Implementing Low-Density Parity-Check Codes in the Mars Relay Network

Abstract

This paper describes the implementation of Low-Density Parity-Check (LDPC) channel codes in the Mars Relay Network; specifically in the NASA MAVEN orbiter and the ESA ExoMars Trace Gas Orbiter. LDPC codes, also known as Gallager codes, were invented in the early 1960s. At the time, available computational hardware was not powerful enough to implement these codes, so they became “forgotten.” They were later “rediscovered” in the 1990s, whereupon it was demonstrated that practical LDPC decoders performed substantially better than standard convolutional codes. This spurred research and development of LDPC codes at JPL, which culminated in the implementation of a rate 1/2 LDPC decoder and encoder in the Electra proximity radio delivered to MAVEN in 2012 and subsequently to other Mars spacecraft. The paper starts with a brief history of LDPC code development and then goes on to describe the development and validation of recent firmware and software updates to facilitate LDPC code operation with adaptive data rate control. The LDPC codes developed for Electra were verified to produce approximately 3 dB coding gain relative to NASA's standard convolutional code, increasing data return by up to a factor of two. The paper concludes with an assessment of in-flight performance of LDPC-enabled relay passes between MAVEN and the Mars 2020 rover.