array antenna comprising a plurality of antenna elements, the antenna-received signals comprise per-element signals, and wherein each information signal comprises or is derived from a respective one of the per-element signals, such that each optical communication signal among the plurality of optical communication signals corresponds to the antenna-received signals incoming on a particular element of the phase array antenna.

14. A satellite configured for operation in a satellite communications system (SCS), the satellite comprising: a laser communication terminal configured to: receive a plurality of optical communication signals via free space propagation from a ground segment of the SCS, each optical communication signal having a corresponding optical carrier frequency and conveying one or more information signals for relaying via the satellite; and receive an optical pilot signal in conjunction with the plurality of optical communication signals, the optical pilot signal conveying a reference tone; and an optical receiver configured to: generate a plurality of optical reference carriers at the corresponding optical carrier frequencies of the plurality of optical communication signals by recovering the reference tone from the optical pilot signal, locking a local laser to a carrier phase and frequency of the optical pilot signal, and inputting light from the locked local laser into an optical frequency comb modulated via a local oscillator (LO) signal derived from the reference tone; and recover the information signals by performing coherent detection on each optical communication signal among the plurality of optical communication signals, using the corresponding optical reference carrier.

15. The satellite according to claim 14, further comprising a plurality of forward transponders configured to amplify the information signals for transmission from a user antenna system of the satellite.

16. The satellite according to claim 14 or 15, wherein the plurality of optical communication signals are received as a composite optical signal from a respective optical ground station (OGS) in the ground segment, the composite optical signal formed via wavelength division multiplexing (WDM), and wherein the optical receiver includes an optical demultiplexer for demultiplexing the plurality of optical communication signals.

17. The satellite according to claim 16, wherein the composite optical signal is a first composite optical signal among two or more composite optical signals received by the satellite, and wherein the satellite is configured to perform coherent detection on the information signals conveyed in each composite optical signal, based on generating a respective plurality of optical reference carriers for each composite optical signal.

18. The satellite according to claim 17, wherein the satellite is configured to use the optical frequency comb to generate all respective pluralities of optical reference carriers.

19. The satellite according to claim 17, wherein the optical frequency comb is a first one among two or more optical frequency combs, with the satellite configured to use each optical frequency comb to generate one or more pluralities of optical reference carriers corresponding to one or more composite optical signals received by the satellite.

20. The satellite according to claim 19, wherein the optical pilot signal is used commonly for each of the two or more optical frequency combs.

21. The satellite according to any one of claims 14-20, wherein each optical communication signal is modulated to convey two or more information signals at respective radiofrequency (RF) or intermediate frequency (IF) carrier frequencies, and wherein the satellite is configured to perform coherent detection on each optical communication signal based on being configured to generate one or more electrical-domain signals responsive to detected modulation of the optical communication signal and performing electrical-domain frequency demultiplexing to separate the two or more information signals.

22. The satellite according to claim 21, wherein each information signal comprises a forward beam signal conveying information for one or more terminals operating in a beam coverage area associated with the forward beam signal.

23. The satellite according to claim 22, wherein the satellite includes a plurality of forward transponders that are configured to perform bent-pipe transmission of each forward beam signal.

24. The satellite according to any one of claims 14-21 , wherein the information signals are weighted for beamforming transmission from respective antenna elements comprised in a phased array antenna onboard the satellite, and wherein the satellite is configured to transmit respective radiofrequency (RF) downlink signals from the respective antenna elements, each RF downlink signal comprising or derived from a corresponding one of the information signals, with resulting superpositions of the transmitted RF downlink signals forming spot beams in the far field, with each spot beam corresponding to a respective beam coverage area within an overall satellite service area associated with the satellite.

25. A satellite configured for operation in a satellite of a satellite communications system (SCS), the satellite comprising: an antenna system configured to receive radiofrequency (RF) uplink signals from respective ones among a plurality of terminals operating in a satellite service area associated with the satellite, the antenna system operative to output corresponding antenna-received signals; and an optical transmitter configured to: generate a plurality of optical reference carriers by inputting light from a local laser into an optical frequency comb, with the optical frequency comb modulated via a local oscillator (LO) signal derived from a reference tone conveyed by an optical pilot signal received from an optical ground station (OGS) of the SCS; impress respective ones among a plurality of information signals onto respective ones of the optical reference carriers, to form a plurality of optical communication signals, each information signal comprising or derived from one or more respective ones among the antenna-received signals; and output the plurality of optical communication signals for transmission via a laser communication terminal (LCT) of the satellite, to the OGS.

26. The satellite according to claim 25, wherein, for transmitting the plurality of optical communication signals to the OGS, the optical transmitter is configured to form a composite optical signal via wavelength division multiplexing (WDM) of the plurality of opticalcommunication signals and provide the composite optical signal to the LCT, for transmission towards the OGS.

27. The satellite according to claim 25 or claim 26, wherein the antenna system is a phased array antenna comprising a plurality of antenna elements, and wherein the antenna-received signals comprise per-element signals, and wherein each information signal comprises or is derived from a respective one of the per-element signals, such that each optical communication signal among the plurality of optical communication signals corresponds to the antenna-received signals output from a particular one of the antenna elements.

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