Closed-Loop Space-Ground Cognitive Software-Defined Radio for LEO Satellite Broadband: Cooperative FrFT-Riemannian Sensing, Contractive Banach Predistortion, and Dual-Doppler Soft Handover

Abstract

Connecting rural communities, maritime vessels, and emergency responders to Low Earth Orbit (LEO) satellite broadband constellations (such as Starlink, OneWeb, Kuiper, and 3GPP Release-18/19 Non-Terrestrial Networks) currently relies on vendor-locked user terminals equipped with proprietary modems, temperature-stabilized oscillators, and heavily backed-off linear power amplifiers. Transitioning to an open-architecture, low-SWaP-C (Size, Weight, Power, and Cost) Commodity Off-The-Shelf Software-Defined Radio (COTS SDR) baseband transceiver paired with a regenerative satellite SDR payload (interfacing directly at L/S-band or behind standard LNB/BUC RF front-ends at Ku/Ka-band) requires overcoming four coupled physical-layer challenges: (i) severe 27,000 km/h orbital Doppler shifts (+/-45 to +/-300 kHz), chirp rates (-1.5 kHz/s), uncompensated oscillator drift, and terrain-occluded hidden nodes that degrade Geostationary (GEO) incumbent spectrum sensing; (ii) nonlinear Solid-State Power Amplifier (SSPA) saturation that distorts 32APSK constellations and leaks intermodulation interference into protected GEO bands, including during 0-deg in-line LEO-GEO conjunctions where spatial beamforming fails; (iii) space-ground round-trip delays (10 to 50 ms) that render reactive link adaptation stale and cause 45-115 ms single-RF-chain handover blackouts; and (iv) high-latency neural network controllers that exceed sub-millisecond symbol timing budgets and lack deterministic ITU interference guarantees. This paper presents CPG-CogSDR, an open-architecture, closed-loop space-ground baseband transceiver that compensates for low-cost RF front-end impairments through three coupled signal-processing stages. First, a dual space-ground ephemeris-conditioned Fractional Fourier Transform (FrFT) and Affine-Invariant Symmetric Positive Definite (SPD) Riemannian manifold detector (E-FRFT-RM), paired with a W=3 kinematic-elevation observability filter (KEO-SRG) and a 64-bit space-ground cooperative Riemannian geodesic barycenter (SGC-RGF), achieves 99.67% F1-score and 0.33% false-positive rate in 93.61 us (52.5x faster than 1D CNN baselines) while improving hidden-node recall from 70.00% to 100.00%. Second, a contractive Banach digital predistorter and weighted subspace spectral nuller (CB-DPD-SNS) contracts with Lipschitz constant L <= 0.088 < 1 to cancel 99.05%-99.14% of SSPA distortion (0.055%-0.058% EVM), form -75.36 to -76.43 dBc GEO protection notches on a single RF chain, and recover +5.96 dB of uplink power margin (74% power reduction). Third, an AR(2) minimum-outage look-ahead link adapter and Dual-FrFT DAPS soft handover controller (HP-PACM-HO) combines autoregressive scintillation prediction with a dual-angle FrFT bank and 4-tap Lagrange FIFO buffer to increase pass goodput by +39.84% to +43.85% with 0.0 ms handover interruption. Evaluated across a 17-block 3GPP NTN / DVB-S2X baseband simulator, a real-time UDP loopback testbed, and a 45-waypoint public SatNOGS/NORAD RTL-SDR over-the-air (OTA) anchor, the transceiver passes all 12 IEEE Std 1012-2016 verification checks and confirms cross-band dimensionless scaling (subspace angle 4.07 deg < 6 deg, Kolmogorov-Smirnov p = 0.8245 > 0.05).