Optical frequency reuse
While investigating a possibility that mPOWER constellation might be using laser based communication terminals (LCT) for maintaining high throughput uplink/downlink with gateways, I was surprised by current state-of-the art LCTs that are in the range of 1-2Gbps. That seems like a big disconnect with terabit satellites like Viasat-3 or mPOWER that would use Ka band or V band only. Mounting hundreds or even thousands LCTs on each satellite to reach RF based terabit satelites does not look like viable option.
While optical communication seems to be much better for high throughput, long distance communication than RF, how can RF based satellites offer a thousand times more throughput?
Phased array antennas enable complex beam forming - essentially multiple beams supported by a single array. That, combined with frequency reuse enables much higher throughput rates.
Main cause of this disconnect can be traced to a single frequency used for space based optical communications. LCT demonstrators so far have focused mainly on single frequency/wavelength lasers plus standard optical instruments (essentially telescopes) for beam focusing. But long distance optical fibers already use WDM (Wavelength Division Multiplexing), enabling use of 40 or 80 channels (different frequency bands) over the same fiber. Such dense WDM, if coupled into the same system as OPTEL-ยต, would bring capacity of a single gateway link from 2Gbps up to 80 or 160Gbps, which seems to be adequate for terabit communication satellites. So there is a clear upgrade path for LCT to rapidly expand their capacity to/from orbit.
Another issue for application of laser communication terminals is related to mechanical interface required for telescope attitude and pointing. That is rather small issue for cis-lunar or deep space probes due to small angular velocity of their communication target on Earth. But LEO satellite might require one degree per second antenna tracking capability both on a ground station and a satellite. That puts high strain on the movable parts (telescope mount) in continuous operation. For LCT to really become feasible, some equivalent of flat antenna (phased array antenna) in optical wavelength range needs to be provided.
That is another area where equivalent of optical "phased array" and metamaterial antennae are required. Phased array optics has been for some time a part of photonics, but recent developments of photonic integrated circuits, especially through possible Earth based applications (such as LIDARs for autonomous cars and UAVs), enables multiple laser beams.
While optical communication seems to be much better for high throughput, long distance communication than RF, how can RF based satellites offer a thousand times more throughput?
Phased array antennas enable complex beam forming - essentially multiple beams supported by a single array. That, combined with frequency reuse enables much higher throughput rates.
Main cause of this disconnect can be traced to a single frequency used for space based optical communications. LCT demonstrators so far have focused mainly on single frequency/wavelength lasers plus standard optical instruments (essentially telescopes) for beam focusing. But long distance optical fibers already use WDM (Wavelength Division Multiplexing), enabling use of 40 or 80 channels (different frequency bands) over the same fiber. Such dense WDM, if coupled into the same system as OPTEL-ยต, would bring capacity of a single gateway link from 2Gbps up to 80 or 160Gbps, which seems to be adequate for terabit communication satellites. So there is a clear upgrade path for LCT to rapidly expand their capacity to/from orbit.
Another issue for application of laser communication terminals is related to mechanical interface required for telescope attitude and pointing. That is rather small issue for cis-lunar or deep space probes due to small angular velocity of their communication target on Earth. But LEO satellite might require one degree per second antenna tracking capability both on a ground station and a satellite. That puts high strain on the movable parts (telescope mount) in continuous operation. For LCT to really become feasible, some equivalent of flat antenna (phased array antenna) in optical wavelength range needs to be provided.
That is another area where equivalent of optical "phased array" and metamaterial antennae are required. Phased array optics has been for some time a part of photonics, but recent developments of photonic integrated circuits, especially through possible Earth based applications (such as LIDARs for autonomous cars and UAVs), enables multiple laser beams.
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