As the demand for satellite data transmission increases, higher capacity optical links need to be developed to allow satellites to be connected directly to ground stations (GST). The advantages of Low Earth Orbit (LEO) direct-to-Earth links are smaller latency when compared to relay systems using Geostationary Orbit (GEO) satellites, i.e. LEO-to-GEO and GEO-to-GST, and an increased available bandwidth offered by the optical spectrum with respect to radio frequency (RF) which allows for much higher link capacity. The increase in data rate of optical satellite to ground links towards 100 Gbps will require implementing optical coherent transceivers with capability to compensate for Doppler shift and atmospheric channel impairments. An important figure of merit which needs to be carefully considered in a satellite system is the equipment power consumption. The power consumption of coherent receivers used for terrestrial applications is closely related to the bit rate, with a receiver back-end digital signal processing being responsible for the vast majority of the power consumed.In this paper we propose a hybrid approach to signal processing consisting of simplified digital and analogue elements allowing for significant power reduction. Moreover, one of the attractive aspects of the proposed approach is that it does not require an increased complexity for an increase in baud rate. It will be discussed that the analogue approach to the frequency and phase recovery would allow a saving of approximately 40% to 50% of power on the overall DSP block at baud rates between 10 Gbaud and 100 Gbaud.

An Optical Injection Phase-Lock Loop coherent receiver has been tested against various levels of deep atmospheric fading to experimentally evaluate its feasibility in a ground-to-satellite optical communications application.