  Free-space optical communications have held the promise of revolutionizing
space communications for a long time. The benefits of increasing the bitrate
while reducing the volume, mass and energy of the space terminals have
attracted the attention of many researchers for a long time. In the last few
years, more and more technology demonstrations have been taking place with
participants from both the public and the private sector. The National
Institute of Information and Communications Technology (NICT) in Japan has a
long experience in this field. SOTA (Small Optical TrAnsponder) was the last
NICT space lasercom mission, designed to demonstrate the potential of this
technology applied to microsatellites. Since the beginning of SOTA mission in
2014, NICT regularly established communication using the Optical Ground
Stations (OGS) located in the Headquarters at Koganei (Tokyo) to receive the
SOTA signals, with over one hundred successful links. All the goals of the SOTA
mission were fulfilled, including up to 10-Mbit/s downlinks using two different
wavelengths and apertures, coarse and fine tracking of the OGS beacon,
space-to-ground transmission of the on-board-camera images, experiments with
different error correcting codes, interoperability with other international
OGS, and experiments on quantum communications. The SOTA mission ended on
November 2016, more than doubling the designed lifetime of 1-year. In this
paper, the SOTA characteristics and basic operation are explained, along with
the most relevant technological demonstrations.
