Publications

Free-space optical communication links can experience signal power fluctuations due to channel effects such as turbulence and pointing jitter. Systems can ensure reliable, error-free communication over fading channels by using physical-layer techniques (e.g., forward error correction with codeword interleaving) and link-layer techniques (e.g., erasure coding or ARQ). In this work, Shannon capacity analysis is used to compare the fundamental performance of different coding architectures in a variety of link conditions. For systems using coherent receivers we find that, in channels with benign to moderate fade statistics, there can be a ~3 dB link budget advantage to using physical-layer interleaving instead of deferring fade mitigation to the link layer. On the other hand, in very strong fluctuations or when system robustness is paramount, it can be advantageous to use link layer codes.

Demand for increased capacity in deep-space to Earth communications systems continues to rise as sensor data rates climb and mission requirements expand. Optical freespace laser communications systems offer the potential for operating at data rates 10 to 1000 times that of current radiofrequency systems. A key element in an optical communications system is the Earth receiver. This paper reviews the design of a distributed photon-counting receiver array composed of four meter-class telescopes, developed as a part of the Mars Laser Communications Demonstration (MLCD) project. This design offers a cost-effective and adaptable alternative approach to traditional large, single-aperture receive elements while preserving the expected improvement in data rates enabled by free-space laser communications systems. Key challenges in developing distributed receivers and details of the MLCD design are discussed.