Viterbi Winning Path Metric for CPM Carrier Offset Estimation
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Solution Overview
Problem
Existing systems face challenges in accurately estimating carrier frequency offset (CFO) and carrier phase offset (CPO) in continuous phase modulated (CPM) signals, especially in low Signal-to-Noise Ratio (SNR) environments without pilot or preamble signals, which complicates frequency lock and demodulation, particularly in telemetry and deep space communications.
Innovation Solution
A method involving the computation of an objective function to estimate CFO and CPO by performing frequency and phase shifting on CPM signal blocks, followed by Viterbi demodulation to maximize the winning path metric, allowing for blind estimation without requiring preambles or pilots, and using a traceback process to recover the transmitted symbol sequence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Maximum Likelihood Sequence Estimation (MLSE) using Viterbi demodulator is used to demodulate CPM signals, then demodulation accuracy is improved, but the system requires accurate knowledge of carrier frequency offset (CFO) and carrier phase offset (CPO) which are typically unknown in blind reception scenarios
Solution Approach 1:
The patent applies preliminary action by performing CFO and CPO estimation before Viterbi demodulation. The system first computes an objective function based on received signal samples to estimate the frequency and phase offsets, then uses these estimates to correct the signal before applying Viterbi demodulation. This preliminary correction enables the subsequent demodulation to achieve high accuracy without requiring prior knowledge of the offsets.
2Measurement precision
If pilot or preamble signals are included in CPM transmission to facilitate frequency and phase estimation, then estimation accuracy is improved, but spectral efficiency is reduced due to the overhead of these non-information-carrying signals
Solution Approach 1:
The patent applies self-service by enabling the CPM signal itself to carry the information needed for frequency and phase estimation, without requiring separate pilot or preamble signals. The objective function is computed directly from the received signal samples, allowing the signal to estimate its own parameters. This eliminates the need for dedicated estimation signals, thereby maintaining high spectral efficiency while achieving accurate CFO and CPO estimation.
3Reliability
If Phase Locked Loops (PLLs) are used to estimate and correct carrier frequency and phase offsets, then frequency lock capability is improved, but the system becomes complex and difficult to design especially at low SNRs
Solution Approach 1:
The patent replaces the traditional Phase Locked Loop (PLL) mechanism with a direct computational approach. Instead of using analog or digital PLL circuits that require careful design and tuning, the system computes an objective function from the received signal samples to directly estimate CFO and CPO. This substitution of the mechanical PLL system with a computational method simplifies the design, reduces complexity, and improves performance at low SNRs while maintaining frequency lock capability.
4Reliability
If the CPM signal is designed to have no pilots or preambles for security purposes, then communication security is improved, but blind frequency lock becomes extremely challenging especially in the presence of Doppler shift
Solution Approach 1:
The patent applies parameter changes by formulating an objective function that directly estimates frequency and phase parameters from the received signal samples without requiring pilots or preambles. The objective function is designed to be robust against Doppler shift by operating on the phase differences between consecutive symbols. This approach maintains communication security while overcoming the difficulty of blind frequency lock in challenging environments.
Data Source
AI summary
A system and method for estimating carrier frequency offset Δf and carrier phase offset φ0 inherent in a received CPM signal. Samples of a continuous phase modulated (CPM) signal are received. A maximum of an objective function J is determined over a two-dimensional region parameterized by frequency offset v and phase offset w. The coordinates vmax and wmax of a maximizing point in the region represent estimates of the carrier frequency offset Δf and the carrier phase offset φ0. To evaluate the objective function J at a point (v,w), apply a frequency shift of amount −v and a phase shift of amount −w to the received samples to obtain modified samples, and perform Viterbi demodulation on the modified samples to obtain a winning path metric value at a final time. The winning path metric value is the objective function value J(v,w).