Iterative Equalization and Turbo Decoding for HF Multipath Channels

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Solution Overview

Problem

High Frequency (HF) communications systems face performance degradation due to the difficulty in implementing differential modulation as constellation size increases, especially with Phase Shift Key (PSK) or Quadrature Amplitude Modulation (QAM), which is challenging for recursive inner codes in multipath/fading channels.

Innovation Solution

The system employs coherent modulation instead of differential modulation, using a serial concatenation of a convolutional code and a recursive inner rate one code, with interleaving to remove correlation effects from multipath fading channels, enabling iterative equalization and decoding.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If differential modulation is used with recursive inner code, then interleaver gains are obtained in multipath/fading channels, but implementation difficulty increases significantly as constellation size increases

Engineering Contradiction:

Improveperformance in multipath/fading channelsVSAvoidimplementation difficulty of differential modulation

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent changes the modulation parameter from differential modulation to coherent modulation. This parameter change allows the system to maintain the benefits of recursive inner codes and interleaver gains while avoiding the implementation difficulties that arise with larger constellation sizes in differential modulation schemes.

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If coherent modulation is used with iterative equalization and decoding, then implementation complexity is reduced, but the system traditionally requires differential modulation to achieve recursive inner code gains

Engineering Contradiction:

Improvemodulation implementation complexityVSAvoidinterleaver gains from recursive inner code

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent segments the coding structure into an outer convolutional code and an inner recursive code, with the inner code operating at rate 1. This segmentation allows the inner recursive code to provide interleaver gains while the outer code handles error correction, and enables coherent modulation to be used instead of differential modulation.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements iterative equalization and decoding with feedback loops where the outer decoder provides feedback to the equalizer and the inner decoder provides feedback to the outer decoder. This feedback mechanism enables the system to achieve both coherent modulation and recursive inner code gains simultaneously.

Inventive Principle:

Principle #23Feedback

Data Source

PatentUS7620881B2System and method for communicating data using iterative equalizing and decoding and recursive inner code

Publication Date: 2009.11.17 L3HARRIS GLOBAL COMMUNICATIONS INC

AI summary

A system and method for communicating data includes an inner encoder for encoding data with the recursive inner rate one code. A modulator coherently modulates a communications signal that carries the data that has been encoded. An interleaver is operative with the inner encoder and modulator to aid in removing correlation of multipath fading channels on consecutive bits. A convolutional encoder is followed by a random interleaver. The modulator can be operative with mini-probe sequences. An equalizer and decoder receives the communications signal and iteratively decodes any recursive inner rate one code and convolutional code in a turbo fashion.