Turbo Code Iterative Decoding With Hybrid Log-MAP Lookup Tables
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Solution Overview
Problem
The computational complexity of the Maximum A Posteriori (MAP) algorithm in turbo code decoding is high, making it difficult to implement effectively in practice, despite being the optimal decoding algorithm for turbo codes.
Innovation Solution
The method involves generating a look-up table for transcendent functions representing correcting factors, performing calculations to obtain forward and backward metrics, and using the Log-MAP algorithm with optional Max-Log-MAP algorithm implementations to reduce complexity, thereby facilitating iterative decoding in digital systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the Maximum A Posteriori (MAP) algorithm is used for turbo code decoding, then decoding performance is optimized, but computational complexity increases significantly
Solution Approach 1:
The patent transforms the MAP algorithm from the linear domain to the logarithmic domain by applying logarithmic transformation to the probability calculations. This parameter change converts multiplication operations into addition operations, significantly reducing computational complexity while maintaining decoding performance. The log-likelihood ratios are computed using logarithmic transformations of the forward and backward metrics.
Solution Approach 2:
The patent introduces lookup tables (LUTs) that store pre-computed values of the correcting factor function. Instead of performing complex transcendental function calculations during decoding, the system retrieves pre-computed values from the LUT, dramatically reducing computational complexity. The LUT acts as a disposable pre-computed resource that replaces expensive real-time calculations.
2Device complexity
If the Log-MAP algorithm is implemented using lookup tables, then computational complexity is reduced, but memory requirements increase
Solution Approach 1:
The patent implements a hybrid approach where the Log-MAP algorithm is partially implemented using lookup tables for the correcting factor, while other parts of the algorithm may use Max-Log-MAP approximations or simplified calculations. This partial use of LUTs provides a trade-off between computational complexity reduction and memory resource consumption, allowing system designers to adjust the degree of LUT usage based on available resources.
3Measurement precision
If iterative decoding is performed with multiple calculations, then decoding accuracy improves, but processing time increases
Solution Approach 1:
The patent performs preliminary computation of the correcting factor values and stores them in lookup tables before the actual decoding process. By pre-computing these values, the system avoids repeated complex calculations during iterative decoding, thereby reducing processing time while maintaining the ability to perform multiple iterations for improved decoding accuracy.
Data Source
AI summary
In a digital system using a turbo code, a method for performing iterative decoding in accordance with a Log-MAP Algorithm comprises the steps of:—generating a look-up table comprising a plurality of values representative of a correcting factor;—performing a first calculation to obtain a forward metric;—performing a second calculation to obtain a backward metric;—performing a third calculation to obtain a log-likelihood ratio for every information bit to be decoded. In accordance with the method, at least one and no more than two of such calculations are performed by the use of said look-up table for implementing the Log-MAP decoding algorithm and the remaining calculations are performed implementing a Max-Log-MAP decoding algorithm.