LEO Satellite Synchronization Pre-correction for Doppler Shifts
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Solution Overview
Problem
Non-geosynchronous satellite communication systems, such as those using low-earth orbits, face challenges in time or frequency synchronization due to varying satellite velocities and carrier frequencies, leading to differential Doppler frequency shifts and signal propagation delays among user terminals within a satellite beam coverage.
Innovation Solution
The implementation of open loop and closed loop pre-correction methods to adjust the transmission times and carrier frequencies of radio signals from user terminals, ensuring that signals arrive at the gateway with minimal time-of-arrival differentials and frequency offset differentials, using local GPS references and ephemeris data for accurate synchronization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If non-geosynchronous satellites are used to provide communication coverage, then the number of satellites and coverage area are improved, but differential Doppler frequency shifts and signal propagation delays occur among user terminals
Solution Approach 1:
The patent applies preliminary action by computing pre-correction time and frequency values before signal transmission. The gateway determines these correction values based on satellite ephemeris data and user terminal positions, then transmits them to user terminals in advance. This allows user terminals to pre-adjust their transmission parameters, eliminating differential Doppler shifts and propagation delays before they affect communication synchronization.
Solution Approach 2:
The patent implements parameter changes by dynamically adjusting transmission time and frequency parameters based on satellite position and user terminal location. The gateway computes pre-correction values that modify the original transmission parameters, transforming the signal characteristics to compensate for expected Doppler effects and propagation delays. This enables precise synchronization despite the moving satellite platform.
2Measurement precision
If pre-correction values are computed and transmitted to user terminals, then time and frequency synchronization is improved, but system complexity and computational requirements increase
Solution Approach 1:
The gateway serves as an intermediary that centralizes the complex computational tasks. Instead of requiring each user terminal to independently calculate pre-correction values, the gateway computes these values using satellite ephemeris data and user position information, then distributes them to all user terminals. This intermediary approach simplifies the overall system architecture by consolidating complexity in a single location while maintaining precise synchronization across all terminals.
3Reliability
If guard times or bands are increased to accommodate differential delays and offsets, then communication reliability is improved, but system capacity and efficiency decrease
Solution Approach 1:
The patent converts the harmful effect of differential Doppler shifts and propagation delays into a benefit by computing pre-correction values that actively compensate for these effects. Instead of treating differential delays as a problem requiring larger guard times, the system uses satellite ephemeris data and user position information to calculate correction values that eliminate the differentials. This transforms what would be harmful variations into precise synchronization, increasing both reliability and system capacity by removing the need for excessive guard times.
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
This approach effectively reduces differential time delays and frequency offsets among user terminals, improving communication efficiency and capacity by ensuring synchronized signal arrival at the gateway, thereby minimizing the need for large guard times or bands and reducing overhead.
Implementation Method 1
different velocities of a satellite relative to a communication device (such as a gateway or a user terminal (UT)) on the ground at different times as well as different carrier frequencies for different beams. For example, a feeder link between a gateway on the ground and a LEO satellite may experience a variation in the Doppler frequency shift that varies with time and carrier frequency.
Implementation Method 2
The feeder link may also experience a variation in the propagation delay of radio frequency (RF) signals between the satellite and the gateway that varies with time.
Data Source
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AI summary
Method and apparatus for time or frequency synchronization of radio signals transmitted by user terminals in communication with a gateway through a satellite is provided. The satellite may be part of a non-synchronous satellite communication system, such as a low-earth orbit (LEO) satellite communication system for data, voice or video communications. Times of transmission of return link radio signals from the user terminals may be adjusted such that the signals arrive at the satellite or at the gateway without large time delay differentials. Carrier frequencies of return link radio signals transmitted from the user terminals may be adjusted such that the signals arrive at the satellite or at the gateway without large frequency offset differentials.