Session P4a: T&F Transfer Utilizing Existing and Emerging Communication Systems

Precise Time Transfer for High Throughput Satellite Communications Links
Janis Surof and Juraj Poliak, German Aerospace Center (DLR)
Location: Seaview A/B
Date/Time: Thursday, Jan. 30, 8:57 a.m.

Future satellite constellations for high throughput communications with optical links are seen to provide broadband network coverage and serve as a backup of the ground fibre network. A single channel of an optical satellite link is foreseen to support data transmissions up to 100 Gbps. The physical characteristics of such optical links make them robust against jamming and spoofing and their broad bandwidth enables precise time transfer. To increase the exploitation of such communications constellation satellites we propose to simultaneously perform time transfer within the network.
Current time transfer concepts like frequency comb pulse transmission offer advantages such as high accuracy and sensitivity, but require a large bandwidth and specific hardware which increases the implementation effort. Phase modulation techniques of continuous wave beams with high rate pseudo random noise (PRN) transmission in combination of optical correlation utilize a single channel but employ a receiver structure which is not yet compatible with the communications setup. Furthermore, the US Space Development Agency defined a time transfer standard which enables time transfer (synchronization) within a data channel but the stability remains in the ns-range. In future communications constellations, e.g. IRIS2, HydRON, or 6G-NTN, satellite synchronization at tens or hundreds ps-level enables more elaborate networking schemes, improves general performance and implementation of Doppler-tracking GNSS-independent positioning. To unlock such functionalities, adaptation of optical transceiver functions is necessary. This paves the road towards a unique optical transceiver solution enabling besides high time-transfer accuracies also high data rates whilst at the same time improving synchronization capabilities for communications constellations with negligible overhead.
This study elaborates the requirements on time transfer functionalities being integrated into an optical communications link. Compared to pure time transfer systems the constraints are mainly defined by the data exchange. The time transfer shall be implemented at physical layer and independent of the modulation and coding scheme, thus supporting on-off-keying, higher order modulation formats like phase shift keying or quadrature amplitude modulation and polarization multiplexing. This supports compatibility to current and future optical inter-satellite links as well as, to certain extent, backwards compatibility to already existing systems. The time transfer shall be performed with minimum effort and only with the hardware which is readily available with its main addition being an ultra-stable oscillator as the system frequency reference and adaptation of the received data processing. Further, the overhead of the time transfer shall be minimized in order not to significantly limit the data throughput. As the link budget is defined by the data channel, there is more power being received compared to a pure time transfer system which is used to optimize the overhead.
Given the requirements, a communications channel including precise time transfer is designed and presented. A pure digital system is envisioned where all processing steps are performed in the digital domain. The processing units of the transceiver are referenced to an ultra-stable oscillator (USO). The transmitter signal is generated by expanding the commonly used frame structure consisting of a frame header and user data with a short data field for time transfer. The header shall contain a known pseudo random noise sequence which is used to detect the beginning of a new frame. The time transfer data field contains the respective time of transmission and observables to compute the two-way time transfer. At the receiver the signal is detected and demodulated depending on the respective modulation. The time transfer observables are gained at the frame recovery of the data processing chain, where the time of arrival is estimated with the timing recovery algorithm output. It is an open loop transmission concept; hence no further acquisition is required as soon as the data transfer is established and inherently robust to Doppler as the data transfer itself has to be designed such that it can cope Doppler shifts. Further processing of the estimation points allows to reach sub-sample resolution. Finally, receiving and transmission time are combined and compared to the counter measurement to calculate the two-way time transfer.
To verify the feasibility of precise time transfer via an optical high throughput channel the system is set up in the laboratory. A binary phase shift keying transmission system with data rates up to 25 Gcps is tested in a common mode scenario and the time transfer precision is evaluated. The system consists of commercial of the shelf components and the signal processing is performed offline after sampling the data with a real-time scope. Lower data rates up to 5 Gbps, resembling the current state-of-art of optical inter-satellite communications, are compared to high rates for future developments. The reception power is chosen such that the forward error coding limit of the respective data rate is met. To optimize the overhead, various header lengths are tested whilst maintaining ps-level time transfer stability at minimum effort.
The presented results show that pure digital time transfer could be integrated into optical inter-satellite communications systems with picosecond stability. The targeted data overhead and additional processing are acceptable for the communications system to add a significant functionality to the communications satellites.