  The design of a downlink communication system for returning scientific data
from an interstellar flyby probe is reviewed in this tutorial white paper. It
its assumed that the probe is ballistic, and data is downloaded during a period
following encounter with the target star and its exoplanet(s). Performance
indices of interest to scientific investigators include the total
launch-to-completion data latency and the total volume of data reliably
recovered. Issues considered include the interaction between the speed and mass
of the probe and the duration of downlink transmission. Optical communication
using pulse-position modulation (PPM) with error-correction coding (ECC) is
assumed. A very large receiver collection area on or near Earth is composed of
individual incoherently-combined diffraction-limited apertures. Other important
issues in the design including transmit and receive pointing accuracy and beam
size and receiver field of view are reviewed. Numerical examples assume a
mission to Proxima Centauri (the nearest star to our Sun) initially launched by
directed-energy propulsion from the vicinity of Earth.
