Low-Density Parity-Check (LDPC) codes are a class of forward error correction (FEC) linear block codes that provide near-capacity performance for power-efficient communications. Optimum decoding requires accurate combining ratio estimation to scale the input signal in an additive white Gaussian noise channel (AWGN). Test data and analysis show that the performance of the optimal LDPC decoding algorithm is severely degraded when encountering pulsed radio frequency interference (RFI) from sources such as ground-based radars.

This research effort first reveals that the LDPC performance degradation under pulsed RFI is not due to the burst of errors but to the inaccurate combining ratio estimation. Although an ideal combining ratio estimator (symbol-wise) could mitigate the degradation caused by pulsed RFI, it is not practical for implementation. Some near optimal LDPC decoding algorithms that do not require combining ratio estimation are investigated.

One near optimum decoding algorithm “min-scale” is proposed in the robust decoder design to mitigate pulsed RFI. Analysis shows that min-scale only loses about 0.2 dB performance compared to the optimal decoding algorithm under nominal conditions; in a pulsed RFI environment, the designed performance can be achieved if the system design has adequate margin.

Without combining ratio estimation, the decoder design complexity is reduced and the risk of performance degradation due to combining ratio estimation error is eliminated. This robust decoder design enables reliable communications to space vehicles that may encounter strong pulsed RFI signals from high-power radar systems.

This work was done by Jianjun Ni and Chatwin Lansdowne of Johnson Space Center. MSC-25776-1

NASA Tech Briefs Magazine

This article first appeared in the September, 2015 issue of NASA Tech Briefs Magazine.

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