Software-Defined Lasercomm - Development of a Laser Communications Relay Demo Transmitter

Optical links hold great promise for future high-rate space communications. A number of existing systems have demonstrated the feasibility of closing the link for both space-ground and space-space. The Laser Comm Relay Demo (LCRD) satellite is designed to further advance the use of optical communications in space. This paper presents a detailed implementation for the digital and RF subsystems of a LCRD-compliant ground transmitter. The transmitter is designed to be very inexpensive and very flexible. The flexibility is desirable since the signal processing required to overcome the atmospheric distortion is not yet fully understood. The combination of fast and slow fades requires deep interleaving and powerful error correction. All the signal processing is implemented entirely in software executing on mid-range servers without GPUs. Maintaining real-time throughput of up to 2.880 Gbps in software is challenging. We describe the parallel processing architecture that takes advantage of modern multi-core processors and provide details for the bottleneck blocks - the DVB-S2 encoder and convolutional interleaver. Reliable operation is enabled by careful assignment of threads to cores and management of thread priorities. Conversion to an RF signal driving the waveforms is performed by a relatively simple companion FPGA utilizing a set of Gigabit Transceivers as DACs. The system passes internal testing with low implementation loss and is pending integration tests at NASA facilities.