Curt M. Schieler

Multi-aperture digital coherent combining for free-space optical communication receivers

Space-to-ground optical communication systems can benefit from reducing the size, weight, and power profiles of space terminals. One way of reducing the required power-aperture product on a space platform is to implement effective, but costly, single-aperture ground terminals with large collection areas. In contrast, we present a ground terminal receiver architecture in which many small less-expensive apertures are efficiently combined to create a large effective aperture while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of four lasercom signals, at power levels below 0.1 photons/bit/aperture.

Large-volume data delivery from low-Earth orbit to ground using efficient single-mode optical receivers

Space systems operating in low-Earth orbit are often constrained by how much data can be delivered from space to ground. Traditional data delivery approaches are often limited by either large link losses associated with transmission via a geosynchronous relay satellite or short contact times and spectrum-constrained data rates associated with direct-to-Earth radio-frequency links. Direct-to-Earth optical communication links from low-Earth orbit based on fiber telecommunications technologies that can operate at high data rates (> 100 Gb/s per wavelength channel) can enable the delivery of extremely large volumes of data from space to ground. We analyze the performance of such systems and discuss the performance gains that are enabled by coupling the received signal to an efficient single-mode-fiber-based receiver, even in the presence of turbulence-induced losses.

A New Optical Communication Architecture for Delivering Extremely Large Volumes of Data from Space to Ground

Space-based laser communication has been demonstrated at rates ranging from 10’s of Mbps to a few Gbps for near-Earth crosslinks and for direct-to-Earth downlinks from ranges as far as the Moon. We describe a novel space-to-Earth communication architecture that can deliver many terabytes of data regularly, if the user is willing to accept certain amounts of delay. With careful design of space and ground terminals, and by tapping the recent advances in integrated extremely high rate modems developed by the fiber telecommunications industry, we believe that space terminal cost, ground terminal cost, and operations costs can be kept much lower than present day radio-frequency or proposed optical systems while increasing the amount of data delivery by orders of magnitude.

Experimental demonstration of multi-aperture digital coherent combining over a 3.2-km free-space link

The next generation free-space optical communications infrastructure will need to support a wide variety of space-to-ground links. As a result of the limited size, weight, and power on space-borne assets, the ground terminals need to scale efficiently to large collection areas to support extremely long link distances or high data rates. Recent advances in integrated digital coherent receivers enable the coherent combining (i.e., full-field addition) of signals from several small apertures to synthesize an effective single large aperture. In this work, we experimentally demonstrate the coherent combining of signals received by four independent receive chains after propagation through a 3:2-km atmospheric channel. Measured results show the practicality of coherently combining the four received signals via digital signal processing after transmission through a turbulent atmosphere. In particular, near-lossless combining is demonstrated using the technique of maximal ratio combining.

Experimental demonstration of photon efficient coherent temporal combining for data rate scaling

The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals at LEO, GEO, and deep space to the ground. Efficiently enabling such a diverse mission set requires a common ground station architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while supporting a wide range of data rates. One method for achieving excellent sensitivity performance is to use integrated digital coherent receivers. Additionally, coherent receivers provide full-field information, which enables efficient temporal coherent combining of block repeated signals. This method allows system designers to trade excess link margin for increased data rate without requiring hardware modifications. We present experimental results that show a 45-dB scaling in data rate over a 41-dB range of input powers by block-repeating and combining a PRBS sequence up to 36,017 times.

Multi-aperture digital coherent combining for next-generation optical communication receivers

Space terminals for free-space optical communication systems are under constant pressure to reduce their size, weight, and power profiles. Ground terminals with large collection areas are costly, but provide a means to reduce the aperture-power product on a space platform required to close a given link. We present a ground terminal receiver architecture in which many small apertures are coherently combined while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of low-flux lasercom signals.

Data volume analysis of a 100+ Gb/s LEO-to-ground optical link with ARQ

Space-based optical links can, in principle, support high data rates by using power efficient communication schemes and unconstrained spectrum. In particular, direct links from low-Earth orbit (LEO) to ground have the potential to support very high rates due to the short link distances involved. In this work, we consider an architecture for LEO-to-ground links that operate at peak rates of 100+Gb=s. Such rates are routinely achieved over fiber channels using power-efficient, fiber-coupled transceivers; however, free-space systems that use these devices may need additional error control to ensure reliable communication over an atmospheric channel. We analyze the data volume, or throughput, that can be delivered by a LEO-to-ground system using fiber-coupled transceivers in conjunction with automatic repeat request (ARQ) protocols. We show that many terabytes per day can be delivered error-free from LEO to a single ground terminal for a variety of orbit and ground terminal geometries.

Experimental comparison of 3-mode and single-mode coupling over a 1.6-km free-space link

Free-space optical communications links have the perpetual challenge of coupling light from free-space to a detector or fiber for subsequent detection. It is especially challenging to couple light from free-space into single-mode fiber (SMF) in the presence of atmospheric tilt due to its small acceptance angle; however, SMF coupling is desirable because of the availability of extremely sensitive digital coherent receivers developed by the fiber-telecom industry. In this work, we experimentally compare three-mode and single-mode coupling after propagating through 1.6 km of free-space with and without the use of a fast-steering mirror (FSM) control loop to mitigate atmospherically induced tilt. Here, the 3-mode fiber is a 3-mode photonic lantern multiplexer (PLM) that passively couples light into three SMF outputs. With the FSM control loop active, coupling into the PLM and the SMF yielded nearly identical coupling efficiencies, as expected. Experimental results with the FSM control loop off show that coupling from free-space to PLM increases the average power received, and mitigates the negative impacts of tilt-induced fading relative to coupling directly to SMF.

Data delivery performance of space-to-ground optical communication systems employing rate-constrained feedback protocols

Space-based optical communication systems that transmit directly to Earth must provision for changing conditions such as received power fluctuations that can occur due to atmospheric turbulence. One way of ensuring error-free communication in this environment is to introduce link-layer feedback protocols that use an Earth-toSpace uplink to request retransmission of erroneous or missing frames. In this paper, we consider near-Earth systems that use low-bandwidth uplinks to supply feedback for automatic repeat request (ARQ) protocols. Constraining the uplink signaling bandwidth can reduce the complexity of the space terminal, but it also decreases the efficacy of feedback schemes. Using a Markov-based model of the link-layer channel, we give an analytical result for the downlink performance penalty of a system employing a data-rate-constrained selective-repeat ARQ protocol. We find that the tradeoff between downlink performance and feedback rate is primarily influenced by the coherence time of the atmospheric channel.