Symbol Timing Frequency Synchronization With Dual-Loop Error Correction
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Solution Overview
Problem
In communication systems, low-cost oscillators used in user terminals often result in significant frequency errors at high frequencies, leading to potential miss detection or interference, particularly in satellite and terrestrial networks where frequency accuracy is critical for orthogonality of channels.
Innovation Solution
An apparatus for synchronizing frequency to a symbol timing using a master oscillator, accumulator, and frequency controller, which includes a phase lock loop with both inner and outer loops to zero out frequency errors, employing a Voltage Controlled Crystal Oscillator (VCXO) or Numerically Controlled Oscillator (NCO) to control sampling and interpolation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If low-cost oscillators are used in user terminals, then the cost of terminals is reduced, but frequency errors increase significantly at high frequencies
Solution Approach 1:
The patent implements a feedback mechanism where the terminal measures its frequency offset relative to the hub and sends this information back to the hub. The hub then adjusts its frequency accordingly, creating a closed-loop system that compensates for the low-cost oscillator's inherent inaccuracies without requiring expensive precision components in the terminal.
Solution Approach 2:
The hub acts as an intermediary that receives frequency offset measurements from the terminal and performs the complex frequency adjustment calculations. This mediator approach allows the terminal to use simple, low-cost oscillators while the hub's more powerful processing compensates for the accuracy limitations.
2Measurement precision
If frequency correction is performed by the terminal before transmission, then frequency accuracy is improved, but the complexity of the terminal increases
Solution Approach 1:
Instead of implementing complex frequency correction algorithms in the terminal, the patent uses a feedback approach where the terminal simply measures and reports its frequency offset. The actual correction is performed by the hub, which has more processing capability, thereby maintaining frequency accuracy while minimizing terminal complexity.
Solution Approach 2:
The terminal performs the simple task of measuring its own frequency offset relative to the hub's transmissions and reports this information. This self-service measurement approach allows the terminal to remain simple while still contributing to the frequency correction process.
3Measurement precision
If phase locking is used for frequency synchronization, then frequency accuracy is improved, but acquisition time increases
Solution Approach 1:
The patent performs preliminary frequency offset measurement by having the terminal measure its offset relative to the hub before actual data transmission begins. This preliminary action allows the system to quickly establish frequency alignment without requiring lengthy phase locking procedures during active communication.
Solution Approach 2:
The patent replaces the traditional mechanical/electronic phase locking mechanism with a digital measurement and reporting approach. The terminal measures frequency offset in the digital domain and communicates this information to the hub, which then performs the frequency adjustment, eliminating the need for complex phase locking hardware and reducing acquisition time.
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 solution effectively corrects frequency errors, ensuring accurate synchronization and reducing interference, with acquisition times improved to milliseconds in some cases, even under challenging signal-to-noise ratios, such as in Internet of Things (IoT) devices.
Implementation Method 1
In the apparatus the master oscillator may include a Voltage Controlled Crystal Oscillator (VCXO) to control a sampling of the input signal by an Analog-to-Digital-Converter (A/D)
Implementation Method 2
In some embodiments, for example, in a purely digital implementation, the master oscillator may include a Numerically Controlled Oscillator (NCO) that modifies input sampling via a sample interpolator
Implementation Method 3
The apparatus may include a phase lock loop comprising an inner loop comprising a second order loop, wherein an output of the second order loop controls an interpolator
Data Source
AI summary
An apparatus for synchronizing frequency to a symbol timing, the apparatus including: a master oscillator to generate a master clock signal; an interpolator to accumulate a frequency error estimate between a symbol timing frequency and the master clock signal; and a frequency controller to transfer a portion of the frequency error estimate to the master oscillator to obtain a lock between the symbol timing and the master clock signal of the master oscillator in a manner that zeros-out the frequency error estimate.