CPM Phase Tracking Using Constellation Mapping at Low Es/N0
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Solution Overview
Problem
Existing synchronization methods for signals using continuous phase modulation, such as GMSK, face challenges with high phase noise at low Es/N0 ratios and the Doppler effect, leading to reduced accuracy and increased bit error rates, especially in satellite communications.
Innovation Solution
A method that determines a complex vector representative of the signal's amplitude and phase, using a mapping function to associate samples with constellation points, calculates phase offset estimates, and corrects the phase using a phase-locked loop and all-pass filter to maintain synchronization, even under low Es/N0 conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If NDA method is used for synchronization, then carrier frequency and symbol rate can be estimated, but phase noise becomes very high at low Es/N0 ratios requiring reduced loop bandwidth which conflicts with Doppler offset requirements
Solution Approach 1:
The patent introduces a pilot subcarrier as an intermediary element to enable accurate phase and frequency estimation without the harmful squaring operation. The pilot signal serves as a reference that allows the receiver to estimate carrier frequency and symbol rate while avoiding the phase noise generation inherent in NDA methods.
Solution Approach 2:
The patent uses a simplified version of the NDA method applied to a pilot signal rather than the full data signal. By copying the estimation approach but applying it to a dedicated pilot carrier, the system achieves frequency and rate estimation without the harmful effects of squaring the modulated data signal.
2Measurement precision
If pilot subcarrier is added to enable synchronization, then phase and frequency estimation improves, but device complexity increases and power amplifier saturation is compromised
Solution Approach 1:
The patent makes the power amplifier work in saturation mode for high efficiency while the pilot subcarrier handles the synchronization function separately. The pilot signal is transmitted at lower power or with different characteristics, allowing the main data signal to utilize full power amplifier saturation without compromising synchronization accuracy.
3Adaptability or versatility
If power back-off is applied to accommodate pilot subcarrier, then subcarrier and data signal can coexist, but useful power for data transmission is reduced
Solution Approach 1:
The patent applies different power levels and transmission characteristics to different signals. The pilot subcarrier is transmitted with appropriate power for synchronization, while the main data signal can utilize the full power amplifier saturation mode, optimizing the useful power available for data transmission without interfering with the pilot function.
Data Source
AI summary
A method for tracking the phase of a signal modulated by a continuous phase modulation includes: a complex vector representative of the received signal is determined for each symbol period with an offset of a half symbol period relative to the symbol rate of the receiver; a level measurement is performed on two complex samples distributed with an offset roughly of a half symbol period either side of the instant of estimation of the complex vector; the complex vectors are associated with one of the points of the constellation of the modulation making it possible to find the point which is closest; average estimation of the offset of the phase is calculated by calculating the difference between the average estimation of the phase of the signal and the theoretical phase value associated with each point of the constellation; an absolute estimation and a differential estimation of the offset of the phase is calculated.