Low-density parity-check (LDPC) codes often suffer from an “error floor” in which increasing signal-to-noise ratios do not lead to substantially better error rate performance. Error floors of LDPC codes are often not an inherent problem of the code design, but rather, a limitation of the standard decoding algorithms.

In this new technology, small but critical variations in standard belief propagation decoding algorithms dramatically lower the error floor of LDPC codes, including the set of AR4JA LDPC codes that are recommended by the Consultative Committee for Space Data Systems (CCSDS), an international standards organization.

A modification of standard belief propagation message passing is made in a special way that allows the new decoder to converge to the correct codeword with a higher probability than was previously possible. The key innovation that leads to the lower error floors, particularly for the rate 4/5 AR4JA LDPC code and for any of the AR4JA codes when used with higher order modulations, is the introduction of a “partial hard-limiter” for decoder messages coming from the check nodes of the decoding graph.

Nearly every space mission NASA flies uses channel codes to protect the reliability of transmitted bits. LDPC codes are the way of the future, as they are contained in the standards not just for civilian space use, but also for terrestrial WiFi, WiMAX, and digital video broadcasting.

This work was done by Jon Hamkins of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

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