Objective: The objective of this activity is to develop and test a high data rate transceiver (TX-RX) demonstrator for coherent optical space-to-ground feeder links, with a high spectral efficiency. The demonstrator shall support coherent optical links with a data rate exceeding tens of Gbps. It shall demonstrate the increased spectral efficiency achieved by using advanced digital signal processing (DSP) techniques at both transmitter and receiver. Targeted Improvements: Improvement of 1.5 times of the spectral efficiencycompared to coherent FSO links with no advanced DSP, and a times four (x4) increase compared to non-coherent links. The target spectral efficiency shall exceed 5 bits/s/Hz. Description:So far, the technological development of free-space optical (FSO) communications has followed the developments of optical fibre communications with some years of delay. Hence, existing techniques already usedin coherent terrestrial optical networks may be used to anticipate future system criticalities in optical satellite systems, like the spectrum scarcity in extremely high throughput feeder links. Nowadays, the available optical C-band may not offer enough channelsfor downlink with a suitable separation. Atmospheric effects may be particularly severe at certain wavelengths, and dense wavelength division multiplexing may become impractical because of the high power required onboard. Therefore, a spectral efficiency increasewill soon be necessary in the next generation of optical satellite networks. This activity shall consider coherent space-to-ground FSO links supporting a data throughput of tens of Gbps. The transmitter and receiver used for the demonstrator shall therefore implement all the DSP required by coherent transmission schemes. On top of that, other advanced DSP techniques used in coherent terrestrial optical fibre networks shall be identified and adapted, aiming at increasing the throughput and the spectral efficiency of the link. As a minimum, the following techniques shall be tested: wavelength diversity, time-frequency packing, probabilistic constellation shaping. The implementation complexity shall also be taken into account, limiting as much as possible the complexity increase required on board the satellite. The demonstrator shall implement the DSP part in hardware and emulate the optical head and data transport.