CPM Receiver Synchronization for Satellite AIS Doppler Correction
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Solution Overview
Problem
Satellite-based Automatic Identification Systems (AIS) face challenges such as packet collisions, Doppler shift due to satellite motion, and lower signal-to-noise ratio, leading to high packet error rates (PER) and bit error rates (BER), especially in heavy traffic and near equatorial regions.
Innovation Solution
A method for demodulating received signals using continuous phase modulation (CPM) that involves normalizing samples to unity, estimating and compensating for time and frequency offsets, and employing a two-stage frequency offset estimation process, along with Soft Input Soft Output (SISO) algorithms for improved packet decoding and post-processing techniques to correct errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional demodulation methods are used for satellite-based AIS, then the system can operate with basic functionality, but packet error rate and bit error rate increase significantly due to Doppler shift and time offsets
Solution Approach 1:
The patent applies preliminary action by performing time offset and frequency offset estimation and compensation before the main demodulation process. The receiver first estimates the time offset based on signal autocorrelation, then compensates for it, followed by frequency offset estimation and compensation. This preliminary synchronization prepares the signal for accurate demodulation, directly reducing packet error rate and bit error rate caused by satellite motion effects.
Solution Approach 2:
The patent replaces traditional mechanical synchronization methods with signal processing-based estimation techniques. Instead of using fixed timing references or mechanical synchronization, the system uses autocorrelation-based time offset estimation and pilot signal-based frequency offset estimation, substituting mechanical approaches with electronic signal processing to achieve better accuracy in satellite-based AIS reception.
2Measurement precision
If signal processing is performed without normalization, then processing steps are simpler, but estimation accuracy of time and frequency offsets deteriorates
Solution Approach 1:
The patent applies preliminary action by normalizing the received signal samples before performing time offset and frequency offset estimation. The normalization step scales the signal to have unit energy or unit amplitude, which improves the accuracy of subsequent autocorrelation-based time offset estimation and pilot-based frequency offset estimation. This preliminary preparation enhances measurement precision without significantly increasing overall system complexity.
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
Significantly reduces PER and BER in satellite-based AIS systems, especially in heavy traffic and equatorial regions, while also enhancing processing efficiency and allowing for effective interference cancellation.
Implementation Method 1
satellite motion with respect to the emitters induces a significant Doppler shift of the carrier frequency
Data Source
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AI summary
The invention relates to a method for demodulating a received signal relating to a sequence of transmitted symbols that have been modulated by continuous phase modulation, the method comprising the steps of: normalizing samples of a sequence of samples generated from the received signal, to obtain a normalized sequence of samples, wherein an amplitude of each sample of the normalized sequence of samples has an absolute value equal to unity, estimating, on the basis of the normalized sequence of samples, a time offset and a frequency offset of the received signal, and using the estimated time offset and the estimated frequency offset for compensating the normalized sequence of samples for the time and frequency offsets, to obtain a compensated sequence of samples, and determining a sequence of symbols corresponding to the transmitted sequence of symbols on the basis of the compensated sequence of samples. The invention further relates to a receiver for demodulating a received signal relating to a sequence of transmitted symbols that have been modulated by continuous phase modulation.