WLAN Time Calibration Through Frame Exchange Merging
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Solution Overview
Problem
The existing FTM mechanism in WLANs requires one-to-one frame exchange between stations, which is time-consuming and leads to high air interface overheads.
Innovation Solution
A method where a first station receives frames from a second station, stores the receiving moments, and calculates the clock offset based on the stored moments and sending moments, ignoring the short transmission delay between stations, thus reducing the need for additional frames and overheads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If one-to-one frame exchange is used in FTM mechanism, then time synchronization can be achieved, but air interface overheads increase and time consumption increases
Solution Approach 1:
The patent merges multiple frame exchanges into a single frame by having the second station send both the first frame (containing sending moment) and the second frame (containing receiving moment) in one transmission, and having the first station send both the third frame (containing receiving moment of first frame) and fourth frame (containing sending moment of second frame) in one acknowledgment. This reduces the number of separate frame exchanges while maintaining the ability to calculate clock offset through multiple time points.
Solution Approach 2:
The patent segments the frame exchange process into distinct functional components: the first frame carries sending moment information, the second frame carries receiving moment information, and the acknowledgment frame carries both receiving moment of first frame and sending moment of second frame. This segmentation allows efficient packing of multiple time measurement points into fewer frames, reducing air interface overheads while preserving synchronization precision.
2Measurement precision
If multiple frames are exchanged for time calibration, then accurate clock offset can be calculated, but transmission time increases
Solution Approach 1:
The patent applies preliminary action by having the first station pre-store the receiving moment of the first frame when it is received. This pre-stored information is then readily available when the second frame arrives, eliminating the need for re-transmission or re-measurement. The clock offset calculation can proceed immediately using the pre-stored receiving moment and the sending moment carried in the second frame, thus reducing time calibration duration while maintaining accuracy.
3Measurement precision
If transmission delay is considered in calculation, then accuracy improves, but calculation complexity increases
Solution Approach 1:
The patent changes the parameter of transmission delay from a variable that requires measurement and compensation to a negligible constant that can be ignored. By operating in a short-range WLAN environment where transmission delay is consistently minimal and stable, the patent simplifies the clock offset calculation to merely the difference between receiving and sending moments, without requiring complex transmission delay measurements or compensations, thus reducing calculation complexity while maintaining sufficient accuracy.
Data Source
AI summary
A time calibration method includes a first station receives a first frame from a second station; the first station stores a receiving moment of the first frame; the first station receives a second frame from the second station, where the second frame includes a sending moment at which the second station sends the first frame; and the first station obtains a clock offset between a first clock of the first station and a second clock of the second station based on the receiving moment of the first frame and the sending moment of the first frame.