Kalman Filter PDV Compensation in Sync-E PTP Clocks
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Solution Overview
Problem
Packet delay variations (PDVs) in packet networks hinder accurate time synchronization in IEEE 1588 Precision Time Protocol (PTP) systems, despite frequency synchronization being achieved through Synchronous Ethernet (Sync-E), leading to clock noise that can render recovered clocks unusable for end-system applications.
Innovation Solution
The method combines Sync-E for frequency synchronization with IEEE 1588 PTP for time synchronization, using a linear Kalman filter to compensate for PDVs by estimating the variance of measurement noise and adjusting the slave clock's time offset, thereby improving clock alignment and quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If IEEE 1588 PTP is used for time synchronization over packet networks, then time offset alignment can be achieved, but packet delay variations (PDVs) introduce clock noise that degrades synchronization accuracy
Solution Approach 1:
The patent introduces Sync-E as an intermediary frequency synchronization mechanism that operates in parallel with PTP time synchronization. By separating frequency alignment (handled by Sync-E) from time offset measurement (handled by PTP), the system mitigates the impact of PDVs on overall synchronization quality, as the frequency-stabilized clock is less susceptible to packet delay variations
Solution Approach 2:
The patent changes the operational parameters of the slave clock by first stabilizing its frequency through Sync-E before performing time offset measurements with PTP. This parameter sequencing—frequency stabilization prior to time synchronization—improves measurement accuracy by reducing the variability introduced by PDVs during the time offset calculation
2Stability of the object's composition
If Sync-E is used for frequency synchronization, then frequency alignment is achieved, but time offset determination remains challenging due to PDV exposure in PTP messages
Solution Approach 1:
The patent segments the synchronization task into two distinct phases: frequency alignment (performed by Sync-E) and time offset determination (performed by PTP). This segmentation allows each subsystem to optimize for its specific function, with Sync-E handling frequency stability and PTP handling time offset, thereby reducing the overall complexity of achieving both goals simultaneously
3Ease of manufacture
If PTP messages are sent end-to-end without intermediate compensation, then implementation is simple, but PDV compensation is inadequate leading to poor time synchronization
Solution Approach 1:
The patent positions Sync-E as an intermediary layer that provides frequency stabilization to the PTP time synchronization process. This intermediary frequency reference acts as a foundation that improves the robustness of end-to-end PTP measurements, allowing the system to maintain implementation simplicity while achieving better time synchronization accuracy through the combined Sync-E/PTP approach
Data Source
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AI summary
This invention relates to methods and devices for time and frequency synchronization. The invention has particular application where time and frequency synchronization over packet networks using, for example, the IEEE 1588 Precision Time Protocol (PTP) is being carried out. The primary challenge in clock distribution over packet networks is the variable transit delays experienced by timing packets, packet delay variations (PDVs). Embodiments of the invention provide a method for time offset alignment with PDV compensation where a synchronized frequency signal is available at a slave device via Synchronous Ethernet and is used to determine the compensation parameters for the PDV.