Ultra-high-speed data relay systems

Abstract

The SpaceDataHighway, the first operational service of high-speed data relay system based on optical inter-satellite links, has set a new milestone in space optical communications. Data relay systems are becoming crucial in applications such as Earth observation, where huge amounts of data need to be sent to Earth reliably and with low latency. Optical communications plays a major role in such high-speed applications, since no-regulations are needed, because the lack of interference among users, and the huge amount of available bandwidth. Since the end of the 1990's, several experiments have shown the feasibility of such technology with several demonstrations from LEO, GEO and the Moon. The current state-of-the-art relay system architecture involves LEO and GEO satellites with optical inter-satellite links, and direct Ka-Band RF links from GEO to the Earth. Next generation systems may involve also UAVs, and may rely only on optical communications to exploit the full potential of these frequencies. The main challenges of using optical links are the turbulence effects, when the link traverses the Earth's atmosphere, and the degrading impact of platform microvibrations because of the inherently small divergence of the transmitted beam. Such aspects have to be taken into account when designing future systems. Together with the modulation, the forward error correction (FEC) defines the communications performance of the system. Following CCSDS coding recommendations, the performance of several coding schemes is analyzed; concretely Reed-Solomon codes, convolutional codes, turbo codes and low-density parity check codes are taken into account. One of the main characteristics of the atmospheric channel is the correlation of fading events, which requires further data protection to compensate for erasure events. Interleaving and packet level coding in combination with FEC are compared through simulations. Finally, different approaches for data correction are considered. The complexity on board the GEO satellite can specially limit the use of the most advanced decoding schemes and data-protection for the upcoming generations of relay systems. The trade-off between performance and complexity is crucial in order to allow further system enhancements in terms of capacity, without endangering the whole system availability.

An Information-Theoretic Comparison Between Coherent and IM/DD Transmissions for Free Space Optical Communications

“…In [7], a detailed and complete simulator is employed to evaluate the complex space-and time-varying channel response due to atmospheric turbulence, accounting for the different optical rays in the received lightwave beam (which implies physically faithful but long simulations). As demonstrated in [7], most of the residual phase variations after the AO correction can be attributed to an effect called atmospheric piston [14], that it is not corrected by traditional AO systems. Nevertheless, such kind of phase changes due to the atmosphere are relatively stable and it is reasonable to consider them as a quasi-constant phase, that can be included in the model of a non-ideal local oscillator which however is not considered in (4).…”