Hybrid coded modulation for reliable and power-efficient coherent free-space optics

With the advent of the "high-speed space Internet" (a term coined by NASA) and the expected 50,000 low Earth orbit (LEO) satellites to be launched within 10 years, multi-gigabit per second satellite-based links are of key importance to satisfy the ever-increasing data rates. The high data rates required in both terrestrial and satellite links will result in a capacity crunch, where traffic requirements are much higher than the available resources to transport the data.

Free-space optics (FSO) is a technology that can help overcome the capacity crunch benefiting from the vast amounts of bandwidth available in the THz-regime of the electromagnetic spectrum and can provide high-throughput connectivity for DCIs, backhauling, and satellites. Typical FSO systems encode information in the optical signal power: bits are represented by the presence or absence of light. Modern high-performance communication systems, however, are nonbinary coherent systems, where multiple bits are modulated on the signal's amplitude and phase to increase the spectral efficiency. Nonbinary coherent systems are more prone to errors for a given signal-to-noise ratio, requiring the use of complex forward error correction (FEC) schemes. The combination of nonbinary modulation and FEC is called coded modulation (CM) and it will be an essential building block of future FSO systems targeting speeds beyond 800 Gbps per wavelength.

CM receivers can be based on soft decisions (SDs) or hard decisions (HDs). SD-CM receivers are the best solution in terms of performance, but their energy efficiency in multi-Gbps systems is in the order of 100 pJ/bit, resulting in high power consumption (80 W for 800 Gbps). This problem is especially critical in ground-to-satellite and satellite-to- satellite links, where the LEO satellite receiver is powered by compact solar arrays with a limited power budget. HD-CM receivers, on the other hand, result in energy efficiencies that are two orders of magnitude better (1 pJ/bit). However, their performance is worse than that of their SD-CM counterparts. Furthermore, the reliability of FSO links cannot be guaranteed, mainly because weather conditions strongly affect the performance of such systems. This weather- dependent channel makes off-the-shelf CM systems unsuitable for FSO links. Commercially-available modules for coherent optical fibres could be used in terrestrial FSO links, however, they were never designed for the FSO channel.

In this project, we will tackle the problems of power efficiency and reliability by developing novel hybrid CM architectures for FSO links in close collaboration with industry, going from theoretical foundations to hardware implementations. We propose to jointly design innovative hybrid CM architectures tailored to the properties of the FSO channel, where SDs are partially used by the receiver, but the digital signal processing (DSP) functionalities stay inherently HD. Such architectures are expected to be the first ones to offer excellent performance-complexity tradeoffs. Furthermore, we will study rate adaptation to cope with varying weather conditions, to increase reliability, increase net transmission rates, and/or decrease transmit power. We will achieve this goal by combining the hybrid architectures with signal shaping (geometrical or probabilistic) and adaptive FEC. Finally, we will develop hardware-based proof-of-concept demonstrators for the proposed concepts in collaboration with our industrial partners to facilitate the utilisation and commercialisation of the proposed research by the industry.

We connect to the world via information received through television, social media, and the internet. As errors always happen during transmission of this information, modern communications systems use error-correction techniques, which are becoming increasingly complex and power-hungry and are starting to become the bottleneck for next generation communication systems. In this project we will propose new and robust techniques to deal with these errors for links based on transmission of light over the air, which are heavily affected by varying weather conditions, enabling the deployment of communication systems providing connectivity to areas not reachable by wired solutions.