NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We have built an endto-end free-space photon counting testbed to demonstrate many of the key technologies required for a deep space optical receiver. The testbed consists of two independent receivers, each using a Geiger-mode avalanche photodiode detector array. A hardware aggregator combines the photon arrivals from the two receivers and the aggregated photon stream is decoded in real time with a hardware turbo decoder. We have demonstrated signal acquisition, clock synchronization, and error free communications at data rates up to 14 million bits per second while operating within 1 dB of the channel capacity with an efficiency of greater than 1 bit per incident photon.
The DVB-S2 coding standard has seen widespread use in many radio frequency (RF) communications applications. The availability of commercial-off-the-shelf (COTS) intellectual property (IP) that can be used to rapidly prototype and field communications systems makes this well-performing, standards-based approach to forward error correction (FEC) coding extremely attractive. In this paper, we evaluate the application of the DVB-S2 coding standard to an asymmetric satellite communications channel. The uplink comprises a fading optical link employing binary differential phase-shift keyed (DPSK) modulation, while the downlink comprises an RF link employing 16-ary amplitude and phase shift keyed (16-APSK) modulation. To simplify the payload implementation, hard-decision uplink demodulation is considered with uplink channel state information transmitted on the downlink for soft-decision decoding in the ground-based receiver. Additionally, we outline many of the trade offs in the overall system design, and some performance results of a baseline design are presented.
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