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

Satellite Internet from space 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 unavailable. A major challenge in this effort is the transfer of large amounts of data between satellites and ground stations from the ground. The data rates required for this are in the order of Tbit/s, which can hardly be met by current RF technologies due to the low bandwidth. “Free-space optical communications technologies offer a potential solution, as they can achieve unprecedentedly high throughput by leveraging unlicensed optical bandwidth and thereby greatly reduce the number of ground stations needed.” says Yannik Horst, lead author of the study and researcher in Prof. Leuthold’s group at ETH Zurich. However, optical modulation techniques and methods to mitigate atmospheric turbulence have yet to be found and are the 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 line speed of 1 Terabit per second over a wireless distance of 53.42 km on a single optical carrier , a five-factor improvement in data rate and distance over previous experiments. To successfully carry out these experiments, the three partners have pooled their specific expertise. Thales Alenia Space, a French aerospace company, developed the space terminal with the precise targeting system. Onera, the French national office for aerospace studies and research, built the optical ground station with its adaptive optics system to mitigate atmospheric turbulence and correct the perturbed wavefront in phase. ETH Zurich provided one of its main areas of expertise in advanced optical communication and was responsible for the development of the high-speed coherent optical transmitter and receiver.

In this study, they tested advanced optical modulation formats for space-to-earth applications. It has been found that advanced modulation formats can be received with good quality despite turbulence in the atmosphere. However, depending on weather conditions, losses could be high. “When power is limited, one should therefore resort to more robust modulation formats as they offer greater sensitivity,” explains Horst.

In this matter, Leuthold’s group presents constellation modulation as a solution for transmitting high data rates despite the low signal-to-noise ratio. The group introduced a new four-dimensional modulation format called 4D-BPSK (Binary Phase Shift Keying) which was initially tested in a communications link within this study. “Using 4D-BPSK, we can transmit information with a sensitivity of 4.3 photons per bit at a bit-error-rate (BER) of 110^-3” says Horst, “we use this modulation format to show the successful transmission of information up to 210 Gbit/s within a single carrier”. In addition, it has a measured sensitivity advantage of 1.4 dB (1.7 dB simulated) and 0.7 dB (0.75 dB simulated) versus 4 QAM polarization multiplexed (Quadrature Amplitude Modulation) and 4 QAM polarization switching (PS) respectively at a BER of 110^-3. This is new as the PS 4 QAM modulation format has so far been considered the most energy 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 adaptive optics-based atmospheric turbulence mitigation technique can improve the received optical power by up to 28 dB and 7 dB compared with a simple correction of the static aberration or tip-tilt correction, respectively. In addition, adaptive optics is compatible with complex modulation formats, where information is encoded on both the amplitude and phase of the light, without compromising performance.

In future applications, 1 channel 1 Terabit per second transmission can be easily scaled to a 50 channel transmission link using wavelength division multiplexing. This could involve a spatial link > 50 Terabits per second using conventional technologies. Commercialization of the product is now up to the industrial partners, says Leuthold. Instead, ETH scientists will continue their efforts to enhance the Internet with new modulation techniques and device concepts.