Tightly-coupled VS Loosely-coupled accelerators for data compression in space applications: a NOEL-V case study

The growing complexity of space systems and missions is causing a rapid increase of onboard data size, to the point where efficient data compression algorithms become essential. The resource constrained environment requires careful consideration of how to optimize this procedure. On the other hand, NOEL–V is emerging as a promising space-grade processor, offering both high performance and reliability for critical space applications. Being built on the RISC–V ecosystem, NOEL–V inherits its ability to enhance performance through the integration of hardware accelerators with the processor. Tightly-coupled and loosely-coupled accelerators (TCAs and LCAs respectively) offer different benefits: the former are more compact but require more effort to integrate them, while the latter should be carefully designed to minimize their area. This work will compare those two approaches regarding the optimization of the CCSDS 121.0 algorithm, a lossless data compression technique recommended as a standard by the Consultative Committee for Space Data Systems. Synopsys ASIP Designer has been used to design and integrate the accelerators into a NOEL–V model. Two variants of TCAs will be proposed, each targeting specific bottlenecks of the algorithm, reaching a total clock cycle reduction of 83%, with an area increment of only 9% (based on 65 nm low power ASIC technology). The proposed memory-mapped LCA performs the entire compression procedure, reaching a speed-up factor of 480× and a 48% area increment with respect to the reference implementation. With a throughput of 7.83 Gb/s, the LCA turned out to be the far superior design in terms of Throughput-Area Ratio, but the TCAs proved to be a valid choice for lossless data compression optimization in area-constrained environments.