M-QAM Carrier Phase and Frequency Offset Tracking

Solution Overview

Problem

Existing digital communication systems using quadrature amplitude modulation (QAM) face challenges in accurately estimating and tracking carrier phase and frequency offsets due to sensitivity to random noise, leading to poor fidelity in received data.

Innovation Solution

A method employing a grid search for initial phase estimation and binary trust-weighted Kalman filter-based tracking, using a cubic polynomial approximation of the complex exponential function to compute phase and frequency offsets, ensuring robustness against noise and improving initial estimate accuracy.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If PLL-based or Kalman filter-based phase/frequency offset estimation and tracking algorithms are used, then carrier phase and frequency offset tracking is achieved, but the system becomes highly sensitive to random noise and other sources of distortion

Engineering Contradiction:

Improvecarrier phase and frequency offset estimation accuracyVSAvoidrobustness against random noise and distortion

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent applies preliminary action by performing a grid search over candidate phase offset values before final estimation. The receiver tests multiple candidate phase values (e.g., -45° to +45° in 5° steps) and selects the one that maximizes a figure of merit metric, establishing a reliable initial estimate that is less sensitive to noise before proceeding with tracking algorithms.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements feedback through iterative refinement of phase and frequency offset estimates. The receiver continuously monitors the figure of merit metric and adjusts candidate phase values based on previous results, using feedback from the channel estimator to refine estimates and improve robustness against noise and distortion in subsequent iterations.

Inventive Principle:

Principle #23Feedback

2Loss of time

If conventional phase/frequency offset estimation algorithms are used, then initial estimates can be obtained, but the fidelity of receiving transmitted bits deteriorates under noisy conditions

Engineering Contradiction:

Improveacquisition time for initial estimatesVSAvoidfidelity of received bits

Core Design Contradiction:

Loss of timeVSManufacturing precision

Solution Approach 1:

The patent applies preliminary action by performing a grid search over candidate phase offset values before final estimation. The receiver tests multiple candidate phase values (e.g., -45° to +45° in 5° steps) and selects the one that maximizes a figure of merit metric, establishing a reliable initial estimate that is less sensitive to noise before proceeding with tracking algorithms.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements feedback through iterative refinement of phase and frequency offset estimates. The receiver continuously monitors the figure of merit metric and adjusts candidate phase values based on previous results, using feedback from the channel estimator to refine estimates and improve robustness against noise and distortion in subsequent iterations.

Inventive Principle:

Principle #23Feedback

3Measurement precision

If a grid search with multiple candidate phase offset values is performed, then initial phase offset estimation accuracy is improved, but computational complexity increases

Engineering Contradiction:

Improveinitial phase offset estimation accuracyVSAvoidcomputational complexity of phase estimation

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent applies segmentation by dividing the phase offset search space into discrete candidate values (e.g., -45° to +45° in 5° steps, creating 19 candidates). This segmentation allows the receiver to systematically evaluate each candidate using a figure of merit metric based on constellation diagram geometry, improving accuracy while keeping computational complexity manageable through structured division of the search space.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent applies parameter changes by transforming the continuous phase offset estimation problem into a discrete optimization problem. The receiver evaluates multiple discrete candidate phase values and selects the optimal one based on a figure of merit metric, changing the parameter space from continuous to discrete to balance accuracy and computational complexity.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS11146439B2Robust carrier phase and frequency tracking for M-QAM demodulation

Publication Date: 2021.10.12 NATIONAL INSTRUMENTS CORP

AI summary

A method for determining coarse carrier phase and frequency offsets of an initial block of received M-QAM symbols includes creating a grid of discrete candidate phase offset values and for each candidate value: applying the candidate value to each symbol, applying a respective hard decision to each applied symbol, and computing a figure of merit based thereon. The candidate value having the best figure of merit is selected as an initial phase offset estimate. An initial frequency offset estimate is computed using the symbols updated with the initial phase offset estimate, their respective hard decisions, and an approximation of the complex exponential function. To track carrier phase and frequency offsets associated with a series of symbol blocks, for each symbol of a current block, set a binary trust weight based on comparison of a computed parameter with a threshold and use the binary trust weights to compute a phase offset error and a frequency offset error for the current block.