Diversity Transforms for OTFS-based LEO Non-Terrestrial Networks

Low Earth orbit (LEO) satellite constellations are a key enabler for non-terrestrial networks (NTNs) in sixth-generation (6G) systems. However, achieving high reliability over NTN links for mission-critical applications remains challenging, particularly for high-mobility terminals. Existing transmission diversity techniques based on selection combining or maximal ratio combining (MRC) provide no guaranteed recovery under simultaneous channel outages. Moreover, conventional retransmission schemes in the NTN are incompatible with strict latency constraints in 6G. This paper introduces a diversity transform that integrates forward error correction (FEC) coding with transmission diversity to improve the reliability of orthogonal time frequency space (OTFS)-modulated LEO NTN downlinks. It guarantees block‑erasure recovery up to a prescribed number of subchannels, with residual errors corrected by the outer FEC, thereby eliminating retransmissions. The transform is inserted between the FEC encoder and the modulator, enabling compatibility with various coding schemes and channel models. In this system, a data word is first partitioned into subwords, each independently FEC encoded. A diversity mapper then distributes these encoded subwords across the available subchannels using a binary transform matrix derived from a Reed–Solomon code. This structure guarantees block-erasure recovery whenever a sufficient number of subchannels survive, with residual errors corrected by the outer FEC. An ephemeris-driven reliability prediction framework is developed, combining orbital mechanics, atmospheric models, and a finite-blocklength error probability analysis derived for OTFS transmission over transparent satellite relays. The framework enables proactive outage prediction without channel state information feedback. Simulation results show that the proposed diversity transform provides lower block error rate and reduced latency compared to standalone FEC and retransmission schemes, especially under blockage and high-Doppler conditions characteristic of LEO NTN links.