Closed-Loop Doppler Tracking for Inter-Satellite Links
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Solution Overview
Problem
High Doppler shifts in satellite communications between low earth orbit (LEO) and geostationary earth orbit (GEO) satellites and between satellites and terrestrial devices pose challenges for coherent optical systems, as existing methods are inadequate for tracking and correcting these shifts, especially in high data rate links.
Innovation Solution
A closed-loop Doppler tracking system using position-based local oscillator adjustment and digital signal processing (DSP) to track and correct Doppler shifts, employing methods such as M-th power operation, Fast Fourier Transform (FFT), and phase ambiguity calculation, with dual lasers for handling rapid fluctuations and sign flips.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If terrestrial coherent optical system methods are used, then high data rate links at 100 Gbps or higher are achieved, but the system cannot handle large Doppler shifts experienced in satellite communications
Solution Approach 1:
The system changes the operational parameters of the local oscillator, allowing it to tune across a wide frequency range (±13 GHz) to track and compensate for large Doppler shifts. This parameter adjustment enables the terrestrial optical system to adapt to satellite communication conditions while maintaining high data rates.
Solution Approach 2:
The local oscillator is made dynamically adjustable rather than fixed, enabling real-time frequency tracking of the Doppler-shifted signal. This dynamic adaptation allows the system to maintain synchronization with satellite signals despite large and varying Doppler shifts, resolving the contradiction between high data rate performance and Doppler tolerance.
2Device complexity
If fixed local oscillator frequency is used, then system simplicity is maintained, but Doppler tracking and correction capability is lost
Solution Approach 1:
The system implements a feedback mechanism where the received signal's frequency information is used to adjust the local oscillator frequency. This closed-loop feedback enables automatic Doppler tracking and correction, providing both the tracking accuracy needed and a relatively simple implementation through standard feedback control techniques.
3Reliability
If wide Doppler tracking range is implemented, then satellite communication reliability is improved, but system complexity and computational requirements increase
Solution Approach 1:
The system performs preliminary Doppler estimation and local oscillator frequency adjustment before full signal processing. This preliminary action reduces the computational burden during main signal processing while ensuring reliable Doppler compensation, thus improving reliability without proportionally increasing overall system complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables efficient capture and correction of wide Doppler ranges up to +/−13 GHz, ensuring reliable high data rate communications in satellite links by accurately adjusting local oscillator frequencies and digitally correcting residual shifts.
Implementation Method 1
determine a Doppler shift or carrier frequency offset in the input signal based on the relative velocity; adjust a local oscillator frequency based on a Doppler measured using the position information
Data Source
AI summary
An apparatus can include transceiver circuitry to receive an input signal from a target apparatus. The apparatus can further include a processing circuitry to determine position information of a source object and a target object. Based on the position information, the processing circuitry can calculate a relative velocity and determine a Doppler shift or carrier frequency offset in the input signal based on the relative velocity. The processing circuitry can adjust a local oscillator frequency based on a Doppler measured using the position information in an initial link acquisition phase. The processing circuitry can track the Doppler continuously over a range of tens of gigahertz accounting for Doppler phase ambiguities, and correct for a tracked Doppler shift by partially adjusting a local oscillator frequency and by correcting a residual Doppler shift digitally.