Chapter Summary

Chapter Summary

Key Points

• 1.

  LEO satellites orbit at 7.5 km/s. Doppler at 28 GHz: up to $\pm 690$ kHz — an order of magnitude beyond 5G NR mmWave subcarrier spacing. OFDM SINR ceiling at LEO Doppler: $-10$ dB (unusable). OTFS SINR: full 20 dB (no Doppler penalty). The architectural gap is decisive.

• 2.

  LEO channel is DD-sparse ($P = 1$-$3$ paths). Almost all energy in LOS path. OTFS's DD-domain processing captures the channel in 1-3 occupied DD bins among $\sim 10^6$ total — sparsity ratio $\sim 10^{-6}$. Detection: focus on occupied bins; rate proportional to path gains.

• 3.

  Multi-satellite macro-diversity provides 99.99% reliability. $S = 4$-$5$ simultaneously-visible satellites; coherent combining at UE. Per-satellite outage 5% → aggregate outage $\sim 10^{-7}$. Link availability improves from "good" (90%) to "near-perfect" (99.99%). Soft handover between satellites: no service interruption.

• 4.

  CommIT contribution: Buzzi-Caire-Colavolpe 2024. First quantitative establishment of OTFS as the 6G NTN waveform. Key numbers: 40 Mbps per UE, 99.99% availability, 8 ms latency — 4× rate and 100× reliability vs GEO; 3× reliability vs OFDM- based LEO (Starlink baseline).

• 5.

  LEO-OTFS is the only viable option at mmWave. Below mmWave (sub-6 GHz), OFDM is competitive at LEO. Above 20 GHz, OFDM Doppler penalty is catastrophic; OTFS is mandatory. 6G mmWave NTN will be OTFS-based.

• 6.

  6G NTN integrates terrestrial, LEO, and HAPS. Single waveform (OTFS) across all tiers. UE handles Doppler range from stationary (GEO) to 7.5 km/s (LEO) with one receiver architecture. Universal broadband — every person on Earth has connectivity.

• 7.

  Deployment roadmap: 3GPP Rel. 17-19 (2022-2026): OFDM-based NTN. Rel. 20 (2026-2028): OTFS study item. Rel. 21 (2028-2030): OTFS as LEO standard. 2030+: full 6G NTN rollout with OTFS. Starlink Gen 3, Kuiper, OneWeb Next: commercial OTFS-LEO platforms.

• 8.

  The universal coverage vision: 10,000-satellite constellation

  • cell-free OTFS + multi-tier integration = 99.99% global broadband coverage at 50+ Mbps, 10 ms latency. Eliminates the digital divide. The 2030s 6G promise.