XDSL Time Synchronization via Sinusoidal FFT Offset Correction
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Solution Overview
Problem
In xDSL systems, achieving accurate time synchronization between customer premises equipment (CPE) and central office (CO) equipment is challenging due to unequal downlink and uplink delays, which are affected by complex channel delays and errors in frame synchronization, especially in long loops with noise and bridging taps.
Innovation Solution
A method and apparatus for time synchronization in DSL systems that involve transmitting and receiving symbols to calculate the offset between CPE and CO clocks, using sinusoidal signals and Fast Fourier Transform (FFT) to correct time stamps and adjust clocks for synchronization, accounting for propagation delays and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If frame synchronization algorithm is used to detect frame boundary and record time stamps, then time synchronization can be implemented, but synchronization precision is restricted by sampling rate and frame synchronization error
Solution Approach 1:
The patent replaces the traditional frame synchronization algorithm (which relies on digital signal processing and sampling) with a physical layer solution using sinusoidal training signals and FFT-based frequency detection. This substitution eliminates the need for complex frame boundary detection algorithms and reduces synchronization errors by operating at the physical signal level rather than the digital frame level.
Solution Approach 2:
The patent introduces sinusoidal training signals with specific frequencies and amplitudes to enable precise time stamp recording. By changing the signal characteristics to include these known sinusoidal patterns, the system can accurately determine signal arrival times through frequency detection, thereby improving time synchronization precision without being constrained by sampling rate limitations.
2Ease of operation
If simple delay measurement is used to calculate clock offset, then calculation is straightforward, but accuracy deteriorates when downlink delay不等于uplink delay due to complex channel conditions
Solution Approach 1:
The patent introduces sinusoidal training signals as intermediaries to measure propagation delay. These training signals are transmitted from CO to CPE and back, allowing both ends to detect the signal characteristics and calculate delay based on frequency shifts and phase differences. This intermediary approach provides more accurate delay measurement compared to simple timestamp comparison, especially when uplink and downlink delays differ.
Solution Approach 2:
The system implements a feedback mechanism where the CPE measures the upstream delay and communicates this information back to the CO. The CO then uses this feedback to calculate the downstream delay more accurately, taking into account the asymmetry between uplink and downlink channels. This feedback loop enables precise clock offset calculation even in complex channel conditions.
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 accurately synchronizes CPE and CO clocks by correcting time stamp errors and calculating channel delays, improving synchronization precision and reducing noise influence, even in complex xDSL environments.
Implementation Method 1
using sinusoidal signals and Fast Fourier Transform (FFT) to correct time stamps
Data Source
AI summary
The present invention provides a method, an apparatus, and a system for time synchronization of an xDSL. The method includes: transmitting, by a customer premises equipment, a first symbol to a central office equipment, and obtaining time Ts2 indicating the moment that the first symbol is transmitted; receiving, by the CPE, a second symbol transmitted by the CO, and obtaining time Ts1 indicating the moment that the second symbol is received; obtaining, by the CPE obtains time Tm2 indicating the moment that the first symbol is received by the CO and time Tm1 indicating the moment that the second symbol is transmitted by the CO; the CPE calculates an offset between a clock of the CPE and a clock of the CO according to Ts1, Ts2, Tm1 and Tm2; and the CPE adjusts the clock of the CPE according to the offset to achieve synchronization.