New developments for high-speed internet from space enabled by coherent modulation and adaptive optics

Internet from space via satellite has become a topic of interest. Commercial satellite networks, such as SpaceX Starlink and Telesat LightspeedTM, aim to provide high-speed Internet to remote and rural areas where traditional communication technologies are not available. A major challenge in this endeavour is the transfer of large amount of data between satellites and ground-stations from earth. The data rates required for this are in the order of Tbit/s, which can be hardly met by current radio-frequency technologies due to low bandwidth. “Free-space optical communication technologies offer a potential solution, as they can achieve unprecedented high throughputs by exploiting the license-free optical bandwidth und thus, greatly reduce the number of needed ground stations.” says Yannik Horst, lead author of the study and researcher in the group of Prof. Leuthold at ETH Zurich. Yet, optical modulation techniques and methods to mitigate atmospheric turbulence need to be found and are a subject of ongoing research.

In a new paper published in Light: Science & Applications, a team from ETH Zurich, Thales Alenia Space and Onera demonstrated a 1 Terabit per second line-rate over wireless distance of 53.42 km on a single optical carrier, an improvement of a factor five in data-rate and distance over previous experiments. To successfully carry out these experiments, the three partners combined their specific competencies. Thales Alenia Space, a French aerospace company, developed the space terminal with the precise pointing system. Onera, the French national office for aerospace studies and research, built the optical ground station with its adaptive optics system to mitigate the atmospheric turbulence and correct the phase-perturbated wavefront. ETH Zurich provided one of its key areas of expertise in advanced optical communication and was responsible for the development of the high-speed optical coherent transmitter and receiver.

In this study, they tested advanced optical modulation formats for space-earth applications. It was found that advanced modulation formats can be received with good quality despite of turbulences in atmosphere. Depending on weather conditions losses might be high though. “When power is limited, one should therefore resort to more robust modulation formats as they offer a higher sensitivity”, explains Horst.

In this matter, Leuthold’s group presents constellation modulation as a solution to transmit high data-rates despite of low signal-to-noise ratio. The group has introduced a new four-dimensional modulation format called 4D-BPSK (Binary Phase Shift Keying) that was first put to test in a communication link within this study. “Utilizing 4D-BPSK, we can transmit information with a sensitivity of 4.3 photons per bit at a bit-error-rate (BER) of 1∙10^-3” says Horst, “we use this modulation format to show the successful transmission of up to 210 Gbit/s of information within a single-carrier.” Furthermore, it has a measured sensitivity advantage of 1.4 dB (1.7 dB simulated) and 0.7 dB (0.75 dB simulated) over polarization-multiplexed 4 QAM (Quadrature Amplitude Modulation) and polarization switched (PS) 4 QAM respectively at a BER of 1∙10^-3. This is a novelty as PS 4 QAM modulation format was so far considered the most power-efficient modulation format for coherent modulation systems.

To compensate for the high losses in the turbulent 53 km free-space channel, they found that the atmospheric turbulence mitigation technique based on adaptive optics can improve the received optical power by up to 28 dB and 7 dB when compared against a simple static aberration correction or a tip-tilt correction, respectively. On top, adaptive optics is compatible with complex modulation formats, where information is encoded onto both, the amplitude and phase of the light, without performance penalty.

In future applications, the 1 channel 1 Terabit per second transmission can be easily scaled to a 50-channel transmission link by using wavelength division multiplexing. This might result in a > 50 Terabits per second space link using conventional technologies. The commercialization of the product is now up to the industrial partners, says Leuthold. Instead, the scientists from ETH will continue in their effort to boost the Internet with novel modulation techniques and device concepts.